本文選題：堿性基性巖脈 + 酸性火山巖��； 參考：《中國科學(xué)院研究生院(海洋研究所)》2015年碩士論文

【摘要】：東昆侖造山帶是青藏高原北部一條重要的構(gòu)造巖漿帶,廣泛分布晚三疊世火山巖(T3e)。本論文以東昆侖造山帶晚三疊世基性巖脈(輝綠巖)和酸性火山巖(流紋質(zhì)凝灰?guī)r、流紋斑巖)為研究對象,通過鋯石U-Pb年代學(xué)、主微量元素和同位素地球化學(xué)分析測試方法,對該套(次)火山巖的成因及構(gòu)造意義取得了初步認(rèn)識(shí)。東昆侖造山帶晚三疊世(次)火山巖的噴發(fā)年齡為228-218 Ma。其中基性巖脈侵入更老(~8-9 Myrs)、更大的A型花崗巖巖基內(nèi),代表演化的堿性玄武質(zhì)巖漿。這些巖脈的微量元素配分模式與現(xiàn)今洋島玄武巖(OIB)的特征相似,但也具有微弱的大陸地殼特征(即富集Rb、Pb,虧損Nb、Ta、Ti)。它們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)的同位素特征表明該基性巖脈起源于交代富集的大陸巖石圈地幔,并在上升過程中經(jīng)歷了地殼混染。酸性火山巖具有高LREE/HREE比值(如,[La/Yb]N=5.71-17.00),負(fù)Eu異常和Sr、P強(qiáng)烈虧損的特征,類似于典型的大陸上地殼組成(UCC)。高的87Sr/86Sr(0.7213-0.7550),相對UCC虧損的εNd(t)(-3.83~-5.09)和εHf(t)(-3.06~-3.83),以及和同期基性巖脈、高Nb-Ta流紋巖(東昆侖造山帶晚三疊世殼-幔混合型火山巖)重疊的同位素比值,表明酸性火山巖是由45-50%地殼物質(zhì)和50-55%幔源基性巖漿(很可能是基性巖脈的母巖漿)混合的產(chǎn)物。其中幔源基性巖漿作為新生地殼物質(zhì)底侵或侵入至下地殼,該新生地殼受后來底侵巖漿的加熱發(fā)生部分熔融產(chǎn)生酸性火山巖。我們提出東昆侖造山帶晚三疊世基性巖脈和酸性火山巖的形成與后碰撞伸展及相關(guān)的造山垮塌有關(guān),這樣的構(gòu)造背景對引起軟流圈上涌發(fā)生減壓熔融并導(dǎo)致先前交代富集的巖石圈地幔及地殼熔融的過程至關(guān)重要。本研究對理解造山帶背景下從洋殼俯沖到陸-陸碰撞再到后碰撞過程中巖漿的起源和新生陸殼增生的過程提供了依據(jù)。
[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.
【學(xué)位授予單位】：中國科學(xué)院研究生院(海洋研究所)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2015
【分類號】：P588.14

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