American Mineralogisr, Volume 64, pages l5-31, 1979

Petrology of an eclogite-metarodingite suite at Cima di Gagnone, Ticino, Switzerland

BrRNnno W. EveNs

Department of Geological Sciences AK-20, Uniuersity of WashingtonSeat t le, llashingt on 98 I 9 5

VoLxnaa.n Tnouusnonrr

Institut fur Kristallographie und Petrographie, Eidgenbssische Technische HochschuleETH-Zent rum. C H -8092 Zilrich. Switzerland

eNn Wor-RRe*r RrcHrrn

M ine ralog is ch- P et rog raphis ches I ns t i tut de r U niue r s i tiit W ienA-1010 Wien. Austria

Abstract

Mafic rocks composed essential ly of garnet * cl inopyroxene occur in associat ion withmetamorphic ultramafic rocks in the Penninic Adula-Cima Lunga Nappe around the summitof Cima di Gagnone, Ticino, Switzerland. The mafic rocks are interpreted as basalt ic volcan-ics and minor intrusives and their mildly and strongly rodingit ized products, later metamor-phosed under eclogite facies condit ions. They show continuous vai iat ion in their bulk chem-ical, mineralogical, and textural propert ies from eclogite (metabasalt ic, Cpx with Jd > l0percent) to metarodingite (CaO o 24 weight percent, NarO ( 0. I weight percent). Garnets inthis series vary continuously from 84 percent pyralspite, l6 percent ugrandite ( in eclogite), to42 percent pyralspite, 58 percent ugrandite; some highly calcic metarodingites contain garnetwith 80 percent ugrandite. Cl inopyroxenes vary correspondingly from omphacite (45 percentJd) to diopside (( l percent Jd) and eventual ly to fassaite (23 percent Cats). The metaro-dingites dif fer from grospydites and kyanite eclogites in kimberl i tes by the absence of kyaniteand presence of diopside rather than omphacite; they also dif fer in bulk composit ion.

Abundances of some trace and minor elements appear to have been l i t t le affected by theprocess of rodingit izat ion or by the succeeding episodes of eclogite and amphibol i te faciesmetamorphism. They suggest sea-f loor tholei i t ic basalt aff ini t ies and a possible oceanic originfor the ultramafic-mafic rock suite at Cima di Gagnone. An eclogite facies garnet lherzol i teassemblage is preserved in one of the ultramafic bodies.

The Kp for Fe1o1u/Mg part i t ioning between garnet and cl inopyroxene averages 6 in theeclogites, and r ises, with increase in grossular component in the garnet, to l5 in the morecalcic metarodingites. The Riheim and Green temperature cal ibrat ion applied to eclogiteswith garnets containing up to 25 percent grossular component, and a new cal ibrat ion byGanguly suitable for low-Na pyroxene-calcic garnet pairs appl ied to the metarodingites. givecomparable values and suggest condit ions for the eclogite facies metamorphism were l ikely toh a v e b e e n 8 0 0 " C , P x 2 5 k b a r . S i m i l a r v a l u e s h a v e p r e v i o u s l y b e e n o b t a i n e d f l o r t h e G a g n o n egarnet lherzol i te. These condit ions do not overlap those estimated for the Tert iary CentralAlpine Barrovian-style metamorphism in the area (600*100'C, P ( l0 kbar).

The Central Alpine metamorphic overprint did not reequil ibrate Fe and Mg between garnetand pyroxene. However, i t part ial ly amphibol i t ized the eclogites and was responsible forhornblende and epidote growth in the metarodingites. Contact reaction zones (probablypolymetamorphic) between metarodingite boudins and ultramafic rock contain hornblende,diopside, epidote, sphene, and chlori te; contact zones against eclogite contain the sameminerals, except for garnet instead of diopside. Phlogopite and staurol i te have been found inamphibol i t ized eclogite.

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l 6 EVANS ET AL.: ECLOCITE-METARODINGITE SUITE

Assuming metasomatic rodingit izat ion to be an exclusive accompaniment of serpentiniza-tion, the mafic-ultramafic rock suite at Cima di Gagnone must, at an early stage in itsmetamorphic history, have been in a low I, low P environment. This was lol lowed bysubduction and eclogite facies metamorphism, then tectonic uprise to shal lower depths, andregional Barrovian-style metamorphism.

General introduction

This is a petrological study of a suite of mafic rockssurrounding and enclosed in lenses of metamorphicultramafic rock in amphibolite facies gneisses in thevicinity of Cima di Gagnone, Valle Verzasca, Ticino,Switzerland. The mafic rocks comprise amphibolites,eclogites, metamorphosed rodingites, and rocks tran-sit ional between them, together with the products ofcontact reaction between mafic and ultramafic rock.This paper describes the completely gradational set ofbulk chemical, textural, and mineralogical propertiesof the eclogite-metarodingite suite. These mafic rockstogether with associated ultramafics record geologic,metamorphic, and tectonic events pre-dating the Ter-tiary Central Alpine, Barrovian-style regional meta-morphism.

Regional introduction

The Cima di Gagnone area is located in the Lepon-tine Alps 25 km N of Locarno, between the Ticinoand Verzasca Valleys. It is situated in the heart of theCentra l Alp ine metamorphic bel t c lose to the s i l l ima-nite isograd. Structurally, the area l ies within thelower Pennine Nappes, between the N-S trendingaxial culmination of the Ticino (Leventina Nappe) tothe E and the steeply plunging Maggia Zone to the W(Fig. l). It includes the boundary zone between theSimano and the Cima Lunga ( :Adula) Nappes(Preiswerk et al., 1934} Isoclinally folded rocks inthis area possess a flat-lying regional foliation andaxial planes that dip progressively steeper south to-wards the S, where they merge into the steep, E-Wtrending zones close to the Insubric Line (Wenk,r 9ss).

Boundaries between structural units (nappes) inthis general area have traditionally been drawn alongmetacarbonate zones, on the assumption, based onthe lower-grade Pennine nappes, that the metacarbo-nate rocks are metamorphosed Mesozoic sediments.At Cima di Gagnone, they are impure calcareoussediments varying in thickness from a few centimetersto several tens of meters. The metacarbonate rocksare accompanied by a distinctive suite of rock typesthat are largely absent from the monotonousquartzo-feldspathic gneieses of the over- and under-lying nappes, uiz. ultramafic lenses and macrobou-

dins, amphibolites (some eclogitic), and semi-pelit icgneisses. Alpe Arami, l5 km SE of Cima di Gagnone,is probably the best-known locality in this zone. Thissuite has been tentatively interpreted as an associa-tion of highly metamorphosed oceanic rocks belong-ing to the Tethyan plate. The lherzolit ic compositionof the ultramafics (which include garnet peridotite)has suggested, on the other hand, correlation with thesubcontinental mantle of the south European plate.The age of the suite-Mesozoic or pre-Mesozoic-issti l l unknown. Dal Vesco (1953) gave special atten-tion to the mafic/ultramafic components of the suite.Additional information on the regional geology maybe found in Wenk (1943, 1967, p. 418-430), Evansand Trommsdorff (1974), Heinrich (1978), andSt?iuble (1978).

