Petrology and thermobarometry of mafic granulites and migmatites from the Chafalote Metamorphic Suite: New insights into the Neoproterozoic P–T evolution of the Uruguayan—Sul-Rio-Grandense shield     

A.O.M.S. Gross  G.T.R. Droop  C.C. Porcher  L.A.D. Fernandes 《Precambrian Research》2009,170(3-4):157-174

This paper reports a study of the metamorphic evolution of pelitic, semi-pelitic migmatites and mafic granulites of the Chafalote Metamorphic Suite (CMS), Uruguay, which represents the southernmost exposures of high-grade metamorphic rocks in the Dom Feliciano Belt, Uruguain—Sul-Rio-Grandense shield, South America. This belt is one of the Brasiliano orogens that crop out along the Brazilian and Uruguayan Atlantic margin, and the CMS is one of several disconnected segments of supracrustal rock in a dominantly granitic terrain. Petrological evidence from CMS mafic granulites and semi-pelitic migmatites indicates four distinct metamorphic assemblages. The early prograde assemblage (M₁) is preserved only as inclusions in porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by near-isothermal decompression (M₃), which resulted in symplectites and coronitic textures in the mafic granulites and compositional zoning of Ca in garnet (decreasing rimwards) and plagioclase (increasing rimwards) in the semi-pelitic migmatites. The retrograde metamorphic assemblage (M₄) is represented by hydration reaction textures replacing minerals of the M₂ and M₃ assemblages. Average P–T calculations using the program THERMOCALC and conventional thermobarometric methods yield peak-metamorphic (M₂) P–T conditions of 7–10 kbar and 830–950 °C, near-decompressional (M₃) P–T conditions of 4.8–5.5 kbar and 788–830 °C and M₄ retrograde P–T conditions of 3–6 kbar and 600–750 °C. The calculated P–T path for the CMS rocks is ‘clockwise’ and incorporates a near-isothermal decompression segment followed by minor cooling, consistent with a history of crustal thickening followed by extensional collapse at ca. 650–600 Ma. The metamorphism recorded by rocks of this crustal segment may be correlated with 650 Ma metamorphism in the Coastal Terrane of the Kaoko Belt in Namibia, being the first unequivocal match between South America and Africa provided by crystalline rocks south of the Congo Craton.  相似文献   

Vesiculation and crystallization under instantaneous decompression: Numerical study and comparison with laboratory experiments     

A. Toramaru  T. Miwa 《Journal of Volcanology and Geothermal Research》2008

Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate.  相似文献   

A decompressional P–T path, Reinbolt Hills, East Antarctica     

G. T. NICHOLS  R. F. BERRY 《Journal of Metamorphic Geology》1991,9(3):257-266

Granulites exposed in the Reinbolt Hills, East Antarctica, are part of the extensive Late Proterozoic granulite complex of East Antarctica, which includes the Rauer Group to the east and the northern Prince Charles Mountains to the west. The deformation history includes three pervasive deformation phases. No chemical or mineralogical distinction between these phases has been detected and this is interpreted to be the result of complete re-equilibration at the end of the third deformation phase. Two late deformation phases post-date the metamorphism and record a medium-temperature cooling path. A short segment of the P–T path of these rocks was inferred from mineral reactions that occurred during these late deformation phases. The path passes from 800°C, 7 kbar to 690°C, 5 kbar, indicating strong decompression, which is typical of a thrust-dominated crustal thickening followed by rapid erosion or extensional collapse.  相似文献   

Decompressed eclogites in the Sveconorwegian (–Grenvillian) orogen of SW Sweden: petrology and tectonic implications     

C. MÖLLER 《Journal of Metamorphic Geology》1998,16(5):641-656

Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.  相似文献   

A corundum–quartz assemblage from the Eastern Ghats Granulite Belt,India: evidence for high P–T metamorphism?     

R. K. SHAW  & M. ARIMA 《Journal of Metamorphic Geology》1998,16(2):189-196

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.  相似文献   

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway     

H. S. TOMKINS  I. S. WILLIAMS  D. J. ELLIS 《Journal of Metamorphic Geology》2005,23(4):201-215

Regionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M₁ metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.  相似文献   

Solar System Processes Underlying Planetary Formation, Geodynamics, and the Georeactor     

