A non-compressive tectonic regime for the evolution of Archean Gabbro Anorthosite Complexes: Evidence from the Singhbhum Craton, Eastern India.

The oval-shaped Archean Singhbhum Craton from Eastern India preserves geological records from the Hadean Eon to recent. The craton mainly comprises Archean TTGs (Tonalite-Trondhjemite-Granodiorite) and granitoids surrounded by metavolcano-sedimentary sequences are interlayered by banded iron formations followed by mafic and felsic magmatism and subsequent sedimentation cycles. The well-preserved Mayurbhanj Gabbro Anorthosite Complex (GAC) is a Mesoarchean mafic lithounit located on the eastern margin of the Singhbhum craton with coeval anorogenic Mayurbhanj Granite/Granophyre. The Mayurbhanj GAC exposures are found around the Mesoarchean Simlipal basin. These mafic rocks are free from structural deformations and mineral recrystallization. According to their mineralogical features and abundances, the GAC rocks are classified into leucogabbro-norite, leucogabbro, gabbronorite and gabbro. The variability in the anorthite content (An36 to An84) and the paucity of igneous amphibole preclude the involvement of a hydrous mantle source in these rocks' evolution. The GAC samples exhibit a transitioning trend between tholeiitic to the calc-alkaline field, where major oxides and trace element data point towards the fractional crystallization of olivine and pyroxene. The Mayurbhanj GAC show Large Ion Lithophile Elements enrichment over High Field Strength Elements depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The rocks display enrichment in light rare earth elements over heavy rare earth elements with a prominent positive Eu anomaly. The geochemical proxies like elevated Th/Yb, Nb/Yb, variable Gd/YbCN and varying Dy/Yb and Dy/Dy* ratios suggest a possible origin of Mayurbhanj GAC from a heterogeneous depleted mantle source. The Sr-Nd isotopic systematics of the studied rocks reveal that the parental melts have assimilated with Paleoarchean felsic crust. We evidence that the GAC rocks' arc-like geochemical signatures were caused by crustal contamination and perhaps not due to subduction-related origin. Combining the field, mineralogical, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC conceivably evolved from an anhydrous heterogeneous mantle source that has undergone crustal assimilation in a non-compressive tectonic regime.