Journal
GEOLOGY
Volume 43, Issue 2, Pages 179-182Publisher
GEOLOGICAL SOC AMER, INC
DOI: 10.1130/G36245.1
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Funding
- Australian Research Council (ARC)
- ARC Centre of Excellence for Core to Crust Fluid Systems
- Macquarie University Australian Postgraduate Award
- DEST (Department of Education, Science and Training) Systemic Infrastructure grants
- ARC LIEF (Linkage Infrastructure, Equipment, and Facilities)
- National Collaborative Research Infrastructure Strategy (NCRIS), industry partners, and Macquarie University
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Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (T-RD) of 234 +/- 3 Ma, while T-RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 +/- 7 Ma, epsilon(Hf) = 9.7 +/- 4.6, and delta O-18 = 4.8%-8.2%. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (less than or similar to 10m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.
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