New Geochronological Results from Glaucophane bearing Metabasalts and Metadacites from the Nan - Uttaradit mafic-ultramafic complex, NE-Thailand

The Nan – Uttaradit mafic-ultramafic complex, associated with which are two outcrops of epidote blueschists, forms the linear core of the Nan back-arc basin. We have also found, as float, higher grade blue amphibole – garnet gneisses and white mica – garnet schists, from which we report newly obtained U-Pb zircon and allanite protolith ages, with K/Ar metamorphic ages from phengitic white micas.

The two outcrops of epidote blueschists are some 130 km apart; in the stream Huai Sak, 20 km east of Nan Noi town, and along a mountain ridge just north of highway 102, some 15 km west of Uttaradit city. The gneiss float samples were found in the stream Huai Phi Rong, 1 km east of Huai Sak, and on point bars of the Wa river east of Mae Charim town.

The blueschists, commonly retrogressed to greenschists, have the mineral assemblage Gln/Rbk/Act – Ep – Chl – Ph – Ab – Qz ± Ttn ± Rt ± Ilm ± Hem. Whole-rock geochemistry points towards basic igneous protoliths of tholeiitic affinity. The gneisses are coarse grained, with garnets up to 1 cm diameter. They have mineral assemblages Grt + Gln/Rbk/Win + Ep + Ph + Chl + Qz ± Stp ± Ap ± Ttn ± Rt ± Zrc. Geochemistry indicates dacite to andesite protoliths of calc-alkaline affinity.

Zircons large enough to analyse have been found only in the gneisses and garnet – white mica schists. They are euhedral to subhedral grains, 30 to 100 μm in length, with magmatic oscillatory zoning. U–Pb isotopic compositions of zircons from 11 samples were obtained using LA-(MC)ICP-MS. There are no indications of metamorphic rims, with all ages in the range 330 to 310 Ma. One sample also contained an older cluster around 360 Ma. Allanite, of magmatic origin, occurs in metabasites and gneisses as euhedral to subhedral grains, 100 to 400 μm in length, some with metamict cores and patchy zoning. U-Pb analysis by LA-MC- ICP-MS constrains their ages to 340 – 320 Ma, in good agreement with the zircon dates.

To determine the age of the HPLT event that affected these rocks, white micas and amphiboles were separated from five samples for K/Ar dating. Mineral inhomogeneity means that no reliable ages were obtained from the amphiboles, which will now be dated using the Ar/Ar method. However, two phengitic white mica samples gave consistent ages of ~327 and ~317 Ma.

It is concluded that subduction of the Nan basin was ongoing by the mid Carboniferous, with some igneous rocks being subducted very soon after emplacement. Further, if the Nan basin is indeed a back arc basin formed by rifting off the Sukhothai terrane from Indochina, then the precursor volcanic arc must have been formed at least in the Early Carboniferous and more likely in the Late Devonian. It is notable that the subduction of the Nan basin began at least some 100 my before the first recognized events of the Indosinian orogeny, which occurred around the end of the Middle Triassic.