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1.
Two groups of granitoids associated with gold mineralization in the Appalachian orogen of southwestern New Brunswick are recognized: a Late Silurian to Early Devonian (423–396 Ma) granodioritic to monzogranitic series (GMS), and a Late Devonian (370–360 Ma) granitic series (GS). The GMS granitoids are relatively low in silica, calc-alkaline, metaluminous to weakly peraluminous, and show characteristics of normal (oxidized) to reduced I-type granites depending on the properties of country rocks. They may have been derived from partial melting of lower crustal rocks triggered by underplated basaltic magmas; and country rocks bearing reduced organic carbon and/or graphite may have played an important role in the reduction of normal I-type intrusions to reduced I-type, which is essential in the formation of intrusion-related gold systems. In contrast, the GS granites, although calc-alkaline and metaluminous to peraluminous, are relatively rich in silica, incompatible elements, and high field strength elements. They are fractionated I-type granites, and are probably related to the coeval Mount Douglas granite in the Saint George batholith through fractional crystallization. Their parental magmas may have been derived from partial melting of quartzofeldspathic sources at relatively low temperatures. Both GMS and GS intrusions are orogenic, although some of them display the affinity of those emplaced into a within-plate environment. The origin of intrusion-related gold systems in this region appears to be controlled by several factors, including magma sources, magmatic processes, redox conditions (country-rock nature), and local structural regimes.  相似文献   

2.
We have simulated the dehydration-melting of a natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) in piston-cylinder experiments at 10 kbar and 750–1000°C, for 1–9 days. The solidus temperature is lower than 750°C; garnet appears at 850°C. The overall reaction is: Hb+PL+Cpx+Al-Hb+Ca-Hb+Ga+Opx. Three stages of reaction are: (1) melting dominated by the growth of clinopyroxene and garnet, with little change in composition of liquid or garnet, (2) a reversal of this reaction between 875°C and 900°C, with decreases in the amounts of liquid and garnet, and (3) a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase. Garnet is enriched in pyrope and zoned from Fe-cores to Mg-edges (range 3 mol % pyrope); liquid composition is enriched first in An (to 950°C) and then in Ab. The liquids are more calcic and aluminous than natural tonalites, which is attributed to the plagioclase composition (An90). The formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite — not H2O-saturated conditions. The results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet-absent field (900–1000°C), but with a solidus backbend at 7–9 kbar, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation in a closed system must make H2O available for H2O-undersaturated melting right down to the H2O-saturated solidus, below 700°C, which defines a large low-temperature PT area where hydrous granitoid melts can be generated with residual garnet and hornblende.  相似文献   

3.
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   

4.
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   

5.
Spectacular exposures of granulite-migmatite occur in the Chilka Lake area of the Eastern Ghats belt. The garnetiferous granite gneiss of peraluminous granitic composition, often contains restitic metapelite inclusions and is demonstrably a product of biotite-dehydration melting in pelitic rocks. On the other hand, older layers and bands of charnockitic rocks frequently occur as dismembered patches within the peraluminous granite, thus imparting a measled appearance of the granite exposures. The partial melting and emplacement of the peraluminous granite represent the Grenvillian thermal event, as evidenced by Rb-Sr whole rock and Pb-Pb zircon dating. On the other hand, minor patches of charnockite represent migmatized relict, as evidenced by some older zircons, in addition to those of Grenvillian age.  相似文献   

6.
Atsushi Kamei   《Gondwana Research》2002,5(4):813-822
Cretaceous granitic rocks are widely distributed in the northern to central parts of Kyushu Island, southwest Japan arc, and are divided into two types, granite and tonalite-granodiorite. These rocks are clearly defined by their mineral assemblage, that is, the granite contains mostly muscovite and/or garnet and does not have hornblende, whereas the tonalite-granodiorite is always observed with hornblende. Many workers suggest that most of the granite has low initial Sr isotopic ratios (SrI90 Ma: 0.7052-0.7059) whose values are similar to those of tonalite-granodiorite (SrI90 Ma: 0.7050-0.7056).

The SiO2 contents of the granite (67.9 to 78.0 wt.%) are higher than that of tonalite-granodiorite (54.2 to 68.2 wt.%). The granite is characterized by peraluminous compositions, while the tonalite-granodiorite shows metaluminous to peraluminous compositions. The petrogenetic relation between granite and tonalite-granodiorite is examined by a modal batch melting model, which referred to results of already published experiments. The result of the model suggests that the petrogenesis of the peraluminous granitic magma can be explained by an anatexis of metaluminous tonalite in the tonalite-granodiorite under lower to middle crustal conditions and variable fH2O.

