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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.
http://www.sciencedirect.com/science/article/pii/S1674987112000655   总被引：1，自引：1，他引：0  
Four different varieties of charnockitic rocks,with different modes of formation,from the Mesoproterozoic Natal belt are described and new C isotope data presented.Excellent coastal exposures in a number of quarries and river sections make this part of the Natal belt a good location for observing charnockitic field relationships.Whereas there has been much debate on genesis of charnockites and the use of the term charnockite.it is generally recognized that the stabilization of orthopyroxene relative to biotite in granitoid rocks is a function of low aH2O（±high CO2）,high temperature,and composition （especially Fe/（Fe +Mg））.From the Natal belt exposures,it is evident that syn-emplacement.magmatic crystallization of chamockite can arise from mantle-derived differentiated melts that are inherently hot and dry（as in the Oribi Gorge granites and Munster enderbite）,as well as from wet granitic melts that have been affected through interaction with dry country rock to produce localized charnockitic marginal facies in plutons（as in the Portobello Granite）.Two varieties of post-emplacement sub-solidus chamockites are also evident.These include charnockitic aureoles developed in leucocratic,biotite.garnet granite adjacent to cross-cutting enderbitic veins that are attributed to metamorphic-metasomatic processes（as in the Nicholson’s Point granite,a part of the Margate Granite Suite）,as well as nebulous,patchy charnockitic veins in the Margate Granite that are attributed to anatectic metamorphic processes under low-aHO fluid conditions during a metamorphic event.These varieties of chamockite show that the required physical conditions of their genesis can be achieved through a number of geological processes,providing some important implications for the classification of charnockites,and for the interpretation of charnockite genesis in areas where poor exposure obscures field relationships.  相似文献   

3.
Summary The low-pressure emplacement of a quartz diorite body in the metapelitic rocks of the Gennargentu Igneous Complex (Sardinia, Italy) produced a contact metamorphic aureole and resulted in migmatisation of part of the aureole through partial melting. The leucosome, formed by dehydration melting involving biotite, is characterised by granophyric intergrowth and abundant magnetite crystals. A large portion of the high temperature contact aureole shows petrographic features that are intermediate between quartz diorite and migmatite s.s. (i.e. hybrid rocks). A fluid inclusion study has been performed on quartz crystals from the quartz diorite and related contact aureole rocks, i.e. migmatite sensu stricto (s.s.) and hybrid rocks. Three types of fluid inclusions have been identified: I) monophase V inclusions, II) L + V, either L-rich or V-rich aqueous saline inclusions and III) multiphase V + L + S inclusions. Microthermometric data characterised the trapped fluid as a complex aqueous system varying from H2O–NaCl–CaCl2 in the quartz diorite to H2O–NaCl–CaCl2–FeCl2 in the migmatite and hybrid rocks. Fluid salinities range from high saline fluids (50 wt% NaCl eq.) to almost pure aqueous fluid. Liquid-vapour homogenisation temperatures range from 100 to over 400 °C with an average peak around 300 °C. Temperatures of melting of daughter minerals are between 300 and 500 °C. Highly saline liquid- and vapour-rich inclusions coexist with melt inclusions and have been interpreted as brine exsolved from the crystallising magma. Fluid inclusion data indicate the formation of fluid of high iron activity during the low-pressure partial melting and a fluid mixing process in the hybrid rocks.  相似文献   

4.
Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO2-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (?5 to ?20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO2 which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent.  相似文献   

