Pyroxenttes: High-pressure segregates or recycled oceanic crust?

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... Combined with the Re-Os systematics of modern MORB and the ancient mid-ocean ridge setting as discussed above, a metasomatic mechanism related to melt/peridotite reaction might be a plausible interpretation for the suprachondritic Re-Os systematics of the three fertile samples. When silicate melts reacted with the pre-existing peridotite residue, olivine could be consumed and substituted by new pyroxene together with garnet or hornblende participation at various melt/rock ratios (Rapp et al., 1999;Zhang and Xu, 2012), and sometimes secondary sulphide (Wittig et al., 2010): ...
... Thus, we attribute the observed high Fe/Mn and Zr/Hf ratios to partial melting of pyroxenite/eclogite. However, most pyroxenites have lower LREE and K 2 O contents than peridotites, whereas OIB-type magmas are commonly alkaline in character and enriched in LREE (Zhang and Xu, 2012). Thus, we suggest that a mixed source is the most likely candidate for the Zhejiang basaltic magmas. ...
Cenozoic continental basalts from Zhejiang Province, southeast China are tholeiitic to weakly alkalic in composition, with moderate MgO contents (6-11 wt.%) and an average Mg# of 62. They display typical OIB-like trace element features, including enrichment in most incompatible elements, both LILE and LREE, and negative K, Pb, Zr, Hf anomalies. In particular, they are characterized by high Fe/Mn (73 ± 5), La/Yb (19 ± 6) and Nb/Ta (18.8 ± 0.4) ratios, which can be attributed to the presence of residual clinopyroxene, garnet and rutile in the mantle source. Based on these minerals, the following hybrid source rocks are hypothesized: garnet pyroxenite/eclogite and peridotite. Clinopyroxene-liquid thermobarometry indicates clinopyroxene crystallization temperatures of > 1257 °C. This is higher than the assumed temperature at the base of the sub-continental lithospheric mantle (SCLM) (~ 1220 °C) beneath Zhejiang, thus the magmas were presumably derived from the asthenosphere. Some typical geochemical features such as negative K, Pb anomalies, positive Ba, Sr, Nb, Ta anomalies and the extremely high Os isotopic signatures, suggest participation of EM-like mantle sources, indicative of ancient subducted oceanic crust. (87Sr/86Sr)i (0.7037-0.7046) and 143Nd/144Nd (0.512832-0.512990) isotope ratios point to the presence of mixed components in the source region, i.e. DMM, EM1 and EM2. Recent seismic tomographic images of the mantle beneath Zhejiang suggest the presence of a subducted slab of oceanic lithosphere in the transition zone. Based on the combined geophysical and geochemical evidence, we propose that the major source of the Zhejiang basaltic magmas was the ancient subducted oceanic slab in the transition zone with an EM-like signature. The other magma sources include depleted asthenospheric peridotite possessing a DMM-like signature. The dynamics of this upwelling hybrid magma was apparently related to westward subduction of the Pacific plate underneath the eastern Asian continent. This process may have triggered the widespread Cenozoic volcanism related to the lithospheric thinning in East China in the Mesozoic and Cenozoic.
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The origin of ultramafic rocks, especially those in suture zones, has been a focus because they are not only important mantle sources of magma, but also provide substantial information on metamorphism and melt/fluid–peridotite interaction. Ultramafic rocks in Hujialin, in the central part of the Sulu orogen, include peridotite and pyroxenite. Although many papers on their origin and tectonic evolution have been published in the past few decades, these questions are still highly debated. Here, we present mineralogy, mineral composition, and bulk-rocks of these ultramafic rocks to evaluate their origin and tectonic evolution. The garnet clinopyroxenite is low in heavy rare-earth elements (HREE, 5.97-10.6 ppm) and has convex spoon-shaped chondrite-normalized REE patterns, suggesting the garnet formed later, and its precursor is clinopyroxenite. It is high in incompatible elements (i.e., Cs, Rb, Ba) and shows negative to positive U, Nb, and Ta anomalies, without pronounced positive Sr or Eu anomalies. Clinopyroxene in garnet clinopyroxenite contains high MgO (Mg# 0.90–0.97). The mineral chemistry and bulk-rock compositions are similar to those of reactive clinopyroxenite, suggesting that it originally formed via peridotite–melt interaction, and that such silicic and calcic melt might derive from the subducted Yangtze continent (YZC). Dunite contains olivine with high Fo (93.0–94.1), low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%), chromite with high Cr# (0.75–0.96), TiO2 (up to 0.88 wt.%), and Na2O (0.01–0.10 wt.%). It has negatively sloped chondrite-normalized REE patterns. Mineral chemistry and bulk rocks suggest dunite likely represent residual ancient lithosperic mantle peridotite beneath the North China Craton (NCC) that was overprinted by aqueous fluids. The lack of prograde and retrograde metamorphic minerals in dunite and irregular shaped mineral inclusions in chromite suggest dunite did not subduct to deep levels. Dunite mingled with garnet clinopyroxenite during exhumation of the latter at shallow depths. These ultramafic rocks, especially hydrated peridotite, may be important sources of Au for the Jiaodong gold province in the NCC.
A ultramafic pyroxenite pluton has been discovered in Zhongzaohuo area in the East Kunlun orogen Recently. This paper reports the results of petrological, geochemical and genetic mineralogy research on the pyroxenite pluton. The rock is mainly composed of clinopyroxene, orthopyroxene and amphibole, and minor plagioclase, quartz, biotite and iron opaque minerals. Amphibole and biotite were formed during retrograde metamorphism. The discriminant analysis results suggest that the Opx are magmatogenic, thus the rock should be named pyroxenite rather than granulite. The rock has high MgO, CaO and low Al2O3 and enriched in Rb and Th and depleted in Nb and Ti, showing clear evidence for an enriched mantle source. Field occurrence of the pyroxenite pluton suggests that the pyroxenite pluton was formed after the mylonization of the surrounding rocks. Combined with the tectonic evolution of East Kunlun, we come to the conclusion that the subduction of an Paleo-Tethys(A'nyemaqen) oceanic slab at the Middle Permian led to fluid and Si-rich? melt metasomatism, inducing partial melting of an enriched lithospheric mantle(peridotites) to form the ultramafic pyroxenite magma. The pyroxenite magma underplated the overlying lower crust, captured the metamorphic zircons of the granulite and exchanged some trace elements, but didn't result in the lower crust partial melting to form any felsic magma. The pyroxenite magma emplaced alone eventually.
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