西藏东南缘早二叠世长英质凝灰岩锆石U-Pb年龄和Hf同位素特征

Zircon U-Pb and Hf Isotopic Composition of Permian Felsic Tuffs in Southeastern Margin of Lhasa,Tibet

西藏东南缘记录的唐加-松多古特提斯增生杂岩带对认识古特提斯洋晚古生代的构造演化提供了新的证据.针对该杂岩带中新发现的长英质凝灰岩开展了全岩主、微量元素,锆石LA-ICP-MS U-Pb定年和锆石Hf同位素组成的研究.结果显示冲尼凝灰岩喷发于278~275 Ma,具有较高的SiO_(2)含量(63.47%~72.65%)、Al_(2)O_(3)含量(14.53%~21.31%),较低的K_(2)O含量(1.30%~2.51%)和TiO_(2)含量(0.50%~1.17%),MgO含量较低,介于0.92%~2.00%,Mg^(#)范围在19.9~34.2(均低于40).富集大离子亲石元素(LILE)、亏损高场强元素(HFSE).锆石具有较高的ε_(Hf)(t)值(+10.2~+14.4)和相对年轻的地壳模式年龄T_(DM)^(c)=351~621 Ma,认为冲尼凝灰岩是唐加-松多古特提斯洋向北俯冲背景下的新生地壳部分熔融的产物,洋盆俯冲消减的开始时代不晚于早二叠世,并且在早二叠世拉萨地体东南缘存在新生地壳生长事件.

The Tangjia-Sumdo Paleo-Tethys accretionary complex belt,recorded in the southeastern margin of Lhasa,provides new evidence for the understanding of the Late Paleozoic tectonic evolution of the Paleo-Tethys Ocean.In this paper,it reports new data for the felsic tuff in this accretionary complex belt,including petrology,major and trace element compositions,zircon U-Pb age,and in-situ Hf isotopic compositions.U-Pb zircon dating indicates that the timing of eruption of the Chongni tuffs was ca.278-275 Ma.They are characterized by relatively high SiO_(2)(63.47%-72.65%),and high Al_(2)O_(3)(14.53%-21.31%),relatively low K_(2)O(1.30%-2.51%),TiO_(2)(0.50%-1.17%),MgO(0.92%-2.00%),and Mg^(#)(19.9-34.2).These tuffs exhibit LILE enrichment and HFSE depletetion.The zircons participating in the weighted age calculation have positive ε_(Hf)(t) values of+10.2 to +14.4 and relatively young zircon Hf crustal model ages(T_(DM)^(c)=351-621 Ma).It agrees that Chongni tuffs were derived from the partial melting of the juvenile crust under the background of the northward subduction of the Tangjia-Sumdo Paleo-Tethys Ocean slab,and the beginning time of the ocean slab subduction is not later than Early Permian,before which there were the events that crust grew in the southeastern margin of Lhasa.