The grade of metamorphism in the Gagnone areahas been determined in the course of numerous stud-ies on progressive regional metamorphism in theCentral Alps (Trommsdorff and Evans, 1974, andothers). The assemblage kyanite * staurolite * mus-covite 4 quartz characterizes the metapelitic gneisses,whereas in quartz veins all three AlzSiOu polymorphscan be found (Heinr ich, 1978), a l though ear ly kyani tedominates over late andalusite and fibrolite (e.g., atPasso di Gagnone, Fig. 2). Metacarbonate rocks con-tain calcite, dolomite, tremolite, diopside, quartz, cal-cic plagioclase, and scapolite. Almandine-andesineamphibolite can be shown in many examples to haveformed from an earlier eclogitic assemblage. Ultra-mafic rocks are composed of olivine, anthophyll ite orcummingtonite, tremolite, chlorite, chrome-spinel,and at least two generations of enstatite. The meta-morphism involved at least an earlier high-pressureeclogite facies (defined by the parageneses omphacite* pyralspite and olivine * pyrope) and a later lower-pressure amphibolite facies event. This last event, theCentral Alpine metamorphism, was Tertiary in age,as has long been known from field and more recentlyfrom geochronological studies (Jiiger et al., 1967;Frey et al., 1974).

Field relations of the mafic rocks

This study concentrates on the mafic rocks occur-ring in contact association with the ultramafic bodiesaround Cima di Gagnone (Fig. 2). Layers of variably

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EVANS ET AL: ECLOGITE-METARODINGITE SUITE

Fig. l . Tectonic map of the southern Lepont ine Alps. Smal larea around Cima di Gagnone is enlarged in Fig 2 Alpe Arami is at A

amphibol i t ized ec logi te occur a long the margins ofthe ultramafic bodies and in the nearby semi-pelit icgneisses. Metamorphosed rodingites (i.e., calc-sil icaterocks formed by Ca-enrichment of dikes or countryrock dur ing serpent in izat ion) and rocks t ransi t ionalbetween eclogite and metarodingite occur inside theultramafics. They form thin boudinaged sheets, al-most everywhere paral le l to the composi t ional band-ing and foliation in the ultramafics.

The eclogites, which we wil l define here as con-s is t ing of pyra lspi te and pyroxene wi th more than l0percent jadeite component, are generally bandedrocks forming layers (Fig. 3) typically one to threemeters thick and tens of meters long. The bandedappearance is due to variations in the relativeamounts of the pr inc ipal minerals and of amphibole,epidote, and symplektite. A weak foliation parallel tothe banding is marked by the alignment of pyroxene,hornblende, and zois i te . The gra in s ize of the pr inc i -pal constituents is I to 2 mm, rounded garnets beingthe largest and most conspicuous.

The metarodingites are pink, relatively fine-grainedhighly calcic rocks, in places faintly banded, and are

commonly cross-cut by irregular thin veins of garnet,pyroxene (less common), and secondary hornblende.Compared to the eclogites, the metarodingites formthinner bodies, se ldom exceeding one meter in th ick-ness, and are strongly boudinaged. Recognizablemetarodingi te boudins may be as smal l as one or twocentimeters across.

Dark, amphibole-rich contact reaction zones (Fig.4) surround a l l metarodingi te boudins. There arenormally three such reaction zones around each bou-din; from metarodingite to ultramafic rock, these are:( l ) a green zone consist ing of hornblende-epidotesymplekt i te and d iopside sharply bounded againstthe metarodingite, (2) a black zone of hornblendeand epidote (locally with megacrysts of sphene), and(3) a coarse-grained heterogeneous zone of actinoliteand chlor i te , wi th chlor i te becoming more abundantoutwards. Reaction zones between eclogite and ultra-mafic rock are not so simple and clear as thosearound metarodingite. From eclogite to ultramafic,they are made up of the sequence: ( l ) garnet * sym-plektite (after omphacite), (2) hornblende, passingoutwards in to act inol i te , and (3) ch lor i te*act inol i te .

t 8 EVANS ET AL: ECLOG]TE-METARODINGITE SUITE

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The principal difference between the reaction zonesagainst metarodingi te and eclogi te is the abrupt d is-appearance at the beginning ofzone I ofgarnet in theformer and of omphaci te in the la t ter .

A sizeable fraction of the garnet-bearing maficboudins in the region possess st ructura l and textura lcharacteristi cs transitional between eclogite and Ca-r ich metarodingi te. These rocks, as wi l l be shown

Fig. 3 Cross-sect ion of the western, hanging-wal l s ide of u l t ra-maf ic lens I 63, showing typical mode of occurrence of eclogi te andmetarodingi te Abbreviated specimen numbers are given (e.g , f r l li s 163 -4 - l l . Tab le l ) .

F ig. 4 Sketch of hand specimen of zoned metarodingi te lens

from ul t ramaf ic lens 163. Zones 1.2. and 3 are descr ibed in thetext .

below, have correspondingly intermediate bulkchemical and mineralogical properties. They are in-terpreted as rocks in which the original process ofrodingitization was incomplete. The cut-off point be-tween rocks label led metarodingi te and t ransi t ionalcan only be arb i t rary. Transi t ional rocks have not asyet been found spatially separating eclogite and meta-rodingi te in one s ingle maf ic layer . Indeed, a meta-rodingi te body wi th an ec logi te in ter ior , such asmight form by metamorphism of a basal t ic d ike wi throdingitized margins, has not yet been discovered inthe area.

In a few places, gradations from eclogite into py-

ralspitic garnet rock and from metarodingite intougranditic garnet rock have been observed.

Microscopic petrography

Eclogite

Di f ferent specimens show a process of amphibol i t i -zation developed to varying degrees. The early ec-

logi t ic assemblage inc ludes garnet , omphaci te, andrut i le . Cl inozois i te and pale brownish-green horn-

b lende probably a lso belong to the ec logi te assem-blage (Heinr ich, 1978). Unl ike at Alpe Arami (Dal

Vesco, 1953; Ernst, 1977), kyanite eclogite is rare,a l though pseudomorphs of corundum * phlogopi te

af ter kyani te are not uncommon (Heinr ich, 1978).Accessory minerals include zircon, apatite, andquarrz. Evenly distributed garnets tend to be porphy-

roblast ic , and except in thei r marginal zones, conta int iny inc lus ions, some making S-shaped and p lanar

t ra i ls . Some garnets are ato l l -shaped, even in samplesthat escaped amphibolit ization. Omphacite may beelongate and in some cases poikiloblastic to smallergarnets.

Amphibol i t izat ion reduces the modal amount ofgarnet and pyroxene, and is concentrated at the mar-gins of the eclogite layers or parallel to the bandingand foliation within the body. The garnets tend to

EVANS ET AL. ECLOGITE_METARODINGITE SUITE

become ato l l -shaped or h ighly i r regular , but in manyrocks seem to remain part of the assemblage, that is,show evidence of textura l equi l ibr ium wi th the horn-b lende. The products of ident i f iab le garnet break-down are epidote and hornblende, in some cases insymplekt i t ic in tergrowth wi th zoned poik i lob last icp lagioc lase. Omphaci te is in i t ia l ly par t ia l ly a l tered toa fine-grained symplektite which afterwards becomesreplaced by hornblende. Format ion of a more d iop-sidic clinopyroxene also seems to accompany increas-ing amphibol i t izat ion. Epidote gra ins tend to bezoned to more ferric compositions outwards; the lessferr ic examples are id ioblast ic and e longate. Rut i lebecomes rimmed and replaced by sphene, but onoccasion has surv ived tota l amphibol i t izat ion. Wi thincreasing amphibol i t izat ion, the hornblende changesfrom very pale brown to green. phlogopi te hasformed along favored S-surfaces in some of the am-phibol i t ized ec logi tes. One h ighly amphibol i t ized ec-logi te was found to conta in smal l amounts of s tauro-l i te .

