The petrogenesis of early Paleozoic high-Ba–Sr intrusions in the North Qinling terrane, China, and tectonic implications

Abstract

High-Ba–Sr (HiBaSr) intrusions provide important insights into early Paleozoic magmatism and tectonic evolution of the Qinling orogenic belt, China. Here, new zircon U–Pb ages, whole-rock major and trace element data, and Sr–Nd–Hf isotopic compositions are presented for three HiBaSr dioritic intrusions (Yangjiazhuang and Tangzang quartz diorite intrusions and Honghuapu diorite intrusion) in the North Qinling terrane. The Yangjiazhuang quartz diorites and Honghuapu diorites are high-K calc-alkalic to shoshonitic rocks, whereas the Tangzang quartz diorites are Na-rich calc-alkalic rocks. LA–ICP–MS zircon U–Pb dating indicates that the K-rich dioritic intrusions were formed at 439–438 Ma, and the Na-rich intrusion at 429–425 Ma. Rocks of the three intrusions are metaluminous to weakly peraluminous, and are characterized by high Ba and Sr contents of 792–3050 and 579–1385 ppm, respectively, with obvious enrichment in LILEs (e.g., Rb, Ba, Th, and U) and depletion in HFSEs (e.g., Nb, Ta, and Ti). They exhibit relatively uniform Sr and Nd isotopic compositions with (87Sr/86Sr)i ratios of 0.7032–0.7059, εNd(t) values of −0.65 to −2.78, and two-stage Nd model ages (TDM2) of 1.23–1.39 Ga, indicative of an enriched mantle source. The K-rich intrusions exhibit minor Nd–Hf isotopic decoupling (ΔεHf(t) = 2.93–5.01) and have low Nb/U ratios (2.11–3.01) and Mg numbers (48.5–52.1), suggesting that the Yangjiazhuang quartz diorites and Honghuapu diorites were formed through fractional crystallization of partial melts of enriched mantle metasomatized by terrigenous-sediment-derived fluids. The Na-rich Tangzang quartz diorites exhibit distinct ΔεHf(t) values (5.72–9.35) and elemental contents (e.g., high Na2O and Nb), and highly variable Rb/Y, Nb/Y, Th/Zr, and Nb/Zr ratios, indicating that they formed through crustal assimilation and fractional crystallization of partial melts of enriched mantle previously fertilized by terrigenous sediment-derived fluids and slab melts. Petrogenetic models for the HiBaSr dioritic rocks and Late Ordovician–Silurian igneous rocks of the western North Qinling terrane indicate that the K-rich HiBaSr intrusions formed in a slab subduction setting, whereas the Na-rich intrusion was controlled by slab break-off after continental collision. Together with granulite-facies metamorphism in the region, the emplacement of the HiBaSr dioritic rocks indicates that the closure of the Shangdan Ocean and initial collision between the North China Craton and Yangtze Block occurred at ca 433 Ma, with subsequent slab break-off triggering extensive Silurian magmatism at ca 430–402 Ma.