本文選題：Sr-Nd-Pb-Os-Mg-O同位素 + 鋯石U-Pb-Hf同位素��； 參考：《中國地質(zhì)大學(xué)(北京)》2017年博士論文

【摘要】：青藏高原南部的拉薩地塊在后碰撞構(gòu)造-巖漿演化階段發(fā)育了一套與東西向伸展構(gòu)造伴生的漸新世-中新世鉀質(zhì)-超鉀質(zhì)火山巖和埃達(dá)克質(zhì)含礦斑巖。作為青藏高原演化過程中深部動(dòng)力學(xué)過程的巖漿活動(dòng)響應(yīng),這套后碰撞巖漿活動(dòng)記錄為探討高原隆升機(jī)制、揭示巖石圈深部物質(zhì)組成、以及示蹤特提斯洋殼俯沖消減和印度-歐亞大陸匯聚過程中的沉積物俯沖交代作用提供了重要視角,具有很高的科研價(jià)值。然而,對(duì)于藏南幔源超鉀質(zhì)巖石的成因、其地幔源區(qū)交代富集作用和中新世藏南深部動(dòng)力學(xué)過程的理解卻明顯不足。與此同時(shí),鉀質(zhì)中酸性火山巖被認(rèn)為具有和后碰撞埃達(dá)克質(zhì)侵入巖相似的地球化學(xué)特征,因此被視為一種特殊的埃達(dá)克質(zhì)巖石,但是這種觀點(diǎn)顯然忽略了兩者之間可能存在的成因差異。本文以藏南幔源超鉀質(zhì)火山巖(包括捕獲的地幔橄欖巖包體和殼源包體)和同時(shí)代的鉀質(zhì)中酸性巖石為主要研究對(duì)象,進(jìn)行了系統(tǒng)的鋯石U-Pb年代學(xué)和原位Lu-Hf同位素研究,并對(duì)全巖Sr-Nd-Pb-Os和Mg-O同位素組成進(jìn)行了準(zhǔn)確測(cè)定。借助鋯石U-Pb定年、微量元素和Lu-Hf同位素分析,本文從藏南超鉀質(zhì)火山巖中發(fā)現(xiàn)了大量大陸地殼起源的元古代-新生代鋯石捕擄晶。通過對(duì)比拉薩地塊的碎屑鋯石和巖漿活動(dòng)記錄,本文發(fā)現(xiàn)超鉀質(zhì)巖漿在演化過程中受到了拉薩地塊地殼物質(zhì)的混染。超鉀質(zhì)巖石鋯石捕擄晶的DyN/YbN和U/Yb從~55 Ma開始發(fā)生系統(tǒng)性的升高,這種現(xiàn)象被解讀為記錄了印度-歐亞大陸匯聚導(dǎo)致拉薩地塊地殼自早古近紀(jì)以來的持續(xù)加厚過程,表明幔源巖漿捕獲的鋯石捕擄晶可以用來反演上覆地殼的演化。全巖Os-Sr-Mg同位素分析進(jìn)一步證實(shí)了地殼混染作用對(duì)后碰撞幔源超鉀質(zhì)巖漿活動(dòng)的同位素組成具有深遠(yuǎn)影響,同時(shí)還發(fā)現(xiàn)新特提斯洋殼富含碳酸鹽的沉積物在北向俯沖過程中對(duì)藏南巖石圈地幔的交代與改造作用。受到地殼混染影響較小的超鉀質(zhì)巖石樣品具有低的187Os/188Os,這些低187Os/188Os樣品的δ~(26)Mg與~(87)Sr/~(86)Sr呈負(fù)相關(guān)關(guān)系,與Hf/Sm則為正相關(guān)關(guān)系,表明高鎂碳酸鹽(例如白云石)的交代作用能夠有效改變地幔源區(qū)的Mg同位素組成。拉薩地塊超鉀質(zhì)巖石的δ~(26)Mg和Hf/Sm在空間分布上向北逐漸降低的趨勢(shì)表明地幔碳酸鹽交代作用會(huì)隨著新特提斯洋板片俯沖深度的增加而逐漸增強(qiáng)。盡管鉀質(zhì)火山巖和埃達(dá)克質(zhì)侵入巖均展現(xiàn)出高Sr/Y和La/Yb、低Y和Yb的地球化學(xué)特征,但是這兩種殼源后碰撞巖漿巖具有不同的時(shí)空分布和同位素變化特征。后碰撞埃達(dá)克質(zhì)侵入巖主要出露在南部拉薩地塊,起源于加厚下地殼的部分熔融,其非放射性成因的~(143)Nd/~(144)Nd以及其同巖漿鋯石εHf(t)從35Ma開始迅速減低的趨勢(shì)早于幔源超鉀質(zhì)巖漿活動(dòng)的開始,表明俯沖印度大陸地殼物質(zhì)對(duì)埃達(dá)克質(zhì)巖漿活動(dòng)有重要貢獻(xiàn)。另一方面,同時(shí)代的鉀質(zhì)中酸性火山巖與幔源超鉀質(zhì)巖石擁有相同的時(shí)空分布特征,這兩種巖石的全巖Pb同位素和鋯石Hf同位素組成不僅證實(shí)了兩者與拉薩地塊的親緣性,還表明深俯沖的印度大陸地殼物質(zhì)在巖石形成過程中并未扮演主要角色。通過對(duì)比拉薩地塊不同地區(qū)的后碰撞巖漿活動(dòng)記錄,本文發(fā)現(xiàn)早中新世鉀質(zhì)-超鉀質(zhì)巖石在野外呈現(xiàn)類似雙峰式火山巖的分布形式,兩者在同位素組成上的差異表明幔源超鉀質(zhì)巖漿對(duì)同時(shí)代鉀質(zhì)火山巖的直接貢獻(xiàn)很少;隨著拉薩地塊后碰撞巖漿活動(dòng)向東部遷移,晚中新世的鉀質(zhì)火山巖擁有與超鉀質(zhì)巖石相近的同位素組成,暗示兩者具有更加密切的成因聯(lián)系�？紤]到與青藏高原加速隆升相關(guān)的一系列地質(zhì)事件在中新世集中爆發(fā),包括剝蝕-沉積速率加劇、海水~(87)Sr/~(86)Sr比值劇烈變化、以及構(gòu)造應(yīng)力格架轉(zhuǎn)變等,拉薩地塊漸新世-中新世的鉀質(zhì)-超鉀質(zhì)巖漿活動(dòng)可能與該時(shí)期藏南加厚巖石圈地幔底部對(duì)流減薄或拆沉作用有關(guān)。
[Abstract]:The Lhasa massif in the southern part of the Qinghai Tibet Plateau developed a set of Oligocene Miocene potash and hyper kaliplic volcanic rocks associated with east-west extensional tectonics in the post collisional tectonic magmatic evolution stage. As a response to the magmatic activity of the deep dynamic process during the evolution of the Qinghai Tibet Plateau, the post collisional magmatism record was recorded. In order to explore the mechanism