Precambrian Research 99 (2000) 113–146www.elsevier.com/locate/precamres
U–Pb geochronology of zircon and monazite fromMesoproterozoic granitic gneisses of the northern
Blue Ridge, Virginia and Maryland, USA
John N. Aleinikoff a,*, William C. Burton b, Peter T. Lyttle c,Arthur E. Nelson b, C. Scott Southworth b
a US Geological Survey, MS 963, Denver, CO 80225, USAb US Geological Survey, MS 926A, Reston, VA 22092, USAc US Geological Survey, MS 908, Reston, VA 22092, USA
Received 29 March 1999; accepted 23 July 1999
Abstract
Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province inVirginia and Maryland. Lithology, structure, and U–Pb geochronology have been used to subdivide the gneisses into threegroups. The oldest rocks, Group 1, are layered granitic gneiss (1153±6 Ma), hornblende monzonite gneiss (1149±19 Ma),porphyroblastic granite gneiss (1144±2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?).These gneisses contain three Proterozoic deformational fabrics. Because of complex U–Pb systematics due to extensiveovergrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolutionion microprobe (SHRIMP), whereas all other ages are based on conventional U–Pb geochronology. Group 2 rocks areleucocratic and biotitic varieties of Marshall Metagranite, dated at 1112±3 Ma and 1111±2 Ma respectively. Group 3rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077±4 Ma) and quartz-plagioclase gneiss(1077±4 Ma); (2) white leucocratic metagranite (1060±2 Ma), pink leucocratic metagranite (1059±2), biotite granitegneiss (1055±4 Ma), and megacrystic metagranite (1055±2 Ma). Groups 2 and 3 gneisses contain only the two youngerProterozoic deformational fabrics. Ages of monazite, separated from seven samples, indicate growth during both igneousand metamorphic (thermal ) events. However, ages obtained from individual grains may be mixtures of different agecomponents, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite(imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070,1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granuliteevent at about 1035 Ma did not exceed about 750°C. Lack of evidence for 1110 Ma growth of monazite in porphyroblasticgranite gneiss suggests that the Short Hill fault might be a Grenvillian structure that was reactivated in the Paleozoic. Thetiming of Proterozoic deformations is constrained by crystallization ages of the gneissic rocks. D1 occurred between about1145 and 1075 Ma (or possibly between about 1145 and 1128 Ma). D2 and D3 must be younger than about 1050 Ma.Ages of Mesoproterozoic granitic rocks of the northern Blue Ridge are similar to rocks in other Grenville terranes of theeastern USA, including the Adirondacks and Hudson Highlands. However, comparisons with conventional U–Pb ages ofgranulite-grade rocks from the central and southern Appalachians may be specious because these ages may actually bemixtures of ages of cores and overgrowths. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Blue Ridge; Grenville; Mesoproterozoic; Monazite; U–Pb absolute age; Zircon
* Corresponding author. Fax: +1-303-236-4930.E-mail address: [email protected] (J.N. Aleinikoff )
0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0301-9268 ( 99 ) 00056-X
114 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
1. Introduction quadrangles that have been partially or completelymapped as part of the Washington West andFrederick 30∞×60∞ quadrangle geologic mappingThe northern Blue Ridge Physiographic
Province, about 150 km in length from central by the US Geological Survey in cooperation withthe Maryland Geological Survey. Previous geo-Virginia to south-central Maryland, is composed
primarily of Mesoproterozoic granulite facies chronologic work on Mesoproterozoic rocks tothe south (cited in Bartholomew and Lewis, 1984;gneisses and mantling greenschist facies metasedi-
mentary and metavolcanic cover sequence rocks Clarke, 1984) indicated that these basementgneisses are about 1.0–1.2 Ga, with large uncer-of Neoproterozoic to Early Cambrian age. This
package of rocks comprises a small part of the tainty. During the course of the new mapping,Mesoproterozoic orthogneisses were subdividedBlue Ridge Province, an upland belt that extends
nearly 900 km from Georgia to Pennsylvania into 13 varieties, on the basis of lithology andstructure [presence of foliation(s)], 12 of which(Fig. 1). Recent detailed 1:24 000-scale maps
( Kline et al., 1991; Fauth and Brezinski, 1994; were collected for U–Pb zircon geochronology. Inaddition, monazite from seven samples was datedSouthworth, 1994, 1995; Burton et al., 1995;
Southworth and Brezinski, 1996; Nelson, 1997) to either confirm the age of emplacement of theigneous protolith or determine the timing ofhave delineated more than 15 Mesoproterozoic
basement units. Geologic maps at 1:100 000 scale metamorphism.(Burton et al., 1992; Lyttle et al., 1999; Southworthet al., unpublished data) synthesize the geology ofthis region (Fig. 2). 2. Geologic setting
The focus of this paper is the determination ofages of granitic gneisses found within 20 7.5∞ Mesoproterozoic gneisses and Neoproterozoic
to Early Cambrian cover rocks of the northernBlue Ridge are bounded on the west by foldedPaleozoic sedimentary rocks of the Great Valleyof the Valley and Ridge Province and on the eastby gently west-dipping Upper Triassic and LowerJurassic clastic rocks and basalt of the Culpeperbasin. Both basement gneisses and cover sequencerocks were deformed in the late(?) Paleozoic toform the Blue Ridge–South Mountain anticlino-rium, a complex fold that is overturned to thewest. The Mesoproterozoic gneisses of our studyarea are an extension of the Lovingston Massif ofcentral Virginia (Sinha and Bartholomew, 1984),a relatively shallow terrane of amphibolite to gran-ulite facies rocks, located to the east of the deeperlevel granulite facies Pedlar Massif. However, acharnockite (described below) in the northernBlue Ridge may be correlative with rocks of thePedlar River Charnockite Suite (Sinha andBartholomew, 1984).
The term ‘Marshall Granite’ was originally usedby Jonas (1928) for widespread Precambrian gran-ite in the Blue Ridge basement of northern Virginiaand Maryland. This granite was formally namedFig. 1. Areas of exposed Mesoproterozoic rocks in the eastern
North America. the Marshall Metagranite by Espenshade (1986),
115J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
who recognized both fine- and coarse-grained vari- is distinguished by an extremely variable migmati-tic texture on an outcrop scale. Textures rangeeties (Ym, Ymc, Fig. 2) in the Marshall
Quadrangle. In the adjacent Orlean 7.5∞ quadran- from a well-foliated gneissic layering to totallymassive, with diffuse or irregular boundariesgle, Clarke (1984) mapped four varieties of granitic
gneiss [including Flint Hill Gneiss (Yfh, Fig. 2), between the textural domains. Complex fold pat-terns and cross-cutting relationships are typicallyporphyroblastic gneiss (Ypg), augen gneiss (Ypb),
and Marshall Metagranite (Ym)] and Howard seen in outcrop. Layered granitic gneiss may be ahybrid rock, such as layered felsic volcanic rock(1991) recognized three varieties of granitic gneiss
and a metanorite in his regional study of that was partially migmatized during injection ofporphyroblastic granite gneiss (Ypg), with whichMesoproterozoic rocks in northern Virginia.
Mesoproterozoic rocks that make up the core it is frequently associated.Hornblende monzonite gneiss (Yhg) is largelyof the northern Blue Ridge anticlinorium consist
of weakly to strongly foliated gneisses exhibiting restricted to an area west of the Short Hill faultin the northern part of the study area. It is typicallyhigh-grade metamorphic mineral assemblages and
textures. The orthogneisses can be divided into a dark green, medium-grained, well-foliated rockin which hornblende is the dominant mafic mineraltwo groups on the basis of lithology: (1) moder-
ately to strongly foliated granitic gneiss and weakly (about 10% by volume), quartz content is typicallyonly 15–20%, and microcline is as much as 50%.to moderately foliated granite gneiss that comprise
over 90% by volume of the Mesoproterozoic base- Lighter-colored, more leucocratic phases also exist.It has a metaluminous composition, in contrast toment (Fig. 2), and (2) volumetrically minor non-
granitic rocks including quartz-plagioclase gneiss all of the other granitic compositions, which areperaluminous (Burton and Southworth, 1996).(Yqp, Fig. 2) and garnet-graphite paragneiss,
hornblende-pyroxene metanorite, and quartzite Porphyroblastic granite gneiss (Ypg) is wide-spread over the study area. It is yellowish-brown(undivided non-granitic rocks on Fig. 2) (Burton
et al., 1992). Protoliths of the paragneiss, metanor- weathering and consists of ovoid porphyroblastsof microcline 1–3 cm long in a matrix of finer-ite, and quartzite are considered to be older than
the granitic protoliths (Burton and Southworth, grained plagioclase and gray quartz. Biotite is thedominant mafic mineral and garnet is locally1993).
A total of 11 of the 13 varieties of common. Despite its coarse grain size, this rock istypically well-foliated, as shown by the flatteningMesoproterozoic granitic gneiss that have been
mapped in the northern Blue Ridge anticlinorium and preferred elongation direction of the porphyro-blasts. At a number of localities, this porphyro-(Fig. 2) contain biotite as the dominant mafic
mineral, and some are locally very leucocratic. The blastic granite gneiss is intruded by irregulardikelets of younger granitic material, including gar-terms ‘metagranite’ (a poorly foliated, generally
mica-poor granite), ‘granite gneiss’ (a well-foliated, netiferous metagranite (Ygt), pink leucocraticmetagranite (Yml ), and possibly Marshall Meta-relatively mica-rich granite) and ‘granitic gneiss’
(a well-foliated, relatively mica-rich intermediate granite (Ym).Coarse-grained metagranite (Ymc) has a dis-to felsic rock) are used to differentiate degree of
foliation and possible protolith in these metaig- tinctive texture consisting of 1–2 cm long, denselypacked white or pink, sub- to euhedral microclineneous rocks. All names for rock types are informal
except Marshall Metagranite (Espenshade, 1986). porphyroblasts and lesser interstitial plagioclaseand distinctive blue quartz. This rock typically hasSamples are described below in order of decreasing
age. This order is determined primarily by U–Pb a coarse, indistinct Mesoproterozoic foliation con-sisting of crudely aligned porphyroblasts. It occursgeochronology, although local field relations, as
noted below, have aided relative age assignments largely as map-scale inclusions within MarshallMetagranite (Ym), and was originally mapped(also see Fig. 2).
Layered granitic gneiss (Ylg) is mostly medium- as a coarser-grained facies of the Marshall(Espenshade, 1986; Kline et al., 1991).grained and white, gray, or pink weathering, and
116 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fig. 2. Generalized geologic map of the northern Blue Ridge geologic province, Virginia–Maryland: (a) northern portion; (b) south-ern portion.