Rocks of tonalite to granodiorite in a subduction system generally have low initial Sr isotopic ratios (<0.705), which imply that the magma derived from a highly metaluminous origin such as mafic magma, mafic lower crust and/or subducting slab. In this study, it is regarded that the peraluminous granitic magma can be directly produced by an anatexis of metaluminous tonalite. Therefore, peraluminous granite with low initial Sr isotopic ratios may imply to a first step of recycling of the granitic layer in an active plate margin.  相似文献   


7.
The stability and partial melting of synthetic pargasite in the presence of enstatitic orthopyroxene (opx), forsterite, diopsidic clinopyroxene (cpx), plagioclase (An50), and water has been studied in the range of 0.4–6.0 kb and 750–1000°C in the system Na2O-CaO-MgO-Al2O3-SiO2-H2O with a fixed bulk composition of pargasite+5 opx. The addition of orthopyroxene effectively reduces the stability field of pargasite by approximately 200°C at 1 kb. The invariant point involving pargasite coexisting with water-saturated liquid and anhydrous phase shifts from about 0.85 kb and 1025°C to 2.5±0.5 kb and 925±25°C with the addition of opx. Based on the solidus mineral assemblage and direct chemical analysis of quenched glass, the vapor-saturated liquid has a composition close to that of intermediate plagioclase. A layered silicate, interpreted to be Na-phlogopite, has an upper-thermal stability that nearly equals that of pargasite in the field of partial melting and coexists with liquid, pargasite, cpx, and forsterite at 6 kb, 1000°C. These results support the hypothesis that mantle metasomatism could involve formation of pargasitic amphibole from a silicate melt at depths as shallow as 8–10 km.  相似文献   

8.
Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt (<5–25%) by 925°C, but undergo extensive (30–50%) melting between 950 and 1000°C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900°C. It is as fertile a melt source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000°C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000°C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba, Eu, K/Ba, and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (<500–800 ppm), have high K/Ba and low Ba/Rb (est.) (62–124 and 1–3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting, are Ba-rich [2000–2600 ppm, K/Ba=16–22; Ba/Rb (est.) =25–47]. Melts of KH-8 have variable Ba contents; <500 ppm Ba at low temperature but >900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,87Sr/86Sr=0.757) is refractory, while samples with far less radiogenic Sr (87Sr/86Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having87Sr/86Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite.  相似文献   

9.
Melting experiments on a mantle-derived nodule assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300° C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250° C, representing 20–30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20–30% melting at 30 kbar have high K2O (3.07–5.05 wt.%), low SiO2 (35.0–39.2 wt.%), high K/K + Na (0.54–0.71), K + Na/Al (0.99–1.08) and Mg/ Mg + FeT of 0.59–0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline mafic magmas derived from a pyrolitic mantle source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.  相似文献   

10.
The southernmost Guyana Shield-Uatumã subdomain, northeastern Amazonas State, Brazil is dominantly formed by granitoid and volcanic rocks from the Água Branca Suite (ABS), undivided Granite Stocks (GS) and São Gabriel volcano–plutonic system (SGS). The ABS is characterized by a granite series that exhibits comparatively low Fe/(Fe + Mg) ratio, low (Nb/Zr)N, high Sr values and high Rb/Zr ratio. Its rocks display metaluminous to weakly peraluminous (A/CNK 0.94–1.06), high-K calc-alkaline, I normal-type character and have moderately to strongly fractionated rare earth elements (REE) pattern. The SG granites and SGS effusive–ignimbrite–granite association is metaluminous to weakly peraluminous (A/CNK 0.84–1.18), high-K calc-alkaline, has moderately to weakly fractionated REE trend, higher Fe/(Fe + Mg) ratio, lower Sr content and lower Rb/Zr ratio. The ABS geochemical signature is consistent with formation from volcanic arc rocks and small participation of collisional setting rocks, whereas the SG and SGS have post-collisional tectonic rocks-related geochemical signature. This model is in harmony with a post-collisional extensional regime, started with the 1.90–1.89 Ga Água Branca magmatism, and culminated with the 1.89–1.88 Ga São Gabriel system at an early stage of intracratonic reactivation, which included intrusion of mafic dikes. The Uatumã subdomain was related to mantle underplating with continental uplift and its origin involved contributions of 2.3–2.44 Ga Archean-contaminated Trans-Amazonian, 2.13–2.21 Ga Trans-Amazonian, 1.93–1.94/2.0 Ga Tapajós-Parima. Foliation styles point out that part of the Água Branca granitoids recorded later deformational effects, likely related to the Rio Negro Province formation.  相似文献   

