本文選題：安多陸塊　切入點：花崗片麻巖　出處：《中國科學(xué)院大學(xué)(中國科學(xué)院廣州地球化學(xué)研究所)》2017年碩士論文　論文類型：學(xué)位論文

【摘要】：青藏高原是自顯生宙以來由一系列微陸塊、增生帶和島弧連續(xù)地拼貼到歐亞大陸的南部邊緣組成。作為青藏高原內(nèi)部最古老的地體,安多陸塊以透鏡體形式夾持于羌塘地體和拉薩地體之間,發(fā)育大量新元古代至早中生代不同類型的巖漿巖,記錄了不同時期的地質(zhì)演化過程。在本研究中,我們對安多陸塊新元古代、早古生代和早中生代三期主要的巖漿巖開展了詳細的野外調(diào)查和室內(nèi)巖相學(xué)研究,并進行了精確的鋯石U-Pb年代學(xué)和Lu-Hf同位素研究、全巖主-微量元素地球化學(xué)和Sr-Nd同位素地球化學(xué)分析,旨在揭示安多陸塊三期巖漿巖的組成、形成時代、巖石成因和構(gòu)造環(huán)境,恢復(fù)地質(zhì)演化過程,并探討安多陸塊大陸地殼的形成、改造和生長過程。我們對安多陸塊一新元古代花崗質(zhì)片麻巖體的鋯石U-Pb同位素定年結(jié)果揭示其原巖侵位年齡為8~(87) Ma。該花崗質(zhì)片麻巖體具有高的SiO2(65.56~79.82wt.%),A/CNK(1.39~1.68),CIPW剛玉含量(2.74~5.10 wt.%)。結(jié)合富鋁礦物的出現(xiàn)(如白云母),推測其原巖為強過鋁質(zhì)S-型花崗巖。這些花崗質(zhì)片麻巖具有低的CaO/(MgO+FeO~T)、Al2O3/TiO2、Rb/S和Rb/Ba比值,高的CaO/Na2O(0.3),富集的εNd(t)(-5.51~-2.76)和εHf(t)(-15.79~4.56)同位素特征,表明安多新元古代花崗片麻巖原巖很可能是由變泥質(zhì)巖部分熔融形成。大量存在的繼承鋯石也支持這一解釋。這些特點表明,安多新元古代花崗片麻巖原巖很可能是由變泥質(zhì)巖部分熔融形成,樣品高的MgO、Cr、Ni含量揭示其巖漿源區(qū)可能有幔源物質(zhì)或新生地殼物質(zhì)的加入。鋯石U-Pb同位素定年揭示安多陸塊早古生代花崗質(zhì)片麻巖的侵位年齡介于484~500 Ma。這些花崗質(zhì)片麻巖呈現(xiàn)出高且變化較大的SiO2(68.46~~(87).52%)含量,顯示富K(Na2O/K2O1),低的P2O5(0.1%)和弱過鋁質(zhì)的地球化學(xué)特征。結(jié)合角閃石礦物的存在,其這些特征說明其原巖為I-型花崗巖。片麻巖具有富集的同位素(εNd(t)=-10.98~-4.54;εHf(t)=-10.38~1.31)特征,推測其為安多基底部分熔融形成。早古生代花崗質(zhì)片麻巖具有比安多新元古代片麻巖高的εNd(t)和εHf(t)值,結(jié)合同期發(fā)育的鎂鐵質(zhì)巖包體,揭示在其形成的過程中有地幔物質(zhì)的加入。鑒于一些樣品有很高的SiO2、FeO~T/MgO和(K2O+Na2O)/CaO,我們認為這些片麻巖的原巖在形成過程中發(fā)生了一定的巖漿分異作用。通過對安多陸塊早中生代巖漿巖的研究,我們發(fā)現(xiàn)了morb型輝長巖,高鎂閃長巖,鈣堿性閃長巖和花崗閃長巖這四種不同類型的巖石。系統(tǒng)的鋯石u-pb同位素定年顯示這些巖漿巖的結(jié)晶年齡是一致的,主要集中在174-177ma。我們系統(tǒng)的巖石地球化學(xué)研究揭示這四種早中生代巖漿巖具有明顯不同的成因:(1)輝長巖:輝長巖具有相對平坦的稀土配分模式((la/yb)n=0.70~1.13)和虧損的nd和hf同位素組成(εnd(t)=4.40~5.50;εhf(t)=10.33~15.55),其sr-nd同位素組成和ree配分模式類似于班公-怒江洋蛇綠巖,指示安多輝長巖可能來自虧損的軟流圈地幔源區(qū)。安多輝長巖富集大離子親石元素(lile:k、rb、pb)、虧損高場強元素(hfse:nb、ta、ti),高的ba/la(8~15),ba/th(29~150),u/th(0.57~2.73)和低的th/yb(0.05~0.33),顯示含水流體的交代作用在其巖石形成過程中發(fā)揮了重要作用。這些地球化學(xué)特征和富水礦物角閃石的存在,說明了被含水流體交代的富集的巖石圈地幔也參與了安多輝長巖形成過程。故安多陸塊輝長巖可能是由被富集巖石圈地幔交代的軟流圈地幔部分熔融形成。輝長巖具有低的sm/yb和dy/yb,指示了輝長巖熔體熔融深度是在巖石圈地幔的尖晶石相二輝橄欖巖穩(wěn)定區(qū)形成的,形成在較淺的深度(80km)。(2)高鎂閃長巖:安多閃長巖有高的mgo(8.30~10.24wt.%)和mg#(70~74),高的cr(400~547ppm)和ni(120~152ppm),低的hree、sr/y和la/yb比值,類似于贊岐巖的地球化學(xué)特征。其高的k2o(k2o/na2o=3.19~5.04)和ba(1000ppm)含量,la/sm,th/yb和ba/la,低的ba/th和sr/nd比率以及富集的同位素nd和hf(εnd(t)=-10.80~-10.50;εhf(t)=-11.95~-6.47)特征,闡明了安多高鎂閃長巖主要是由俯沖的大洋沉積物熔體和地幔楔橄欖巖熔體反應(yīng)形成。(3)鈣堿性閃長巖:相比于高鎂閃長巖,安多鈣堿性閃長巖有低的mgo、mg#、cr、ni和高的feot。其nd同位素組成(εnd(t)=-7.5~-7.3;εhf(t)=-0.04~-12.33)略高于安多地塊的基底巖石,顯示安多鈣堿性閃長巖是由安多基底部分熔融且有地幔巖漿顯著加入形成。(4)鈣堿性花崗閃長巖:鈣堿性閃長巖富集的nd同位素組成(εnd(t)=-10.80~-10.60)和初始的~(87)sr/86sr((86sr/~(87)sr)i=0.7303~0.7306)接近于安多基底的sr-nd同位素組成(εnd(t)=-7.49~-9.56,(86sr/~(87)sr)i=0.7268~0.7383),說明安多鈣堿性閃長巖為安多基底部分熔融形成。結(jié)合前人研究成果,我們認為安多陸塊三期巖漿巖都形成于大陸邊緣弧構(gòu)造環(huán)境,即俯沖-增生造山系統(tǒng)。安多大陸地殼形成于新元古代早期,此后在早古生和早中生代均經(jīng)歷了顯著的大陸地殼生長與改造作用。