of uplift of the plateau, to reveal the composition of the deep lithosphere, and to trace the subduction and subduction of the subduction of the Tethys oceanic crust and the subduction and metasomatism of the sediment in the India Eurasia continent, it is of great value for scientific research. However, the genesis of the mantle source rocks in the mantle source in Zangnan, and the metasomatism and enrichment of the mantle source region. The deep understanding of the deep dynamic process in the middle Miocene of Zangnan is obviously inadequate. At the same time, the acid volcanic rocks in the potash are considered to have similar geochemical characteristics with the post collision of the edacitic intrusive rocks, so they are considered a special kind of edacic rocks, but this view obviously ignores the possible cause of the difference between the two. In this paper, a systematic study of zircon U-Pb chronology and in situ Lu-Hf isotopes was carried out on the mantle derived ultra potassic volcanic rocks (including the captured mantle peridotite xenoliths and crust source enclaves) and the intermediate acid rocks of the same epoch, and the composition of Sr-Nd-Pb-Os and Mg-O isotopes of the whole rock was accurately measured with the aid of zirconium. The dating of stone U-Pb, trace elements and Lu-Hf isotopes found a large number of Proterozoic Cenozoic zircon captive crystals from the Zangnan super potassic volcanic rocks. By comparing the detrital zircon and magmatic records of the Lhasa massif, this paper found that the super potassic magma was subjected to the crustal substance of the Lhasa massif during the evolution process. The DyN/YbN and U/Yb of the zircon captive crystals of the super potassic rocks began to rise systematically from ~55 Ma. This phenomenon was interpreted as a record of the continuous thickening of the crust of the Lhasa massif since the early Palaeogene of India and Eurasia, indicating that the captive zirconium captive crystals captured by the mantle derived magma could be used to invert the overlying crust. The Os-Sr-Mg isotopic analysis of the whole rock further confirms that the crustal contamination has a profound influence on the isotopic composition of the ultra potassic magmatism of the post collision mantle source. Meanwhile, it is also found that the carbonate deposits of the new Tethys oceanic crust have been metasomatic and reformed in the Zangnan lithosphere mantle during the North subduction. The smaller super potash rock samples have low 187Os/188Os, and the delta ~ (26) Mg of these low 187Os/188Os samples is negatively correlated with ~ (87) Sr/~ (86) Sr, and is positively correlated with Hf/Sm, indicating that the metasomatism of high magnesium carbonate (such as dolomite) can