5.
Experiments were conducted at 1 GPa on four starting materials to investigate the effects of variable mineral proportions on the melting systematics of compositionally fertile peridotitic assemblages. Starting materials were constructed by recombining Kilbourne Hole xenolith mineral separates by weight into four mixtures with mineral proportions olivine (Ol): orthopyroxene (Opx): clinopyroxene (Cpx): spinel (Sp) of 0.50:0.07:0.40:0.03 (FER-B), 0.50:0.46:0.01:0.03 (FER-C), 0.50:0.30:0.10:0.10 (FER-D), and 0.50:0.235:0.235: 0.03 (FER-E). Experiments were performed on a 1.27-cm (0.5 in.) piston-cylinder apparatus over the temperature interval 1270–1390 °C, using a variation of the diamond aggregate melt extraction technique employing vitreous carbon spheres in place of diamonds as the melt extraction layer. The solidus temperatures are similar for all the starting materials, with an average value of 1250 °C. In FER-D and -E, the near-solidus melting reaction for a lherzolite assemblage was determined to be of the form Cpx + Opx + Sp → melt + Ol. A subsequent reaction of the form Opx + Sp → melt + Ol was determined for FER-D after the exhaustion of Cpx. Over the entire temperature interval investigated for FER-B and -C, reactions were determined to be of the form Cpx + Sp → melt + Ol and Opx + Sp → melt + Ol, respectively. Melt percent (F) vs temperature (T) curves are concave up for all starting materials, demonstrating that isobaric melt productivity increases with progressive batch melting. At any given melt fraction, (dF/dT)P increases with increasing amount of Cpx in the starting material, indicating that the modal proportion of Cpx is one of the primary controls on isobaric melt productivity of upwelling peridotite. The concave up melt productivity functions for peridotitic assemblages determined in this study suggest that assuming linear or concave down melt productivity functions for modeling mantle melting may not be appropriate. Received: 2 August 1999 / Accepted: 7 June 2000  相似文献   

6.
High temperature (>900 °C) metamorphism affected the New Russia gneiss complex in the aureole of the Marcy anorthosite, Adirondack Highlands, New York. Dehydration melting of pargasitic hornblende and plagioclase in metagabbro during contact metamorphism produced garnet among other phases, an indicator that pressure exceeded 700 MPa during anatexis. Partial melting also occurred in mangerite and charnockite. Minerals that equilibrated during melting yield barometric estimates of 970 ± 100 MPa (garnet–orthopyroxene–plagioclase–quartz in metagabbro and mangerite) and 735 ± 100 and 985 ± 100 MPa (garnet–hornblende–plagioclase–quartz, metagabbro and mangerite, respectively). From these results we infer that the Marcy anorthosite was emplaced at a depth of at least 23 km and probably near 32 km. Received: 9 February 2000 / Accepted: 4 April 2000  相似文献   

