Browsing by Author "Liu, P.-P."
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Item Extremely high Li and low δ 7Li signatures in the lithospheric mantle(Chemical Geology, 2012-01) Su, B.-X.; Zhang, H.-F.; Deloule, E.; Sakyi, P.A.; Xiao, Y.; Tang, Y.-J.; Hu, Y.; Ying, J.-F.; Liu, P.-P.Geochemical behavior of lithium (Li) in mantle processes is generally explained by: (1) Li diffuses into minerals from melts, faster in clinopyroxene than in olivine, creating isotopically lighter-Li clinopyroxene and heavier-Li olivine; (2) Partitioning of Li changes with temperature, and as cooling proceeds Li diffuses into clinopyroxene from olivine, also resulting in low-δ 7Li clinopyroxene and high-δ 7Li olivine, similar to the result of process (1). Our results obtained from highly carbonatite-metasomatized peridotite xenoliths from the Western Qinling reveal that the minerals of these xenoliths contain extremely high Li in a range of 2-16ppm for olivine, 2-43ppm for orthopyroxene, and up to 75ppm for clinopyroxene. Secondly, olivines have much lower δ 7Li (down to -42‰) relative to co-existent pyroxenes. Above observations provide important insights into the behavior of Li during mantle processes. One explanation is that the fractionation of Li strongly depends on the metasomatic medium. Alkaline melts, particularly Na-rich carbonatitic melts, which have high Li contents (~200ppm) but mantle level δ 7Li (3-5‰), tend to effectively transfer 6Li into olivine and 7Li into pyroxene, while silicate melts behave on the contrary. Alternative might be the existence of a reservoir with a high Li content but low δ 7Li, like eclogite, in the mantle since the 7Li released into the overlying mantle wedge during dehydration. This study reveals the distinctive behavior of Li between the carbonatite and silicate metasomatism in the mantle. © 2011.Item Late Paleozoic mafic-ultramafic intrusions in southern Central Asian Orogenic Belt (NW China): Insight into magmatic Ni-Cu sulfide mineralization in orogenic setting(Ore Geology Reviews, 2013-06) Su, B.-X.; Qin, K.-Z.; Tang, D.-M.; Sakyi, P.A.; Liu, P.-P.; Sun, H.; Xiao, Q.-H.The mafic-ultramafic intrusions hosting magmatic Ni-Cu sulfide ore deposits, usually present within the Precambrian cratons and on their margin, have been extensively studied, whereas those occurring within the Phanerozoic orogenic belts have received little attention. The southern margin of the Central Asian Orogenic Belt is a region of late Paleozoic mafic-ultramafic intrusions emplaced during post-orogenic stage. In order to recognize the mineralization features, this study compares barren or weakly mineralized mafic-ultramafic intrusions in the Beishan with those hosting Ni-Cu sulfide ore deposit in the Altay and Eastern Tianshan in petrology, mineralogy and geochemistry aspects. Several mineralization features can be recognized with respects to petrology, mineralogy and geochemistry from comprehensive comparisons. Among the mineralization indicators, we highlight strong alteration accompanying sulfide emplacement, post-erosion exposure of magma conduit, abundant primary hydrous minerals and potential transportation mechanism of ore-forming elements. Other indicators are the presence of orthopyroxene and pyroxenite for potential sulfide formation through crystal fractionation, restricted Fo and Ni ranges in olivines for sulfide segregation, and some other special geochemical characteristics related to crustal contamination. The variation in mineralization could be ascribed to different degrees of partial melting, crustal assimilation and subduction-related modification of a mantle source. We suggest that the Ni-Cu sulfide mineralization of mafic-ultramafic intrusions within orogenic belts is remarkably different from that of cratonic and continental marginal settings in respects of petrogenesis and mineralizing mechanism. Magmatic conduit-type Ni-Cu sulfide ore deposit is a target for future prospecting and exploration for mafic-ultramafic intrusions in the orogenic belts. © 2012 Elsevier B.V.Item Occurrence of an Alaskan-type complex in the Middle Tianshan Massif, Central Asian Orogenic Belt: Inferences from petrological and mineralogical studies(International Geology Review, 2012-02) Su, B.-X.; Qin, K.-Z.; Sakyi, P.A.; Malaviarachchi, S.P.K.; Liu, P.-P.; Tang, D.-M.; Xiao, Q.-H.; Sun, H.; Ma, Y.-G.; Mao, Q.The Xiadong mafic-ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southern margin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite, hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxene is an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite, and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes are Ca-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO 2 and Al 2O 3 contents and high Cr# and Fe 2+/(Fe 2++Mg) values. Primary and secondary Hbls show wide compositional variations. These petrological and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide, which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite supports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that the MTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports the hypothesis of southward subduction of the Palaeozoic Junggar Ocean. © 2012 Copyright Taylor and Francis Group, LLC.Item Petrological, geochemical and geochronological constraints on the origin of the Xiadong Ural-Alaskan type complex in NW China and tectonic implication for the evolution of southern Central Asian Orogenic Belt(Lithos, 2014) Su, B.