J. Marvin Herndon 《Earth, Moon, and Planets》2006,99(1-4):53-89

Only three processes, operant during the formation of the Solar System, are responsible for the diversity of matter in the Solar System and are directly responsible for planetary internal-structures, including planetocentric nuclear fission reactors, and for dynamical processes, including and especially, geodynamics. These processes are: (i) Low-pressure, low-temperature condensation from solar matter in the remote reaches of the Solar System or in the interstellar medium; (ii) High-pressure, high-temperature condensation from solar matter associated with planetary-formation by raining out from the interiors of giant-gaseous protoplanets, and; (iii) Stripping of the primordial volatile components from the inner portion of the Solar System by super-intense solar wind associated with T-Tauri phase mass-ejections, presumably during the thermonuclear ignition of the Sun. As described herein, these processes lead logically, in a causally related manner, to a coherent vision of planetary formation with profound implications including, but not limited to, (a) Earth formation as a giant gaseous Jupiter-like planet with vast amounts of stored energy of protoplanetary compression in its rock-plus-alloy kernel; (b) Removal of approximately 300 Earth-masses of primordial volatile gases from the Earth, which began Earth’s decompression process, making available the stored energy of protoplanetary compression for driving geodynamic processes, which I have described by the new whole-Earth decompression dynamics and which is responsible for emplacing heat at the mantle-crust-interface at the base of the crust through the process I have described, called mantle decompression thermal-tsunami; and, (c) Uranium accumulations at the planetary centers capable of self-sustained nuclear fission chain reactions.  相似文献   

南海中央海盆扩张期后海山链岩浆活动的热模拟研究     

林巍  张健  李家彪 《海洋科学》2013,37(4):81-87

壳幔黏性结构是影响南海中央海盆扩张期后海山链岩浆活动的主要因素,研究其岩浆活动发育机制具有重要的科学意义。利用重、磁、测深及岩样分析数据提供的地壳结构和热力学参数,通过FEM数值模拟,研究了南海中央海盆珍贝-黄岩海山链之下黏性结构与岩浆熔融、运移活动的关系。根据黏性主要受压力和温度控制,但熔融期间的散热及脱水会增加黏性,设计了三种不同的黏性结构模型。计算结果表明：三种垂向黏性结构模型在不同的温度条件下,都可以使地幔熔融区最大熔融程度达到20%~25%。岩浆熔融程度与地幔热结构、熔融潜热和含水量有关,并受扩张速率影响,慢速扩张脊下的岩浆熔融程度相对较低。岩浆运移在扩张期后主要依靠减压熔融浮力,垂直上升至熔融区顶面后沿上倾面向脊轴运移。接近脊轴熔融带,部分熔融程度高,远离轴部熔融带,部分熔融程度较低。主要结论：南海中央海盆海底扩张期,海山链之下10 km深度具备形成拉斑玄武岩浆的条件;扩张期末,该地区25 km深度具备形成碱性玄武岩浆的条件。  相似文献   

地层抬升过程中的砂体回弹作用及其油气成藏效应   总被引：13，自引：0，他引：13  

姜振学庞雄奇  金之钧关德范 李玉喜 《地球科学》2004,29(4):420-426,482

地层抬升是盆地构造运动的主要表现形式之一,以往多强调其对油气藏的破坏作用,而忽略了其对油气成藏贡献的研究．通过对不同粒级砂体在不同温度、压力条件下回弹量实验模拟,建立了其在地层抬升过程中不同地质条件下砂体回弹量模板;以大庆长垣为典型解剖实例,结合地层埋藏史、油气生排烃史研究,进而建立地层抬升造成的减压增容效应与油气成藏的定量关系模型,定量评估出大庆长垣地层抬升造成的砂体回弹效应对油气成藏贡献是13．43％～40．46％．对油气成藏机理进行了新的探讨,可为油气资源分布预测研究提供更科学的依据．  相似文献   

P–T conditions of decompression of the Limpopo high-grade terrane: record from shear zones     

C. A. Smit  D. D. Van Reenen  T. V. Gerya  L. L. Perchuk 《Journal of Metamorphic Geology》2001,19(3):249-268

The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high‐grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low‐strain gneisses, bound by an imbricate system of kilometre‐wide deep crustal shear zones characterized by the presence of high‐strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high‐grade terrane. Low‐ and high‐strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low‐strain gneisses as a uniform orientation of relict Al‐rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg‐rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high‐strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid‐shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg‐rich garnet porphyroblasts, and syn‐decompression M2 pencil‐shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near‐isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil‐shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low‐strain gneisses from thrust sheets show similar M2 decompression cooling and near‐isobaric cooling reaction textures that formed within the same P–T range, but under low‐strain conditions, as shown by their pseudo‐idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low‐strain conditions in gneisses from thrust sheets and high‐strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post‐decompression shearing under epidote–amphibolite facies conditions.  相似文献