Triassic Tectonic Affinity to Indochina-East Malaya Block for West Sumatra and Paleo-Tethys Implications: Constraints From Late Triassic Igneous Rocks

Abstract

Mesozoic igneous rocks from Sumatra provide crucial insights into the temporal-spatial connection with the Eastern Tethys Domain. This study presents new zircon ages and whole-rock elemental data for the Late Triassic igneous rocks in West Sumatra. These igneous rocks yield consistent ages of 227–211 Ma. Granitoid and rhyolite exhibit A-type affinities with high K₂O and FeOt contents, and are depleted in HFSEs. They have similar ε_Nd (t) values of −8.3 to −5.0, ε_Hf (t) values of −8.0 to +1.4, and initial ratios for ²⁰⁶Pb/²⁰⁴Pb (18.71–19.17), ²⁰⁷Pb/²⁰⁴Pb (15.74–15.77) and ²⁰⁸Pb/²⁰⁴Pb (39.28–39.48). They were derived from an “ancient” meta-mafic source with a juvenile mafic crust component. Mafic-intermediate igneous rocks can be classified as medium- to high-K calc-alkaline series and are characterized by depletion in Nb, Ta, and Ti. Mafic-intermediate volcanic rocks show higher ε_Nd (t) values of −0.7 to +1.3 and ε_Hf (t) values of +3.5 to +4.2 than the diabases with ε_Nd (t) of −9.0 to −8.5 and ε_Hf (t) of −5.5 to −1.2. All these igneous rocks share similar initial ratios for ²⁰⁶Pb/²⁰⁴Pb (18.62–18.84), ²⁰⁷Pb/²⁰⁴Pb (15.68–15.75) and ²⁰⁸Pb/²⁰⁴Pb (38.67–39.05), suggesting a metasomatized lithospheric mantle origin. These Late Triassic igneous rocks share similar isotopic compositions with the synchronous post-collisional igneous rocks along the Eastern Paleo-Tethys giant igneous belt and were formed in a post-collisional extensional setting. Regional stratigraphy and detrital zircon observations indicate that West Sumatra constitutes the southern part of the Indochina-East Malaya Block, and the Eastern Paleo-Tethys zone likely extends into Sumatra.