东昆仑造山带晚三叠世（次）火山岩的成因及构造意义

发布时间：2018-06-01 01:41

本文选题：碱性基性岩脉 + 酸性火山岩　；参考：《中国科学院研究生院(海洋研究所)》2015年硕士论文

【摘要】：东昆仑造山带是青藏高原北部一条重要的构造岩浆带,广泛分布晚三叠世火山岩(T3e)。本论文以东昆仑造山带晚三叠世基性岩脉(辉绿岩)和酸性火山岩(流纹质凝灰岩、流纹斑岩)为研究对象,通过锆石U-Pb年代学、主微量元素和同位素地球化学分析测试方法,对该套(次)火山岩的成因及构造意义取得了初步认识。东昆仑造山带晚三叠世(次)火山岩的喷发年龄为228-218 Ma。其中基性岩脉侵入更老(~8-9 Myrs)、更大的A型花岗岩岩基内,代表演化的碱性玄武质岩浆。这些岩脉的微量元素配分模式与现今洋岛玄武岩(OIB)的特征相似,但也具有微弱的大陆地壳特征(即富集Rb、Pb,亏损Nb、Ta、Ti)。它们的微量元素特征以及高87Sr/86Sr(0.7076-0.7104),低εNd(t)(-2.18~-3.46)和低εHf(t)(-2.85~-4.59)的同位素特征表明该基性岩脉起源于交代富集的大陆岩石圈地幔,并在上升过程中经历了地壳混染。酸性火山岩具有高LREE/HREE比值(如,[La/Yb]N=5.71-17.00),负Eu异常和Sr、P强烈亏损的特征,类似于典型的大陆上地壳组成(UCC)。高的87Sr/86Sr(0.7213-0.7550),相对UCC亏损的εNd(t)(-3.83~-5.09)和εHf(t)(-3.06~-3.83),以及和同期基性岩脉、高Nb-Ta流纹岩(东昆仑造山带晚三叠世壳-幔混合型火山岩)重叠的同位素比值,表明酸性火山岩是由45-50%地壳物质和50-55%幔源基性岩浆(很可能是基性岩脉的母岩浆)混合的产物。其中幔源基性岩浆作为新生地壳物质底侵或侵入至下地壳,该新生地壳受后来底侵岩浆的加热发生部分熔融产生酸性火山岩。我们提出东昆仑造山带晚三叠世基性岩脉和酸性火山岩的形成与后碰撞伸展及相关的造山垮塌有关,这样的构造背景对引起软流圈上涌发生减压熔融并导致先前交代富集的岩石圈地幔及地壳熔融的过程至关重要。本研究对理解造山带背景下从洋壳俯冲到陆-陆碰撞再到后碰撞过程中岩浆的起源和新生陆壳增生的过程提供了依据。
[Abstract]:The East Kunlun orogenic belt is an important tectonic magma belt in the northern part of the Qinghai-Xizang Plateau, widely distributed in late Triassic volcanic rocks. In this paper, the late Triassic basic vein (diabase) and acid volcanic rock (rhyolitic tuff, stream porphyry) in the east Kunlun orogenic belt were studied. The zircon U-Pb chronology, main trace elements and isotopic geochemistry were used. A preliminary understanding of the genesis and tectonic significance of this suite of volcanic rocks has been obtained. The eruption age of the late Triassic volcanic rocks in the East Kunlun orogenic belt is 228-218 Ma. Among them, the basic vein intruded into the older, 8-9 Myrsl, and the larger A-type granitic rocks represented the evolution of alkaline Black Tortoise magma. The microelement distribution patterns of these veins are similar to those of the present oceanic basalt OIBs, but they also have weak continental crustal characteristics (i.e., enrichment of RbPb, depletion of NbPb and depletion of NbPb). Their trace element characteristics and isotopic characteristics of high 87s Sr / 86 Sr ~ (6 +) 0.7076-0.7104 ~ (4), low 蔚 -Ndndltni-2.18 ~ (18) ~ 3.46) and low 蔚 ~ (-HfT) -2.85 ~ (-4.59) indicate that the basic vein originated from the metasomatic enriched continental lithospheric mantle and experienced crustal mixing during the uplift process. Acid volcanic rocks are characterized by high LREE/HREE ratios (e.g., [La/Yb] NV 5.71-17.00), negative EU anomalies and strong SRP depletion, similar to typical upper continental crustal composition. The isotopic ratios of the high 87s / 86s rout 0.7213-0.7550, relative to the UCC depleted 蔚 Ndndltndlt-3.83- 5.09) and the 蔚 Hftntler-3.06- 3.83G, as well as the high Nb-Ta rhyolite (late Triassic crust-mantle mixed volcanic rocks of the eastern Kunlun orogenic belt) overlapped with the corresponding basic veins. It is shown that acidic volcanic rocks are the product of mixing 45-50% crustal material with 50-55% mantle derived basic magma (probably the parent magma of the basic-vein). Among them, the mantle derived basic magma, as the material of the new crust, undergoes or invades to the lower crust, and the new crust is partially melted to produce acidic volcanic rock by the heating of the later underplating magma. We suggest that the formation of the late Triassic basic vein and acid volcanic rocks in the East Kunlun orogenic belt is related to the post-collision extension and related orogenic collapse. Such a tectonic setting is crucial to the process of subdued melting of the upwelling of the asthenosphere and the melting of the previously metasomatic enriched lithospheric mantle and crust. This study provides a basis for understanding the origin of magma from oceanic crust subduction to continental collision to post collision in the background of orogenic belt and the process of new continental crust proliferation.
【学位授予单位】：中国科学院研究生院(海洋研究所)
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P588.14

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