大青山太古宙高级变质岩近水平顺层伸展变形——地球上早期的伸展构造

内蒙古大青山—乌拉山位于华北克拉通北缘,是我国高级区发育较好的区域之一。区内高级变质杂岩总体上呈近东西向延伸,经历多期变质变形作用改造。近水平顺层伸展变形是高级变质杂岩中最早的塑性流动变形作用,发生在下地壳麻粒岩相环境下。变形作用伴随麻粒岩相变质作用、深熔作用,形成了顺层滑动韧性变形带、穹隆构造、层内底辟褶皱和L构造岩等构造形迹组合。这期变形作用不仅导致区内的孔兹岩系和麻粒岩系以近水平构造面接触,也造成孔兹岩系中各个地层单位具有以透镜状岩片堆叠的地层结构,并且还伴随有不同类型的深熔片麻岩形成。近水平顺层伸展变形作用的确立,对研究早前寒武纪高级区地壳形成演化和高级变质地层构造格架的建立具有重要的理论意义。     

怀安蔓菁沟早前寒武纪高压麻粒岩混杂岩带地质特征、岩石学和同位素年代学

华北克拉通北缘中段怀安蔓菁沟高压麻粒岩混杂岩带产在太古宙怀安杂岩南缘与花岗岩带交界处,由高压基性麻粒岩、辉长质麻粒岩、英云闪长质麻粒岩和少量夕线石榴片麻岩相间排列的席状岩层构成,岩层间被高应变带或剪切带分隔。高压基性麻粒岩是石榴辉石麻粒岩。据石榴石斑晶内包裹的早期矿物(Cpx+Q)估算的早期高压变质作用条件:T=800℃,P>1.4GPa。环绕斑晶的后成合晶反应边矿物组合(P1+Opx+Hb+Cpx)的变质条件为:T=820℃,P为0.7～0.9GPa。全岩Sm-Nd等时线年龄2.65Ga,矿物Sm-Nd等时线年龄1.82Ga,锆石U-Pb一致线年龄1.83Ga。高压基性麻粒岩的原岩代表晚太古代陆壳的最下部,大约在2.7Ga从上地幔分异出来,可能经壳下垫托作用加在早期陆壳底部,随后经历高压变质作用。早元古代晚期,由于地壳规模的大型逆冲作用,使其上升,并经受褶皱形变、剪切推覆和退变质等作用的改造,形成高压麻粒岩混杂岩带。     

华北东部岩石圈减薄中的下地壳过程：岩浆底侵、置换与拆沉作用

最近研究表明，华北中生代岩石圈减薄不仅是岩石囤地幔减薄，而且下地壳也发生了一定程度的减薄和置换。本文强调下地壳过程，如岩浆底侵、置换和拆沉作用是理解岩石囤减薄机制的关键因素之一。火山岩及其捕虏体的信息似乎支持在华北东部南、北缘存在局部造山带型的下地壳与岩石囤的拆沉作用。但是华北东部整体上的减薄机制难以用造山带的拆沉模式来解释。华北东部克拉通内部的火山岩中的下地壳捕虏体有两类，一类是经历了麻粒岩相变质的底侵辉长岩和辉石岩以及榴辉岩相变质的石榴辉石岩，形成时代约在140～120Ma；另一类是经过了中生代变质叠加的前寒武纪麻粒岩。中生代华北东部曾发生过大规模的岩浆底侵作用，现今的下地壳很可能已大部分不是前寒武纪的下地壳，它们由中生代变质的辉长岩、镁铁质岩石以及经历了很强烈改造的前寒武纪下地壳麻粒岩组成。此外，在华北东部普遍存在着化学成分类似于“埃达克岩”的中生代高锶花岗岩类岩石，它们的形成与岩浆底侵作用和镁铁质下地壳的部分熔融有关，其熔融残留应是榴辉岩或石榴角闪岩。尽管如此，要通过下地壳部分熔融形成一个厚的密度很大的榴辉岩层，由它的重力不均衡来带动80～120公里厚的岩石圈地幔一起拆沉进入软流圈的机制很难发生在华北克拉通内部。岩浆底侵和置换作用是下地壳过程非常重要的形式，与岩石圈的减薄具有密切联系，其具体机制尚不完善。     

