Tektonisches Verhalten und geologische Bedeutung von Kalksilikatfels-Lagen und -Spindeln im Damara-Orogen Südwest-Afrikas

Zusammenfassung Beschrieben wird das Verhalten von frühdiagenetisch gebildeten schichtparallelen Kalksilikatfels-Lagen, -Linsen, -Knollen und diffusen Ansammlungen während der Gesteinsdeformation in Teilen des Damara-Orogens. In Abhängigkeit von ihrer relativen Löslichkeit und/oder Kompetenz sowie vom Winkel zwischen Schichtung und Schieferung können Lagen und Linsen als solche erhalten bleiben oder in spindelförmige Körper zerlegt werden (Verkürzungsboudinage). Knollen werden zu Ellipsoiden oder Spindeln deformiert und diffuse Kalksilikatfels-Ansammlungen in eine Vielzahl von Spindeln aufgelöst. Alle im Zusammenhang mit der Anlage der Schieferung gebildeten Spindeln und Ellipsoïde sind streng in der Schieferung orientiert. Es wird deutlich, daß die Spindelform der Kalksilikatfels-Körper ein Produkt der Gesteinsdeformation ist.In Gebieten, in denen der Verdacht besteht, daß parallel zu einem deutlichen Materialwechsel eine Schieferung verläuft, kann aus der Existenz von Spindeln auf das Vorhandensein dieser Schieferung geschlossen werden.Die ungleichmäßige Verteilung von Kalksilikatfels-Körpern innerhalb der Khomas-Serie ermöglicht eine Unterteilung dieser bisher ungegliederten, bis zu 6000 m mächtigen Gesteinsfolge.

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The behaviour during rock deformation of diagenetically formed layers, lenses, nodules (concretions) and diffuse concentrations of calc-silicate rock is described from parts of the Damara belt. Depending on their relative solubility and/or competency and on the angle between bedding and cleavage planes the layers and lenses may either more or less retain their shape or be cut up into spindle-shaped bodies (boudinage due to shortening). Nodules are deformed to ellipsoides, while diffuse concentrations of calc-silicate rock are transformed into a number of spindles. All spindles and ellipsoides which are formed during the formation of the cleavage are strictly orientated parallel to the cleavage planes. It is therefore obvious that the spindleshape of these bodies is a consequence of rock deformation. Thus, in areas in which a cleavage is suspected to lie parallel to a distinct bedding, the occurrence of spindles may confirm the existence of this cleavage.The non-statistical occurrence of calc-silicate rock bodies within the Khomas Series may render possible a first subdivision of this up to 6000 m thick succession.

Diese Arbeit ist im Sonderforschungsbereich 48 Entwicklung, Bestand und Eigenschaften der Erdkruste, insbesondere der Geosynklinalräume, Göttingen, entstanden.     

Reaction textures in scapolite–wollastonite–grossular calc-silicate rock from the Kerala Khondalite Belt, Southern India: evidence for high-temperature metamorphism and initial cooling

Scapolite–wollastonite–grossular bearing calc-silicate rocks from the Vellanad area in the Kerala Khondalite Belt (KKB) of Southern India preserve a number of reaction textures which help to deduce their P–T–fluid history. Textures include calcite+plagioclase±quartz symplectites after scapolite, grossular+quartz coronas between wollastonite and plagioclase, grossular coronas between wollastonite and plagioclase+calcite that replace former scapolite, and grossular blebs replacing anorthite+calcite+quartz pseudomorphs of scapolite. Garnet coronas are also observed between clinopyroxene and wollastonite or scapolite or plagioclase. The reactions, apart from those involving clinopyroxene, can be modelled in the simple CaO–Al₂O₃–SiO₂–CO₂ system and interpreted using partial reaction grids constructed for the activities of end-members in the analysed phases. The reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures. For the compositions of the phases present in this study, the medium pressure calc-silicate assemblages are defined by the stable pseudo-invariant points [Qtz], [Mei] and [Grs]. The textural features interpreted using these activity-corrected grids indicate a phase of isobaric cooling from about 835°C to 750°C at 6 kbar in the Vellanad area. This is inconsistent with earlier studies on other lithologies from the KKB, most of which imply a post-peak P–T path involving near-isothermal decompression. However, as the temperatures obtained for the KKB from the calc-silicates are higher than those previously deduced from metapelites and garnet–orthopyroxene assemblages, the phase of near-isobaric cooling reported here is inferred to have proceeded prior to the onset of the decompression documented from studies of other rock types.     

