Petrogenesis of middle Ordovician peraluminous granites in the Baoshan block: Implications for the early Paleozoic tectonic evolution along East Gondwana

Abstract

Peraluminous granitic magmatism in the Baoshan block is long-lasting roughly from 500Ma to 450Ma. The petrogenesis and geodynamics for this long-lived magmatism remain controversial. To address this controversy, this study reports the zircon U–Pb age and Hf-isotope, and bulk-rock major and trace element data of the granites from the Shuangmaidi and Mengmao areas in the Baoshan block. LA-ICPMS zircon U–Pb dating reveals that the granitic rocks from the two areas were emplaced between 470 and 459Ma. These rocks are high silicic and strongly peraluminous, with SiO2=73.6–77.6wt.%, A/CNK ratios of 1.0–1.6, and CIPW normative corundum contents of 0.7–5.3wt.%. They are enriched in LREEs, LILEs (e.g., Rb, Th, U, and K) and Pb, and depleted in HFSEs (e.g., Nb, Ta, P, Zr, and Ti), Eu, Sr, and Ba. The εHf(t) values for co-magmatic zircons of the Shuangmaidi coarse- and fine-grained porphyritic granites show wide ranges from −11.6 to +5.2 and from −8.1 to +7.0, concentrating in −7.1 to +0.5 and −8.1 to +0.7, respectively; and those of the Mengmao granites concentrate between −4.6 and −0.5. The primary magmas of these granites can be mainly attributed to the partial melting of ancient metasedimentary rocks, while small amounts of mantle-derived components were introduced into the magma sources for the Shuangmaidi granites. The primary magma of the Shuangmaidi granites experienced biotite-dominant mineral fractionation, and that of the Mengmao granite mainly fractionated K-feldspar and plagioclase. Combining our data with the regional sedimentary unconformity, multi-type magmatism, and high-pressure metamorphism in the Baoshan and its periphery blocks, we propose that these ca. 470–460Ma peraluminous granites were formed in the tectonic setting of the thickened lithospheric delamination following the final amalgamation of outboard Asian microcontinents onto the East Gondwana margin at ca. 490–475Ma. Our study favors that the long-lasted (ca. 500–450Ma) peraluminous granitic rocks in the Baoshan block constitute of the early Paleozoic magmatic belt along the East Gondwana marginal blocks like Himalaya, Lhasa, and Qiangtang. This magmatism was produced in successive stages, including proto-Tethyan slab rollback period to ca. 500Ma, slab break-off at ca. 500–490Ma, lithospheric thickening at ca. 490–475Ma, and lithospheric delamination at ca. 475–460Ma; and then it vanished at ca. 450Ma signifying the end of proto-Tethyan accretionary orogenesis.