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1.
Mineral assemblages, rock and mineral chemistry, and mineral reactions, in calc-silicate rocks from Koduru area, Andhra Pradesh, India are discussed. Mineralogical and bulk chemical differences indicate 3 calc-silicate rock types — type I with K feldspar+calcite+wollastonite+quartz+scapolite+diopsidess +andraditess+sphene, has relatively high rock oxidation ratios. Type II is a highly calcic variety with high rock MgFe ratios, and has K feldspar+calcite+wollastonite+quartz+scapolite + diopsidess±grossularitess+sphene+zoisite. Type III has K feldspar +calcite+wollastonite+quartz+scapolite+diopsidess +sphene+hornblende+magnetite, and has relatively low oxidation ratio and low MgFe ratio. The 3 calc-silicate rock types have originated as mixtures of limestone/dolomite/marl.Diopside was produced by a reaction involving Ca-amphibole +calcite+quartz, and reversed during retrogression. Andraditess in type I rocks was produced at the expense of hedenbergitic component of pyroxene in a continuous reaction as a consequence of increase in the oxygen content of the original sediment relative to type III. Calcite+quartz reacted to give wollastonite. During cooling an influx of water caused scapolite to alter to zoisite.  相似文献   

2.
The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely narrow range of composition, about an average of Ms93.5Pg6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH2O slowly increased from initial values less than Ptotal + rock strength, under conditions of low permeability, the actual value of PH2O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening of thermal gradients following the onset of fractional melting. The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz. It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase. Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are much lower in sodium. The plagioclases are of “low” structural type, although is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite is indicated.  相似文献   

3.
The trioctahedral mica ephesite, Na(LiAl2) [Al2Si2O10] (OH)2, has a large -T stability field in the quaternary system NaAlSiO4-LiAlSiO4-Al2O3-H2O. At temperatures below 400–500° C it coexists with diaspore, while at higher temperatures it occurs with corundum, until it decomposes to nepheline +eucryptite+corundum+H2O at 600–800° C (Fig. 1). Nature faithfully reflects these phase relations; ephesite is found to coexist with diaspore or corundum in silicadeficient metamorphosed rocks or in hydrothermally altered nepheline-syenite pegmatite.Thermodynamic analysis of phase relations of ephesite in the silica saturated portion of the quinary system NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O shows that the assemblage quartz+ephesite is always metastable with respect to paragonite+spodumene or paragonite+petalite at temperatures down to approximately 300° C (Fig. 3). At lower temperatures, a number of other phases like bikitaite, cookeite, Na-montmorillonite, and analcime are stabilized. Stability and compatibility relations involving these phases are presently not amenable to thermodynamic treatment due to lack of suitable data. Nevertheless, the absence of the assemblage quartz+ephesite in nature seems to vindicate our conclusion that it is metastable down to at least 300° C.The frequently encountered assemblage quartzspodumene (or petalite)-microcline-albite of some lithium pegmatites contains muscovite (±lepidolite), rather than paragonite. The absence of paragonite in such rocks is best explained by the inherent metastability of the phase-pair paragonite+microcline with respect to muscovite+albite. The pegmatite bulk compositions plot in the four-phase field spodumene (petalite)-microcline-muscovite-albite, cutting out paragonite from the observed assemblage Thus, absence of paragonite-spodumene or paragonitepetalite in nature reflects lack of suitable bulk compositions in rocks.  相似文献   

4.
Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular-shaped bodies of coarse-grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer-grained granite. In the central part of the uplift, wollastonite-bearing calc-silicate rocks occur within the Valley Spring Gneiss. The development of these calc-silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc-silicate rocks equilibrated under H2O-rich conditions with χCO2 <0.10. Fluid inclusions present within the calc-silicate minerals are H2O-rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO2 in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H2O-rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm3. Isochores for these fluid inclusions, when combined with the stability of the solid-solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc-silicates were emplaced at a pressure of at least 1–2 kbar.  相似文献   

