V016-0001
A Mantle Source for the Khasi Mafics, Shillong Plateau – Insights into crustal contamination
Wednesday, 9 December 2020
Poster
Gaurav Kumar, Indian Institute of Science Education and Research Bhopal, Bhopal, India, Nidha Azeez, Indian Institute of Science Education and Research Bhopal, Earth and Enviornmental Sciences, Bhopal, India, D.S.M. Pdah, Directorate of Mineral Resources, Government of Meghalaya, Shillong, India, M.a. Khonglah, Lad Smit, Shillong, India and Arundhuti Ghatak, Indian Institute of Science Education and Research Bhopal, Department of Earth & Environmental Sciences, Bhopal, India
Abstract:
The Khasi mafics are a group of Proterozoic basic to meta-basic intrusive rocks, with negligible to low-grade metamorphism, and exposed as sills around Khasi and Jaintia Hills in the Shillong Plateau, Meghalaya, India. This is a comprehensive study, including detailed sampling across several exposures, their petrography, and major-trace element geochemistry. The petrographic characteristics indicate two groups in the context of their alteration from negligible to significant alteration. The foliated and non-foliated varieties of altered samples are also present. Foliated-unaltered samples are composed of largely subhedral and medium to coarse-grained hornblende, actinolite, saussuritized plagioclase, chlorite and opaque minerals. These samples showed relict textures of clinopyroxenes with a reaction rim of amphibole, poikilitic texture in amphibole and chlorite grains and quartz inclusions. The Foliated-altered samples also have hornblende, chlorite, actinolite, altered plagioclase and opaque minerals, along with recrystallized quartz, epidote, biotite and calcite. The mineral textures have shown that the alterations are a result of high-temperature hydrothermal activity, which can be attributed to the intrusion of the neighboring Mylliem granite pluton.
Major element analysis shows low-high MgO (0.83-10.9 wt%) and moderate to high CaO (3.75-12.2 wt%). The rocks are enriched in FeO(T) (12.8-21.6wt%) and TiO2(5.02-0.92 wt%). The major element data and Mg# (9.83 to 59.81)indicate the absence of fractional crystallization, and the presence of batch melting with differential contamination. The least contaminated samples (based on Mg#) having a flat REE pattern indicatean OIB or a lower mantle source, and distinct from the depleted mantle. Typical arc signatures, as proposed in previous studies, are absent for these rocks. Th/Yb vs Nb/Yb ratios indicate magma-crust interaction. The most contaminated samples show upper crustal contamination character, which is also indicated by values of [Th/Nb]PM and [La/Nb]PM. The results of this study indicate a primitive mantle source with upper crustal contamination for the Khasi metabasics. Batch melting models of the least contaminated samples indicate these rocks to have been derived by 5-8% batch melting of a spinel lherzolite.
