Occurrence of the high grade Thabsila metamorphic complex within the low grade Three Pagodas shear zone, Kanchanaburi Province, western Thailand: Petrology and geochronology

摘要

Within the NW-SE trending Three Pagodas shear zone, which is a result of the India-Eurasia collision, a narrow lenticular basement slice of high grade metamorphic rocks, called Thabsila metamorphic complex, is exposed. The Thabsila metamorphic complex is juxtaposed by fault contacts to the very low to non-metamorphic rocks of the shear zone. The high grade basement slice can be subdivided into four units based on lithology: (1) Unit A is composed of marble, mica schist and quartzite, (2) unit B comprises mylonites, (3) unit C is composed of calcsilicate, and (4) unit D comprises various varieties of gneisses. Classic geothermobarometry and pseudosection calculations reveal a P-T variation among the four units. Unit A experienced medium amphibolite facies conditions of 550-650 ℃ and 5-6.5 kbar while units B, C and D experienced upper amphibolite facies metamorphism, around 640-710 ℃ and ～5.5-8 kbar. Age of metamorphism and the cooling history were constrained from LA-ICP-MS U-Pb zircon age and Rb-Sr biotite isochron age. Metamorphic zircon rims yield a metamorphic age of ～51-57 Ma, while Rb-Sr bio-tite cooling ages are ～32-36 Ma. These P-T-t data suggest that the Thabsila gneiss experienced peak upper amphibolite facies metamorphism around ～51-57 Ma during the early collision between India and Eurasia. Subsequently, whilst lateral southeastward extrusion of the Indochina terrane during Oligo-cene, it was exhumed due to strike-slip faulting along the Three Pagodas shear zone in the transtensional regime. The observed deformation stages Dl (constriction) and D2 (sinistral shearing) postdate the peak metamorphism and can be related to the exhumation stage. Final cooling of the basement rocks down to a temperature of ～350-300 ℃ is indicated by the biotite Rb-Sr ages at around 32-36 Ma.