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1.
Petrogenetic grids are a powerful tool for understanding metamorphic terrains and many theoretical grids have been suggested for the process of granulite formation in metapelitic rocks, via fluid-absent biotite melting reactions. However, application of these grids has been difficult due to the lack of suitable experimental constraints. We present here an experimentally determined and tightly constrained petrogenetic grid for KFMASH system metapelites which extends from 840–1000°C and 5.0–12.5 kbar. Sixty four experiments on three KFMASH, mineral-mix, bulk compositions (X Mg=0.62, 0.74, 0.86) provide phase composition and assemblage data from which a grid can be derived and constrained. Reversal experiments and consideration of the phase composition data show the experiments to be close to equilibrium. The KFMASH univariant fluid-absent biotite melting reactions occur between 850 and 870°C at 5 kbar and between 900 and 915°C at 10 kbar. These reactions are connected to equilibria beyond the stability of biotite to develop a fixed framework within which the phase assemblage evolution of metapelitic rocks can be interpreted. The effect of minor components on phase equilibria is evaluated using the experimentally determined grid as a simple-system reference. The temperature at which melting occurs in metapelites is strongly controlled by the concentrations of titanium and fluorine in biotite. Pressure-temperature pseudosections presented for each of the experimental compositions show both the univariant and divariant reactions available to a particular bulk composition, clearly illustrating the possible evolution of the phase assemblage. The pseudosections also constrain the stability limits of  相似文献   

2.
In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als.The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C.We demonstrate that, even under fluid-absent conditions, a large proportion (40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites.The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.  相似文献   

3.
We conducted fluid-absent partial melting experiments, at 0.5 and 1.0?GPa in the temperature range 750 to 1000?°C, to investigate the influence of bulk rock Mg ? [100Mg/(Mg+Fe)] and the effects of additional TiO2 on the granulite-grade anatectic evolution of relatively magnesian metapelites and metagreywackes. In these experiments, melting began between 780 and 830?°C by the incongruent breakdown of biotite to produce quartz-saturated, granulite-facies residual mineral assemblages in equilibrium with H2O-undersaturated granitic melt. The glass (quenched melt) compositions produced in this study vary little. Generally, the glasses have compositions similar to those of many natural strongly peraluminous leucogranites. The solidus temperatures in both rock types increase with increasing Mg ?, but are unaffected by the presence or absence of a TiO2 component. At 0.5?GPa the metapelites melted at temperatures up to 50?°C lower than the equivalent metagreywackes, but at 1?GPa there was no discernible difference. This study suggests that the fluid-absent solidus has a steep positive dP/dT slope in metapelites and steep negative dP/dT slope in metagreywackes. The pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability. In TiO2-free compositions this was found to increase by 15 to 20?°C in the metapelites and by 30 to 40?°C in the metagreywackes, as a function of increasing Mg ? from 49 to 81. The presence of a TiO2 component increases the upper limit of biotite stability by ～50?°C in the metapelites and by ～80?°C in the metagreywackes, over that observed in the equivalent TiO2-free compositions. In consequence, in the TiO2-free samples large pulses of melt (up to 35 wt%) are produced over narrow temperature ranges (as little as 15?°C in these experiments) between 830 and 875?°C. In the TiO2-bearing samples the major pulse of melt production occurs more gradually between 830 and >900?°C.  相似文献   