117J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fig. 2. (continued)
118 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
One of the most abundant Mesoproterozoic weathering, medium- to fine-grained and generallywell-foliated, with a biotite content of 10–15%.granitic rocks in the Blue Ridge basement is theGarnetiferous metagranite (Ygt) and leucocraticMarshall Metagranite (Ym), first described bymetagranite (Yg) are interlayered with the biotiteJonas (1928) and later by Espenshade (1986). Itgranite gneiss (Ybg).occurs mostly to the east of the Short Hill fault
Megacrystic metagranite (Ypb) occurs over a(Fig. 2). It is an orange or pink weathering, weaklylarge area in the southwestern part of the studyto moderately foliated, medium-grained biotitearea (Fig. 2). It is a coarse-grained, porphyroblas-granite gneiss in which biotite content ranges fromtic rock that resembles porphyroblastic granite10–15%. Leucocratic and more biotitic subvarietiesgneiss (Ypg) and, locally, coarse-grained meta-also have been recognized. Thin pink or whitegranite (Ymc); however, Ypb is separated fromfiner-grained aplite and coarser-grained pegmatitethe other two coarse-grained lithologies by aveins commonly intrude this granite and locallynorth–south-trending belt of the Neoproterozoicdefine a layering parallel to foliation.Robertson River Intrusive Suite (Tollo andQuartz-plagioclase gneiss (Yqp) is restricted toAleinikoff, 1992). Compositional and texturalthe northern part of the basement as narrow lensesdifferences that distinguish megacrystic metagrani-within biotite granite gneiss (Ybg), leucocratictic (Ypb, an adamellite with single large microclinemetagranite (Yg), and garnetiferous metagranitecrystals) from porphyroblastic granite gneiss (Ypg,(Ygt). It is a white to gray-weathering felsic rocka granite with porphyroblasts composed of micro-that ranges from massive to well-foliated, withcline and quartz aggregates) are recognizablelittle or no potassium feldspar and minor (0–10%)(Clarke, 1984).biotite. It has a trondhjemitic chemical composi-
Medium- to coarse-grained, massive totion (Burton and Southworth, 1996).well-foliated quartz–hornblende–orthopyroxene–Garnetiferous metagranite (Ygt) is one of thepotassium feldspar–plagioclase rock or charnock-predominant rock types at the north end of theite (Yc) occurs in linear and pod-like bodies that
exposed core of the Blue Ridge anticlinorium. It are mapped primarily on the basis of float. Theis white, light gray or cream weathering, medium- charnockite is distinctive in having a dark green,to fine-grained, and massive to weakly foliated. It fresh surface and a crusty, pitted, orange–yellowis similar in appearance to leucocratic granite (Yg, weathering rind 1–2 cm thick. Massive charnockitesee below); these rocks are differentiated primarily is found as a pod-like body within well-foliatedon the presence or absence of scattered almandine hornblende monzonite gneiss (Yhg) northwest ofgarnets. Biotite content ranges from 0 to 10%. Hillsboro (Fig. 2), whereas farther south well-
White leucocratic metagranite (Yg) is similar foliated charnockite occurs as linear bodies withinin appearance to garnetiferous metagranite (Ygt), various granitic gneisses.except that it lacks garnets. White aplite and Pink to white aplites and pegmatites commonlypegmatite sills also locally define layering in this intrude the other granitic rocks on an outcroprock. scale (not shown in Fig. 2). They range in mor-
Pink leucocratic metagranite (Yml ) borders the phology from thin (1–2 cm) planar dikes and sillsMarshall Metagranite (Ym) (Fig. 2) and has the to thicker (several meters) irregular bodies.same orange to pink weathering aspect and grain Because some of these bodies are highly deformedsize but a more variable biotite content (0 to 10– and others cross-cut all Mesoproterozoic struc-15%). Its texture ranges from totally massive to tures, they probably have a range of intrusive ages.locally well-foliated and/or lineated. The map No dating of these rocks was attempted.pattern of Yml (Fig. 2) suggests a thick sill-likebody that intruded between the MarshallMetagranite and gneisses of the western Blue Ridge 3. Mesoproterozoic structures and metamorphismand enclosed a septum of porphyroblastic granitegneiss (Ypg). Mesoproterozoic structures and mineral assem-
blages, produced during the 1.1–1.0 Ga GrenvilleBiotite granite gneiss (Ybg) is orange to gray
119J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
orogeny, are discernible in granitic and non- Outcrop-scale, open to tight, upright folds in folia-tion and migmatitic layering (D3) are locally devel-granitic rocks despite the fact that rocks of the
northern Blue Ridge anticlinorium developed a oped and have gently north- or south-plunginghinges. These folds arch D2 foliation and areregional cleavage and experienced metamorphic
retrogression during late (?) Paleozoic deforma- partly responsible for its NE to NW range in strike(Burton and Southworth, 1996).tion. All of the granites sampled in this study have
a Grenvillian metamorphic foliation, indicating Mineral assemblages, in textural equilibriumthat indicates granulite-facies and upper amphibo-that deformation occurred or continued after intru-
sion of the youngest granitic protoliths. Grenvillian lite-facies metamorphism, are found in some rocksin the map area. These include hornblende–ortho-foliation is defined by platy or tabular mafic miner-
als such as biotite or hornblende, flattened, pyroxene–microcline-bearing charnockite (Yc,Fig. 2), and brown hornblende–orthopyroxene-unstrained quartz and feldspar in a granoblastic-
elongate fabric and, locally, a migmatitic layering bearing metanorite (included in non-granitic rocks,Fig. 2). Kline et al. (1994) studied relict peridotiticdefined by thin, lit–par–lit sills of aplite or pegma-
tite. Gneissic banding composed of alternating thin (olivine–pyroxene-bearing) and gabbroic (horn-blende–pyroxene) mineral assemblages in a smallmafic and felsic layers is rare but is seen in layered
granitic gneiss (Ylg). Open to isoclinal folds in composite ultramafic body that they interpreted asigneous. Hornblende from this body yielded a latefoliation and ptygmatic folds in cross-cutting aplite
and pegmatite veins indicate ductile conditions Grenvillian 40Ar/39Ar age of 995 Ma, which Klineet al. (1994) interpret as a cooling age followingduring deformation. A stretching lineation con-
sisting of biotite streaks and/or rodded quartz and intrusion of the ultramafic body; 40Ar/39Ar agesof 1.0 to 0.92 Ga from hornblende in hornblendefeldspar occurs locally on foliation surfaces. This
lineation is best developed in the biotite-bearing monzonite (Yhg) are interpreted as a cooling agesfollowing regional metamorphism ( Kunk et al.,granitic gneisses such as Marshall Metagranite
(Ym) and biotite granite gneiss (Ybg). Where 1993). No P–T–t studies have been conducted onthese rocks, primarily because their mineralogicisoclinal folds and mineral lineations occur
together, the fold hinges and stretching lineations compositions are not amenable to such an analysis.are collinear, suggesting a common origin (Burtonet al., 1994; Burton and Southworth, 1996).
Orientations of Grenvillian structures and cross- 4. U–Pb geochronologycutting relationships suggest multiple episodes ofdeformation. An older episode of foliation devel- 4.1. Proceduresopment (D1) is recorded by one exposure ofporphyroblastic granite gneiss (Ypg, Fig. 2), where Approximately 25 kg of each sample were pro-
cessed through the standard mineral separationsa vein of garnetiferous metagranite (Ygt), else-where well-foliated, truncates well-developed, procedure, including crushing, pulverizing, and
concentration of heavy minerals using a Wilfleynorthwest-trending D1 foliation in the host rock(Burton et al., 1994). If unrotated this foliation Table, methylene iodide, and magnetic separator.
All samples were obtained from outcrops exceptrecords NE–SW compression. Northwest-trendingfoliation in hornblende monzonite gneiss (Yhg) sample Bl-1-94 which was collected from a large
pile of rock that had been removed from a pastureand layered granitic gneiss (Ylg) is also inferredto be of D1 age (Fig. 2). Grenvillian D2 foliation as in situ float. The most important criteria for
selection of zircons for analysis was clarity (ashas two main trends, northwest and northeast,with vertical to northeast or southeast dips respec- clear as possible with a minimum of cracks and
inclusions) and color (in general, light tan grainstively. The collinear folds and mineral stretchinglineations are developed in this foliation and contain less uranium than darker grains).
Although ideally we would prefer to select euhedralplunge southeast, perhaps indicating SE–NWtransport after initial development of D2 foliation. zircons, in many instances fragments of grains
120 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146T
able
1U
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122 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146T
able
1(c
onti
nued
)
Gra
ina
Wei
ght
Con
cent
rati
ons
Mea
sure
dP
bco
mpo
siti
onb
Rat
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(%er
ror)c
Age
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(mg)
(ppm
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UP
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b/206P
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b/206P
b/207P
b/207P
b/204P
b204P
b207P
b208P
b238U
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b
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5497
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12.9
880.
282
140.
1851
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4)1.
914
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5)0.
074
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.07)
1095
1086
1068
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onaz
iteC
4e0.
023
2180
1744
2027
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68.4
12.2
950.
249
480.
1802
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9)1.
865
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0)0.
075
07(0
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1068
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1070
5sm
onaz
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123
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237
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884
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3)0.
075
10(0
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1077
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onaz
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076
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241
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9)1.
884
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9)0.
074
99(0
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onaz
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2296
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208
770.
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2)1.
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7)0.
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7)0.
075
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0)0.
074
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0.29
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1.89
3(0
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0.07
519
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9)10
8110
7910
74
WW
29-9
0(Y
m)
6a(−
100+
200
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0.02
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67.7
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89.8
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(0.2
1)1.
882
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2)0.
076
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1063
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6b(−
150+
200
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0.04
350
1.9
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18.2
260.
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7)1.
861
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8)0.
075
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0.01
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0.00
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8)0.
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7)
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6e(−
150+
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0.01
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3.7
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0)1.
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m)
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100+
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10.
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1218
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12.9
7310
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0.18
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5)11
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197b
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0+15
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213
118
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7c(−
100+
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30.
027
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177
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2391
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3310
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0.16
92(0
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1.78
9(0
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0.07
665
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0)10
0810
4111
127d
(−10
0+15
0)E
C4
0.02
341
9.9
81.1
214
66.6
1956
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9.35
080.
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4)1.
974
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7)0.
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58(0
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1105
1107
1110
7e(−
100+
150
)EC
50.
053
456.
186
.95
4636
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04.1
12.8
1210
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0.18
79(0
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1(0
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0.07
65(0
.51
)11
1011
0911
077f
(−15
0+20
0)E
10.
020
529.
010
0.9
1167
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32.9
11.5
848.
8254
0.18
26(0
.22)
1.92
4(0
.26)
0.07
642
(0.1
2)10
8110
8911
067g
(−15
0+20
0)E
20.
028
831.
316
2.0
1063
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53.2
11.2
657.
5897
0.18
27(0
.16)
1.92
7(0
.17)
0.07
646
(0.0
6)10
8210
9011
07
123J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146B
R16
74(Y
qp)
8a(−
100+
150
)E1
0.03
977
5.9
145.
982
6.18
857.
2510
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10.9
410.
1809
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6)
1.87
7(0
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0.07
525
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2)
1072
1073
1075
8b(−
100+
150
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0.02
775
5.9
133.
153
87.9
8254
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15.9
740.
1794
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8)
1.85
8(0
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0.07
513
(0.0
6)
1064
1066
1072
8c(−
100+
150
)E3
0.02
611
4019
6.7
1253
728
623
13.2
4519
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0.17
81(0
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3)0.
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40.
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8.1
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60.
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BR
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0+15
0)C
20.
062
612.
212
4.4
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13.2
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0.17
88(0
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841
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5)0.
074
69(0
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6010
6010
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(−10
0+15
0)C
30.
126
334.
169
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1214
624
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13.3
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0.17
70(0
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821
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2)0.
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3)
1051
1053
1057
9d(−
100+
150
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0.04
756
3.4
116.
648
86.6
9495
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3.71
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5)
1.83
6(0
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0.07
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7)
1058
1058
1059
BR
1313
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(Ym
l)10
a(−
100+
150
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10.
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223.
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2662
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86.4
13.0
935.
2305
0.17
75(0
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827
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9)0.
074
64(0
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5310
5510
5910
b(−
100+
150
)EC
20.
016
219.
444
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880.
1015
14.8
11.7
674.
9132
0.18
06(0
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882
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2)0.
075
60(0
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)10
7010
7510
8410
c(−
100+
150
)EC
30.
027
255.
949
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4903
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408
13.2
204.
9739
0.17
60(0
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)1.
809
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0)0.
074
58(0
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)10
4510
4810
5710
d(−
100+
150
)EC
40.
037
234.
247
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8743
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693
13.3
784.
0809
0.17
81(0
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)1.
831
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1)0.
074
59(0
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)10
5710
5710
5810
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onaz
ite1e
0.04
224
6.2
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1671
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56.4
12.3
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260.
1725
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6)
1.75
4(0
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0.07
374
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9)
1026
1029
1034
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mon
azit
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0.03
517
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4413
83.7
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0.08
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0.18
10(0
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864
(0.5
1)0.
0747
(0.2
7)
1072
1068
1060
BR
554-
90(Y
bg)
11a
(−10
0+15
0)C
10.
059
748.
