Petrogenesis and tectonic implications of the neoproterozoic mafic-ultramafic rocks in the western Jiangnan Orogen: Insights from in situ analysis of clinopyroxenes

• Published in: Lithos 2021-07, Vol.392-393, p.106156, Article 106156
• Main Authors: Kou, Caihua, Liu, Yanxue, Li, Tingdong, Ding, Xiaozhong, Zhang, Heng, Liu, Yong
• Format: Article
• Language: English
• Subjects: In situ study of clinopyroxene; Origin of clinopyroxene; Parental magma; Source nature; Tectonic setting
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2021.106156

The Huaqiao clinopyroxenite and Maodu gabbro are Neoproterozoic mafic-ultramafic rocks located in the western Jiangnan Orogen. Both rocks contain fresh clinopyroxenes. The in situ analysis of fresh clinopyroxenes from these rocks provides essential information about their parental magmas and mantle sources. Here, we present in situ major and trace element compositions of clinopyroxenes from these rocks to determine their petrogenesis and tectonic setting. Clinopyroxene grains from the Huaqiao clinopyroxenite exhibit relatively high values of Mg# (80.6–84.1), Cr2O3 (0.38–1.00 wt%), TiO2 (0.44–1.84 wt%), Ni (324–460 ppm), V (314–611 ppm), and Co (37–46 ppm); the calculated Mg# values of the parental liquid(s) are 52–58 (average 55). Clinopyroxene grains from the Maodu gabbro display lower Mg# (71.5–79.1), Cr2O3 (0–0.88 wt%), Al2O3 (1.72–4.75 wt%), and Na2O (0.24–0.88 wt%) contents but higher TiO2 (0.88–1.89 wt%) contents than those from the Huaqiao clinopyroxenite; they show relatively high Cr (15–4145 ppm, with an average of 1774 ppm), V (34–663 ppm, with an average of 535 ppm), and Co (32–60 ppm) contents; the Mg# values of the parental liquid(s) are estimated to be 39–49 (average 45). Therefore, we infer that clinopyroxene grains from the Huaqiao clinopyroxenite and Maodu gabbro crystallized from evolved basaltic melts. Mineral-melt partition coefficients reveal that the estimated parental magma composition in equilibrium with clinopyroxene for the Huaqiao clinopyroxenite exhibits slight enrichments in light rare earth elements (LREEs, LaN/YbN = 1.12–2.71; subscript N: chondrite normalized) with flat heavy REEs (HREEs, GdN/YbN = 1.15–1.85). It displays negative high field strength element (HFSE, such as Nb, Zr, and Hf) anomalies, resembling those of typical arc basalt. The calculated parental melt for the Maodu gabbro is characterized by pronounced enrichments in LREEs with LaN/YbN ratios of 11.36–19.02, and in large ion lithophile elements (LILEs) relative to HFSEs and lacks significant Nb, Zr, and Hf troughs, comparable to those of ocean island basalts (OIBs). Hence, we infer that the magma source of the Huaqiao clinopyroxenite was an arc mantle metasomatized by subduction melts and/or fluids from the subducted slab formed in a subduction setting; the nature of the source for the Maodu gabbro was the asthenospheric mantle that experienced upwelling and partial melting in an extensional setting. •We determined the in situ elemental compositions of clinop