Isobaric cooling and anti-clockwise P–T paths in the Variscan Odenwald Crystalline Complex, Germany     

T. M. Will  E. Schmädicke 《Journal of Metamorphic Geology》2003,21(5):469-480

Mineral assemblages in Al₂O₃‐rich, SiO₂‐ and K₂O‐poor metapelitic rocks from the western Odenwald Crystalline Complex (Variscan Mid‐German Crystalline Rise, southern Germany) include corundum, spinel, cordierite, sillimanite, garnet and staurolite. Quartz is absent from almost all samples. Therefore, the applicability of conventional geothermobarometry is very limited or even impossible. Detailed petrographic investigation on selected samples permits inference of the sequence of appearance and disappearance of several mineral assemblages. The recognition of such partial re‐equilibration stages and their associated mineral assemblages, together with mineral stabilities predicted from KFMASH pseudosections, enables the determination of the pressure‐temperature (P–T) trajectories experienced by these rocks during the Variscan metamorphism. The rocks were metamorphosed under low‐P/high‐T conditions and underwent an anti‐clockwise P–T evolution. A pressure increase from about 2 kbar to 4 ± 0.5 kbar was accompanied by heating. Peak metamorphic conditions were reached at pressures of 4 ± 0.5 kbar and temperatures of at least 640 °C, probably even higher. The retrograde evolution is characterised by near‐isobaric cooling from ≥ 640 °C to approximately 550 °C. The rocks underwent the anti‐clockwise evolution in a subduction‐related magmatic arc setting. The close spatial association of the low‐P/high‐T rocks with recently discovered metabasic eclogites in the eastern part of the Odenwald Crystalline Complex may indicate a fossil paired metamorphic belt in the Central European Variscides.  相似文献   

庐山星子群变质岩的变质作用P-T条件研究   总被引：1，自引：0，他引：1  

张海祥  张伯友  孙大中  朱炳泉 《矿物岩石》2003,23(2):32-36

对分布于扬子块体南缘古元古代星子群变质宏的变质作用研究表明，它们普遍经历了角闪岩相的变质，其变质温度为480℃-580℃，压力在300MPa-500MPa之间。这与扬子块体南缘其他元古代地层仅经历了轻微变质的事实截然不同，表明星子群变质岩具有独特的构造演化历史。越来越多的研究发现，星子群变质岩是由地壳伸展作用引发的、来源于下地壳的前寒武纪变质基底，它并不是来源于华夏块体的“飞来峰”。  相似文献   

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France   总被引：6，自引：0，他引：6  

MICHAEL BROWN  R. DAVID DALLMEYER 《Journal of Metamorphic Geology》1996,14(3):361-379

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.  相似文献   

新疆西南天山木扎尔特一带低压麻粒岩相变质作用P-T轨迹及其地质意义   总被引：11，自引：5，他引：11  

李强  张立飞 《岩石学报》2004,20(3):583-594

本文首次报道在新疆西南天山木扎尔特一带发现了二辉石麻粒岩和麻粒岩相变质的堇青石榴矽线石片麻岩。二辉石麻粒岩的矿物组合为单斜辉石-斜方辉石-黑云母-角闪石-斜长石-石英。堇青石榴矽线石片麻岩矿物组合为堇青石-矽线石-石榴石-黑云母-斜长石-石英。岩石学和矿物学特征表明它们是典型的低压麻粒岩相变质岩石，其变质作用经历了两期演化：a．峰期麻粒岩相变质，T=681～705℃，P=5．4～5．8kbar；b．峰后角闪岩相退变阶段，T：571～637℃．P=4．7～5．3kbar。其变质作用P-T轨迹具有逆时针近等压降温(IBC)的特点，代表该地区可能为塔里木板块向伊犁．中天山板块俯冲过程中，在陆壳一侧所产生的陆源岩浆弧区域，由于受到下部岩浆热源的影响，在拉伸环境下出现低压麻粒岩相变质。通过分析低压麻粒岩相岩石与其南部高压一超高压变质带的大地构造位置和年代关系，我们认为该地区的低压麻粒岩相变质岩石可能与其南部的西天山高压一超高压变质带组成了双变质带。  相似文献   

