Carboniferous-Triassic felsic igneous rocks and typical mineral deposits in the Truong Son orogenic belt, SE Asia: Implications for Paleo-Tethyan tectonic evolution and metallogeny

Abstract

The Truong Son orogenic belt (TSOB) is part of the eastern Tethyan mineral deposit domain, and preserves a near-complete record of Paleo-Tethyan orogeny-related magmatism and mineralization. By compiling and evaluating the data of plutonic and (sub)volcanic rocks and associated deposits on a regional scale, and combining with new data of typical igneous rocks and mineral deposits, we establish a unified Paleo-Tethyan evolutionary model to help understanding the metallogeny in the TSOB. The geochronological data reveal the felsic magmatism and associated mineralization occurred in Carboniferous–Triassic (ca. 317–202 Ma) with a magmatic lull at ca. 270–263 Ma. Before the lull, most igneous rocks are high-Mg#, sodic, metaluminous I-type granitoids (or volcanic equivalents) with depleted Hf isotopes, which were derived from a juvenile crustal source with significant mantle contribution. After the lull, the igneous rocks (ca. 262–235 Ma) consist of metaluminous I-type granitoids and abundant S-type granitoids with geochemical features of low-Mg#, potassic, peraluminous and enriched Hf isotopes, suggesting their derivation from ancient continental crust without obvious mantle contribution. This magmatic lull corresponds to the tectonic transition from subduction to initial collision, which reasonably accounts for the changing of petrogenetic types and magma sources of the associated igneous rocks in the TSOB. The slightly depleted Hf isotopes of the ca. 234–202 Ma rocks possibly suggest another episode of tectonic transition in Late Triassic. The distinct mineralization styles and S isotopes of ore deposits of each episode indicate that mineralization was related to the tectonic evolution and magmatism in each stage. Here, we propose an evolutionary model for the Paleo-Tethys orogeny in the TSOB, as follows: (1) low-angle subduction beneath the TSOB during Late Carboniferous-Late Permian, favored the formation of juvenile crust-derived adakitic rocks and high-Mg# I-type felsic rocks as well as associated porphyry Cu ± Au, skarn Fe ± Cu ± Au and epithermal Au–Ag mineralization; (2) subsequent collision during Late Permian-Middle Triassic not only accounted for the generation of Precambrian crust-derived I-type granitoids and intrusion-related Au ± Ag deposits, but also resulted in the formation of low-Mg# S-type rocks, minor volcanic rocks and associated porphyry and/or skarn Sn ± Zn ± Pb deposits; (3) possible post-collisional extension during Late Triassic may be favorable for the formation of metaluminous, high-K (±high Mg#) I-type granitoids and volcanic rocks, and Carlin Au, porphyry and/or skarn mineralization.