| 西藏冈底斯西段鲁尔玛晚三叠世二长闪长岩的成因 |
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| 引用本文： | 刘洪,张林奎,黄瀚霄,李光明,欧阳渊,吕梦鸿,刘函,兰双双,闫国强.西藏冈底斯西段鲁尔玛晚三叠世二长闪长岩的成因[J].地球科学,2019,44(7):2339-2352. |
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| 作者姓名： | 刘洪 张林奎 黄瀚霄 李光明 欧阳渊 吕梦鸿 刘函 兰双双 闫国强 |
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| 作者单位： | 1.中国地质调查局成都地质调查中心, 四川成都 610081 |
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| 基金项目： | 中国地质调查项目DD20190542中国地质调查项目DD20190147国家重点研发计划项目2016YFC0600308国家重点研发计划项目SQ2018YFC060162中国地质调查项目DD20160015 |
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| 摘 要： | 晚三叠世及以前岩浆活动研究的缺乏严重制约了雅鲁藏布新特提斯洋演化模式的讨论.为探讨冈底斯西段打加错地区晚三叠世岩浆活动的地球动力学背景，对鲁尔玛二长闪长岩开展了岩石学、地球化学和年代学研究工作.利用LA-ICP-MS方法测得2件二长闪长岩样品的锆石206Pb/238U年龄加权平均值为212.1±0.6 Ma(MSWD=0.97)和212.8±0.2 Ma(MSWD=0.74)，表明鲁尔玛中性岩浆活动发生在晚三叠世.二长闪长岩主要由斜长石、钾长石、普通角闪石等组成，并含少量石英、普通辉石、黑云母、磁铁矿、磷灰石等，具有中等的SiO2(50.75%~54.69%)含量，高的K2O(2.71%~3.99%)和总碱(K2O+Na2O=5.84%~8.65%)含量，以及中等的A12O3(13.77%~19.17%)含量，较高的CaO(4.90%~10.14%)含量.里特曼指数(σ43)为3.40~7.65，A/CNK值为0.56~1.00，表明鲁尔玛二长闪长岩属于准铝质钾玄岩系列岩石.岩石富集轻稀土元素(LREE)和大离子亲石元素(LILE)，相对亏损重稀土元素和高场强元素(HFSE)，轻重稀土分异较弱(LREE/HREE=7.45~11.10)，并具轻微的Eu负异常(δEu=0.83~0.95)，无Ce异常(0.93~1.04).相对较低的87Sr/86Sr初始比值((87Sr/86Sr)t=0.705 532~0.706 135，平均0.705 852)、正的εHf(t)值(4.97~14.10，平均8.77)、年轻的地壳模式年龄(TDM2=348~930 Ma，平均为686 Ma)，(143Nd/144Nd)t和εHf(t)分别为0.512 639~0.512 669(平均为0.512 657)和0.08~0.67(平均为0.43)，(206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t分别为:18.235~18.521(平均为18.411)、15.593~15.651(平均为15.617)和39.948~38.579(平均为38.401)，指示二长闪长岩起源于新生地壳.Sr-Nd-Pb-Hf同位素及微量元素特征表明鲁尔玛二长闪长岩为新生地壳部分熔融的产物.研究结果揭示，鲁尔玛二长闪长岩体是晚三叠雅鲁藏布新特提斯洋北向俯冲初期，新生地壳熔融形成的准铝质碱性钾玄岩系岩石.该岩体的研究表明，在晚三叠世(~213 Ma)，冈底斯西段的打加错地区，雅鲁藏布新特提斯洋壳已经向北俯冲到南拉萨微陆块之下.
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| 关 键 词： | 拉萨地块 打加错 锆石U-Pb年代学 Sr-Nd-Pb-Hf同位素 岩石成因 地球化学 |
| 收稿时间： | 2019-01-31 |
Petrogenesis of Late Triassic Luerma Monzodiorite in Western Gangdise,Tibet, China |
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| Abstract: | The study on the evolution model of the New Tethys Ocean in Yarlung Zangbo has been seriously restricted due to the lack of in-depth research on magmatism during and even before the Late Triassic In order to discuss the geodynamic background of the Late Triassic magmatism in Dajia Co area of western Gangdise, this paper presents a study on petrology, geochemistry and chronology of the Ruerma monzodiorite. LA-ICP-MS zircon U-Pb analyses suggest that monzodiorite has two weighted mean 206Pb/238U ages of 212.1±0.6 Ma (mean square weighted deviation=0.97), and 212.8±0.2 Ma (mean square weighted deviation=0.74), which suggests that the Luerma magmatism event took place during the late Triassic epoch. The rocks contain plagioclase, potassium feldspar and amphibole, and contain a small amount of quartz, pyroxene, biotite, magnetite and apatite. They have moderate contents of SiO2 (50.75%-54.69%), and A12O3 (13.77%-19.17%), high contents of K2O (2.71%-3.99%), total K2O+Na2O (5.84%-8.65%), and CaO (4.90%-10.14%), with a Reitman index (σ43) of 3.40 to 7.65, and A/CNK values of 0.56 to 1.00. These characteristics suggest that the Luerma monzodiorite is a quasi-aluminous and alkaline-shoshonite series. The rocks are enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements(HFSE), with LREE/HREE ratios of 7.45 to 11.10. They have weakly negative Eu anomalies of 0.83 to 0.95 without obvious Ce anomalies (δCe=0.93-1.04). The relatively low initial 87Sr/86Sr ratios of 0.705 532 to 0.706 135, positive εHf(t) values of 4.97 to 14.10, and two-stage Hf model ages (TDM2) ranging from 348 Ma to 930 Ma, indicate that the rocks derived from a juvenile crust. The monzodiorite's (143Nd/144Nd)t values range from 0.512 639 to 0.512 669, their εHf(t) values range from 0.08 to 0.67, their (206Pb/204Pb)t values range from 18.235 to 18.521, their (207Pb/204Pb)t values range from 15.593 to 15.651, and their (208Pb/204Pb)t values range from 39.948 to 38.579. These data indicate that the Luerma monzodiorite was formed by partial melting of the juvenile crust. We propose that the Luerma monzodiorite was formed during the northward subduction of Yarlu Zangbo oceanic crust of the Neo Tethys Ocean. |
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