胶西北郭家岭花岗闪长岩侵位深度:来自角闪石温压计和流体包裹体的证据

本文应用角闪石地质温压计和流体包裹体对胶东西北部与成矿关系密切的郭家岭花岗闪长岩结晶的温压条件、侵位深度进行了讨论。郭家岭花岗闪长岩的矿物组合为斜长石、角闪石、黑云母、石英、碱性长石、铁-钛氧化物、榍石和次生绿泥石,符合角闪石温压计的使用前提。应用角闪石-斜长石温度计和角闪石全铝压力计所获得的温压条件分别为,郭家岭岩体为694~733℃,1.3~2.4kbar,丛家岩体为702~719℃,2.4~3.9kbar,北截岩体为723~727℃,3.8~4.2kbar。未变形岩石的石英中含有5种类型包裹体,其中CO₂-H₂O-NaCl包裹体广泛分布,最具有代表性。由CO₂-H₂O-NaCl包裹体等容线相图计算的温压范围与岩石所获得的温压范围一致,表明角闪石温压计获得的结果是比较可信的。岩体的侵位深度变化较大,其中郭家岭岩体平均为6km,丛家岩体平均为10km,北截岩体平均为13km,显示从东到西侵位越来越深结晶温度越来越高的趋势。不同侵位深度的岩体同时出露到地表暗示了早白垩世以来胶东地区的剥蚀抬升速率可能是不均匀的,西部相对东部抬升更快剥蚀更多。     

高压超高压变泥质岩适用的地质温压计评述

高压超高压变泥质岩形成温压条件及p-T演化轨迹是高压超高压变质作用研究的重要内容。本文介绍了高压超高压变泥质岩适用的几种地质温压计,包括石榴子石-多硅白云母Fe~(2+)-Mg交换温度计、石榴子石-绿泥石Fe~(2+)-Mg交换温度计、硬绿泥石-绿泥石Fe~(2+)-Mg交换温度计、多硅白云母Si含量压力计、金红石Zr含量温度计、榍石Zr含量温压计、锆石Ti含量温度计、石英Ti含量温度计以及氧同位素温度计,并对上述温压计的适用条件及使用时的注意事项做了简要评述。     

华中高压—超高压变质富铝岩石的温压条件研究及其地质意义

华中高压－超高压变质富铝岩石为陆壳岩石经变质而成，其典型的岩石类型为蓝晶石黄玉石英石。矿物温压计估算其ｐ－Ｔ条件为Ｔ－７００，ｐ＝１．２ＧＰａ；高温高压实验结果表明，蓝晶石＋黄玉组合在Ｔ＝１０００－１２００℃，ｐ＝３．１－３．７７ＧＰａ范围内可以稳定存在，是高压－超高压变质作用产物。     

四川木里长枪穹隆石英流体包裹体特征及其地质意义

通过木里长枪构造-热穹隆体砂泥质变质岩及分泌石英脉中较系统的石英流体包裹体测试与研究,表明穹隆体中石英流体包裹体主要有NaCL-H2O、纯H2O及CO2-H2O、纯CO2等四类,有三个形成或捕获阶段,分别与穹隆体变形变质阶段吻合,其所限定的穹隆体变质条件及抬升P-T轨迹与变形-变质演化研究结果基本一致.     

安徽五河-凤阳地区基于流体包裹体研究的成矿作用温压条件分析

五河-凤阳地区具有较好的成矿地质构造条件,迄今为止已在该区域发现了近20处小型金矿床或矿化点,另外近几年也在该区域发现了如江山铅锌金银矿床等多处中小型铅锌金银矿床。本文结合典型矿床所采集的流体包裹体样品的测试研究,对五河-凤阳地区成矿作用的温压条件进行了分析。     

