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Geoscience ›› 2020, Vol. 34 ›› Issue (03): 450-465.DOI: 10.19657/j.geoscience.1000-8527.2020.010

• Petrology • Previous Articles     Next Articles

Geochronology, Geochemistry and Sr-Nd-Hf Isotopic Compositions of the Wuhua Gabbros in Langshan, Inner Mongolia: Constraints for Mantle Sources and Petrogenesis

ZHANG Yun1,2(), SUN Lixin1,2, ZHANG Tianfu1,2, SUN Yiwei1,2, ZHANG Qi1,2, LI Yanfeng1,2, YANG Zeli1,2, LIU Wengang1,2   

  1. 1. Tianjin Center of Geological Survey, China Geological Survey, Tianjin 300170, China
    2. North China Center for Geoscience Innovation, Tianjin 300170, China
  • Received:2018-11-02 Revised:2019-12-11 Online:2020-07-04 Published:2020-07-05

Abstract:

Two types of gabbros (i.e., olivine and hornblende gabbros) are exposed in the Wuhua gabbroic intrusions. LA-ICP-MS zircon U-Pb dating indicates that the Wuhua gabbros were emplaced in the late Early Carboniferous ((319.8±1.8) Ma to (325.4±1.6) Ma). The Wuhua gabbros are tholeiitic, with high Mg# va-lues (mostly 76.70-80.35), high Al2O3 (13.55%-25.13%) and low TiO2 (0.18%-1.35%). The rocks have low total REE contents (∑REE=8.92×10-6-40.10×10-6), and are featured by LREE enrichments ((La/Yb)N=1.74-3.06) and positive Eu anomaly (δEu=1.156-3.86). The Pb and LILE (K, Rb, Ba, Sr) enrichments and HFSE (Nb, Ta, Zr, Hf, Th) depletions, and the relatively low Zr content (4.38×10-6-26.6×10-6) and low Zr/Y ratio (1.08-2.35) resemble those of typical subduction-related arc volcanic rocks. The Sr-Nd-Hf isotopic data show that the Wuhua gabbros were derived from an enriched mantle source, and that crustal assimilation was likely insignificant. According to geochemical data and previous regional geological studies, we consider that the Wuhua gabbros may have formed from partial melting of the hydrated and enriched lithospheric mantle wedge. The Wuhua gabbros were likely formed in the incipient subduction phase, when the western section of the northern North China Plate margin started to become an Andean-type continental arc in the late Early Carboniferous. This tectonic phase may have ended before the Early Permian.

Key words: gabbro, zircon U-Pb geochronology, geochemistry, Sr-Nd-Hf isotope, Andean-type active continental margin, Langshan, Inner Mongolia

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