Abstract
The Hongshishan mafic-ultramafic complex is situated in the north of the Beishan orogenic collage and the southern part of the Central Asian Orogenic Belt. This paper outlines the petrological, geochemical, and mineralogical data of the Hongshishan ultramafic–mafic complex in the Beishan orogenic collage to constrain its tectonic setting and mineralization. The lithological units of the complex include dunite, clinopyroxene peridotite, pyroxenite, and gabbro. The complex showed concentric zonation, from clinopyroxene peridotite and dunite in the core to pyroxenite and gabbro in the margin. These ultramafic–mafic rocks are characterized by cumulate and layering textures. Field observations, petrography, and significant elemental composition variation, a decreasing sequence of ferromagnesian minerals (Mg#), olivine Fo, and spinel Cr#, all show fractional crystallization trends from dunites through clinopyroxene peridotite and pyroxenite, to gabbros. There are systematic trends among the primary oxides, e.g., CaO, TiO2, and Al2O3, with MgO, suggesting a fractional crystallization trend. SiO2 and Al2O3 increased, which coupled with decreasing MgO, suggested olivine fractionation. The negative correlations of CaO and Al2O3 with MgO meant the accumulation of spinel and mafic minerals. The compositions of olivines from the dunite and clinopyroxene peridotite in the Hongshishan plot within the Alaskan Global trend fields displayed a typical fractional crystallization trend similar to olivines in an Alaskan-type complex. The clinopyroxenes in the clinopyroxene peridotite primarily occur as a diopside and appear in the field of an Alaskan-type complex. The absence of orthopyroxene, less hydrous, and free of fluid inclusions in the chrome spinels means the absence of a magmatic origin of chromite-bearing peridotites in hydrous parental melts or scarce hydrous melts. Serpentinization, carbonatization, subduction modification, and enrichment may account for the LILE-enrichment and HFSE-depletion of peridotite rocks. Negative Eu anomalies and REE fractionations of mafic-ultramafic rocks may not be directly attributed to crustal assimilation. Petrological, mineralogical, and geochemical characteristics indicated the Hongshishan complex is not the member compositions of a typical ophiolite. However, it displays many similarities to Alaskan-type mafic-ultramafic intrusions related to subduction or arc magmas setting at ∼366.1 Ma and suffered subduction modification and enrichment. The Hongshishan complex is a unique Ir-Ru-rich chromite deposit in the southern margin of the Altaids orogenic belt. Chromites occur primarily in light yellow dunites, with banded, lenticular, veined, thin-bedded, and brecciated textures. Part of the chromite enrichment in IPGE (Os, Ir, Ru) and the chondrite-normalized spider diagram of PGE showed steep right-facing sloped patterns similar to those of the PGE-rich ophiolitic chromites.
Highlights
Alaskan-type complexes show the following characteristics: they are related to the subduction environment and composed of a central dunite body grades outward into wehrlite, clinopyroxenite and gabbroic lithologies, occasionally occurring olivine clinopyroxenite, hornblendite clinopyroxenite, hornblendite, and hornblendite gabbro, characterized by the dominance of olivine, clinopyroxene, hornblende and the absence of orthopyroxene and plagioclase
We present new zircon U–Pb isotopic data, silicate chemical compositions, chromian minerals of spinels, olivines, and clinopyroxenes, which should provide new insights and realizations regarding tectonic evolution in the Beishan orogenic collage
The absence of orthopyroxene and the fact that most chrome spinels were less hydrous and free of fluid inclusions meant a magmatic origin of chromite-bearing peridotites was absent in hydrous parental melts or scarce hydrous melts
Summary
Alaskan-type complexes show the following characteristics: they are related to the subduction environment and composed of a central dunite body grades outward into wehrlite, clinopyroxenite and gabbroic lithologies, occasionally occurring olivine clinopyroxenite, hornblendite clinopyroxenite, hornblendite, and hornblendite gabbro, characterized by the dominance of olivine, clinopyroxene, hornblende and the absence of orthopyroxene and plagioclase. Various hypotheses have been suggested to describe the petrogenesis of Alaskan-type complexes such as fractional melting in the mantle (Irvine, 1967; Taylor, 1967), fractional crystallization from magma mixtures (Sha, 1995; Farahat and Helmy, 2006; Habtoor et al, 2016), fractional crystallization from a common hydrous parental magma without significant crustal contamination (Farahat and Helmy, 2006; Tian et al, 2011), depleted mantle metasomatized by subduction-related melts/ fluids (Su et al, 2014), and tectonic emplacement of fragments of a pre-existing body (Efimov, 1998) These hypotheses have generated some controversy and contradictions. The viewpoint of depleted mantle metasomatized by subductionrelated melts/fluids favors multiple magmatic pulses instead of fractional crystallization within a common parental magma (Su et al, 2014)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