513
0.7
4119
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43.7
12.9
4815
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0.17
70(0
.21
)1.
814
(0.2
2)0.
074
35(0
.04
)10
5110
5110
5111
b(−
100+
150
)C2
0.04
077
0.5
133.
613
240
5500
913
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16.9
610.
1779
(0.2
6)
1.82
9(0
.27)
0.07
457
(0.0
5)
1055
1056
1057
11c
(−10
0+15
0)C
30.
050
740.
412
8.1
1721
198
569
13.4
0216
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0.17
71(0
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819
(0.1
9)0.
074
47(0
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)10
5110
5210
5411
d(−
100+
150
)C4
0.05
980
5.3
139.
615
117
3455
713
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18.2
960.
1784
(0.1
8)
1.83
6(0
.20)
0.07
462
(0.0
8)
1058
1058
1058
11e
(−10
0+15
0)C
60.
040
768.
714
0.0
1299
.014
04.1
11.8
1310
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0.17
75(0
.38
)1.
824
(0.4
0)0.
0745
(0.1
3)
1053
1054
1055
11f
mon
azit
ee0.
109
4109
3839
4007
853
001
13.5
590.
195
530.
1743
(0.3
2)
1.76
6(0
.32)
0.07
348
(0.0
3)
1036
1033
1027
U39
-90
(Ypb
)12
a(−
100+
150
)EC
10.
048
115.
820
.95
2312
.224
947
13.3
129.
5401
0.17
81(0
.41
)1.
831
(0.4
2)0.
074
55(0
.12
)10
5710
5710
5612
b(−
100+
150
)EC
20.
035
230.
641
.72
3820
.079
97.2
13.1
149.
5300
0.17
77(0
.18
)1.
824
(0.2
0)0.
074
42(0
.09
)10
5410
5410
5412
c(−
100+
150
)EC
30.
029
211.
138
.58
2487
.085
14.5
13.1
218.
9308
0.17
85(0
.34
)1.
835
(0.3
5)0.
074
54(0
.09
)10
5910
5810
56
Bl-
1-94
(Yc)
13a
(−10
0+15
0)E
qCA
1e0.
011
84.1
817
.24
467.
2076
6.04
9.59
394.
8564
0.18
25(0
.92
)1.
905
(0.9
6)0.
0757
(0.2
6)
1080
1083
1088
13b
(−10
0+15
0)E
qCA
2e0.
007
205.
945
.26
434.
255
4.16
9.15
95.
4736
0.19
48(0
.53
)2.
089
(0.5
9)0.
0778
(0.2
4)
1147
1145
1141
13c
(−10
0+15
0)E
lCA
3e0.
007
320.
866
.11
641.
2081
3.29
10.3
027.
1280
0.19
14(0
.60
)2.
019
(0.8
1)0.
0765
(0.5
0)
1129
1122
1108
13d
(−10
0+15
0)E
lCA
4e0.
008
192.
843
.71
303.
5035
1.88
8.18
94.
4549
0.18
86(0
.60
)1.
991
(0.6
3)0.
0766
(0.2
0)
1114
1113
1110
13e
(−10
0+15
0)E
lCA
5e0.
004
278.
770
.32
178.
5119
7.34
6.50
813.
4549
0.18
80(0
.68
)1.
964
(0.7
8)0.
0758
(0.3
5)
1111
1103
1089
13f
(−10
0+15
0)E
lCA
6e0.
005
457.
011
5.2
229.
5824
5.23
7.22
113.
8139
0.19
56(0
.35
)2.
097
(0.3
9)0.
0777
(0.1
7)
1152
1148
1140
13g
(−10
0+15
0)E
lCA
7e0.
003
535.
012
0.6
320.
2037
3.39
8.34
824.
5009
0.18
87(0
.54
)1.
990
(0.5
7)0.
0765
(0.1
8)
1114
1112
1108
13h
(−10
0+15
0)E
lCA
8e0.
007
241.
967
.84
147.
5215
9.54
5.91
082.
7626
0.19
13(0
.86
)2.
014
(0.9
5)0.
0764
(0.3
7)
1128
1120
1105
124 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Tab
le1
(con
tinu
ed)
Gra
ina
Wei
ght
Con
cent
rati
ons
Mea
sure
dP
bco
mpo
siti
onb
Rat
ios
(%er
ror)c
Age
s(M
a)d
(mg)
(ppm
)
UP
B206P
b/206P
b/206P
b/206P
b/206P
b/207P
b/207P
b/206P
b/207P
b/207P
b/204P
b204P
b207P
b208P
b238U
235U
206P
b238U
235U
206P
b
13i
(−10
0+15
0)E
lCA
10e
0.00
544
7.4
85.7
670
1.50
1423
.810
.488
7.56
880.
1846
(0.7
3)1.
932
(0.7
5)0.
0759
(0.1
6)
1092
1092
1093
13j
(−10
0+15
0)E
lCA
11e
0.00
542
6.7
85.7
149
5.70
779.
429.
6401
6.11
860.
1852
(0.7
8)1.
942
(0.8
1)0.
0760
(0.1
9)
1095
1096
1096
BR
1529
(Yhm
)14
a(−
100+
150
)EC
10.
037
308.
358
.46
6354
.912
110
12.8
568.
7357
0.18
46(0
.46)
1.95
0(0
.48)
0.07
661
(0.1
1)10
9210
9911
1114
b(−
100+
150
)EC
20.
033
326.
360
.47
6420
.113
399
12.8
998.
6632
0.18
04(0
.28)
1.90
2(0
.37)
0.07
65(0
.24
)10
6910
8211
0714
c(−
100+
150
)EC
30.
041
264.
251
.73
1651
.218
98.6
11.9
846.
5731
0.18
25(0
.38)
1.91
1(0
.39)
0.07
596
(0.0
8)10
8010
8510
9414
d(−
100+
150
)EC
40.
015
224.
344
.53
1644
.627
56.4
12.2
026.
8663
0.18
68(0
.38)
1.97
8(0
.43)
0.07
68(0
.20
)11
0411
0811
1614
e(−
100+
150
)EC
60.
017
356.
669
.49
1701
.732
10.7
12.4
127.
0162
0.18
42(0
.38)
1.93
4(0
.40)
0.07
615
(0.1
0)10
9010
9310
9914
f(+
100)
EqC
1e0.
014
302.
970
.19
2196
.839
94.7
12.8
232.
3841
0.17
88(0
.33)
1.83
5(0
.34)
0.07
442
(0.0
9)10
6110
5810
5314
g(+
100)
EqC
2e0.
011
288.
757
.03
3040
.013
254
12.6
839.
7141
0.19
41(0
.31)
2.08
2(0
.33)
0.07
778
(0.1
0)11
4411
4311
4114
h(+
100)
EqC
3e0.
007
462.
885
.78
2042
.044
50.1
12.7
889.
0337
0.18
05(0
.32)
1.86
7(0
.36)
0.07
50(0
.16
)10
7010
6910
6914
i(+
100)
EqC
4e0.
007
637.
614
2.9
5719
.013
420
13.2
432.
5488
0.17
70(0
.22)
1.81
7(0
.23)
0.07
445
(0.0
7)10
5010
5210
5414
j(+
100)
EqC
5e0.
005
1916
316.
516
66.4
1989
.512
.107
16.2
510.
1665
(0.2
2)1.
732
(0.2
4)0.
075
45(0
.08)
993
1020
1081
14k
(+10
0)E
C1
0.01
628
5.0
58.5
165
0.23
990.
9811
.056
5.75
850.
1854
(0.3
6)1.
946
(0.3
8)0.
076
11(0
.12)
1096
1097
1098
14l
(+10
0)E
C2
0.01
531
0.8
71.1
934
2.32
409.
618.
9153
4.27
190.
1892
(0.3
8)2.
023
(0.4
3)0.
0776
(0.1
8)
1117
1123
1136
14m
(+10
0)E
C3
0.01
329
7.5
58.3
015
38.3
3995
812
.854
9.10
180.
1917
(0.3
8)2.
047
(0.4
1)0.
0774
(0.1
4)
1131
1131
1133
14n
(+10
0)E
C4
0.01
633
3.1
47.3
411
39.5
4016
.612
.395
8.39
400.
1371
(0.3
3)1.
458
(0.3
7)0.
0771
(0.1
5)
828
913
1125
HF
2-90
(Yhm
)15
a(−
100+
150
)EC
10.
047
416.
678
.66
8466
.519
558
13.1
307.
0939
0.18
02(0
.29)
1.87
4(0
.29)
0.07
543
(0.0
6)10
6810
7210
8015
b(−
100+
150
)EC
20.
052
454.
987
.29
6470
.322
312
13.1
426.
4584
0.18
10(0
.19)
1.88
3(0
.20)
0.07
546
(0.0
5)10
7310
7510
81
125J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–14615
c(−
100+
150)
EC
40.
024
369.
172
.32
2283
.040
15.4
12.5
027.
0672
0.18
57(0
.21)
1.95
7(0
.23)
0.07
645
(0.0
9)10
9811
0111
0715
d(−
100+
150)
EC
50.
029
423.
279
.45
3028
.651
97.3
12.6
3310
.271
0.18
49(0
.23)
1.94
8(0
.25)
0.07
642
(0.1
1)10
9410
9811
0615
e(−
100+
150)
EC
70.
032
439.
182
.99
4321
.687
45.8
13.0
476.
6158
0.17
86(0
.41)
1.84
7(0
.42)
0.07
502
(0.0
5)10
5910
6210
6915
f(−
150+
200)
EC
80.
017
513.
510
2.0
1184
.313
38.6
11.6
365.
8951
0.18
14(0
.20)
1.88
4(0
.25)
0.07
53(0
.14)
1075
1075
1077
15g
(−15
0+20
0)E
C9
0.00
646
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90.9
313
23.0
2168
.812
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7.63
870.
1869
(0.4
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973
(0.5
0)0.
0766
(0.2
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0511
0611
1015
h(+
100
)EC
1e0.
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448.
282
.02
1053
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877.
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0.01
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867
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3)0.
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13(0
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1068
1069
1072
15j
(+10
0)E
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0.01
267
3.7
130.
720
81.5
6458
.212
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210.
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821
(0.2
2)0.
074
74(0
.11)
1049
1053
1061
15k
(+10
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0.01
255
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106.
617
29.5
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(0.2
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1071
1081
15l
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0.01
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142.
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60.3
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213
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1055
1053
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1060
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15n
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9.6
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216
54.7
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840
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6010
5915
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1000
EC
9e0.
017
452.
593
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870.
8310
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10.8
507.
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39(0
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0.07
806
(0.0
8)11
4211
4411
4915
q(+
100
)EC
10e
0.01
038
9.1
79.0
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6.02
560.
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9539
5.50
480.
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075
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1057
1062
1070
15r
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413
6.6
2481
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51.7
12.6
237.
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0.17
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5(0
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0.07
456
(0.1
1)10
5410
5510
5715
s(+
100
)EC
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0.01
023
7.4
85.1
582
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84.4
964.
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1.72
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1813
(0.2
7)1.
893
(0.5
4)0.
0757
(0.4
4)10
7410
7910
8815
t(+
100
)EC
13e
0.01
831
5.3
67.4
252
4.60
588.
119.
8548
5.92
300.
1898
(0.2
2)2.
024
(0.2
6)0.
0774
(0.1
3)11
2011
2411
3115
u(+
100
)EC
14e
0.02
537
4.8
77.1
289
3.78
1014
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.166
5.22
520.
1834
(0.2
1)1.
911
(0.2
4)0.
075
55(0
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1086
1085
1083
aAll
zirc
ons
wer
eab
rade
d[A
lein
ikoff
etal
.,19
90b;
mod
ified
from
Kro
gh(1
982)
].A
bbre
viat
ions
:E(e
long
ate)
,C(c
lear
),E
q(e
quan
t),P
(pit
ted/
fros
ted
),U
z(u
nzon
ed),
Sp(s
pott
ed),
FO
Z(f
aint
osci
llato
ryzo
ning
).b
Bla
nk(w
hich
decr
ease
dfr
omab
out
50to
10pg
,50
%,
duri
ngth
eco
urse
ofth
isst
udy)
and
frac
tion
atio
n(0
.140
.03%
)co
rrec
ted.
