西藏嘉黎断裂带凯蒙蛇绿岩的年代学、地球化学特征及大地构造意义  

Age,geochemistry and its tectonic significance of Kaimeng ophiolites in Jiali fault belt,Tibet.

作　　者：和钟铧[1] 杨德明[1] 王天武[1] 

机构地区：[1]吉林大学地球科学学院,长春130026

出　　处：《岩石学报》2006年第3期653-660,共8页Acta Petrologica Sinica

基　　金：本文受国土资源部地质调查项目(H46002002);吉林大学'985'工程联合资助.

摘　　要：凯蒙蛇绿岩主要分布于嘉黎断裂内凯蒙沟两侧的东西向山脊上,沿嘉黎断裂带呈东西向线状分布,组成蛇绿岩的岩石类型主要有纯橄岩、方辉橄榄岩、单辉橄榄岩、橄辉岩、橄长岩和辉长岩等。蛇绿岩具有贫硅、贫碱、低钛,而富铁、镁的特点。稀土总量很低,∑REE值变化于0．415×10^(-6)～3．273×10^(-6)之间,其中方辉橄榄岩和橄长岩的轻稀土略有富集,而单辉橄榄岩和辉长岩的轻稀土略有亏损,所有岩石都具明显的正铕异常。辉长岩的Th-Ta比值大体相等,显示蛇绿岩形成于洋中脊构造环境,而在原始地幔标准化配分曲线上具有明显的Rb、Ba、Sr低度富集和Nb的相对亏损,显示岛弧(IAT)特点,这种兼具IAT和MORB地球化学特点特征预示着凯蒙蛇绿岩形成于不成熟弧后盆地环境。蛇绿岩中橄长岩SHRIMP锆石U-Pb年龄为218．2±4．6Ma,说明在晚三叠世时期洋壳就已出现。凯蒙蛇绿岩的确定表明嘉黎断裂在早期(T_3-J_1)可能是冈底斯构造带上的一条重要缝合带。Kaimeng ophiolites locate in Jiali fault belt and expose along Kaimeng ridge with E-W trend. The ophiolites mainly consist of harzburgite, clinopyroxene peridotite, pyrolite, troctolite and gabbo. The characteristics of Kaimeng ophiolites are low content in Si, K2O ＋ Na2O and Ti, and high content in Mg and Fe. Kaimeng ophiolites have low ∑REE（ ∑REE =0. 415 × 10^-6 - 3. 273 × 10^-6） and show ∑ LREE-rich in the harzburgite and troctolites, but ∑ LREE-depletion in the clinopyroxene peridotite and gabboes. All rocks show obvious Eu positive anomaly. The Th-Ta ratios of gabbo are almost equal, which show that Kaimeng ophiolites were originated from the MORB, but in the curves of the trace element distribution patterns normalized by the primitive mantle, Kaimeng ophiolites exhibit Rb, Ba, Sr-rich and Nb-depletion, showing that the ophiolites were formed in the island-arc setting（IAT）. Therefore, this group of rocks not only shows MORB feature, but also exhibits IAT characteristics, which reveal that Kaimeng ophiolites should be formed in the immature back-arc basin tectonic setting. The SHRIMP U-Pb dating results on zircon from the troctolite is 218.2 ±4.6Ma, showing that the ocean crust appeared in the late Triassic. The definition of Kaimeng ophiolite shows that Jiali fault may be an important suture in Gangdese tectonic belt during T3 -J1 .

关 键 词：凯蒙蛇绿岩 地球化学特征 SHRIMP锆石U-PB定年 构造意义 嘉黎断裂带 西藏 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]