西藏札佐晚白垩世中期埃达克岩年代学、地球化学及其构造意义

Chronology, geochemistry and tectonic significance of middle stage of Late Cretaceous adakite in Zhazuo aera, Tibet

关键词:  南冈底斯  晚白垩世  埃达克岩  Lu-Hf同位素  洋脊俯冲

Key words:  south Gangdese  Late Cretaceous  adakite  Lu-Hf  Ridge subduction

摘要:

为解释南冈底斯晚白垩世埃达克质岩石成因及其地球动力学机制，本文对西藏扎囊县札佐地区二长花岗岩开展了锆石U-Pb年代学、Hf同位素和全岩地球化学分析。札佐二长花岗岩锆石U-Pb定年为80.43±0.62 Ma，其SiO₂为66.19%~66.84%，Al₂O₃为15.17%~15.48%，MgO为1.67%~1.91%，Mg#为47.4~51.5，K₂O为3.86%~4.09%，A/CNK=0.91~1.01，属准铝质高钾钙碱性岩石。岩石轻稀土富集明显，高Sr（492×10^-6~670.2×10^-6），低Y（8.27×10^-6~14.99×10^-6），Yb（1.07×10^-6~1.79×10^-6），高Sr/Y（35.0~81.0），高La/Yb（17.4~21.4），弱负Eu异常，具埃达克岩地球化学特征。相对富集大离子亲石元素（LILE），亏损高场强元素（HFSE）和重稀土。锆石ε_Hf（t）值为10.5~14.1，单阶段Hf模式年龄（t_DM1）为184.8~326.1 Ma，平均为203.4 Ma，二阶段Hf模式年龄（t_DM2）为247.2~476.0 Ma，平均为287.1 Ma，略大于侵位年龄，指示岩浆物质来源于俯冲洋壳，并可能卷入俯冲沉积物。岩石中地幔组分印记Mg#值和相容元素Ni、Cr含量较高，表明熔体在上升过程中与上覆地幔楔发生反应。研究分析表明，在新特提斯洋洋脊俯冲作用下，高温热流透过板片窗导致洋壳（及俯冲沉积物）部分熔融形成札佐埃达克质二长花岗岩。同时表明，在80 Ma左右，新特提斯洋仍处于洋脊俯冲阶段。

Abstract:

In order to explain late Cretaceous adaktic rocks in South Gangdese and the geodynamic mechanism, we present zircon U-Pb chronology, Lu-Hf isotope and geochemistry analysis for the monzogranite from the Zhazuo area, Zhanang, Tibet. The zircon U-Pb dating yieldes 80.43±0.62 Ma for the Zhazuo monzogranite. The rocks are high-K Calc-alkaline metaluminous, with SiO₂ (66.19%~66.84%), Al₂O₃ (15.17%~15.48%), MgO (1.67%~1.91%)，Mg# (47.4~51.5)，K₂O (3.86%~4.09%)，A/CNK (0.91~1.01). The rocks show typical adakitic features, with strongly riched in LREE, high Sr (492×10^-6~670.2×10^-6), low Y (8.27×10^-6~14.99×10^-6) and Yb (1.07×10^-6~1.79×10^-6), high Sr/Y (35.0~81.0) and La/Yb (17.4~21.4) and slightly negative Eu anomalies. They are enriched in LILE and depleted in HFSE, HREE. Zircon ε_Hf(t) values range from 10.5 to 14.1, with t_DM1 ranging from 184.8 Ma to 326.1 Ma and t_DM2 ranging from 247.2 Ma to 476.0 Ma, slightly older than the emplaced age, indicating that the magma is derived from subducted oceanic crust probably with some subducted sediments.The Mg# value of the mantle component imprint in the rock and the content of compatible elements Ni and Cr are high, indicating that the melts have interacted with the mantle during ascent. Research analysis shows that the high heat flow flows through the slab window, which induce partial melting of oceanic crust and some subducted sediments, and forms the adakitic monzogranite in the Zhazuo area under the geodynamic setting of ridge subduction. It further indicates that Neo-Tethys is still in the ridge subduction stage at about 80 Ma.