( B )

Fig. 5. (A) Photomicrograph of sugary- textured metarodingi te(163 -5 -9 ) . Con ta i ns ga rne t (w i t h i nc l us i ons ) , d i ops ide and i lmen i t e ;p lane-polar ized l ight . (B) Pholomicrograph of metarodingi te (31)wi th d iopside vein let , p lane-polar ized l ight .

F i g 6 . Pho rom ic rog raph o f me ta rod ing i t e ( 31 -5 -3D) , show ingcontact wi th a l terat ion zone l , p lane-polar ized l ight . Unal teredrock contains garnet and diopside; a l tered zone contains diopside,textural ly unchanged, and hornblende * epidote symplekt i le a l terga rnet .

M etarodingite

In th in sect ion a typ ical rock conta ins roughlyequal amounts of co lor less, pale p ink, or pale yel lowgarnet and color less d iopsid ic c l inopyroxene, andsubordinate quant i t ies of epidote and hornblende.Hornblende occurs instead of garnet in a few moremagnesian samples, and in these epidote is a lso moreabundant . Sphene is a fa i r ly consistent accessory,commonly cored by rut i le , a l though rut i le may occurwi thout sphene. l lmeni te is present in some samples,green spinef only rarely (e.g. l6O-78). Except close toand wi th in the marginal react ion zones, ear l ier andlater assemblages cannot be c lear ly d is t inguished.Secondary hornblende, chlor i te , i lmeni te, vesuvian-i te , and calc i te (Dal Vesco, 1953) do not occur per-vasively . The absence of kyani te and the presence ofd iopside rather than omphaci te d is t inguish theserocks from grospydites.

Cl inopyroxene and epidote occur as 0.1-mm grainsin a feebly schis tose mosaic (F ig. 5a) . Garnet tends tobe coarser-gra ined, i r regular in out l ine, and in somecases poik i lob last ic ; i t does not form spher ica l por-phyroblasts as in the ec logi tes. Thin veins paral le l toor obl ique to the fo l ia t ion are composed of garnet orIess commonly c l inopyroxene, hornblende, or epi -dote, a l l somewhat coarser than in the neighbor ingrock (F ig. 5b) . Hornblende general ly occurs in ter-s t i t ia l ly or poik i lob last ica l ly in the garnet-bear ingmetarodingi tes.

The sharp boundary against zone 1 of the react ioncontact (F ig. a) is marked by the replacement ofgarnet ( inc luding veins) by hornblende-epidote sym-plekt i te (F ig. 6) , or less commonly d iopside-epidotesymplekt i te . The composi t ional layer ing and thecl inopyroxene fabr ic of the metarodingi te cont inue

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right through this zone. With an increase in grain-sizeoutwards, the symplektite pseudomorphs after garnetgradually disappear in favor of a coarse-grained xe-noblastic granular hornblende-epidote aggregate(zone 2). The epidotes are noticeably zoned towardsmore ferric rims. Megacrysts of hornblende, epidote,sphene, and ilmenite occur patchily in the outer partsof zone 2. Passage into zone 3 (chlorite zone) isaccompanied by a change towards an idioblastic acti-nol i t ic amphibole and the d isappearance of epidote.The chlorite zone proper, closest to the ultramafic,contains tremolite. The transition into ultramaficschist is marked by the incoming of o l iv ine and or-thopyroxene.

At least zone I and probably also zone 2 wasoriginally metarodingi t ic (i.e. garnetiferous, etc.).Bulk analyses (W. Richter , unpubl ished) show thatthe transition from metarodingite to reaction zone Iwas largely isochemical; it is the front of the hydra-tion reaction: garnet + HrO - epidote * hornblende.The transition from zone I to zone 2 may have in-volved some mass transfer, but is in part an ex-pression of the reaction: clinopyroxene * H2O -epidote * hornblende. Mass transfer of the type com-monf y found at ultramafic contacts (e.g. Brady, 1977)was obviously responsible for zone 3.

Bulk chemistry

Analyses of rocks spanning the entire range fromeclogite to Ca-rich metarodingite (excluding marginalreaction contacts) are assembled in Table I in orderof increasing CaO content. The eclogite end-members

Fig. 8. Ti -Zr-Y diagram af ter Pearce and Cann (1973). Datafrom Table I (A) f ie ld of low-K thole i i tes; (B) ocean-f loor basal ts,low-K thole i i tes. and calc-alkal i basal ts; (C) calc-alkal i basal ts: (D)

ocean is land and cont inental basal ts.

(e.g. , 163-4- l l , 163-4-9, 163-4-8) are basal t ic in thei rchemistry, except for their extremely low KrO. Tran-s i t ion towards metarodingi te has involved pr inc ipal lyan increase in CaO coupled with a decrease in NarO(Fig. 7) , to the point where the bulk chemist ry can nolonger be matched by any magmatic rock (nor bygrospydite; Sobolev et al., 1968). Alumina, CrrO3,TiOr, MgO, total iron, MnO, PrOu, and the traceelements Nb, Zr , Y, and Sr do not show any obvioussystemat ic changes in th is t ransi t ion. Si l ica d imin-ishes s l ight ly . A lumina and MgO vary ant ipa-thet ica l ly , but the reason for th is is not known. Thechemical transition from metabasaltic eclogite tometarodingite is reflected in the norm (Table I ) by anincrease in Di f An from 55 to 95 percent and adecrease in Ab * Ne from 2l to 0.2 percent. Thedegree of sil ica undersaturation in the norm has beensomewhat exaggerated by counting all Fe as FeO.

Despite the rocks having undergone rodingitiza-tion and eclogite facies and amphibolite facies meta-morphism, corre lat ion in the sui te among the t raceelements Ti, P, Zr, and Y, no doubt reflective oforiginal igneous fractionation, has remained excellent(Figs. 8 and 9). Relative abundances of these ele-ments are the same in eclogites and metarodingites.Their proportions are consistent with the trace ele-ment chemistry of ocean-floor basalts, as depictdd byPearce and Cann (1973). Y/Nb andP/Zr ratios sug-gest tholeiit ic affinit ies (Pearce and Cann, 1973; Win-chester and Floyd, 1976). Strontium shows no corre-lation with CaO and apparently did not increasedur ing rodingi t izat ion.

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22 EVANS ET AL,: ECLOG]TE-METARODINGITE SUITE

composi t ions, namely, whether they are ec logi t ic (ba-sal t ic) , rodingi t ic , or composi t ional ly in termediate.

Garnel

The analyses of cores and rims of garnet in 24specimens of mafic rock from eight different ultrama-fic lenses in the Gagnone region (Fig. 2), plotted inFigure 10, have tota ls wi th in the range 99.0 to 101.0percent and cation contents relative to 24 oxygensw i th in t he l im i t s : s i , 5 .93 -6 .07 ;A l * c r * Fe3+ ,3 .95 -4. l5 ,Ca * Mg * Mn * Fe2+, 5.92-6.02. Ferr ic i ronwas calculated so as to make Ca * Mg * Mn * Fd+: 6/16 of to ta l cat ions, as requi red by the garnetformula. The values given for FqO' (and correspond-ing est imates of percent andradi te) in the analyses ofgarnet from five typical Gagnone rocks (Table 2)must be regarded as h ighly uncer ta in.

