Time-space evolution of an ancient continent, a window to changing crustal architecture: Insights from granitoids of Singhbhum Craton, eastern India

Abstract

Archean cratons display spatial and temporal changes in the composition and thickness of the crust, which has important implications for intracrustal differentiation, attainment of long-term stability, and operation or absence of plate tectonics. The geodynamic reason(s) for these changes, though, is a controversial subject. To address this issue, the present contribution investigates the spatial and temporal changes in granitoid compositions of the well-exposed Singhbhum Craton. It reports new zircon U-Pb and Lu-Hf, and whole-rock geochemical data on Paleoarchean granitoids from some of the less-studied parts of the Singhbhum Craton. The new information is then collated with previously reported data from the central part (cratonic core) of the Singhbhum Craton to illustrate the spatial and temporal changes in granitoid chemistry, using selected major oxide and pressure-sensitive elemental ratios. Temporal changes in the characteristics of the granitoids suggest that the crust building in Singhbhum Craton possibly started in a relatively thin oceanic plateau with the emplacement of low-pressure TTGs and diorites during ∼3.53–3.47 Ga. The ∼3.47–3.42 Ga period marks a gradual increase in crustal thickness, inferred from progressively increasing Al2O3 and pressure-sensitive trace element ratios of the TTGs. Subsequently, as a result of protracted mantle upwelling and consequent juvenile TTG addition, the composition of the bulk continental crust differentiated into a more evolved felsic composition by ∼3.35 Ga, when the first potassic granite was emplaced. Spatial distribution of the granitoids suggests partial convective overturn (contemporaneous sinking of greenstones and diapiric rise of shallow- to mid-crustal granitoid domes) played a role in bringing the older as well as juvenile TTGs to a melting depth, generating ∼3.35–3.25 Ga spatially restricted high-silica, high-potassic, low-pressure K-rich granite and transitional granitoid domes. TTG magmatism also continued contemporaneously during this period. During ∼3.33–3.32 Ga, continental crust in Singhbhum Craton reached the maximum thickness, inferred from the formation of high-pressure (or low-HREE) TTGs. Afterward, possibly delamination of dense lower crustal residue caused the crust to become thinner, as recorded by progressively low-pressure K-rich granite and transitional granitoid emplacement until ∼3.28 Ga. Near the end of Paleoarchean (∼3.26–3.25 Ga), the crust became thicker again, further yielding high-pressure TTGs and K-rich granites. We suggest an oceanic plateau-like setting (characterized by relatively inefficient heat extraction compared to a modern-day subduction zone) where plume magmatism and delamination of anatectic residues of crustal melting caused the time-transgressive thickening and thinning of the continental crust, respectively. This process resulted in change in melting depth and, in turn, in the granitoid chemistry. Therefore, the Paleoarchean crustal architecture and the bulk crustal composition of the Singhbhum Craton controlled the temporal change in granitoid chemistry in an ongoing tectonic regime (partial convective overturn-dominated setting) without any distinct tectonic shift.