Spatiotemporal variations of Early to Middle Paleozoic magmatism in northern Beishan: Implications for accretionary and collisional processes in the southern Central Asian Orogenic belt

Abstract

Spatiotemporal variations in magmatic compositions reflect changes in magma sources and their formation conditions, offering insights into geodynamic processes. This study presents new zircon U–Pb and geochemical data for early–mid Paleozoic granitoids in northern Beishan, located within the southern Central Asian Orogenic Belt. Late Ordovician (451 Ma) granodiorites are I-type and formed through water-fluxed melting of amphibolites. Early Devonian (403–397 Ma) granites show I-, S-, and A-type affinities with enriched Sr–Nd isotopes, indicating ancient crustal sources. Late Devonian to early Carboniferous (366–357 Ma) granitoids exhibit I-type characteristics. They and coeval intermediate-felsic intrusives show similar Sr–Nd–Hf isotopes, with 87Sr/86Sr(i) from 0.7045 to 0.7075, εNd(t) from −2.93–0.92, and zircon εHf(t) predominantly ranging from 2.9 to 6.5. Geochemical studies suggest their derivation from metasomatized mantle, with fractional crystallization playing a key role. Available data reveal three magmatic stages: ∼500–420 Ma, ∼420–390 Ma, and ∼ 370–350 Ma. Stage I mainly consists of (quartz) diorites, granodiorites, and granites. They show increasing K2O/Na2O and zircon saturation temperatures from south to north, with the Hanshan unit characterized by A-type granites. These variations reflect northward subduction of the Beishan Ocean, leading to water-fluxed crustal melting in the Mazongshan arc and dehydration melting in the Hanshan back-arc. In Stage II, complex mafic–felsic rock associations composed of (quartz) diorites, high-silica granites, gabbros, and intermediate–mafic volcanic rocks, along with high temperatures required for S- and A-type granite formation, suggest increased mantle heat input from slab breakoff following Beishan Ocean closure. Stage III rocks, confined to the Mazongshan unit, consist of diorites, tonalites, granodiorites, and granites associated with southward subduction of the Jijitaizi–Xiaohuangshan Ocean. This study demonstrates that multistage subduction–accretion contributed to Beishan orogen formation, highlighting magmatic compositional variations as key to understanding accretionary orogens.