Origin of Chromitites in the Songshugou Peridotite Massif, Qinling Orogen (Central China): Mineralogical and Geochemical Evidence

• Published in: Journal of earth science (Wuhan, China) Vol. 30; no. 3; pp. 476 - 493
• Main Authors: Rui, Huichao, Jiao, Jiangang, Xia, Mingzhe, Yang, Jingsui, Xia, Zhaode
• Format: Journal Article
• Language: English
• Published: Wuhan China University of Geosciences 01.06.2019; Springer Nature B.V; Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China%School of Earth Science and Resources,Chang’an University,Xi’an 710054,China%Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China; School of Earth Sciences,China University of Geosciences,Wuhan 430074,China
• Subjects: Aluminum oxide; Biogeosciences; Chlorite; Chromite; Chromium oxides; Depletion; Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; Geotechnical Engineering & Applied Earth Sciences; Grains; Inclusions; Lava; Magma; Manganese; Massifs; Melts; Metamorphism; Metamorphism (geology); Mineral assemblages; Minerals; Nickel; Olivine; Ores; Orogenic Magmatism and Metamorphism Dedicated to IGCP 649: Diamonds and Recycled Mantle; Orogeny; Peridotite; Ratios; Silicates; Special Issue on Ophiolite; Subduction; Subduction (geology); Subduction zones; Titanium dioxide; Trace elements; Tremolite; Zinc; China; peridotite; chromite trace element; Qinling Orogen; parental magma; Songshugou; chromitite
• ISSN: 1674-487X; 1867-111X
• DOI: 10.1007/s12583-019-1227-8

The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine- and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr 2 O 3 contents (0.08 wt.%–0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (<0.1 wt.%). Chromites in the Songshugou peridotite massif are high-Cr type, with Cr # and Mg # values ranging of 67.5–87.6, and 23.4–41.2, respectively. The Cr-chlorite, formed by reactions between olivine and chromite in the presence of fluid under middle temperature, indicates the Songshugou peridotite massif has undergone alteration/metamorphism process during emplacement. Chromite grains are modified by these processes, resulting in the various degrees of enrichment of Fe 2 O 3 , Cr 2 O 3 , Zn, Co and Mn, depletion of MgO, Al 2 O 3 , Ga, Ti and Ni. Due to low silicate/chromite ratios in the massive ores, chromites from them are slightly influenced by alteration/metamorphism and thus preserve the pristine magmatic compositions. The parental magma calculated based on them has 11.17 wt.%–13.57 wt.% A1 2 O 3 and 0.15 wt.%–0.27 wt.% TiO 2 , which is similar to the parental melts of high-Cr chromitites from elsewhere and comparable with those of boninites. Combined with informations from previous studies, major and trace elements geochemistry of chromite, as well as the nature of the parental magma, it can be revealed that the Songshugou chromitities formed in a supra-subduction zone environment.