Timing and nature of the Jinshajiang Paleo-Tethys: Constraints from zircon U-Pb age and Hf isotope of the Dongzhulin layered gabbro from Jinshajiang ophiolite belt, northwestern Yunnan
滇西北金沙江古特提斯洋早期演化时限及其性质:东竹林层状辉长岩锆石U-Pb年龄及Hf同位素约束

The Jinshajiang ophiolite belt is generally regarded as one of the Paleo-Tethys sutures in northwestern Yunnan.Zircon UPb age and Hf isotope of one layered gabbro collected from Dongzhulin in the Jinshajiang ophiolite belt were studied,aiming to constrain the timing and nature of the Jinshajiang Paleo-Tethys oceanic basin.LA-ICP-MS zircon U-Pb dating results yield a weighted mean 206Pb/238U age of 354 ± 3Ma for the Dongzhulin layered gabbro,indicating that the Jinshajiang Paleo-Tethys oceanic lithosphere formed in the earliest Carboniferous.Zircon from the Dongzhulin layered gabbro exhibits a narrow range of initial Hf isotope ratios(εHf(t) =10.3 ～12.6),with corresponding tDM1 of 478 ～576Ma.The εHf(t) values are obviously lower than that of depleted mantle reservoir at 354Ma and the tDM1 are obviously older than the timing of zircon growth,suggesting that their source is metasomatic mantle rather than N-MORB-like depleted mantle.Early subduction of Tethys,which could carry crustal components to the mantle,might responsible for the character of the Jinshajiang Paleo-Tethys mantle.These features,combined with the tectonic framework and spatial allocation of the archipelagic arc basin systems on the Nujiang-Lancangjiang-Jinshajiang area,southwestern China,suggest that the Jinshajiang Paleo-Tethys was mostly produced in a back-arc basin.Consequently,the Jinshajiang Paleo-Tethys was not the main ocean of the Paleo-Tethys,but an important branch of the Paleo-Tethys that once separated the Zhongzan-Zhongdian block and ChangduSimao block.