西太平洋典型边缘海盆的岩浆活动

在发育有全球最大、最复杂的弧—沟—盆体系的西太平洋地区,集中了全球75％左右的边缘海盆(弧后盆地).根据磁异常条带年龄,这些边缘海盆可粗略分为3个扩张幕.主要根据DS-DP,ODP和IODP计划实施以来所获得的成果,结合其他海洋调查航次研究成果,系统阐述了分属3个扩张幕的西菲律宾海盆(第一扩张幕)、南海—四国海盆(第二扩张幕)和冲绳海槽(第三扩张幕)—马里亚纳海槽内的岩浆活动特点.西菲律宾海盆(扩张时代为65～35 Ma BP)从原先的赤道位置迁移至现今的位置,其内存在如似正常洋中脊玄武岩(NMORB)、洋岛玄武岩(OIB)及弧火山岩等多种岩石类型,其地球动力学背景分别与弧后扩张、地幔柱及火山弧等背景有关,其复杂的构造演化样式需要进一步研究;四国海盆(扩张时代为27 ～ 15 Ma BP)是由古伊豆—小笠原—马里亚纳弧(IBM)裂解形成的,其内除发育正常(N)—富集(E)的洋中脊玄武岩(NMORB-EMORB)外,还在扩张停止的同时出现了板内火山作用,形成了中K-超K碱性玄武岩.四国海盆的扩张模式并没有从岩石学和地质年代学角度进行明确制约,板内火山作用的地球动力学背景也不甚清楚.南海(扩张时代为32～15.5 MaBP)是由来自华南地块的一些微陆块向东南裂离后的海底扩张所形成,并在海底扩张后2 ～8 Ma出现板内火山作用,截止目前,并没有获取到洋壳基底样品,主要获取到了南海海山似OIB的玄武岩,未来需要从岩石学和地质年代学角度对南海海底扩张动力学和时代以及扩张期后的板内火山作用动力学背景进行进一步制约.马里亚纳海槽(扩张时代为5 Ma BP至今)为一年青的洋内弧后盆地,其北段处于裂解增进阶段,其内出露有似MORB(中南段)及介于似MORB与似岛弧岩石之间过渡类型的玄武岩(增进端);虽然在扩张时代上与马里亚纳海槽相当,但冲绳海槽(扩张时代为4 Ma BP至今)为一陆缘、初生弧后盆地,从西南往东北方向,不同区段处于不同的伸展发育阶段,西南段出露有似MORB岩石,中段岩石主要为玄武质岩石和流纹质岩石组成双峰组合,而东北段为中酸性火山岩.正在活动的马里亚纳海槽与冲绳海槽的岩浆作用研究应和其伴随的火山岛弧及其相邻的海沟处正在俯冲的洋壳板块结合起来,完整理解板块俯冲输入(subduction input)与弧及弧后输出(volcanic output)之间的关系,这将为揭示西太平洋地区构造演化提供重要证据.即将在西太平洋地区实施的IODP 349 ～ 352航次,为我国科学家提供了研究西太平洋地区构造演化的契机.     

安徽蚌埠西贾庄铅锌金银矿床赋矿地层和岩浆岩年代学、Hf同位素、地球化学特征及其意义

安徽省北部蚌埠隆起位于华北克拉通东南缘,郯庐断裂带西侧。蚌埠隆起和胶东金矿区现今位置被认为是郯庐断裂带大规模左行平移引起,推测两区有相似的成矿背景。蚌埠隆起东西向长约160km,以固镇-永平岗断裂为界分为东西两段。前人已对蚌埠隆起东段的成岩成矿作用开展了元素地球化学和成矿流体等方面的研究,而蚌埠隆起西段由于第四系覆盖,一直未发现岩浆岩和相关成矿作用,被认为是无矿区,未有相关研究。西贾庄矿床是近年来蚌埠隆起西段新发现的矽卡岩型铅锌金银矿床,为区内成岩成矿作用研究提供了良好的对象。本次工作在矿床地质特征研究基础上,系统开展了矿床赋矿地层和岩浆岩的年代学、地球化学和Hf同位素的研究,确定了成矿岩体的源区、性质和演化,丰富和完善了区内成岩成矿作用研究。西贾庄矿床的赋矿地层为黑云母斜长片麻岩,成岩时代为2466±12Ma,确定其为五河群西堌堆组地层,其原岩来自2. 65～2. 85Ga新生地壳的重熔;花岗闪长斑岩为成矿岩体,成岩时代为113. 0±1. 0Ma～114. 9±1. 7Ma,石英二长岩为成矿期后脉岩,成岩时代为112. 1±0. 8Ma,因此限定成矿时代约为112～115Ma。花岗闪长斑岩和石英二长岩的地球化学特征基本相同,均属高钾钙碱性、准铝质-弱过铝质的Ⅰ型花岗岩类。这两类侵入岩的ε_(Hf)(t)=-16. 6～-12. 5,t_(DM2)=1963～2264Ma,表明它们的岩浆源区为老地壳(主要为古元古代)的部分熔融,推测其形成于古太平洋板块俯冲作用下的弧后伸展环境。通过对比,西贾庄矿床侵入岩与蚌埠隆起东段早白垩世晚期侵入岩体具有相同的构造背景和源区,蚌埠隆起西段在覆盖层之下仍有一定的成矿潜力。     

