High-pressure mafic granulite and supracrustal rocks in the southern Hengshan area, North China Craton: Metamorphic P-T-t evolution and geotectonic significance

Abstract

Amphibolite- to granulite-facies rocks are widely distributed in the Hengshan Complex, middle Trans-North China Orogen, and the high-pressure (HP) mafic granulite has been recently identified in the southern Hengshan area. The HP mafic granulite and amphibolite occur as rootless tectonic boudins/lenses within the TTG (tonalite–trondhjemite–granodiorite) gneiss/metapelite, indicative of typical “block-in-matrix” texture of metamorphic-tectonic mélange. Three to four generations of metamorphic mineral assemblages that correspond to the prograde (M1), peak (M2), and retrograde (M3-M4) stages, are recognized in these rocks. Conventional geothermobarometry and phase equilibrium modeling yield peak P–T conditions of 10.8–13.8 kbar/754–799 °C for the HP mafic granulite and 7.3–9.0 kbar/690–725 °C for the supracrustal rocks, respectively. A clockwise P–T path with near-isothermal decompression (ITD) and subsequent near-isobaric cooling (IBC) segments is reconstructed for the HP mafic granulite, indicating a dynamic subduction–collision–exhumation process that unfolded during an orogenic event. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircon yields metamorphic ages of ca. 1.91–1.83 Ga, representing the long-lived tectono-metamorphic event caused by collision between the Eastern and Western Blocks along the Trans-North China Orogen. It is hypothesized that the tectono-metamorphic mélange in the area originated from the tectonically juxtaposition of metamorphic rocks that had diverse protoliths, different peak P–T conditions and discrepant metamorphic ages. This complexity may be a hallmark of Paleoproterozoic orogens, drawing intriguing parallels with the intricate characteristics observed in Phanerozoic orogenic belts.