4.
To understand the petrogenesis of peraluminous granites syntectonicto the Dorsal de Canguçu Transcurrent Shear Zone in theSul-rio-grandense Shield, Brazil, melting experiments were performedon one of the potential protoliths, a cordierite-bearing semi-peliticmetasedimentary gneiss (PE-1). Experiments were conducted atpressures of 5, 10 and 15 kbar, at temperatures of 700–900°C,and under fluid-absent and 5% H2O-present conditions. The experimentsshow that fluid-absent melting begins at near-solidus conditions,around 700°C, promoted by participation of retrogressivephengitic muscovite in the reaction Mus + Kf ± Qz = melt± Fe–Ti oxide ± Als, producing a very smallamount of melt (<9%) with widely ranging composition. Allhypersolidus experiments (>800°C) produced S-type graniticmelts promoted by participation of biotite or cordierite inthe reactions Bio + Pl + Crd + Qz = Px + Fe–Ti oxide +melt at 5 kbar, and Bio + Pl + Crd ± Qz = Grt + Als ±Kf + melt at 10 and 15 kbar, both producing a high amount ofmelt (10–63% by volume). The melt compositions obtainedat 900°C and 15 kbar under fluid-absent conditions, promotedby biotite or cordierite breakdown, are similar to the syntectonicgranites. However, it is unlikely that the granites were formedat this pressure (corresponding to a depth of melting of  相似文献   

5.
Experiments at 750 °C, 200 MPa(H2O), a (H2O)=1, and fO2∼Ni-NiO established that the equilibrium among tourmaline, biotite, cordierite, and melt (± spinel, aluminosilicate, or corundum) occurs with ∼2 wt% B2O3 in strongly peraluminous melt with an aluminosity, measured by the parameter ASI, of >1.2. The experiments demonstrate the relationship of tourmaline stability to the activity product of the tourmaline components boron and aluminum, which are inversely related to one another. Tourmaline is unstable in metaluminous to mildly peraluminous melts (ASI <1.2) at 750 °C regardless of their boron content. For a given aluminosity, addition of components such as F requires a greater boron content of melt at this equilibrium. The stability of tourmaline increases with decreasing temperatures below 750 °C. At the inception of melting, tourmaline breaks down incongruently to assemblages containing crystalline AFM silicates (biotite, cordierite, garnet, sillimanite), aluminates (spinel, corundum), and B-enriched but Fe-Mg-poor melt. Granitic melts are likely to be undersaturated in tourmaline from the start of their crystallization, and their initial boron contents will be limited by the abundance of tourmaline in their source rocks. Quartzofeldspathic (gneissic, metapelitic) rocks that reached conditions of the granulite facies and still contain (prograde) tourmaline are rare, and probably have never yielded a partial melt. Most leucogranitic magmas will initially crystallize biotite, cordierite, or garnet, but not tourmaline. With crystallization, the Fe-Mg content of melt decreases, and the B2O3 content increases until the tourmaline-biotite and/or tourmaline-cordierite (or garnet) equilibria are attained. The B2O3 content of melt is buffered as long as these equilibria continue to operate, but low initial Fe-Mg contents of the magmas limit the quantity of boron that can be consumed by these reactions to <1 wt% B2O3. Normally, leucogranitic magmas contain insufficient Fe and Mg to conserve all boron as tourmaline and thus lose a large fraction of magmatic boron to wallrocks. Leucogranites and pegmatites with tourmaline as an early and only AFM silicate mineral probably contained >2 wt% B2O3 in their bulk magmas. Received: 6 August 1996 / Accepted: 21 July 1997  相似文献   

6.
Partition coefficients (DBemineral/melt) for beryllium between hydrous granitic melt and alkali feldspars, plagioclase feldspars, quartz, dark mica, and white mica were determined by experiment at 200 MPa H2O as a function of temperature (650-900°C), activity of Be in melt (trace levels to beryl saturation), bulk composition, and thermal run direction. At trace levels, Be is compatible in plagioclase of An31 (1.84 at 700°C) and muscovite (1.35 at 700°C) but incompatible in biotite (0.39-0.54 from 650-800°C), alkali feldspar (0.38-0.19 from 680-850°C), quartz (0.24 at 800°C), and albite (0.10 at 750°C). The partition coefficients are different at saturation of the melt in beryl: lower in the case of plagioclase of An31 (0.89 at 700°C), muscovite (0.87 at 700°C), biotite (0.18-0.08 from 675-800°C), alkali feldspar (0.18-0.14 from 680-700°C), and quartz (0.17-0.08 from 750-800°C), but higher in the case of albite (0.37 at 750°C).With other data sources, these new partition coefficients were utilized to track, first, the distribution of Be between aluminous quartzofeldspathic source rocks and their anatectic melts, and second, the dispersion or concentration of Be in melt through igneous crystal fractionation of different magma types (e.g., S-type, I-type) up to beryl-saturated granitic pegmatites and, finally, into their hydrothermal aureoles. Among the rock-forming minerals, cordierite, calcic oligoclase, and muscovite (in this order) control the fate of Be because of the compatibility of Be in these phases. In general, beryl-bearing pegmatites can arise only after extended crystal fractionation of large magma batches (to F, fraction of melt remaining, ≤0.05); granitic magmas that originate from cordierite-bearing protoliths or that contain large modal quantities of calcic oligoclase will not achieve beryl saturation at any point in their evolution.  相似文献   

