Typology and Geochemistry of Microgranular Enclaves Hosted in Malanjkhand Granitoids, Central India

Abstract: The Cu(±Mo±Au) hosting Malanjkhand granitoids (MG, -2400 Ma) represent Palaeoproterozoic felsic magmatism in Central India, which constitute part of the Bundelkhand Protocontinent. The enclaves hosted in MG can be classified broadly into two categories (1) Microgranular enclaves (2) Country-rock xenoliths. The microgranular enclaves (ME) are mesocratic to melanocratic, fine to coarse grained, porphyritic and commonly showing typical igneous textures. Field and microstructural evidences of ME and MG suggest the interaction between coeval enclave (mafic) and granite magmas. Modal compositions of ME are mostly granodiorite, quartz monzodiorite and monzogranite, whereas MG belong to quartz monzodiorite and monzogranite both being related to the calc-alkaline granodiorite (medium-K) series but evolve into the field of granitoids generated by crustal fusion. The primary enclave magma generating various ME can be represented by a ME (SiO2=48.58, MgO=8.59 and Fe2O3t=15.15 wt%), geochemically similar to calc-alkaline basalt. On various bi-variate geochemical plots MG and ME show a wide compositional variation suggesting mixing of enclave and granite magmas. On a multicationic R1-R2 plot the tie lines joining the ME and respective host MG suggest that sufficient chemical disequilibria exist between them. Using experimental melt compositions in terms of SiO2 and K2O contents, the MG melt is likely to be derived from a tonalitic source, which subsequently experienced fractional crystallization, evolving within a high-K calc-alkaline field, whereas the ME plot in various fields suggesting the involvement of magma-mixing phenomena and to a certain extent potassium migration leading to a different degree of chemical disequilibria with their host MG, rather than crystal-liquid separation. Contents of selected trace elements (Ni, Rb, Sr, Y, Zr) of few pairs of ME-MG suggest partial to complete equilibration between them. However, Ti and Y contents, closer to primitive mantle, are indicative of mantle contribution to the source rocks from which ME and MG are derived.