The garnets show an uninterrupted range in com-position from 84 percent pyralspite, l6 percentugrandite to at least 58 percent ugrandite, mainly butnot exclusively as a function of percent CaO in therock (F ig. l l ) . Thus, the ec logi tes conta in pyra lspi te,whereas the metarodins i tes and intermediate rocks

100 r50 200 25OZr, ppm

Fig. 9. Plot ofTi against Zr ,af ter Pearce and Cann (1973). Darafrom Table l . (A) Fie ld of low-K thole i i tes: (B) ocean-f loor basal ts,low-K thole i i tes, and calc-alkal i basal ts; (C) calc-alkal i basal ts; (D)ocean-f loor basal ts. F ie ld D should probably be extended to in-c lude more di f ferent iated, h igher Ti and Zr rocks.

Mineral chemistry

Microprobe analyses have been made of garnet,cl inopyroxene, hornblende, epidote, and sphene. Thechemistry of the first three strongly reflect bulk-rock

a _ 3 1o_ 33o_r60+_t63._245e_248o_249a _ 2 6 1

ALM + SP GR+ANDFig. 10. Core and r im composi t ions of garnet f rom Gagnone eclogi tes, metarodingi tes, and t ransi t ional rocks. Symbols refer to

di f ferent u l t ramaf ic host bodies (Fig. 2) .

q+- o

6 t

} Aa

EVANS ET AL.' ECLOGITE-METARODINGITE SUITE

Table 2. Microprobe analyses of garnet

z 5

Ec log l te153K

Ec log i te248-6

Transitional249-J

Metarodlnglte Metarodlngite163-P 163-M

s 102

TLO2

A1203

CrrO..

F 2o3TEU

Ih0

Mgo

Ca0

xo t e

.04

2 2 , 5 9

.00

.o2

19 . 86

. 3 9

a . ) z

9 . 2 7

3 9 . 5 3

, 1 0

2 2 . 5 6

. 0 0

, ) u

, , t o

5 . 4 0

38 . 07

. 1 0

2 2 . 0 4

.00

.00

Z I . 3 L

4 . 0 4

L 5 . 5 4

39 . 10

.04

2 2 . 5 6

. 0 4

1 7 . 7 3

o 1

6 . 8 9

L Z . ) d

39 ,26

. 1 0

2L,75

.001 1

L 4 . 2 8

, 9 6

6 . 4 4

I q R 1

5 t . 2 5

. 0 0

2 L . 9 0

. 0 0

. 2 3

L8 ,7 4

1 . 3 0

6 , 3 3

1 l o ?

J ' . O I

. 1 1

2 2 . 2 L

. 0 2

. 2 2

L 4 . Z J

6 . 9 9

15 . 90

4 0 . 0 6

. r t2 2 , 5 9

. 1 0

, 0 4

L 2 . 8 9

1 0

8 , 3 6

15 . 60

J 6 . O v

t . L 2

18 . 20

. 0 0

4 . L Z

4 , 4 3

1 . 0 0

5 . L t

28.66

99 .97 100 .46 99 .24 99 .72 9 9 . 3 3 9 9 . 6 6 99 .57 99 .94 99 .39

Si

Cr

T1

! et+

Fe- '

Mn

MC

Ca

5 . 9 6 1

4 .04 r

.000

.005

.002

,050

L . 9 2 7

r . 507

5 . 9 6 5

4 . 0 1 3

,000

,011-

.01,6

2 . 7 I 0

.064

, r o s

1 . 0 3 5

s . 9 5 0

4 , 061

.000

.oL2

.000

z . I 6 >

. 045

.94Lt 1 1 r ,

5 . 0 0 3

3 .962

.002

.013

,025

1 . 8 0 0

,029

L . 5 7 6

2 . 5 7 8

q oon

3 .982

,0L2

.0L2

.005

L .6L2

.024

1 . 8 5 3

2 . 500

5 . 980

3 . 3 3 6

.000

,131

. 4 7 9

. i30

, 7 3 0

4 , t 4 0

Cations relatlve to

5 . 9 6 6 5 . 9 9 2

4 .026 3 .913

.005 . 000

.005 . 011

,006 . 084

2,263 L822

.118 , I24

r . 5 6 7 L . 4 6 5

2 .057 2 .585

24 oxygeae

6 . 018

J . Y O I

. 000

.000

. 0 2 7

2 , 4 0 5

.L69

L . q q I

L .96L

FeZ03 and FeO based on total- lron and ideal garnet fornula. C, core composltlon. R, rln compoeltlon.

conta in ugrandi te or garnet of in termediate composi-t ion. The most calc ic metarodingi tes (e.g. l63M) con-ta in grossular (F ig. l0) wi th less than 20 percent py *A lm .

The garnets are a lmost ternary in composi t ion;percent andradite averages 0.9 and exceeds 3.7 onlyin the most grossular- r ich garnets ( )75 percentgross), where calculated amounts reach l2 percent;percent spessartine averages 1.3 and does not exceed3.8. The re lat ive proport ions of pyrope and a lman-dine components vary considerably, a l though thespread is noticeably smaller within groups of samplesfrom the same ultramafic lens (Fig. l0).

Zoning in the garnets tends to be concentrated inthe marginal parts and takes the form of an increasein FelMg rat io towards the r im; Ca zoning is notpronounced and consists , in most cases, of a smal ldecrease towards the r im.

Crystall ization temperatures evidently were abovethe garnet solvus (e.9., Ganguly and Kenn edy, 1974).No rocks have been found with two microprobe-

detectable garnet populations related to a grossular-pyralspi te solvus, which is apparent ly asymmetr ic to-wards pyralspi te (Newton et al . ,1977: Cressey, 1978).However, evidence of exsolut ion in one of the meta-

70

ts\+

o(rs

50

40

a

l o r , , , , , | , , I- 1 0 t 2 t 4 t 6 t 8 2 0 ? ? 2 4 2 6 2 8

% Coo RockFig, I l . Correlat ion between garnet composi t ion and whole rock

CaO percent.

I

. l

; !' l t

: !

a .I

a A EVANS ET AL.: ECLOGITE-METARODINCITE SUITE

Co

Fig. 12. Na us. Ca per formula uni t in analyzed Gagnone pyrox-

enes Percentages refer to content of "or thopyroxene" end-mem-

ber.

rodingite garnets (sample 33-5-l), with 24 percentpyrope, 24 percent almandine, and 5 I percent grossu-

lar * andradite, was observed in transmission elec-

t ron micrographs (Ghose et a l . ,1976). Garnets in ter-mediate in composition between grossular andpyralspi te are, of course, not uncommon in kyani te

eclogi te and grospydi te xenol i ths in k imber l i te (e.9. ,

Sobolev et a l . , 19681 Chinner and Cornel l , 1974).

Condi t ions no more extreme than those of the h igh-greenschist facies or low amphibolite facies are, in

fact, capable of permitting compositions midway be-

tween grossular and almandine to crystall ize (Acker-

mand et a l . .1972' . Her i tsch. 1973). A solvus is more

likely to be encountered, however, as the ratio py-

rope/a lmandine increases (Ganguly and Kennedy,1974).

