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An EMP and TEM-AEM Study of Margarite, Muscovite

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An EMP and TEM-AEM Study of Margarite, Muscovite JOURNAL OF PETROLOGY VOLUME 37 NUMBER 2 PAGES 201-233 1996 ANNE FEENSTRA* MINERALOGISCH-PETROGRAPHISCHES INSTITUT, UNIVERSITY OF BERNE, BALTZERSTRASSE 1, CH-J012 BERNE, SWITZERLAND An EMP and TEM-AEM Study of Margarite, Muscovite and Paragonite in Polymetamorphic Metabauxites of Naxos (Cyclades, Greece) and the Implications of Fine-scale Mica Interlayering and Multiple Mica Generations Coexisting white micas and plagioclase were studied by electron covite-paragonite solvus thermometry. Chemical data for Co— microprobe (EMP), and transmission and analytical electron Na micas from this study and literature data indicate that microscopy (TEM-AEM) in greenschist- to amphibolite- naturally coexisting margarite—paragonite pairs display con- grade metabauxites from Naxos. The TEM—AEM studies siderably less mutual solubility than suggested by experimental indicate that sub-micron scale (0-01-1-0 jim thick) semi- work. The variable and irregular Na partitioning between coherent intergrowths ofmargarite, paragonite and muscovite are margarite and muscovite as observed in many metamorphic common up to lower amphibolite conditions. If unrecognized, rocks could largely be related to opposing effects of pressure on such small-scale mica interlayering can easily lead to incorrect Na solubility in margarite and paragonite and/or non-equili- interpretation of EMP data. Muscovite and paragonite in M2 brium between micas. greenschist-grade Naxos rocks are mainly relics of an earlier high-pressure metamorphism (M.\). Owing to the medium- KEY WORDS: Ca—Na—K mica; margarite; mctabauxite; Naxos; sub- pressure M2 event, margarite occurs in middle greenschist-grade micron-scale mica interlayering metabauxites and gradually is replaced by plagioclase + cor- undum in amphibolite-grade metabauxites. The margarite dis- IV VI 3+ VI plays minor Al3 (Fe ,Al)SL3 O-i and considerable INTRODUCTION (Na,K) SiCa-iALj substitution, resulting in up to 44 mol% Since 1970, many new occurrences of the brittle paragonite and 6 mol % muscovite in solution. The composi- mica margarite, Ca2Al2(Al2Si2)Oio(OH)2, have tional variation of muscovite is mainly described by been reported from localities throughout the world. VI 2+ VI v VI 3+ (Fe ,Mg)Si Al-! AL, and (Fc AL,) exchanges, the Most are in metamorphosed marls and calcic pelites latter becoming dominant at amphibolite grade. Muscovite is but margarite is also found in metamorphosed significantly richer in Fe than margarite or paragonite. Ca—Na— basites, anorthosites, impure marbles and bauxites K partitioning data indicate that margarite commonly has a [see reviews by Frey et al. (1982) and Guidotti significantly higher Na/(Na + K + Ca) value than coexisting (1984)]. Till now relatively little attention has been muscovite or plagioclase. Exceptions are found in several given to the classical occurrence of margarite in greenschist-grade rocks, in which M/ formed muscovite may emery rocks (Smith, 1850, 1851), where the mineral have failed to equilibrate with M^ margarite. The sluggishness coexists with corundum and Fe(—Ti)-oxides. of K-rich micas to recrystallize and adjust compositionally to From the literature, two main occurrences of changing P—T conditions is also reflected in the results ofmus- margarite can be distinguished (see Frey et al., 1982; •Pre»ent address: Iiutitut fUr Petrologie, Geozentram UnivenitSt Wien, AJtharutrajse 14, A-1090 Wien, Austria. © Oxford University Press 1996 JOURNAL OF PETROLOGY VOLUME 37 NUMBER 2 APRIL 1996 Guidotti, 1984). The first type involves margarite as addresses the problems involved with electron a prograde rock-forming mineral in lower green- microprobe analysis of white micas interlayered on a schist to amphibolite facies rocks, either as a matrix submicron scale. The final part of the paper focuses mineral, or at higher grades, sometimes with a por- on the somewhat puzzling phase relations among the phyroblastic habit. Best documented is the regional Ca-, Na- and K-micas and discusses the complica- distribution and petrologic significance of prograde tions of fine-scale mica interlayering and multiple margarite in metamarls and calcic metapelites from mica generations in polymetamorphic rocks. A the Swiss and Austrian Alps (e.g. Ackermand & second paper (A. Feenstra, in preparation) will deal Morteani, 1973; Hock, 1974; Frey & Orville, 1974; with the complete petrologic phase relations in calcic Frey, 1978; Hoinkes, 1978; Frey et al., 1982; Frank, metabauxites, including a model for the progressive 1983; Bucher et al., 1983). Prograde margarite regional metamorphism of such Al-excess rocks. occurrences have also been reported from several other parts of the world, e.g. from France (Sagon, 