Late Paleozoic Tectonomagmatic Evolution of the Eastern Tianshan, Northwest China: Insights from Geochronology and Geochemistry of Volcanic Rocks from the Dananhu–Lop Nur Area

Abstract

The final closure time of the Paleozoic ocean in the Eastern Tianshan is debated, and the geotectonics are still poorly constrained; hence, research on late Paleozoic igneous rocks has important implications for the evolutionary processes involved in this area’s tectonic history. However, limited attention has been given to the volcanic rocks in the late Paleozoic strata across the Eastern Tianshan because of their rarity and identification difficulty. This article focuses on the key site of an accretionary orogen in Central Asia, the Kalatage (Shaerhu)-Dananhu–Lop Nur area, to promote the study of tectonomagmatic evolution. The interbedded volcanic rock layers in late Paleozoic strata, including volcanic edifice deposits, are mainly rhyolitic volcanic, andesitic/dacitic, and basaltic rocks. These felsic, intermediate, and mafic rocks have zircon laser ablation ICP-MS U-Pb ages of 362.5–303 Ma. Geochemically, all samples have similar primitive mantle–normalized trace-element spider diagrams and chondrite-normalized rare earth element plots. The felsic and intermediate volcanic rocks are classified as I-type rocks and formed in a volcanic arc, and the mafic rocks belong to the tholeiite series and formed in island arc (earlier) and intraplate (later) settings. By the time the mafic rocks with intraplate basalt characteristics formed at 303±13 Ma, the Paleo-Asian Ocean had completely closed, and the region had entered the intraplate evolution stage. On the basis of other findings regarding the ages of igneous rocks and synthetic analysis, the northward subduction of the Paleo-Asian oceanic slab can preferably explain the relationship between the distinctive tectonic environment and the age variation trend in the study area. The process generated subduction-, collision- and postcollision-related magmatism from the Devonian to the late Carboniferous, and after 310 Ma, the Kalatage-Dananhu–Lop Nur area synchronously underwent a tectonic transition from subduction and collision to an intracontinental environment.