Cambrian–Ordovician Arc-Related Magmatism in the Central Southeast Asian Continents and Its Significance on Early Palaeozoic Tectonics of the Indochina Block

Abstract

Knowledge of the early Palaeozoic tectonic evolution of the Indochina Block is critical for reconstructions of the Gondwana supercontinent in relation with other microcontinents. In the central Indochina Block, the Early Palaeozoic Tam Ky–Phuoc Son Suture Zone (TPSZ) and Po Ko Suture Zone (PKSZ) demarcate the boundary between the Truong Son Belt in the north and Kon Tum Massif in the south. In the southern parts of Truong Son Belt, we have identified meta-mafic and meta-intermediate rocks associated with plagiogranite, metapelitic and meta-siliceous rocks. Here we present integrated field, geochemical and geochronological results of the meta-gabbroic and amphibolitic samples in the southern Truong Son Belt to unravel the tectonic evolution of the Indochina Block during the Early Palaeozoic. The MC–LA–ICP–MS zircon U–Pb ages indicate magmatism at ca. 504–460 Ma. Based on geochemical composition, the samples range from gabbro to gabbro-diorite. The gabbroic samples have slightly higher Mg# (58.9–62.8) than those of the gabbro-dioritic samples (Mg# = 56.3–61.5), and both types of rock show low TiO₂ contents (0.68–0.98 wt%), displaying enriched LREE ((La/Sm) _N  = 1.66–3.23) and LILE (e.g., Rb, Ba, Th and U), and pronounced negative HFSE (e.g., Nb, Ta and Ti anomalies) with low εHf(t) zircon values (from −6.34 to +0.71). These geochemical characteristics suggest that the metabasic rocks are of subduction-related arc magmatic processes, and also indicate infiltration of sediment melt from subducting slab to mantle-wedge prior to its partial melting. The new data suggests that northward subduction under the Truong Son Belt might have initiated during the Early Cambrian, and lasted until around the Middle Ordovician. On a regional scale, the ages of the Early Palaeozoic arc magmatism in the Truong Son Belt are comparable with the ages of tectonic events that occurred in Himalaya, West Qiangtang and East Tibet at the northern margin of the Gondwana supercontinent.