首页> 外文期刊>Acta geologica Sinica: Journal of the Geological Society of China >Geochronologic Constraints on the Magmatic Underplating of the Gangdise Belt in the India-Eurasia Collision: Evidence of SHRIMP II Zircon U-Pb Dating
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Geochronologic Constraints on the Magmatic Underplating of the Gangdise Belt in the India-Eurasia Collision: Evidence of SHRIMP II Zircon U-Pb Dating

机译:印度洋-欧亚大陆碰撞恒河带岩浆下的地质年代学约束:SHRIMP II锆石U-Pb约会的证据

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Abundant small mafic intrusions occur associated with granitoids along the Gangdise magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends approx 1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations., along the Lhasa-Xigaze segment of the mafic intrusive zone, and was followed by zircon SHRIMP II U-Pb dating. In addition to the ages of two samples previously published (47.0 +- 1 Ma and 48.9 +- 1.1 Ma), the isotopic ages of the remaining four gabbro samples are 51.6 +- 1.3Ma, 52.5 +- 3.0 Ma, 50.2 +- 4.2Ma and 49.9 +- 1.1Ma. The range of these ages (47-52.5 Ma) provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP II U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma (weighted average age 503 +- 10 Ma), thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick "crust-mantle transitional zone" characterized by Vp = 6.85-6.9 km/s.
机译:沿岗迪斯岩浆带上的花岗岩类伴有大量的铁质基性侵入。除了在花岗岩类岩体中有许多离散的辉长岩体外,辉长辉绿岩带也沿着Gangdise带的南缘出现在Yarlung Zangbo缝合线的北部。镁铁质侵入带在空间上对应于一个强烈的航磁异常,其延伸约1400 km。镁铁质侵入体由间断分布的小物体和辉长岩和白云石的堤防组成,并积聚了辉石岩,橄榄石辉石岩,变质辉石辉石和角闪石。许多证据表明,冈底斯辉长辉石-辉石岩的组合很可能是地幔衍生的岩浆底盘作用的结果。在沿镁铁质侵入带的拉萨-日喀则段的六个位置对镁铁质岩石进行了详细的现场调查和系统采样,然后进行了锆石SHRIMP II U-Pb测年。除了先前发布的两个样本的年龄(47.0±1 Ma和48.9±1.1Ma)之外,其余四个辉长岩样品的同位素年龄分别为51.6±1.3Ma,52.5±3.0Ma,50.2±4.2。 Ma和49.9 +-1.1Ma。这些年龄范围(47-52.5 Ma)为大约在冈底斯带下方岩浆下新世始新时期提供了地质年代学约束。 50毫安这次地下活动是印度与欧亚大陆碰撞发生的前一千五百万年,与岩浆混合过程同时发生。 SHRIMP II U-Pb同位素分析还发现了一些锆石晶粒中的几个老年龄,大多数年龄在479-526 Ma之间(加权平均年龄503 +-10 Ma),因此可以得出有关先前存在的下地壳的信息。发生了镁铁质岩浆的下沉。人们认为,岩浆下沉是印度洋-欧亚大陆碰撞期间地壳生长的主要机制之一,可能与时间上对应的形成了以Vp = 6.85-为特征的14-16 km厚的“地壳-幔幔过渡带” 6.9公里/秒

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