浙西北早白垩世花岗质岩石成因与构造演化

发布时间：2018-07-04 17:23

  本文选题：同位素年代学 + 岩石地球化学特征　； 参考：《浙江大学》2016年博士论文

【摘要】：花岗岩是大陆上地壳的主要组成部分,记录着陆壳形成、壳-慢相互作用和岩石圈演化的丰富信息。浙西北地区位于扬子地块与华夏地块拼合带——江绍断裂带的北西侧,是研究华南中生代构造-岩浆活动的重要组成部分。本文对浙西北地区的13个早白垩世花岗质侵入体进行了系统的野外地质调查及岩相学、年代学、地球化学、全岩Sr-Nd同位素和锆石Lu-Hf同位素研究,并取得了如下主要成果和认识：1.野外地质调查、岩相学以及锆石U-Pb同位素测年结果表明,浙西北地区早白垩世的花岗质侵入岩的形成时代主要在集中148～125 Ma之间,早期(148～135 Ma)岩石类型以花岗闪长岩和花岗斑岩为主,晚期(135-125 Ma)以黑云母花岗岩为主。2.根据岩石地球化学特征,可将浙西北早白垩世的花岗质岩石分为I型、S型、A型和高分异花岗岩四类。其中I-S型花岗岩对应了早期的花岗闪长岩和花岗斑岩,具有相对较低的Si02含量(平均约68%)和稀土含量,具轻微的Eu负异常和较强的轻重稀土分馏,亏损高场强元素Nb～Ta、Ti,而I型花岗岩比S型花岗岩更富Sr。这两类岩石的形成与区域内早中侏罗世挤压造山导致的地壳加厚有关。A型花岗岩对应了晚期的黑云母花岗岩,具有较高的SiO2含量(平均约75%)和高FeOT/MgO值,具强烈的Eu负异常,富集稀土元素,轻重稀土分馏不明显,富集大离子亲石元素Rb、Th、U,强烈亏损Ba、Sr、Ti、P,岩石形成于造山后的岩石圈伸展环境下。3.全岩Sr-Nd和锆石Hf同位素特征显示,浙西北早白垩世的花岗质岩石主要由成分相当于上溪群、双桥山群和双溪坞群的中-新元古代变质基底经部分熔融而形成。地球化学特征显示浙西北早白垩世I型花岗岩形成于相对较高的压力条件下,S型花岗次之,而A型花岗岩形成于较低的压力条件下。4.通过对比浙西北与华南其他地区晚中生代岩浆作用的特征,结合区域地质研究成果和地球物理学研究的资料,建立浙西北晚中生代构造-岩浆演化模型：早侏罗-晚侏罗世中期(180～150 Ma),受古太平洋板块俯冲的影响,浙西北地区发生陆内造山作用,地壳增厚并隆升剥蚀；150～135 Ma期间,随着古太平洋俯冲板块的后撤以及岩石圈的拆沉,岩石圈进入伸展阶段,软流圈地慢物质沿构造薄弱带(“十杭带”)上涌,地壳基底物质部分熔融形成Ⅰ型、S型和高分异花岗岩。135 Ma之后,随着岩石圈进入强烈的伸展阶段以及地壳的减薄,地壳大规模重熔形成广泛分布的中酸性火山岩和A型花岗岩。
[Abstract]:Granite is a major component of the upper continental crust, which records rich information on the formation of landing crust, crust-slow interaction and lithospheric evolution. Northwest Zhejiang is located in the north and west of the Yangtze and Huaxia block assemblage belt-Jiangshao fault zone, which is an important part of the study of Mesozoic tectonic-magmatic activity in South China. In this paper, 13 early Cretaceous granitic intrusions in northwestern Zhejiang Province have been systematically investigated in the field and studied in lithology, geochronology, geochemistry, whole-rock Sr-Nd isotopes and zircon Lu-Hf isotopes. And has made the following main achievements and understanding: 1. Field geological survey, lithology and zircon U-Pb isotopic dating indicate that the age of granitic intrusive rocks in the early Cretaceous in northwestern Zhejiang was mainly between 148Ma and 125Ma. The early stage (148A 135 Ma) was dominated by granodiorite and granitic porphyry, and the late stage (135-125 Ma) was mainly biotite granite. According to the petrogeochemical characteristics, the granitic rocks of the early Cretaceous in northwest Zhejiang can be divided into four types: type I and S type A and high grade isomorphic granites. The I-S-type granite corresponds to the early granodiorite and granitic porphyry, with relatively low SiO2 content (about 68% on average) and rare earth content, with slight negative EU anomaly and strong fractionation of heavy and heavy rare earth elements. The depletion of high field strength element NbSU Tahuangtii, while the I-type granite is more Sr-rich than S-type granite. The formation of these two types of rocks is related to the crustal thickening caused by the early and middle Jurassic compressional orogeny in the region. The A-type granites correspond to the late biotite granites, with high Sio 2 content (mean 75%) and high FeOT / MgO value, with strong negative EU anomalies. The enrichment of rare earth elements, the fractionation of light and heavy rare earth elements is not obvious, the enrichment of large ion lithophile elements RbPh, and the strong depletion of Ba-Sr-TiP-, the rocks are formed in the post-orogenic lithospheric extensional environment of .3. The Sr-Nd and zircon HF isotopic characteristics show that the granitic rocks of the early Cretaceous in northwestern Zhejiang Province were mainly formed by partial melting of the Meso-Neoproterozoic metamorphic basement of the Shangxi Group, Shuangqiao Mountain Group and Shuangxiwu Group. Geochemical characteristics show that the early Cretaceous I-type granites formed in northwestern Zhejiang Province under relatively high pressure conditions, followed by S-type granitic granites, while A-type granites formed under lower pressure conditions .4. By comparing the characteristics of late Mesozoic magmatism between the western and northern Zhejiang and other areas of South China, combined with the results of regional geological research and geophysical data, The late Mesozoic tectonic-magmatic evolution model of northwestern Zhejiang was established: early Jurassic-Late Jurassic (180-150Ma). Due to the subduction of paleo-Pacific plate, intracontinental orogeny occurred in northwestern Zhejiang, crust thickening and uplift and denudation occurred during 150-135Ma. With the retreat of the paleo-Pacific subduction plate and the delamination of the lithosphere, the lithosphere enters the extensional stage, and the asthenospheric slow material surges up along the tectonic weak zone ("Shihang belt"). After the partial melting of the basement material of the crust formed type 鈪，

本文编号：2096810