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11.
The name Mérida Glaciation is proposed to designate the alpine glaciation which affected the central Veneruelan Andes; during the Late Pleistrocenc. Two main morainie levels have been recognired: one between 2600 and 2700 m, and another between 3000 and 3500 m elevation. The snow line during the last glacial advance was lowered approximately 1200 m below the present snow line (3700 m). Rodiocarbon dating indicates that the moraines are older than 10,000 years B.P., and probahly older than 13,000 years B.P. The lower morainie level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin). 相似文献
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CASTRO ANTONIO; CORRETGE L. GUILLERMO; DE LA ROSA JESUS D.; FERNANDEZ CARLOS; LOPEZ SUSANA; GARCIA-MORENO OLGA; CHACON HELENA 《Journal of Petrology》2003,44(7):1309-1344
The Sanabria appinitic rocks and host migmatites form an unusual,non-peri-batholithic complex in which all the typical membersof the appinite suite are present. It differs from most appiniticcomplexes in the deeper level of emplacement and the close temporaland spatial association with migmatites. Consequently, manyin situ relationships that resulted from the invasion of maficmagma into a crustal anatectic zone are extremely well preserved.The complex shows unequivocal relations between members of theappinitic suite and between these and migmatites derived byanatexis of a gneissic formation (Ollo de Sapo gneiss). Theserelations point to derivation of monzodiorites and biotite dioritesby hydrous basalt fractionation combined with fluid-assistedmelting of the crustal rocks surrounding the appinitic intrusions.This hydrous basic magma may be derived from an enriched regionof the mantle associated with subduction. Petrogenetic modelshave been tested using a combination of field relations andgeochemical data. Despite the complexity of the processes involved,it is concluded that water played an important role in the petrogenesisof the intermediate and mafic magmas. Reaction between monzodioritemelts and the host migmatites was responsible for the generationof a range of intermediate rocks within the complex. The needfor water to facilitate magma generation in both the mantleand the crust suggests that melting is linked with subduction.This interpretation has important implications because appiniticmagmatism may be considered as indicative of subduction processesinvolved not only in the generation of the mafic end-membersof the suite, but also in the generation of batholiths withwhich the appinitic rocks are spatially and temporally associated. KEY WORDS: appinite; monzodiorite; migmatite; Variscan orogen; Iberian massif 相似文献
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CASTRO ANTONIO; GUILLERMO CORRETGE L.; EL-BIAD MOHAMMED; EL-HMIDI HASSAN; FERNANDEZ CARLOS; PATINO DOUCE ALBERTO E. 《Journal of Petrology》2000,41(10):1471-1488
We have studied experimentally the melting relationships ofthe Ollo de Sapo gneiss (OSG), an important crustal protolithfor the Iberian leucogranites, of possible volcanoclastic origin.The results of this study are compared with previously determinedPTt paths, allowing us to interpret the mechanisms of meltingand granitoid production during the Hercynian orogenic cycle.Phase relationships determined in fluid-absent experiments indicatethat the OSG is a fertile source for peraluminous leucogranites.The slope of the fluid-absent solidus is strongly controlledby the breakdown of Ms in the presence of Qtz, Pl and Kfs. Thissolidus curve has a positive slope ranging from dP/dT = 30 bar/°Cat low P (<6 kbar) to dP/dT = 70 bar/°C at higher P (6–15kbar). The relationships between the Ms vapour-absent solidusand the PTt metamorphic paths in different sectors of the Iberianmassif have two important implications: (1) melt productivityis strongly favoured at low P; (2) anatexis in the Iberian massifprobably took place by decompression associated with crustalthinning and extension. These results are in agreement withthe relationships between granite production and tectonic deformationphases observed in the Iberian massif. Our results emphasizethat anatexis is a process that is strongly controlled bothby the phase relationships of the crustal protoliths and bythe thermal structure of the continental crust. Consequently,one must be careful when assigning potential crustal protolithsto particular granite associations exclusively on the basisof geochemical comparisons. KEY WORDS: anatexis; Hercynian orogen; Iberian massif 相似文献