Clinopyroxene

A variation in jadeite component of pyroxene from

more than 40 percent to almost zero parallels the

decrease in bulk NarO of the rocks in the passage

from eclogite to metarodingite. Cats component(CaAlzSiO.) is uniformly low, with the exception of

the few highly calcic metarodingites, which contain

diopside zoned marginally to fassaite with more than

20 percent Cats. These relationships are i l lustrated in

Figure l2 in terms of Ca and Na atoms per formulaunit of 6 oxygens and in Figure 13 in terms of theproportions of Di * Hed, Jd, and Cats. These dia-grams give the results of full microprobe analyses of

67 clinopyroxenes in 26 samples of eclogite/meta-

rodingite from nine different ultramafic lenses. Total

cation contents per formula unit ranged from 3.988

to 4.020 (average 4.006), and Si contents, excluding

the fassai t ic pyroxenes (F ig. l3) , ranged f rom l '931

to 1.993 (average 1.965). I l lust rat ive analyses (Table

3) are of clinopyroxene grains in contact with the

garnets previously selected for Table 2.The calculat ion of molecular components in the

pyroxenes is based on the assumpt ion that a l l i ron is

ferrous. Al though th is is unl ike ly to be ent i re ly t rue,

the most reasonable procedure for estimating the

proportions of ferrous and ferric iron in microprobe

analyses of pyroxenes, one in which total cations are

equated to 4 and total oxygens to 6 (Rybutn el al.,

1976), is so cr i t ica l ly dependent on the accuracy of

the Si determination that the result is nearly worth-

less for the calculation of FezO' content in low-iron

pyroxenes. In our case, this procedure yielded acmite

contents ranging up to 6 percent (average 2), and also

several negative contents of Fe3+, particularly for the

omphacites. Thus, we have preferred to regard all Fe

as ferrous, compute first Jd from the number of Na

atoms, then TiCats (CaTiAlrOu) from Ti, Cats from

the remain ing Al , Di * Hed f rom the remain ing Ca,

and En * Fs f rom the remain ing Mg + Fe * Mn'

This ignores the Si determinat ion. The (Mg,Fe)rSi rO.

content of the clinopyroxenes so calculated (Fig' | 2)

averages 3.8 percent and exceeds 5 percent in only

one sample (160-4-8) . Our calculated Cats com-

ponents are s imi lar to , but of course not ident ica l to ,

Cats calculated from four-coordinated Al (i.e. 2.0 -

Si ) , e .g. , Lover ing and Whi te (1969) '

CATS JD

Fig. 13. Molecular proport ions of d iopside, jadei te, and Cats

components in analyzed Cagnone pyroxenes.

0.70.0L

04

EVANS ET AL.: ECLOGITE-METARODINGITE SIJITE

Table 3. Microprobe analyses of cl inopyroxene

E c l o g i t e163K

Ec log i te Trans i t iona l248-6 249-J

Metarod ing i te1 6 3 - P

Metarod ing i te163-M

sio2

Tio2

At^0^

Cr 2Q3

Fe0

MnO

Mgo

Ca0

Naro

K2o

5 5 . 4 8

. 1 6

11 . 41

. 0 6

3 . 4 5

. 0 1

9 . 7 4

1 4 . 8 9

5 . 7 0

0 . 00

5 3 . 2 7

. L l

7 . 0 6

.0 t -

3 . 7 L

. 0 3

L 2 . 6 6

r 9 . 9 3

2 . 7 8

. 0 0

5 4 . 2 4

. 1 0

3 . 1 0

. 0 2

4 . 5 8

1 4 . 1 1

2 2 . 7 2

I . 1 7

. 0 0

53 .82

l ?

r . 9 5

. 0 0

2 . L 8

. 0 0

L 6 . 9 4

2 4 . 8 4

. 3 4

. 0 0

5 1 . 3 1

4 . 3 6

n . d ,

2 . 8 8

. 0 4

1 5 . 4 8

2 5 . 2 9

. 0 4

4 5 . 9 0

2 . I 7

10 . 01

n . d .

4 . 6 0

. 1 1

1 2 . 5 4

2 4 . 4 9

. 0 5

n . d .

1 0 0 . 9 0 9 9 . 6 2 100 . 17 1 0 0 . 2 0 99 ,84 9 9 . 8 8

S i

^ , r v. . V I

AT

1 l

F e

Mn

Mg

C a

Na

xY

L . 9 5 4

. 0 4 6

.428

. 0 0 2

, 0 0 4

. t 02

, 0 0 0

. 5 1 0

. 5 6 2

. 3 9 0

a \ ?

1 . 0 4 6

1 . 8 8 1

. 1 1 9

. 0 7 0

. o r2

. 0 8 8

. 0 0 1

.846

. 9 9 4

. 0 0 3

. 9 9 1

1 . 0 1 7

1 . 7 0 3

t o 7

. 14I

. 0 6 1

.143

.003

. 6 9 4

. 9 7 3

.004

. 9 7 7

L . 0 4 2

Cat ions re lat ive to 6

1 . 9 3 3 I . 9 7 7

.067 . 023

. 2 3 5 . 1 1 0

. 0 0 0 . 0 0 1

. 0 0 5 . 0 0 3

. I r 2 . 1 4 0

. 0 0 1 . 0 0 4

. 6 8 5 . 7 6 7

. 7 7 5 . 8 8 8

. 196 . 083

. 9 7 L . 9 7 r

r - . 038 1 . 025

oxygens

1 . 9 5 4

. 0 4 6

.037

. 0 0 0

. 0 0 4

. 0 6 6

. 0 0 0

.916

. 9 6 6

. 0 2 4

. 9 9 0

1 . 0 2 3

Minor e lements exhib i t no obvious dependency onthe re lat ive proport ions of jadei te and d iopside com-ponent. TiO, averages 0.13 weight percent, Cr2Og0.02 percent, MnO 0.03 percent, whereas KrO wasnot detected (<0.01 percent). The rare fassaitic clino-pyroxenes have, l ike their associated garnet, consid-erably h igher T iO, (Table 3) .

The metamorphic histories of the eclogites andmetarodingites are believed to have been the same,namely an eclogite facies metamorphism followed byan amphibolite facies metamorphism, the effects ofthe la t ter being more not iceable at the margins of themafic bodies. The occurrence at Gagnone of highAlv l omphaci t ic pyroxene and h igh Al Iv tscherma-kitic pyroxene (fassaite) is a consequence ofthe rangein bulk composi t ions of the maf ic rocks. Their con-ventional correlation with high and low p/Tenviron-ments respect ive ly (Whi te, 1964) is of course onlyvalid in appropriate multiphase assemblages where

heterogeneous equi l ibr ia contro l l ing the AI IV/AIv lratio of pyroxene can operate. The fact that bothjadeitic and tschermakitic pyroxenes can occur inhigh-pressure metamorphic facies is not in confl ictwith experimental data on their intrinsic stabil it ies(Hays, 1966; Kushiro, 1969).

Amphibole

Amphiboles from Gagnone eclogites and amphi-bolit ized eclogites are complex hornblendes in whichthe edenite component becomes more significant asthe associated clinopyroxene becomes more jadeitic(F ig. la) . Amphiboles f rom the metarodingi tes ( (10percent Jd in Cpx) show a trend from tremolit ichornblende to pargasi t ic hornblende (Fig. l4) . In theeclogites, the amphibole mostly appears to be in tex-tural equil ibrium with pyroxene and garnet; on theother hand, most amphiboles do not form a part ofthe primary metarodingite texture.

EVANS ET AL. ECLOGITE-METARODINGITE SUITE

0.8

\+b 0.6=

0%t0,5 t .0 1 .5 2 .O

AL IV

F ig . 14 . Na * K us . A l r v i n Cagnone amph ibo les . Con lou rs

represent percenlage of jadei te component in coexist ing pyroxene

T, t remol i te; Ts, tschermaki te; P. pargasi te; E, edeni te.