1967), Norway (Andreasson & Lagerblad, 1980), GEOLOGICAL SETTING AND NE Japan (Okuyama-Kusunose, 1985) and the METAMORPHIC PETROLOGY Rocky Mountains in Canada (Gal & Ghent, 1991). The island of Naxos is part of the Attic-Cycladic The second type, which has received much Metamorphic Complex (ACMC), which stretches attention in the literature, involves margarite from mainland Greece to SW Turkey (Fig. 1). The occurring as a pseudomorphic replacement of Al-rich ACMC is essentially a nappe pile of predominantly minerals. Margaritization of andalusite and kyanite Mesozoic sedimentary and volcanic rocks metamor- porphyroblasts is fairly common (e.g. Guidotti & phosed at various conditions. Petrologic and geo- Cheney, 1976; Guidotti et al., 1979; Cooper, 1980; chronologic studies (e.g. Andriessen et al., 1979; Van Enami, 1980; Baltatzis & Katagas, 1981; Morand, der Maar & Jansen, 1983; Feenstra, 1985; Diirr, 1988), but cases of replacement of sillimanite, cor- 1986; Wybrans & McDougall, 1986, 1988; Dixon et undum, chloritoid, staurolite, plagioclase, (clino)- al., 1987; Schliestedt et al., 1987; Okrusch & Brocker, zoisite and muscovite by margarite have also been 1990) indicate that the ACMC rocks have experi- described (e.g. Gibson, 1979; Teale, 1979: Frey et al., enced two main Alpine metamorphisms; an early 1982; Yardley & Baltatzis, 1985; Grew et al., 1986; Alpine high-pressure phase, M\, which ended at 40- Stahle et al., 1986). Textural evidence commonly 50 Ma and a medium-pressure Barrovian-type phase suggests that such margarite is not a peak meta- at 20-25 Ma (see Fig. 1). On virtually all Cycladic morphic mineral but developed during the waning islands, the Eocene M\ phase is expressed by blue- (retrograde) stages of the metamorphic cycle, and/or schist facies mineralogies (locally with eclogite-facies during polymetamorphism. The margarite probably assemblages), which are in varying degrees over- largely formed by means of local ion-exchange reac- printed by the M2 event. The high-P mineral tions between the Al-rich precursor and Ca-rich assemblages formed during the collision of micro- fluids. The fact that Al-silicates, chloritoid and plates with the Eurasian continent (e.g. Robertson & staurolite have high Al/Si ratios similar to those of Dixon, 1984: Diirr, 1986: Jacobshagen, 1994). margarite may facilitate margaritization of these During the M2 event, greenschist-grade rocks were minerals. formed on most islands, but metamorphic conditions In rocks high in Al and Ca, margarite thus forms locally reached upper amphibolite grade with asso- over a wide range of physico-chemical conditions, ciated migmatitization (e.g. on Naxos and Paros). during progressive regional metamorphism as well as Pre-Alpine basement is locally exposed as the lowest retrogressive and polymetamorphic events. Problems tectonic unit in windows on Ios, Sikinos and Naxos can thus arise regarding the petrogenetic significance (see Fig. 1). The basement consists of (leuco)gneisses of margarite in cases where it is unclear from tex- affected by Hercynian (~300 Ma) medium-pressure tural and chemical evidence at which stage of the P- metamorphism and pre-Hercynian intrusive rocks T—t history of a rock the mineral formed. (Van der Maar etal., 1981; Van der Maar & Jansen, 1983; Andriessen et al., 1987). On Naxos, prograde margarite is an important constituent of emery deposits ranging in meta- Isoclinally folded sequences of metasediments morphic grade from middle greenschist (~450°C) to (pelitic and psammitic schists and gneisses, quart- middle amphibolite facies (~620°C). This paper zites, calcitic and dolomitic marbles) and meta- deals with the mineralogical aspects of margarite volcanics (amphibolites, basic schists) are the and associated muscovite, paragonite and plagioclase dominant lithologies of the ACMC. Metacarbonate in the Naxos metabauxites, discusses the Ca-Na-K units are widespread and contain in certain strati- partitioning among these minerals, and also graphic horizons metabauxite lenses (up to ~8 m 202 FEENSTRA Ca-Na-K MICAS IN METABAUXITES OF NAXOS ANDROSW250 KEA MYKONOS © Cret HIM 2a 37°. 39° 22 Fig. 1. Metamorphic map of the Cyclades and dUtribution of metabauxites and karstbauxites in the Aegean region (Feenjtra, 1985). dia, diaipore-bearing metabauxites; cor, corundum-bearing metabauxites; Cret, Cretaceous kantbauxita; Jur, Jurasiic karstbauxites; Jur + Cret, Jurassic and Cretaceoui karstbauxites; Mei, Meiozoic karstbauxites of which the exact age ii unknown; PK, Parnasjos- Kiona zone; AC, Attic-Cydadic Metamorphic Complex; B, pre-Alpine basement; 1, M, glaucophane schist facia metamorphiim; 2a, A/2 greenschist fades metamorphism; 2b, M2 amphibolite fades metamorphism; 2c, M% migmatite; 3a, M3 granodiorite; 3b, M5 contact metamorphism; V, Pliocene volcanism. thick), which are of karstic origin.
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