Ass
umed
blan
kco
mpo
siti
onis
204:
206:
207:
208=
1:18
.8:1
5.65
:38.
65.
c2s
unce
rtai
ntie
s.d
Com
mon
lead
corr
ecti
ons
from
Stac
eyan
dK
ram
ers
(197
5)
mod
el.
eSin
gle
grai
n.fM
ixed
frac
tion
ofel
onga
tean
deq
uant
grai
ns.
126 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
were chosen because they had the fewest imperfec- ages for zircon samples cited below are concordiaintercept ages calculated using a best-fit regressiontions. All grains thought to be igneous in origin
have deformed, pitted faces, indicative of the high- through a linear data array. In rare cases ofoverlapping data with little or no spread in Pb/Ugrade metamorphic conditions these rocks have
undergone during the Grenville orogeny, but they ages, the age is calculated using the weightedaverage of the 207Pb/206Pb ages. U–Pb data frompreserve euhedral oscillatory zoning. In addition,
equant, multifaceted clear to light tan grains monazite are treated similarly; however, when onlyone grain was analyzed from a sample and the(thought to have formed during metamorphism)
were obtained from two samples and one sample results are reversely discordant (i.e. plot above theconcordia curve), the age of the monazite is takenyielded spherical, abraded grains interpreted as
detrital. to be the 207Pb/235U age [assuming that the reversediscordance is caused by excess 206Pb (Parrish,All analyzed zircons were hand-picked under
alcohol, abraded moderately (to remove crystal 1990)].Zircons from sample HF2-90 were also analyzedfaces, resulting in lozenge-shaped grains) or
severely (resulting in spheres) (Aleinikoff et al., using SHRIMP II (Table 2) at the Research Schoolof Earth Sciences, Australian National University,1990b, modified from Krogh, 1982), and leached
in 7 N HNO3 and 6.5 N HCl prior to dissolution. Canberra. Selected grains were mounted in epoxy,ground to half-thickness and polished with 6 mmZircon fractions weighed between 1 and 126 mg.
Most fractions were composed of about five to ten and 3 mm diamond paste. Zircon standard AS3(1099 Ma zircon from gabbroic anorthosite fromgrains, although some samples with complex iso-
topic systematics necessitated analysis of individual the Duluth Complex, Paces and Miller, 1993) wasused to calibrate 206Pb/238U ages. All grains weregrains (Table 1). Individual monazite grains were
hand-picked for clarity, color ( light yellow to dark imaged in cathodoluminescence (CL) and photo-graphed in both transmitted and reflected lightorange), and lack of cracks or inclusions. Monazite
was leached in 3N HNO3 and 3N HCl prior to prior to SHRIMP analysis to identify cores andovergrowths and crack- and inclusion-free areas.dissolution.
All zircons were loaded into PFA microcapsules Analytical procedures followed the methodsdescribed in Compston et al. (1984) and Williamsin a standard Parr bomb (modified from Parrish,
1987), to which was added a mixed 205Pb–233U– and Claesson (1987). SHRIMP results are dis-played as: (1) a Tera–Wasserburg plot (238U/206Pb236U spike, concentrated HF, and concentrated
HNO3. Dissolution required about 3 days in an versus 207Pb/206Pb; Tera and Wasserburg, 1972)using data that has not been corrected for commonoven at about 210°C. Monazite was dissolved in
12 N HCl in a screw-top container on a hot plate Pb content — this plot merely provides informa-tion for an assessment of which data points shouldat 150°C for several days. Extraction of Pb and U
from zircon and monazite followed procedures of be used for the age calculation; (2) weightedaverages plot of 207Pb/206Pb ages corrected forKrogh (1973) and Parrish (1987), with minor
modifications. Isotopic ratios of U, loaded on a common Pb.single Re filament with suspended graphite (aqua-daug), and Pb, loaded on a single Re filament 4.2. Results: zircon U–Pb geochronologywith silica gel and 0.5N H3PO4, were measured ona VG Micromass 54E mass spectrometer with a U–Pb ages obtained by conventional isotope
dilution–thermal ionization mass spectrometrysingle Faraday cup collector and Daly multiplier,using the ANALYST program of Ludwig (1992). (ID-TIMS) for 15 samples of felsic metaigneous
gneisses span a range of about 100 m.y. (fromMeasured Pb isotopic ratios were corrected for thepresence of common Pb using the Stacey and about 1150–1050 Ma), and are divisible into three
groups, based on age and degree of deformation.Kramers (1975) model. Data reduction and plot-ting were done using the PBDAT and ISOPLOT Group 1 includes five samples that are mostly
strongly foliated and range in age from 1153±6programs, respectively (Ludwig, 1991a,b). Most
127J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Table 2SHRIMP U–Th–Pb data for sample HF2-90
Sample Spot loc., Ub Thb Th/U 206Pb/ 238U/ Errord 207Pb/ Errord % 207Pb/ Errordgrain colora (ppm) (ppm) 204Pb 206Pbc 206Pbc concordant 206Pbe (Ma) (Ma)
1.1 c, p-rd 681 215 0.32 7231 5.1010 0.0954 0.081 65 0.000 76 97 1189 322.1 r, rd 801 274 0.34 5554 5.1190 0.1464 0.079 80 0.001 57 102 1127 463.1 r, cl 906 810 0.89 5435 5.6647 0.0863 0.075 27 0.000 75 104 1004 283.2 c, cl 1580 824 0.52 642 5.1582 0.1173 0.098 80 0.000 42 100 1112 404.1 r, cl 3564 56 0.01 45 249 5.7026 0.1104 0.074 35 0.000 40 100 1042 114.2 c, cl 508 452 0.89 1414 5.1741 0.1552 0.090 02 0.001 05 94 1197 655.1 r, cl 1243 797 0.64 6957 5.4567 0.1326 0.074 80 0.000 45 108 1007 196.1 r, rd 963 836 0.87 12 191 5.5358 0.0906 0.075 42 0.000 62 102 1048 227.1 c, cl 382 125 0.33 4004 5.2572 0.1034 0.081 15 0.001 30 98 1137 528.1 c, rd 463 189 0.41 5307 5.1182 0.1230 0.082 33 0.000 83 97 1189 319.1 c, cl 835 463 0.55 7503 5.6268 0.0772 0.076 41 0.000 51 100 1055 209.2 r, cl 506 196 0.39 2538 5.1766 0.0531 0.080 63 0.000 82 106 1069 3310.1 c, rd 1121 428 0.38 13 077 4.9905 0.1019 0.080 40 0.000 37 100 1180 1611.1 c, cl 266 99 0.37 1980 4.9444 0.1036 0.081 45 0.001 21 112 1048 5612.1 r, rd 892 664 0.74 7675 5.5752 0.1176 0.076 27 0.000 46 101 1053 2413.1 c, rd 424 181 0.43 2577 5.0342 0.1975 0.084 96 0.000 93 98 1184 4914.1 c, rd 532 223 0.42 3178 5.0529 0.1394 0.081 24 0.000 88 104 1115 3515.1 c, cl 512 250 0.49 3277 4.8513 0.0659 0.080 36 0.002 11 110 1096 6416.1 c, cl 463 162 0.35 4084 4.8386 0.1496 0.079 81 0.000 85 109 1104 3117.1 c, cl 324 169 0.52 3704 4.9759 0.0943 0.081 14 0.001 56 104 1129 5518.1 c, p-rd 321 125 0.39 2086 5.2739 0.1169 0.082 97 0.001 22 101 1099 4719.1 c, cl 572 279 0.49 5110 5.0639 0.0847 0.079 71 0.000 75 104 1119 4120.1 c, rd 895 325 0.36 6868 4.9370 0.1242 0.079 24 0.000 69 105 1126 2120.2 r, rd 1469 827 0.56 12 618 5.3270 0.1617 0.075 46 0.000 42 106 1050 1721.1 c, cl 397 70 0.18 16 633 5.4894 0.1078 0.0765 0.001 29 99 1086 4321.2 r, cl 2319 328 0.14 23 725 5.5645 0.0781 0.075 11 0.0005 101 1055 2022.1 c, cl 374 234 0.63 2898 5.1237 0.1046 0.078 37 0.002 34 111 1026 8123.1 c, rd 846 482 0.57 7543 5.5385 0.1067 0.075 57 0.0007 103 1033 26
a Abbreviations: c (core), r (rim), p-rd (pale red), rd (red), cl (colorless).b Concentrations are probably 20%.c Uncorrected for common Pb content.d 1s, absolute uncertainty.e Corrected for common Pb content using 204Pb content.
to about 1140 Ma. One of these samples (HF2-90; in Group 1. The geochronologic results are dis-cussed in order of decreasing age.hornblende monzonite gneiss) was also dated by
in situ analysis using SHRIMP. Group 2 includestwo samples that are moderately to stronglydeformed and have identical ages of 1111±2 and 4.2.1. Group 1
A sample of layered granitic gneiss (Ylg; sample1112±3 Ma. Group 3 is composed of six samplesthat are weakly to moderately deformed and range number U250-92) was collected from a well-
exposed, low outcrop in a pasture (sample localityin age from 1077±4 to 1055±2 Ma (subjectivelysubdivided into subgroups 3A and 3B). In addi- 1, Fig. 2). The outcrop is composed of finely
layered felsic rock that appears to be of volcaniction, zircon from charnockite (sample Bl-1-94) didnot yield a precise age due to very complicated origin, with other layers and dikelets of porphyro-
blastic granite gneiss (Ypg) and garnetiferous met-isotopic systematics, although we suspect (on thebasis of 207Pb/206Pb ages) that this rock belongs agranite (Ygt). It is impossible to determine the
128 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
exact nature of the cross-cutting relationships in pitted and slightly bent faces. We tentatively inter-pret this morphology to indicate that these zirconsthis outcrop.
Zircons from sample U250-92 have a wide range are igneous in origin. The remaining one-third ofthe grains are spherical to lozenge shaped, withof shapes [Fig. 3(a)] and colors. Approximately
two-thirds of the grains are medium to dark brown, frosted surfaces. These grains have a wide rangein color, including pale orange, deep red, and veryeuhedral, and prismatic, with length-to-width
ratios l/w=2–7. Many of the more elongate grains dark brown. The morphology and color variabilityof these zircons are quite suggestive of a detritalare doubly terminated with somewhat rounded
tips, and nearly all are moderately deformed with origin. They lack the typical characteristics of
Fig. 3. Scanning electron microscope images of representative zircons from granitic gneisses of the northern Blue Ridge. Grains in(a) and (b) are from Group 1, (c) is from Group 2, (d) and (e) from Group 3, (f ) is unknown age but thought to be in Group 1.(a) Elongate and spherical grains (detrital origin?) in sample U250 (layered granitic gneiss). (b) Elongate, partially resorbed andovergrown grains from sample J329-90 (porphyroblastic granite gneiss). (c) Elongate grains from sample WW22-90 (MarshallMetagranite). (d) Elongate and equant grains from sample HF1-90 (garnetiferous metagranite). Equant grains have faces, suggestiveof metamorphic origin. (e) Elongate grains from sample BR11-90 (white leucocratic metagranite). (f ) Elongate grains from sampleBl-1-94 (charnockite).
