Abbott, 1984

Abbott, 1984 - American Mineralogist, Volume 69, pages...

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American Mineralogist, Volume 69, pages 250-251, 1984 The greenschist-amphibolite transition in the CFM projection Rtcua,Ro N. Annorr, Jn. Department of Geology Appalachian State University, Boone, North Carolina 28608 Abstract The CFM projection (C = CaO+Na2O+K2O-AlzOr, F : FeO-FezO:, M = MgO) was originally designed for illustrating mineral assemblages in upper amphibolite and granulite facies quartz-feldspar-magnetite-bearing metabasites. By introducing chlorite, the projec- tion can also be used to illustrate mineral assemblages in greenschist facies quartz- feldspar-magnetite-bearing metabasites. The greenschist-amphibolite facies transition takes place in the context of Abbott's (1982) lowest grade CFM topology. In metabasites, the change from the greenschist facies to the amphibolite facies commonly proceeds with no change in the mineral assemblage other than (l) a change in the compositions of the minerals and (2) a change in the modal proportions of the minerals (Laird, 1980; Thomp- son et al., 1982; Laird in Robinson et al., 1982). The common assemblage is amphibole (Amp) + chlorite (Chl) + plagioclase (Pla) + epidote (Epi) + quartz (Qtz)+Fe3+ oxide (hematite or magnetite : MCt)tTi-mineral (ilmen- ite or sphene)-+K-mica (muscovite of biotite = Bio)tcar- bonate (calcite or ankerite)rgarnet (Gar). In the green- schist facies, the amphibole is actinolite (Act) and the plagioclase is albite. In the amphibolite facies, the amphi- bole is hornblende (Hnb) and the plagioclase is oligoclase or a more calcic plagioclase. The whole-rock reaction relating the two facies is polyvariant and has been treated recently in an elegant fashion by Thompson et al. (1982) and by Laird in Robinson et al. (1982).In these works, the changes from actinolite to hornblende and from albite to oligoclase were modelled as continuous reactions. Abbott (1982) presented a petrogenetic grid for amphib- olite and granulite facies metabasites, based on phase relationships in the CFM projection (Q : CaO+Na2O +K2O-Al2Or, F: FeO-Fe2O3, M = MgO). The CFM projection is useful for illustrating mineral assemblages in metabasites containing qvar1"z, magnetite and one or two feldspars. The grid consists of 26 P-T regions, each characterized
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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Abbott, 1984 - American Mineralogist, Volume 69, pages...

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