A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

被引:115
作者
Borisova, Anastassia Y. [1 ,2 ]
Ceuleneer, Georges [1 ]
Kamenetsky, Vadim S. [3 ]
Arai, Shoji [4 ]
Bejina, Frederic [5 ,6 ]
Abily, Benedicte [1 ]
Bindeman, Ilya N. [7 ]
Polve, Mireille [1 ]
De Parseval, Philippe [1 ]
Aigouy, Thierry [1 ]
Pokrovski, Gleb S. [1 ]
机构
[1] Univ Toulouse 3, Observ Midi Pyrenees, GET, CNRS,IRD,CNES,UMR 5563, F-31400 Toulouse, France
[2] Moscow MV Lomonosov State Univ, Dept Geol, Moscow 119991, Russia
[3] Univ Tasmania, ARC Ctr Excellence Ore Deposits, CODES, Hobart, Tas 7001, Australia
[4] Kanazawa Univ, Dept Earth Sci, Kanazawa, Ishikawa 9201192, Japan
[5] Univ Toulouse 3, UPS, OMP, IRAP, F-31400 Toulouse, France
[6] CNRS, IRAP, F-31400 Toulouse, France
[7] Univ Oregon, Eugene, OR 97403 USA
基金
澳大利亚研究理事会;
关键词
Oman ophiolite; chromitite; chromian spinel; MORB; inclusions; Moho Transition Zone; hydrothermal fluid; serpentinites; protolith; oxygen isotopes; AFC; ABSORPTION FINE-STRUCTURE; OXYGEN-ISOTOPE FRACTIONATION; MELT INCLUSIONS; SAMAIL OPHIOLITE; OCEANIC-CRUST; HIGH-PRESSURE; CR-SPINEL; HYDROTHERMAL CIRCULATION; THERMODYNAMIC PROPERTIES; OLIVINE PHENOCRYSTS;
D O I
10.1093/petrology/egs054
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of similar to 500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1-50 mu m) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f(O2), of similar to 3 log units below the quartz-fayalite-magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950 degrees C at subsolidus conditions, were overgrown by more oxidized host chromite (f(O2) approximate to QFM) in association with microcrystal inclusions of silicates (plagioclase An(86), clinopyroxene, and pargasite) that were formed between 950 and 1050 degrees C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to similar to 0 center dot 1 degrees C a(-1)). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.
引用
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页码:2411 / 2440
页数:30
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