129J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
metamorphic zircons ( low U, multiple facets, clear collinear, and thus no definitive age informationwas obtained from the multigrain fractions beyondand colorless) and thus it is unlikely that they
formed during metamorphism. It is possible that the suggestion that the rock is of ‘Grenville’ age.A total of 12 zircons were analyzed individually,the elongate zircons are also detrital. A volcanic
origin is suggested for this rock because of its fine in an attempt to eliminate the possibility of mixinggrains of different ages [Fig. 4(b)]. Five singlegrain size, layering, and the presence of spherical
zircons interpreted as detrital. grains (1g, 1j, 1k, 1o, 1n) form a collinear array(two are slightly discordant, two are slightlyU–Pb data from four multigrain fractions of
elongate zircon from sample U250-92 (Table 1) reversely discordant, and one is very discordant).A linear regression through these five data pointsare discordant, with 207Pb/206Pb ages between 1120
and 1151 Ma and Pb/U ages ranging from 1013 yields intercept ages of 1152.7±5.6 and287±131 Ma. Grain 1l (not shown on Fig. 4) hasto 1090 Ma [Table 1, Fig. 4(a)]. The data are nota 207Pb/206Pb age of 1356 Ma (Table 1) and proba-bly is detrital (although it may be a xenocryst, i.e.inherited by melting or assimilation processes). Sixother grains (1e, 1f, 1h, 1i, 1m, 1p) have younger207Pb/206Pb ages (1086–1131 Ma, Table 1); thedata are discordant and scattered [Fig. 4(b)]. Wesuggest that the isotopic systematics in these zir-cons were modified by Pb-loss and possibly theformation of metamorphic overgrowths during theGrenville orogeny at about 1.05 Ga (see Section 5.2on ages of metamorphic monazite). The collineararray of five apparently relatively undisturbedgrains leads to the conclusion that the protolith ofthe layered gneiss is about 1150 Ma. This inter-pretation is in agreement with field relationsobserved at other locations that suggest that thelayered gneiss is intruded by the porphyroblasticgranite gneiss and the garnetiferous metagranite.However, if all of the zircons are detrital, then theage of the layered granitic gneiss is unknown, butthought to be older than the porphyroblastic gran-ite gneiss and garnetiferous metagranite (seebelow).
Six grains of monazite from sample U250-92were analyzed individually, yielding 207Pb/206Pbages of 1028 to 1112 Ma (Table 1). Isotopic datafrom three monazites (1r, 1q, and 1v) are collinearwith intercept ages of 1098±3 and 600±118 Ma[Fig. 4(c)]. Isotopic data from two monazites (1sand 1u) form a line with intercept ages of 1057±5and 624±81 Ma. A regression calculated throughmonazite 1t and anchored at 600±50 Ma resultsin an upper intercept age of 1121±6 Ma. It ispossible that some of these grains contain mixturesFig. 4. U–Pb data for sample U250-92 ( layered granitic gneiss):of ages (see discussion of monazite ages in(a) zircon multigrain fractions; (b) zircon single grains; (c)
monazites. Section 5.2; cf. Hawkins and Bowring, 1997;
130 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fanning and Aleinikoff, 1998), but three grains were subdivided into two populations on the basisof color (as seen in the epoxy ion probe mount):yielded concordant ages of about 1099 Ma,
1096 Ma, and 1056 Ma [1v, 1q and 1u respectively, (1) deep to pale red, and (2) very pale red tocolorless. U–Pb data from 28 spots on a total ofFig. 4(c)]. The age of 1121±6 Ma is regarded as
a minimum age for the protolith of the layered 23 grains result in two groupings of ages [Table 2,Fig. 6(c)]. However, a priori discrimination ofgranitic gneiss.
Two samples of hornblende monzonite gneiss zones based on color, morphology, or CL zoningdid not result in distinct age groupings. Data from(Yhg, sample localities 14 and 15, Fig. 2) were
analyzed for U–Pb zircon geochronology by con- red and colorless grains that show oscillatoryzoning yield an age of 1149±19 Ma (MSWD=ventional isotope dilution methods. Outcrop pat-
terns and foliation in this unit strike NW, parallel 1.12), whereas overgrowths on red grains, plusseveral wholly colorless grains and a few cores ofto regional D1 foliation (Fig. 2). Garnetiferous
metagranite (Ygt) truncates the hornblende mon- red grains, are dated at 1042±11 Ma (MSWD=0.71) [Fig. 6(c)]. Thus, cores and overgrowthszonite gneiss in map pattern at the north end of
the study area in Maryland. Although no cross- were found to be both old and young, suggestingcomplex igneous and metamorphic growth pro-cutting relationships have been observed in
outcrops, the truncation may be an intrusive con- cesses. We interpret the older age as the time ofemplacement of the protolith of the gneiss and thetact. The significant D1 foliation and the contact
with garnetiferous metagranite suggest that the younger age as the time of regional progrademetamorphism of the rock. Surprisingly, these ageshornblende monzonite gneiss is older.
The zircon populations in both samples of are in excellent agreement with the oldest andyoungest ages as determined by isotope dilu-hornblende monzonite gneiss are heterogeneous.
Morphologic varieties exist in shape (equant to tion U–Pb geochronology (1149±2 Ma and1053±2 Ma, Table 1), implying that some of theelongate, l/w=1–6), color (pink, deep red, light
to dark brown), crystallinity (anhedral to euhe- grains contain only one age component. Althoughin situ spot analysis by SHRIMP allows the nearlydral ), and crystal shape (simple elongate prisms
with or without pyramidal terminations to multi- unequivocal interpretation of the age of a particu-lar zone within a grain, the uncertainties are largefaceted equant grains). CL imaging (Fig. 5) reveals
that many of the grains in sample HF2-90 are due to the very small amount of material beinganalyzed. Conventional U–Pb analysis yields pre-overgrown by one or more zones of younger
zircon. 207Pb/206Pb ages of multigrain fractions cise ages but interpretation of the data is severelylimited by a lack of understanding of exactly whatand single zircons range from 1149±2 to
1053±2 Ma [Table 1, Fig. 6(a)]. was analyzed (i.e. how many age domains arecontained within the zircon). The most robustIsotopic data from zircon from a second sample
[BR1529, Fig. 6(b)] collected about 20 km north interpretations are those based on results fromboth techniques.of HF2-90 are also complicated. 207Pb/206Pb ages
of multigrain fractions and single zircons range Two samples of porphyroblastic granite gneiss(Ypg) were collected and dated. Sample J329-90,from 1141±2 to 1053±2 Ma (Table 1). It is likely
that most grains in both samples of hornblende from the southern end of the study area ( locality2, Jeffersonton 7.5∞ quadrangle), and BR1700, frommonzonite gneiss contain mixtures of zircon that
may differ in age by as much as 100 m.y. Standard the central part of the study area ( locality 3,Bluemont 7.5∞ quadrangle), are identical in appear-concordia plots [Fig. 6(a) and (b)] of HF2-90 and
BR1529 do not yield easily interpretable ages. ance. This rock also is similar in appearance to amegacrystic metagranite (Ypb) in the southwest-The scatter of conventional U–Pb data for
zircons from the hornblende monzonite gneiss ern part of the study area (Fig. 2), although subtletextural and chemical differences permitted Nelsonindicated that in situ SHRIMP analysis would be
necessary to decipher the isotopic systematics. (1997) to map them as different units, substanti-ated by U–Pb geochronology (see below). At local-Prior to SHRIMP analysis, zircons from HF2-90
131J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fig. 5. CL images of zircon from hornblende monzonite gneiss sample HF2-90. Note oscillatory zoned cores overgrown by unzonedrims. White ellipses indicate locations of SHRIMP analyses.
ity 2, the porphyroblastic granite gneiss is cut by three slightly discordant points (one of which isless than 1% reversely discordant). A best-fitnumerous dikes of leucocratic granitic rock of
unknown age. At locality 3, the porphyroblastic regression through the data yields intercept agesof 1143.8±1.5 and 491±159 Ma [Fig. 7(a)]. Wegranite gneiss crops out within about 100 m of
charnockite. However, because the charnockite interpret the upper intercept as the crystallizationage of the protolith of the porphyroblastic graniteoccurs only as float at this locality, the contact
relations between the charnockite and the porpyro- gneiss; the lower intercept suggests loss of Pb inthe Paleozoic but due to the large uncertainty weblastic granite gneiss are unknown.
Zircons from both samples of porphyroblastic are unable to evaluate when exactly that occurred.Monazite in this rock occurs in two color varieties:granite gneiss are medium brown and stubby to
elongate with l/w=3–6 [Fig. 3(b)]. The zircon light yellow and dark yellow. One grain of eachwas dated, yielding slightly discordant U–Pb datapopulation ranges in shape from subhedral to
euhedral; most grains are deformed, with rounded with 207Pb/206Pb ages of 1106 Ma and 1060 Marespectively. However, as discussed below in detail,terminations. Many of the grains are fragments of
zircon crystals. Four fractions from J329-90 form these ages may be mixtures of different ages ofgrowth domains.a collinear array with one concordant point and
132 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fig. 6. U–Pb data for samples of hornblende monzonite gneiss: (a) zircon from sample HF2-90; (b) zircon from sample BR1529; (c)Tera–Wasserburg and weighted averages plots of SHRIMP data from sample HF2-90.
133J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
about 1140 Ma, on the basis of identical mineral-ogic and textural appearance and the age of onezircon. This interpretation implies that the youngerzircons are metamorphic in origin and that themultigrain fractions are composed of mixtures ofgrains of different ages. Two grains of monazite,light yellow and dark yellow, yield ages of 1127 Maand 1060 Ma respectively [Table 1, Fig. 7(b)]. Thesignificance of these ages, possibly the result ofmixing of different age components, is discussedin detail in Section 5.2.
Zircons from a sample of coarse-grained granitegneiss (Ymc, sample LN398, locality 4, Fig. 2) aremedium–dark brown. They are prismatic (l/w=2–5), with rounded tips and edges. U–Pb isotopicdata from five multigrain fractions from the(−100+150) size fraction are slightly (0.5–1%)discordant and are not collinear [Fig. 8(a)]. Theyhave 207Pb/206Pb ages ranging from 1120–1128 Maand 206Pb/238U ages ranging from 1104–1116 Ma.A regression through the data, anchored at400±100 Ma, yields an upper intercept age of1129±8 Ma, with a large MSWD of 20 [Fig. 8(a)].Fig. 7. U–Pb data for samples of porphyroblastic granite gneiss:
(a) zircon and monazite from sample J329-90; (b) zircon andmonazite from sample BR1700.
U–Pb data from sample BR1700 are much morescattered than the isotopic data from sampleJ329-90 [Fig. 7(b)]. Five multigrain fractions areall somewhat discordant (1.4–2%), with 207Pb/206Pb ages ranging from 1099 to 1126 Ma. Thenon-linear data array suggests that these fractionsare composed of zircons that formed at two ormore times. These different ages may occur eitheras discrete grains or as metamorphic overgrowths.Because of the possibility of mixing of grains ofdifferent ages, we analyzed six zircons individually.U–Pb data from three of these grains plot on areference chord between 350 and 1050 Ma, onegrain is quite discordant with a 207Pb/206Pb age of1078 Ma, one grain is slightly discordant with a207Pb/206Pb age of 1105 Ma and one grain isconcordant at about 1139±4 Ma [Table 1,Fig. 7(b)]. This oldest zircon is essentially the sameage as the upper intercept age (1144±2 Ma) ofthe other sample of porphyroblastic granite gneiss Fig. 8. U–Pb data for sample LN398 (coarse-grained metagran-(J329-90). Although we are unable to determine ite): (a) (−100+150) multigrain fractions; (b) (−100+150)
single grains and (−200) multigrain fractions.an age for sample BR1700, we suggest that it is
134 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Four multigrain fractions from the (−200) size to anhedral, subequant to elongate (l/w=1–5) andhighly fractured [Fig. 3(c)]. Four of five fractionsfraction were analyzed because the smaller grains
tend to be less deformed and cracked, and presum- from the (−100+150) size fraction from sampleWW29-90 yield U–Pb data that are collinear withably less susceptible to Pb-loss. In addition, four
elongate, clear grains from the (−100+150) frac- intercept ages of 1112±3 and 336±72 Ma[Fig. 9(a)]; the fifth fraction plots to the left of thetion were analyzed individually (Table 1). Of these
eight analyses, three of the (−200) fractions (4j– discordia, with a 207Pb/206Pb age of 1085 Ma.Isotopic data from two grains of monazite (one of4l ) plus one coarser grain (4g) plot collinearly,
with intercept ages of 1127.7±9.9 and 550±99 Ma which is concordant) form a line with interceptsat 1051±2 and 406±129 Ma. Zircons from sample(MSWD=1.5) [Fig. 8(b)]. The other single grains
are concordant or slightly discordant with WW22-90 have isotopic systematics that are sim-ilar to the data from sample WW29-90 [Fig. 9(b)].207Pb/206Pb ages of 1141, 1107, and 1059 Ma. The
fourth (−200) fraction has a 207Pb/206Pb age of A best-fit discordia through five of seven fractionshas intercept ages of 1111±2 and 201±89 Ma.1098 Ma. By analogy with the isotope dilution U–
Pb systematics of zircon from the hornblende The two fractions plotting off the discordia have207Pb/206Pb ages of 1112 Ma and 1119 Ma respec-monzonite gneiss, the oldest grain (4i; 207Pb/206Pb
age of 1141±3 Ma) could be the minimum age for tively. No monazite was recovered from sampleWW22-90.the time of emplacement of the coarse-grained
granite gneiss. However, the paucity of data atabout 1140 Ma makes this conclusion suspect. The 4.2.3. Group 3
Quartz-plagioclase gneiss (Yqp) occurs as thinsingle grain with the concordant age of 1059 Maprobably represents the time of overgrowth. All lenses in the northern part of the study areaother grains contain mixtures of ages betweenabout 1140 and 1060 Ma. However, becausenumerous single grains and multigrain fractionshave 207Pb/206Pb ages between about 1118 and1128 Ma, it is possible that the time of emplace-ment was 15–20 m.y. younger than the main plu-tonic event dated (see above) at about 1145 Ma.In this scenario, the grain with 1141 Ma age wouldbe interpreted as xenocrystic. We suggest an ageof about 1140 Ma for the coarse-grained granitegneiss.
