扬子北缘米仓山地区新元古代碱性杂岩地球化学特征及地质意义

发布时间：2018-05-22 12:15

  本文选题：扬子地块 + 米仓山　； 参考：《西北大学》2017年硕士论文

【摘要】：扬子地块北缘新元古代经历了复杂地质过程,并发育大量的镁铁质-酸性-碱性侵入岩体。米仓山位于扬子地块北缘,关于该区的新元古代碱性杂岩的成因机制及深部动力学背景目前仍存在极大争议。本论文通过对扬子地块北缘米仓山地区碱性杂岩体展开详细的岩石学、全岩主微量、Sr-Nd同位素及锆石U-Pb年代学的研究,基本厘清了该区新元古代侵入杂岩体的时空格架,并结合地球化学探讨该区碱性杂岩的成因机制、构造环境及深部动力学过程。论文的研究工作取得如下成果和认识:1、碱性岩(磷霞岩和正长岩)和闪长岩都形成于新元古代早期(～865Ma),是同一构造背景下不同部位的岩浆产物。其中磷霞岩(高镁和低镁)具有较低的硅含量,并且Na_2OK_2O,高碱,属于钠质碱性岩。稀土元素含量较低,富集大离子亲石元素,亏损高场强元素。初始锶比值变化范围较大,正εNd(t)值(+0.8～-+3.27);水磨正长岩(富钠和富钾)具有高SiO_2、(Na_2O + K_2O)和里特曼指数(σ),低TiO_2、MgO、Al_2O_3,属于碱性准铝质岩系。稀土元素含量相对较低,其中轻稀土元素相对富集,重稀土元素相对亏损,弱负Eu异常,富集大离子亲石元素,亏损高场强元素。较低初始锶比值,正εNd(t)值(+0.89～+1.29)。二者起源于亏损地幔源,经结晶分异演化,在后续岩浆上升过程中可能遭受了地壳的同化混染,并且正长岩演化过程不同于磷霞岩,可能是超基性-碱性岩浆演化期间不同阶段的产物,形成于陆缘弧内裂谷的构造环境。石英闪长岩具有高SiO_2、Al_2O_3、TFe_2O_3和Na_2O,低K_2O、CaO和A/CNK,属于钠质碱性准铝质岩系。较低稀土元素,负Eu异常,富集Rb、Ba、Pb等,亏损Nb、Ta等,中等初始锶比值,正εNd(t)值(+2.92～+3.66),是新生玄武质下地壳部分熔融的产物,在其上升演化的过程中明显遭受了上地壳的同化混染,形成于大陆边缘弧的构造环境。2、辉长岩和花岗岩形成于新元古代中期(～745Ma),其中中子园辉长岩具有低SiO_2和Na_2O,且Na_2OK_2O,高Al_2O_3、TFe_2O_3和CaO,属于钙碱性基性岩系。较低稀土元素总量,富集大离子亲石元素,亏损高场强元素,无明显Eu异常,具有低初始锶值,正εNd(t)值(+1.6～+4.5),其起源于石榴石二辉橄榄岩亏损地幔源,其熔融程度大于15%,在岩浆上升演化阶段遭受了地壳混染作用,是弧后盆地构造环境下的产物。二长花岗岩具有高硅,低TiO_2、MgO、CaO、P2O5和Mg#值。稀土元素含量较低,轻重稀土元素分馏明显,富集轻稀土元素及Rb、U和Th,亏损Ba、Nb和P,负Eu异常,属于高分异Ⅰ型花岗岩类。正εNd(t)值(+3.6～+5.2),二阶段模式年龄值为0.96～1.07Ga,是新生玄武质下地壳部分熔融的产物,在其上升演化的过程中明显遭受了上地壳的同化混染,形成于弧后伸展的构造环境。3、综合相关区域地质构造热时间,扬子北缘在新元古代时期(900Ma～700Ma)总体上可能处于俯冲相关的构造背景,米仓山地区新元古代时期的岩浆活动可以大致划分为三个阶段,经历了从早期陆缘弧(裂谷)(～860Ma)到火山弧(～830Ma)再到弧后伸展(～750Ma)的构造演化。扬子北缘米仓山地区碱性侵入杂岩是俯冲相关构造环境,是米仓山地区新元古代时期两阶段的岩浆事件,为进一步探讨扬子北缘新元古代期间岩浆作用的成因机制和深部动力学提供重要的岩石学约束。
[Abstract]:The Neoproterozoic Era of the northern margin of the Yangtze block has experienced complex geological processes and developed a large number of mafic acid alkaline intrusive rocks. M Cangshan is located in the northern margin of the Yangtze block. The genetic mechanism and deep dynamic background of the Neoproterozoic alkaline complex in this area are still very controversial. This paper has passed to the northern margin of the Yangtze massif, Cangshan. The detailed petrology of the regional alkaline complex, the main trace of the whole rock, the Sr-Nd isotopes and the zircon U-Pb chronology have basically clarified the space-time framework of the Neoproterozoic intrusive complex in this area, and discussed the genetic mechanism, the tectonic environment and the deep dynamic process of the alkaline complex in this area with the geochemistry. The research work of this paper has been obtained. The following achievements and understandings are as follows: 1, the alkaline rocks (Lin Xiayan and zenyan) and diorite are formed in the early Neoproterozoic (~ 865Ma) and are the magma products of different parts of the same tectonic setting. Among them, the Phosphorite (high magnesium and low magnesium) has lower silicon content and Na_2OK_2O, high alkali, and sodium alkaline rock. The rare earth element is low and rich in enrichment. The initial strontium ratio has a high variation range, the initial strontium ratio has a larger variation range, the positive epsilon Nd (T) value (+0.8 ~ -+3.27), the high SiO_2, (Na_2O + K_2O) and Litman index (Na_2O + K_2O), and the low TiO_2, MgO, Al_2O_3, and low rare earth elements in the water mill. The rare-earth elements are relative loss, weak negative Eu anomaly, enrichment of large ion stone elements, high field strong elements, low initial strontium ratio and positive epsilon