P–T–t constraints on the Inthanon metamorphic core complex and implications for the evolution of the Western Gneiss Belt, northern Thailand

The Inthanon metamorphic core complex in northern Thailand comprises gneisses, schists, migmatites and calc-silicates which are intruded by a variety of granitoids of different age. New P-T estimates, U-Th-Pb total age and U-Pb age data of monazite indicate a multi stage history of the core complex.

Garnet mica-schists and gneisses are exposed in the western part of the Inthanon metamorphic core complex. Garnets show two episodes of growth, with some displaying garnet breakdown and corona formation containing plagioclase, quartz, biotite, and muscovite. The garnet core (grt1) records medium to high-grade metamorphism in the Late Triassic to the Early Jurassic. The second garnet growth (grt2) can be distinguished by a significantly lower Ca content and formed during a high-grade metamorphic event in the Late Cretaceous. Both garnet generations contain abundant inclusions of biotite, muscovite, and monazite, which are used for the reconstruction of the P-T-t history. The monazite included in grt1 yields an age of ~230 Ma. Metamorphic conditions during the first episode of garnet crystallization are 0.7–0.8 GPa at 530–570 °C. The second garnet growth occurred in the upper amphibolite facies at pressures of 0.4–0.5 GPa and temperatures of 640–670 °C. The monazite inclusions in grt2 and monazite within the garnet coronae yield an age of about 80 Ma. The monazite in the matrix, which exhibits complex chemical zoning indicative of recrystallization and re-precipitation during multiple stages of metamorphism, yields a main population at 230 Ma and a typical lead loss trend to a few clusters of 80 Ma dates.

Orthogneiss domains are characterized by a mylonitic texture with large K-feldspar augen and porphyroclasts surrounded by a fine-grained, foliated matrix of quartz and feldspar. This domain is mainly exposed in the core zone of the Inthanon complex and tends to become more mylonitic towards the east. Temperature conditions of 650–700 °C and 0.4 to 0.7 GPa were calculated by pseudosection modelling and using the Ti-in biotite-geothermometer. The monazite texture from the orthogneiss domain displays patchy zoning indicative of several resorptions and reprecipitations. The monazite exhibits an older age range of 230–210 Ma, with younger clusters yielding ca. 75 Ma, and shows a lead loss trend to ca. 30 Ma. The orthogneiss domain is extensively injected by concordant foliated biotite-garnet leucogranite with a monazite U-Pb age around 40 Ma. This age is considered as the upper age limit for the early stages of ductile shearing in the core complex.

Our data indicate that the Western Gneiss Belt in Thailand underwent several tectono-metamorphic events: (1) a medium P-T regional metamorphic phase in the Late Triassic to the Early Jurassic related to Sukhothai-Sibumasu collision (~200 Ma), followed by (2) a widespread younger overprint at upper amphibolite facies in the Late Cretaceous connected with local plutonic activity 75‑65 Ma. (3) Local Late Eocene–Oligocene magmatism and Chiang Mai basin development within deformation zones led to the emplacement of orthogneisses and local metamorphic overprint as seen e.g. in the Mae Ping and Three Pagoda shear zones.