Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: Implications for the transition stage from a compressional to an extensional regime for the North China Craton

Abstract

The 2.5Ga metavolcanic rocks in Changyukou, Northwestern Hebei, can be classified into three groups based on major and trace elements: high-Mg basalts, tholeiitic basalts, and the calc-alkaline series (basaltic andesites-andesites and dacites-rhyolites).Both high-Mg basalts and tholeiitic basalts have negative anomalies of Nb, Zr, Ti and Heavy Rare Earth Elements (HREE) as well as enrichments of Sr, K, Pb, Ba and Light Rare Earth Elements (LREE) and show typical subduction zone affinities. The petrogenesis of high-Mg basalts can be ascribed to high-degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in Large Ion Lithophile Elements (LILE) and LREE by slab-derived hydrous fluids/melts/supercritical fluids, as well as the subsequent magma mixing processes of different sources at different source depths, with little or no influence of polybaric fractional crystallization. The flat HREE of tholeiitic basalts indicates they may also originate from the spinel stability field, but from obviously shallower depths than the source of high-Mg basalts. They may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate.We obtain the compositions of the Archean lower crust of the North China Craton based on the Archean Wutai-Jining section by compiling the average tonalite–trondhjemite–granodiorite (TTG) components, average mafic granulite components, and average sedimentary rock components. The modeling results show that the generation of high-Al basalts, basaltic andesites and andesites can be attributed to assimilation by high-Mg basalts (primary basalts) of relatively high-Al2O3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases, while Al2O3-rich plagioclase crystallization is suppressed under high-pressure and nearly water-saturated conditions. Dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites (high-Al basalts) and andesites. Mixing of magmas at various stages along the fractionation course of basaltic andesites (high-Al basalts) toward rhyolites promotes the trend of the calc-alkaline series.To reconcile the 2.55 to 2.5Ga TTGs derived from overthickened crust, the 2.51 to 2.50Ga calc-alkaline volcanic rocks derived from thickened crust, tholeiitic basalts representing low pressure and an extensional tectonic setting, 2493Ma leucosyenogranites derived from overthickened crust, 2437Ma biotite-monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened, as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites and widespread extension and rifting setting within the NCC from 2300Ma, we propose a model of an evolving subduction process. Among them, the composition of the 2.5Ga Changyukou volcanic rocks and potassic granites as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites may reveal that the tectonic setting in Northwest Hebei was in a transition stage from a subduction-related compressional regime to an extensional regime related to plate rollback.