Geochronological and geochemical constraints on volcanic and subvolcanic rocks of the Sukhothai Arc-Nan Suture-Indochina collisional zone and their tectonic implications

The tectonic assembly of Thailand via collision during the protracted Indosinian Orogeny and closure of the Paleotethys Ocean has generated a variety of geological interpretations. Although mapping of igneous rocks has been undertaken across the region, limited recent work has been undertaken to consider the role of magmatic events in the tectonic evolution of Thailand. This study focuses on establishing the composition, origin and age of volcanic and subvolcanic rocks across northern Thailand to evaluate current tectonic models.
This study presents the whole rock geochemistry, zircon U/Pb geochronology and oxygen isotopic analysis on a suite of basaltic andesite, andesite and rhyolite rocks along a west-east transect across northern Thailand covering the Sukhothai Arc, Nan-Uttaradit Suture zone and Indochina Terrane. Geochemically, the rocks are medium-K calc-alkaline, which is traditionally linked to a subduction zone origin. Trace element geochemistry shows characteristic signatures observed in modern subduction zone-arc settings. Samples from the western end of the transect in the Sukhothai Arc yielded U/Pb (zircon) ages of 259.6 ± 2.5 Ma (late Middle Permian) whereas ages for samples from the eastern Indochina Terrane range between 238.8 ± 3.8 and 232.5 ± 1.3 Ma (late Middle Triassic to early Late Triassic). The Middle to Late Triassic ages correlate with published work in the region, however, the Middle Permian age is significantly older. This suggests a longer and more complex history for the magmatic arc than previously recognised. Oxygen isotopes have provided new insights into the subsurface processes and show an increasing trend in crustal recycling along the transect. The Sukhothai Arc indicates a mantle origin, whereas the Loei region of the Indochina Terrane indicates long term mixing of mantle-derived melts within the crust.
The geochemical data and petrographic work presented in this study supports the existence of an east-dipping subduction zone beneath the thinned continental edge of Indochina during the Middle to Late Triassic. The 260 Ma age of the Lampang (arc) sample, approximately 20 to 40 Myr older than other volcanic rocks dated in the region, indicates that mantle-derived magmatism was occurring as early as the late Middle Permian.