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扬子克拉通北缘新元古代岛弧花岗岩类成分极性及成因的地球化学探讨 总被引：3，自引：1，他引：3
对扬子克拉通北缘新元古代岛弧花岗岩类成分极性的地球化学研究表明,在SiO_2含量相近的条件下,由北向南岩石中K_2O、K_2O+Na_2O、K_2O/Na_2O逐渐增加,而CaO逐渐降低;从北向南,微量元素Rb、Ba、Th、Zr、Nb、Ta、 ∑REE及Rb/Th、Rb/Nb、∑LREE/∑HREE、(La/Yb)_N 逐渐增加或具增加趋势,而 Sr、Co、Ni和Sr/Ba具逐渐降低或具降低趋势;同位素组成由北向南δ~18O、εs_r(t)系统增高,而ε_(Nd)(t)逐渐降低。这些组分的空间变化规律与环太平洋带位于俯冲带上盘向陆一侧的岛弧或大陆边缘岩浆中出现的成分极性规律相一致.分析认为,大陆壳物质以不同比例的加入是影响俯冲带岛弧花岗质岩浆成分极性的一个重要因素,向陆一侧岩浆中陆壳物质所占的比例逐渐增高,而幔源物质所占比例逐渐降低。 相似文献
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本文重点简述了特提斯构造域内古，中，新三个演化阶段的蛇绿混杂岩与岛弧带的时空展布及其沟－弧－盆体系，所识别出的蛇绿混杂岩，洋中脊拉斑玄武岩，大洋沉积物的岛弧带等地质记录，提供了东特提斯早期大洋岩石圈板块运动的有力证据。同时，与岛弧有关的不同时期不同阶段的各种弧前盆地，弧间盆地和弧后贫地成为造山带板块会聚边缘特征的标志。 相似文献
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北秦岭古聚会带壳幔再循环 总被引：10，自引：2，他引：10
以同构造期代表古洋壳残片的蛇绿岩及产于古岛弧的玄武岩为基础，通过Ｎｄ，Ｐｂ同位素与微量元素示踪及岩浆源区分析，揭示出北秦岭元古宙上地幔以强亏损（εＮｄ（ｔ）＋６．３～＋７．３）和高的Ｙｂ／Ｈｆ，Ｎｂ／Ｌａ和Ｔｈ／Ｌａ比值为特征，北秦岭地壳和上地幔明显具有Ｐｂ同位素比值高的特征．北秦岭丹凤群岛弧火山岩、二郎坪群弧后玄武岩以及松树沟蛇绿岩中变拉斑玄武岩εＮｄ（ｔ）、放射成因Ｐｂ同位素、Ｙ／Ｔｂ和Ｔｉ－ＭｇＯ研究表明，本区玄武岩存在两类性质不同的岩浆源．一类与亏损的北秦岭岩石圈上地幔源区有关；另一类与携带海洋沉积物的洋壳板块俯冲参与有关．由此，论证了北秦岭古聚会带壳幔之间物质再循环 相似文献
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Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from 1900 14C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO2) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO2) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO2 contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past 2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO2 range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions. 相似文献
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The work is dedicated to most important abiotic processes of the Early Precambrian, effect of which is recorded in continental crust, and to complementary processes in subcontinental mantle. We intend to figure out when a certain process was triggered first in the past and what indications suggest its further activity, evolution and possible cessation in subsequent geological history. Considerations are based on described natural objects characterizing particular geological events and enabling the cause-and-effect interpretation in order to understand different viewpoints known from publications. Considered in the work are the early Precambrian greenstone belts and ophiolites, island-arc systems and ecologites, magmatism unconnected with subduction zones (rifting-related, plateau basalts, dykes, kimberlites) and anorthosites representing a group of heterochronous intrusions of complicated genesis. Main considerations are premised with a brief review of the earliest geodynamic phenomena associated with meteorite impacts by termination of the planetary accretion. 相似文献
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西秦岭天水地区李子园群变质火山岩的地球化学特征及其地质意义 总被引：16，自引：3，他引：16
西秦岭天水地区的早古生代李子园群为一套中浅变质的沉积-火山岩系。沉积岩系主要由变质碎屑岩和碳酸盐岩组成,火山岩系主要由变玄武岩、变玄武安山岩和变安山岩组成,包括岛弧型火山岩和玻安岩。岛弧型火山岩SiO2含量介于48.79%~54.64%之间,TiO2含量较低(0.29%~0.88%);稀土元素分布型式呈LREE略富集型,富集大离子亲石元素(LILE)Cs、Sr、Th、U,相对亏损Rb、K和高场强元素(HFSE)Nb、P、Sm、Ti和Y,具Nb负异常,类似于低钾岛弧拉斑玄武岩和钙碱性玄武岩特征。玻安岩具有中等的SiO2含量(53.59%~59.28%),低的TiO2含量(0.24%~0.48%);相对中等的MgO含量(4.90%~4.96%)、较低的CaO/Al2O3比值(0.39~0.54)和较高的Mg#值(0.54~0.58)、Al2O3/TiO2比值(33.88~64.29);同时具有较低的Ti/Zr比值(15~83)、V/Zr比值(2.18~8.35)和较高的Zr/Y比值(3.82~12.08),相对富集大离子亲石元素(LILE),特别是Rb、Ba、Th,而亏损高场强元素(HFSE),如Nb、Ta、P、Ti、Y、Yb,显示为亏损MREE的U型稀土元素分布型式。岛弧型火山岩和玻安岩的存在表明,李子园群及其中的中基性火山岩系形成于俯冲带之上的岛弧或弧前环境。 相似文献
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义敦印支岛弧褶皱带地质构造演化复杂,岩浆活动十分活跃,成矿地质地球化学条件优越,金、银、铜、铅、锌、锡等金属矿产资源丰富。西南三江为有色、贵金属资源富集区,义敦为花岗岩型锡-银多金属矿集区。根据区域成矿环境及研究区成矿条件分析,找矿潜力巨大,有望取得找矿突破。 相似文献