Arc-like volcanic rocks in NW Laos: Geochronological and geochemical constraints and their tectonic implications

摘要

The Loei Fold Belt, located at the western margin of the Indochina Block, is considered to be tectonically linked to the subduction of the Paleotethyan Ocean and subsequent collision. In northwestern Laos, the volcanic rocks are traditionally mapped as Permian-Early Triassic sequences. Our LA-ICP-MS zircon U-Pb dating results show that andesitic, rhyolitic and tuffaceous samples from the Muang Feuang region of the Loei Fold Belt have zircon U-Pb ages of 330.4 ± 2.2 Ma, 334.9 ± 1.7 Ma and 349.6 + 1.7 Ma, which are interpreted as their eruption ages, respectively. These ages are indicative of an Early Carboniferous origin. The basaltic and basaltic-andesitic samples from the Pak Lay region yield zircon U-Pb ages of 314.6+ 2.7 Ma and 315.4 ±3.8 Ma, respectively, and have a Late Carboniferous origin. These samples exhibit depletion in HFSEs (e.g., Nb, Ta, Ti) with high LILE/HFSE ratios and exhibit a geochemical affinity to continental arc volcanic rocks. These signatures suggest the presence of an Early to Late Carboniferous active continental margin along the western margin of the Indochina Block. The Loei Fold Belt may extend north into NW Laos.