LT-HP metamorphic rocks from the Nan River suture zone, Thailand

This petrological study involves a detailed examination of blueschists and other LT-HP metamorphic rocks which were tectonically emplaced within the mafic – ultramafic belt of the Nan River suture zone of northern Thailand. The blueschists do not contain garnet, lawsonite, jadeite or pumpellyite and, therefore, are assigned to the epidote – blueschist subfacies. Associated meta-cherts sometimes contain piemontite or bright blue winchite. Thrust sheets of blueschists interlayered with greenschists occur in two localities: one large outcrop is patchily exposed west of Uttaradit city, the other 135 km to the north along two stream sections on the southern flank of Doi Phuk Sung. In addition, rather rare float samples of garnet – white mica ± blue amphibole gneisses have been found in one of the Doi Phuk Sung stream sections and on three different point bars of the Wa River, ca. 40 km further to the north. These gneisses are given the field term “exotics” as their outcrop is unknown and obviously of higher metamorphic grade than any other rocks exposed in the region. The blueschists contain the mineral assemblage Amp (glaucophane – riebeckite, barroisite – winchite) + Ep + Wm + Chl + Ab + Qz ± Ttn ± Rt ± Zrn. The greenschists are typically composed of Amp (actinolite, barroisite) + Chl + Ep + Ab + Qz +Wm ± Ttn ± Rt ± Hem. The “exotics” fall into three groups based on their mineralogy: (1) Grt – Wm + Ab + Chl + Qz ± Stp ± Ep ± Ttn ± Aln ± Zrn ± Ap; (2) Amp (hornblende) – Grt – Wm + Ab + Chl + Qz ± Stp ± Ep ± Ttn ± Aln ± Zrn ± Ap; and (3) Amp (glaucophane – riebeckite, barroisite, winchite) – Grt – Wm + Ab + Chl + Qz ± Stp ± Ep ± Ttn ± Aln ± Zrn ± Ap. The blueschists and greenschists from both localities show identical history of 3 phases of deformation and relative timing of mineral growth. Evidence of D1, preserved as rootless isoclinal folds of quartz rich seams, was almost entirely destroyed by the development of an intense pervasive schistosity during D2, associated with tight to isoclinal folds. D3 produced a crenulation cleavage with little recrystallisation. The peak P-T conditions of blueschists were estimated by constructing pseudosections, indicating temperatures of approximately 450 to 500 °C and pressures of 0.8 to 1.2 GPa. The exotics preserve direct evidence of gneissic segregation banding and a single deformation event. In many cases amphiboles are visually zoned with green cores and violet – blue rims. This is interpreted to indicate that they originated in the greenschist/amphibolite facies and then passed into the blueschist/eclogite facies. Garnets are generally fractured and fragmented and usually display strong prograde compositional zoning where Xprp and Xalm are increasing, Xgrs and Xsps decreasing from core to rim. Peak P-T conditions were estimated by using the Grt-Amp thermometer and calculated pseudosections with temperatures of approximately 550-600°C and pressures of 0.8 to 1.2 GPa.