Magmatic and metamorphic imprints from the root of an Archean continental arc: Evidence from the Qianhuai microblock in the North China Craton

Abstract

The Archean tectonic framework of the North China Craton (NCC) involved the assembly of several micro-continents along zones of ocean closure during the late Neoarchean, followed by high-grade metamorphism associated with the final assembly and cratonization of continental blocks during Late Paleoproterozoic. Here we investigate a metamorphosed igneous suite comprising pyroxenite, gabbro and trondhjemite in association with charnockite from the margin of the Qianhuai micro-block with a view to understand the magmatic and metamorphic history and crustal evolution in the NCC during late Neoarchean. We present new data from field investigations, petrology, geochemistry and zircon U-Pb, REE and Lu-Hf analyses. Mineral phase equilibria modeling shows that the pyroxenite formed at P-T conditions of 1.5–1.6 GPa and 900–1000 °C, possibly representing underplating of high temperature basaltic magma at the base of the continental arc, with the subsequent metamorphic conditions estimated as 1.2–1.4 GPa and 840–870 °C from the metagabbros. Both metagabbro and pyroxenite show an anticlockwise P-T path characterized by initial heating followed by moderate to high-pressure metamorphism and cooling. The base of a thickened arc is a viable tectonic settings to account for this scenario. Geochemical data suggest that the pyroxenite-gabbro-trondhjemite association represent the fractionated sequence from a common mafic magma, whereas the charnockite corresponds to anhydrous lower crustal melts. The high degree of hydration of the mafic and ultramafic units suggest retrograde metamorphism under hydrous conditions associated with slab subduction. The mafic-ultramafic rocks fall within the field of island arc tholeiite and the trondhjemite falls in the field of oceanic arc basalt. Geochemical data indicate arc signature of the rocks and the subduction processes involved in the generation of precursor basaltic melts that underplated the overriding arc crust and triggered lower crustal melting.The U-Pb data on magmatic zircon grains from the trondhjemite, metagabbro, pyroxenite and charnockite yield emplacement ages of 2481 Ma, 2449 Ma and 2464 and 2480 Ma and suggest subduction-related magmatism during Neoarchean-early Paleoproterozoic transition, followed by high grade metamorphism at ca. 1.94 Ga as indicated by the lower intercept ages. Zircon REE data confirm the magmatic crystallization history. The zircon Lu-Hf data show consistently positive εHf(t) values (0.5–11.7) and plot between CHUR and new crust line suggesting that depleted mantle sources contributed significantly to arc building and crustal growth. The computed model ages suggest that the magma source involved Neoarchean-Mesoarchean juvenile components. Our study provides important insights into magmatic and metamorphic processes at the base of an Archean continental arc and contributes to the understanding of the formation and growth of micro-continents within the NCC.