7.
Tourmaline-out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite-facies metapelites in the Yanai area, Ryoke metamorphic belt, SW Japan. The rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade, and the chemical zoning of tourmaline is controlled by X□AlNa–1Mg–1 and MgTiYAl–2 vectors in low- to medium-grade zones where muscovite is stable, whereas it is controlled by Mg(OH)YAl–1O–1, CaMgOX□–1 YAl–1(OH)–1 and MgTiYAl–2 vectors in further higher–grade, muscovite-unstable zones. The size of tourmaline increases drastically where breakdown of muscovite+quartz takes place, probably due to the growth of tourmaline during breakdown of muscovite. On the high-temperature side of the tourmaline-out isograd, depletion of whole-rock boron is observed. Escape of boron-bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion, although locally trapped, boron-bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism.  相似文献   

8.
The Sauwald and Mühl zones of the prebatholithic, Moldanubian, middle crust in northern Austria contain metapelites and metaluminous to weakly peraluminous metagreywackes, respectively. Both zones were affected by low-pressure, high-temperature metamorphism and anatexis. The metapelites of the Sauwald zone became in-situ diatexites, probably by fluid-absent reactions involving the breakdown of muscovite and the partial breakdown of biotite. The biotite-plagioclase-quartz gneisses of the Mühl zone experienced only slight melting. Following this event, and while the mid crust was still hot, additional heat was locally advected into the Mühl zone by the intrusion of the Weinsberg granite. This brought about fluid-present partial melting of the biotite-plagioclasequartz gneisses, producing relatively large volumes of metaluminous to weakly peraluminous, I-type Schlieren granite. This cool, wet, restite-rich magma remained close to its site of generation. Thus, infracrustal I-type granitoids may be formed anywhere in the crust, and not always at high T. Under special circumstances the heat and fluids from granitic magmas can spawn secondary granites. Also, relatively low initial 87Sr/86Sr values (of around 0.707) in I-type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle-derived magma. The Weinsberg granite magma came from the lower crust (P probably <700 MPa), where widespread fluid-absent breakdown of biotite-plagioclase-quartz assemblages occurred. The necessary high heat flow was probably provided by newly underplated mafic magmas. However, these seem not to have mixed or mingled with the crustally derived Weinsberg magmas. Deep equivalents of the Mühl-zone metagreywackes may have formed the Weinsberg protolith. Fluid-absent experiments show that the melting temperature probably exceeded 850°C and that a garnet-bearing, orthopyroxene-rich residue should be present in the lower crust. Fluid-present experiments demonstrate that the availability of free H2O can radically alter the characteristics of the partial melts, from apparent S-type mineralogy (with fluid-absent melting) to I-type mineralogy (with wet melting).  相似文献   

9.
Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and 87Sr/86Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in εNd-87Sr/86Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component.  相似文献   

