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1.
Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (NMg=38–42) + biotite (NMg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and afl H2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (NMg=45–48) + K-feldspar (Or70–80) + quartz + plagioclase (An28) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns: – a transition zone with the assemblage biotite (NMg= 49–51) + hornblende (NMg = 47–50) + plagioclase + quartz + K-feldspar + ilmenite + magnetite; – a KPQ (K-feldspar–plagioclase–quartz) zone with the assemblage K-feldspar + plagioclase + orthopyroxene (NMg=45–48) + quartz + ilmenite + magnetite; – a charnockite core with the assemblage K-feldspar + plagioclase + orthopyroxene (NMg = 39–41) + biotite (NMg=48–52) + quartz + ilmenite + magnetite. Systematic changes in the bulk chemistry and mineralogy across the four zones suggest that along with metasomatic transformation, this process may have been complicated by partial melting in the charnockite core. This melting would have been coeval with metasomatic processes on the periphery of the charnockite patch. There is also good evidence in the charnockitic core that a second mineral assemblage, consisting of orthopyroxene (NMg= 36–42) + biotite (NMg=50–51) + K-feldspar (Or70–80) + quartz + plagioclase (An28–26), could have crystallised from a partial melt during cooling from 720 to 660 °C at decreasing afl H2O from 0.67 to 0.5. Post-magmatic evolution of charnockite at T < 700 °C resulted in fluids being released during the crystallisation of the charnockitic core. These gave rise to the formation of late stage rim myrmekites along K-feldspar grain boundaries as well as late stage biotite, cummingtonite, and carbonates. Received: 15 September 1999 / Accepted: 8 June 2000  相似文献   

2.
The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH2O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.  相似文献   

3.
Arrested charnockite formation at Kottavattam, southern India   总被引：7，自引：0，他引：7  
Abstract At Kottavattam, southern Kerala (India), late Proterozoic homogeneous leptynitic garnet–biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse-grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation. Charnockitization post-dates the polyphase deformation, regional high-grade metamorphism and anatexis, and evidently occurred at a late stage of the Pan-African tectonothermal history. Geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar Plith–T conditions (650–700°C, 5–6 kbar) in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases (XCO2= 0.7–0.6; XH2O= 0.2–0.3; XN2= 0.1; log fO2= -17.5). Mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow, outward-migrating transition zones adjacent to the gneiss, followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone. Compared to the host gneiss, the charnockite shows higher concentrations of K, Na, Sr, Ba and Zn and lower concentrations of Mg, Fe, Ti, V, Y, Zr and the HREE, with a complementary pattern in the narrow transition zones of biotite breakdown. The Plith–T–XH2O data and chemical zonation patterns indicate charnockitization through subsolidus-dehydration reaction in an open system. Subsequent residence of the carbonic fluids in the charnockite resulted in low-grade alteration causing modification of the syn-charnockitic elemental distribution patterns and the properties of entrapped fluids. We favour an internally controlled process of arrested charnockitization in which, during near-isothermal uplift, the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from ‘fluid-absent’in the gneiss to ‘fluid-present’in the fracture zones and to the development of an initial fluid-pressure gradient, triggering the dehydration reaction.  相似文献   

4.
Isochemical conversion of garnet-biotite bearing paragneiss to charnockite in the Precambrian Khondalite belt of southern Kerala is described from Ponmudi area. Petrographic evidences indicate the formation of hypersthene by the breakdown of biotite in the presence of quartz following the reaction: Biotite + quartz → hypersthene + K-feldspar + vapour. The estimated pressure — temperature conditions of metamorphism are around 5–7 kbars and 750° ± 40°C. Presence of CO2-rich, mixed CO2-H2O and H2O-rich inclusions were noticed in gneiss as well as in charnockites. Charnockites contain abundant CO2-rich inclusions.  相似文献   