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Significance of MORB-derived Amphibolites from the Aracena Metamorphic Belt, Southwest Spain 总被引：2，自引：0，他引：2
CASTRO ANTONIO; FERNANDEZ CARLOS; DE LA ROSA JESUS D.; MORENO-VENTAS IAKI; ROGERS GRAEME 《Journal of Petrology》1996,37(2):235-260
The Aracena metamorphic belt, in the southwest Iberian Massif,is characterized by the presence of MORB-derived amphibolitesand continental rocks deformed and metamorphosed during theHercynian orogeny. Geochemical relationships of these amphibolitesindicate the existence of a multiple fractionation process froma set of parental magmas, implying the existence of a multi-chambersystem beneath the ridge where the basalt protolith was extruded.Neodymium isotopic ratios are typical of MORB, and oxygen isotopesindicate that these amphibolites have been derived from theuppermost part of the oceanic crust. Thermal evolution, revealedfrom the study of chemical variations in the amphibole chemistry,is interpreted as resulting from subduction in a low-pressureregime in which the thermal structure of the continental hanging-wallplayed an important role. This continental wall was previouslyheated by subduction of a slab window resulting from migrationof a triple junction along the continental edge during plateconvergence. Three petrologic arguments support this tectonicmodel. These are: (1) the low-pressure inverted metamorphicgradient of amphibolites of the oceanic domain; (2) the high-temperature-low-pressure metamorphism of the continental hanging wall; (3)the early intrusion of boninites into the continental domain.
*Corresponding author. 相似文献
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LENOIR XAVIER; GARRIDO CARLOS J.; BODINIER JEAN-LOUIS; DAUTRIA JEAN-MARIE; GERVILLA FERNANDO 《Journal of Petrology》2001,42(1):141-158
Evidence for a major heating event accompanied by decompressionwas recently reported from crustal rocks drilled in the Alboranbasin. The metamorphic evolution recorded by these rocks impliescomplete removal of lithospheric mantle during the Cenozoic,a process that is confirmed by geophysical modelling indicatingthin lithosphere beneath the Alboran domain. In this region,the Ronda lherzolite massif (Betic Cordillera, southern Spain)provides a unique opportunity for the observation of mantleprocesses associated with lithospheric thinning. A strikingfeature of the Ronda peridotite is a narrow recrystallizationfront, which has been ascribed to kilometre-scale porous meltflow through the massif. The front separates the spinel tectonitedomain, interpreted as old, veined lithospheric mantle, fromthe granular domain where the lithospheric microstructures,mineralogical assemblages and geochemical signatures were obliteratedby grain growth coeval with pervasive infiltration of basalticmelts. On the basis of trace-element abundances in peridotitescollected over a distance of 12 km along the recrystallizationfront, our study confirms that the front is a relatively sharp( 相似文献
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Intervals of soft‐sediment deformation features, including vertical fluid escape and load structures, are common and well‐exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine‐grained material transported through the flame onto the palaeo‐seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment‐rich fluids. Flames are oriented parallel to the deep‐water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north‐west‐facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep‐water succession. The driving mechanisms that resulted in the final configuration of the soft‐sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground‐penetrating radar profiles confirm the overall north‐east orientation of the flame structures and provide a basis for recognition of potential larger‐scale examples of flames in seismic reflection data sets. 相似文献
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GARRIDO CARLOS J.; BODINIER JEAN-LOUIS; BURG JEAN-PIERRE; ZEILINGER GEROLD; HUSSAIN S. SHAHID; DAWOOD HAMID; CHAUDHRY M. NAWAZ; GERVILLA FERNANDO 《Journal of Petrology》2006,47(10):1873-1914
We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of LaN, YbN and(La/Sm)N with Eu* [Eu* = 2 x EuN/(SmN + GdN), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)Nwith Eu*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclase–garnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 25–30 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middle–upper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middle–upper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the ‘arc paradox’ of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth 相似文献