Were South India and North China Craton attached during the Paleoproterozoic (Orosirian) Nuna Assembly? Novel geochemical and isotopic investigations of A2-type granites from the Khammam Schist Belt, Eastern Dharwar Craton, India

Abstract

The paper presents novel geochemical and geochronological data from granites in Khammam, Eastern Dharwar Craton (EDC), India. The studied granites contain major mineral phases like quartz, alkali-feldspar, plagioclase, biotite, and muscovite in decreasing order of abundance. The accessory phases are epidote, titanite, and zircon. The samples comprise 70 ‐–77 wt% SiO₂ and 12–15 wt% Al₂O₃. The K₂O and Na₂O concentrations range from 2.57 to 5.65 wt% and 1.17 to 2.69 wt%, respectively. They are enriched in Rb, Th, and Pb and depleted in Nb, Ta, and Ti. On a chondrite-normalized plot, the samples exhibit a rightward trend with a negative Eu anomaly. Zircon saturation in silicate melts yields a temperature (T_Zr) of 859 ‐ 978 °C. The microstructure and U - Pb isotopic analysis of zircon grains (n = 79) reveals the presence of magmatic and polymetamorphic grains with ²⁰⁷Pb/²⁰⁶Pb age clusters at 1844 Ma (number of analyses, n = 7) - 1858 Ma (n = 8), 1737 Ma (n = 5) -1768 Ma (n = 4), 1619 Ma (n = 7) - 1634 Ma (n = 6), and 1554 Ma (n = 5), respectively. The magmatic zircons exhibit ε(Hf) values between 3 and 18.9 with a two-stage model age of 2.03 Ga. In contrast, the metamorphic zircons exhibit ε(Hf) values between −5.6 and 18, yielding a two-stage model age of 1.97 Ga. The geochemical and geochronological studies indicate that the rocks are A₂-type granite emplaced during the accretion of the eastern block of the North China Craton (NCC) and EDC between 1844 Ma and 1858 Ma. The zircons from 1737–1768 Ma and 1620 Ma show the time of metamorphic growth during Antarctica-Nellore Schist Belt (EDC) accretion. Finally, the U-Pb zircon ages from 1554 Ma represent Nuna's final amalgamation. The results of this study posit an association between EDC and NCC during Nuna assembly.