5.
Incipient charnockite formation at Kurunegala in Sri Lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss. Mineral equilibria in the charnockite assemblage record pressure-temperature (P-T) conditions of 738±60° C and 6.9±1.2 kbar at-17.0±1.2 log fO2 and aH2O=0.18±0.16. Wholerock trace-element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of Sm>Rb>Pb>U>Sr>Nd, with a fractionation of Rb/Sr, Sm/Nd and Th/U ratios, and crystallization of the charnockite assemblage at 535±5 Ma. Major element (Fe–Mg–Ca) and Sm–Nd equilibration between minerals occurred at 524±9 Ma, whereas, Pb and Rb–Sr underwent continued exchange to 501±5 Ma and 486±1 Ma, respectively. Trace-element data for both amphibolite and charnockite minerals show that depletion on a whole-rock scale can be accounted for either by changes in mineral modes or trace-element abundances, within the immediate area of dehydration. The fractionation of Sm/Nd on a whole-rock scale is controlled by the breakdown of amphibole, without the growth of a major new host-phase for Sm in the charnockite. Rubidium and Sr are dependent on the relative behaviour of biotite, plagioclase and alkali-feldspar. Modelling of dehydration-melting involving the breakdown of amphibole, biotite, and alkali-feldspar reproduces the observed Sm/Nd and Rb/Sr fractionation, and indicates the loss of small melt fractions, on a cm scale, from the charnockite. These observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation.  相似文献   

6.
Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.  相似文献   

7.
Abstract Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2 abundances determined from fluid inclusions by stepped-heating mass spectrometry. Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low-pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (for XH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of ?8% and from the gneiss, 2 m from the charnockite, by values of ?15%. The large CO2 abundances and relatively heavy carbon-isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon-isotope front through the rock more rapidly than the reaction front. The carbon-front/reaction-front separation at Kalanjur is significantly larger than the value determined from a graphite-bearing incipient charnockite nearby, consistent with the predictions of one-dimensional advection models. Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2 abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C of c.?5.5% in the gneiss to ?9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2-saturated partial melt in response to influx of an isotopically light carbonic fluid. Thus, incipient charnockites from the high-grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non-pervasive CO2 influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.  相似文献   

8.
The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.  相似文献   

9.
Arrested charnockite formation in southern India and Sri Lanka   总被引：7，自引：3，他引：7  
Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic gray gneiss produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of mixed-facies exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO2 and Na2O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar. Lower P(H2O) in the Ponmudi-type metamorphism was probably the definitive factor.CO2-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO2 from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO2-streaming hypothesis. CO2-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H2 leakage or some systematic contamination.Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO2 drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO2-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO2-streaming hypothesis for the Ponmudi-type localities.  相似文献   

10.
The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO2-rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO2-rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.  相似文献   

11.
M.A. Cosca  R. Caby  F. Bussy   《Tectonophysics》2005,402(1-4):93
In situ UV-laser ablation 40Ar/39Ar geochronological and geochemical data, together with rock and mineral compositional data, have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the UHP whiteschists of the Dora Maira Massif, Italy. Several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation. Whole rock XRF data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith. Minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss. The pseudotachylyte contains newly crystallized grains of biotite and K-feldspar in a matrix of glass with partially fused grains of quartz, zircon, apatite, and titanite. Electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer-scale changes in mineralogy. UV-laser ablation ICP-MS traverses across the mylonitic gneiss–pseudotachylyte contact are consistent with cataclastic communition of REE carriers such as epidote, monazite, allanite, zircon, and apatite before melting as an efficient mechanism of REE homogenization in the pseudotachylyte. The 40Ar/39Ar data from one band of pseudotachylyte indicate formation at 20.1 ± 0.5 Ma, when the mylonitic gneisses were already in a near surface position. The variable effects of top-to-the-west shear deformation within outcrops of the coesite-bearing unit are reflected in localized zones of protomylonite, cataclasite, ultracataclasite, and pseudotachylyte. Preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the UHP rocks. It is speculated that deeper crustal seismic events potentially played a role in the unroofing of the UHP rocks at earlier stages in their exhumation history.  相似文献   

