Late Triassic post-collisional granites related to Paleotethyan evolution in northwestern Lao PDR: Geochronological and geochemical evidence

Abstract

The late Paleozoic to early Mesozoic granites exposed in northwestern Lao PDR provide important constraints on the tectonic evolution of the Eastern Paleotethyan Ocean and regional correlation with the giant granitic belt in Southeast Asia. New geochronological data show that the granites have Late Triassic zircon U–Pb ages of 231–220 Ma. They are dominated by monzogranite and biotite granite with an I-type geochemical affinity. These granites are enriched in LREEs and LILEs and depleted in HFSEs. The geochemical variations for these granites indicate the fractional crystallization of plagioclase, K-feldspar, biotite, apatite, and Fe–Ti oxides. Their initial 87Sr/86Sr ratios range from 0.7021 to 0.7105 and εNd (t) values from −1.6 to −7.3. Zircon in-situ εHf (t) values are in range of −6.0–+5.3 (peaks at −2.8 and +2.5, respectively), Hf model ages of 0.67–1.64 Ga (peaks at 0.83 Ga and 1.45 Ga, respectively), and δ18O values of 6.6‰–8.0‰, similar to the Late Triassic granitoids from the Eastern Province in Southeast Asia. These granites originated from a mixed source of ancient metamorphic rocks with juvenile mafic crust. The Late Triassic granites in northwestern Lao PDR formed in a post-collisional setting in response to the thickened crustal collapse during the assembly of the Sibumasu with Indochina blocks. These granites can southerly link with the Eastern granite province of the Eastern Paleotethyan Domain in Southeast Asia.