Early Permian passive-margin magmatism in the Sibumasu Block, northern Thailand: A product of the slab-pull force of the Paleo-Tethys Ocean?

Continental rift-related magmatism provides important clues to continental rifting and breakup, but the petro-genesis and geodynamic processes remain debatable. Here we report a detailed study on zircon and apatite U-Pb dating, whole-rock geochemistry, and Sr-Nd-Hf isotopes for the hornblende gabbros in northern Thailand. Zircon and apatite U-Pb age data reveal emplacement of the hornblende gabbros at ca. 290 Ma. The hornblende gabbros are alkaline in composition and characterized by slight negative Nb-Ta-Zr-Hf and marked negative Ti anomalies. They have enriched Sr-Nd-Hf isotopic values [whole-rock (87Sr/86Sr)i = 0.705970-0.706756; epsilon Nd(t) = +0.1 to +0.7; zircon epsilon Hf(t) = +0.2 to +9.2]. Their geochemical affinities to within-plate basalts suggest a continental rifting setting. These combined features reveal that the hornblende gabbros in northern Thailand originated from a subcontinental lithospheric mantle previously modified by subducted sediments. Our results combined with published data indicate that the continental rift-related magmatism in the Sibumasu Block took place over a long period of ca. 309-282 Ma. We suggest that the slab pull force of the Paleo-Tethys oceanic lithosphere is a likely mechanism for the Late Carboniferous-Early Permian magmatic event in the passive margin of the Sibumasu Block.