榴辉岩中传统地质温压计新解：来自PT视剖面图的证据

石榴石-单斜辉石(GC)温度计和石榴石-单斜辉石-多硅白云母(GCP)压力计是确定榴辉岩形成温压条件的最常用方法,二者主要依据石榴石、绿辉石和多硅白云母中相组分之间的交换和转换变质反应.依据MORB成分计算的PT视剖面图表明,在不同榴辉岩矿物组合中,控制3个矿物相成分变化的相组分之间的变质反应不同.在低温含绿泥石、滑石和蓝闪石榴辉岩组合中,石榴石和绿辉石的镁含量主要受到含水矿物脱水反应的控制,并都随温度升高而升高,二者之间的铁镁交换反应并不起主要作用.因此,在自然界含有蓝闪石等含水矿物的低温榴辉岩中,由于绿辉石相对富镁而常常导致GC温度计结果偏低.在含有硬柱石的高压-超高压榴辉岩中,石榴石中的钙含量受到硬柱石的控制,随着压力升高或温度降低,硬柱石含量增加,使石榴石中钙降低,此时石榴石-绿辉石-多硅白云母之间的转换反应对石榴石成分的影响会很微弱,由于石榴石相对贫钙而导致GCP压力计结果偏低.在含有蓝晶石的中温高压-超高压榴辉岩中,矿物成分的变化受到石榴石-绿辉石之间的铁镁交换反应和石榴石．绿辉石．多硅白云母-蓝晶石-石英/柯石英之间的一系列转换反应控制,因此,GC和GCP温压计都能给出相对合理的结果.在低压普通角闪石榴辉岩中,石榴石和绿辉石中的镁含量主要反应压力变化,有时并不指示变质作用温度.在含有蓝闪石等含水矿物的低温榴辉岩中,Thermocalc程序中的平均温压(avPT)方法可以给出比较合适的温度,但压力结果与GCP压力计一样也会偏低一些.在蓝闪石和绿帘石等含水矿物消失后的中温蓝晶石榴辉岩中,avPT方法难以给出合理的PT信息.相对来说,视剖面图方法能够给出最多的PT信息,是目前确定变质岩PT条件的最好方法.     

拉曼光谱方法测量流体包裹体的内压及其应用

通过金刚石压腔和显微激光拉曼光谱方法,获得了流体包裹体中H2O-NaCl体系的O—H、碳酸根的C—O、硫酸根的S—O和正庚烷-环己烷的C—H振动拉曼位移与温度和压力关系的表达式。该研究结果可以用于实测流体包裹体在不同温度下的内压,并可由此获得流体包裹体的p-t线或等容线。这不仅有助于精确获得流体包裹体的形成温度和形成压力,也可为研究和确定流体包裹体中流体的组成和性质提供依据。     

铜陵地区中酸性侵入岩及其包体的成因和矿物温压计的应用

铜陵地区中酸性侵入岩可划分为橄榄安粗岩系列和高钾钙碱性系列。前者岩石组合为辉石二长闪长岩+二长岩+石英二长岩,后者为辉长闪长岩+石英二长闪长岩+花岗闪长岩。两个系列岩石的组成矿物种类相似,但在不同岩石中的含量不同,主要造岩矿物为斜长石、辉石、角闪石、黑云母、钾长石、石英。橄榄安粗岩系列岩石中含有大量的深源包体,包括辉石堆积岩包体、角闪石堆积岩包体和角闪石辉长质堆积岩包体,主要矿物为辉石、角闪石,其次为尖晶石、斜长石、磷灰石、金云母;高钾钙碱性系列岩石中含大量的微粒闪长质包体、镁铁质石英二长闪长质包体和富云母包体,主要矿物为斜长石、角闪石、黑云母。在考虑温压计使用条件的前提下,选择合适的温压计计算了两个系列侵入岩及包体形成的温压条件。结果表明,橄榄安粗岩系列侵入岩侵位深度(4～6km)略小于高钾钙碱性系列侵入岩(6～7km),各种堆积包体形成于45～65km的深位岩浆房,微粒闪长质包体形成于12～15km的浅位岩浆房,镁铁质石英二长闪长质包体是早期侵入岩的边缘相,而富云母包体可能为地壳部分熔融的残余相。     

Prograde garnet-bearing ultramafic rocks from the Tromsø Nappe, northern Scandinavian Caledonides

Garnet-bearing peridotitic rocks closely associated with eclogite within the Tromsø Nappe of the northern Scandinavian Caledonides show good evidence for prograde metamorphism. Early stages are recognized as inclusions of hornblende and chlorite in the cores of large garnet poikiloblasts. Closer to the garnet rim, clinopyroxene and Cr-poor spinel appear as additional inclusion phases. Four suites of spinel inclusions can be distinguished based on optical properties and chemical composition. The innermost suite (suite 1) has the lowest Cr# and highest Mg#. Further rimward, the spinel inclusions gradually change in composition, with increasing Cr# and decreasing Mg#. Spinel is rare in the matrix, but locally chromitic spinel occurs as larger grains. Garnet poikiloblasts are rimmed by a kelyphite zone consisting of Hbl + Cr-poor Spl or Opx ± Cpx + Cr-poor Spl, and locally an inner zone of Na-rich Hbl + Chl. Matrix assemblage in the garnet-bearing peridotitic rocks is Hbl + Chl + Cpx + Ol ± Cr-rich spinel, defining a strong foliation wrapping around garnets and associated kelyphites. Thin layers of garnet-orthopyroxenite and garnet–hornblende–zoisite–chlorite rocks are presumably coeval with the matrix foliation of the peridotitic rocks.

