Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Mill Creek Nappe
Avondale Massif
West Chester Massif
Mill Creek Nappe
Street Road Fault
Embreeville thrust
Advances in constraining the timing of deformation and metamorphism: Salinic through Acadian transpressional tectonics in the Central Appalachians
Howell Bosbyshell1, LeeAnn Srogi1 and Gale Blackmer2
1West Chester University 2Pennsylvania Geological Survey
ReferencesAleinikoff, J.L, Schenck, W. S., Plank, M. O., Srogi, L. A., Fanning, C. M., Kamo, S. L., and Bosbyshell, H., 2006, Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite: Geological Society of America Bulletin, v. 118, p. 39-64.
Blackmer, G. C., 2005, Preliminary bedrock geologic map of a portion of the Wilmington 30- by 60-minute quadrangle, southeastern Pennsylvania: Pennsylvania Geological Survey, 4th ser., Open-File Report OFBM 05–01.0, 16 p., Portable Document Format (PDF) file.
Bosbyshell, H., Blackmer, G.C., Srogi, L., Mathur, R., Crawford, M., Valencia, V., and Schenck, W.S., 2012, Detrital zircon ages from the Wissa-hickon Formation, southeast Pennsylvania and northern Delaware: Regional tectonic implications: Geological Society of America Abstracts with Programs, v. 44, no. 2, p. 87.
Bosbyshell, H., 2008, Bedrock geologic map of a portion of the Philadelphia quadrangle, Montgomery and Philadelphia Counties, Penn-sylvania: Pennsylvania Geological Survey, 4th ser., Open-File Report OFBM 08–05.0, 21 p., Portable Document Format (PDF).
Faill, R. T., 1997a, A Geologic history of the North-Central Appalachians. Part 1. Orogensis from the Mesoproterozoic through the Taconic orogeny: American Journal of Science, v. 297, pp. 551-619.
Hanan, B. B., and Sinha, A. K., 1989, Petrology and tectonic affinity of the Baltimore mafic complex, Maryland: in Mittwede, S. K., and Stoddard, E. F., eds., Ultramafic rocks of the Appalachian Piedmont: Geological Society of America Special Paper 231, p. 1-18.
Pyle, J.M., Bosbyshell, H., and Blackmer, G.C., 2006, Refining the metamorphic and tectonic history of the southeastern Pennsylvania Piedmont: Recent results from monazite and zircon geochronology and accessory-phase thermometry, in Pazzaglia, F.J., ed., Excursions in Geology and History: Field Trips in the Middle Atlantic States: Geological Society of America Field Guide 8, p. 83-112.
Srogi, L., Blackmer, G.C. and Bosbyshell H., 2007, Contrasting metamorphic histories within the Wissahickon Formation: Evidence for subdivision and tectonic implications: Geological Society of America Abstracts with Programs, v. 39, no. 1, p. 86.
?
?
?
?
?
?
?
?
?
?
?
?
? ?
? ?
??
Geologic Map of Southeastern Pennsylvania and Northern Delaware
Pa.
Del.Md.
S t r e e t R o a d F a ul t
E mb
r e e v i l l e T h r u s t
H u n t i n g d o n V a l l e y S Z
UD
U
D
U
DU
D
Pa.Del.
Wilmington Complex
Coastal Plain
Avondale MassifWest C
hester Massif
Honey Brook Upland
Mine Ridge
Mill Creek
Anticli
ne
M a r t i c L i n e
Woodville
Nappe
Legend
Stockton Fm.
Triassic rocks
10 Km
N
S
EW
40º
41º41º75º
78º80º
77º42º
79º
40º
76º
80º
75º
76º
77º
78º42º
75º
79º
DEL.
Octoraro FormationChickies, Harpers, Antietam Fms.undifferentiated meta-clastic rocksUndifferentiatedmeta-carbonate rocks
Lower PaleozoicRocks of known and probable Laurentian affinity
Cambrian West Grove Metamorphic Suite
Peters Creek Schist
Mt. Cuba Gneiss
Laurels and Doe Run Schist
Glenarm GroupPrecambrian
Anorthosite
Quartz monzonite gneiss
Felsic and intermediate gneiss
Graphitic felsic gneiss
West Chester MassifGranulite faciesAmphibolite facies
Avondale massif
Undifferentiated gneissSycamore Mills Fm.
Kennett Square Amphibolite
White Clay Creek Amphibolite
Silurian plutonic/metaplutonic rocks
Ordovician metaigneous rocksGabbro Granite/
granodiorite
Volcanic Plutonic
Undifferentiatedultramafic rocks
Wissahickon Fm
Ordovician meta-clastic and meta-volcanic rocks
Rocks of arc affinity
This poster focuses on the West Grove Meta-morphic Suite (WGMS)- the Mt. Cuba Gneiss and Doe Run Schist and the arc-related Wissahickon Formation.
