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

Timing and nature of the Jinshajiang PaleoTethys:Constraints from zircon U-Pb age and Hf isotope of the Dongzhulin layered gabbro from Jinshajiang ophiolite belt,northwestern Yunnan

滇西北金沙江蛇绿岩带是古特提斯最重要的缝合带记录之一,本文对该带内的东竹林层状辉长岩进行了年代学、岩石地球化学及锆石Hf同位素研究。LA-ICP-MS锆石U-Pb测年结果显示东竹林层状辉长岩形成于354±3Ma,表明金沙江古特提斯洋在早石炭世已扩张形成洋壳,暗示其裂解时期应为更早的泥盆纪。单颗粒锆石原位Hf同位素分析得到东竹林层状辉长岩锆石εHf(t)=10.3～12.6,平均值为11.5,明显低于结晶时亏损地幔值;单阶段亏损地幔Hf模式年龄tDM1为478～576Ma,平均值为523Ma,明显大于成岩年龄354Ma。锆石Hf同位素结果显示金沙江古特提斯洋地幔受到了富集组分的影响。岩石微量元素特征显示富集组分可能来自特提斯连续演化过程中早期的俯冲作用带入的壳源物质。结合区域演化特征,认为金沙江古特提斯洋是在弧后盆地基础上发展起来的洋盆,它不能构成古特提斯的主大洋,而是古特提斯洋的一个重要分支,分隔着中咱-中甸地块与昌都-思茅地块。

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.