7.
《Precambrian Research》2001,105(2-4):247-267
The Umba igneous complex consists of an enderbite–charnockite suite, including porphyritic variety of charnockites, and a porphyritic granite. Both are intruded by irregular veins or minor bodies of later reddish granite. The porphyritic charnockites locally contains abundant xenoliths of country rocks and its contamination by sedimentary material is expressed by a minor content of garnet, that increases in amount in areas with sedimentary inclusions. The Umba igneous complex and the Umba block metasediments were deformed together during two episodes of deformation. The first one was a major episode of thrusting with the formation of a penetrative shear foliation (S1), which dips gently eastwards, and a gently SE-plunging lineation. Coeval with this thrusting, the boundary between the Umba block and the Poriya Guba series in the southeast developed as a strike-slip shear zone, that juxtaposed the two blocks along a tectonic melange zone. The S1-shearing deformed the enderbite–charnockite suite, and probably also the porphyritic granite, into plate-like, eastward-dipping bodies. Predating the shearing, the metasediments underwent high-grade metamorphism and anatexis leading to a high degree of partial melting. This anatexis is also found in the enderbite–charnockite suite, but in a much smaller scale and mainly in the marginal parts of the bodies. The second episode of deformation formed narrow localized extensional shear zones (S2), which are developed in all rock units. The S1-shearing in the tectonic melange zone occurred under high-pressure metamorphism during cooling at constant pressure (T=806–818°C, P=9.3–9.5 kbar) and then at decreasing pressure due to tectonic uplift. Both seem to have gone through the same deformation events as the metasediments. The S2-extension occurred under decompression (P=7.5–8.0 kbar, T=860–840°C) caused by uplift or tectonic erosion of the thrust pile. Though indistinguishable in the field the enderbite–charnockite suite form a discontinuous suite with a trondhjemitic trend for the former, and a calc–alkaline trend for the latter. Geochemical study shows that the charnockite group is more strongly differentiated than the enderbite group and that magmatic differentiation in the charnockites were controlled by K-feldspar fractionation. The enderbites, on the other hand, lack differentiation and are considered to have crystallized rapidly from its magma source. The charnockites came from a different source that, judging from the high K/Rb ratio, formed at a deeper crustal level than the enderbites. Both members of the enderbite–charnockite suite formed due to subduction in an island arc setting, and Sm–Nd model ages of 2.1–1.9 Ga indicate that the Palaeoproterozoic suite has a juvenile character. Conventional U-Pb zircon dating of the porphyritic charnockite has given discordant ages of 1912.5±7.7 Ma, 1949±7 Ma and 1966±9. Our preferred interpretation is that the 1912.5±7.7 Ma age represents the age of intrusion, or maximum intrusion age of the charnockites, and that the 1949±7 Ma and 1966±9 Ma ages for the abraded type represent ages or mixed ages of inherited zircons from the contaminating Umba block metasediments. The youngest detrital zircons in these metasediments have similar ages. Their source could have been early magmatic arc intrusives, which were eroded shortly after their formation. If the Umba metasediments were deposited in a magmatic arc setting their initial deformation in an evolving arc may have provided the necessary heat flow for anatexis and high-grade peak metamorphism of the metasediments. Therefore, the intrusions of the enderbite–charnokite suite during the later evolution of the magmatic arc could have post-dated the peak of metamorphism, but still pre-date collision and thrusting leading to tectonic telescoping of the units, and thus explain the lower metamorphic grade in the Umba igneous complex compared to the metasediments. If the 1912.5±7.7 Ma age represents the maximum time of intrusion, the true intrusion age might be slightly younger.  相似文献   

8.
http://www.sciencedirect.com/science/article/pii/S1674987112000643   总被引：2，自引：1，他引：1  
Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust.Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block,southern India,and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature（UHT） metamorphism have been reported.Proximal to the UHT rocks are patches and lenses of charnockite（Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm） occurring within Opx-free Grt-Bt gneiss（Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt） which we report in this study.The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of～820℃and～9 kbar.Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T（～760℃and～7.5 kbar）. which is consistent with the results obtained from geothermobarometry（710—760℃,6.7—7.5 kbar）. but significantly lower than the peak temperatures（>1000℃） recorded from the UHT rocks in this locality,suggesting that charnockitization is a post-peak event.The modeling of T versus molar H2O content in the rock（M（H2O）） demonstrates that the Opx-bearing assemblage in charnockite and Opxfree assemblage in Grt-Bt gneiss are both stable at M（H2O） = 0.3 mol%-0.6 mol%.and there is no significant difference in water activity between the two domains.Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid.T-XFe3+（= Fe2O3/（FeO + Fe2O3） in mole） pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene:Opx is stable at XFe3+ <0.03 in charnockite.while Opx-free assemblage in Grt-Bt gneiss is stabilized at XFe3+ >0.12.Such low oxygen fugacity conditions of XFe3+ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid（e.g.,H2O + CH4） during the retrograde stage.  相似文献   

9.
The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite, diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to 6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence, the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the Novopavlovsk complex in the Azov Region.  相似文献   

