摘要
The Rajahmundry Trap Basalts （RTB） are erupted through fault-controlled fissures in the Krishna-Godavari Basin （K-G Basin） of Godavari Triple Junction, occurring as a unique outcrop sandwiched between Cretaceous and Tertiary sediments along the east coast of India. Detailed geochemical studies have revealed that RTB are mid-Ti （1.74-1.92） to high-Ti （2.04-2.81） basalts with a distinct quartz tholeiitic parentage. MgO （6.2-13.12 wt.%）, Mg# （29-50） and Zr （109-202 ppm） suggest that these basalts evolved by fractional crystallization during the ascent of the parent magma along deep-seated fractures. Mod- erate to high fractionation of HREE, as indicated by （Gd/Yb）N ratios （1.71-2.31） of RTB, suggest their generation through 3-5% melting of a Fe-rich mantle corresponding to the stability fields of spinel and garnet peridotite at depths of 60-100 km. Low K2O/P2O5 （0.26-1.26）, high TiO2/P2O5 （6.74-16.79）, La/Nb （0.89-1.45）, Nb/Th 〉 8 （8.35-13）, negative anomalies at Rb reflect minimum contamination by granitic continental crust. （Nb/La）PM ratios （0.66-1.1） of RTB are attributed to endogenic contamination resulted through recycling of subducted oceanic slab into the mantle. Pronounced Ba enrichment with relative depletion in Rb indicates assimilation of Infra- and Inter-trappean sediments of estuarine to shallow marine character. Geochemical compositions such as A1203/TiO2 （3.88-6.83）, medium to high TiO2 （1.74 -2.81 wt.%）, positive Nb anomalies and LREE enrichment of these RTB attest to their mantle plume origin and indicate the generation of parent magma from a plume-related enriched mantle source with EM 1 signature. Ba/Th （46-247）, Ba/La （3.96-28.51） and Th/Nb （0.08-0.13） ratios suggest that the source enrichment process was marked by recycling of subduction-processed oceanic crust and lithospheric components into the mantle. Zr/Hf （37-41） and Zr/Ba （0.51-3.24） indicate involvement of an asthenospheric mantle source. The Rajahmundry basalts show affinity towards FOZO （focal zone mantle） and PSCL （post-Archaean subcontinental lithosphere） which reflect mixing between asthenospheric and lithospheric mantle components in their source. Origin of RTB magma is attributed to plume-lithosphere interaction and the upward movement of melt is facilitated by intrabasinal deep-seated faults in the K-G Basin.
The Rajahmundry Trap Basalts （RTB） are erupted through fault-controlled fissures in the Krishna-Godavari Basin （K-G Basin） of Godavari Triple Junction, occurring as a unique outcrop sandwiched between Cretaceous and Tertiary sediments along the east coast of India. Detailed geochemical studies have revealed that RTB are mid-Ti （1.74-1.92） to high-Ti （2.04-2.81） basalts with a distinct quartz tholeiitic parentage. MgO （6.2-13.12 wt.%）, Mg# （29-50） and Zr （109-202 ppm） suggest that these basalts evolved by fractional crystallization during the ascent of the parent magma along deep-seated fractures. Mod- erate to high fractionation of HREE, as indicated by （Gd/Yb）N ratios （1.71-2.31） of RTB, suggest their generation through 3-5% melting of a Fe-rich mantle corresponding to the stability fields of spinel and garnet peridotite at depths of 60-100 km. Low K2O/P2O5 （0.26-1.26）, high TiO2/P2O5 （6.74-16.79）, La/Nb （0.89-1.45）, Nb/Th 〉 8 （8.35-13）, negative anomalies at Rb reflect minimum contamination by granitic continental crust. （Nb/La）PM ratios （0.66-1.1） of RTB are attributed to endogenic contamination resulted through recycling of subducted oceanic slab into the mantle. Pronounced Ba enrichment with relative depletion in Rb indicates assimilation of Infra- and Inter-trappean sediments of estuarine to shallow marine character. Geochemical compositions such as A1203/TiO2 （3.88-6.83）, medium to high TiO2 （1.74 -2.81 wt.%）, positive Nb anomalies and LREE enrichment of these RTB attest to their mantle plume origin and indicate the generation of parent magma from a plume-related enriched mantle source with EM 1 signature. Ba/Th （46-247）, Ba/La （3.96-28.51） and Th/Nb （0.08-0.13） ratios suggest that the source enrichment process was marked by recycling of subduction-processed oceanic crust and lithospheric components into the mantle. Zr/Hf （37-41） and Zr/Ba （0.51-3.24） indicate involvement of an asthenospheric mantle source. The Rajahmundry basalts show affinity towards FOZO （focal zone mantle） and PSCL （post-Archaean subcontinental lithosphere） which reflect mixing between asthenospheric and lithospheric mantle components in their source. Origin of RTB magma is attributed to plume-lithosphere interaction and the upward movement of melt is facilitated by intrabasinal deep-seated faults in the K-G Basin.
基金
CSIR for providing the funds to NGRI to carry out this research work in MIP-6201-28(CM)
