哀牢山古特提斯洋缝合时限：晚二叠世花岗岩类锆石U-Pb年代学与地球化学制约

Closure time of the Ailaoshan Paleo-Tethys Ocean: Constraints from the zircon U-Pb dating and geochemistry of the Late Permian granitoids

金沙江-哀牢山缝合带系三江特提斯构造域东缘一条重要古特提斯缝合带,其南段哀牢山缝合带闭合的时限及其岩浆活动响应,尚缺乏精确的年代学与岩石地球化学制约。本文对出露于哀牢山缝合带北部老王寨矿区的花岗岩类进行了锆石U-Pb年代学、Hf同位素与全岩微量元素地球化学研究。2件花岗斑岩(D12-1与D13-1)206Pb/238U加权平均年龄为247.7±1.8Ma与255.0~251.7Ma,1件石英斑岩(L12-8)为255.1±3.4Ma,表明一次晚二叠世的岩浆事件。微量元素地化结果显示清晰的Rb、U、Th、Pb正异常和Ba、Sr、Ti、Nb负异常以及轻重稀土强烈分异(LREE/HREE=7.2~18.2)与负Eu异常(0.74~0.78)特征,结合前人主量元素分析表现出的过铝质与高钾钙碱质结果,认为其主体系加厚地壳的富铝沉积岩区深熔形成的S型花岗岩。花岗斑岩εHf(t)值总体集中于0.8~3.3,也含有少量异常正值(7.2)和负值(-5.1和-2.9),解释其源区为亏损地幔与地壳(沉积源区特征明显)的混合。石英斑岩εHf(t)值分布很窄,结果均为负值(-5.5^-2.3),认为其可能系下地壳先存弧岩浆岩源区部分熔融的结果。花岗斑岩锆石较石英斑岩锆石显示重稀土元素分馏相对显著与Ti温度计结果偏高的特征,表现出与其源区中地幔组分的涉入作用相关。总体加厚地壳背景下成生的岩浆岩涉入地幔组分,反映岩浆活动发生于哀牢山缝合带的后碰撞伸展背景。综合新近发表的俯冲型与碰撞型岩浆岩锆石U-Pb定年数据,将哀牢山缝合带闭合的时间限定在晚二叠世(~260Ma),其早于西侧昌宁-孟连洋与北段金沙江洋闭合的时限(分别为中三叠世~235Ma和早三叠世~245Ma)。

The Jinshajiang-Ailaoshan Paleo-Tethys suture zone is an important tectonic boundary developed along the western margin of the South China block. The closure time and related magmatism in the south segment, commonly known as the Ailaoshan, were less constrained for lacking available geochronological and geochemical data. This paper reports zircon U-Pb dating, Hf isotopic compositions and trace element chemistry of the granite porphyry and quartz porphyry from the Laowangzhai area in the northern Ailaoshan. Two granite porphyry samples record 206Pb/238 U weighted mean age of 247. 7 ± 1.8Ma and 255.0 - 251.7Ma, and one quartz porphyry sample yields age of 255. 1 ± 3.4Ma, indicating that these porphyries were emplaced in Late Permian. Whole rock trace elements result shows distinctive enrichment in the Rb, U, Th, and Pb, and depletion in Ba, Sr, Ti, and Nb, and strong fractionated REE （LREE/HREE =7.2 - 18. 2 ） and negative Eu anomalies （0. 22 -0. 24）. In combination with the peraluminous and high-K calc-alkaline properties revealed by previous major elements analysis, the granite porphyry was classified as the S-type granite which was produced under a thickened crust setting. The zircon εHf（t） of the granite porphyry are mainly of slightly positive values and clustered in 0. 8 - 3.3, with several elevated positive （7. 2） and negative values （ - 5. 1 and - 2. 9）. It was explained that the primary magma was derived from a mixture from mantle component and ancient crust materials （sediments source-dominant）. The zircon εHf （t） of the quartz porphyry sample are characterized by clustered negative values （ -5.5 - -2. 3 ） and it was interpreted by partial melting of crustal materials, which is most likely the arc magmatic rocks in lower crust formed during the Ailaoshan oceanic slab subduction. The more fractionated HREE and higher temperature calculated by the zircon Ti thermometry in the granite porphyry than those in the quartz porphyry are consistent with the explanation that the granite porphyry has more mantle contribution as melting. The mantle melting probably induced by mantle upwelling under a thickened crust condition may denote a post-collision extensional setting after the closure of Ailaoshan Paleo-Tethys Ocean. It was summarized that the transition from subduction-related to collision-related magmatisms along the Ailaoshan suture was at - 260Ma. This time is earlier than that of its north extension, Jinshajiang suture （-245Ma） and that of the paralleled Changning-Menglian suture （-235Ma） to the west.