-X.; Qin, K.-Z.; Zhou, M.-F.; Sakyi, P.A.; Thakurta, J.; Tang, D.-M.; Liu, P.-P.The Xiadong mafic-ultramafic complex is located in the Central Tianshan Terrane, the southern part of the Central Asian Orogenic Belt. It is composed of dunite, hornblende clinopyroxenite, hornblendite and hornblende gabbro. Several dykes of hornblendite, hornblende gabbro and gabbroic diorite cut the main body. These rocks are characterized by adcumulate textures. The dunites contain high-Fo (92.3-96.6) olivines with rare orthopyroxene and plagioclase. Hornblende, chromite, and magnetite are common phases in all of the rock types. Geochemically, the rocks of the Xiadong complex display high MgO contents and extremely low trace element abundances. Although the rocks of the main body show higher MgO contents and lower trace element abundance compared to the dykes, both are characterized by flat REE patterns and enrichment of LILE relative to HSFE. These petrological, mineralogical and geochemical features indicate that the Xiadong complex is a typical Ural-Alaskan type complex. The cross-cutting and intrusive relationships of the rock units and their distinct geochemical features suggest that the main body represents a different pulse from the intrusive dyke. The positive whole-rock εNd(t) values, together with variable (87Sr/86Sr)i ratios, show a depleted mantle source metasomatized by subduction-related material. Zircon U-Pb dating on one hornblende gabbro from the main body of the complex yields a U-Pb 206Pb/238U age of 479Ma, and zircons from three samples of the dykes yield ages of 477, 379 and 313Ma. All the zircons have positive εHf(t) and slightly higher δ18O than normal mantle, and their Hf-O isotopic values are correlated to their ages. The older samples are comparable to those from the Permian mafic-ultramafic complexes in the Beishan Terrane, which were derived from a mantle source metasomatized by subduction of the South Tianshan ocean, while the younger samples are similar to those from the Paleozoic complexes in the Eastern Tianshan, which record the subduction features of the Junggar ocean. These results suggest that the Central Tianshan was a continental arc from at least the Ordovician to the Carboniferous. The lithospheric mantle beneath the Central Tianshan was depleted in Sr-Nd-Hf isotopic compositions and was metasomatized by melts and/or fluids from the subduction of the South Tianshan and Junggar oceans in Paleozoic. The formation of the Xiadong Ural-Alaskan type complex was the product of arc magmatism via high-degree melting of the lithospheric mantle beneath the southern Central Asian Orogenic Belt. © 2013 Elsevier B.V.Item Subduction-induced mantle heterogeneity beneath Eastern Tianshan and Beishan: Insights from Nd-Sr-Hf-O isotopic mapping of Late Paleozoic mafic-ultramafic complexes(Lithos, 2012-03) Su, B.-X.; Qin, K.-Z.; Sun, H.; Tang, D.-M.; Sakyi, P.A.; Chu, Z.-Y.; Liu, P.-P.; Xiao, Q.-H.Extensive late Paleozoic mafic-ultramafic complexes in the Eastern Tianshan and Beishan regions in the southern Central Asian Orogenic Belt (CAOB) represent an ideal opportunity to assess the characteristics of the mantle in this area. Whole-rock Nd-Sr and zircon Hf-O isotopic data from the mafic-ultramafic complexes of the Jueluotage Belt, Middle Tianshan Massif and Beishan Rift were compiled for isotopic mapping. ε Nd(t) and ( 87Sr/ 86Sr) i values range from +5.5 to +9.5 and 0.702 to 0.706 respectively in the Jueluotage Belt, and from -2 to +11 and 0.703 to 0.710 respectively in the Middle Tianshan and Beishan. Zircon ε Hf (t) and δ 18O values are in the range of +6.0-+17.0 and 4.14%-8.00% respectively in the Jueluotage Belt, +1-+8 and 5.17%-6.30% respectively in the Middle Tianshan, and show large variations of -1-+17 and 4%-12% respectively in the Beishan. Spatially, the ( 87Sr/ 86Sr) i ratios of mafic-ultramafic complexes increase from the Jueluotage Belt and Beishan to the Middle Tianshan, while the ε Nd(t) values show an opposite trend and reflect a mixing character between depleted mantle and EM-II reservoirs. Zircon ε Hf (t) and δ 18O values exhibit an apparent southward decreasing and increasing trends respectively, from the Jueluotage Belt, through Middle Tianshan, to Beishan Rift. Their corresponding Hf model ages increase continuously from north to south. These observations demonstrate that the mantle sources beneath the Eastern Tianshan and Beishan are compositionally heterogeneous, which presumably result from significant and variable degrees of subduction-related modification. We therefore suggest that the late Paleozoic subcontinental lithospheric mantle beneath the Jueluotage Belt was modified by slab-derived melts and fluids during the subduction of the Junggar Ocean, whereas the mantle beneath the Beishan Rift was influenced mainly by slab-derived fluids during the subduction of the South Tianshan Ocean. The unique similarities of the Middle Tianshan mafic-ultramafic complexes reflect a two-stage modification of their mantle source, namely the sequential subduction of the early South Tianshan Ocean, followed by subduction of the Junggar Ocean. Thus, the subduction-induced mantle heterogeneity beneath the CAOB is probably regional in scope. © 2011 Elsevier B.V..