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991.
Raphael A. J. Wüst Colin R. Ward R. Marc Bustin Michelle I. Hawke 《International Journal of Coal Geology》2002,49(4)
Since the Carboniferous, tropical latitudes have been the site of formation of many economic coal deposits, most of which have a restricted range of mineralogical composition as a result of their depositional environment, climatic conditions, and diagenesis. Mineralogical and microscopic investigations of tropical peats from Tasek Bera, Peninsular Malaysia, were performed in order to better understand some of these factors controlling the nature, distribution and association of inorganic matter in peat-forming environments. Distribution and nature of the inorganic fraction of peat deposits give insight into the weathering conditions and detrital input into the mire system. Because the inorganic composition of peat deposits is determined by plant communities, height of water table, and climate, the results of the quantitative and qualitative analysis can be used to reconstruct palaeoclimatic conditions.Tasek Bera is a peat-accumulating basin in humid tropical Malaysia with organic deposits of low- to high-ash yield and thus representative of many ancient peat-forming environments. Clay minerals dominate the mineralogical composition of the peat and organic-rich sediments, while quartz and clays dominate the underlying siliciclastic deposits. Kaolinite is the most abundant clay mineral in the organic deposits with minor amounts of illite and vermiculite. Particle size analyses indicate that >50% of the inorganic detrital fraction is <2 μm. Most detrital quartz grains range in size from fine silt to fine sand. The fine sand fraction accounts for a maximum of 5 wt.% of the inorganic constituents. In addition, abundant biogenic and non-biogenic, Al- and Si-rich amorphous matter occur. In the ombrotrophic (low-nutrient) environment, biogenic inorganic material contributes up to >75% of the ash constituents. As a consequence, the vegetational communities make an important contribution to the inorganic and overall ash composition of peats and coals. The ash content of the often inundated peat consists on average of 10% opaline silica from diatoms and sponge spicules, while the ash of the top deposits may have up to 50% biogenic silica. Hence, Al- and Si-hydroxides and the opaline silica from diatoms and sponges represent a large repository of Al and Si, which may form the basis of mineral transformation, neoformation and alteration processes during coalification of the peat deposits. As a result, most coal deposits from paleotropical environments are anticipated to have little to no biogenic inorganic material but high amounts of secondary clays, such as kaolinite (detrital kaolinite, resilisified kaolinite, or desilisified gibbsite) or illite, and various amounts of detrital and authigenetic quartz. 相似文献
992.
Marc Constantin 《Contributions to Mineralogy and Petrology》1999,136(1-2):111-130
Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight
on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral
and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other
EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile
elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature
of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading
ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic
processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite
massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic
wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been
injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that
crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition.
Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession
of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location.
In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have
flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate
gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic
activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in
peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific
Rise.
Received: 13 October 1997 / Accepted: 5 February 1999 相似文献
993.
Zaineb?HajjarEmail author Amina?Wafik Marc?Constantin Mohamed?Bhilisse 《Arabian Journal of Geosciences》2016,9(6):471
The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile. 相似文献
994.
Martin Simon Pavel Pitra Philippe Yamato Marc Poujol 《Journal of Metamorphic Geology》2023,41(1):181-203
In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh-pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well-preserved phengite-bearing eclogite sample from the Nordfjord region. The sample was investigated using phase-equilibrium modelling, trace-element analyses of garnet, trace- and major-element thermobarometry and quartz-in-garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near-isobaric heating to 660–680°C. Near-isothermal decompression to 10–14 kbar is recorded in fine-grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end-member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny. 相似文献