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Fluid flow patterns and infiltration isograds in melilite marbles from the Bufa del Diente contact metamorphic aureole, north-east Mexico 总被引：2，自引：0，他引：2
In the inner aureole of the Bufa del Diente alkali syenite (north-east Mexico), thin calcareous argillite bands horizontally embedded in impure marbles acted as contact-metamorphic aquifers for hypersaline brines of magmatic origin. Thick-bedded marbles were largely impervious. From 180 m up to the intrusion contact, argillites were completely decarbonated, resulting in melilite + wollastonite + phlogopite + perovskite-bearing parageneses. In marbles, this assemblage is confined to a narrow 7-12-m-wide infiltration zone adjacent to the contact. Up to this distance, calcite + wollastonite + diopside + alkali feldspar + titanite was stable, indicating that the fluid evolution in these marbles was internally buffered. Brine infiltration from the metaargillite aquifer into the marbles occurred perpendicular to the marble-metaargillite boundaries and was confined to a zone 4-6 cm wide above the boundaries. This is documented by the three reactions Cc + Di = Mel + CO2, (1) Cc + Kfs + Di + H2O = Phl + Wo + CO2, (2) Cc + Ttn = Prv + Wo + CO2, (3) Melilites (Ak32-45Gh13-32Sm32-40 to Ak52-72Gh0-1Sm28-48) occur as rims around diopsides and become continuously thicker towards the metaargillite beds. Fluid inclusion observations suggest that the infiltrating brine was hypersaline (NaCl + KClcq～ 65 wt%) and that the reactions took place at the water-rich side of the H2O-CO2-salts immiscibility field at about 600d? C (2, 3) and 660 to 680d? C (1) at P～ 1200 bar and Xco2～ 0.02. Mass balance calculations show that the amount of brine infiltrated from the aquifer into the marble was very low and decreased continuously with increasing distance from the boundary. The maximum width of brine infiltration was about 6 cm. This confirms that brine flow was largely parallel to the aquifer, not perpendicular to it. The CO2 produced by the decarbonation reactions probably escaped as an immiscible low-density H2O-CO2 fluid of Xco2≤ 0.5 into overlying marble via grain-edge flow. The metaargillite-marble boundary acted as a semipermeable membrane 6 cm in thickness keeping back the brine in the aquifer and losing the in-situ produced low-density CO2-rich fluid. 相似文献
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Rare earth element (REE) and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the Variscan fold belt in NE Bavaria, Germany. The metapelites include a continuous prograde, mainly low-P (3–5 kbar) metamorphic profile from greenschist (c. 400 °C) to lower granulite facies conditions (c. 700 °C). The LREE (La–Sm) are incorporated preferentially in monoclinic monazite (REO9 polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO8 polyhedron). The major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La, Ce and Nd (La0.20–0.23, Ce0.41–0.45, Nd0.15–0.18)PO4, all other elements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 with some Dy and Gd solid solutions (Y0.76–0.80, Dy0.05–0.07, Gd0.04–0.06). In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3–5 from greenschist to granulite facies rocks. Monazite crystals often show zonation with cores low in HREE and rims high in HREE that is interpreted as growth zonation attained during prograde metamorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3–4 with increasing metamorphic grade. Prograde zonation in single crystals of xenotime was not observed. The XHREE+Y in monazite and XLREE in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 °C to 700 °C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U–Pb age determination. The combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature–time paths. 相似文献
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Deposition of a 2700-m-thick clastic platform succession in a N-S striking basin in northern Chile began in the Early Devonian during a global sea-level rise. A transition to terrestrial facies took place at the Early-Late Carboniferous boundary when the Gondwana glaciation began and global sea-level dropped. On the platform, interbedded cross-bedded or bioturbated sandstones, offshore tidal dunes and sand waves, and mudstones and tempestites suggest switching intertidal and shallow or deep subtidal environments. However, evidence for subaerial erosion indicates a significant regression during the Early Devonian. In an adjacent and deeper N-S striking sub-basin to the W, up to 3600 m of turbidites were deposited from the Late Devonian to the Late Carboniferous by mainly southerly palaeocurrents. Turbidites accumulated in coarse-grained proximal sand lobes in the N, and in fine-grained lobe fringe and basin plain environments in the S, with alternating upward-thinning and upward-thickening cycles typical of tectonically controlled aggradational turbidite systems. The sedimentological data indicate that the deeper basin depositional system evolved to a large extent independently from the platform system. Sediment in the deeper basin is less mature and more poorly sorted than that on the platform, suggesting that detritus bypassed the platform and was shed directly from the source areas into the western basin. The only depositional link between the platform and deeper basin systems seems to be longshore platform currents which may have funnelled minor quantities of mature sand into the deeper basin via bypass canyons. Although platform and deeper basin evolved in a common extensional tectonic setting, the platform reflects eustatic changes of sea-level whereas deposition in the deeper basin records syndepositional tectonics. 相似文献
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The Adula Nappe is a slice of Pre-Mesozoic continental basementaffected by Early Alpine (Mesozoic or Lower Tertiary) high-pressuremetamorphism. Mineral compositions in mafic rocks containingomphacite + garnet + quartz record a continuous regional trendof increasing recrystalliza tion temperatures and pressuresthat can be ascribed to this regional high-pressure metamorphicevent. P-T estimates derived from mineral compositions gradefrom about 12 kb and 500 ?C or less in the north of the nappeto more than 20 kb/800 ?C in the south. The regional P-T trend is associated with a mineralogical transitionfrom assemblages containing additional albite and abundant amphiboles,epidote minerals, and white micas in the north (omphacite-garnetamphibolites) to kyanite eclogites containing smaller amountsof hornblende and zoi.site in the south. Textures and mineralcompositional data show that these hydrous and anhydrous silicatesassociated with omphacite + garnet + quartz arc primary partsof the high-pressure assem blages. Observed phase relationsbetween these primary silicates, theoretical Schreinemakersanalysis, and the thermobarometric results, together indicatethat the regional transition from omphacite amphibolites tokyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H2O-conserving) (15) par+epi+hbl+qtz=omp+kya+H2O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (aH2O) as indicated by isofacialmineral assemblages, and the H2O character of the reaction (15),are interpreted to reflect largely H and predominantly fluid-absenthigh-pressure metamorphism within the northern part of the nappe.The omphacite amphibolites and paragonite eclogites in thisarea are thought to have formed by H2O reactions from Pre-Mesozoichigh-grade amphibolites, i.e. from protoliths of similar bulkH2O-countent. The second ‘isograd’ (12) is interpreted to markthe regional transition from largely fluid-absent metamorphismin the north to fluid-present metamorphism in the south, wheremetamorphic pressures and temperatures in excess of 12-15kband 500-600?C were sufficient for prograde in-situ dehydrationof similar hydrous protoliths to kyanite eclogites. The observationof abundant veins, filled with quartz+kyanite+omphacite, suggeststhat a free fluid coexisted locally with the kyanite eclogitesof the southern Adula Nappe at some time during progressivedehydration. 相似文献
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Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain‐size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea‐level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea‐level. Backflow re‐mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross‐bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low‐gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits. 相似文献
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