5.
The Navachab gold deposit in the Damara belt of central Namibia is hosted by a near-vertical sequence of amphibolite facies shelf-type metasediments, including marble, calc-silicate rock, and biotite schist. Petrologic and geochemical data were collected in the ore, alteration halos, and the wall rock to evaluate transport of elements and interaction between the wall rock and the mineralizing fluid. The semi-massive sulfide lenses and quartz–sulfide veins are characterized by a complex polymetallic ore assemblage, comprising pyrrhotite, chalcopyrite, sphalerite, and arsenopyrite, native bismuth, gold, bismuthinite, and bismuth tellurides. Mass balance calculations indicate the addition of up to several orders of magnitude of Au, Bi, As, Ag, and Cu. The mineralized zones also record up to eightfold higher Mn and Fe concentrations. The semi-massive sulfide lenses are situated in the banded calc-silicate rock. Petrologic and textural data indicate that they represent hydraulic breccias that contain up to 50 vol.% ore minerals, and that are dominated by a high-temperature (T) alteration assemblage of garnet–clinopyroxene–K-feldspar–quartz. The quartz–sulfide veins crosscut all lithological units. Their thickness and mineralogy is strongly controlled by the composition and rheological behavior of the wall rocks. In the biotite schist and calc-silicate rock, they are up to several decimeters thick and quartz-rich, whereas in the marble, the same veins are only a few millimeters thick and dominated by sulfides. The associated alteration halos comprise (1) an actinolite–quartz alteration in the biotite schist, (2) a garnet–clinopyroxene–K-feldspar–quartz alteration in the marble and calc-silicate rock, and (3) a garnet–biotite alteration that is recorded in all rock types except the marble. The hydrothermal overprint was associated with large-scale carbonate dissolution and a dramatic increase in CO2 in the ore fluid. Decarbonation of wall rocks, as well as a low REE content of the ore fluid resulted in the mobilization of the REE, and the decoupling of the LREE from the HREE. The alteration halos not only parallel the mineralized zones, but may also follow up single layers away from the mineralization. Alteration is far more pronounced facing upward, indicating that the rocks were steep when veining occurred. The petrologic and geochemical data indicate that the actinolite–quartz– and garnet–clinopyroxene–K-feldspar–quartz alterations formed in equilibrium with a fluid (super-) saturated in Si, and were mainly controlled by the composition of the wall rocks. In contrast, the garnet–biotite alteration formed by interaction with a fluid undersaturated in Si, and was mainly controlled by the fluid composition. This points to major differences in fluid–rock ratios and changes in fluid composition during alteration. The alteration systematics and geometry of the hydrothermal vein system are consistent with cyclic fluctuations in fluid pressure during fault valve action. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

6.
Batu Hijau is a world-class gold-rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc-silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc-silicate rocks occur at the contact with copper-gold-bearing tonalite porphyries. The rocks are fine-grained and granular with green, reddish-brown and white layers. The green layers consist mostly of fine-grained clinopyroxene (diopside and hedenbergite) and the reddish-brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc-silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc-silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340–360 C (high salinity 35–45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280–300 C (low salinity 1–10 NaCl wt% eqn.). Late calcite ± quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe-rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc-silicate rocks associated with massive magnetite-chalcopyrite-pyrite assemblage indicates the occurrence of calcic-exoskarn surrounding the Batu Hijau porphyry copper-gold deposit.  相似文献   

7.
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   

8.
《Precambrian Research》2003,120(1-2):37-53
The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.  相似文献   

9.
Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite(1) (MnO 1%), magnetite(2) (MnO 15%) and magnetite(3) (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X CO 2 (<0.2), and the latter equilibrated with a CO2-rich fluid. Rhodochrosite+magnetite(1)+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f O 2 through complex mineral-fluid interactions.  相似文献   

10.
Stratabound tungsten mineralization in regional metamorphic calc-silicate rocks of probably Lower Paleozoic age is described from the polymetamorphic Austroalpine Crystalline Complex (ACC) of the Eastern Alps. Scheelite-bearing calc-silicate rocks which are often associated with marbles and tourmalinites are intercalated conformably with metaclastic rocks. Alkalipoor calc-silicate rocks with high amounts of clinozoisite/ zoisite, grossular, quartz, plagioclase, etc. are the most important host rocks for tungsten mineralization. These unusual calc-silicate rocks are products of regional metaorphism and are interpreted as reaction skarns. They have formed in the presence of a water-dominated fluid phase with very low XCO2.In the Koralpe estimated P-T conditions are 650–700 °C at 5–7 kb. The mineralogical composition and the mineral zoning of the calc-silicate rocks is controlled by the degree of the Hercynian and Eoalpine metamorphism. There are no signs of graniteelated skarn formation. Tungsten preconcentration is thought to be syngenetic/syndiagenetic. It is genetically linked to exhalative hydrothermal processes in other Lower Paleozoic terrains of the Eastern Alps.  相似文献   

