Petrotectonics of plutonic rocks in the eastern Mae Chan area, Chiang Rai, northern Thailand

Abstract

The plutonic rocks in Doi Pha Rua and Doi Sak, the eastern part of Tha Khao Pluek Sub-District, Mae Chan District, Chiang Rai Province, exhibit compositions ranging from felsic to mafic rocks. Petrography and geochemistry are essential tools for classifying rocks into four magmatic groups. Group I is monzogranite, granodiorite, and tonalite, with titanite as a minor constituent. They are peraluminous and have medium-K transitional to high-K calc-alkaline affinities that show both I-type and S-type. Their N-MORB-normalized multi-element patterns exhibit LILE enrichment and a negative Nb anomaly, which are typical of magmas formed at active continental margins. Group II is tonalite, with titanite as a minor constituent. They are peraluminous, tholeiitic series, characteristic of I-type granite. Group II does not exhibit a negative Nb anomaly in N-MORB normalized multi-element patterns and might have occurred in a post-collision environment. Group III is cumulate gabbro and tholeiitic series. Their chondrite-normalized REE patterns show positive Eu anomalies. Group IV is microgabbro and has chondrite-normalized REE and N-MORB normalized multi-element patterns that are very similar to those of Group I. Groups I and II are informative to tectonic environments of formation, while Group III and IV are cumulative (not represent magma) and isotropic, respectively. One Group IV sample is meaningless for interpretation. The Group I is volcanic arc granite, and Group II might have been post-collision granite. The Mae Chan granitic pluton is a part of Eastern Granitic Belt (EGB) and represents magmatism along the boundary between the Sukhothai Arc and the Inthanon Zone.