11.
Peraluminous potassium-rich granitoids in the Semail Ophiolite   总被引：11，自引：0，他引：11  
Potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the Semail ophiolite, predominantly in the northern and less frequently in the central and southern part. New chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites, occurring as late intrusives in the gabboro and mantle sequence. Based on mineral chemistry, the P-T conditions of andalusite-cordierite bearing pegmatoidal granites were calculated, giving pressures of 1.8–2.2 kbar and a solidus within the stability field of andalusite. Hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675° C at 2 kbar. At 700° C, excess H2O, there is already 70–80% melt from the granitic starting compositions. Potassic granites in the ophiolite can be the result of extreme fractional crystallization in the late intrusive series. Granitic melts can also be produced by a two-stage process, in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series. Two-mica granites and pegmatites in the underlying metamorphic sole, possibly melting products of amphibolites and biotite gneisses, are contemporaneous. These escaping fluids could also explain the Pb and Sr isotopic compositions of the leucocratic dikes, indicating variable crustal contamination.  相似文献   

12.
Materials balance calculations are presented to document the progressive bulk chemical changes in metagreywacke and metapelite with separation of increments of granite of minimum melt composition. During partial melting, enrichment of Fe relative to Mg in granite and strong absorption of water leave residual rocks with increasing proportions of such Mg-rich phases as cordierite and progressively dehydrated. Extraction of most granitic compositions from metapelites results in increasing the Al value of residua, while extraction of an alkali granite melt from metagreywacke is necessary to change the Al values in their residua (from peraluminous to sub- or metaluminous). Under essentially constant temperature conditions, complex isograds may develop locally reflecting the bulk chemical changes produced by melt removal. Natural amphibolite/granulite facies rocks from three locations in Canada are compared with the calulated trends.  相似文献   

13.
The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al2O3–K2O–FeO–Fe2O3–P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3–O–P+5 and Al+3–O–P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K–O–Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.  相似文献   

14.
The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H2O-CO2vapour. Starting materials were glasses of intermediate composition(65 wt.% Si02 representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH2O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH2O, 725–750?Cat aH2O0?5, and 800–875?C at aH2O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH2O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H2O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH2O in the dacite. At aH2O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH2O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH2O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH2O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH2O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH2O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.  相似文献   

15.
Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<2 (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<2O/Na2O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Ybcn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Ybcn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.  相似文献   

16.
This study examines hydration–diffusion in the metaluminous haplogranite system at 200 MPa H2O and 800–300°C. At 800°C hydration is accompanied by melting and uphill diffusion of sodium from anhydrous glass toward the region of hydration and melting, whereas potassium diffuses away from the hydration front and into anhydrous glass. Silicon and aluminum are simply diluted upon hydration. There is no change in molecular Al/(Na + K) throughout the entire hydration-diffusion aureole and, therefore, (1) there is no loss of alkalis to the vapor, and (2) K migrates to replace Na in order to maintain local charge balance required by IVAl. Alkali diffusion occurs over a viscosity contrast from 104.1 Pa s in hydrous liquid to 1011.8–1013.5 Pa s in anhydrous glass. From these results, we interpret that: (1) Na is structurally or energetically favored over K as a charge-balancing cation for IVAl in hydrous granitic liquids, whereas the opposite behavior has been observed for anhydrous melts, and (2) the diffusion of alkalis through silicate melts is largely independent of viscosity. Results from 600°C are similar to those at 800°C, but hydration at 300°C involves a loss of Na and concomitant increase in molar Al/(Na + K) in the hydration zone due to hydrogen-alkali exchange between fluid and glass. Hydration behavior at 400°C is transitional between those at 300°C and 600°C, suggesting that the change in hydration mechanism occurs near the glass transition.  相似文献   

