Petrogenesis and geochemistry of circa 2.5 Ga granitoids in the Zanhuang Massif: Implications for magmatic source and Neoarchean metamorphism of the North China Craton

Abstract

The tectonic framework of the North China Craton (NCC) during late Archean to early Paleoproterozoic (circa 2.5Ga) is still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. Circa 2.5Ga magmatic and metamorphic activities are widely spread throughout the NCC, which can be used as an efficient target to better understand the tectonic evolution at this period. In this study, based on a detailed field, structural, geochemical, geochronological and Sm–Nd isotopic study, we focus our work on the Haozhuang granitoids in the Zanhuang Massif located at the eastern margin of the Central Orogenic Belt of the NCC. The granitoids mainly include undeformed pegmatite and granodiorite. One pegmatite and two granodiorite samples yield zircon 207Pb/206Pb ages of 2513±29Ma, 2511±36Ma and 2528±18Ma, respectively. The granodiorites show metaluminous and shoshonitic to high-K calc-alkaline series characteristics with A-type granite affinity. The circa 2.5Ga granodiorites have highly negative εNd(t) values (−29.22~−33.12) and TDM model ages between 2671Ma and 3151Ma. This work shows clearly, from whole-rock major and trace elements and Sm–Nd isotopic studies, that the Haozhuang granodiorites were derived from partial melting of old and thickened TTG crust rather than mantle sources, and formed in a subduction-related tectonic setting. With geochemical comparison studies to other similar-aged granitic rocks in the Zanhuang Massif, we suggest that these granitic rocks possibly have a certain correlation during the magma evolution. Coupled with our previous geochemical and isotopic studies on circa 2.5Ga mafic dike swarms, we propose that the similar-aged granitic rocks and mafic dike swarms were produced by an east-dipping subduction polarity reversal event following an arc–continent collision between the Fuping/Wutai island arc and Eastern Block of the NCC above a west-dipping slab. The east-dipping subduction resulted in partial melting of the enriched lithospheric mantle, firstly forming the circa 2.5Ga mafic dikes inducing the widespread circa 2.5Ga metamorphism in the Central Orogenic Belt and the Eastern Block, and then the parental magma of these mafic dike swarms heated the old and thickened TTG crust causing the formation of similar-aged granitic rocks at a later stage.