Late Paleoproterozoic polymetamorphism in the north Yinshan Block, North China Craton and its tectonic implications

Forearc extension can result in low-P/T type metamorphism, but only a few such metamorphic belts are reported from modern and ancient arcs, and no reports are from early Precambrian terranes. The Yinshan Block (YB) in the northwestern North China Craton (NCC) experienced complicated Paleoproterozoic magmatism and metamorphism, but its tectonic evolution is inadequately constrained from a detailed metamorphic perspective. This study systematically investigates pelitic migmatites from a newly-identified Hadatu suite in the north YB for constraining the metamorphic and tectonic evolution. Two phases of metamorphism (M-I and -II) are recognized. M-I is the upper-amphibolite to granulite facies of the low-P/T type (31-32 ℃/km) dominated by post-peak cooling evolution. M-II is the greenschist facies overprinting of the medium-P/T type (24-25 ℃/km) characterized by a clockwise P-T path. Zircon and monazite U-Pb dating suggests that the two phases of metamorphism occurred in 1.92-1.91 Ga and 1.88-1.86 Ga, and the Hadatu suite was deposited during 1.96-1.92 Ga. The M-I metamorphism might correlate forearc extension as a result of slab rollback and mantle upwelling, and appears to be one of the early Precambrian forearc metamorphic records. The M-II metamorphism was ascribed to a collision orogeny in the north NCC. We propose that the early Precambrian tectonic evolution of the YB and also its adjacent Khondalite Belt involves five stages: the Neoarchean cratonization at ~2.5 Ga, early Paleoproterozoic extension during 2.46-2.30 Ga and 2.22-2.00 Ga, the first-phase collision during 2.0-1.95 Ga, oceanic subduction during 1.96-1.88 Ga, and the second-phase collision event during 1.88-1.86 Ga.