10.
The system KAlO2–MgO–SiO2–H2O–CO2 has long been used as a model for the processes of granulite-facies metamorphism and the development of orthopyroxene-bearing mineral assemblages through the breakdown of biotite-bearing assemblages. There has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures of 342 to 1500 MPa with T between 710 and 1045 °C and X Fl H2O between 0.05 and 1.00), accurately locating most of the dehydration and melting equilibria in P-T-X Fl H2O space. The most important primary result is that the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be almost coincident with the fluid-absent reaction (Phl + Qtz = En + Sa + melt) in the CO2-free subsystem. In conjunction with the results of previous measurements of CO2 solubility in silicate melts and phase equilibrium experiments, our theoretical analysis and experiments suggest that CO2 cannot act as a flux for partial melting. Crustal melting in the presence of H2O–CO2 mixed fluids will always occur at temperatures higher than with pure H2O fluid present. Magmas produced by such melting will be granitic (s.l.) in composition, with relatively high SiO2 and low MgO contents, irrespective of the H2O–CO2 ratio in any coexisting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO2. The channelled passage of hot CO2-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated) systems, minor partial melting can also occur in veins initially filled with CO2-rich fluid, as dehydration and local disequilibrium drive the fluid towards H2O-rich compositions. However, CO2 is unlikely to be a significant agent in promoting regional granulite-grade metamorphism, melting, magma generation, metasomatism or long-range silicate mass transfer in Earth's crust. The most viable model for the development of granulite-facies rocks involves the processes of fluid-absent partial melting and withdrawal of the melt phase to higher crustal levels. Received: 28 November 1996 / Accepted: 25 June 1997  相似文献   

11.
The results of skarn-forming processes at contacts of the multiphase Southern California Batholith with carbonate rocks accessible to study in quarries in Riverside, California, involve prograde metasomatic transformations of marmorized dolomites and calcareous rocks in contact with granitic melts and contaminated magmas. The processes of contact assimilation are proved to have been controlled by the emplacement of granitic melts overheated relative to subliquidus melts (with the overheated melts prone to approach the composition of granodiorite, syenite, and gabbro) into skarnified marbles. The degree of magma overheating was evaluated based on G.F. Smith’s data on linear melting temperature variations for anhydrous intrusive rocks with various SiO2 concentrations (<750°C for granites and >1100°C for contaminated rocks, ΔT 350°). This corresponds to the thermal regime of the development of mineralogically contrasting hypabyssal skarn aureoles: magnesian at contacts with granite magmas and calcic at contacts with melts of high basicity. The peripheral parts of the aureoles ubiquitously contain preserved zones of forsterite calciphyres and periclase marbles, whereas skarns at mafic intrusions consist of high-temperature silicates of decreasing Mg contents: monticellite, merwinite, melilite, and spurrite. Prograde and retrograde mineralforming processes in the metasomatic rocks and their facies affiliation are analyzed, and the chemical composition of the minerals are examined. The Riverside skarn aureoles are compared with other compositionally contrasting skarn aureoles that developed in contacts with granite magmas and melts of increasing basisity.  相似文献   

12.
Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from <0.1 to <20 ppm, with silver-gold ratios of around 4 to 7. Mineralogical studies show a systematic distribution of three hydrothermal mineral assemblages. At distances >50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances <10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (<4.1 equivalent wt.% NaCl and <4 wt.% CO2), near neutral pH, reduced and, at times, boiling. Received: 19 May 1998 / Accepted: 8 March 1999  相似文献   

13.
Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H2O-poor melt and a silicate-poor H2O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H2O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an XH2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H2O, and the conjugate water-rich melt about 47 wt% H2O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H2O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. Received: 15 September 1999 / Accepted: 10 December 1999  相似文献   

14.
The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.  相似文献   

15.
A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz = orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this. These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures >500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet, orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade, low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for the Mafic Formation; and that this dehydration event did not involve partial melting. Received: 15 February 2000 / Accepted: 26 June 2000  相似文献   