Simplified Geologic Map of the Central Appalachian Piedmont
Mesozoic Basin
Wissahickon FmWilmington Complex
James Run arc/BMC
Coastal Plain
Mt. Cuba GneissLaurels & Doe Run SchistMetasedimentary rocksClastics and carbonatesGrenville gneiss
0 km 25
A
N
39 o30'N
40 o30'N
40 oN
75o W
76o WPa.Md.
MAP AREA
Modified from Faill (1997), Hanan and Sinha (1989).
R o s e m
o n
t
S
Z
CC-7205
B-22
Ge-06-33
Wiss-1N
P-05-02
WG
WG-43
C-207GW-05-54
44069
-216
Bc21-bWG-101
GW-05-154Doe Run SchistUnionville, Pa
n = 124
C-207Doe Run Schist
Chatham, Pan = 90
500 1000 1500 2000 2500Ma
Bc21-bMt. Cuba Gneiss
Ashland, Den = 95
44069Mt. Cuba Gneiss
Yorklyn, Den = 49
Aleinikoff et al. 2006
Wg-101Mt. Cuba GneissWest Grove, Pa
n = 95
0 500 1000 1500 2000 2500Ma
525Ma
0 500 1000 1500 2000 2500 3000Ma
The West Grove Metamorphic Suite is no longer considered part of the Wissahickon Formation, based on differences in tectono-thermal history and detrital zircon provenance. (Bosbyshell et al, 2012).
West Grove Metamorphic Suite Wissahickon Formation
CC46-2Wissahickon Fm.Glen Riddle, Pa
n = 116
Wiss-1NWissahickon Fm.
Wissahickon CreekPhiladelphia, Pa
n = 139
P-05-02Wissahickon Fm.
Fairmont ParkPhiladelphia, Pa
n = 143
610 Ma 1200 Ma
1470 Ma960 Ma
554 Ma
574 Ma
1050 Ma960 Ma
1035 Ma
546 Ma
1050 Ma
945 Ma
1020 Ma
965 Ma
547 Ma
895 Ma
1195 Ma
Doe Run Schist
WG-216GW-05-154C-207
340
360
380
400
420
440
460
480
520
540
560
Age
(Ma)
staurolite
sillimanite
WG-216
Width of field ≈ 6mm for both photos
Ca
434
430
471
491
427
418
m1
The Embreeville Thrust places the West Grove Metamorphic Suite and all struc-turally higher rocks above the green-schist grade Peters Creek Formation.
staurolite
sillimanitegarnet
C-207
Ca x-ray map of garnet from WG-216. Low Ca rim is in-terpreted to correspond to garnet growth during high T staurolite breakdown (Srogi et al., 2007). Late garnet and sil after st (photomicrographs) is syn- to post-tectonic. Isopleths of this garnet rim compositon inter-sect at approximately 700C and 5 kb.
400 500 600 700 800Temperature [C]
4000
5000
6000
7000
8000
9000
10000
Pres
sure
[Bar
]
pl ilm grt bt sil qtz liq qfm
st-bearingassemblages
WG-216
kyan
itesil
liman
ite
-sta
urol
ite
-bio
tite
+liquid-H
2O
+cordierite
+sill
iman
ite
-chloritoid
+ rt- ilm
+chlorite
-chlorite
T h
e r i
a k - D o m i n o
Monazite with young, low-Y, high-Th rims is present in the matrix and as inclusions in the high-Ca garnet rims in WG-216. High-T metamor-phism can be no older than 410 ± 11 Ma. Textural considerations on the timing of monazite growth similarly constrain the age of high-T metamorphism and deformation in C-207 to 411 ± 11 Ma.
alm
grs
pyr
sps
Mt. Cuba Gneiss
2 mm
Rose
mon
t She
ar Z
one
340
360
380
400
420
440
460
480
Age
(Ma)
Monazite is complexly zoned and may contain two age populations: 419 ± 9.7 Ma and 437 ± 11.0 Ma. Both inclusions and matrix grains contain compositional domains which yield both ages. While there are instances where textures are con-sistent with this age difference, such examples are not ubiquitous. Thus it is not clear that these re-sults indicate two distinct periods of monazite growth. The average of all analyses is 427 ± 14 Ma, similar to previously reported TIMS and SHRIMP results (above, Aleinikoff, 2006).
