Three episodes of Precambrian mafic magmatism in the southern Central Tianshan Block (NW China): Insight into an evolving geodynamic model

Abstract

Precambrian microcontinents are an essential component in the tectonic evolution and architecture of the Central Asian Orogenic Belt (CAOB). However, the early crustal evolution and origin of Precambrian microcontinents remain poorly constrained due to the scarcity of early Precambrian rocks. The Chinese Central Tianshan Block (CTB) constitutes a microcontinent in the western CAOB with Precambrian basement rocks along its southern margin. The basement rocks, comprising amphibolite-facies supracrustal assemblages of possible volcano-sedimentary origin, are exposed in the Wuwamen Complex, which occurs as tectonic slivers in the late Paleozoic Wuwamen ophiolitic mélange. Here we present in situ zircon U–Pb and Lu–Hf isotopic data and the whole-rock geochemistry of three suites of (meta-)mafic rocks from the Wuwamen Complex and an associated Neoproterozoic gabbro body. The zircon dating revealed that the Wuwamen Complex contains two suites of meta-mafic rocks with protolith formation at ca. 2.55 Ga and 2.21 Ga followed by an amphibolite-facies metamorphic overprint at ca. 1.83 Ga. In addition, an individual gabbro body adjacent to the Wuwamen Complex yielded an emplacement age of ca. 0.69 Ga, reflecting a mid-Neoproterozoic mafic magmatic event in the CTB. Furthermore, zircon U–Pb dating of a crosscutting diorite dyke and a gneissic diorite sourced from a composite batholith in the CTB yielded weighted mean intrusive ages of ca. 320 Ma and 454 Ma, respectively. In conjunction with the field observation, the emplacement ages suggest that the Wuwamen Complex represents a part of the continental arc basement of the CTB. The three suites of (meta-)mafic rocks show geochemical characteristics similar to typical arc-related basalts. Along with the evolved zircon Hf isotopic compositions, it is suggested that these rocks originated from partial melting of a metasomatized sub-continental lithospheric mantle during different events. In conjunction with previous studies, the here presented new data further suggest that the CTB and other microcontinents in the western CAOB record similar Precambrian magmatic and metamorphic episodes, which are comparable to those of the northern Tarim during Proterozoic times. The here outlined new model indicates that the microcontinents of the western CAOB and the Tarim Craton may have shared a common geodynamic evolution.