Nd (T) value (+0.89 to +1.29). The two originates from the source of the depleted mantle, and the evolution of crystallization differentiation may suffer from the assimilation and mixed dyeing of the crust during the subsequent magma ascending process, and the evolution process of the syenite is not. The same to the phosphorite, may be the product of the different stages of the ultrabasic basic magma evolution, which formed in the tectonic environment of the continental margin rift. The quartz diorite has high SiO_2, Al_2O_3, TFe_2O_3 and Na_2O, low K_2O, CaO and A/CNK, which belongs to the alkaline alkaline aluminous rock system of sodium, and the lower rare earth elements, negative Eu anomalies, enrichment Rb, Ba, Pb, etc. The medium initial strontium ratio and positive epsilon Nd (T) value (+2.92 ~ +3.66) are the products of partial melting of the new basaltic lower crust. In the process of its ascending and evolution, the upper crust was obviously subjected to assimilation and contamination of the upper crust and formed in the tectonic environment of the continental margin arc, and the gabbro and granite formed in the middle Neoproterozoic (to 745Ma), of which the gabbro of the neutron orchards is in the middle of the Neoproterozoic. Low SiO_2 and Na_2O, and Na_2OK_2O, high Al_2O_3, TFe_2O_3 and CaO, belong to the basic Calc basic rock series. The total amount of low rare-earth elements, rich in large ion stone elements, loss of high field and strong elements, no obvious Eu anomalies, low initial strontium, and positive e Nd (+1.6 to +4.5) value (+1.6 to +4.5), originated from the source of the garnet peridotite depleted mantle, which has a large melting degree. 15%, in the stage of the magma rising and evolution, the crustal contamination is the product of the crustal contamination, which is the product of the tectonic environment in the back arc basin. The two granites have high silicon, low TiO_2, MgO, CaO, P2O5 and Mg# values. The REE content is low, the heavy rare earth elements are fractionated, the light rare earth elements and Rb, U and Th are enriched, and the loss Ba, Nb and P, negative Eu anomalies, belong to high fractionated I. The type granitoids, the positive epsilon Nd (T) value (+3.6 ~ +5.2), the two stage model age value of 0.96 to 1.07Ga, are the products of the partial melting of the new basaltic lower crust. In the process of its ascending and evolution, the assimilation and contamination of the upper crust was obviously affected, and formed in the tectonic setting of.3 in the post arc extension. The Neoproterozoic Era (900Ma ~ 700Ma), in general, may be in the subduction related tectonic setting. The magmatic activity of the Neoproterozoic Era in Cangshan area can be roughly divided into three stages, which experienced the tectonic evolution from early continental margin arc (rift) to 860Ma to volcanic arc (to 830Ma) and then to post arc extension (to 750Ma). The alkaline intrusive complex is a subduction related tectonic environment, which is the two stage of the Neoproterozoic magma event in the Cangshan area, which provides important petrological constraints for further exploration of the genesis and deep dynamics of the Neoproterozoic magmatism during the northern margin of the Yangtze River.
【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】：P588.15
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本文编号：1922019