Metamorphic P–T paths and zircon u–pb ages of Paleoproterozoic metabasic dykes in eastern Hebei and northern Liaoning: Implications for the tectonic evolution of the North China Craton

Abstract

Metabasic dykes are described from Qianxi (JD1462) and Qinglong (JD15158) in the eastern Hebei and Qingyuan (15Q18) in the northern Liaoning that clearly crosscut the foliation of Neoarchean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and supracrustal rocks in the Longgang Block. Petrographic observations and phase modeling using THERMOCALC suggest that these metabasic dykes have experienced metamorphism of high-pressure (HP) granulite facies and amphibolite facies characteristic of clockwise P–T paths. Their peak P–T conditions were constrained to be 11.5 ± 0.6 kbar/815 ± 62 °C for sample JD1462, 8–9 kbar/570–585 °C for JD15158 and 10.7 ± 0.6 kbar/780 ± 70 °C for 15Q18, corresponding to apparent geothermal gradients of 18–19 °C/km. Zircon U–Pb dating yields metamorphic ages of ∼1.82 Ga for samples JD15158 and JD1462, interpreted to represent the peak stage of amphibolite facies and the post-peak decompression stage of HP granulite assemblage. Zircons in sample 15Q18 yield four age groups of ∼1.84 Ga, 1.94 Ga, 2.12 Ga and 2.49 Ga, interpreted to represent the ages, respectively, of the post-peak decompression stage of HP granulite assemblage, a mixed result from zircons of ∼1.84 Ga and ∼2.12 Ga, the emplacement of basic dykes, and the captured zircon grain from country TTGs. Thus, the metamorphic evolution of metabasic dykes may correlate to a crustal thickening event occurred close to 1.84–1.82 Ga, which is separated from the collision event at ∼1.95 Ga prevailed in the North China Craton. The metabasic dykes show weakly right-declined REE patterns and are depleted in Nb, Ta and Ti, inferred to represent a within-plate extensional setting at ∼2.12 Ga, consistent with the magmatic events occurred in the Trans-North China Orogen and Jiao-Liao-Ji Belt. As a consequence, the Archean Longgang Block was largely influenced by the Paleoproterozoic magmatic and metamorphic events.