Early Permian plutons from the northern North China Block: constraints on continental arc evolution and convergent margin magmatism related to the Central Asian Orogenic Belt

Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro-diorite-quartz diorite-granodiorite-tonalite-granite plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process during the late Carboniferous to early Permian, from 324 +/- 6 to 274 +/- 6 Ma, and lasted for at least 50 Ma. In this paper, the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial Sr-87/Sr-86 ratios from 0.70520 to 0.70615 and have negative whole-rock epsilon(Nd)(t) values ranging from -17.4 to -9.3 and zircon epsilon(Hf)(t) values of -23.2 to -10.5. The gabbros exhibit higher epsilon(Nd)(t) values from -11.1 to -9.3 and epsilon(Hf)(t) values from -16.5 to -10.5, and one granodiorite exhibits an even lower epsilon(Nd)(t) value of -17.4 and zircon epsilon(Hf)(t) values of -23.2 to -15.1. Geochemical, Sr-Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust-mantle interaction in the northern NCB. Petrological associations as well as geochemical and Sr-Nd-Hf isotopic results show that the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB likely occurred during a period from similar to 270 to similar to 250 Ma, i.e, in the late Permian to earliest Triassic.