16.
The Profitis Ilias gold deposit, located on the western part of Milos Island, Greece, is the first epithermal gold deposit discovered in the Pliocene–Pleistocene Aegean volcanic arc. Estimated ore reserves are 5 million tonnes grading 4.4 g/tonne Au and 43 g/tonne Ag. The deposit is closely associated with a horst and graben structure, and occurs in a series of steep interconnected crustiform-banded quartz veins up to 3 m wide, extending to depths of at least 300 m. The mineralisation occurs in three stages and is hosted by 3.5–2.5 Ma old silicified and sericitised rhyolitic lapilli-tuffs and ignimbrites. It consists of pyrite, galena, chalcopyrite, electrum and native gold. Additionally, adularia occurs with quartz mainly in veins. Homogenisation temperatures of primary liquid-rich inclusions vary from 145 to 399 °C for the ore stage, and 112 to 263 °C for the post-ore stage. Salinities range between 0.1 and 11.4 wt% NaCl equiv. and 0.93 to 8.5 wt% NaCl equiv. for the ore stage and the post-ore stage, respectively. Rare vapour-rich inclusions in ore stage quartz homogenise between 368 and 399 °C and estimates of eutectic melting (−25 to −38 °C) indicate the presence of Ca and Mg in the ore fluids. Sample elevation versus fluid inclusion Th–salinity relationships show (1) a high-salinity trend, where moderate-temperature (300–250 °C) and moderate-salinity brines (∼3 wt% NaCl equiv.) trend to high-salinity (up to 15 wt% NaCl equiv.) fluids with lower (∼25–50 °C) homogenisation temperatures, and (2) a high-Th trend where moderate-salinity and moderate-temperature brines (200–250 °C; 3 wt% NaCl equiv.) develop into low-salinity (<1 wt% NaCl equiv.), high-temperature (>350 °C) fluids. These trends are best explained by extreme boiling and vapourisation phenomena between 200 and 250 °C. The 430–450 m asl (metres above sea level) level marks the transition between a lower liquid-dominated segment of the system where only the steep high-salinity trend is seen, and an upper vapour-dominated segment where the high-Th trend or a combination of both are seen. There is a close spatial association between mineable gold grades and the upper segment of the system. Depth-to-boiling curves suggest that the paleo-surface was ∼200 m above the present summit of Profitis Ilias. Comparison of the mineralisation and fluid geochemistry at Profitis Ilias with that of the nearby modern geothermal system indicates that the processes of metal mineralisation have probably been continuous since the Late Pliocene. Received: 24 February 2000 / Accepted: 15 July 2000  相似文献   

17.
Mo-Bi mineralization occurs in subvertical and subhorizontal quartz-muscovite-± K-feldspar veins surrounded by early albitic and later K-feldspathic alteration halos in monzogranite of the Archean Preissac pluton, Abitibi region, Québec, Canada. Molybdenite is intergrown with muscovite in the veins or associated with K-feldspar in the alteration halos. Mineralized veins contain five main types of fluid inclusions: aqueous liquid and liquid-vapor inclusions, aqueous carbonic liquid-liquid-vapor inclusions, carbonic liquid and vapor inclusions, halite-bearing aqueous liquid and liquid-vapor inclusions, trapped mineral-bearing aqueous liquid and liquid-vapor inclusions. The carbonic solid in frozen carbonic and aqueous-carbonic inclusions melts in most cases at −56.7 ± 0.1 °C indicating that the carbonic fluid consists largely of CO2. All aqueous inclusion types and the aqueous phase in carbonic inclusions have low initial melting temperatures (≥70 °C), requiring the presence of salts other than NaCl. Leachate analyses show that the bulk fluid contains variable proportions of Na, K, Ca, Cl, and traces of Mg and Li. The following solids were identified in the fluid inclusions by SEM-EDS analysis: halite, calcite, muscovite, millerite (NiS), barite and antarcticite (CaCl2 · 6H2O). All are interpreted to be trapped phases except halite which is a daughter mineral, and antarcticite which formed during sample preparation (freezing). Aqueous inclusions homogenize to liquid at temperatures between 75 °C and 400 °C; the mode is 375 °C. Aqueous-carbonic inclusions homogenize to liquid or vapor between 210 °C and 400 °C. Halite-bearing aqueous inclusions homogenize by halite dissolution at approximately 170 °C. Aqueous inclusions containing trapped solids exhibit liquid-vapor homogenization at temperatures similar to those of halite-bearing aqueous inclusions. Temperatures of vein formation, based on oxygen isotopic fractionation between quartz and muscovite, range from 342 °C to 584 °C. The corresponding oxygen isotope composition of the aqueous fluid in equilibrium with these minerals ranges from 1.2 to 5.5 per mil with a mean of 3.9 per mil, suggesting that the liquid had a significant meteoric component. Isochores for aqueous fluid inclusions intersect the modal isotopic isotherm of 425 °C at pressures between 590 and 1900 bar. A model is proposed in which molybdenite was deposited owing to decreasing temperature and/or pressure from CO2-bearing, moderate to high salinity fluids of mixed magmatic-meteoric origin that were in equilibrium with K-feldspar and muscovite. These fluids resulted from the degassing of a monzogranitic magma and evolved through interaction with volcanic (komatiitic) and sedimentary country rocks. Received: 6 February 1997 / Accepted: 28 January 1998  相似文献   

