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High-grade Metamorphism (high-grade + metamorphism)
Selected AbstractsNiches of the pre-photosynthetic biosphere and geologic preservation of Earth's earliest ecologyGEOBIOLOGY, Issue 2 2007NORMAN H. SLEEP ABSTRACT The tree of terrestrial life probably roots in non-photosynthetic microbes. Chemoautotrophs were the first primary producers, and the globally dominant niches in terms of primary productivity were determined by availability of carbon dioxide and hydrogen for methanogenesis and sulfite reduction. Methanogen niches were most abundant where CO2 -rich ocean water flowed through serpentinite. Black smoker vents from basalt supplied comparable amount of H2. Hydrogen from arc volcanoes supported a significant methanogenic niche at the Earth's surface. SO2 from arc volcanoes reacted with organic matter and hydrogen, providing a significant surface niche. Methane ascended to the upper atmosphere where photolysis produced C-rich haze and CO, and H escaped into space. The CO and C-rich haze supported secondary surface niches. None of these ecologies were bountiful; less than 1% of the CO2 vented by ridge axes, arcs, and metamorphism became organic matter before it was buried in carbonate. In contrast, a photosynthetic biosphere leaves copious amounts of organic carbon, locally concentrated in sediments. Black shales are a classic geologic biosignature for photosynthesis that can survive subduction and high-grade metamorphism. [source] Petrology of corundum-spinel-sapphirine-anorthite rocks (sakenites) from the type locality in southern MadagascarJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2008M. M. RAITH Abstract ,Sakenites' constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum-, spinel- and sapphirine-bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite-sillimanite-cordierite gneisses of the Ihosy granulite unit in the NW of the Pan-African Bongolava-Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite-rich sediments or calcareous bauxites that were altered by Ca or Si infiltration-metasomatism prior to or coeval with the development of the anorthite-corundum assemblage. P,T pseudosection modelling of metapelitic gneisses suggests peak-conditions around 800 °C and 6,7 kbar for the regional high-grade metamorphism and deformation in the NW part of the Bongolava-Ranotsara shear zone. The well-annealed granoblastic-polygonal textures indicate complete chemical and textural re-equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P,T conditions (750,700 °C, 6 kbar), the influx of Mg-, Si- and K-bearing fluids into the anorthite-corundum rocks caused significant metasomatic changes. In zones infiltrated by ,primary' potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg-Al minerals, thereby producing corundum-, spinel- and sapphirine-bearing phlogopitites. Further advance of the resulting ,residual' Mg- and Si-bearing fluids into anorthite-corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava-Ranotsara ductile shear zone c. 530,500 Ma ago. [source] Oxide and sulphide isograds along a Late Archean, deep-crustal profile in Tamil Nadu, south IndiaJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2005D. E. HARLOV Abstract Oxide,sulphide,Fe,Mg,silicate and titanite,ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high-grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo-ilmenite in the high-grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe,Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite,pyrite micro-veins along silicate grain boundaries formed over a wide range of post-peak metamorphic temperatures and pressures ranging from high-grade SO2 to low-grade H2S-dominated conditions. Oxygen fugacities estimated from the orthopyroxene,magnetite,quartz, orthopyroxene,hematite,quartz, and magnetite,hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO4 during high-grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase. [source] Timing of high-grade metamorphism: Early Palaeozoic U,Pb formation ages of titanite indicate long-standing high- T conditions at the western margin of Gondwana (Argentina, 26,29°S)JOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2003F. Lucassen Abstract Concordant U,Pb ages of c. 530,510 Ma and c. 470,420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high- T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26,29°S). The U,Pb ages document the time of titanite formation at high- T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were > c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U,Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U,Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U,Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation-enhanced recrystallization of titanite and/or different thermal peaks during a long-standing, geographically fixed, high- T regime in the mid-crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region. [source] Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central AustraliaJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2001I. S. Buick Abstract Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800,875 °C and >9,10 kbar) and subsequent decompression in a kilometre-scale, E-W striking zone of noncoaxial, high-grade (c. 700,735 °C and 5.8,6.4 kbar) deformation. However, new SHRIMP U-Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta-volcaniclastics) yield 206Pb/238U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield 206Pb/238U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400,300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475,460 Ma & 400,300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300,1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed. [source] Metamorphic diogenite Grosvenor Mountains 95555: Mineral chemistry of orthopyroxene and spinel and comparisons to the diogenite suiteMETEORITICS & PLANETARY SCIENCE, Issue 4 2000J. J. PAPIKE It does not show the usual brecciated appearance of other diogenites or wide compositional variability of orthopyroxene or spinel. Electron microprobe analysis (EMPA) and secondary ion mass spectrometer (SIMS) analysis of orthopyroxene and EMPA of spinel show limited compositional variability. Compositions for orthopyroxene for Fe/(Fe + Mg) atomic, Al, Zr, Y, and Yb fall in the middle of the compositional ranges of the diogenite suite. Apparently, GRO 95555 formed at sufficient depth or location to escape brecciation by meteorite bombardment and in a thermal regime that led to high-grade metamorphism resulting in homogeneous mineral compositions. [source] Paleoproterozoic, High-Metamorphic, Metasedimentary Units of Siberian CratonACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 5 2009Lena URMANTSEVA Abstract: Sensitive, high-resolution ion microprobe zircon U,Pb ages of Paleoproterozoic, high-grade, metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet,biotite, hypersthene,biotite, and cordierite-bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4,3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show core-rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ,2.7, ,2.3, and 1.95,2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton. [source] | ||||||||||