Qingyuan high-grade granite–greenstone terrain in the Eastern North China Craton: Root of a Neoarchaean arc

Abstract

The Qingyuan high-grade granite–greenstone terrain is one of the first recognized granite–greenstone terrains in the Eastern North China Craton. Similar to other Archaean terrains, its tectonic environment is debated: was it an intra-continental rift or a modern analogy of arc? Occurrence and petrogenesis of major igneous series, the ultramafic–mafic and felsic volcanic rocks (>2510Ma) and the plutons of the quartz diorite (2570–2510Ma), TTG (2570–2510Ma) and quartz monzodiorite (2510–2490Ma) series in Xinbin area are investigated. The mafic intrusives and volcanic rocks have high MgO content (5.4–7.5wt.%) and Mg-number (48–61). They show slightly depleted to flat trace element patterns. The ultramafic rocks (serpentinite) could be genetically related to the mafic rocks. The meta-dacite–rhyolite is adakitic with enriched light and middle REEs and LILEs, but significantly depleted HFSEs. The quartz diorite has high Mg-number (60–64), moderately enriched light and middle REEs and LILEs. The TTG shows distinct light but moderate middle REE-enrichment, prominent Nb–Ta-depletion but Zr–Hf-enrichment (Zr/Sm>100). The quartz monzodiorite has moderate light and middle REE-enrichment, significant Nb–Ta-depletion, and negative Eu-/Sr-anomalies. The TTG has more depleted Sr–Nd isotopes (εNdt=+2–+6; 87Sr/86Srt=~0.700) than all the others (εNdt=0–+2; 87Sr/86Srt=0.701–703). Their petrogenesis can be explained by an Archaean-style subduction defined as a mantle wedge–absent flat–‘hot’-subduction with significant vertical tectonism in the overriding slab: the ultramafic–mafic rocks were originated from primitive mantle; the meta-dacite–rhyolite was originated from the eclogite facies overriding crust; the quartz diorite was a mixture of melts from mantle and the overriding crust; the TTG was from the subducting slab under amphibolite to amphibole-bearing eclogite facies; and the quartz monzodiorite was from the subducting slab after the derivation of the TTG. The Qingyuan terrain represents remnants of the root of a Neoarchaean arc: its anticlockwise P–T-path recorded the initiation of the arc to its cession and exhumation/cratonization at ~2480Ma.