10.
Fluid-absent melting experiments on a biotite (20 wt.%) andhornblende (2 wt.%) bearing tonalitic gneiss were conductedat 6 kbar (900–975C), 10 kbar (875–1075C), and14 kbar (950–975C) to study melt productivity from weaklyperaluminous quartzofeldspathic metamorphic rocks. At 6 kbar,biotite dehydration–melting is completed at 975C viaincongruent melting reactions that produce orthopyroxene, twooxides, and {small tilde}25 wt.% granitic melt. At 6 kbar, hornblendedisappears at 900C, probably in reaction with biotite. At 10kbar, biotite dehydration–melting produces <10 wt.%melt up to 950C via incongruent melting reactions that produceorthopyroxene, garnet, and granitic melt. Hornblende disappearsin the satne temperature interval either by resorption or byreaction with biotite. Widespread biotite dehydration–meltingoccurs between 950 and 975C and produces orthopyroxene, twooxides, and {small tilde}20 wt.% fluorine-rich (up to 0•31wt.%) granitic melt. At 14 kbar only a trace of melt is presentat 950C, and the amounts of hornblende and biotite are virtuallythe same as in the starting material. At 975C, hornblende isgone and {small tilde}10 wt.% granitic melt is produced by meltingof both biotite and hornblende. Our results show that hornblende-bearing assemblages cannotgo through dehydration–melting on their own (althoughthey can in combination with biotite) if the Ca content in thesource rock is too low to stabilize clinopyroxene. In such rocks,hornblende will either resorb or melt by reaction with biotite.Under fluid-absent conditions, intrusion of hot, mantle-derivedmagmas into the lower crust is necessary to initiate widespreaddehydration–melting in rocks with compositions similarto those discussed here. We argue that the high thermal stabilityof biotite in our starting material is caused mainly by theincorporation of fluorine. The relatively high F content inbiotite in the starting material (0•47 wt.%) suggests thatthe rock has experienced dehydroxylation in its past. F enrichmentby a previous fluid-absent partial melting event is excludedbecause of the lack of phases such as orthopyroxene and garnetwhich would have been produced. Our experiments show that thedehydration–melting of such F-enriched biotite producesF-rich granitic liquids, with compositions within the rangeof A-types granites, and leaves behind a granulitic residuedominated by orthopyroxene, quartz, and plagioclase. This studytherefore supports the notion that A-type granites can be generatedby H2O-undersaturated melting of rocks of tonalitic composition(Creaser et al., 1991), but does not require that these sourcerocks should be residual after a previous melting event.  相似文献   

11.
Degree of partial melting of pelitic migmatites from the Aoyama area, Ryoke metamorphic belt, SW Japan is determined utilizing whole-rock trace element compositions. The key samples used in this study were taken from the migmatite front of this area and have interboudin partitions filled with tourmaline-bearing leucosome. These samples are almost perfectly separated into leucosome (melt) and surrounding matrix (solid). This textural feature enables an estimate of the melting degree by a simple mass-balance calculation, giving the result of 5–11 wt.% of partial melting. Similar calculations applied to the migmatite samples, which assume average migmatite compositions to be the residue solid fraction, give degree of melt extraction of 12–14 wt.% from the migmatite zone. The similarity of the estimated melting degree of 5–11 wt.% with that in other tourmaline–leucogranites, such as Harney Peak leucogranite and Himalayan leucogranites, in spite of differences in formation process implies that the production of tourmaline leucogranites is limited to low degrees of partial melting around 10 wt.%, probably controlled by the breakdown of sink minerals for boron such as muscovite and tourmaline at a relatively early stage of partial melting. Because the amount of boron originally available in the pelitic source rock is limited (on average 100 ppm), 10 wt.% of melting locally requires almost complete breakdown of boron sink mineral(s) in the source rock, in order to provide sufficient boron into the melt to saturate it in tourmaline. This, in turn, means that boron-depleted metapelite regions are important candidates for the source regions of tourmaline leucogranites.  相似文献   

