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Nanoscale study of lamellar exsolutions in clinopyroxene from olivine gabbro; recording crystallization sequences in iron-rich layered intrusions

Gao Wenyuan, Cristiana L. Ciobanu, Nigel J. Cook, Ashley Slattery, Huang Fei and Wang Daoheng
Nanoscale study of lamellar exsolutions in clinopyroxene from olivine gabbro; recording crystallization sequences in iron-rich layered intrusions
American Mineralogist (February 2019) 104 (2): 244-261

Abstract

Pyroxene exsolutions and associated Fe-Ti oxides and spinels are described in a sample of olivine gabbro representing the Middle Zone of the Panzhihua layered intrusion, Southwest China, part of the Emeishan LIP. High-angle annular dark-field scanning transmission electron microscope imaging, electron diffraction, and energy dispersive spectroscopy reveal complex multi-stage exsolution relationships in the host clinopyroxene. The studied assemblage is common in gabbroic rocks and comprises subcalcic diopside and lamellar clinoenstatite (<1 wt% Ca). Two sets of exsolved clinopyroxene lamellae are observed. Only one is, however, well developed as lamellae oriented approximately parallel to (801) of diopside, making an angle of approximately 10 to 11 degrees with the (100) planes, or the c axis, of both phases. These are the so-called "100" lamellae with a perfect fit along a-crystallographic axes when viewed down to [010] zone axis. Crosscutting exsolutions of Fe-(Ti) oxides are relatively common throughout the same host clinopyroxene. Apart from ilmenite and magnetite with variable Ti-content, hercynite is a minor yet ubiquitous phase. The nanoscale study indicates a sequence of fine-scale processes: from higher-T ( approximately 1030-1100 degrees C): (I) (clino)enstatite exsolutions in low-Ca diopside; followed by (II) slightly Ca-richer diopside overgrowths and high-T titanomagnetite exsolution in diopside; to lower-T (<450 degrees C) (III) titanomagnetite exsolutions into ulvospinel + magnetite; followed by (IV) sub-solidus re-equilibration in clinopyroxenes and among Fe-Ti oxides + hercynite. Using exact phase boundary theory, pressures of lamellar exsolution within the host diopside are estimated as approximately 2 GPa with an error of + or -< or =1 GPa. The present study of complex exsolutions in clinopyroxene demonstrates that a nanoscale approach can help constrain P-T-X evolution during formation of layered intrusions.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 104
Serial Issue: 2
Title: Nanoscale study of lamellar exsolutions in clinopyroxene from olivine gabbro; recording crystallization sequences in iron-rich layered intrusions
Affiliation: Northeastern University, School of Resources and Civil Engineering, Shenyang, China
Pages: 244-261
Published: 201902
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 62
Accession Number: 2019-044209
Categories: Mineralogy of silicatesIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
N26°00'00" - N27°00'00", E101°00'00" - E102°30'00"
Secondary Affiliation: University of Adelaide, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201923
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