Petrochemical study of post-Triassic basalts from the Nan Suture, northern Thailand

Abstract

Mafic-ultramafic rocks along the Nan Suture, a continental suture between Shan-Thai and Indo-China cratons, include ocean-island basalts, backarc basin basalts and andesites, island-arc basalts and andesites, supra-subduction cumulates, and continental intraplate basalts. The first four compositional groups formed in the Carboniferous to Permo-Triassic, prior to the Late Triassic continental collision, whereas the other erupted in post-Triassic time, possibly the Cenozoic. The post-Triassic lavas form a discontinuous narrow belt, disconformably overlying metagabbros/amphibolites and serpentinite melange. They are much less deformed and altered relative to the older rocks. The post-Triassic lavas are evolved mildly alkalic rocks, characterized by SiO 2 = 51.2–58.0 wt %, MgO = 3.4–6.3 wt%, Nb/Y = 0.8–1.0 , Ti/V > 50, ( La/Yb) n = 7.4–11.6 , and FeO, TiO 2 and V depletion with progressive fractionation. They may be classified as hawaiite, mugearite and benmoreite with Na 2 O/K 2 O = 1.8–2.4 . They are phyric with olivine (Fo 68.0–83.6), calcic clinopyroxene ( mg# = 0.70–0.86 , plagioclase, magnetite and ilmenite phenocrysts and microphenocrysts embedded in fine-grained matrix made up mainly of plagioclase, clinopyroxene and olivine. Orthopyroxene ( mg# = 0.65–0.70) occurs as an additional microphenocryst phase in the highly evolved samples. These basalts are chemically broadly comparable with alkalic basalts such as those erupted in the postshield stages of Haleakala in the Hawaiian chain, and are interpreted to have erupted in continental environment as the Late Cenozoic basalts in mainland SE Asia.