Mineral isotope evidence for the contemporaneous process of Mesozoic granite emplacement and gneiss metamorphism in the Dabie orogen

Kinetics of isotopic equilibrium in the mineral radiometric systems of igneous and metamorphic rocks is an important issue in geochronology. It turns out that temperature is the most important factor in dictating isotopic equilibrium or disequilibrium with respect to diffusion mechanism. Contemporaneous occurrence of Mesozoic granites and gneisses in the Dabie orogen of China allows us to evaluate the thermal effect of magma emplacement and associated metamorphism on mineral radiometric systems. Zircon U-Pb, mineral Rb-Sr and O isotope analyses were carried out for a Cretaceous granite and its host gneiss (foliated granite) from North Dabie. Zircon U-Pb dating gave consistently concordant ages of 127 ± 3 Ma and 128 ± 2 Ma for the granite and the gneiss, respectively. A direct correspondence in equilibrium state is observed between the O and Rb-Sr isotope systems of both granitic and gneissic minerals. Mineral O isotope temperatures correlate with O diffusion closure temperatures under conditions of slow cooling, indicating attainment and preservation of O isotope equilibrium in these minerals. The mineral Rb-Sr isochron of granite, constructed by biotite, feldspar, apatite and whole-rock with the O isotope equilibrium, yields a meaningful age of 118 ± 3 Ma, which is in accordance with the mineral Rb-Sr isochron age of 122 ± 1 Ma for the host gneiss. The consistency in both U-Pb and Rb-Sr ages between the granite and the gneiss suggests a contemporaneous process of crystallizing the zircons and resetting the Rb-Sr radiometric systems during magma emplacement and granite foliation. Whereas the zircon U-Pb ages for both granite and gneiss are interpreted as the timing of magma crystallization, the young Rb-Sr isochron ages record the timing of Sr diffusion closure during the slow cooling. Protolith of the gneiss crystallized shortly before intrusion of the granite, so that it was able to be foliated by voluminous emplacement of coeval mafic to felsic magmas derived by anatexis of orogenic lithospheric keel. Therefore, extensional collapse of collision-thickened crust at Early Cretaceous is suggested to trigger the post-collisional magmatism, which in turn serves as an essential driving force for the contemporaneous high-T deformation/metamorphism.     

北阿尔金地区古元古代ca.2.0Ga岩浆-变质事件

中国有三个主要的克拉通,分别是华北、华南和塔里木,它们在显生宙经造山过程聚集到一起。塔里木克拉通位于中国的西北部,面积超过60万平方千米,其北侧为中亚造山带,南侧为西昆仑造山带和阿尔金造山带。塔里木克拉通的前寒武纪岩石主要出露在其南北两侧边缘,包括库鲁克塔格、敦煌、阿尔金、铁克里克和阿克苏地块,它们记录了塔里木克拉通早期的构造演化。北阿尔金地块的阿克塔什塔格地区位于塔里木克拉通的东南边缘。该地区最老的岩石被称为米兰群或阿克塔什塔格杂岩。主要岩石包括太古宙的TTG岩石和表壳岩,以及古元古代的片麻状花岗岩,另有少量变质基性岩呈包体状出露在强变形的长英质侵入体中。本文对该区闪长质片麻岩开展了锆石SHRIMP U-Pb定年,同时还对变质基性岩进行了锆石LA-ICP-MS U-Pb测年和地球化学分析,目的是要约束北阿尔金地区古元古代的岩浆-变质事件。闪长质片麻岩的结晶年龄为2.04～2.03Ga,它们形成于岛弧环境。地球化学分析表明,变质基性岩的原岩是拉斑玄武岩。它们有类似于E-MORB的平坦的稀土配分模式,Nb、Ta、Zr、Hf不亏损,说明它们形成于大洋板内环境。在变质基性岩中还识别出两期变质锆石,其中2.05～2.01Ga的早期锆石代表了麻粒岩相的变质作用,而1.98～1.96Ga的晚期锆石可能与角闪岩相的退变质作用有关。无论该区岩浆作用还是变质作用都与约2.0Ga发生的俯冲增生造山事件有关。     