12.
周文孝  葛梦春 《地球科学》2013,38(4):715-724
通过大比例尺填图和剖面研究, 将内蒙古锡林浩特地区的"锡林郭勒杂岩"解体分为3大部分: 一套表壳岩、晚元古代基性-超基性侵入岩和早古生代酸性侵入岩.在此基础上根据变质岩的岩性组合和变形变质特点, 可将其中的表壳岩化分为4个岩性段: 黑云(石榴石)斜长片麻岩, 间夹多层条纹状斜长角闪岩, 在其顶部多见薄层含磁铁石英岩; 中粗粒黑云斜长片麻岩和细粒长英质片麻岩; 混合岩化条带状黑云(角闪)斜长片麻岩, 含石榴石黑云二长片麻岩夹条纹状斜长角闪岩组合; 夕线石黑云斜长片麻岩, 条纹状黑云斜长片麻岩, 间夹含石榴石黑云母石英片岩.通过对比区域内宝音图群、艾勒格庙组、白乃庙群等古老地块的岩石组合, 发现在原岩建造、变质级别、沉积环境等特征上均有区别, 应将这套表壳岩单独厘定为锡林浩特岩群, 它们组成了锡林郭勒微陆块前寒武变质基底岩系, 其地质特征的研究对锡林郭勒微陆块基底的形成和演化及华北板块与西伯利亚板块的构造关系演化有重要意义.   相似文献   

13.
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   

14.
The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H2O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks.  相似文献   

15.
The occurrence of a charnockitised felsic gneiss adjacent to a marble/calc-silicate horizon at Nuliyam, southern India, has been cited in recent literature as a classic example of the dehydration of crustal rocks resulting from the advective infiltration of CO2-rich fluids generated from a local carbonate source. Petrographic study of the Nuliyam calc-silicate, however, reveals it to consist of abundant wollastonite and scapolite and contain locally discordant veins rich in wollastonite. At the pressure—temperature conditions proposed for charnockite formation in recent studies, 5 kbar and 725°C, this wollastonite-bearing mineral assemblage was stable in the presence of a fluid phase only if X CO2 was near 0.25 and could not have coexisted with the fluid causing biotite breakdown and charnockite development in adjacent rocks (X CO2>0.85). The stable coexistence of wollastonite and scapolite prohibits the calc-silicate from being a source for fluid driving charnockitisation at the required P-T conditions. Textural observations such as the limited replacement of wollastonite by calcite+quartz symplectites and mosaics, are consistent with late fluid infiltration into the calc-silicate. The extensive isotopic, chemical and mineral abundance data of Jackson and Santosh (1992) are re-interpreted and integrated with these observations to develop a model involving the infiltration of an externally derived CO2-rich fluid during high-temperature decompression. Increased charnockite development next to the calc-silicate has arisen because the calc-silicate acted as a relatively unreactive and impermeable barrier to fluid transport and caused fluid ponding beneath antiformal closures. The Nuliyam charnockite/calc-silicate locality is an example of a structural trap in a metamorphic setting rather than a site where charnockite formation can be attributed to local fluid sources.  相似文献   

16.
We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Opx?+?Ilm?+?Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs?+?Qtz?+?Pl?+?Bt?+?Grt?+?Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T?>?900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H2O (M(H2O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H2O)?=?0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H2O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.  相似文献   

17.
阿斯哈金矿位于东昆中隆起带东段,是东昆仑重要的金、铁多金属成矿带。金矿的容矿围岩为印支早期闪长岩和黑云母花岗岩,NNE向和NW向断裂为主要的容矿构造,Ⅰ号脉为该金矿主要的矿脉之一,云煌岩与金矿脉空间关系密切。流体包裹体主要有富CO2三相和气液两相2种类型。流体盐度（w(NaCl)）为1.83%~8.13%,流体密度为0.69~0.87 g/cm3,成矿温度为155.3~425.6 ℃。成矿Ⅰ阶段流体为低盐度、富CO2的高温流体;成矿Ⅱ阶段富CO2型和气液两相流体包裹体共存,发生了以CO2逸失为特征的不混溶或沸腾,致使残余流体盐度升高;成矿Ⅲ阶段为气液两相包裹体。激光拉曼光谱分析表明,流体气相成分主要有CO2、CH4、N2。结合氢、氧和硫同位素组成分析认为,成矿流体主要为幔源流体,晚期有大气水的加入。通过等容线图解法估算成矿压力为98~132 MPa,估算成矿深度为8.16~9.58 km。通过与典型造山型金矿特征对比,阿斯哈金矿为中成造山型金矿,矿床形成于早印支期陆内造山由挤压向伸展转换时期。  相似文献   

