摘要
New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic （ca. 1100 Ma） and late Cretaceous （ca. 90 Ma） kimberlites in the Timmasamudram cluster （TKC） of the Wajrakarur kimberlite field （WKF）, Eastern Dbarwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry （phlogopite, spinel, perovskite and clinopyroxene）, bulk-rock chem- istry （SiO2, K20, low TiO2, Ba/Nb and La/Sm）, and perovskite Nd isotopic compositions, the TI（-1 （mac- rocrystic variety） and TI（-4 （Macrocrystic variety） kimberlites in this cluster are here classified as orangeites （i.e. Group II kimberlites）, with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites （e.g., TK-2, TK-3 and the TK-1 microcrystic variant）, are more similar to other 1100 Ma, Group l-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on pub- lished carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TI（C kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite. Depleted mantle （TDM） Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasomatic enrichment of their source regions are broadly similar to that of the Meso- proterozoic kimberlites of the EDC. The younger, late Cretaceous （ca. 90 Ma） TI（-1 （macrocrystic variant） and TK-4 kimberlites, as well as the orangeites from the 8astar Craton, share similar Nd model ages of 1100 Ma, consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent. The presence of late Cretaceous diamondiferous orangeite activity, presumably related to the location of the Marion hotspot in southern India at the time, suggests that thick lithosphere was preserved, at least locally, up to the late Cretaceous, and was not entirely destroyed during the breakup of Gondwana, as inferred by some recent geophvsical models.
New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic （ca. 1100 Ma） and late Cretaceous （ca. 90 Ma） kimberlites in the Timmasamudram cluster （TKC） of the Wajrakarur kimberlite field （WKF）, Eastern Dbarwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry （phlogopite, spinel, perovskite and clinopyroxene）, bulk-rock chem- istry （SiO2, K20, low TiO2, Ba/Nb and La/Sm）, and perovskite Nd isotopic compositions, the TI（-1 （mac- rocrystic variety） and TI（-4 （Macrocrystic variety） kimberlites in this cluster are here classified as orangeites （i.e. Group II kimberlites）, with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites （e.g., TK-2, TK-3 and the TK-1 microcrystic variant）, are more similar to other 1100 Ma, Group l-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on pub- lished carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TI（C kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite. Depleted mantle （TDM） Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasomatic enrichment of their source regions are broadly similar to that of the Meso- proterozoic kimberlites of the EDC. The younger, late Cretaceous （ca. 90 Ma） TI（-1 （macrocrystic variant） and TK-4 kimberlites, as well as the orangeites from the 8astar Craton, share similar Nd model ages of 1100 Ma, consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent. The presence of late Cretaceous diamondiferous orangeite activity, presumably related to the location of the Marion hotspot in southern India at the time, suggests that thick lithosphere was preserved, at least locally, up to the late Cretaceous, and was not entirely destroyed during the breakup of Gondwana, as inferred by some recent geophvsical models.
基金
financial support from the Department of Science and Technology,New Delhi,in the form of a major research project grant under the Fast Track Scheme for Young Scientists(No. SR/FTP/ES-175/2010)
the Head,Department of Geology,Banaras Hindu University,DST-SERB(1R/S4/ESF-18/2013)
New Delhi and Alexander von Humboldt Foundation,Germany,for support
the National Research Foundation
The Centre of Excellence for Integrated Mineral and Energy Resource Analysis(CIMERA) at the University of Johannesburg
