The Tak Batholith, Thailand: the evolution of contrasting granite types and implications for tectonic setting

Abstract

The Tak Batholith, Thailand, is made up of four zoned plutons, the youngest of which is 210 Ma old. The composition of the plutons changes from granodioritic in the oldest, through monzonitic and monzogranitic to syenogranitic in the youngest. The earliest pluton (Eastern) shows an extended compositional range and is calc-alkaline-granodioritic, medium K in the sense of Lameyre and Bowden. It is similar to classic Andean Cordilleran Batholith plutons. The three later plutons (Western, Mae Salit and Tak), the first of which also shows an extended compositional range, are all calc-alkaline- monzonitic, high K plutons characterised by the early precipitation of U, Th, REE rich accessory minerals and K-feldspar. In this sense they are very different from the Eastern pluton, in which the dominant early felsic phase crystallising is plagioclase and accessory mineral precipitation tends to be late. The changes in major and trace element composition of the plutons with time are similar to that seen in an increasing K volcanic series at an active continental margin. Indeed, the final member is chemically similar to shoshonitic volcanic rocks and by analogy the plutonic sequence is considered to be due to a change from subduction to strike-slip and then uplift, which brought mantle of different composition into the source region beneath the Tak Batholith. In such situations simple classifications relating a batholith to end-member source and geotectonic setting may be misleading.