The amphibole chemist ry was determined by mi-croprobe analys is of 25 samples f rom 8 u l t ramaf iclenses. Those selected for Table 4 are of hornblendesin contact or near contact wi th the garnets and pyrox-enes in Tables 2 and3. Recast on an anhydrous basisof 23 oxygens, the range in cat ion contents was: Ca,1.32 to 1.9 (ec logi tes) , 1 .9 to 2.03 (metarodingi tes) ;Na * K ,0 .34 to l . l ( ec log i t es ) ,0 . l l t o 0 .69 (me ta -rod ing i t es ) ; A l l v ,0 .37 to 2 .00 ;and A l v l , 0 .28 to 1 .28 .I ron was taken as ent i re ly ferrous in these calcu-lat ions. Mgl(Mg t Fe) , wi th one except ion, fe l l be-tween 0.83 and 0.63. The average content of minorelements is d is t inct ly h igher than in the c l inopyrox-enes, uiz. TiO, 0.45 percent, CrzO' 0.04 percent, MnO0. 13 percent , and KrO 0.26 percent .

Epidote

The eclogi tes conta in two var iet ies of epidote: ( l ) acoarse-gra ined id ioblast ic e longate c l inozois i te wi th 4to 5 percent CarFerSirOr, (OH) endmember and (2) axenoblast ic epidote wi th l3 to l7 percent of th iscomponent. The former variety often defines a schis-tos i ty and appears to be in textura l equi l ibr ium wi thpyroxene, garnet , and amphibole. The lat ter appearsto postdate the garnet and pyroxene. Some samplesof ec logi te conta in both var iet ies of epidote.

Epidote in metarodingi tes seems most ly of second-ary or ig in. l ts composi t ion ranges f rom l3 to 24percent CarFerSi .O,r (OH). These var iat ions in epi -

dote composi t ion presumably ref lect in par t theFcOr/FeO rat io of the rock. There is no corre lat ionbetween epidote composi t ion and the pr inc ipal var ia-

t ions in pyroxene, garnet , and amphibole noted ear-l i e r .

S phene

Analyses showed subst i tu t ions of Fe3+ * Al ior T if rom 4.5 to l0 percent . The nature of the coupledsubst i tu t ion was not determined.

lron-magnesium partit ioning

Because the Gagnone eclogi tes and metarodingi tesare essent ia l ly two-phase (garnet * c l inopyroxene)rocks, and the paragenet ic re lat ions of hornblendeand epidote minerals are not ent i re ly c lear , we ef fec-t ive ly have h igh-var iance assemblages and con-sequent ly l i t t le opportuni ty to der ive P,T in format ionfrom considerat ions of heterogeneous equi l ibr ia . Onthe other hand, as is wel l known, the f ract ionat ipn ofFe and Mg among s i l icate minerals at metamorphictemperatures, par t icu lar ly when garnet is involved, isre lat ive ly sensi t ive 10 temperature.

Garnet-pyroxene

The r im composi t ions of coexis t ing garnet andcl inopyroxene f rom Gagnone eclogi tes and metarod-

Tab le 4 M i c rop robe ana l yses o f amph ibo le

E c l o g i t e

2 4 8 - 6

Trans iE io t ra l Rod ing i te

249-J 163-P

rU.€\o

\v

o 4

E c l o g i t e1 6 3 K

s i02

Tio 2orzo3

Cr "o .

Fe0

MnO

Ca0

Na20

Kzo

4 6 . 2 7

. 0 5

8 . 1 s

. 0 4

1 4 . 1 6

8 . 9 1

3 . 5 0

. 3 0

4 7 . 8 9

-36

L7.22

. 0 7

8 5 4

. 0 9

1 5 . 3 3

1 1 1 9

2 . 0 8

, 2 1

4 2 6 4

7 4 . 4 L

. 0 0

1 0 . 1 9

. 1 3

1 4 . 1 9

1 2 . 1 9

2 . 0 9

. 1 0

4 8 . 2 6 5 4 . 1 3

. 3 3 1 3

9 . 6 4 3 . 9 0

. 0 3 . 0 3

9 . r 2 7 . 1 8

. r 4 . 7 2

1 5 . 0 3 1 8 . 5 6

1 2 . 5 1 1 2 . 7 2

1 . 1 0 . 4 0

. 1 9 . 0 3

94.23 9 6 . 9 8 9 6 . 3 5 9 7 . 2 0 9 7 . O 7

Cat lons re la t i ve to 23 oxygens (anhydrous)

si.. , r v

C r

T i

F e

Mn

Mg

Na

K

xY

6 . 5 2 7

1 . 4 7 9

1 . 1 4 0

. 005

. 0 5 2

. 9 6 0

. 004

3 . 1 0 0

1 . 3 4 6

. 9 5 9

0 5 5

2 . 3 6 0

6 . 8 6 5 6 . 9 4 9 7 . 6 2 9

1 . 1 3 5 1 . 0 1 1 . 3 7 r

. 7 6 r . 6 3 4 . 2 7 7

. 008 .003 . 003

. 039 .036 . 014

1 . 0 2 3 1 . 1 0 5 . 8 4 7

. 0 r1 .016 . 015

3 . 2 7 5 3 . 2 4 4 3 . 8 9 8

r . 7 L 9 1 . 9 4 1 r . 9 2 0

571 .309 .7L2

.017 .034 .005

2 . 3 3 3 2 . 2 8 4 2 . 0 3 7

5 7 r 7 5 . 0 3 8 5 . 0 5 4

6 . 2 4 2

t . 7 5 8

- 7 2 7

. 000

. 0 5 8

t - 2 4 7

-016

3 . 0 9 7

2 . 0 0 6

. 5 9 2

, 0 1 8

2 . 6 7 6

EVANS ET AL.: ECLOGITE-METARODINGITE SUITE

ingites are plotted logarithmicalty in Figure 15. Thevalues of Kp (FelM g Gt)/(Fe/Mg Cpx) range from5.5 to 15. Inc lus ion of analyses of garnet and pyrox-ene cores would extend the spread to Kp values ashigh as 30. This could mean that core pairs recordlower temperatures or higher pressures of equil ibra-t ion than r im pai rs (Banno, 1970) or , more l ike ly inour opin ion, that they do not record a re l iab le ex-change equi l ibr ium. In general , the c l inopyroxenesare more homogeneous than the garnets, and it ispossib le that , unl ike the garnet in ter iors, the pyrox-ene interiors participated in the exchange equil ibriumthat the r im pai rs record. The enr ichment of Mg inthe garnet r ims would be consistent wi th th is hypoth-E S I S .

Aside from real variations in physical conditions,the spread in values of Kp (Fig. l5) may be at t r ibutedto a combination of the following: imperfect ex-change equil ibration between the points analyzed,analytical error, variabil ity in pss+ /ps2+ ratio partic_ularly in pyroxene, and the influence of other com-ponents in the two phases, notably grossular andjadeite. The correlations between Kp for rim pyrox-ene-garnet pairs and these additional componentsare i l lust rated in F igures l6 and 17. The posi t ivecorrelation between Kp and percent grossular (clMysen and Heier, 1972) parallels the effect alreadywell known for natural garnet-biotite and garner-hornblende pai rs (e.g. Ganguly and Kennedy, 1974,and others), and ascribed to non-ideality in groqsu-

o . ^Y I U

*i

5,0

4 0

3 0

I z ozd

- l )

6V

{.s ' .O/ '/ .$s

a /

/ ' /,/ /.

