Univ Adelaide, Adelaide Microscopy, Adelaide, SA 5005, Australia;Northeastern Univ, Key Lab, Minist Educ Safe Min Deep Met Mines, Shenyang 110819, Liaoning, Peoples R China;Northeastern Univ, Key Lab, Minist Educ Safe Min Deep Met Mines, Shenyang 110819, Liaoning, Peoples R China;Northeastern Univ, Key Lab, Minist Educ Safe Min Deep Met Mines, Shenyang 110819, Liaoning, Peoples R China;Univ Adelaide, Sch Chem Engn, Adelaide, SA 5005, Australia;Univ Adelaide, Sch Chem Engn, Adelaide, SA 5005, Australia;
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 similar to 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  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 (similar to 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 similar to 2 GPa with an error of +/- <= 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.
High-angle annular dark-field scanning transmission electron microscopy;clinopyroxene;titanomagnetite;liquid-magmatic ore deposits;sequence of exsolution;