Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes |
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| Authors: | LIANG Zi XIAO Yan Joyashish THAKURTA SU Benxun CHEN Chen BAI Yang and Patrick A SAKYI |
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| Institution: | 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, China;2. University of Chinese Academy of Sciences, China;3. Department of Geosciences, Western Michigan University, Kalamazoo, USA;4. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, China;5. Department of Earth Science, School of Physical and Mathematical Sciences, University of Ghana, Legon-Accra, Ghana |
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| Abstract: | 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 K?z?lda? 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 K?z?lda? 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%), Al2O3 (0.67–2.15wt%), Cr2O3 (0.10–0.42wt%) and Na2O (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. |
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| Keywords: | olivine dunite clinopyroxene lamella intergrowth of clinopyroxene and magnetite maficultramafic complex |
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