Petrogenesis of Murgab gabrro-diorite from Pamir:Evidence from zircon U-Pb dating, Hf isotopes and lithogeochemistry
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摘要:
穆尔尕布岩体位于塔吉克斯坦帕米尔地区中部，中帕米尔和南帕米尔之间的Rushan-Pshart缝合带中，岩石类型主要由辉长岩和少量闪长岩组成，呈岩株状侵入于新元古代（?）萨雷吉尔加组浅变质碎屑岩中。根据LA-ICPMS锆石U-Pb定年结果，穆尔尕布岩体中辉长岩的年龄为（232.0±1.5）Ma，闪长岩的年龄为（231.5±1.9）Ma，两者在误差范围内一致，代表了该岩体的形成时代。辉长岩和闪长岩中锆石的εHf（t）值变化范围分别为4.8~12.1、6.4~10，加权平均值为8.1±1.5（MSWD=6.5）和7.9±0.8（MSWD=2.4），显示其原岩来源于地幔物质，其单阶段Hf模式年龄TDM1分别为477~621 Ma，391~672 Ma，指示其原岩为寒武纪-前寒武纪基底。岩石地球化学研究表明，辉长岩类具有贫碱、低Al、富Mg特征，属于低钾（拉斑）系列，闪长岩类则显示富Si、Al，贫Mg、低Ti的特征，属于钙碱性-高钾钙碱性系列；两者的稀土和微量元素特征相似，稀土总量高，呈轻稀土富集的右倾型配分型式，无Eu异常或轻微正Eu异常，微量元素富集大离子亲石元素，亏损高场强元素，指示穆尔尕布岩体可能形成于岛弧环境。综合区域地质资料，认为在晚三叠世Rushan洋陆俯冲尚未结束，表明洋盆闭合时限晚于232 Ma。
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关键词:
- 帕米尔 /
- 穆尔尕布岩体 /
- LA-ICP-MS锆石U-Pb年龄 /
- 锆石Hf同位素
Abstract:Located in the central part of Pamir, Tajikistan, near the Rushan-Pshart suture zone, the Murgab pluton separates the middle Pamir from southern Pamir. Petrologically, the pluton consists of gabrro and diorite which intruded into the epimetamorphic detrital rocks of late Proterozoic (?). LA-ICP-MS zircon U-Pb dating shows that the age of gabbro and that of diorite is 232±1.5 Ma and 231.5±1.9 Ma respectively, which represents the formation age of this pluton. The εHf (t) values of the zircon are in the range of 4.8-12.1 and 6.4-10, with a weighted average of 8.1±1.5 (MSWD=6.5) and 7.9±0.8(MSWD=2.4), suggesting that its original rock was derived from mantle materials. The one-stage Hf model ages (TDM1) are 477-621 Ma, 391-672 Ma respectively indicating an original rock of Cambrian-Precambrian basement. The lithogeochemical data shows that gabbros are characterized by rich Mg and, poor Al and alkali, thus belonging to low-potassium rocks, whereas the diorites are rich in Si, Al and poor in Mg, Ti, hence belonging to calc-alkaline to high K calc-alkaline rocks. The gabbros and diorites share similar REE and trace elements features, which are characterized by high total REE content and rich LREE with slight normal Eu anomaly. The trace elements are characterized by rich LILEs and poor HFES. Based on geochemical characteristics, the authors infer that the pluton formed in an island-arc environment. The subduction was continuing during late Triassic, which indicates that the Rushan Ocean basin was closed at least later than 232 Ma.
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Key words:
- Pamir /
- Murgab complex /
- zircon LA-ICP-MS U-Pb dating /
- zircon Hf isotopes
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图 4 穆尔尕布辉长岩-闪长岩稀土元素球粒陨石标准化配分型式图（a）及微量元素原始地幔标准化蛛网图（b）（球粒陨石标准值、原始地幔标准值据Sun et al., 1989）
Figure 4.
表 1 帕米尔地区穆尔尕布基性杂岩体主量元素(%)、微量和稀土元素(10-6)分析结果
Table 1. Abundances of major elements (%), trace elements and rare earth elements (10-6) of the Murgub mafic complex in Pamir area
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表 2 穆尔尕布辉长-闪长岩中锆石LA-ICP-MS U-Pb分析结果
Table 2. LA-ICP-MS zircon U-Pb dating results of Murgab gabrro and diorite
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表 3 帕米尔穆尔尕布辉长-闪长岩锆石Lu-Hf同位素组成
Table 3. Zircon Lu-Hf isotope data for gabbro and diorite of Murgab, Pamir
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