High-Temperature Metasomatism of a Two Pyroxene Granulite in the Tekkali Area, Eastern Ghats, India

Two pyroxene granulite occurs within the migmatite in Tekkali area, Andhra Pradesh in the form of elongated and/or flattened lenses for several kilometers along strike. Two zones have been identified in the large granulite lenses; inner zone has an assemblage of Opx+Cpx+PI+11m (Type I), while the outer zone is of Opx+Cpx+Pl+Bt+Kfs+Qtz (Type II). Such a zonation can be attributed to nietaqomatic alteration of the host migmatite. The smaller lenses of two pyroxene granulite, generally consisting of Type II assemblage, have been entirely metasomatised.

A two pyroxene granulite lens with a width of ca. 5 metres hosted by plutonic diatexite was selected for chemical investigation of the metasomatism. Its core portion with the assemblage of Type 1 is considered to retain the initial bulk composition. From core to margin of the lens, elements such as K, Ba and Rb, drastically increase evidencing their movement from the host diatexite. The alteralion transformed the chemical feature of the granulite from the one of rift-related basalt composition to volcanic are hasalt.

Geothermometric estimation indicates that biotite in Type II was crystallized at high temperature condition (∼870°C), which i s consistent with that of the host diatexite emplacement. Retrograde conditions obtained from Type I are -800-900°C for exsolution of pyroxenes, and ∼5.8 kbar and -700°C for the isobaric cooling (IBC) event, resulting in garnet+quartz corona formation. The comparable IBC condition is traced from the host diatexite. The petrographic and geochemical observations involve several important implications: (1) Before the IBC stage at which garnet+quartz corona formed in inner part, metasomatism had occurred in outer part, because of the infiltration of alkali-enriched fluid including volatile at high temperature condition. (2) During (I), the host diatexite is assumed to have provided the mobile elements effectively. It is therefore possible that the two pyroxene granulite had undergone granulite grade metamorphism, simultaneously with or before the emplacement of the host rock. (3) Rift-related basaltic magma is proposed as a probable precursor of the two pyroxene granulite.