克拉通边缘岩石圈金属再富集与金-钼-稀土元素成矿作用

克拉通是大规模成矿的重要构造环境,其边缘产出了众多世界级规模的金、钼、稀土元素矿床。然而,克拉通如何控制巨型矿床的形成与分布尚不十分清楚。文章基于作者和前人的研究成果,探讨了扬子和华北克拉通岩石圈早期金属富集与后期金属活化问题。在全球范围,多数克拉通在其形成之后长期保持稳定,但部分克拉通(如华北、扬子)在克拉通化之后又经历了早期(元古代)增生与晚期(中生代—新生代)改造。在克拉通化及其之后,处于克拉通边缘的大洋岩石圈或克拉通块体间的有限洋盆发生板片俯冲,释放出含金属组分(REE、Cu、Au)的富CO2流体,交代亏损的大陆岩石圈地幔(SCLM),并使之发生交代和金属再富集。俯冲诱发的弧岩浆在大陆下地壳底侵可形成新生下地壳,伴随着少量硫化物的堆积而发生金属(Au、Cu)再富集。由于克拉通相对稳定,新生下地壳在进变质脱水过程中仍能保存部分金属,释放的(含Au)变质流体很可能被封存或固结在地壳的某个部位。在克拉通破坏改造期,软流圈上涌改变克拉通SCLM热结构并诱发其部分熔融,产生富REE的碳酸岩熔体和富水的基性岩浆(如煌斑岩)。前者在浅部地壳侵位并出溶成矿流体,形成碳酸岩型REE矿床;后者在深部地壳脱挥发分(H2O+CO2),诱发新生下地壳重熔和含Au硫化物(和/或含Au流体囊)活化,形成富Au岩浆系统或流体系统。这些深地壳熔/流体沿克拉通边界或岩石圈不连续运移至上部地壳,岩浆系统直接出溶成矿流体,形成以斑岩体为中心的斑岩型Au矿,含Au富CO2流体流沿断裂网络系统活动并沉淀金属,形成石英脉型和蚀变岩型Au矿。伴随克拉通破坏改造,克拉通边界断裂或基底断裂重新活化,并诱发古老下地壳熔融,产生含Mo岩浆系统。这个理论框架不同于已有的造山带成矿理论模式,它解释了克拉通边缘异常富集Au、Mo、REE矿床及其成矿规律,可用于类似克拉通地区的成矿预测。     

班韦卢地块和伊鲁米德带区域地质及构造演化特征

"变质克拉通"作用为稳定克拉通的被动边缘受到陆内碰撞作用的影响而发生变质作用和岩浆作用。班韦卢地块位于赞比亚东北部,被认为是坦桑尼亚克拉通或刚果克拉通的变质克拉通边缘,伊鲁米德带位于班韦卢地块的东南部,为其元古宙变质克拉通边缘。本文通过对两者的基底、地层和岩浆岩特征的系统整理研究,发现元古宙变质基底从北向南随变质作用的增强,由片岩类渐变为片麻岩类,这可能是由于两者构造层位的不同和遭受变质克拉通作用的期次不同共同造成或其中之一造成的。穆瓦超群大规模沉积作用年龄为1800Ma左右,姆波洛科索群沉积碎屑主要来源于盆地南部的变质基底,锆石年龄显示含少量太古宙地壳组分。多期次的岩浆岩具有相似的地球化学性质,其来源可能为太古宙地壳的局部熔融作用。根据地层和岩浆岩特征,可以将构造演化分为3个阶段:班韦卢地块和伊鲁米德带"变质克拉通"阶段(≥1880Ma)、稳定沉积阶段(1880Ma~1050Ma)和伊鲁米德带"变质克拉通"阶段(≤1050Ma),不同阶段地幔岩浆加入量较少,但可作为热源导致太古宙地壳多次熔融而形成元古宙地质体。     