effectively change the Mg isotopic composition of the mantle source region. (26) the trend of decreasing the spatial distribution of Mg and Hf/Sm indicates that the mantle carbonate metasomatism will gradually increase with the increase of the subduction depth of the new Tethys plate. Although the potassium volcanic rocks and the intrusive rocks all show high Sr/Y and La/Yb, low Y and Yb geochemistry characteristics, but these two shells collide with magma. The post collisional EDK intrusive rocks are mainly exposed in the southern Lhasa block, which originated from partial melting of the thickened lower crust. The non radioactive ~ (143) Nd/~ (144) Nd and its magmatic zircon epsilon Hf (T) rapidly decreased from 35Ma to the mantle derived ultra potassic magmatism. At the beginning, it shows that the subduction of the crust in the continental crust of India has an important contribution to the arkic magmatic activity. On the other hand, the same epoch potassium medium acid volcanic rocks have the same temporal and spatial distribution characteristics with the mantle derived super potassic rocks. The total rock Pb isotopes of the two rocks and the zircon Hf isopsin not only confirmed the relationship between the two types of rocks and the Lhasa massif. It is also shown that the deep subduction of the crust of the India continental crust did not play a major role in the process of rock formation. By comparing the records of post collision magma activities in different regions of the Lhasa massif, it was found that the early Miocene potassic super potassic rocks were distributed in the field similar to the Shuangfeng type volcanic rocks in the field, both of which were on the isotopic composition. The difference shows that the direct contribution of the mantle derived super potassic magma to the same epoch potassium volcanic rocks is very little. With the migration of the post collisional magmatic activity to the East, the late Miocene potassic volcanic rocks have a similar isotopic composition with the super potassic rocks, suggesting that the two have a more close genetic connection. The acceleration uplift of the Qinghai Tibet Plateau is considered. A series of related geological events erupted in the Miocene, including the intensification of erosion and deposition, the violent change of the ~ (87) Sr/~ (86) Sr ratio and the tectonic stress transition, and the potassium and super potassium magmatic activity of the Oligocene Miocene in the Lhasa massif may be thinned or removed from the bottom of the thick lithosphere mantle at the time of Zangnan. It's relevant.
【學(xué)位授予單位】：中國地質(zhì)大學(xué)(北京)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：P588.1
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本文編號(hào)：1943792