12.
Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.  相似文献   

13.
Effects of H2O on the Disequilibrium Breakdown of Muscovite+Quartz   总被引：1，自引：0，他引：1  
We have examined in detail the effect of H2O on the texturesand mechanisms produced during the breakdown of muscovite+quartzunder experimental disequilibrium conditions using low-porosityrock samples. Under H2O conditions (1 wt.% H2O added) muscovitereacts completely in <5 months at 757?C to a metastable assemblageof peraluminous meltmullite+biotite, a reaction which delaysthe formation of the stable equilibrium assemblage. In contrast,muscovite in the H2O undersaturated experiments breaks downby both the stable dehydration reaction producing K feldspar+sillimanite+biotiteand by metastable melting reactions in the same sample. Thecom position of the metastable melt is controlled by a numberof kinetic factors which change as a function of time and reactionprogress. Early melts are highly siliceous and sodic due tothe rapid dissolution of quartz relative to muscovite, coupledwith the incongruent melting of the paragonite component ofmuscovite and crystallization of biotite. The delayed nucleationof mullite results in Al supersaturation of the melt, whichinhibits the rate of the melting reaction. Once mullite nucleatesthe degree of Al melt supersaturation is decreased and the rateof muscovite dissolution increases. After complete reactionof muscovite the melt chemistry continues to change as Si diffusesinto the pseudomorphs from adjacent quartz. Even after 5 monthsat 757?C large compositional gradients in Si and Al still persistwithin the melt. In the H2O experiments metastable melting occursinitially, catalysed by traces of grain boundary fluid or byfluid released from the dehydration of muscovite. However, oncemelting starts any fluid is strongly fractionated into the meltphase, reducing µH2O Under these conditions further meltingis inhibited and the stable dehydration reaction will continue.Examination of examples of natural muscovite reacted under disequilibriumconditions in xenolithic and contact metamorphic rocks at lowpressures suggests that metastable melting is an important processunder certain geological conditions. It is possible that itis widespread in contact metamorphic rocks, but the texturesindicative of metastable melting reactions are obscured duringthe extended cooling histories of such rocks.  相似文献   

14.
 Major and trace element models of recently published vapour-absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting, and to distinguish between plagioclase-limited and biotite-limited, biotite dehydration melting. In the case of granitoids from the Pan-African Damara mobile belt (Namibia), many of the leucogranites and Salem-type granitoids may be modelled by biotite dehydration melting. The low Rb/Sr granitoids (e.g. Donkerhuk Onanis, Salem Onanis, Donkerhuk Nomatsaus, Salem Goas) probably reflect feldspar-limited, biotite dehydration melting (a pelitic source) whereas the high Rb/Sr suites (e.g. Bloedkoppie leucogranite, Stinkbank leucogranite, Salem Swakopmund, Leucocratic Stink bank granite) reflect biotite-limited, biotite dehydration melting (a greywacke source). Alaskites from the Damara belt have major element compositions which are consistent with muscovite dehydration melting, and their positive Eu anomalies are linked to high K2O reflecting K-feldspar entrainment. Combined Zr and LREE (light rare earth element) solubility models indicate that insufficient time (probably less than 104 years) had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of Zr and the LREEs. In contrast, the leucogranites and Salem-type granites have attained their equilibrium inventories of these trace elements. Combined Fe2O3 and MgO contents in some samples from two granitoids (the Salem Goas and Donkerhuk Onanis intrusions) are higher than those readily attainable by biotite dehydration melting indicating either: (1) that they contain a contribution from melts generated by incipient garnet breakdown or; (2) that they contain small amounts of an entrained ferromagnesian phase. Received: 24 April 1995/Accepted: 11 December 1995  相似文献   

15.
Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean 39Ar-40Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar 39Ar-40Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates.  相似文献   

16.
Tourmaline bearing leucogranite occurs as a pluton with pegmatitic veins intruding the Archaean granodiorite in the Bastipadu area, Kurnool district of Andhra Pradesh. We present field and petrographic relations, mineral chemistry and geochemical data for the leucogranite. It is essentially a two-mica granite, composed of quartz, perthite, microcline, albite, tourmaline and muscovite along with minor biotite and titanite. The euhedral tourmalines are regularly distributed in the rock. The geochemical studies show that the leucogranite is calc-alkaline, peraluminous to metaluminous which formed in a syn-collisional to volcanic arc-related setting. It displays strong ‘S’ type signatures with high K/Na ratios, moderately fractionated light rare earth elements, relatively flat heavy rare earth elements with \(\hbox {[Ce/Yb]}_\mathrm{N} \le 27.8\) and a strong negative Eu anomaly. The geochemical characteristics indicate that the leucogranite melt might have been generated from partial melting of metasediments. Electron probe microanalyser data show the presence of alkali group tourmaline in leucogranite represented by schorl and dravite. Tourmaline compositions plot in the Li-poor granitoids and associated pegmatites and aplites and metapelites/metasammites fields. Partial melting of boron-enriched source rocks is linked with the development of tourmalines in the leucogranite.  相似文献   

