Blueschists and blue amphibole gneisses in the Nan suture zone, NE Thailand

The Nan River mafic-ultramafic belt was identified when detailed geological mapping of NE Thailand began in the 1970s, and suspected to represent a suture zone. However, in the absence of an obvious ophiolite, its tectonic status was not confirmed until two short papers (Barr et al., 1985 and Barr & Macdonald, 1987) reported the discovery of associated  blueschists. Unfortunately military restrictions on access to detailed topographic maps meant they that they did not state an exact location and the outcrops were “lost” to Thai geologists and no further research was conducted. The two “lost” blueschist localities (south of Nan and west of Uttaradit) were recently re-discovered and related winchite – barroisite schist units identified. Additionally, garnet – glaucophane/riebeckite – white mica – quartz – magnetite – titanite – rutile ± albite ± stilpnomelane bearing gneisses were found among the bedload of a stream cutting through these schists. These gneisses are believed to be derived from “exotic” blocks in a mapped, but poorly exposed thrust sheet of tectonic melange, but to date no in place examples have been found. Similar blueschists/greenschists, gneisses and related garnet – white mica schists have been found further north as cobbles on point bars of the Wa river (west Nan), which cuts through a different section of the mafic – ultramafic unit in a mountainous and inaccessible national park.

At both in-place blueschist locations the schists have undergone two episodes of deformation, producing well developed schistosities and tight folding. The blue amphiboles are crossitic in composition. They do not contain garnet nor lawsonite, but abundant epidote and white mica with elevated phengite content. They are interbanded with winchite – barroisite bearing schists. The observed mineral assemblages are poorly suited to apply well established geothermobarometers. However, a PT window of the metramorphic overprint could be established with ca. 450 to 550 °C and 0.6 to 1.0 GPa. Geothermobarometry of the blue amphibole and garnet bearing exotic gneisses from the first blueschist locality (south Nan) indicates peak T conditions of ca. 550°C and a max. P of ca. 1 GPa. Comparable blue amphibole and garnet bearing gneisses from the second locality (Wa river) indicate similar peak PT conditions.

In-situ U-Pb zircon analyses from 6 blue amphibole – phengite bearing gneiss samples gave weighted mean ²⁰⁶Pb/²³⁸U dates ranging from 312 to 326 Ma, which is interpreted as the age of the protolith. Accessory phases within the blueschists and gneisses include variously zircon, titanite, rutile, allanite and monazite. Planned analysis of these phases should provide the age of HP/LT metamorphism.