Evolution of supracrustal rocks of the Indochina Block: Evidence from new detrital zircon U–Pb ages of the Kontum Massif, Central Vietnam

Abstract

The Kontum Massif and the Truong Son Belt, central Vietnam are the magmatic–metamorphic massifs (belts) of the Indochina Block. These two massifs (belts) underwent two independent orogenic events during Ordovician–Silurian and Permian–Triassic ages. However, due to the strong overprint of these two orogenic events, the evidence of any preexisting (e.g., Precambrian) tectono–thermal events have become extremely poor. Hence, the Precambrian age components of the Indochina Block have not been fully revealed, and their implication is not well–understood. It is well known that such ‘lost memories’ of the older continental rocks or source crustal materials are sometimes preserved in the sedimentary basins. Keeping that goal in mind, we have investigated the LA–ICP–MS detrital zircon U–Pb dating for three sedimentary and metasedimentary rocks in the Kontum Massif and southern part of the Truong Son Belt, Indochina Block, central Vietnam to unravel the Precambrian episodes of the Indochina Block, if any. Two Triassic (meta)–sedimentary rocks reveal two significant age clusters of latest Carboniferous to Triassic (~ 300–230 Ma) and Early Paleozoic (~ 480–410 Ma), with the conspicuous lack of the 400–320 Ma zircon grains. Abundant Permian detrital zircon grains in Triassic metaquartzite from the Kontum Massif document that the subduction–related magmatism before the continental collision played an important role in the growth of the Kontum Massif. In contrast to these two (meta)–sedimentary rocks, the detrital zircon spectra of metaquartzite from the Kontum Massif show the most significant age peak of ~ 1780 Ma and abundant Archean zircon grains (16%). Furthermore, the oldest detrital zircon shows the Paleoarchean age (²⁰⁷Pb/²⁰⁶Pb spot age of 3314 ± 22 Ma). The metaquartzite was metamorphosed in the Early Paleozoic (~ 414 Ma). The youngest detrital zircon grain yields the ²⁰⁷Pb/²⁰⁶Pb spot age of 1359 ± 85 Ma. Hence, the approximate depositional timing of the protolith of metaquartzite was in Mesoproterozoic time. The newly obtained detrital zircon data indicate a significant contribution of the Paleoarchean to Paleoproterozoic crustal materials during the deposition of the quartzite in the Kontum Massif. The Kontum Massif had developed as a different unit in the Precambrian period from the Truong Son Belt as well as the southwestern Yangtze Block due to their different Precambrian provenance. This study reveals a new clue that the Kontum Massif was one of the Precambrian blocks in the Southeast Asian massifs.