17.
Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration‐melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite‐biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid‐absent solidus. For the metapelite, temperatures ranged between 650 and 800 °C at Pc=700 MPa to investigate the muscovite‐dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 °C at Pc=1000 MPa to explore biotite dehydration‐melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (ε.) 1.37×10?5 s?1. In the presence of deformation, the positive ΔV and associated high dilational strain of the muscovite dehydration‐melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration‐melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt‐enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite‐dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite‐dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite‐derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.  相似文献   

18.
The Pemali tin deposit is located in a Triassic granite pluton the magmatic evolution of which is characterized by a decrease of compatible Ca, Mg, Ti, P and Zr in the sequence: medium- to coarse-grained biotite granite, megacrystic medium-grained biotite granite, two-mica granite/muscovite granite. The tin mineralization is confined to the two-mica granite and consists of disseminated cassiterite as well as greisen-bordered veins. The highly evolved muscovite granite is tin-barren and is distinguished from the two-mica granite by its low mica content and low loss-on-ignition values. The fluid inclusions in quartz and fluorite of the two-mica granite and of the greisen homogenize in the 115–410 °C temperature range; the salinities are in the range of 0.4–23 equiv wt% NaCl and the CO2 concentrations are < 2 mole%.  相似文献   

19.
Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO2-N2 fluids (5–30 mol. % N2) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.  相似文献   

20.
Lead contents of S-type granites and their petrogenetic significance   总被引：2，自引：0，他引：2  
An evaluation of Pb and Ba contents in S-type granites can provide important information on the processes of crustal partial melting. Primary low-T S-type granites, which form mainly by fluid-absent muscovite melting, may acquire a significant enrichment in Pb when compared to higher-T S-type granites for a given Ba content. We consider the following factors are responsible for this enrichment: Muscovite is a major carrier of Pb in amphibolite facies metapelites, and thus large quantities of Pb can be liberated upon its breakdown. The typical restite assemblage of Qz?+?Bt?+?Sil?±?Pl?±?Grt?±?Kfsp that forms during low-T, fluid-absent muscovite melting can take up only minor amounts of this Pb. This is because the crystal/melt Pb distribution coefficients for these restite minerals are low to very low. Only K-feldspar is moderately compatible for Pb, with a crystal/melt distribution coefficient of ~3, but its modal content in restites is usually low. At the same time, the restite assemblage will retain much Ba owing to the very high Ba uptake in both biotite and K-feldspar, which is an order of magnitude higher than for Pb. Thus, during a low-T anatectic event involving a low degree of crustal melting, Pb (as an incompatible element) can become strongly enriched in the partial melt relative to Ba and also relative to source rock values. In the case of higher-T anatexis and larger partial melt amounts, the Pb becomes less enriched and the Ba less depleted or even enriched relative to source rock values. During fractional crystallization of a S-type granite magma, Ba behaves strongly compatibly and Pb weakly compatibly. The concentrations of both elements decrease along the liquid line of decent. Owing to this sympathetic fractionation behavior, the primary, source-related Pb–Ba fingerprint (with weak or strong Pb enrichment) remains in evolved S-type granites. This facilitates a distinction between primary low-T S-type granites, which are related to muscovite melting, and secondary low-T S-type granites that evolve through fractional crystallization from a higher-T parental magma. We show in this paper that a simple logarithmic Pb versus Ba diagram can be a valuable aid for interpreting the petrogenesis of S-type granite suites.  相似文献   

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