Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

Abstract

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.