Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China

Abstract

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO₂ contents (1.47% -3.59%) and low MgO (3.52% -7.88%), K₂O (0.12% -1.21%). Large ionic lithophile elements (LILEs) (Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO₂ ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of ⁸⁷Sr/⁸⁶Sr (I_Sr) (0.7052-0.7097) and ε_Nd(t) (-0.70 to -5.35), while the Pb isotopic compositions with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of 16.908-17.982, 15.487-15.721, 37.276-38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.

Keywords

Mafic rocks / Sugetbrak Formation / Rodinia supercontinent / Neoproterozoic / Sr-Nd-Pb isotopes / Mantle plume / Geological survey engineering / Tarim Block / Xinjiang

CRediT authorship contribution statement

Xiang-kun Zhu conceived of the presented idea. Hong-zhe Xie, Xun Wang, Yuan He, Wei-bing Shen wrote the manuscript in consultation. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgement

This work was funded by the Key Laboratory of DeepEarth Dynamics of Ministry of Natural Resources (J1901-201), and China Geological Survey Program (D20090002). The authors are grateful to Dr. Zhao-fu Gao and Dr. Qiu-hong Xie and three anonymous reviewers for their constructive comments and suggestions which substantially improved this work in science and presentation.

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