从地壳上地幔构造看大陆碰撞带岩石圈的克拉通化

本篇讨论超级大陆汇聚后逐渐变为克拉通或扩大克拉通的作用过程,即指经及大陆碰撞地体汇聚后新形成的大陆块逐渐转变为刚性克拉通的作用过程。增生大陆岩石圈的克拉通化的作用后果,包括大陆地壳密度的增加,岩石圈地幔的增厚和大地热流值的下降,使大陆岩石圈逐渐刚性强化。大陆碰撞后形成的大陆块必须经过克拉通化的过程,才能逐渐成为刚性克拉通。作用过程主要包括:①上地壳沉积碎屑岩石结晶岩化和中地壳岩石角闪岩化;②下地壳岩石基性化;③大陆碰撞带下凹莫霍面的磨平;④岩石圈地幔底侵加厚形成陆根。从大陆碰撞带转变为克拉通的过程也是岩石圈地幔不断增厚而地壳缓慢变硬变冷的过程。这个过程包含以下作用:区域变质作用,交代作用和岩石圈幔源岩浆的底侵。这个过程时间尺度比碰撞造山作用大一个级次。长期的底侵作用使地壳岩石密度和强度不断加大,改变岩层的矿物成分和局部结构。当大陆岩石圈克拉通化到一定程度之后,由于下方软流圈的热能供应逐渐减缓,使岩石圈地温梯度缓慢下降,最终结果会形成大陆根。由于显生宙大陆碰撞带岩石圈强度弱,大陆碰撞时更容易造成岩石圈变形,因此大陆碰撞的板内效应主要发生在大陆内的显生宙碰撞带。显生宙大陆碰撞带如果再次受到大陆碰撞板内效应的作用,其克拉通化的过程必然会推迟。     

吉林南部新太古代末地壳深熔作用——来自变质石英闪长岩及淡色花岗岩的证据

深熔作用是大陆地壳分异、元素迁移富集和混合岩化作用的主要机制和关键地质过程.吉南地区出露的太古宙基底普遍经历了角闪岩相-麻粒岩相变质及深熔作用,长英质淡色体及淡色花岗岩广泛分布.吉南和龙花岗-绿岩地体出露的太古宙变质石英闪长岩及相关的长英质浅色体和含斜方辉石(角闪石)淡色伟晶花岗岩的野外地质特征、相互关系及岩相学特征指...     

关于变质深熔作用与成岩成矿关系的思考

变质作用与地壳形成演化过程密切伴生 ,并与成岩、成矿作用具密切关系。从动力学系统分析 ,区域变质作用混合岩化作用变质深熔作用是温压递进性变质作用。变质深熔作用是变质过程一个重要组成部分 ,它具有独特的温压、热力学、动力学及地球化学特性 ,文中定义为变质深熔系统 ,简称MAS ,它是由物质来源、能量来源、作用形式 ,物质转移与富集 ,形成时间与位置等要素组成。文中列举与变质深熔作用有关的花岗质岩石及矿床实例 ,并据MAS要素分析其成岩、成矿机制 ,剖析了哀牢山变质带、云开变质带花岗质岩石属变质深熔成因 ,变质带中伟晶岩矿床、剪切带金矿床形成与变质深熔作用具密切关系。当前国际上十分重视在矿床形成和物质转移中的变质作用研究 ,因此深入开展变质深熔作用与成岩、成矿关系研究具重要理论和实践指导意义。     

辽西地区台里韧性剪切带中长英质脉体的变形演化机制研究

华北克拉通构造演化及破坏机制一直是地学界的热点话题,中生代华北克拉通东部主要受扬子板块的俯冲使区域内岩浆-变质事件频繁,形成一系列近EW向构造,包括位于辽西地区的一条中生代NEE向台里韧性剪切带。该韧性剪切带中发育有S-C组构、旋转碎斑、石香肠、长英质脉体等丰富的构造现象。通过野外观测和室内综合分析,识别出其中长英质脉体的演化序列为"分异脉→香肠化→透镜化→碎斑化→细粒化→丝带化"六个变形阶段。对各变形阶段样品切制垂直面理平行线理的薄片,并对薄片基质层进行二维有限应变测量分析与统计,包括真应变差(ε_1-ε_2)等值线图、岩层厚度比(S)等值线图、运动学涡度(W_k)等值线图。其中W_k在演化前期主要小于0.71,演化中期主要大于0.71,演化后期主要小于0.71,表明长英质脉体演化过程前期主要为纯剪切作用,中期为简单剪切作用,后期为纯剪切作用,同时W_k为正值和S小于1反映研究区内韧性剪切带在伸展减薄的动力学背景下形成,这对了解华北克拉通东部中生代构造演化提供了新的约束。     

