Cambrian mafic and granitic intrusions in the Mazar-Tianshuihai terrane, West Kunlun Orogenic Belt: Constraints on the subduction orientation of the Proto-Tethys Ocean

Abstract

The West Kunlun Orogenic Belt (WKOB) carries key information about the evolution of the Proto-Tethys Ocean. In order to have a better understanding of the early Paleozoic tectonic evolution of the WKOB and to unravel the process of the Tarim assemblage to Gondwana, we report field observations, petrography, ages, chemical and Sr-Nd-Hf isotopic compositions of the newly identified Cambrian mafic and granitic intrusions in the Mazar Terrane-Tianshuihai terrane (MZT-TSHT) at the interior of the WKOB. Our zircon U–Pb dating reveals that the mafic and granitic rocks in Tianshuihai Terrane (TSHT) emplaced at ca. 530 Ma, and that the mafic rocks in the Mazar Terrane (MZT) emplaced at ca. 500–490 Ma. Whole-rock major and trace element geochemistry defines a tholeiitic signature for these mafic rocks. Isotopically, the TSHT mafic rocks have depleted Nd isotope compositions with initial εNd(t) isotopes ranging from 2.74 to 3.60, whereas the MZT mafic rocks exhibit enriched whole-rock Nd isotope compositions with εNd(t) values ranging from −12.90 to −9.71. The distinct chemical and isotopic signatures argue that the Cambrian mafic rocks in the TSHT were derived from a depleted mantle source in an initial subduction setting, whereas the mafic rocks from MZT were derived from an enriched sub-continental lithospheric mantle source in an arc setting. The Cambrian granites in the TSHT show typical fractionated I-type granite elemental signatures, in line with their Nd-Sr-Hf isotope compositions. These Cambrian granites were most likely generated by partial melting of Mesoproterozoic (0.9–1.3 Ga) crustal sources due to underplating of the coeval mafic magma. Our results in combination with previous studies suggest that the Cambrian mafic and granitic rocks in the MZT-TSHT were generated by the southward subduction of the Proto-Tethys Ocean initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive early Paleozoic accretionary wedge in WKOB. The Proto-Tethys Ocean finally closed at ca. 440 Ma as demonstrated by the amphibolite facies metamorphism of the accretionary complex and the middle to late Devonian molasses unconformably overlying the accretionary complex. This process caused the blocks in East Asia, including the Tarim, Qaidam and North Qilian, Yangtze and Indochina, to dock at the northern fringe of the Eastern Gondwana.