接触变质带中的脱碳作用SCIEI北大核心CSCD

地球大气中的碳含量控制着地球长期(百万年尺度)和短期气候。最近的一些工作表明,地质历史中温室气候的时期和高强度的岩浆作用之间可能存在时间上的重合。从而提出假设,即火山和侵入岩与地壳碳酸盐岩之间相互作用释放的二氧化碳可能在调节外生系统的长期碳收支方面发挥了重要作用。例如,在Pangea超大陆裂解的过程中,特提斯体系大陆弧与白垩纪环太平洋弧的活动时间上重合。这些剧烈的大陆弧活动导致的接触变质作用释放的CO_(2)可能有助于形成白垩纪的温暖气候。除了这些宏观假设之外,接触变质作用产生的碳通量仍有待岩石学研究来解决。岩浆的加热和流体的渗透会促使围岩发生脱碳反应。富含H_(2)O的流体渗透会显著降低CO_(2)或其它含碳组分的活度,并有效地从系统中移除反应产物,从而促使通过发生更多的脱碳反应来恢复化学平衡。因此,渗透流体的通量和模式,无论是岩浆还是天水来源,对研究碳通量至关重要。在本文中,我们回顾了岩浆侵入周围的热量和流体传输以及碳酸盐岩和碳质变泥质岩组合的脱碳反应。对现代火山弧的观察表明,如果岩浆与上覆板块的碳酸盐岩地层相互作用,则会产生大量的碳通量。白垩纪大陆弧中丰富的矽卡岩和钙硅酸盐地体为量化从露头到造山尺度的碳通量提供了多样的野外研究机会。最后,本文概述了未来研究将要探索或解决的技术问题,包括流体来源和流动模式、含盐流体的相平衡以及脱碳的时间尺度等。     

Wollastonite and scapolite in Precambrian calc-silicate granulites from Australia and Antarctica

Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO₂ activity at a low value, so that any coexisting fluid phase must be H₂O rich and CO₂ poor ( X _co2 = 0.2-0.3). In contrast, the H₂O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO₂ activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid.     

胶北南山口古元古代高压基性麻粒岩和钙硅酸盐岩的岩石地球化学特征探讨

胶北莱西古元古代的高压基性麻粒岩和钙硅酸盐岩的基本矿物组合分别为以铁铝榴石为主的石榴石-普通辉石-铁紫苏辉石和钙铝榴石-黝帘石-葡萄石-钠长石.矿物岩石学研究表明钙硅酸盐岩是由含石榴石高压基性麻粒岩经退变质和钙质交代作用形成.南山口高压基性麻粒岩记录了麻粒岩相变质作用前、麻粒岩相变质作用、退变质和钙硅酸盐岩化共同作用以及完全钙硅酸盐岩化的四个阶段的地质作用,其矿物组合分别为Cpx+ Pl+ Qtz(M1),Grt+ Cpx+ Rt+ Qtz(M2),Cpx+Pl+ Opx+ Ilm+ Mgt+ Ep(M3)和Grs+ Zo+ Prh+ Ab+ Cal(M4).微量元素研究表明,高压基性麻粒岩中大离子亲石元素Ba、Rb、K、Rb、Th富集,而高场强元素Nb、Zr、Ti、Y亏损,具有轻稀土富集的右倾型稀土配分曲线.稀土元素和微量元素配分图解显示了岛孤拉斑玄武岩的特征.主元素、微量元素的构造判别图解进一步分析表明高压基性麻粒岩及其钙硅酸盐岩的原岩形成于大陆边缘的岛弧环境.综合高压基性麻粒岩岩石学、元素地球化学特征认为,莱西高压基性麻粒岩的原岩是拉斑玄武岩质岩石,可能是形成于孤后扩张背景下基性的侵入岩或喷出岩.岩石形成以后,在胶-辽-吉带碰撞闭合过程中,经历了麻粒岩相变质作用,又在后来的抬升过程中经历退变质和钙硅酸盐岩化作用.     

