Recycled crustal zircons from podiform chromitites in the Luobusa ophiolite, southern Tibet
Recycled crustal zircons from podiform chromitites in the Luobusa ophiolite, southern Tibet
复制标题
DOI：
10.1111/iar.12011
复制
发表时间：
2013-03
期刊：
影响因子：
1.5
通讯作者：
S. Yamamoto;T. Komiya;H. Yamamoto;Y. Kaneko;M. Terabayashi;I. Katayama;T. Iizuka;S. Maruyama;Jingsui Yang;Y. Kon;T. Hirata
中科院分区：
文献类型：
--
作者：
S. Yamamoto;T. Komiya;H. Yamamoto;Y. Kaneko;M. Terabayashi;I. Katayama;T. Iizuka;S. Maruyama;Jingsui Yang;Y. Kon;T. Hirata
利用激光烧蚀微探针-电感耦合等离子体质谱(LA-IC-PMS)测定了藏南罗布萨蛇绿岩中豆荚状铬铁矿的锆石U-Pb年龄，以确定豆荚状铬铁矿与寄主地幔橄榄岩之间的年龄关系。LA-IC-PMS斑点分析结合阴极发光图像给出了从白垩纪到晚太古代(约100-2700 Ma)的广泛年龄范围。康科迪亚曲线上的最小年龄约为100 Ma，略低于俯冲带的交代(岩浆)事件(120±10 Ma)，表明锆石遭受了一定程度的铅损失。然而，所发现的大多数年龄比铬铁矿和蛇绿岩建造的年龄要早得多。激光拉曼光谱分析表明，从铬铁矿中回收的锆石含有地壳矿物包裹体，如石英和钾长石，但没有地幔矿物(如橄榄石、辉石和铬铁矿)，表明它们来自地壳成因。结果表明，铬铁矿中的地壳锆石具有捕虏晶成因，并在形成过程中长期存在于地幔橄榄岩中，然后被夹带到铬铁矿中。这表明新特提斯洋底的地幔橄榄岩受到了地壳物质混染的影响。我们的结果与以前的报道一致，即印度洋中洋脊玄武岩具有地壳物质污染的同位素特征。根据这些结果，以及以前对冈瓦那地质的同位素研究，我们得出结论，豆荚状铬铁矿中的古锆石可能提供了地壳物质通过上地幔循环的证据。
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.