滇西古特提斯造山带的威尔逊旋回：岩浆活动记录和深部过程讨论

滇西古特提斯造山带威尔逊旋回的岩浆活动记录保存在昌宁－孟连带和金沙江－哀牢山带及其周围地区。在大陆拉张阶段，思茅地块之下的大陆岩石圈地幔是明显亏损的，指示它已经历过一次或几次部分熔融事件。洋盆阶段的ＭＯＲＢ源于强烈亏损的大洋岩石圈地幔源区，但与地幔柱发生广泛的混合作用，与现代印度洋脊下的大洋岩石圈地幔相似。从俯冲阶段到碰撞阶段，大陆地幔则变得越来越富集。文中还讨论了滇西古特提斯大陆岩石圈地幔和大洋岩石圈地幔的性质以及大陆岩石圈地幔转化为大洋岩石圈地幔的过程，推测板块扩张的起因可能与下地幔的活动有关。     

高温下非传统稳定同位素分馏

过去十几年来,非传统稳定同位素地球化学在高温地质过程的研究中取得了的重大进展。多接收诱导耦合等离子质谱(MC-ICP-MS)的应用引发了稳定同位素分析方法的重大突破,使得精确测定重元素的同位素比值成为可能。本文总结了以Li、Fe和Mg同位素为代表的非传统稳定同位素在岩石地球化学研究中的应用。Li同位素目前被广泛地用于地幔地球化学、俯冲带物质再循环和变质作用的研究中,可以用来示踪岩浆的源区性质和扩散等动力学过程。不同价态的Fe在矿物熔体相之间的分配可以产生Fe同位素分馏,可以发生在地幔交代、部分熔融、分离结晶等过程中。岩浆岩的Mg同位素则大致反映其源区的特征,地幔的Mg同位素组成比较均一,这为研究低温地球化学过程中Mg同位素的分馏提供一个均一的背景。此外,Cl,Si,Cu,Ca,U等等同位素体系也具有广阔的应用前景。对同位素分馏机制的实验研究和理论模拟为理解非传统稳定同位素数据提供了必要的指导。实验表明,高温下具有不同的迁移速度的轻、重同位素可以产生显著的动力学同位素分馏,这一分馏可以在化学扩散、蒸发和凝华等过程中发生;同位素在矿物和熔体以及流体相中化学环境的差异使得不同相之间可以发生平衡分馏。而最近的硅酸盐岩浆的热扩散和热迁移实验则揭示了一种"新"的岩浆分异和同位素分馏机制。沿着温度梯度,硅酸盐岩浆可以发生显著的元素和同位素分异,湿的安山岩可以通过这种方式演变成花岗质成分,因此这个过程可能对陆壳的产生和演化有重大影响。如果温度梯度在岩浆作用中能长期存在,热扩散就可以产生稳定同位素的分馏,这一机制有别于传统的平衡和动力学同位素分馏。 而多个稳定同位素体系的正相关关系是示踪热迁移过程的最有力证据。在热扩散过程中,流体承载的物质的浓度和它的索瑞系数有关。但是这个系数对体系的很多参数非常敏感,变化极大,因此对热扩散效应的研究产生极大的困难。对热扩散实验的镁、钙和铁同位素测量表明,同位素比值的变化与体系的化学组成以及总温度无关,只和温度变化的幅度有关,这意味着即使元素的索瑞系数变化多端,某一元素的同位素之间的索瑞系数的差别总为常数。这一发现有助于简化对热扩散和索瑞系数这一基础物理问题的研究 。     

Geochronology and geochemistry of early Paleozoic granitic and coeval mafic rocks from the Tannuola terrane (Tuva,Russia): Implications for transition from a subduction to post-collisional setting in the northern part of the Central Asian Orogenic Belt

Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks.     

U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe,eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite (younger orthogneisses) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as older orthogneisses. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation ²⁰⁷Pb/²⁰⁶Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (²⁰⁷Pb/²⁰⁶Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these older orthogneisses represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides.     

Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro–Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi–Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi–Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.     

华北地区晚中生代重大构造转折的地质证据

华北地区在侏罗纪和白垩纪分别发生了两次不同性质的岩浆活动,早期形成一套高锶石英闪长岩,另一期为钾玄岩系。两套岩石分别代表地壳加厚和减薄的构造背景,两次岩浆活动的转折期大致在130Ma左右,此外,华北地区自垩纪广泛分布的碱性岩同样表明区域内在白垩纪曾发生过强烈的岩石圈伸展作用。这一地质特征与区内盆地地震剖面、造山带构造活动年龄、变质核杂岩的年龄、早向坚世太平洋板块运动方向和运动速率的改变以及郯庐断裂左旋运动年龄等地质资料相佐证。因此华北地区岩石圈减薄作用主要发生在早向垩世时期,晚侏罗世——早白垩世是华北地区中生代重大构造发生的转折点。