Abstract:
In Thailand, alkaline granitoid rocks are less widespread and crop out in less extend as small stocks or plutons. The study areas are located in 2 vicinities of Tha Takiap and Mae Yan areas which respectively are a part of the eastern and central granite belts. Petrographic study of the rocks from both areas show that alkaline feldspar is the principal mineral phase with small amount of quartz and plagioclase. Compositionally, the investigated rocks fall in the range of alkali feldspar syenite to quartz-alkali feldspar syenite. High concentration of Na2O+K2O and high ratio of FeOtot/(FeOtot+MgO) vs SiO2 indicate that most of the rocks from both areas are syenite and ferroan granite respectively. High ratio of Ga/Al and Y/Nb suggest that the rocks from both areas belong to A2 subtype of A-type granite category. These rocks originated from alkaline magmatism related to extensional tectonic setting. Flat chondrite-normalized REE pattern and positive anomaly of Ta on primitive mantle-normalized spider diagrams are resemble to those of Xiaolonghe granite, southwestern China. These similarities imply that the genesis of Tha Takiap syenitic rocks is related to back-arc extension tectonic setting and the magmatic source is mainly influenced by mantle-derived material. On the other hand, the enrichment of LREE on the chondrite-normalized REE patterns of Mae Yan syenitic rocks and positive anomalies of Th, U, and Pb as well as negative anomalies of Nb and Ta on the primitive mantle-normalized diagrams indicate that the dominant source of partial melting could be derived from continental crust related to subduction tectonic comparable to, post-collisional potassic rock in Xungba basin, southern Tibet.