44069 (monazite)Wissahickon Formation
TIMS data
206
Pb/23
8U
207 Pb/235 U
429
427
426
424
422
4200.0672
0.0676
0.0680
0.0684
0.0688
0.512 0.514 0.516 0.518 0.520 0.522 0.524 0.526 0.528
Concordia Age = 424.9 ± 0.4 Maprobability of concordance = 0.42
P-T conditions in Mt. Cuba Gneiss in the Mill Creek nappe, Yorklyn, De (loc. 44069) are estimated to have exceeded 750°C, based on the presence of perthitic feldspars, at pres-sures in the sillimanite stability field, but greater than 0.6 GPa, based on a lack of cordierite (Bosbyshell et al., 2011). SHRIMP and TIMS monazite ages, 426 ± 3 Ma and 424.9 ± 0.4 Ma, respectively (Aleinikoff, et al., 2006), constrain the timing of high-T metamorphism and deformation.
T h
e r i
a k - D o m i n o
550 650 750 850Temperature [C]
4000
5000
6000
7000
8000
9000
10000
Pres
sure
[Bar
]
23.01.2012 - 10:53:11 CPU time: 5h 04m 32.46
- sillimanite
kyan
itesil
liman
ite
- bio
tite
-sta
urol
ite
-staurolite
-bio
tite
+liquid-H
2O
-mus
covi
te
+cordierite
+cordierite
+kyanite
+sill
iman
ite
-chl
+ms
+ rutile- ilmenite
pl ilm grt bt sil qtz
H2O qfm
pl ilm grt bt sil qtz liq qfm
WG-43b
Ca Mn
The dominant foliation in the Mount Cuba Gniess above the Street Road Fault (sample WG-43) is defined by aligned sillimanite and biotite. Staurolite contains inclusions of foliation-parallel sillimanite and cross-cuts sillimanite fabrics. Kyanite occurs as small crystals which overprint sillimanite and high Ca-rims are present on garnet. This suggests a second period of mineral growth during isobaric cooling from peak temperatures.
B22Ca Mn
U
Wissahickon Formation
Age
(Ma)
Age
(Ma)
300
320
340
360
380
400
420
440
460
480
500
380
390
400
410
430
450
470
420
440
460
480
B-22
Rim domains
320
340
360
380
400
420
440
460
480
Outer core
388 ± 12 Ma
(n = 24)
Cores
Rims
CC-7205
Above, syn-tectonic garnet in ultramylonite from the RSZ (loc. CC-7205). P-T estimates of 600C at 8 kb were ob-tained using the composi-tion of this garnet with recrystallized matrix pla-gioclase and biotite. Syn-tectonic monazite is Devonian, 388 ± 12
Ma. The shear zone cuts high-T, low-P pelitic migmatite. Monazite cores record this earlier metamorphism and are similar in age to zircon and monazite in the adjacent Wilmington Complex (Aleinikoff et al., 2006).
0.5 mm
Rosemont Shear Zone
Sample B-22 demonstrates the metamorphic and deformational history of the western, probably lower portion of the Wissa-hickon Formation. Garnet maps show a low-Ca core, which formed during M1, high-T low-P metamorphism (andalusite-sillimanite grade), with a high-Ca overgrowth the formed during M2, higher-P, kyanite-grade metamorphism. The fabric in this rock, S3 wraps the M1 garnet core, but is included within M2 overgrowth. Monazite cores (433 ± 7 Ma) likely record the time of M1. Asymmetric high-U rims on monazite constrain the timing of S3 (average, 415 ± 14 Ma) and are included in high-Ca garnet rims.
While complexly deformed, the eastern Wissahickon Formation is characterized by a classic Barrovian, st-ky-sil metamorhpic field gradient and exhibits little evidence of the Silurian high-T meta-morphism. Monazite ages indicate that this metamorphism is Devonian, approximately 384 Ma(Bosbyshell, 2008).
Ge-06-33P-05-02
Cores
Tectonic Speculation
Modified from Blackmer, 2005; Pyle et al., 2006.
Wilmington ComplexWissahickon Fm.
Iapetus Ganderia ? Avalonia
Ordovician - ~480 Ma
Silurian ~430 Ma
AvaloniaDoe Run Mt.Cuba Wiss Fm.Laurentia
AvaloniaLaurentia
Wilmington Complex is intra-Iapetan arc.
High-T low-P metamorphism in Wissahickon, then Mt. Cuba Granulite metamorphism of Wilmington Complex
Early Devonian ~410 Ma
Salinic Orogeny
Street Road Fault
Dextral transpressionBurial and metamorphism of Doe Run SchistDeformation onset of crustal thickening in Wissahickon Fm.
Rosemont Shear Zone
AvaloniaLaurentia
Street Road FaultEmbreeville Thrust
Rosemont Shear Zone
Early stages of Acadian orogeny
Middle Devonian ~385 MaEnd of Acadian orogeny
Barrovian metamorphism of Wissahickon Fm.