Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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Geochemistry and Petrogenesis of Anorogenic(?) Granitoids of West Garo Hills, Meghalaya


Affiliations
1 Atomic Minerals Directorate for Exploration & Research, Jamshedpur - 831 002, India
2 Atomic Minerals Directorate for Exploration & Research, Shillong, India
3 Atomic Minerals Directorate for Exploration & Research, Bangalore, India
4 Atomic Minerals Directorate for Exploration & Research, Hyderabad, India
     

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The granite in Samingiri - Dilsekgiri area occurs as discordant, isolated pluton within the migmatitic terrain of West Garo Hills district, Meghalaya. The pluton is exposed over 140 sq km (18 km × 8 km). It exhibits structures of solid state and piecemeal stoping effect proximal to the contact and enveloped by a contact metamorphic aureole of albite-epidote-hornfels facies. Modally, it is biotite-monzogranite and biotite-syenogranite with minor biotite, chlorite, epidote and sericite and accessories like zircon, apatite, allanite, pyrite, magnetite and sphene. Geochemically, it is marked by restricted composition (69-76 wt% SiO2), high alkalies, low Ca, metaluminous to strongly peraluminous (Molar Al2O3/CaO+Na2O+K2O = 0.95-1.54), high FeO/MgO, high Ga/Al, high contents of Rb, Sr, Ba, Y, Zr and Ce and depleted in Ti and P. The field observation, mineralogical and geochemical aspects indicate the post-tectonic nature of West Garo pluton more like as A-type granite formed by partial melting of lower crustal blocks followed by low to moderate degree of fractional differentiation. Low Ca, alkaline nature and peraluminous character point to A-type nature of West Garo granite significantly different from other granites of Meghalaya Plateau. Rb-Sr age (616 ± 86 Ma) of granite, however, corresponds to widespread Middle to Upper Pan African activity, a thermal event prevailed during Late Proterozoic - Early Palaeozoic (500-800 Ma) period, manifested in the form of several granitic intrusions in the basement gneissic complex and the overlying Proterozoic metasediments of the Shillong Group in Meghalaya Plateau.

Keywords

West Garo Granite, Shillong Group, Meghalaya, Pan African Activity.
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  • Geochemistry and Petrogenesis of Anorogenic(?) Granitoids of West Garo Hills, Meghalaya

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Authors

D. K. Choudhury
Atomic Minerals Directorate for Exploration & Research, Jamshedpur - 831 002, India
A. K. Pradhan
Atomic Minerals Directorate for Exploration & Research, Shillong, India
S. Zakaulla
Atomic Minerals Directorate for Exploration & Research, Bangalore, India
K. Umamaheswar
Atomic Minerals Directorate for Exploration & Research, Hyderabad, India

Abstract


The granite in Samingiri - Dilsekgiri area occurs as discordant, isolated pluton within the migmatitic terrain of West Garo Hills district, Meghalaya. The pluton is exposed over 140 sq km (18 km × 8 km). It exhibits structures of solid state and piecemeal stoping effect proximal to the contact and enveloped by a contact metamorphic aureole of albite-epidote-hornfels facies. Modally, it is biotite-monzogranite and biotite-syenogranite with minor biotite, chlorite, epidote and sericite and accessories like zircon, apatite, allanite, pyrite, magnetite and sphene. Geochemically, it is marked by restricted composition (69-76 wt% SiO2), high alkalies, low Ca, metaluminous to strongly peraluminous (Molar Al2O3/CaO+Na2O+K2O = 0.95-1.54), high FeO/MgO, high Ga/Al, high contents of Rb, Sr, Ba, Y, Zr and Ce and depleted in Ti and P. The field observation, mineralogical and geochemical aspects indicate the post-tectonic nature of West Garo pluton more like as A-type granite formed by partial melting of lower crustal blocks followed by low to moderate degree of fractional differentiation. Low Ca, alkaline nature and peraluminous character point to A-type nature of West Garo granite significantly different from other granites of Meghalaya Plateau. Rb-Sr age (616 ± 86 Ma) of granite, however, corresponds to widespread Middle to Upper Pan African activity, a thermal event prevailed during Late Proterozoic - Early Palaeozoic (500-800 Ma) period, manifested in the form of several granitic intrusions in the basement gneissic complex and the overlying Proterozoic metasediments of the Shillong Group in Meghalaya Plateau.

Keywords


West Garo Granite, Shillong Group, Meghalaya, Pan African Activity.

References