[Abstract]:The Qinghai Tibet Plateau since Phanerozoic consists of a series of micro continental blocks, accretion zone and island arc continuous collage to the southern edge of the Eurasian continent. As the interior of the Qinghai Tibet Plateau the oldest block block to form the lens body, sandwiched between the Qiangtang terrane and Lhasa terrane, the development of a large number of Neoproterozoic to different types of early Mesozoic magmatite, recorded the geological evolution in different periods. In this study, we on the Amdo landmass of Neoproterozoic, early Paleozoic and early Mesozoic period three main magmatite carried out detailed field investigation and indoor petrographic studies, and the study of zircon U-Pb geochronology and Lu-Hf isotope precise the analysis of whole rock major and trace element geochemistry and Sr-Nd isotope geochemistry, which aims to reveal the Amdo block three magmatite formation age, petrogenesis and tectonic environment, geological evolution recovery The formation process, and to explore the continental crust of the Amdo landmass, transformation and growth process of a continental block. We Amdo Neoproterozoic granitic gneissic rock zircon U-Pb isotopic dating results reveal the protolith emplacement age of 8~ (87) Ma. of the granitic gneiss body has a high SiO2 (65.56~79.82wt.%), A/CNK (1.39~1.68), CIPW (2.74~5.10 wt.%). The content of corundum based aluminum rich minerals (such as Bai Yunmu), the original rock is peraluminum S- type granite. The granitic gneiss with low CaO/ (MgO+FeO~T), Al2O3/TiO2, Rb/S and Rb/ Ba ratio, high CaO/Na2O (0.3), epsilon Nd enrichment (T) (-5.51~-2.76) and epsilon Hf (T) (-15.79~4.56) isotopic characteristics indicate that the original rocks of Amdo Neoproterozoic granite gneiss were probably formed by partial melting into muddy rocks. There are a lot of inherited zircons also support this interpretation. These features show that the Neoproterozoic granite The original rocks gneiss were probably formed by partial melting of varying argillaceous rock samples, high MgO, Cr, Ni content reveals the magma source region may have added mantle materials or new crustal material. The zircon U-Pb isotopic dating of Amdo landmass Early Paleozoic granitic gneiss