In dunitic to harzburgitic compositions large undulatory grains of Ol + Opx ± Chl + Spl apparently define the maximum-P conditions. This assemblage is succeeded by a recrystallized assemblage of Ol ± Tlc ± Mgs, which in turn is overgrown by strain-free poikiloblasts of orthopyroxene, indicating a temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine.

P–T estimates for the inclusion suites of clinopyroxene and spinel in garnet clearly indicate garnet growth and spinel consumption in a regime of increasing P. The inner suite (suite 1) apparently was in equilibrium with garnet, clinopyroxene and olivine at 1.40 GPa, 675 °C, whereas included spinel with maximum Cr# (suite 4) indicate 2.40 GPa at 740 °C. Grt + Opx from garnet-orthopyroxenite give 1.5–1.9 GPa at 740–770 °C, and Grt + Hbl + Zo + Chl from a zoisite-rich rock give 1.75 ± 0.25 GPa at 740 ± 30 °C, interpreted to represent recrystallization during uplift. In dunitic to harzburgitic compositions, early Ol + Opx ± Chl + Spl is succeeded by Ol ± Tlc ± Mgs, which in turn is overgrown by neoblasts of strain-free orthopyroxene, indicating temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine.

The ultramafic rocks in the Tromsø Nappe were locally strongly hydrated before subduction along with associated eclogites and metasedimentary rocks during the early (Ordovician) stages of the Caledonian orogeny.     

高压-超高压变质岩石中石榴石的环带和成因

在俯冲带变质过程中,石榴石是高压-超高压变质榴辉岩和片麻岩的常见变质矿物。由于石榴石具有难熔和流体中的低溶解能力的特点,通常可以很好地保存下来,并且能够保留复杂的化学成分环带,以及不同类型的矿物或流体包裹体,为解释石榴石寄主岩石经历的变质演化历史提供了重要信息。石榴子石的主微量元素成分受控于很多因素,如全岩成分、变质的温压条件、控制石榴子石形成的相关变质反应、与石榴子石共生的矿物种类和成分等。因此,在利用石榴石探讨超高压变质的演化历史时,对石榴石进行系统的主要元素、微量元素、氧同位素以及矿物包裹体分析,以及相互间的成因关系。同时,对石榴石中的锆石或独居石包裹体并进行原位U-Pb定年和微量元素分析,可以为变质石榴石的形成时代提供直接的时间制约。深入研究超高压变质岩中石榴石的生长阶段,不仅可以为含石榴石寄主岩石的变质过程提供岩石学和地球化学证据,而且对于理解石榴石的形成机制、生长规律及其变质化学动力学过程具有重要的科学意义。     

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone,Bangbing area,southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M₁), the peak eclogite facies (M₂), the decompression blueschist facies (M₃) and retrograde greenschist facies (M₄). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.©2021 China Geology Editorial Office.     

利用石榴橄榄岩重建大陆俯冲带的古动力学环境及其演化过程

本文提出一种对于造山带石榴橄榄岩的简单概念分类模型。该模型将造山带石榴橄榄岩划分为地幔楔型和俯冲带型两种类型,根据所记录温压条件及P-T轨迹的差异,地幔楔型石榴橄榄岩进而被划分为A-D四个亚类,分别对应不同厚度的克拉通型岩石圈地幔及普通大陆岩石圈地幔环境中冷的/古老的/稳定的地幔环境及与之对应的相对热的/年轻的/活动地幔环境。我们选择研究程度较高的斯堪的纳维亚加里东期造山带及我国柴北缘造山带中出露的石榴橄榄岩来检验该模型的适用性。出露于挪威西片麻岩地区的Mg-Cr型及Fe-Ti型石榴橄榄岩分别属于地幔楔型的A亚类(来自于古老的/冷的/厚的/亏损的克拉通岩石圈地幔)及壳源俯冲带石榴橄榄岩,而来自Seve,Troms和Linds地体的石榴橄榄岩则属于地幔楔型的C亚类(来自于古老的/冷的/薄的/亏损的大陆岩石圈地幔)后经历俯冲过程变为俯冲带石榴橄榄岩。对我国柴北缘超高压带绿梁山石榴橄榄岩的检验结果相对复杂。前人的研究表明绿梁山石榴橄榄岩存在三种不同的成因方式,对应分类模型中的:1)D亚类橄榄岩(阿拉斯加型岩浆岩堆晶体);2)壳源俯冲带型石榴橄榄岩;3)A亚类来自克拉通岩石圈地幔的太古代石榴橄榄岩。检验结果表明,该分类模型能较好的解释斯堪的纳维亚加里东期造山带中的石榴橄榄岩的出露规律,岩石学及矿物学特征及不同期次矿物之间的年代学关系,但对于我国绿梁山石榴橄榄岩的成因分类仍需进一步研究。假设该分类模型能适用于大多研究程度较高的造山带石榴橄榄岩,起到决定性作用的两个分类参数为地幔楔形成初期的平均温度(T)及造山带岩石圈地幔的厚度(P)。熔融作用发生的时间及深度同样是区分不同类型石榴橄榄岩的重要因素。     

