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Serpentinites are important volatile and fluid mobile element repositories in oceanic lithosphere and subduction zones, and thus provide significant constraints on global geochemical cycles and tectonic evolution at convergent margins. In this contribution, two types of serpentinites from the Mianlue suture zone in the Qinling orogenic belt, central China, are identified on the basis of detailed mineralogical and geochemical study. Serpentinites from the Jianchaling region (Group 1) are composed of lizardite/chrysotile + magnesite + magnetite. Most of these serpentinites (Group 1a), consist of pseudomorphic orthopyroxene and olivine, and are characterized by low Al 2 O 3 /SiO 2 , high MgO/SiO 2 and Ir-type PGEs to Pt ratios, suggesting a residual mantle origin. Meanwhile, the U-shape REE pattern and positive Eu, Sr and Ba anomalies of these serpentinites indicate that serpentinization fluids have interacted with gabbroic cumulates at moderately high temperatures or associate with the chlorinity and redox conditions of the fluid. Considering the limited mobility of U in the hydrating fluids for the Group 1a serpentinites, hydrating fluids for these serpentinites are most likely derived from the dehydrated slab, and have been in equilibrium with subducting sediments. There are also some serpentinites with low-grade metamorphic recrystallization from the Jianchaling region (Group 1b), represented by recrystallized serpentine minerals (antigorite). The trace element compositions of these Group 1b serpentinites suggest that partial dehydration of serpentinites associated with the transformation from lizardite to antigorite in subduction zone is also likely to affect the geochemistry of serpentinites. Serpentinites from the Liangyazi region (Group 2) are composed of antigorite + dolomite + spinel + magnetite. The high Cr number (0.65–0.80) and low Ti concentrations of spinels in Group 2 serpentinites indicate a refractory mantle wedge origin. Fertile major element compositions (e.g., high Al 2 O 3 content and Al 2 O 3 /SiO 2 ) and conjoint enrichment in light rare earth elements and high field strength elements, however, suggest melt-rock interactions before serpentinization. Combined with their geochemical affinity to “subducted serpentinites”, we conclude that their protoliths (refractory mantle wedge peridotite) experienced melt-rock interactions and then were incorporated into the subduction channel before serpentinization. Studies on these two types of serpentinites indicate that serpentinites from the orogenic belt are most likely characterized by multi-source, multi-stage and multi-genesis, further providing important constraints on subduction channel processes. [ABSTRACT FROM AUTHOR]

Qinling ore belt is the largest known molybdenum belt in the world with a total reserve of >5 Mt of Mo metal. Based on the geochemical behaviour of Mo, the structural settings of the Qinling orogenic belt, and geological events in eastern China, we propose that tectonic settings are of critical importance to the formation of these ore deposits. Molybdenum is very rare in the earth with an abundance of ∼0.8 ppm in the continental crust. Both surface- and magmatic-hydrothermal enrichment processes are required for Mo mineralization. It can be easily oxidized to form water-soluble MoO4 – in the surface environment, especially in the Phanaerozoic, and then precipitated under anoxic conditions. Therefore, closed or semi-closed water bodies with large catchment areas and high chemical erosion rates are the most favourable locations for Mo-enriched sediments. The Qinling orogenic belt was located in the tropics during crustal collisions, such that the chemical erosion was presumably intense, whereas the Erlangping back-arc basin was probably a closed or semi-closed water body as a result of plate convergence. More than 90% of the Mo reserves so far discovered in the Qinling molybdenum belt are associated with the Palaeozoic Erlangping back-arc basin. Compiled Re–Os isotopic ages for porphyry deposits (including several carbonate vein deposits) that have been dated show peaks during 220 million years (>0.32 Mt), 145 million years (> 3.5 Mt), and 115 million years (> 0.84 Mt), which correlate well with the three major episodes of granitoid magmatism since the Triassic. The ∼220 million year episode of mineralization, represented by the Huanglongpu carbonate vein-type deposit and the Wenquan porphyry deposit, coincided with the formation of the South Qinling syn-orogenic granites as well as the Dabie ultrahigh-pressure metamorphic rocks, suggests a genetic relationship with the collision between South and North China Blocks. The ∼145 Ma porphyry Mo deposits, representing the main mineralization, are attributed to reactivation by ridge subduction along the lower Yangtze River belt to the east of the Qinling orogen ∼150–140 Ma. The ∼115 Ma Mo deposits likely reflect slab rollback of the northwestwards subducting Pacific plate ∼125–110 Ma. [ABSTRACT FROM AUTHOR]