emplacement ages of 484~500 Ma. granite gneiss exhibits high and large changes in the SiO2 (68.46~~ (87).52%) content, display K rich (Na2O/K2O1), low P2O5 (0.1%) and the weak geochemical characteristics of aluminum. Combined with amphibole, these characteristics show that the original rock is I- type granite gneiss is enriched. Isotope (Nd (T) =-10.98~-4.54 Hf (T) =-10.38~1.31; E) features that its partial melting of Amdo basement formation. Early Paleozoic granitic gneiss with ancient gneiss is higher than epsilon Nd (T) and Amdo Singapore epsilon Hf (T) value, combined with the earlier development of magnesium iron Rock xenoliths, reveal the process of the formation of the adding of mantle material. In view of some samples with high SiO2, FeO~T/MgO and /CaO (K2O+Na2O), we believe that the protolith of gneiss in the forming process of the occurrence of certain magmatic differentiation. Through the study of Amdo block early Mesozoic magmatite, we found MORB gabbro, high Mg diorite, calc alkaline diorite and granodiorite of the four types of rocks. The zircon U-Pb isotopic system dating indicates that the crystallization age of the magmatite is consistent, mainly concentrated in the 174-177ma. study of rock geochemistry of our system to reveal the four kinds of early Mesozoic magmatite has different causes: (1): gabbro gabbro has relatively flat REE pattern ((la/yb) n=0.70~1.13) nd and Hf isotopic composition and loss (epsilon Nd (T) =4.40~5.50; HF (T) =10.33~15. .55), the Sr-Nd REE isotopic composition and distribution patterns similar to the Bangong Lake Nu River ocean ophiolite, indicating possible loss from Amdo gabbro asthenospheric mantle source. Amdo gabbro enrichment of large ion lithophile elements (lile:k, Rb, Pb) and depleted in high field strength elements (hfse:nb, Ta, Ti), high ba/la (8~15), ba/th (29~150), u/th (0.57~2.73) and low th/yb (0.05~0.33), showed an aqueous fluid metasomatism formation plays an important role in the process of its rocks. These geochemical characteristics and water rich amphibole mineral exists, explain the process of forming an aqueous fluid metasomatism enrichment the lithospheric mantle in the continental block. Amdo gabbro gabbro Amdo may be formed by partial melting of the asthenospheric mantle is enriched lithospheric mantle metasomatism. Gabbro with low sm/yb and dy/yb, indicating that the gabbro melting depth is in the lithospheric mantle 灝栨櫠鐭崇浉浜岃緣姍勬宀╃ǔ瀹氬尯褰㈡垚鐨，

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