Browsing by Author "Zhang, H.F."
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Item Compositionally stratified lithosphere and carbonatite metasomatism recorded in mantle xenoliths from the Western Qinling (Central China)(Lithos, 2010-02) Su, B.X.; Zhang, H.F.; Sakyi, P.A.; Ying, J.F.; Tang, Y.J.; Yang, Y.H.; Qin, K.Z.; Xiao, Y.; Zhao, X.M.In Central China, long-distance effects from collision between the North China and Yangtze cratons, uplift of the Tibetan Plateau and subduction of the Pacific Ocean are believed to converge in the Western Qinling. Mantle xenoliths from Baiguan and Haoti kamafugites in the Western Qinling were investigated to understand the lithospheric structure and mantle metasomatism beneath the orogenic belt. The Western Qinling lithosphere with depths of at least 120 km is geothermally hot and compositionally stratified, companied by a step-wise decrease in fertility with depth. The shallower portion of the lithospheric mantle is represented by type 2 xenoliths which lack alteration and deformation, and have fertile characteristics in compositions. The dominant rock type in the mantle section is the type 1 peridotites, which have had undergone relatively higher melt extraction and subsequent metasomatism. In-situ trace element data for minerals in the Baiguan and Haoti xenoliths are presented in this study. Baiguan clinopyroxenes have high LREE and show Ba, Th, U, Pb, Sr enrichment and negative Hf and Y anomalies, but the Haoti clinopyroxenes have large variations in trace elements and complicated distributions in LILE, and show negative Ti, Zr and Hf anomalies. Clinopyroxenes from both xenolith types have Ti/Eu and (La/Yb)N ratios in large ranges of 902-15,813 and 0-20, respectively. The petrological and geochemical characteristics suggest that the Western Qinling peridotites had been subjected to silicate and subsequent carbonatite metasomatism, and that carbonatite metasomatism was predominant in Baiguan but lesser in Haoti. Comparisons with the Western Qinling carbonatites and world-wide carbonatites indicate that carbonatite melts involved in the metasomatism originated from deep mantle and most likely related to carbonatite magma, with contributions from subducted oceanic crust. The type 2 peridotites, comprising clinopyroxene megacrysts and clinopyroxenite display similar trace element abundances and patterns, suggesting a close affinity in petrogenesis. Crown Copyright © 2010.Item Lithium enrichment and isotopic variation in minerals from peridotite xenoliths from northwestern Ethiopian plateau(Journal of the Geological Society of India, 2018-01) Alemayehu, M.; Zhang, H.F.; Sakyi, P.A.; Haji, M.We report Lithium (Li) concentrations and isotopic compositions for co-existing olivine, orthopyroxene (opx), and clinopyroxene (cpx) mineral separates from depleted and metasomatised peridotite xenoliths hosted by basaltic lavas from northwestern Ethiopian plateau (Gundeweyn area). The peridotites contain five lherzolites and one harzburgite and are variably depleted and enriched in LREE relative to HREE. In both depleted and enriched lherzolites, Li is preferentially incorporated into olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4-2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an enriched harzburgiteare higher than those of lherzolites. Olivines from the samples show higher Li abundances than normal mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li enrichments through melt-preroditite interaction. The average δ⁷ Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to +2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. The Li isotopic composition (3.5‰) of olivines from harzburgite fall within the range of olivine from lherzolites but the opxs show low in δ⁷Li (-2.0‰). Overall Li isotopic compositions of olivines from the peridotites fall within the range of normal mantle olivine, δ⁷Li values of ~+4±2‰ within uncertainty, reflecting metasomatism (enrichment) of the peridotites by isotopically heavy Li-rich asthenospheric melt. Li isotope zonation is also observed in most peridotite minerals. Majority of olivine grains display isotopically heavy cores and light rims and the reverse case is observed for some olivine grains. Orthopyroxene and clinopyroxene grains show irregular distribution in δ⁷Li. These features of Li isotopic compositions within and between grains in the samples reflect the effect of diffusion-driven isotopic fractionation during meltperidotite interaction and cooling processes.Item Origin of sapphirine- and garnet-bearing clinopyroxenite xenoliths entrained in the Jiande