Dunite, glimmerite and spinellite in Achankovil Shear Zone, south India: Implications for highly potassic CO2-rich melt influx along an intra-continental shear zone

摘要

In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinet-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39-50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al2O3 (7.8-8.98 wt.%), and low Ni (10-14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (similar to36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K2O (similar to5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = similar to0.96) and extremely low NiO (<0.14 wt.%). Spinets in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = similar to59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO, fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO2-rich melts injected into lower crustal levels. K-Ar ages of 470.5 +/- 9.3 and 464.5 +/- 9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO2-rich fluid/melt influx along shear zones in this part of East Gondwana continent.