康定杂岩Rb-Sr、Sm-Nd同位素系统及其意义

通过对康定—冕宁地区出露的英云闪长岩、黑云角闪斜长片麻岩、角闪变粒岩全岩及其中所分离出的角闪石、黑云母、斜长石、钾长石的Rb-Sr、Sm-Nd同位素的系统测定,结合岩石的锆石U-Pb年龄结果,确定这些变质杂岩由于经历了复杂的形成过程与变质历史,Rb-Sr、Sm-Nd同位素体系难以确定其结晶年龄。由单矿物与全岩Rb-Sr、Sm-Nd体系拟合的~700 M a的等时线年龄反映了角闪岩相-高角闪岩相的变质作用年龄。Sm-Nd同位素体系由于在变质作用过程中的部分开放性,很容易给出无意义的较老的混合年龄。康定杂岩结晶后并没有经历麻粒岩相变质作用,区域上所含的麻粒岩透镜体可能是新元古代(773~721 M a)期间由Rod in ia超大陆裂解产生的新生洋壳向扬子克拉通陆块俯冲消减过程的变质产物。俯冲到一定深度后,由于板片被拉断,软流圈上涌导致变质洋壳板片岩石、先前底侵变质的镁铁质岩石及扬子陆块长英质基底岩石发生部分熔融,以镁铁质岩石熔融产生的熔浆为主(>70%),与长英质基底岩石熔融产生的熔浆混合形成w(Na2O)/w(K2O)>1的TTG组合。     

太行山北段中生代岩基及其包体锆石U-Pb年代学和Hf同位素性质及其地质意义

太行山北段出露大规模中生代岩浆岩带,以中酸性岩为主,普遍含有基性微粒包体。锆石的SHRIMP U-Pb年代学研究表明,包体形成于126Ma左右,与寄主岩石大致同时形成。锆石的LA-MC-ICPMS Lu-Hf同位素原位测量研究表明,基性岩来自富集地幔的部分熔融,并遭受了一定程度的地壳混染;主要的中酸性岩基形成于壳幔岩浆混和过程,而岩基中微粒基性包体是经历分离结晶的基性岩浆注入酸性岩浆房中形成。     

Early Cretaceous Magma Mingling in Xiaocuo, Southeastern China Continental Margin: Implications for Subduction of Paleo-Pacific Plate

Magma mingling has been identified within the continental margin of southeastern China.This study focuses on the relationship between mafic and felsic igneous rocks in composite dikes and plutons in this area,and uses this relationship to examine the tectonic and geodynamic implications of the mingling of mafic and felsic magmas.Mafic magmatic enclaves(MMEs) show complex relationships with the hosting Xiaocuo granite in Fujian area,including lenticular to rounded porphyritic microgranular enclaves containing abundant felsic/mafic phenocrysts,elongate mafic enclaves,and back-veining of the felsic host granite into mafic enclaves.LA-ICP-MS zircon U-Pb analyses show crystallization of the granite and dioritic mafic magmatic enclave during ca.132 and 116 Ma.The host granite and MMEs both show zircon growth during repeated thermal events at-210 Ma and 160-180 Ma.Samples from the magma mingling zone generally contain felsic-derived zircons with well-developed growth zoning and aspect ratios of 2-3,and maficderived zircons with no obvious oscillatory zoning and with higher aspect ratios of 5-10.However,these two groups of zircons show no obvious trace element or age differences.The Hf-isotope compositions show that the host granite and MMEs have similar ε_(Hf)(t) values from negative to positive which suggest a mixed source from partial melting of the Meso-Neoproterozoic with involvement of enriched mantlederived magmas or juvenile components.The lithologies,mineral associations,and geochemical characteristics of the mafic and felsic rocks in this study area indicate that both were intruded together,suggesting Early Cretaceous mantle—crustal interactions along the southeastern China continental margin.The Early Cretaceous magma mingling is correlated to subduction of Paleo-Pacific plate.     

