MICROSTRUCTURES AND COMPOSITION OF MELT INCLUSIONS (NANOGRANITES) IN ANATECTIC METAPELITES

Both glassy and crystallized melt inclusions (MI) occur in peritectic garnets in metapelitic granulites from the Kerala Khondalite Belt of India. The trapped melt is interpreted to have originated by dehydration melting of biotite. An extensive ESEM-BSE mapping study, along with EMPA analysis, has been carried out to characterize these anatectic MI. The inclusions range from 4 to 35 μm in diameter and occur as clusters in garnets. In spite of the long time it took for these rocks to cool below 350 °C (at least 60 m.y.), different degrees of crystallization were observed in the same cluster, ranging from totally crystallized to totally glassy.

The crystallized MI are referred to as “nanogranites” and always contain quartz, Mg-rich biotite (XFe=0.23) and two feldspars in a fine-grained (≥ 10 nm) polycrystalline aggregate. Based on microstructural evidence, biotite crystallized first, preferentially on the walls of the MI, while quartz and feldspars crystallized later, often forming graphic intergrowths and/or melt pseudomorph-like structures (≥ 50 nm) similar to coarser structures (≈ tens of microns scale) observed in the host rocks. The glassy inclusions are rare (about 15% of the total) and smaller in size (≤15 μm in diameter) compared to the crystallized nanogranite MI. Both the crystallized and glassy MI have SiO2 ≥72 wt%, K2O ≥7 wt% and both CaO and Na2O ≤1 wt%. Both MI types often show negative crystal-shape and contain trapped crystalline phases that are accessories in the host rock, including rutile, titanite, zircon, apatite and Zn-rich spinel. Partially crystallized MI has been also recognized, containing an amorphous phase identified as a more differentiated melt. Cl and Ca are preferentially partitioned into this melt phase, and EMPA data show higher CaO content and lower K2O content compared to the melt in glassy MI.

This study supports the idea that all the trapped droplets of anatectic melt had the same original composition, except for the different trapped accessories. In addition, the cooling history produced micro- and nanoscale structures commonly found at larger scales in migmatites, granulites and granites.