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Crustal Anatexis and Granite Petrogenesis During Low-pressure Regional Metamorphism: The Trois Seigneurs Massif, Pyrenees, France 总被引：2，自引：5，他引：2
Detailed mapping of Hercynian basement rocks exposed in theTrois Seigneurs Massif, Pyrenees, France, has demonstrated acontinuous metamorphic sequence developed in Palaeozoic peliticsediments, ranging from chlorite-grade phyllites, through andalusiteand sillimanite mica schists to a zone of migmatites and ultimatelya heterogeneous, peraluminous, biotite- and cordiente-bearinggranitoid (ranging in composition from biotite granite to quartzdiorite) at the deepest tectonic levels exposed. In additionto this ‘deep’ pluton, a syn-metamorphic leucogranitesuite forms pods and sills within the migmatites and mica schistsand a post-metamorphic, homogeneous biotite granodiorite intrudes(and superimposes a contact aureole on) the metasediments. Despitepost-metamorphic deformations, it is clear that the small ({smalltilde} 3 km) separation of low- and high-grade rocks impliesthe existence of very high temperature gradients (80–100?C km –1) during Hercynian metamorphism. Extensive meltingoccurred at {small tilde} 700 ?C at 10–12 km depth, indicatedby the metamorphic mineral assemblages and metamorphic reactionsoccurring in the mica schists. Whole rock XRF analyses of 50 rock samples, including all themain lithologies, indicate that leucogranite compositions areuniform and identical to those of migmatite leucosomes; theyare also close to the major-element composition of experimentallygenerated partial melts of pelitic rocks from the Trois SeigneursMassif. Taken with field relationships, this implies that allleucogranites were generated by partial fusion of pelitic material(< 40 wt. per cent) from the metamorphic sequence, with rapidremoval of the melt by segregation and intrusion to higher structurallevels. The deep biotite granite was probably generated by partialmelting and homogenisation of the same source material, withthe addition of a small magmatic component that was not derivedlocally from the pelites. The late granodiorite was not generatedby anatexis of pelitic material as observed in the metamorphicsequence, and was probably derived by melting of the lower crustat deeper levels than any contemporary exposure of Hercynianbasement in the Pyrenees. Petrological analysis of the metamorphic sequence suggests thatwater activity was externally buffered to high values throughoutthe ‘high-level’ anatexis observed in the TroisSeigneurs sequence. Evidence for this is provided by metacarbonateand metapelite mineral equilibria, by the sequence of metamorphicisograds and by their sharp definition. Moreover, ‘wet’melting conditions are required in order to generate the observedlarge quantities(> 40 wt. per cent) of granitic melt frompelitic material over the small (< 30 ?C) temperature increaseimplied by the section through the migmatite zone. Anatexisof pelitic metasediment was thus promoted by an influx of hydrousfluid into the melting zone. Stable-isotope studies suggestthat this influx was derived from the ground surface, allowingmelts to be continuously saturated as they were generated, andimplying that groundwater infiltration was primarily responsiblefor large-scale anatexis of metasediment at such shallow depths. 相似文献
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Petrographic and petrologic investigations of upper amphibolitefacies metacarbonates from the East Humboldt Range core complex,Nevada, provide important constraints on P-T-XCO2 conditionsand fluid flow during metamorphism. Three marble assemblagesare observed [(1)dol+cc+bt+cpx+q+ruscapkspamph; (2) cc+bt+cpx+plag+q+sphscapksp;(3) cc+cpx+plag+q+sphscapksp], all of which equilibrated withrelatively CO2-rich fluid compositions, at P-T conditions of6 kbar and 600–750C. The most recent equilibration eventis recorded in some calcsilicate gneisses where retrograde amphiboleand epidotegarnet replace clinopyroxene and plagioclase, respectively.This is attributed to infiltration of H2O-rich fluids at and/orafter peak metamorphic temperatures, which continued as therocks were cooled and rapidly uplifted after a Tertiary extension-relatedheating event. Likely sources for the retrograde fluids are the abundant pegmatiticleucogranites in the area. Volumetric fluid-rock ratios of 0.02–0.4are required to generate the retrograde assemblage, and observedleucogranite proportions are more than adequate to provide therequired volume of fluid. Estimates of retrograde fluid fluxesrange from 0.25 to 5102 cm3/cm2 for a transient temperaturegradient of 5C/m, to 3103 to 7104 cm3/cm2 for a temperaturegradient of 35C/km. These gradients are characteristic of askarn-type contact metamorphic environment and a regional crustalgeotherm, respectively. They imply different time-scales andlength-scales for the retrograde fluid flow system, with theformer more akin to a contact metamorphic setting with local,meter-scale retrograde fluid flow, and the latter to a regionalmetamorphic setting with regionally high mid- and lower-crustaltemperatures and fluid flow throughout a significant thicknessof the middle crust. Higher gradients are considered more likelygiven the proximity of leucogranites to retrogressed calc-silicategneisses, and the resultant relatively small fluxes are consistentwith a magmatic source. The length-scale of reaction within the retrograde fluid flowsystem was of the order of meters to hundreds of meters andinvolved both pervasive and (later) fracture-controlled down-temperatureflow. Retrograde fluid flow in this terrance, as well as othersdominated by magmatic volatiles, is in the form of multiplediscrete bursts of fluid released in a discontinuous mannerpotentially over long periods of time (1–10 Ma) with locallyvariable thermal gradients along the flow path. 相似文献
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Silicic Magma Formation in Overthickened Crust: Melting of Charnockite and Leucogranite at 15, 20 and 25 kbar 总被引：23，自引：0，他引：23
Two models of silicic magma formation have been experimentallytested: (1) generation of A-type granite magma by partial meltingof crustal source rocks at depths >50 km; (2) productionof syenite magma by partial melting of quartzofeldspathic rocksat pressures >15 kbar. Melting experiments at 15, 20 and25 kbar were performed on Archaean biotite-bearing charnockiteof opx-bearing granodiorite composition, and on leucogranite.Most experiments were conducted with oxalic acid (OA) to provide2 wt % H2O and 相似文献
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WICKHAM STEPHEN M.; ALBERTS AMY D.; ZANVILEVICH ADA N.; LITVINOVSKY BORIS A.; BINDEMAN ILYA N.; SCHAUBLE EDWIN A. 《Journal of Petrology》1996,37(5):1063-1095
A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral 18O/16O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme18O depletio (18O feldspar<–12). This reflects thevery low 18O of the water involved in these systems, which probablyhad 18O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal 18O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the 18O/16Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing 18O/16O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic 18O value of plutonic igneous rocks.Most sphene and quartz 18O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in 18O/16O, indicating that, on a regionalscale, the magmatic 18O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower 18O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes
1Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK. 相似文献
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