4.2.2. Group 2Two samples of Marshall Metagranite (Ym)
were collected from the Rectortown 7.5∞ quadran-gle (Fig. 2). Sample WW22-90 is a biotitic graniticgneiss whereas sample WW29-90 is a more leu-cocratic granitic gneiss (sample localities 6 and 7,Fig. 2). Field relations suggest that the leucocraticgranitic gneiss cuts the biotitic granitic gneiss. D2foliation with northwest to northeast strikes andsoutheast-plunging folds and mineral lineationsoccurs in the rocks mapped as MarshallMetagranite, suggesting that the emplacement of Fig. 9. U–Pb data for samples of Marshall Metagranite: (a)the granite was post-D1 and pre-D2. Zircons from zircon and monazite from sample WW29-90; (b) zircon from
sample WW22-90.both rocks are medium to dark brown, subhedral
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(Fig. 2). It is a relatively minor rock type but was light brown to dark brown to deep red, and varyin shape from equant (with some faces) to sphericalconsidered important in understanding the evolu-
tion of the terrane because of its chemical similarity (with no planar faces) to prismatic elongate (euhe-dral to subhedral; l/w=1–5) to irregular (anhe-to ~1.3 Ga metatonalite and metatrondhjemite of
the Green Mountain massif, Vermont (Burton dral ). Spherical grains tend to have pitted, frostedsurfaces whereas more elongate grains have pittedand Southworth, 1996). Zircons from sample
BR1674-93 ( locality 8, Fig. 2) are medium to dark faces and rounded edges. Equant grains with multi-faceted, ‘soccer ball’ morphology rather than detri-brown, subequant to elongate (l/w=1–4), anhedral
to euhedral, and moderately fractured. Many tal spherical shape are similar in appearance tometamorphic zircon. The wide range of zircongrains are irregularly shaped, a morphology sug-
gestive of a multistage (xenocrystic) history. morphologies and the aluminous character of therock suggest that many of these zircons may haveIsotopic data from six of seven fractions form a
collinear array with intercept ages of 1077±4 and cycled through a sedimentary source prior to incor-poration in the felsic magma. In order to attempt462±211 Ma (Fig. 10). One fraction, with a
207Pb/206Pb age of 1066 Ma, plots slightly to the to date the time of crystallization, only elongateeuhedral or equant euhedral grains were analyzedleft of the discordia. This fraction has uncharacter-
istically low 206Pb/204Pb (Table 1). The relatively (i.e. spherical, frosted zircons were not selected).Monazite occurs as medium to dark yellow subhe-young Grenvillian age for the quartz-plagioclase
gneiss (Yqp) rules out any definitive correlation dral to anhedral grains. The color variation isgradational, with no distinct color populations.with Green Mountain metatonalite and meta-
trondhjemite. However, some grains have pitted and frostedsurfaces, whereas others are adamantine. A totalGarnetiferous metagranite (Ygt) crops out over
a large area, particularly in the northern half of of 11 grains comprising the two populations ofmonazite were analyzed for U–Pb geochronology.the northern Blue Ridge (Fig. 2). In many locali-
ties, this rock is found cross-cutting porphyroblas- U–Pb isotopic systematics of zircon from gar-netiferous metagranite sample HF1-90 ( locality 5,tic granite gneiss (Ypg), one of the few
unambiguous field relations in the study area. The Fig. 2) are very complicated (Fig. 11). The firstfour fractions (5a–5d) analyzed consisted ofubiquitous occurrence of coarse garnet in this
rock, perhaps indicative of a metasedimentary mixtures of equant and elongate grains, chosenprimarily because they were relatively clear andsource, is unique amongst granitic gneisses of the
northern Blue Ridge. The zircon population from crack-free. The data are scattered and range frommarginally concordant (fraction 5a) to slightlythe garnetiferous metagranite is very hetero-
geneous [Fig. 3(d)]. Grains vary in color from discordant [Fig. 11(a)], having 207Pb/206Pb ages of1077 to 1118 Ma (Table 1). Four additionalmultigrain fractions, composed only of either elon-gate or equant grains {5e: elongate [Fig. 11(a)];5f–5h: equant [Fig. 11(b)]} were analyzed to sepa-rate the different age components of the previousfour fractions. Fraction 5e has a 207Pb/206Pb ageof 1094 Ma, whereas fractions 5f–5h have207Pb/206Pb ages of 1050 Ma, 1049 Ma and1057 Ma respectively. Because single grain 5e hasan older 207Pb/206Pb age than fraction 5a, werealized that even within a carefully hand-pickedfraction of grains of similar morphology it is likelythat grains of different ages were combined. Thus,four individual elongate grains (fractions 5k–5n),Fig. 10. U–Pb data for sample BR1674-93 (quartz-plagioclase
gneiss). plus two individual equant, multifaceted grains
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A total of 11 individual grains of monazite fromgarnetiferous metagranite, representative of thepitted/frosted and clear/adamantine populations,was analyzed for U and Pb isotopes [Fig. 11(c)].The presumption was made that pitted grainsmight be detrital in origin, whereas the clear grainsmight have grown during metamorphism. Grains5r (clear) and 5w (pitted) yielded concordant data,with 207Pb/206Pb ages of 1070 Ma and 1064 Marespectively. Data from five grains (5o, 5s, 5t, 5q,5y) are reversely discordant and data from fourgrains (5p, 5u, 5v, 5x) are normally discordant(total range of 207Pb/235U ages is 986 to 1086 Ma).The data from all 11 monazite grains form a lineararray, with intercept ages of 1070±3 and56±100 Ma. Two important conclusions from themonazite data are: (1) no history prior to 1070 Mais preserved, despite morphologic differences; (2)1070 Ma is the minimum age of the garnetiferousmetagranite, which implies that the equant zircons(upper intercept age of 1048±6 Ma) are metamor-phic in origin.
We are unable to determine the emplacementage of the garnetiferous metagranite because theisotopic data from elongate zircons are scattered.Two elongate fractions (5b and 5k) have207Pb/206Pb ages of 1118 Ma and 1116 Ma respec-tively, similar to the age of the MarshallMetagranite, suggesting that these grains are inher-ited. Fraction 5a is concordant at 1077±4 Ma,and fractions 5c, 5d, and 5e have 207Pb/206Pb agesbetween 1077 and 1116 Ma. Thus, the crystalliza-tion age of the garnetiferous metagranite is inter-preted as being somewhere between about 1077 MaFig. 11. U–Pb data for sample HF1-90 (garnetiferous metagran-
ite): (a) multigrain fractions (mixed elongate and equant) and (age of youngest elongate grains) and aboutsingle elongate grains; (b) multigrain fractions (equant only) 1070 Ma (age of monazite). If this interpretationand single equant grains; (c) monazites (pitted and clear). is correct, then the garnetiferous metagranite (Ygt)
and quartz-plagioclase gneiss (Yqp) are very sim-ilar in age. Alternatively, the garnetiferous meta-granite might be as old as about 1118 Ma.(fractions 5i and 5j) were analyzed. 207Pb/206Pb
Samples of four different granitic rock typesages for the elongate grains range from 1057 to(Yg, Yml, Ybg, and Ypb; Fig. 2) have the youn-1116 Ma. The two equant grains both havegest ages of granitic gneisses in the northern Blue207Pb/206Pb ages of 1047 Ma, one of which isRidge, spanning a range of about 1055–1060 Ma.concordant. A best-fit line calculated through fourAlthough the ages of these rocks are nearly iden-of five fractions (both single grain and multigrain)tical, the morphologies of zircons they contain areof equant zircon has intercept ages of 1048±6 and
−23±287 Ma [Fig. 11(b)]. quite variable. With the exception of Ybg, the
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rocks are poorly foliated and contain only the D2 of which have deformed and pitted faces. A smallfabric. Thus, even prior to U–Pb geochronology, proportion of grains was nearly colorless and clear,these units were considered to be amongst the contained few inclusions or cracks, and was consid-youngest Mesoproterozoic gneisses of the northern ered appropriate for U–Pb geochronology. ThreeBlue Ridge on the basis of their fabrics. of four fractions yielded isotopic data that are
Sample BR11-90 ( locality 9, Fig. 2) is a white collinear, with intercept ages of 1058.2±2 andleucocratic metagranite (Yg) that in the field 128±204 Ma [Fig. 12(b)]. The fourth fraction hasappears to be similar to the garnetiferous meta- an older 207Pb/206Pb age of 1084 Ma (Table 1).granite except that it lacks garnets. However, the Ages from two monazite grains differ by aboutmorphologies of zircon from the two rocks are 25 m.y. Monazite 10f is reversely discordant andquite different. Zircons from the white leucocratic plots on the discordia derived from the zircongranite gneiss are light to medium brown, euhedral, results, suggesting that the cause of reverse discor-doubly terminated prisms (l/w=3–6), with very dance is due to loss of U or gain of radiogenic Pb,few cracks or inclusions (Fig. 3e). U–Pb data from not excess 206Pb. Monazite 10e is slightly normallyfour fractions yielded three concordant points and discordant with a 207Pb/206Pb age of 1034±2 Ma.one slightly discordant point [Fig. 12(a)]. Intercept We interpret these data as indicating two episodesages calculated from a best-fit regression are of monazite growth, one during crystallization of1059.6±2.2 and 381±384 Ma. the granite magma, and a second period during
Sample BR1313-92 ( locality 10, Fig. 2), a pink reheating about 25 m.y. later.leucocratic metagranite (Yml ), contains medium Sample BR554-90 ( locality 11, Fig. 2), a biotiteto dark brown prismatic (l/w=2–4) zircons, many granite gneiss (Ybg), contains dark reddish-brown
zircons in a variety of shapes including prisms(l/w=3–5), spheres, and lozenge-shaped grains.Most of the grains are anhedral, irregular shapes,although a small proportion was subhedral toeuhedral crystals with pitted faces, some of whichwere selected for analysis. All five analyzed frac-tions yielded concordant isotopic data, with207Pb/206Pb ages ranging from 1058 to 1051 Ma[Fig. 13(a)]. Because the data plot on concordiawith very little spread in the Pb/U ages, we calcu-late an age of 1055.2±3.5 Ma using the weightedaverage of the 207Pb/206Pb ages (Table 1), ratherthan determining concordia intercepts of a best-fitregression. Because there is some spread in thedata along concordia, suggesting the possibility ofvery minor Pb loss in the past, this date shouldbe regarded as a minimum age for the biotitegranite gneiss. Monazite from this sample isreversely discordant with a 207Pb/235U of1033±3 Ma.