,/ ,/,/ ,/

, / c , f' . / '?

o . ' , /$o

. , / o o , /

/ / a " /

. ' r ' . ' ' " o // , /'

,/: ,/' 7

o ro 20 "r".3rorJo

50 60 70

Fig. 16. Correlat ion between Kp for Fe, /Mg part i t ioning betweencl inopyroxene and garnet r ims and percent grossular component ingarnet . Circ les, eclogi tes; dots, metarodingi tes. Arbi t rary dashedl ines indicate the t rend of change in Kp at I : 1000K due tovar iat ion in percent gross alone, according to Ganguly (197g).

lar-pyralspite solid solutions (e.g. Hensen et al.,1975). The t rend l ines on Figure l6 express, for I :1000K, the dependence of Ko on percent grossularalone, according to a relation for K1:J(T,P,X) devel-oped by Ganguly (1978), based on tabulated ther-modynamic data and mix ing models for natura l gar-ngts. The negative correlation between r(p andpercent jadei te (F ig. l7) is probably not so much dueto non- ideal i ty of d iopside- jadei te solut ion as to theinverse correlation of grossular and jadeite percent(due to the inverse CaO us. NarO relation in the bulkrock, Fig. 7) combinef, with the correlation betweenKp and percent grossular (F ig. l6) . This explanat ionis supported by sample 160-4-7, which, despite thelow jadeite content of its pyroxene, contains grossu-lar-poor garnet and hence has a Kp near 6 (F ig. l7) .Clearly, then, attempts to derive estimates of physical

20

3 % o

^ &

3o'o ro *110r,r,

30 40

Fig. l7 Correlation between Ko for FelMg partitioning betweencl inopyroxene and garnet r ims and percent jadei te component inpyroxene. Triangles, specimen 160-4-7 which lras garnet with onlyl6-17 percent ugrandi te.

r 0

0 50 0 5 0 r 0 t 5 0 2 0 3 0 4 0 5

Fe / Mg Cpr

Fig. 15. Part i t ioning of tota l Fe and Mg between c l inopyroxeneand garnet rims. Circles, eclogites; dots, metarodingites.

28 EVANS ET AL.: ECLOGITE-METARODINGITE SUITE

0,05 0.07 0.r 0.2 0.3 0.5 0.7 l0( Fe / Mg ) clinopyroxene

Fig. 18. Part i t ioning of Mg and tota l Fe between coexlst lng

cl inopyroxene and amphibole. Circ les, eclogi tes; dots, metarodin-

gi tes

conditions from the Kp(Mg-Fe) of pyroxene-garnetpairs cannot afford to ignore certain phase composi-

tional effects.I ron i t t F igures 15, 16, and 17 is to ta l Fe. Except

when percent grossular exceeds 60, the relative

amount of Fe3+ in garnet is small, whereas in pyrox-

ene it is possible that Fe3+ constitutes a sizeable part

of total iron. Thus, it is probable that the ratio (Fe"+ /Mg Gt)/(Fe'+ /Mg Cpx) is somewhat larger than theplotted Kp values. This uncertainty can have a signifi-

cant effect on estimated temperatures.For Gagnone eclogites, Ko values (total Fe) range

from 5 to I 0 (Figs. l5 and I 6) and thereby include the

values (-6) found for eclogites from Alpe Arami(Ernst , 1977). Wi th in the garnet composi t ional range

of l5 to 25 percent grossular , typ ical of the product

garnets in the exper iments on thole i i t ic bulk composi-

tions by Riheim and Green (1974), Gagnone eclo-gites have Ko values of 6. According to Riheim and

Green, this partit ioning would correspond to 690'C

at l0 kbar (an approximate lower P l imit for eclogite,

Green and Ringwood, 1967) and 793"C at 25 kbar

(an approximate upper P l imit for hornblende eclo-g i te at 800'C, Lambert and Wyl l ie , 1972). The lat ter

figure would be in excellent agreement with estimates

of P and Z made for the garnet peridotite (outcrop

160) at Gagnone (Evans and Trommsdorff, 1978).

However, these temperatures might be somewhat

high because of the ,"t+ /F€+ uncertainty. On the

other hand, the dP/dT slope of the experimental

calibration includes the effect of increasing grossular

content with increasing pressure (at the expense ofpyroxene) and may be too flat (Wood, 1976); hence

the estimates of Z at lower assumed pressure may be

too low.For the Gagnone metarodingites, it is appropriate

to use the relation: I 'K(ln KD + 2.939) : 4801 +

I1.07P(kbar) + 1586. f€" t + 1308X,9* for low-Na py-

roxene, developed by Ganguly (manuscript). For an

average metarodingite (XEl : 0.4, Ko : 10, see Fig.

l6) , Ganguly 's expression g ives 785'C for P : l0

kbar . or 806"C for P : 20 kbar , or 817'C for 25

kbar.What is important is that, despite their different Ko

values, the avai lable thermobarometr ic data indicate

s imi lar equi l ibrat ion condi t ions for the ec logi tes and

the metarodingites (and, of course, the transitionalrocks). This is consistent with the geological evidencefor an eclogite facies event affecting all the mafic

inc lus ions in the Gagnone region.Having no accurate fix on pressure, we can only

infer a temperature minimum (690'C) for this meta-

morphism. Nevertheless, th is ind icates negl ig ib le

over lap wi th condi t ions est imated for the la ter Cen-

t ra l A lp ine regional metamorphism in the Gagnone

r e g i o n ( 6 0 0 + 1 0 0 " C , < l 0 k b a r ; E v a n s a n d

Trommsdorff, 1974). The Central Alpine meta-

morphism evidently caused litt le or no reequil ibra-

tion of Fe and Mg between pyroxene and garnet.

Temperature and pressure estimates for Gagnone ec-

logites, metarodingites, and garnet peridotite con-

verge on the rounded-of f va lues of T : 800"C and P= 25 kbar.

Pyroxene-amphibole

Compositional factors can also be recognized in

the partit ioning of Fe and Mg between pyroxene and

coexist ing amphibole (F ig. l8) . A l though the Ke cor-

relates with jadeite content, this in turn correlates

with several compositional variables in the amphi-

bole, as we have seen earlier (Fig. l ). It is therefore

not so easy from the present data to identify the

principal compositional variable influencing the Kp

for pyroxene-amphibole pairs. Clearly, for temper-

ature comparisons among different areas, such data

should be used with caution. Furthermore, the nature

of the event being measured is not entirely clear' For

Gagnone eclogites the Kp values fall in the range 1.2

to 3.0, and for Gagnone metarodingites the range is

2.3 to 10. Again, the eclogite pairs cover the range

found for pyroxene-amphibole pairs in Alpe Arami

eclogites (Ernst, 1977). Scatter due to the unknown

?

,st a

a (.\/ . t -

r .0

0.7

u.c

n ?

assbss

0.1 ' -0.03

EVANS ET AL: ECLOGITE-METARODINGITE SUITE 29

proportions of Fe8+ in both pyroxene and amphibolehas, of course, not been considered.

Origin of metarodingites

On the basis of our analytical data, the eclogitesmust represent metabasaltic extrusives or intrusives.On the other hand, i t is appropr iate at th is point tolist the evidence and lines of reasoning for believingthat the garnet-diopside (i.e. calc-sil icate) boudinsare metarodingites, the metamorphosed products ofrodingitized dikes. Alternate possibil i t ies are thatthey are metamorphosed calcareous sediment ormetamorphosed garnet or plagioclase pyroxenitedikes or layers in the ultramafic rocks.

(l) The metarodingites are everywhere in contactwith, in fact usually totally enclosed by, the metamor-phosed ultramafic rock. Mafic layers and boudins inthe surrounding semi-pelit ic gneisses are amphibo-lites, amphibolit ized eclogites, or eclogites, but no-where metarodingites. These observations make asedimentary proto l i th unl ike ly .