胶北地块早元古代钙硅酸盐岩与高压基性麻粒岩成因及地质意义

胶北地块发育一套早元古代变质杂岩,主要由浅变质到中高级变质的滨海相至浅海相的沉积岩系列组成,属于胶-辽-吉构造活动带的一部分。其中具有顺时针变质作用PT轨迹的高压基性麻粒岩和高压泥质麻粒岩指示了造成该带闭合的碰撞构造过程。本文研究与高压基性麻粒岩密切共生的钙硅酸盐岩。首先根据矿物反应关系研究,确定钙硅酸盐岩是由含石榴石的基性麻粒岩经钙质交代形成的。进一步采用残留单斜辉石-石榴石组合、退变矿物组合,并根据SACMF体系矿物组合演化的PT视剖面图,计算得出钙硅酸盐岩经历了早期温压条件为780~850℃、1.0~1.1GPa的变质作用和晚期温压条件为400~650℃、0.6~0.75GPa的退变质作用和交代作用,从而得出顺时针的P-T轨迹。这一结果表明,钙硅酸盐岩形成于陆-陆碰撞过程的晚期阶段,是高压基性麻粒岩在折返过程中经历退变质和Ca质交代联合作用的结果。钙硅酸盐岩的形成机制及其与石榴石基性麻粒岩原岩的成因联系的确立,丰富了对于碰撞变质地块抬升过程及其岩石学效应的认识。     

High-grade calcareous metasediments from the Sawtooth Metamorphic Complex,Idaho, USA: evidence for passive margin strata and polymetamorphism within the Idaho batholith

ABSTRACT

The Sawtooth Metamorphic Complex (SMC) of central Idaho contains exposures of metasupracrustal rocks, in a crucial location between the Archaean Wyoming craton to the east and the adjacent Mesozoic terranes to the west, that provide constraints on Precambrian crustal evolution in the northwestern United States. Mineral textures, whole-rock geochemistry, and thermobarometry of calc-silicate gneisses record multiple stages of crustal evolution, including protolith deposition, burial and multiple metamorphic and deformational overprints. Whole-rock signatures are consistent with derivation from post-Archaean, continental sources that have undergone sedimentary maturation and recycling typical of clastic sedimentation in passive-margin environments and with a metamorphosed sequence of calcareous sandstones and marls, containing varying proportions of clay and quartzo-feldspathic detritus. The sandstone-to-marl continuum may reflect a shallow-to-deep water transition in the depositional environment of the calc-silicate protoliths. Two metamorphic events and three deformational events are preserved. The assemblage clinopyroxene + quartz + plagioclase + K-feldspar + rutile represents peak metamorphic conditions (M1), estimated at 750–775°C with relatively oxidized metamorphic fluids, during development of D1a foliation. Deformation twinning in clinopyroxene records a widespread, high strain D1b event at high temperature. A second static thermal event (M2), associated with randomly oriented amphiboles overprinting M1 and D1b features, records conditions of ~550–725°C and H₂O-rich fluids. Late-stage, brittle–ductile D2 deformation is characterized by mylonitic lenses of quartz, fractures within M1 clinopyroxenes that crosscut D1b deformation twins, and localized fracturing of M2 amphiboles. Geochemical and mineral chemical signatures of SMC calc-silicates preserve fingerprints of the original protolith through burial to mid-crustal conditions and high-grade metamorphism and suggest crustal thickening to about 20 km. The SMC may reflect rocks at depth beneath the Idaho batholith. These data, indicative of newly recognized post-Archaean terrain with passive-margin, continental sediments suggest that a continuous Cordilleran passive margin sequence may have extended along the western edge of Laurentia.     

Mineral exploration under cover: Characterizing mineralizing fluid systems

Mineral exploration in areas of little bedrock exposure must increasingly rely on the predictive quality of geological, geochemical and geophysical data. In this contribution analysis of fluid inclusions is used to characterize fluid systems associated with alteration and mineralization in two locations in South Australia. In the Curnamona Province, fluids related to albitization and calc-silicate alteration with associated brecciation evolve from a saline sodic composition to a metalliferous, saline fluid also found associated with sulphide mineralization. The microthermometric distinction of inclusion types is evidence for pressure fluctuation as a cause for breccia formation. In the Central Gawler Craton gold province fluid inclusion studies have characterized the compositional and constrained the physical conditions of mineralizing fluids. Regionally identified low salinity CO₂-bearing fluids at temperatures < 350 °C and pressures < 1.5 kbar indicate a shallow-level orogenic [Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., 1998. Orogenic gold Deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews, 13, 7–27.] or intrusion-related [Lang, J.R., Baker, T., 2001. Intrusion related gold systems: the present level of understanding. Mineralium Deposita, 36, 477–489.] setting for the mineralization.