~2.5 Ga asthenospheric-melt impregnated continental crust: Implications for sporadically eroded lithosphere beneath the Dharwar craton, India

Abstract

In an intraplate continental setting, asthenospheric melts emplaced at the surface usually exhibit lithospheric interaction and crustal contamination signatures. In such a scenario, the occurrence of crustally uncontaminated juvenile magmatic rocks isan intriguing phenomenon on the Earth. In this contribution, we present geochemical and isotopic evidence of such a rare paleotectonic intracratonic event from the western Dharwar craton, India. The focus of this study is a ~2.5 Ga comagmatic suite of High-Mg tholeiitic dikes emplaced near the eastern margin of the Mesoarchean Nuggihalli greenstone belt. The dikes are characterized by high Nb/Th ~11 and low Th/Ce ~0.03 ratios, and radiogenic initial Nd-isotopic compositions εNd(t=2.5 Ga) = +3.02 to +3.16, in contrast to the contemporaneous continental crust in the study area. Unlike in the continental crust, primitive mantle normalized negative Nb and Ti anomalies are absent in these dikes. Therefore, preclude crustal contamination. The La/Nb < 1 and Ba/Nb < 25 ratios in these dikes do not indicate any significant contribution from the sub continental lithospheric mantle (SCLM) as well. The well-preserved mineralogy and primary igneous textures in these dikes essentially imply that (1) regional deformation and metamorphic events in the Dharwar craton predate 2.5 Ga and (2) The isotopic compositions and the trace element attributes of these dikes evidently suggest that lithosphere beneath the Dharwar craton was either very frail and incapacitated to participate in the process, or perforated to channelize faults and fractures to act as potential conduits for magma transport. Consequently, it implicates sporadically eroded lithosphere beneath the Dharwar craton already by ~2.5 Ga