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Cordierites (Crd) with idealized composition (Mg, Fe)2[Al4Si5O18]*n(H2O, CO2) occur as a characteristic, usually accessory phase in many types of felsic peraluminous igneous rocks, but also in medium- to high-grade metamorphic aluminous rocks of amphibolite and granulite facies (BERTOLDI et al., 2004). This is because stability fields of cordierite in the p-T-X space embraces both, normal metamorphic and igneous domains (CLARKE, 1995). Metamorphic rocks of Moslavačka Gora (MG) often contain cordierite (e.g. BARIĆ, 1954 ; KOROLIJA et al., 1986 ; VRAGOVIĆ & MAJER, 1979). PAMIĆ (1990) reports frequent occurences of cordierite in amphibolite facies schists and to a lesser extent in migmatites. GARAŠIĆ (1993) recognizes the complexity of cordierite-bearing rocks and defines them as polymetamorphic. In the MG granites, unaltered cordierite is rarely present (e.g. PAMIĆ, 1990 ; STARIJAŠ et al., 2010 ; PETRINEC, 2013). We have conducted a systematic study of cordierite-bearing aluminous metamorphic rocks from typical MG localities mentioned in the literature (Jaska creek, Tičar creek, Garjevica, Kamenjača-Stelovača, Kamenac). They all show clear foliations, at least on a microscale. Typical paragenesis includes Als (Sill/And), Qtz, Fsp, Bt ( Ms), Crd ( Grt Spl), secondary and accessory phases. According to microstructural characteristics and observed mineral reactions, all of the studied rocks have been determined as anatexites, with a distinct signature of melt generation and its segregation into specific microstructural domains (PETRINEC, 2013). Based on the microstructural characteristics and further supported by chemical data, two major types of cordierite are defined: prograde (Type I) retrograde (Type II) cordierite. The prograde cordierite from the mesocratic domains forms porphyroblasts elongated in the plane of the main foliation (Type Ia). Most of these Crd porphyroblasts are rimmed by biotite and envelope a partly resorbed alumosilicate core (And/Sill), pointing to Crd production through biotite dehydration melting reaction: And + Bt I + Pl I + Qtz = Crd (Ia) + Kfs + Bt II + Pl II + (Ilm) + melt (PETRINEC, 2013). This type is regarded as peritectic cordierite (CLARK, 1995 ; GROPPO et al., 2013). Garnet- and cordierite-bearing anatexites host another microstructural subtype of prograde cordierite (Type Ib) related to melting reaction including garnet (ÁLVAREZ-VALERO et al., 2007). It encompasses Crd+Spl aggregates developed around partly resorbed Grt grains formed through reaction: Grt + Sil + Bt + Pl = Crd (Ib) + Spl + Kfs + melt (PETRINEC, 2013). Third type of prograde cordierite (Type Ic) is cotectic cordierite (CLARKE, 1995, GROPPO et al., 2013) that occurs in the leucocratic (micro) domains of the samples. It forms subhedral grains with rare biotite and quartz inclusions pointing to growth during melt crystallization i.e. at decreasing temperature, through the reaction: melt Qtz + Kfs + Crd + V. In garnet-bearing samples, cordierite pseudomorphs after garnet are also observed (Type II) which are indicative for back reaction of garnet with the co-existing melt that took place along the retrograde p-T path. Preservation of biotite and its presence as inclusion inside Crd grains, interstitial character of K-feldspar inside the pseudomorphs together with other microstructural characteristics point to reaction: Grt (± Spl) + melt = Crd (II) + Bt + Pl + Kfs (PETRINEC, 2013). Due to its mineralogy i.e. the facility to incorporate and/or release volatiles from its structure, cordierite is easily altered whenever it leaves its own p-T-X stability field (CLARKE, 1995). In the MG anatexites, variable extent of cordierite alterations is observed. Type Ia and Ib cordierites are generally well-preserved, almost free of hydrous retrogression. Type Ic cotectic cordierite from the leucosomes is partly retrogressed and replaced by fine-grained white micachlorite/biotite aggregates (pinnite) along grain boundaries. These observations also corroborate the fluid-saturated (sub)system for the formation of cotectic cordierite (Type Ic) while other (sub)types formed in "dry" conditions. The observations from anatectic rocks of Moslavačka Gora point to multiple cordierite-producing reactions, all of them closely related to production of peraluminous melt in these rocks. Additionally, the presence of cotectic i.e. magmatic cordierite in MG anatexites represents a probable connection with peraluminous cordierite-bearing granites emplaced into the shallow crustal level of the Late Cretacous Adria-Europe plate boundary setting. |
