Recycled crustal zircons from podiform chromitites in the <scp>L</scp>uobusa ophiolite, southern <scp>T</scp>ibet

Abstract

Abstract We have measured the U–Pb age of zircon grains separated from podiform chromitites from the L uobusa ophiolite, S outhern T ibet, 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 C retaceous to the L ate A rchean ( ca 100–2700 M a). The minimum ages of ca 100 M a, plotted on the concordia curve, were slightly lower than the metasomatic (magmatic) event in the supra‐subduction zone (120 ± 10 M a), suggesting that the zircons suffered some P b loss. However, most of the ages found are much older than those of the chromitite and ophiolite formation. Laser R aman 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 N eo‐ T ethys O cean was affected by the crustal material contamination. Our results are consistent with previous reports that mid‐oceanic ridge basalts in the I ndian O cean have the isotopic signature of crustal material contamination. From these results, and previous isotopic studies on G ondwana geology, we conclude that ancient zircons from podiform chromitites could provide evidence of crustal material being recycled through the upper mantle.