17.
The Cordilheira Suite is comprised of peraluminous granites that constitute, together with the Quitéria granite, the beginning of the formation of the Pelotas Batholith in the central portion of the Dom Feliciano Belt. The batholith is composed of seven granitic suites with minor occurrences of gabbro, diorite and subvolcanic rocks. Its evolution between 650 and 550 million years ago is characterised by a ranging from metaluminous to peraluminous rocks and from calc-alkaline to alkaline and peralkaline series. The Cordilheira Suite is composed of the Cordilheira, Arroio Francisquinho, Butiá and Três Figueiras granites, which contain muscovite and/or biotite, with the following accessory minerals: tourmaline, garnet, sillimanite, apatite, zircon, monazite and ilmenite. The granitic bodies are elongate; their ascent and emplacement were controlled by high-angle shear zones oriented at N45-70°E. They have mylonitic structures with magmatic foliation accompanied by a low-angle stretching lineation, indicating that these bodies were emplaced under syn-kinematic conditions during a transcurrent event. The granites have high-K calc-alkaline affinity and are peraluminous. The LILE and REE contents are low. The K2O/Na2O and CaO/Na2O ratios are approximately 1 and less than 0.3, respectively. Pseudosections calculated using the Perple X program suggests that the granites were formed at partial melting temperatures between 740 and 820 °C and pressure between 8.5 and 9 kbar. Petrographic and chemical data suggest that the magmas were generated by the partial melting of the migmatitic pelitic gneisses of the Várzea do Capivarita Complex and, to a lesser degree, orthogneisses of the Arroio dos Ratos Complex, which left a granulitic residue. UHT conditions of granulite facies metamorphism were recorded in the Várzea do Capivarita paragneisses by mineral paragenesis and indicate that temperatures between 900 and 1000 °C and pressures between 4 and 8 kbar were required for the partial melting. It is likely that these medium-pressure conditions resulted from the thickening of the continental crust when the Rio de La Plata Craton collided with the Kalahari Craton to form southwestern Gondwana at the end of the Neoproterozoic.  相似文献   

18.
Compressional wave velocities (VP) at above-solidus temperatures and at 1 GPa were obtained for a granite and amphibolite, which are considered to be major constituents of the continental crust. The temperature variation of velocities showed that the VP values of granite decreased with rising temperature, but substantially increased beyond the melting temperature (850–900 °C). Such an increase may be caused by the α–β transition of quartz. The velocities of amphibolite decreased linearly with increasing temperature and dropped sharply at temperatures above the solidus (700 °C), indicating that partial melting of amphibolite acts to significantly lower the seismic velocities.  相似文献   

19.
We have studied the controls on the Aluminum Saturation Index (ASI = molec. Al2O3/[(CaO)+(Na2O)+(K2O)]) and the concentration of normative corundum of granitic liquids saturated in alumina by equilibrating peraluminous minerals with initially metaluminous haplogranitic minimum composition liquids at 700–800 °C and 200 MPa, at, and below H2O saturation. The ASI and normative corundum increase with increasing H2O concentration in the melt (0.04 to 0.10 moles excess Al2O3 per mole of H2O), temperature, and with addition of the non-haplogranitic components Fe, Mg, and B. The ASI parameter and concentration of normative corundum cannot be used to monitor aAl2O3 between different mineral assemblages and melt because other components that affect the solubility of alumina, including H2O, Fe, Mg, and B, do not appear in their formulations. ASI and normative corundum, however, provide petrogenetic information about magmas generated by partial melting of strongly peraluminous protoliths by virtue of their regular and predictable variation with melt composition (e.g., H2O concentration) and temperature. For the application of these data to natural rocks it is necessary to choose as an analogue system the ASI-solubility or normative corundum-solubility relations of the most chemically complex peraluminous mineral present in the rock. Comparison of ASI values of anatectic leucosomes and allochthonous leucogranites with experimentally predicted values suggests low H2O concentrations in melt during crustal partial melting. Rapid melt segregation before equilibration with restitic peraluminous phases is also suggested in some cases.Editorial responsibility: I. Carmichael  相似文献   

20.
Major, trace element compositions and Sr–Nd isotopic characteristics of charnockitic gneisses from the Southern Granulite Terrain (SGT), South India are presented. The study region encompasses the central segment of the Cauvery Shear Zone system (CSZ) and regions within the Madurai Block (MB) immediately south of it (designated here as the CSZ/MB and MB domains). Differences in the compositions and source characteristics between charnockitic rocks of the CSZ vis-à-vis those of the CSZ/MB and MB regions are highlighted. Foremost, the charnockites and enderbites of the CSZ show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic εNd0 and initial-87Sr/86Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. By contrast, modeled at ca. 1.8 Ga and 0.8 Ga, the CSZ/MB and MB charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, present clear evidence for involvement of Archaean crustal components in sources of their magmatic protoliths. There is also evidence for significant intracrustal melting processes within a thickened crust at elevated temperatures between 800 and 1000 °C. Implications to the controversial Archaean–Neoproterozoic terrane boundary problem of the SGT are discussed.  相似文献   

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