Sample U39-90 ( locality 12, Fig. 2), a megacrys-tic metagranite (Ypb) similar in appearance toJ329-90 (Ypg), has zircons that are notable
Fig. 12. U–Pb data for samples of leucocratic metagranite: (a) because they are light brown, transparent, andzircon from white leucocratic metagranite (sample BR11-90);lack cracks or inclusions. Most of the grains are(b) zircon and monazite from pink leucocratic metagranite
(sample BR1313-92). prismatic (l/w=4–7) and have only slightly bent
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gies, only single grains were analyzed, includingtwo equant grains (13a, 13b) and eight elongategrains (13c–13j) (Table 1). 207Pb/206Pb ages rangefrom 1088 to 1141 Ma and the data are scattered(Table 1, Fig. 14). There is no obvious relationshipof morphology to age in zircon from the charnock-ite. For example, equant grains are 1140 and1090 Ma, and elongate grains of very similar shapeand color occur in all three age groups. Becauseof the lack of any independent evidence for theage of this rock, we are unable to determine itsemplacement age. The two older age groups,~1140 Ma and ~1110 Ma, correspond closely tothe ages of the porphyroblastic granite gneiss andMarshall Metagranite respectively, both of whichare widespread throughout the northern BlueRidge. It is likely that this rock is older than1140 Ma (assuming that this age is a mixture ofages of core and overgrowth, analogous to zirconfrom the hornblende monzonite gneiss); however,the older zircons could be xenocrysts. CL imagingand SHRIMP dating are required to unravel thecomplex growth characteristics.Fig. 13. U–Pb data for samples of granitic gneiss: (a) zircon
and monazite from biotite granite gneiss (sample BR554-90);(b) zircon from megacrystic metagranite (sample U39-90).
5. Summary of geochronologic resultsfaces. Three fractions yielded concordant data witha weighted average of the 207Pb/206Pb ages of 5.1. Results: zircon U–Pb geochronology1055±2 Ma [Fig. 12(b)].
In summary, U–Pb ages from zircon indicatethat the protoliths of 12 samples of granitic gneiss4.2.4. Age uncertain
Zircon from charnockite (Yc) was analyzed for from the northern Blue Ridge were emplaced overa period of about 100 m.y., from about 1150 toU and Pb isotopes, but because of very compli-
cated isotopic systematics, no unambiguous agewas determined. Owing to very poor exposure, themapped distribution of charnockite was basedprimarily on diagnostic soil and float. Althoughnot observed in outcrop, a foliation is evident insawn and stained sections of charnockite. Thecharnockite was not found in cross-cutting rela-tionship with any other rock in the study.
Zircons from charnockite sample Bl-1-94 ( local-ity 13, Fig. 2) are pale to dark pink. Many of thesegrains are equant and multifaceted, whereas othersare elongate (l/w=3–6) with slightly bent facesand rounded tips. Many of the grains show com-plex cores and overgrowths when imaged in CL. Fig. 14. U–Pb data for zircons from charnockite (sample Bl-
1-94).Because of the wide variety of zircon morpholo-
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1050 Ma (Table 3). During this time span, there samples are complicated by inheritance and/orovergrowths. Of nomenclatural significance is theappears to have been three pulses of magmatism,
at about 1150–1140 Ma, 1110 Ma, and 1075– decision here to limit the term ‘MarshallMetagranite’ to the foliated biotitic granite gneisses1055 Ma (perhaps divisible into two subgroups at
about 1075 and 1060–1055 Ma). By comparison dated at 1112±3 and 1111±2 Ma; these rocksoccur mostly east of the Short Hill fault (Fig. 2).with samples from other exposures of Grenvillian
crust (i.e. Felchville tonalite, Ratcliffe et al., 1996;Corbin Metagranite, Heatherington, et al., 1996; 5.2. Results: monazite U–Pb geochronologyBaltimore Gneiss, Aleinikoff et al., 1997), the U–Pb isotopic systematics of zircon from most of the Six granite gneiss samples contain monazite; in
five of these samples (BR554-90, BR1313-92,granitic gneisses in the northern Blue Ridge areunusually simple. Only some of the Group 1 BR1700, J329-90, and U250-92), some of the
Table 3Summary of U–Pb zircon and monazite geochronology of Mesoproterozoic rocks of the northern Blue Ridge, Virginia–Maryland
Fig. Sample number Rock type Zircon agea (Ma) Monazite age (Ma) Reliabilityb
Group 3BFig. 13(b) U39-90 megacrystic metagranite (Ypb) 1055±2 1Fig. 13(a) BR554-90 biotite granite gneiss (Ybg) 1055±4 1033±2 1Fig. 12(b) BR1313-92 pink leucocratic metagranite (Yml ) 1059±2 1059±2c 1
1034±2Fig. 12(a) BR11-90 white leucocratic metagranite (Yg) 1060±2 1
Group 3AFig. 11(a)–(c) HF1-90 garnetiferous metagranite (Ygt) 1077±4 1070±3 2
1048±6 (mz)Fig. 10 BR1674-93 quartz-plagioclase gneiss (Yqp) 1077±4 1
Group 2Fig. 9(a) WW29-90 leucocratic Marshall Metagranite (Yin) 1112±3 1051±2 1Fig. 9(b) WW22-90 biotitic Marshall Metagranite (Yin) 1111±2 1
Group 1Fig. 8(a) and (b) LN398 coarse-grained metagranite (Ymc) ~1140 3Figs. 7(b) and 15 BR1700 porphyroblastic granite gneiss (Ypg) ≥1140±4 1060±1 (dark)d 3
~1050 (mz) 1130±1 ( light)d1071±1 (uz)1062±1 (uz)1050±2(uz)
Fig. 7(a) J329-90 porphyroblastic granite gneiss (Ypg) 1144±2 1060±1 ( light)d 11106±1 (dark)d
Fig. 6(a)–(c) HF2-90+BR1529 hornblende monzonite gneiss (Yhm) 1149±19 (S) 11042±11 (S, mz)
Fig. 4(a)–(c) U250-92 layered granitic gneiss (Ylg) 1153±6 1057±5 21098±31121±6
Age uncertainFig. 14 Bl-1-94 charnockite (Yc) >1145(?) 3
a S: SHRIMP age; mz: metamorphic zircon; uz: unzoned.b 1: excellent (concordant data or very good linearity of data; no complications); 2: moderate (some linear data, but with some
complications); 3: complicated (several splits of different ages).c Upper intercept age; data are about 1% reversely discordant.d Possibly average of multi-age components.
140 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
monazite is younger than coexisting zircon and, about 100 grains from sample BR1700 weremounted in epoxy and polished for BSE imagingtherefore, is considered to be metamorphic in
origin. In contrast, the ages of some monazites in on the USGS Jeol 5800LV scanning electronmicroscope (SEM). The monazites were sub-samples HF1-90, BR1313-92, and BR1700 are
within uncertainty of the inferred age of coexist- divided into two groups of 50 grains each on thebasis of color: medium brown or pale yellowing zircon.
The ages of individual monazites, even within (referred to as ‘dark’ and ‘light’ respectively insubsequent text and in Table 1).a sample, do not define a single high-grade,
‘Grenville’ event. For example, monazite in sample BSE imaging (Fig. 15) of both dark and lightmonazite grains from sample BR1700 reveals thatU250-92 (Group 1; about 1145 Ma) has ages of
about 1110, 1100, 1055, and 1028 Ma. The broad some grains have irregular, oscillatory-zoned coresthat are overgrown by one or more unzoned rimsspectrum of monazite ages can be explained as
being due to: (1) numerous periods of monazite or overgrowths. Other grains have an irregularblotchy or spotted zoning pattern (herein referredgrowth, implying thermal events at about 1130,
1110, 1100, 1080, 1060, 1050, and 1030 Ma; (2) to as ‘spotted’) and some are completely unzoned.A total of 12 half-grains of monazite were removedigneous crystallization during emplacement of the
granitic magma and metamorphic growth (partial from the mount, including four light unzonedgrains, two dark unzoned grains, three light grainsresorption and overgrowth) at about 1030 Ma; or
(3) a combination of (1) and (2). with spotted zoning, one light grain with a faintoscillatory-zoned core and spotted-zoned over-During the early stages of the study, the pre-
vailing interpretation was that monazite is a min- growth, and two dark grains with faint oscillatoryzoning within unzoned overgrowths.eral that contains relatively simple U–Pb isotopic
systematics, lacking multiple age components Although analysis of monazite usually yieldsconcordant age data, results from all 12 grains arewithin a single grain. However, Hawkins and
Bowring (1997), Miller et al. (1997), and Fanning discordant (0.8–3.8%). Another unusual featureof the isotopic data is that the common Pb contentsand Aleinikoff (1998) have shown, using isotope
dilution and ion microprobe U–Pb geochronology are quite high (0.2–2 ng) as expressed by low206Pb/204Pb ratios (Table 1). Both of these atypicaland BSE imaging, that some multiply deformed
rocks contain monazite with multiple age domains. characteristics were probably caused by processingin the laboratory. Half-grains (resulting fromIn some cases, oscillatory-zoned cores (interpreted
as igneous in origin) occur within one or more grinding and polishing for SEM-BSE imaging)picked out of the mount may have lost radiogenicunzoned overgrowths. Other grains from the same
sample are completely unzoned, suggesting the Pb due to sample preparation and probably gainedcommon Pb from either adhering epoxy or embed-presence of only one age domain. Thus, some U–
Pb ages of whole, single monazite grains from Blue ded grinding compounds, despite leaching in veryweak HCl and HNO3 prior to dissolution. InRidge granite gneisses may be mixtures of multiple
age components, analogous to U–Pb data from essence, modern Pb-loss probably was induced inthe laboratory and thus, regressions of linear datazircon from sample HF2-90.
Preliminary U–Pb geochronology of monazite arrays are forced through 0±50 Ma. Neither ofthe laboratory problems caused insurmountablefrom sample BR1700 yielded two ages: a light-
colored grain was about 1130 Ma, whereas a dark difficulties in the geochronologic aspects of thestudy because the monazite grains contain verygrain was about 1057 Ma (Table 1). These grains
were chosen for analysis on the basis of examina- high concentrations of U and radiogenic Pb.Isotopic data from monazite of sample BR1700tion in transmitted and reflected light only; they
were relatively free of inclusions and cracks at the can be subdivided into at least three groups(Table 1, Fig. 16): (1) five grains (all light; twolimit of detection of a standard laboratory binocu-
lar microscope. To determine if mixing of age unzoned, three spotted) form a linear array withan upper intercept age of 1071±1 Ma; (2) threecomponents is a factor in Blue Ridge monazites,
141J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
Fig. 15. SEM-BSE images of monazite from sample BR1700 (porphyroblastic granite gneiss), showing representative zoning patterns:(a) grains with spotted zoning; (b) grain with oscillatory-zoned core invaded and overgrown by unzoned material; (c) broad unzonedrim on spotted core; (d) mostly unzoned grain with faint remnant of resorbed core; (e) completely unzoned grain.
grains (two dark, one light; all unzoned) with an of rocks of the northern Blue Ridge did not exceedabout 750°C (closure temperature for the U–Pbupper intercept age of 1062±1 Ma; (3) one light
unzoned grain with a 207Pb/206Pb age of system in monazite, Parrish and Whitehouse,1999). The youngest growth age of monazite in1050±2 Ma. Three other grains, containing mix-
tures of oscillatory-zoned and unzoned material, BR1700 (~1050 Ma) is very similar to the age ofthree single grains of zircon from the same samplehave 207Pb/206Pb ages of 1057, 1066, and 1086 Ma
and do not plot on any linear arrays. We conclude (Tables 1 and 3). No monazite grains in sampleBR1700 were found to be ~1033 Ma, the youngestthat metamorphic monazite grew in sample
BR1700 at about 1070, 1060 and 1050 Ma. These dated monazites in this study from samplesBR1313-92 and BR554-90. Both of these rocksages correspond fairly closely to thermal events
associated with emplacement of plutons (Table 3). occur to the east of the Short Hill fault, whereasthe porphyroblastic granite gneiss primarily cropsBecause the monazites represent several different
ages (i.e. were not reset to some common age), it out to the west of the fault. The potential signifi-cance of this fault is discussed below.is likely that Grenvillian high-grade metamorphism
142 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
tion and, therefore, must be even younger in age(Burton et al., 1994). Comparison of monazitegrowth ages of 1070, 1060, and 1050 Ma withinferred timing of formation of foliations suggeststhat thermal perturbations in the study area arenot necessarily correlative with the formation offabrics. The youngest monazite age (about1035 Ma) may coincide with either D2 or D3.Hornblende 40Ar/39Ar ages of about 900–1000 Main rocks of the Blue Ridge basement ( Kunk et al.,1993) record the last time that thisMesoproterozoic terrane cooled below about500°C.
