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Recycled crustal zircons from podiform chromitites in the Luobusa ophiolite, southern Tibet

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Abstract

We have measured the U–Pb age of zircon grains separated from podiform chromitites from the Luobusa ophiolite, Southern Tibet, using laser ablation microprobe – inductively coupled plasma mass spectrometer (LA-IC-PMS), to determine the age relationship between the podiform chromitites and the host mantle peridotite. Spot analyses with LA-IC-PMS, assisted by cathodoluminescence images gave a wide age range, from the Cretaceous to the Late Archean (ca 100–2700 Ma). The minimum ages of ca 100 Ma, plotted on the concordia curve, were slightly lower than the metasomatic (magmatic) event in the supra-subduction zone (120 ± 10 Ma), suggesting that the zircons suffered some Pb loss. However, most of the ages found are much older than those of the chromitite and ophiolite formation. Laser Raman spectroscopy analyses revealed that the zircons recovered from the chromitites contain crustal mineral inclusions, such as quartz and K-feldspar, but lack mantle minerals (e.g., olivine, pyroxene, and chromite), suggesting that they had a crustal origin. The results indicate that crustal zircons in chromitites had a xenocrystic origin and resided in the mantle peridotite for a long period before being entrained into the chromitite during its formation. This indicates that the mantle peridotite under the Neo-Tethys Ocean was affected by the crustal material contamination. Our results are consistent with previous reports that mid-oceanic ridge basalts in the Indian Ocean have the isotopic signature of crustal material contamination. From these results, and previous isotopic studies on Gondwana geology, we conclude that ancient zircons from podiform chromitites could provide evidence of crustal material being recycled through the upper mantle.

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