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

作　　者：LIANG Zi XIAO yan Joyashish THAKURTA SU Benxun CHEN Chen BAI Yang Patrick A. SAKYI 

机构地区：[1]State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China [2]University of Chinese Academy of Sciences, Beijing 100049, China [3]Department of Geosciences, Western Michigan University, 1903 West Michigan Avenue, Kalamazoo, MI 49008, USA [4]Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China [5]Department of Earth Science, School of Physical and Mathematical Sciences, University of Ghana, Legon-Accra, Ghana

出　　处：《Acta Geologica Sinica(English Edition)》2018年第2期586-599,共14页地质学报（英文版）

基　　金：financially supported by the National Natural Science Foundation of China (Grants 41522203, 41673037 and 41772055);Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant 2017095)

摘　　要：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.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 

分 类 号：P588.125[天文地球—岩石学]