羌塘地区榴辉岩中矿物出溶体及其成因机制探讨

西藏羌塘地区榴辉岩石榴子石和磷灰石中均发育矿物出溶体,结合电子探针和扫描电镜分析及能谱分析,确定了石榴子石中的出溶体有金红石、石英和角闪石,指示其可能经历了高压/超高压变质作用。本研究在磷灰石中发现了金红石出溶,并认为其可能是在高压/超高压条件下元素相互置换的结果,置换方式为Ti~(4+)(Fe~(2+)、Si~(4+))=2Ca~(2+),这些出溶体的形成为榴辉岩曾经历高压/超高压变质作用提供了矿物学证据。     

Field report:Research along the Yarlung Suture Zone in Southern Tibet,a persistent geological frontier

The Yarlung Suture Zone in Southern Tibet marks the boundary between India and Asia-formerly separated by an ocean basin-and is a critical record of the tectonic processes that created the Tibetan Plateau. The Yarlung Suture Zone is also a frontier research area, as difficulty of access has limited research activity, providing ample opportunities for new discoveries. This paper documents field research conducted by the authors along the Yarlung suture zone in eastern Xigaze(Shigatse, Rikaze)County, ~250 km west of the city of Lhasa, in July 2017. The goal of this research was to map the Suture Zone structure in detail, and more specifically to understand the branching relationships between two major fault systems-the Great Counter Thrust and Gangdese Thrust. A summary of early geological exploration is included to provide context for this research.     

Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition

In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks.Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones.     

洋壳深俯冲超高压变质作用研究及其地质意义

洋壳深俯冲经历超高压变质作用的岩石比较稀少,是目前超高压变质作用研究的重要前沿领域,有关这方面的研究对于建立新的冷俯冲带变质作用类型、探讨地幔水流体的成因、建立更广泛的超高压变质岩石的抬升机制具有重要意义。另外,洋壳深俯冲高压变质带往往与低压高温变质带组成双变质带,对双变质带的深入研究对于完整地重塑俯冲碰撞造山带的形成演化过程具有重要意义。简要地介绍了新疆西天山洋壳深俯冲超高压变质带的研究现状和存在的问题。     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

喜马拉雅造山带的高压超高压变质作用与印度-亚洲大陆碰撞

喜马拉雅造山带是印度与亚洲大陆碰撞作用的产物,正在进行造山作用,是研究板块构造的天然实验室.高压和超高压变质岩分布在喜马拉雅造山带的核部.这些变质岩具有不同的形成条件、形成时间和形成过程,为印度与亚洲碰撞带的几何学、运动学和动力学提供了重要的限定.含柯石英的超高压变质岩产出在喜马拉雅造山带的西段,它们形成在古新世与始新世之间(53～46Ma),为印度大陆西北边缘高角度超深俯冲作用的产物,并经历了快速俯冲与快速折返过程.在约5 Myr内,超高压变质岩从＞100km的地幔深度折返到了中地壳深度,且仅仅叠加角闪岩相退变质作用.高压榴辉岩产出在喜马拉雅造山带中段,形成时间约为45Ma,为印度大陆低角度深俯冲作用的产物,经历了至少20Myr的长期折返过程,叠加麻粒岩相退变质作用和部分熔融.高压麻粒岩产出在喜马拉雅造山带的东端,是印度大陆东北缘近平俯冲作用的产物,峰期变质作用时间约为35Ma,经历了约20Myr的长期折返过程,叠加了麻粒岩相和角闪岩相退变质作用,并伴随有多期部分熔融.因此,喜马拉雅造山带的变质作用具有明显的时间与空间变化,显示出大陆深俯冲与折返过程的差异性,以及大陆碰撞造山带形成机制的多样性.