(2) Albeit rarely, metarodingite appears to cross-cut, at low angles due to tectonic flattening, the com-positional layering in the ultramafic rock. An intru-sive origin is therefore indicated.

(3) Their basic mineralogy, s t ructures, and phys-ical appearance in the field resemble the rodingitedikes enclosed in serpentinite in lower-grade sectionsof the upper Pennine nappes (e.g., peters, 1963; DalPiaz, 1967: Dietrich, 1969; Keusen, 1972). Essentiallycomparable are details such as the size of the rockbodies, the boudinage structure, absence of pro-nounced composi t ional layer ing, presence of th incross-cutting garnet and pyroxene veins, and thepresence of a marginal chlorit ic contact reactionzone.

(4) The bulk chemistry of the metarodingitesshows complete gradation into the basaltic chemistryof the eclogites. Such a gradation would be unlikely ifthe protolith were sedimentary. It would, on theother hand, be the predictable result of variable de-grees of metasomatic rodingitization of basalticdikes. Enrichment in CaO and eventual nearly com-plete depletion in alkalis are the principal changesrecognized in studies of rodingitized dikes enclosed inserpent in i te (Coleman, 1967, p.49; Keusen, 1972,Leach and Rodgers, 1978).

(5) The abundances of minor (e.g., TiO2:1.24 to2.46 percent) and trace elements (Table l) through-out the gradational sequence eclogite-metarodingiteare consistent with basaltic chemistry. Indeed, certainabundance ratios show remarkable regularit ies (Fig.

8) and suggest correlation with tholeiit ic sea-floorbasalts. The concentrations of Cr. Zr. and Sr distin-guish these rocks from the accompanying ultramaficsand make i t improbable that they represent boudi-naged pyroxenite layers.

(6) The enclosing ultramafic rocks are chlorite-enstatite t cummingtonite-olivine schists, some ofwhich are totally devoid of or possess only very mi-nor amounts of t remol i te , the only phase in the u l t ra-mafics carrying more than 0.6 percent CaO at thismetamorphic grade (Evans and Trommsdorff, 1974:Rice el al., 1974). Thus, some of the ultramafics areCaO-depleted, although not AlrOs-depleted, relativeto a lherzolite protolith (Trommsdorff and Evans,1969; Steuble, 1978). This CaO deplet ion could wel lhave taken place during serpentinizalion accom-panying rodingitization (Page, 1967: Barnes andO'Nei l , 1969; Coleman, 1977).

(7) In areas of lower grades of metamorphism inthe Pennine Alps, a serpentinite origin for some ofthe olivine-rich ultramafic rocks was establishedthrough the preservation of a microfolded antigorite-magnetite fabric inside olivine porphyroblasts (e.g.Trommsdorff and Evans, 1974, Plate l c). In the Ga-gnone area, traces of a possible serpentinite-stagefabric appear to have been totally removed.

(8) Relics of t itanian clinohumite and the presenceof a characteristic breakdown intergrowth of olivine* i lmeni te in u l t ramaf ic body l l60 (F ig. 2) may, byanalogy wi th t i tan ian c l inohumite-bear ing Pennin icserpentinites, be a further indication of a precursorserpentinite stage for the Gagnone ultramafics (Evansand Trommsdorff, 1978).

Geologic history and significance of the eclogite-meta-rodingite suite

The formation of the metarodingites at least pre-dated the growth of the postkinematic hornblende,diopside, epidote, and chlorite-bearing reactionzones. Similarly, eclogite formation predated an epi-sode of amphibolit ization which can be attributed tothe Central Alpine metamorphism. The fact that themineralogy and textures of the metarodingites showsmooth gradations into those of eclogite, and thatthese variations can be ascribed entirely to composi-tional variations, suggests that the entire suite sharedan eclogite facies metamorphism sometime duringtheir geological history. Geothermometry has con-firmed this fact. It is sti l l not known, however, if thishigh-pressure metamorphism was early Alpine orpre-Mesozoic. It probably correlates with the forma-tion of garnet peridotite at Gagnone (Evans and

30 EVANS ET AL.: ECLOGITE-METARODINGITE SUITE

Trommsdort r , 1978), Alpe Arami (Dal Vesco, 1953;Milckel, 1969; Ernst, 1977) and perhaps MonteDur ia (Fumasol i , 1974). The temperature o l the ec-logite facies metamorphism, as consistently recordedby element partit ioning in both eclogite and meta-rodingite at Gagnone, was higher than that of theCentral Alpine phase, and considerably higher thanthat of the possibly correlative early Alpine high-pressure events in other parts of the Alps (Bearth,1974; Ernst , 1973; Compagnoni , 1977).

If the hypothesis of partial metasomatic rodingiti-zation is accepted for the Gagnone suite, the neces-sary accompanying event of serpentinization musthave taken place prior to eclogite facies metamorph-ism. The involvement of oceanic rocks is suggested bytheir trace-element chemistry. The early geologic his-tory of the Gagnone mafic/ultramafic suite thereforeappears to have involved subduction metamorphismof partially serpentinized oceanic l ithosphere. Thisconclusion does not have to apply, of course, to allrocks in the Cima Lunga Series, for example, thecarbonate and pelit ic metasediments. Except for one'u l t ramaf ic body at Cima di Gagnone (no. 160, F ig.2), traces of these early high-pressure events withinthe ultramafics have been wiped out by thoroughrecrystall ization during the Central Alpine meta-morphism.

Although we believe the balance of evidence favorsan early rodingitization event, the possibil i ty of masstransfer of CaO and a lkal is dur ing the var ious meta-morphisms cannot entirely be dismissed. For ex-ample, in some of the ultramafics we find clear text-ural evidence of the replacement of tremolite bymagnesiocummingtonite (Rice el al., 1974), with noconcomitant crystall ization of a Ca-bearing phaseelsewhere in the rock. Diffusion on the scale of cen-timeters involving several components, perhaps ear-l ier but certainly also during the Central Alpine meta-morphism, was clearly responsible for the high-variance contact reaction zones developed betweenthe ultramafic and mafic rocks. The size of some ofthe metarodingite bodies (e.9. ) 2 m across), how-ever, argues in favor of an infi l trational process ofmass transfer rather than a diffusional one (Fletcher

and Hofmann, 1974). Thus, a low-temperature, near-surface environment for the formation of these rocksseems very reasonable.

The recognition of calc-sil icate rock associatedwith metamorphic ultramafic rock as metamor-phosed rodingite supplies a piece of information of

I Note added in prool Two addi t ional bodies show evidence of

having formerly been garnet peridotite.

considerable value in reconstructing the geological

history of any crystall ine terrane. The association is

indicative of passage through a pressure-temperature

range in which serpentinization was possible. The

subsequent metamorphic history may be easy to re-

construct, as in the case of contact metamorphism(e.g., Frost, 1975), or rather subtle, as in the present

case of subduction zone, eclogite facies metamorph-ism, followed by upward migration of fragmentedmaterial during alpine tectonism, and subsequentBarrovian-style regional metamorphism.

AcknowledgmentsThis work was supported by NSF grant EAR 75-14904 and

Swiss Nat ional fond grant 2.615-0.76. We are grateful to V. Diet-

r ich, A. Gautschi , C. Heinr ich, O. Schel lhorn, and U Sondereg-

ger for XRF analyses, to R. Schmid for photomicrographs. and

R G. Coleman, V. Dietr ich, and P. Misch for cr i t ical reviews.

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