Geochronological and geochemical constraints on the mafic rocks along the Luang Prabang zone: Carboniferous back-arc setting in northwest Laos

Abstract

The key to gaining a better understanding of Paleotethyan tectonic evolution is to determine the tectonic setting of the main suture/tectonic zones and their relationships in SE Asia. The Luang Prabang tectonic zone in NW Laos lies in the area that aligns with the Jinshajiang–Ailaoshan suture zone to the north and the Nan suture zone to the south. Correlations among them have been debated. Samples from a diabase dyke and coarse-grained basalt in the Luang Prabang tectonic zone yield zircon U–Pb weighted mean ages of 335.5±3.3Ma and 304.9±3.9Ma, respectively, suggesting that these mafic rocks formed in the Carboniferous rather than in the Permian, as previously inferred. The zircon in-situ εHf (t) values range from 11.6 to 13.3. Their initial 87Sr/86Sr ratios range from 0.70318 to 0.70522, and εNd (t) ranges from +5.16 to +6.93. The diabase samples have E-MORB-like characteristics with an addition of a subduction-related component, and the basalt samples were derived from the same type of source as N-MORB magmas newly modified by a slab-derived component. The ages of the mafic rocks along the Luang Prabang tectonic zone are similar to the mafic rocks from the Jinshajiang–Ailaoshan, Song Ma and Nan suture zones in SE Asia. A synthesis of available data suggests the development of a Carboniferous continental back-arc basin separating the Sukhothai Terrane and the Indochina Block. The Luang Prabang tectonic zone represents a remnant of a series of synchronous back-arc basins to the northeast of the Paleotethyan Main Ocean. It is proposed that the Luang Prabang tectonic zone links with the Nan suture zone to the southwest and with the Jinshajiang–Ailaoshan suture zone to the north.