Petrogenesis of Granites from Mugu, Kasaragod District, Kerala, India

ABSTRACT The Mugu granitic rocks are located near the village of Angadimogar in the Kasaragod District of Kerala, southern India. The rocks are intruded into the Neoproterozoic granulite terrain classified as the Coorg Block. The Mugu rocks occupy an area of ~2 km2 and are chiefly composed of syeno-granite and quart-monzonite. The Mugu granitic rocks are leucocratic with coarse to medium grains; under the microscope, the rocks display equigranular hypidiomorphic and inequigranular porphyritic textures. The chief rock-forming minerals are K-feldspar, plagioclase, quartz and amphibole; accessories include sphene, zircon and opaques; and secondary minerals are biotite and epidote. Geochemically, the rocks in general are granite-alkali granite in composition and are ferroan, high K calc-alkaline (to alk-calcic), metaluminous to peraluminous and are akin to leucocratic-sodic associations. In the syeno-granite, the SiO2 and K 2 O are in the ranges of 72–73% and 4.02- 4.05%, respectively and in the quartz-monzonite, the SiO2 and K2O range between 59 and 60% (by weight) and between 2.21 and 2.31%, respectively. They are highly alkaline and sodic with Na2O (~4–5%) > K2O (~2-4%). The primitive mantle normalized spider plots for the syeno-granites reveal prominence of Zr, Th, U, Pb, and Ta, contrasted by a deficiency of Nb, Ti, P, and Sr. In contrast, the quartz-monzonites are characterized by a higher concentration of large ion lithophile elements (LILE; Sr and Ba), compared to high field strength elements (HFSE), and display negative anomalies observed in Nb, Zr, and Ti, reflecting the stable nature of immobile elements in magmas originating from arc-related sources. The total REE of these two types of rocks ranges between 70 and 310 ppm and shows enriched LREE and depleted HREE patterns, indicating a higher degree of partial melting. The tectonic trace element indicators suggest that the rocks are emplaced in a subduction related ‘volcanic-arc’ to ‘within-plate’ setting during post-Neoproterozoic.