Thickness and geothermal gradient of Neoarchean continental crust: Inference from the southeastern North China Craton

摘要

The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granocliorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at similar to 2.70 Ga and similar to 2.55 Ga which were sourced from partial melting of juvenile crustal components. The similar to 2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)(N) = 39), whereas the similar to 2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)(N) = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P-T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44-51 km with geothermal gradients of 17 to 20 degrees C/km for the similar to 2.70 Ga TTG gneisses whereas the similar to 2.55 Ga TTG gneisses show lesser crustal thicknesses of 35-43 km with geothermal gradients of 19 to 26 degrees C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (similar to 41 km, similar to 20 degrees C/km) of the modem average continental crust, indicating that a modem-style plate tectonic regime may have played an important role in the formation and