Late Neoarchean monzogranitic–syenogranitic gneisses in the Eastern Hebei–Western Liaoning Province, North China Craton: Petrogenesis and implications for tectonic setting

Abstract

The medium-grained monzogranitic–syenogranitic gneisses expose as roughly northeast–southwest-trending and cover ∼1500km2 in the southern part of the Eastern Hebei–Western Liaoning Province, North China Craton. These plutonic gneisses have leucocratic features and weakly gneissic to massive structures, and were metamorphosed under greenschist to lower amphibolite facies. LA-ICP-MS zircon U-Pb isotopic dating reveals that the magmatic precursors of these monzogranitic–syenogranitic gneisses were emplaced during 2527–2511Ma. They are characterized by high contents of SiO2 (71.64–78.08%) and K2O (3.55–6.40%), and low contents of CaO (0.25–1.71%), MgO (0.01–0.42%), and Na2O (2.29–4.96%). According to their chondrite-normalized REE patterns, the monzogranitic–syenogranitic gneisses can be subdivided into a high REE group and a low REE group. Studies of petrogenesis reveal that magmatic precursors of the high REE group samples were derived from partial melting of the regionally widespread porphyritic or medium-grained dioritic–quartz monzonitic–granodioritic–monzogranitic gneisses under low pressure conditions (less than ca. 10kbar). The magmatic precursors of the low REE group were formed by fractionated crystallization of the high REE group magma, with titanite, allanite, epidote, apatite, and zircon as the major crystallization fractionation phases. Combined with previous investigations on the late Neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the Eastern Hebei–Western Liaoning Province, the magmatic precursors of these monzogranitic–syenogranitic gneisses were most likely formed in a back-arc basin along the north of the Eastern Block, North China Craton.