Two ages notably absent from the isotopic datapreserved in monazite from sample BR1700 are1110 Ma (time of emplacement of the Marshall
Fig. 16. U–Pb data for monazite from sample BR1700 (porphy- Metagranite) and 1033 Ma (age of monazite inroblastic granite gneiss). Filled black ellipses represent data samples BR1313-92 and BR554-90). The mainfrom grains that contained mixtures of faint remnant zoning body of Marshall Metagranite occurs less thanand unzoned overgrowths. The ellipse with horizontal stripes
5 km to the east of porphyroblastic granite gneisshas a 207Pb/206Pb of 1050±2 Ma.(Ypg) sample BR1700 and the two units are within0.5 to 1 km locally. However, separating these twogranitic gneisses is the Short Hill fault, a post-6. Timing of Grenvillian deformation and
metamorphism Middle Cambrian normal fault that places Lowerand Middle Cambrian carbonate shelf limestoneand dolomite of the Tomstown, Waynesboro, andNorthwest-trending D1 foliation is well-devel-
oped in porphyroblastic granite gneiss (Ypg), Elbrook Formations against metabasalt and meta-rhyolite of the Neoproterozoic Catoctin Formationhornblende monzonite gneiss (Yhm), and layered
granitic gneiss (Ylg). These rocks have Group 1 (570 Ma; Aleinikoff et al., 1995) and LowerCambrian Weverton and Harpers Formations. TheU–Pb zircon ages, suggesting that D1 occurred
after about 1145 Ma. D1 foliation is poorly devel- displacement in the Paleozoic cover rocks is limitedto a few kilometers (Southworth and Brezinski,oped in Group 1 coarse-grained metagranite
(Ymc), suggesting either that this rock is some- 1996). This displacement may be sufficient toaccount for the absence of Marshall-age monazitewhat younger than the porphyroblastic granite
[1128±10 Ma, as suggested by some of the zircon in sample BR1700. Alternatively, the absence of1110 and 1033 Ma monazite in the porphyroblasticdata, Fig. 8)] or that this rock was somehow
resistant to foliation development. D1 foliation in granite gneiss suggests that the monazite in theporphyroblastic granite gneiss was not affected byYpg is discordantly cut by Group 3A garnetiferous
metagranite, indicating that the minimum age for the intrusion of Marshall Metagranite. Thus, theShort Hill fault may previously have had sig-D1 is about 1077±4 Ma. Cooling and strain hard-
ening of Group 1 gneisses may have promoted nificant displacement, juxtaposing MarshallMetagranite and younger granitic gneisses againstpreservation of D1 foliation. Northeast- to north-
west-striking D2 foliation and southeast-plunging porphyroblastic granite gneiss long after emplace-ment of the protoliths of the granitic gneisses. Iflineation are well-developed in Group 2 [Marshall
Metagranite (Ym)] and Group 3 [biotite granite so, the small area of rock classified as MarshallMetagranite north of sample BR1700 and west ofgneiss (Ybg)] rocks. D2 deformation must have
occurred, therefore, after the latest intrusion, or the Short Hill fault (Fig. 2) may be reinterpretedas 1055 Ma biotite granite gneiss (Ybg).about 1055 Ma. Upright D3 folds arch D2 folia-
143J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
7. Comparison with recent U–Pb crystallization 1.17 Ga) approaches the age of northern BlueRidge Group 1 plutonism (Aleinikoff et al., 1997).ages from other Grenvillian terranes in the eastern
Specific correlation of rock units in the northernUSABlue Ridge basement with possible equivalents inthe central Blue Ridge is hampered by a lack ofThe three major Mesoproterozoic magmaticrecent, high-precision U–Pb dates for the latterpulses indicated by our geochronologic data fromregion. The central Blue Ridge rocks are similarrocks of the northern Blue Ridge have possiblein that they are largely plutonic and have U–Pbcorrelatives in other parts of the USA portion ofcrystallization ages in the range of 1.15–1.05 Gathe Grenville Province. Three clusters of U–Pb(Sinha and Bartholomew, 1984). However, theages of plutonic rocks in the Adirondackvery complicated growth characteristics and iso-Highlands of northern New York appear to matchtopic systematics in zircon from the hornblendefairly well those determined for the northern Bluemonzonite gneiss suggest that rocks of similarRidge: 1134–1156 Ma mangeritic and charnockiticlithology in the central Blue Ridge probably are
gneisses and hornblende granitic gneisses, equiva- equally complex. Thus, conventional U–Pb agelent to Blue Ridge Group 1; 1095–1100 Ma horn- data on the central Blue Ridge high-grade rocksblende granitic gneiss (equivalent to Group 2); may represent mixtures of age components andand 1050–1075 Ma alaskitic gneiss, anorthosite, should be viewed with some skepticism.and metagabbro (Group 3) (McLelland et al., Zircons from the Corbin Gneiss of northern1988). Syn- to post-intrusive deformational fabrics Georgia have a crystallization age of aboutin the two younger groups of rocks in both areas 1106±13 Ma, very similar to the emplacement ageare evidence for the late Grenvillian Ottawan of northern Blue Ridge Group 2 rocks.Orogeny of Moore and Thompson (1980). Overgrowths formed at about 1040 Ma, slightly
Two episodes of magmatism in the Adirondacks post-dating the northern Blue Ridge Group 3, andand Green Mountain Massif of Vermont have no much later at about 950–980 Ma (Heatherington
et al., 1996; Aleinikoff, unpublished data).known equivalents in the northern Blue Ridge,In the broader tectonic context, rocks of theincluding 1.30–1.35 Ga tonalitic and trondhjemitic
northern Blue Ridge probably belong to the colli-gneisses (McLelland and Chiarenzelli, 1990;sional Shawinigan (~1.19–1.14 Ga) and OttawanRatcliffe et al., 1991), and 1.25 Ga granites(1.08–1.02 Ga) phases of the Grenville Orogeny(McLelland et al., 1988; Aleinikoff et al., 1990a),as defined in the Grenville Province of easternof the Elzevirian Orogeny of Moore andCanada (Rivers, 1997). However, the emplacementThompson (1980). Also, a suite of 0.96 Ga meg-of the Marshall Metagranite at about 1110 Maacrystic granites occurs in Vermont ( Karabinosdoes not fit this regional scheme. Close correspon-and Aleinikoff, 1990), whereas no thermal ordence among ages of Blue Ridge and Adirondackmetamorphic activity younger than 1.0 Ga hasrocks suggests that discrete magmatic pulses withinbeen found in the northern Blue Ridge.this phase of the Grenville Orogeny in the USA
In the Hudson Highlands of southern New may have been widespread. Evidence for plutonismYork, the oldest well-dated rock is the hornblende- associated with or predating the ~1.25 Gabearing Storm King Granite, whose U–Pb crystal- Elzevirian Orogeny is lacking in the northern Bluelization age of about 1130 Ma (Ratcliffe and Ridge, either because the orogeny never occurredAleinikoff, 1990; 1134±2 Ma, Aleinikoff, unpub- in this region or the rocks were subsequentlylished data) falls between Group 1 and Group ages destroyed during later plutonism.of the northern Blue Ridge, whereas the CanadaHill Granite (1010 Ma; Aleinikoff et al., 1982) isyounger than any dated rocks in the northern Blue 8. ConclusionsRidge. Metavolcanic rocks of the Baltimore Gneissare distinctly older (about 1.25 Ga), but the youn- (1) Large volumes of granitic magma were
emplaced into the crust of what is now the coregest event affecting zircons in these rocks (about
144 J.N. Aleinikoff et al. / Precambrian Research 99 (2000) 113–146
of the Blue Ridge–South Mountain anticlinorium between deformations D1 and D2 in the northernBlue Ridge. The final high-grade event, probablyin northern Virginia and Maryland during a period
of about 100 million years, from about 1150 to at about 1035 Ma, did not reset older monaziteages, indicating that the temperature of this event1050 Ma. Within this time span, three pulses of
plutonism occurred: at about 1150–1140 (or (and all previous events except the first one) wasless than about 750°C.1125 Ma) (Group 1); 1110 Ma (Marshall
Metagranite, Group 2); and 1075–1055 Ma (6) Lack of evidence for 1110 Ma growth ofmonazite in porphyroblastic granite gneiss suggests(Group 3, perhaps divisible into two subgroups at
about 1075 and 1060–1055 Ma). The name, that the Short Hill fault might be a Grenvillianstructure that was reactivated in the Paleozoic.Marshall Metagranite, formerly applied to most
of the granitic gneisses of the northern Blue Ridge, (7) Mesoproterozoic rocks of the northern BlueRidge are broadly age-correlative with rocks inis now restricted to weakly to moderately foliated,
medium-grained biotite granitic gneisses that are other Grenville terranes of the USA, particularlythe Adirondacks. The 100 m.y. span of ages ofabout 1110 Ma.
(2) Although most of the granitic gneisses in Blue Ridge granitic gneisses corresponds to theShawinigan and Ottawan pulses of the Grenvillethis study are amenable to dating by conventional
zircon U–Pb geochronology, zircons in hornblende Orogeny in eastern Canada. However, correlationof ages with other high-grade rocks in the centralmonzonite gneiss are very complex mixtures of
cores and overgrowths, necessitating use of the ion Appalachians may be specious because of the verycomplex growth characteristics of the zircons thatmicroprobe to decipher the age components. On
the basis of this experience, similar strategies may have resulted in mixed ages. ConventionalU–Pb geochronology may not be able to deciphershould be considered when determining the ages
of comparable high-grade metamorphic rocks in multi-age isotopic systematics; in situ analysis byion microprobe probably is the only method capa-the central and southern Blue Ridge.
(3) On the basis of presence or absence of ble of analyzing individual age components withincomplexly zoned grains.foliations in the dated samples, D1 is constrained
to have occurred between about 1145 and 1075 Ma(or possibly between about 1145 and 1128 Ma).Even the youngest dated samples contain a folia-
Acknowledgementstion, which means that the younger foliation-producing deformation (D2) must be younger than
Marianne Walter and Rebecca Sauer ably com-about 1050 Ma. 1035 Ma monazite may havepleted mineral separations and chemical extrac-grown in response to D2 or the subsequent defor-tions. We thank Mark Fanning and Ian Williamsmation (D3) that folded D2 foliation.for help using the ion microprobe in Canberra.(4) Complex zoning in monazite may be causedWe thank Wright Horton, Robert Ayuso,by compositional variation during magmatic orFernando Corfu, and Calvin Miller for detailedmetamorphic growth or by inclusion of xenocrysticcritical reviews of earlier versions of the manu-monazite (comparable to inheritance in zircon).script. Their comments resulted in significantBecause the closure temperature of the U–Pbimprovements to the text, figures, and tables; how-system in monazite is greater than temperatures atever, the authors assume sole responsibility for allwhich metamorphic monazite can form, care mustconclusions and interpretations presented herein.be taken (e.g. by using SEM-BSE imaging) to
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