Petrogenesis of ultramafics in the Neoarchean Veligallu greenstone terrane, eastern Dharwar craton, India: Constraints from bulk-rock geochemistry and Lu-Hf isotopes

作者：Khanna Tarun C; Sai V V Sesha; Bizimis Michael; Krishna A Keshav

来源：Precambrian Research, 2016, 285: 186-201.

DOI：10.1016/j.precamres.2016.09.020

• Ultramafic rocks
• Arc cumulates
• Lutetium-Hafnium
• Neoarchean
• Veligallu greenstone terrane
• Dharwar craton

We report on the bulk-rock geochemistry and Lu-Hf isotope systematics, and infer the petrogenesis of the ultramafic rocks in the Neoarchean Veligallu greenstone terrane, eastern Dharwar craton, India. The ultramafics along with the basalts and adakites yield a bulk-rock Lu-Hf isochron age of 2.696 +/- 0.054 Ga, consistent with the available SIMS zircon U-Pb age of 2.697 +/- 5 Ma, reported for the felsic volcanics, in the literature. The rocks have positive initial epsilon(Hf)(96 Ga) = +3.0 to +6.5, consistent with an origin from a long term depleted mantle source relative to a chondritic reservoir at similar to 2.7 Ga. Geochemically, the ultramafic rocks are characterized by high MgO = 24-34 wt.%, Mg# similar to 89, Cr = 2290-3855 ppm and Ni = 604-966 ppm contents; moderate to low SiO2 = 52-49 wt.%, Al2O3 = 9.2-4.7 wt.%, and TiO2 = 0.30-0.14 wt.%. To first order, these chemical compositions broadly resemble komatiites. The rocks exhibit higher than chondrite Al2O3/TiO2 (21-37) ratio, depletions in their middle-REE relative to the heavy-REE i.e. Gd-N/Yb-N < 1, and negative Nb, Ti, and positive Zr (Hf) anomalies in a primitive mantle normalized trace element variation diagram; attributes indicating a "boninite-like affinity". Hence, the Veligallu ultramafics lack the key geochemical characteristics that would indicate a plume origin typically attributed to komatiites. Post-magmatic low grade metamorphism, hydrothermal alteration and element mobility, and/or contamination by Archean upper continental crust cannot be the cause of these geochemical patterns. Present study suggests that these ultramafic rocks are not melts sensu stricto as they have significantly high Mg-number and MgO contents. Rather, these rocks are most likely to be the products of fractional crystallization of orthopyroxene olivine from a basaltic melt, and hence interpreted as cumulates. The spatial and temporal association of these ultramafics having "boninite-like affinity" with the associated arc magmatic sequence comprising of basalts, andesites and adakites in the Veligallu greenstone terrane essentially indicates that the ultramafic rocks originated in a Neoarchean subduction-related intraoceanic setting, therefore they represent arc cumulates.

• 出版日期2016-10