Petrology and Geochemistry of intrusive igneous rock from the Inthanon zone, Northwestern Thailand

Large plutons are common within the Inthanon Zone in Northwestern Thailand. These igneous rocks are also known as Central Granitoids Belt in mainland Southeast Asia. They are interpreted to be part of the suture zone between Sibumasu and Indochina and were emplaced mainly in the Upper Triassic.

Here, we present new petrological and geochemical data for the Central Granitoids Belt.  A geochronological study on selected samples will follow.  The sampled granitoids can be separated into three groups: (1) biotite granite, (2) hornblende granite, (3) syenite/monzonite. The samples consist of various light colored to dark grey granitoids due to the type and amount of mafic minerals (biotite or hornblende) present. The general mineral assemblage of all the intrusive igneous rocks is quartz + plagioclase + K-feldspar + biotite + apatite + zircon ± allanite ± titanite ± ilmenite. The biotite granites are mostly composed of biotite aggregates associated with accessory minerals: zircon, ilmenite, and apatite. The syenite/monzonite group usually contains additional clinopyroxene and hornblende. Plagioclase and hornblende of the syenite/monzonite group commonly exhibit a sieve texture.

The biotite granite group is typically peraluminous and belongs to the high-K calk-alkaline to shoshonitic series. The hornblende granite group is mostly peraluminous and of predominantly shoshonitic affinity. The syenite/monzonites are typically metaluminous but also belong to the shoshonitic series. The chondrite normalized rare earth element (REE) patterns are quite similar for all igneous rocks with elevated LREE, pronounced negative Eu anomaly and a flat HREE segment. The granite tectonic discrimination plots after Pearce et al. (1984) classify most samples as syn-collision granites (syn-COLG) and when using the Batchelor and Bowden (1985) discrimination diagram as syn-, late, and post-collisional.

The intrusive igneous rocks from Northwestern Thailand were presumably emplaced in a syn- to post-collisional setting when the Sibumasu block collided with the Sukhothai terrane and was eventually amalgamated to the Indochina block. This led to the closure of the Palaeotethys along the eastern area of the Sibumasu block.

Batchelor, R.A. and Bowden, P. (1985) Petrogenetic Interpretation of Granitoid Rock Series Using Multicationic Parameters. Chemical Geology, 48, 43-55.

Julian A Pearce, Nigel BW Harris, Andrew G Tindle (1984). Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25, 956-983.