大别山太湖地区榴辉岩峰期变质条件与构造意义   总被引：3，自引：6，他引：3  

石永红  王清晨 《岩石学报》2004,20(3):609-620

大别山太湖地区曾被作为“冷”、“热”榴辉岩带的交界处所在，我们对大别山太湖地区的进行了1：10000地质填图，并详细研究了该地区榴辉岩的岩相学和矿物成分变化，以及KD值、峰期变质P-T条件。研究表明榴辉岩自南向北可分为三类：①朱家冲型榴辉岩；②大坝型榴辉岩；③金河桥型榴辉岩。其中朱家冲型榴辉岩和金河桥型榴辉岩显示出明显的温度、压力差异，它们形成的P-T条件分别为：P=2．14GPa、T=563℃和P=3．76GPa、T=791℃。大坝型榴辉岩的峰期变质P-T条件差别较大，在柯石英-石英相变线附近徘徊，变质环境相当于高压一超高压过渡环境。朱家冲型榴辉岩与大坝型榴辉岩分别对应于前人所划分的“冷”、“热”榴辉岩，并且两者在峰期，变质温、压条件上是连续变化的，没有明显的温、压间断。因此，我们认为本区所谓的“冷”、“热”榴辉岩带界线是不存在的。  相似文献   

基于物方影像匹配和概率松弛的断面自动提取   总被引：4，自引：0，他引：4  

郑顺义  张祖勋  张剑清 《测绘信息与工程》2004,29(2):26-27

提出了基于物方影像匹配的断面提取,为了提高匹配的可靠性,引入了概率松弛法,最终获得了高精度的断面.  相似文献   

Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area   总被引：4，自引：0，他引：4  

K. STÜWE  R. POWELL 《Journal of Metamorphic Geology》1989,7(4):465-483

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°-79°E, 68°-70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south-west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south-west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth. This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre-metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion-driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.  相似文献   

An introduction to ultrahigh-pressure metamorphism   总被引：6，自引：0，他引：6  

J. G. Liou    Ruyuan  Zhang W. G. Ernst 《Island Arc》1994,3(1):1-24

Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).  相似文献   

Mineral composition variation in Alpine Schist, Southern Alps, New Zealand: Implications for recrystallization and exhumation   总被引：1，自引：0，他引：1  

RodneyH.  Grapes Teruo  Watanabe 《Island Arc》1994,3(3):163-181

Abstract Compositional variation of silicates (plagioclase, K-feldspar, epidote, titanite, garnet, white mica, biotite, chlorite), ilmenite, carbonates (calcite, ankerite) and apatite, in quartzofeldspathic lithologies of the Alpine Schist, New Zealand, is discussed in terms of increasing metamorphic grade and possible isograd-producing reactions. The mineral data, in conjunction with geological considerations, are used to determine polychronous P-T arrays of an early high P/T event (c. 16°C/kb; 5°C/km) overprinted by a lower P/T event (c. 50°C/kb; 15°C/km) that provides an estimation of Mesozoic and Cenozoic exhumation of schist of 11 to 13 km and 19 to 22 km respectively. The effects of possible shear heating and recrystallization to form K-feldspar zone schist near the Alpine Fault is consistent with movement along a mid to lower crustal detachment surface during Cenozoic shortening, and near isothermal exhumation of the schists to form the Southern Alps.  相似文献   

金川超镁铁质岩体矿物化学特征及矿物地质温度计、压力计研究   总被引：4，自引：0，他引：4  

徐庆生  宋学信 《甘肃地质》1994,3(1):59-70

根据金川超镁铁质岩体主要造岩矿物和副矿物的共生组合关系及各种矿物的化学成分特点；选用５种较成熟、使用较方便的矿物地质温度计、压力计（主要是辉石温压计和角闪石压力计），对岩体形成的温、压条件进行了估算，其结果为：成岩温度为１０００～１３００℃（上限可到１４００～１５００℃），成岩压力约为５×１０￣８～１１×１０￣８ｐａ。金川超镁铁质岩体的主要岩石类型为尖晶石二辉橄榄岩，这一岩石类型在复杂体系相关系图中的稳定区域大致处于５×ｌ０￣８～１５×１０￣８ｐａ的范围，这与矿物温压计估算的成岩温压条件是一致的。  相似文献