High-K, calc-alkaline I-type granitoids from the composite Yozgat batholith generated in a post-collisional setting following continent-oceanic island arc collision in central Anatolia, Turkey

Summary The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium- to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Şefaatli and Yerk?y, the southwestern part of the batholith can be subdivided into five mappable units: the Ak?akoyunlu quartz monzodiorite (mafic; hornblende K-Ar cooling ages of 77.6–79.3 Ma); the Cankılı monzodiorite (mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite (mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassıağıl monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 69.9–79.8 Ma) and the Karakaya monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 71.3–77.0 Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements (LILE), light rare earth elements (LREE), some high field strength elements (HFSE) (except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite. Supplementary material to this paper is available in electronic form at     

Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999     

U-Pb zircon dating of mafic dykes and its application to the Proterozoic geological history of the Vestfold Hills,East Antarctica

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement complex intruded by at least nine generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyke generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also places doubt on previous wholerock Rb-Sr dating of mafic dyke suites in this and other areas of East Antarctica. The ²⁰⁷Pb/²⁰⁶Pb zircon ages of 2241±4 Ma and 2238±7 Ma for the Homogeneous and Mottled Norites, respectively, provide a younger emplacement age for associated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424±72 Ma) originally interpreted as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754±16 Ma and 1832±72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-rich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and cross-cutting relationships, and yield zircon emplacement ages of 1380±7 Ma and 1241±5 Ma which also define minimum ages for two suites of lamprophyre dykes. Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478±5 Ma to 2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.     

扬子板块西缘新元古代典型中酸性岩浆事件及其深部动力学机制:研究进展与展望

华南板块发育有巨量新元古代岩浆岩,因而是研究罗迪尼亚（Rodinia）超大陆演化期间华南板块地幔属性、地壳演化和壳幔相互作用最理想的场所。虽然在扬子西缘新元古代镁铁质和酸性岩浆作用方面已有大量的研究,但是在系统研究中酸性花岗岩类所代表的不同深部动力学意义的方面还较为薄弱。文章基于团队近期对于扬子板块西缘新元古代典型花岗岩类的研究成果,系统揭示不同深度层次的岩浆作用。最新研究支持扬子西缘新元古代受控于俯冲构造背景,除发生俯冲流体和板片熔体交代地幔作用外,最新识别的ca.850~835 Ma高Mg#闪长岩指示俯冲沉积物熔体也参与了地幔交代作用。Ca.840~835 Ma过铝质花岗岩的发现说明扬子西缘新元古代时期不仅存在新生镁铁质下地壳的熔融,也发生了俯冲背景下成熟大陆地壳物质的重熔。Ca.780 Ma Ⅰ型花岗闪长岩-花岗岩组合揭示了俯冲阶段后期板片回撤断离后软流圈地幔瞬时上涌引发的不同地壳层次的岩浆响应。从ca.800 Ma的增厚下地壳来源的埃达克质花岗岩到ca.750 Ma的酸性地壳来源的A型花岗岩的出现,表明扬子西缘新元古代时期经历了俯冲有关的地壳增厚到俯冲后期弧后扩张背景下的区域性地壳减薄。      

南阿尔金高压-超高温麻粒岩变质作用:大陆地壳超深俯冲与折返过程的记录

南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却.      

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The ε_Hf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding T_DM1 values are in the range of 919–523 Ma. The ε_Hf(t) values of zircon from the DSC I-type granite are between –1 and 3, with T_DM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ¹⁸O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.     

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees

The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association     

The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet

The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO₂ = 42 %) to felsic (SiO₂ = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO₂, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo₈₇ to Fo₉₃, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO₂ and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO₂ and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO₂, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO₂ contents and Mg/Fe ratio to values dictated by equilibrium with the mantle.