The Timing of the Paleo-Asian Oceanic Closure: Geochemical Constraints from the Jigede Gabbro in the Alxa Block

• Published in: Petrology Vol. 27; no. 4; pp. 425 - 437
• Main Authors: Zheng Liu, Zhang, Xin, Tan, Shu-Cheng, Sha, Xin, He, Xiao-Hu, Zhou, Qing
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.07.2019; Springer Nature B.V
• Subjects: Ablation; Aluminum oxide; Anomalies; Dating; Earth and Environmental Science; Earth Sciences; Fluids; Gabbro; Gabbros; Geochemistry; Geochronometry; Inductively coupled plasma mass spectrometry; Intrusion; Isotopes; Laser ablation; Lasers; Magma; Mass spectrometry; Mass spectroscopy; Mineralogy; Permian; Plagioclase; Radiometric dating; Strontium 87; Strontium isotopes; Subduction; Subduction (geology); Tectonics; Titanium dioxide; Zircon; Zirconium; Lithospheric mantle; Paleo-Asian ocean; Gabbro; Permian; Alxa Block
• ISSN: 0869-5911; 1556-2085
• DOI: 10.1134/S0869591119040076

This paper investigates the Jigede gabbros from the Shalazhashan tectonic belt, Alxa Block. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating reveals that they were emplaced in the Middle Permian (ca. 262 Ma). All the gabbro samples collected from the intrusion exhibit low contents of TiO 2 (0.24–0.37 wt %) and (4.87–5.41 wt %), but high levels of MgO (11.15–12.15 wt %), CaO (11.45–12.75 wt %), Al 2 O 3 (14.18–17.08 wt %) and high Mg# (0.81–0.83). Relative to HREEs and LILEs, the gabbros are enriched in LREEs and depleted in Th, Nb, Ce, Zr, Hf, and Ti, with positive Eu, Sr and U anomalies. In contrast to MORB, the Jigede intrusion has higher initial 87 Sr/ 86 Sr values of 0.7046 to 0.7054 and lower ε Nd (T) of + 1.8 to + 4.8. Elemental and isotopic data suggest that the intrusion was likely to have been produced by partial melting of a shallow lithospheric mantle source modified by slab-derived fluids, with subsequent cumulation of plagioclase. Thus, the Jigede intrusion was emplaced in a back-arc setting, in response to the Paleo-Asian oceanic subduction. In addition, the final closure of the Paleo-Asian Ocean did not occur prior to the Middle Permian.