18.
Southern India and Sri-Lanka are the places where "incipient charnockites",i.e.the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing,igneous looking charnockites,have been discovered in the early sixties.The fact that some incipient charnockites occur along a network of brittle fractures,together with CO_2 remnants preserved in mineral inclusions,had called for the role of fluids during charnockite alteration.The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India.In addition to CO_2-rich fluid inclusions in quartz and feldspar,all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions.CO_2 inclusions are more abundant in paragneiss(Kerala)than in orthogneiss(Karnataka/Tamil Nadu).The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite,both of which also share the brine remnants.All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz,feldspar and orthopyroxene.These metasomatic veins again indicate the action of alkali-exchanging fluids(i.e.,saline solutions).Feldspar microveins,which have been found in most "massive" charnockites,along with the CO_2-rich fluid inclusions,suggest a commonality of incipient charnockite and massive charnockite,both types differing in intensity of interaction with metasomatizing pore fluids.  相似文献   

19.
云南哈播斑岩铜(-钼-金)矿床流体包裹体研究   总被引：3，自引：3，他引：0  
哈播斑岩Cu-(Mo-Au)矿床产于哀牢山富碱斑岩带的南段,形成于青藏高原后碰撞阶段构造转换环境,属于陆-陆碰撞型斑岩矿床.根据脉体的交切关系,确定哈播矿床各种脉的演化序列为早期石英脉→石英-黄铜矿脉→石英辉钼矿脉.脉中流体包裹体的岩相学、显微测温和激光拉曼光谱分析等研究结果显示,各期脉中均有富气相包裹体、富液相包裹体和含子矿物多相包裹体,各种包裹体的气相均含有CO2、SO2、H2O等气体.各期脉中多种包裹体并存并具有相似的均一温度范围,富液相包裹体均一温度149～427℃,盐度ω(NaCleq)6.0％～15.0％;富气相包裹体均一温度205～405℃,盐度ω(NaCleq) 3.4％～19.0％;含子矿物多相包裹体均一温度305～516℃,盐度w(NaCleq) 33.5％～61.0％.哈播矿床的初始成矿流体由稳定共存、不混溶的低盐度流体和高盐度流体组成,高盐度流体是哈播矿床成矿元素迁移的主要载体.成矿流体在400℃左右发生“二次沸腾”、分相,温度下降和挥发分持续逃逸可能是Cu-Au成矿的诱因.Mo元素在成矿流体多次沸腾、分相过程中,持续优先分配进入高盐度流体中而逐步富集;温度下降,使含钼硫化物在流体中溶解度降低、沉淀,形成石英-辉钼矿±黄铜矿脉.  相似文献   

20.
喜马拉雅造山带东端的南迦巴瓦岩群是高喜马拉雅结晶岩系的一部分,主要由麻粒岩相和角闪岩相变质的片麻岩、斜长角闪岩、片岩和钙硅酸盐岩组成.长英质片麻岩主要由斜长石、钾长石、石英、石榴石、黑云母和褐帘石组成.片麻岩中的锆石具有核一边结构,由一个大的继承岩浆核和一个窄的变质生长边组成.锆石岩浆核具同心韵律环带.其REE配分模式以HREE富集和负Eu异常为特征,并具有高的Th/U比值.锆石U-Pb年代分析表明,这种继承岩浆锆石给出的加权平均年龄为490～500Ma.地球化学特征表明,这些片麻岩的原岩是花岗岩和花岗闪长岩,形成在俯冲带的岩浆弧构造环境.钙硅酸盐岩中的锆石具有高级变质岩中变质生长锆石的典型特征,即具有相对较低的REE含量,不明显的负Eu异常和较低的Th/U比值.变质锆石所获得的U-Pb加权平均年龄为505Ma.本文和现有的研究结果表明,喜马拉雅造山带是一个复合造山带,它经历了古生代的原始造山作用,在新生代印度与欧亚板块的碰撞过程中发生了再造山作用.喜马拉的古生代造山带作用是原特提斯洋向冈瓦纳大陆北缘俯冲和亚洲微陆块(包括拉萨和羌塘地块)增生的结果,是在冈瓦纳大陆拼合之后其边缘发生的安底斯型造山作用,因此,它并不属于在冈瓦纳超大陆聚合过程中陆-陆碰撞形成的泛非造山带.  相似文献   

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