西秦岭礼县地区新生代钾霞橄黄长岩地球化学与磁学研究

发布时间：2018-06-04 08:15

  本文选题：西秦岭造山带 + 钾霞橄黄长岩　； 参考：《西北大学》2017年硕士论文

【摘要】：钾霞橄黄长岩系是世界上少有的碱性火山岩(超钾质-钾质系列),其岩浆起源深度较大,包含大量的地幔包体和捕虏体,并且该岩系通常和火成碳酸岩共生,因此,钾霞橄黄长岩、包体和共生的碳酸岩均是研究地球深部物质的重要窗口。秦岭造山带地处华北和扬子两大克拉通之间,研究区甘肃西秦岭位于其西延部分,靠近南北向礼县-天水断陷盆地区,发育了大量的新生代超钾质岩浆岩。本文以礼县的好梯、白关、苟家作为研究区,选择区内新生代钾霞橄黄长岩作为研究对象,开展了详细的岩相学、岩石磁学、地球化学、古地磁学、同位素组成研究,以阐明研究区岩浆岩的岩石成因、源区特征、岩石磁学特征、古纬度变化,主要取得了以下结论:前人研究资料表明礼县钾霞橄黄长岩喷发年龄为新生代中新世(7.1-22 Ma)。我们的地球化学研究揭示,该套岩石具有较高的全碱含量K20+Na20(高达6.16wt.%),显示为钾质,碱性的特征。不同于碱性玄武岩,钾霞橄黄长岩显示较低的Si02和A1203含量,较高的MgO,CaO和Ti02的含量。钾霞橄黄长岩的主量元素与MgO没有明显的线性关系,反映了岩石形成过程中结晶分离作用较弱。此外,钾霞橄黄长岩富集轻稀土和大离子亲石元素,并无Nb、Ta和Eu的异常,与OIB的地球化学性质相似。礼县钾霞橄黄长岩的Sr-Nd同位素组成表现为均一,同时其具有较高的Mg#值(平均73)和均一的MgO、Ni和Cr含量,排除了显著的地壳混染作用。结合其低的初始87Sr/86Sr值(0.7042-0.7051)和亏损的εNd(t)值(+3.2～+3.8),我们推测钾霞橄黄长岩来源于被碳酸盐熔体交代的亏损地幔源区,其源区可能为尖晶石与石榴子石混合的地幔橄榄岩。最新的年代学研究结果表明礼县钾霞橄黄长岩是中新世活动的产物,选择礼县地区好梯、苟家、白关岩体作为古地磁学的研究对象,通过系统的岩石磁学实验(饱和等温剩磁特征、三轴等温剩磁的系统热退磁分析和磁滞回线变化曲线),显示礼县钾霞橄黄长岩的载磁矿物主要是磁铁矿,一少部分样品存在磁黄铁矿。本次研究的大部分样品均具有单分量特征,个别样品具有双分量特征。古地磁结果表明,从新生代以来,西秦岭地区在中新世即已接近现代地理位置,古纬度已经接近现今纬度,并没有发生明显的向北运动,表明西秦岭的构造格局在此之前已经定型。但西秦岭造山带可能受到青藏高原隆升作用引起的远距离构造效应的影响,发生了块体的顺时针旋转运动。由于目前研究薄弱,尚有待于进一步探讨。
[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.
【学位授予单位】：西北大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.1;P318.4
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本文编号：1976655