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Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic-Ultramafic Complexes

Acta Geologica Sinica(English Edition) 2018年第2期92卷 586-599页

作者： LIANG Zi XIAO yan joyashish thakurta SU Benxun CHEN Chen BAI Yang Patrick A. SAKYI State Key Laboratory of Lithospheric Evolution Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China University of Chinese Academy of Sciences Beijing 100049 China Department of Geosciences Western Michigan University 1903 West Michigan Avenue Kalamazoo MI 49008 USA Key Laboratory of Mineral Resources Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China Department of Earth Science School of Physical and Mathematical Sciences University of Ghana Legon-Accra Ghana

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the Klzildag ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle- or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5-92.6 in the Gaositai complex, 86.5-90.1 in the Yellow Hill complex, 93.2-93.4 in the Klzlldag ophiolite and 86.9-88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of CaO （23.6-24.3wt%）, SiO2 （52.2-54.0wt%）, A1203 （0.67-2.15wt%）, Cr203 （0.10-0.42wt%） and Na20 （0.14-0.26wt%）. It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca2＋, Fe3＋, Cr3＋ and Na＋, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe3＋ and Cr3＋ preferentially partitioned into magnetite, while Ca2＋ and Na＋ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.

关键词： olivine dunite clinopyroxcne lamella intergrowth of clinopyroxenc and magnetite maficultramafic complex

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Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic-Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China

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Acta Geologica Sinica(English Edition) 2018年第3期92卷 952-971页

作者： SUN Tao QIAN Zhuangzhi thakurta joyashish YANG Shenghong XU Gang DUAN Jun GAO Bo WANG Jing School of Resource Environment and Earth Sciences Yunnan University MOE Key Laboratory of Western China's Mineral Resources and Geological Engineering College of Earth Science and Recourses Chang'an University Department of Geosciences Western Michigan University Department of Geosciences University of Oulu Faculty of Land Resource Engineering Kunming University of Science and Technology

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and *** age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The εNd(t) values in the rock units vary from +6.70 to +9.64, and initial87Sr/86Sr ratios range between 0.7035 and0.7042. Initial206Pb/204Pb,207Pb/204Pb and208Pb/204Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)PMvalues between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)PMratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper ***

关键词： break-off of subducted slab zircon U-Pb dating whole-rock Sr-Nd-Pb isotopes mafic-ultramafic intrusion southern margin of Central Asian Orogenic Belt China

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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

Petrological, geochemical and geochronological constraints o...

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中国科学院地质与地球物理研究所2014年度（第14届）学术年会

作者： Ben-Xun Su Ke-Zhang Qin Mei-Fu Zhou Patrick Asamoah Sakyi joyashish thakurta Dong-Mei Tang Ping-Ping Liu Qing-Hua Xiao He Sun Key Laboratory of Mineral Resources Institute of Geology and GeophysicsChinese Academy of Sciences Department of Earth Sciences University of Hong Kong State Key Laboratory of Continental Dynamics Northwest University Department of Earth Science University of Ghana Department of Geosciences Western Michigan University

The Xiadong ma fic–ultrama fic 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 Mg O contents and extremely low trace element abundances. Although the rocks of the main body show higher Mg O 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–Pb206Pb/238 U age of 479 Ma, and zircons from three samples of the dykes yield ages of 477, 379 and 313 Ma. 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 ma fic–ultrama fic 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

关键词： Ural-Alaskan type complex Central Asian Orogenic Belt Mafic-ultramafic complex Arc magmatism

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