18.
The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO2-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C2H6, vapor CH4, CH4–H2O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and 40Ar/39Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.  相似文献   

19.
Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell, Cornwall) are used to constrain the composition of fluids responsible for late stage greisening and mineralisation. The topaz contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite, K-feldspar, muscovite, ilmenorutile, apatite, columbite, zircon, varlamoffite [(Sn, Fe)(O, OH)2] and qitianlingite [(Fe+2,Mn+2)2(Nb,Ta)2W+6O10]. Primary L + V inclusions in topaz show relatively high T h (mainly 300 to >500 °C) and a narrow range of salinities (23–30 wt % NaCl equivalent) compared with those in greisen quartz (150–450 °C, 0–50 wt % NaCl equivalent). Textures indicate that topaz formed earlier than quartz and the fluid inclusion data are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and a decrease in pressure causing intermittent boiling. The presence of early-formed albite and K-feldspar as inclusions in the topaz is likely to indicate that the greisen-forming fluid became progressively more acid during greisenisation. The most distinctive inclusions in the topaz are wisp- and bleb-shaped quartz, < 50 μm in size, which show textural characteristics indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe, Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 μm in size. The quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks elsewhere which have been interpreted as trapped “silicate melt”. Their compositions are, however, very different to those expected for late stage topaz-normative granitic melts. From their textural and chemical characteristics they are interpreted as representing crystallised silica colloid, probably trapped as a hydro gel during greisenisation. There is also evidence for the colloidal origin of inclusions of varlamoffite in the topaz. These occurrences offer the first reported evidence in natural systems for the formation of colloids in high temperature hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite, pyrrhotite and SnCl within the quartz inclusions. Received: 9 April 1996 / Accepted: 12 November 1996  相似文献   

20.

The Gour Oumelalen area exposes Paleoproterozoic (1.9 Ga) marbles and calc-silicate granulites. Some marbles show a specific mineralogy characterized by the presence of a highly aluminous clinopyroxene with Al2O3 content exceeding16 wt%. This clinopyroxene shows a marked zoning with a hedenbergitic core rimmed by fassaite. Phase relations are expressed by spectacular reaction textures in calc-silicate granulites as Opx + Cpx + Pl + H2O  == > Grt + Qtz ± Am and Cpx + Ilm + Pl ± Mt = > Grt + Qtz + Spn. In olivine-bearing marbles, clinopyroxene and dolomite occur around olivine and calcite. According to thermodynamic modeling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCFMASHTO) system, the first stage of metamorphism is located at 800 °C and 6–7 kbar, which is followed by an increase in pressure at 9–10 kbar and 800 °C and an isobaric cooling at 680 °C. The deduced anticlockwise P–T path is consistent with a granulitic metamorphism occurring in an active continental margin context.

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