本文選題：西秦嶺造山帶 + 鉀霞橄黃長巖　； 參考：《西北大學》2017年碩士論文

【摘要】：鉀霞橄黃長巖系是世界上少有的堿性火山巖(超鉀質(zhì)-鉀質(zhì)系列),其巖漿起源深度較大,包含大量的地幔包體和捕虜體,并且該巖系通常和火成碳酸巖共生,因此,鉀霞橄黃長巖、包體和共生的碳酸巖均是研究地球深部物質(zhì)的重要窗口。秦嶺造山帶地處華北和揚子兩大克拉通之間,研究區(qū)甘肅西秦嶺位于其西延部分,靠近南北向禮縣-天水斷陷盆地區(qū),發(fā)育了大量的新生代超鉀質(zhì)巖漿巖。本文以禮縣的好梯、白關(guān)、茍家作為研究區(qū),選擇區(qū)內(nèi)新生代鉀霞橄黃長巖作為研究對象,開展了詳細的巖相學、巖石磁學、地球化學、古地磁學、同位素組成研究,以闡明研究區(qū)巖漿巖的巖石成因、源區(qū)特征、巖石磁學特征、古緯度變化,主要取得了以下結(jié)論:前人研究資料表明禮縣鉀霞橄黃長巖噴發(fā)年齡為新生代中新世(7.1-22 Ma)。我們的地球化學研究揭示,該套巖石具有較高的全堿含量K20+Na20(高達6.16wt.%),顯示為鉀質(zhì),堿性的特征。不同于堿性玄武巖,鉀霞橄黃長巖顯示較低的Si02和A1203含量,較高的MgO,CaO和Ti02的含量。鉀霞橄黃長巖的主量元素與MgO沒有明顯的線性關(guān)系,反映了巖石形成過程中結(jié)晶分離作用較弱。此外,鉀霞橄黃長巖富集輕稀土和大離子親石元素,并無Nb、Ta和Eu的異常,與OIB的地球化學性質(zhì)相似。禮縣鉀霞橄黃長巖的Sr-Nd同位素組成表現(xiàn)為均一,同時其具有較高的Mg#值(平均73)和均一的MgO、Ni和Cr含量,排除了顯著的地殼混染作用。結(jié)合其低的初始87Sr/86Sr值(0.7042-0.7051)和虧損的εNd(t)值(+3.2～+3.8),我們推測鉀霞橄黃長巖來源于被碳酸鹽熔體交代的虧損地幔源區(qū),其源區(qū)可能為尖晶石與石榴子石混合的地幔橄欖巖。最新的年代學研究結(jié)果表明禮縣鉀霞橄黃長巖是中新世活動的產(chǎn)物,選擇禮縣地區(qū)好梯、茍家、白關(guān)巖體作為古地磁學的研究對象,通過系統(tǒng)的巖石磁學實驗(飽和等溫剩磁特征、三軸等溫剩磁的系統(tǒng)熱退磁分析和磁滯回線變化曲線),顯示禮縣鉀霞橄黃長巖的載磁礦物主要是磁鐵礦,一少部分樣品存在磁黃鐵礦。本次研究的大部分樣品均具有單分量特征,個別樣品具有雙分量特征。古地磁結(jié)果表明,從新生代以來,西秦嶺地區(qū)在中新世即已接近現(xiàn)代地理位置,古緯度已經(jīng)接近現(xiàn)今緯度,并沒有發(fā)生明顯的向北運動,表明西秦嶺的構(gòu)造格局在此之前已經(jīng)定型。但西秦嶺造山帶可能受到青藏高原隆升作用引起的遠距離構(gòu)造效應(yīng)的影響,發(fā)生了塊體的順時針旋轉(zhuǎn)運動。由于目前研究薄弱,尚有待于進一步探討。
[Abstract]:The potash olivine feldspar series is one of the rare alkaline volcanic rocks in the world (ultrapotash-potash series). Its magma originates in a large depth, including a large number of mantle inclusions and xenoliths, and the series is usually associated with igneous carbonatite. Potash olivine feldspar xenoliths and symbiotic carbonates are important windows for the study of material in the deep part of the earth. The Qinling orogenic belt is located between the two major cratons of North China and Yangtze. The west Qinling of Gansu Province is located in the west extension of the belt, which is close to the Nanbei Lixian-Tianshui fault basin area and has developed a large number of Cenozoic ultrapotash magmatic rocks. In this paper, taking Haodi, Baiguan and Goujia in Lixian County as the research area and selecting the Cenozoic potash olivine in the area as the research object, detailed studies on petrography, petromagnetics, geochemistry, paleomagnetism and isotopic composition have been carried out. In order to elucidate the petrogenesis, source characteristics, petromagnetic characteristics and paleolatitude variation of magmatic rocks in the study area, the main conclusions are as follows: the previous research data indicate that the eruption age of Kaliaolihuang feldspar in Lixian County is 7.1-22 Mai of the Cenozoic Miocene. Our geochemical studies show that the rock has a high K _ 20 Na _ 20 (up to 6.16 wt.), showing the characteristics of potassium and alkalinity. Different from alkaline basalt, kaliacite shows lower Si02 and A1203 contents, and higher MgO-CaO and Ti02 contents. There is no obvious linear relationship between the principal elements and MgO, which indicates that the crystallization separation is weak in the process of rock formation. In addition, the potash olivine feldspar is enriched in light rare earth and large ion lithophile elements, and there is no anomaly of NB, Ta and EU, which is similar to the geochemical properties of OIB. The Sr-Nd isotopic composition of the Kuaixia olivine feldspar in Lixian County is homogeneous, and it has a high Mg# value (mean 73) and homogeneous MgO- Ni and Cr content, which excludes the obvious crustal mixing. Combined with the low initial 87Sr/86Sr value of 0.7042-0.7051 and the depleted 蔚 Ndtof (3.2-3.8), we infer that the potash olivine originated from the depleted mantle source which was replaced by carbonate melt, and its source may be the mantle peridotite mixed with spinel and pomegranate. The results of the latest chronological studies show that the potash olivite in Lixian County is the product of Miocene activities. The Haotian, Goujia and Baiguan rocks are selected as the objects of paleomagnetism. Through systematic rock magnetic experiments (saturation isothermal remanence characteristics, system thermal demagnetization analysis of triaxial isothermal remanence and hysteresis loop variation curve), it is shown that magnetite is the main magnetite in the kalixia olivine feldspar in Lixian County. A small portion of the samples contain pyrrhotite. Most of the samples in this study have single component characteristics and individual samples have double component characteristics. The paleomagnetic results show that, since the Cenozoic, the West Qinling region has approached the modern geographical position in the Miocene, and the paleolatitude is close to the present latitude, and no obvious northward movement has taken place. This indicates that the tectonic framework of the West Qinling Mountains has been finalized before this time. However, the West Qinling orogenic belt may be affected by the long-range tectonic effect caused by the uplift of the Qinghai-Tibet Plateau, resulting in the clockwise rotation of the block. Because the present research is weak, it still needs to be further discussed.
【學位授予單位】：西北大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：P588.1;P318.4
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本文編號：1976655