basalts, SE China(Lithos, 2018-04) Xiao, Y.; Zhang, H.F.; Liang, Z.; Su, B.X.; Zhu, B.; Sakyi, P.A.We present petrological and geochemical data of sapphirine- and garnet-bearing clinopyroxenite xenoliths entrained in the Jiande Cenozoic basalts, SE China, to investigate their igneous and metamorphic history, and reconstruct of the thermal-tectonic evolution of the lithospheric mantle. These xenoliths have an unusual mineral association consisting of clinopyroxene + garnet/kelyphite + spinel (±sapphirine). Clinopyroxene has high Mg# (89–93) and displays convex-upward REE pattern. Garnet, partially to completely kelyphitized, is rich in pyrope end-member. It usually includes relics of spinel, suggesting that garnet was formed at the expense of spinel. The spinel has high MgO (20.8–22.9 wt%) and Al2O3 (64.8–67.9 wt%) contents. Sapphirine, forming a rim on spinel, has homogeneous SiO2 (14.5–14.9 wt%), Al2O3 (60.9–61.7 wt%) and MgO (19.7–20.1 wt%) contents, interpreted to be of metamorphic origin. The subsolidus reaction for the formation of sapphirine is as follows: spinel + garnet = sapphirine + clinopyroxene + orthopyroxene. Thus, the earliest mineral assemblage recorded in these xenoliths was spinel + clinopyroxene. The clinopyroxene in the Jiande clinopyroxenite xenoliths has Li abundances (1.04–1.63 ppm) similar to high-P mafic cumulate but much lower than those in crustal eclogite. In addition, the clinopyroxene and garnet do not show positive Eu anomalies. Therefore, the protolith of these three clinopyroxenite xenoliths was most likely a pyroxenite, originating as clinopyroxene + spinel cumulates from mafic melts percolating through the mantle. Many reaction textures such as formation of garnet and sapphirine were developed during decompression possibly coupled with cooling and melt percolation. During this process, the earlier composition of clinopyroxene and spinel also changed. The latest P-T conditions recorded in these xenoliths were at pressure of 8–10 kbar and temperatures of 1069–1094 °C. These observations imply that these rocks have been tectonically uplifted to shallower levels. The uplift process may have been related to lithospheric thinning process accompanied by lithosphere extension and upwelling of the asthenosphere in eastern China.Item The origin of spongy texture in minerals of mantle xenoliths from the Western Qinling, central China(Contributions to Mineralogy and Petrology, 2011-03) Su, B.X.; Zhang, H.F.; Sakyi, P.A.; Yang, Y.H.; Ying, J.F.; Tang, Y.J.; Qin, K.Z.; Xiao, Y.; Zhao, X.M.; Mao, Q.; Ma, Y.G.Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, AlVI, and AlIV/AlVI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas. © 2010 Springer-Verlag.Item Subduction-driven heterogeneity of the lithospheric mantle beneath the Cathaysia block, South China(Journal of Asian Earth Sciences, 2019-10-03) Sakyi, P.A.; Xiao, Y.; Zhang, H.F.; Su, B.X.; Liang, Z.; Zhu, B.To evaluate the modification of the lithospheric mantle by the Paleo-Pacific Ocean subduction beneath the Cathaysia block, we report major and trace elements, and Li isotopes of olivine from Niutoushan, Mingxi, Xilong and Jiande peridotite xenoliths in South China. Spoon-shaped, convex-upward and highly LREE-enriched chondrite-normalized REE patterns of clinopyroxene in the Niutoushan, Mingxi and Xilong xenoliths reflect the interplay between partial melting and subsequent enrichment by metasomatism. Lithium isotope systematics indicate that at least two different metasomatic events affected these peridotites. High Li concentrations in clinopyroxene (up to 59.1 ppm) are ascribed to diffusive Li influx from silicate melts. Olivine grains with lower diffusivity of Li have much higher Li concentrations (0.56–9.76 ppm) and lighter δ7Li values (−52.75 to +10.49‰) than those in the Jiande and other peridotite xenoliths worldwide, showing a prominent carbonatitemetasomatic signature occurring before silicate metasomatism. In addition, increasing Li concentrations of olivine and clinopyroxene with increasing distance from the present subduction trench is consistent with those of low δ26Mg basalts in South China, indicating that the metasomatic agent was related to the westward subduction of the Paleo-Pacific slab. The Jiande lherzolite xenoliths further away from the trench show ‘mantle-like’ Li features and may represent upwelling asthenospheric materials accreted to the continent. Therefore, both types of metasomatism as well as upwelling asthenosphere could have played an important role in the modification of the chemical composition of the mantle beneath the Cathaysia block