The peraluminous, S-type character is further supported by geochemical parameters such as Fe* and MALI (modified alkali lime index). The aluminium saturation index (ASI) (ranging from 1.0 to 1.3), A/CNK ratios (ranging from 1.4 to 2.11) and A/NK ratios (ranging from 1.75 to 2.43) provide evidences for the peraluminous, S-type nature of the Mylliem granitoids. The strong REE fractionation of the Mylliem granitoids is depicted by (Ce/Yb)N values, which show a range of 1.39 to 1.65. An overall strong REE fractionation pattern has been envisaged for Mylliem granitoids. Chondrite-normalized REE diagram characteristically shows an enriched LREE pattern and prominent negative Eu anomaly (Eu/Eu* = 0.16–0.42) indicating the significant role of plagioclase fractionation from the parent magma. Th/U ratios (3.22–6.77) indicate a relatively higher abundance of Th over U. Critical evaluation of geochemical data and variation trends of major oxides/trace elements suggests a significant role of fractional crystallization in the evolution of Mylliem pluton.
Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification. Sphene and apatite occur as accessory minerals. Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An20–An29). Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing.
Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Field studies indicate that the Mylliem granitoids cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments.
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