Geology of the precambrian rocks of the Tyrssad Weiloi area Khasi hills Meghalaya

Abstract

The Precambrian rocks of the Tyrssad-Weiloi area comprises of the quartzo-feldspathic gneiss, the arenaceous and argillaceous metasedi-ments of the Shillong Group, the Khasi greenstones (meta-dolerites) and the porphyritic granites of the South Khasi pluton. The gneiss forms the basement rocks of the area, overlain unconformably by the Shillong Group of rocks. The early intrusives, the Khasi greenstones show intrusive rela-tions only with the Shillong Group, but the later intrusive, the porphy-ritic granites have affected all the older rocks including the greenstones. The granite has a thermal aureole around it. The quartzo-feldspathic gneiss is a para-gneiss, whose composi-tion stands between arkose and pelite. The gneiss has become migmatitic near the contact of the granite. The Shillong Group of rocks and the greenstones suffered low grade regional metamorphism prior to the intru-sion of the granite. The pelitic members of the Shillong Group developed andalusite hornfels within the thermal aureole of the granite. Likewise, the low grade assemblage, actinolite-chlorite-epidote-elbite (greenschist) of the meta-dolerites have recrystallized into hornblende-hornfels at the proximity of the granite. The petro-chemistry of the greenstones resemble the composition of the tholeiitic basalts and dolerites. The South Khasi granite is a normal biotite granite. The presence of complex twins in plagioclase, zonally arranged inclusions in microcline, sharply euhedral zircons etc., support a magmatic origin of the granite. Tectonically, the rocks of the area have suffered at least seven different phases of deformation (D). The first five (D1 - D5) were respon-sible for the development of various types of folds and associated struc-tures observed in the gneiss and in the rocks of the Shillong Group. Deformation D6 was associated with the emplacement of the granite while D7 produced some shear or crush zones and associated pseudotachylites.