Lithofacies, petrography, and geochemistry of the Lamphun oceanic plate stratigraphy: As a record of the subduction history of Paleo-Tethys in Chiang Mai-Chiang Rai Suture Zone of Thailand

Abstract

The Mélange in Lamphun Province, situated in the southern region of the Chiang Mai-Chiang Rai Suture Zone (CMCRSZ), remains inadequately explored. This study employs field investigations and petrographic analysis to delineate the characteristics of the CMCRSZ Mélange, identifying nine distinct rock units. These units include Carboniferous quartz arenite interbedded with fine-grained sedimentary rocks, Permian lithic graywacke, Permian limestone hosting Pseudodaliolina sp. and Schwagerinid sp., Triassic chert containing Anisian radiolarian fossils, and ocean island basalt. This area exhibits complex folding and thrusting, accompanied by granitic intrusions and low-grade metamorphic rocks within a finer-grained sedimentary matrix. Stratigraphic assessments trace the primary material of the accretionary complex to the upper strata of the oceanic plate stratigraphy. Geochemical analyses indicate the mafic rocks as alkaline to subalkaline basalts, indicative of formation through an alkaline magma series, a conclusion supported by rare-earth element patterns. Sr/Nd isotope classification suggests ocean island tectonics for these rocks. The formation of the CMCRSZ Mélange occurred during the Paleo-Tethys oceanic crust subduction beneath the Indochina Terrane at the Permian-Triassic Boundary, accumulating diverse materials in a dynamically evolving tectonic context. Furthermore, S-type Triassic granites emerged post-collision during the Indosinian Orogeny following the thrusting of the CMCRSZ onto Sibumasu.