11.
Fluid infiltration at great depth during regional metamorphism plays a major role in mass transport and is responsible for significant rheological changes in the rock. Calc-silicate rocks of the Kajalbas area of Delhi Fold Belt, Rajasthan, are characterised by foliation parallel alternate bands of amphibole-rich and clinopyroxene–plagioclase feldspar-rich layers of varying thicknesses (mm to decimetre thick). Textural relation suggests that the amphibole grains formed from clinopyroxene and plagioclase in the late phase of regional deformation. Algebraic analysis of the reaction textures and mineral compositions was performed with the computer program C-Space to obtain the balanced chemical reactions that led to the formation of amphibole-rich bands. The computed balanced reaction is 70.74 Clinopyroxene + 27.23 Plagioclase + 22.018 H2O + 5.51 K++ 1.00 Mg2++ 27.15 Fe2+ = 22.02 Amphibole + 67.86 SiO2 aqueous + 36.42 Ca2++ 8.98 Na+. The constructed reaction suggests that aqueous fluid permeated the calc-silicate rock along mm to decimetre thick channels, metasomatized the clinopyroxene–plagioclase bearing rocks to form the amphibole-rich layers. The regional deformation presumably created the fluid channels thereby allowing the metasomatic fluid to enter the rock system. The above reaction has large negative volume change for solid phases indicating reaction-induced permeability. Thermodynamic calculations suggest that the fluid–rock interaction occurred at 665 ±05°C and 6.6 ±0.25 kbar (corresponding to ～20 km depth). Textural modeling integrating the textural features and balanced chemical reaction of the calc-silicate rocks of Mesoproterozoic Phulad Shear Zone thus indicate that extremely channelled fluid flow was reaction enhanced and caused major change in the rock rheology.  相似文献   

12.
The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite?+?clinopyroxene (mg#?=?71–73)?+?grandite (16–21 mol% Adr)?+?quartz?±?calcite, Type II: grandite (25–29 mol% Adr )?+?clinopyroxene (mg#?=?70)?+?calcite?+?quartz, and Type III: grandite (36–38 mol% Adr)?+?clinopyroxene (mg#?=?55–65)?+?epidote?+?scapolite?+?calcite?+?quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite?+?SiO2?→?wollastonite?+?V, 2. calcite?+?clinopyroxene?+?O2?→?grandite?+?SiO2?+?V, 3. scapolite?+?calcite?+?quartz?+?clinopyroxene?+?O2?→?grandite?+?V and 4. epidote?+?calcite?+?quartz?+?clinopyroxene?+?O2?→?grandite?+?V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2?<?0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO2 and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (XCO2?<?0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al2O3 was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-XCO2 and isobaric T-XCO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9?±?1 kbar and 750°?±?30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.  相似文献   

13.
An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.  相似文献   

14.
The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12wt K2O, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 ± 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead — isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.  相似文献   

15.
A new occurrence of Mn-rich rocks was discovered within the high-pressure/low-temperature metamorphic rocks on the Palos peninsula of Syros (Greece). Near the summit of Mount Príonas, a meta-conglomerate consists of calcite (~63 wt%), pink manganian phengite, blue–purple manganian aegirine–jadeite, microcline, albite and quartz. In addition, it contains abundant braunite-rich aggregates (up to ~1.5 cm in diameter) that include hollandite [(Ba0.98–1.02K<0.01Na<0.02Ca<0.03) (Mn 1.02–1.52 3+ Fe 0.38–0.88 3+ Ti0.29–0.92Mn 5.11–5.76 4+ )O16], barite and manganian hematite. Due to metamorphic recrystallization and deformation, the contacts between clasts and matrix are blurred and most clasts have lost their identity. In back-scattered electron images, many aegirine–jadeite grains appear patchy and show variable jadeite contents (Jd10–67). These pyroxenes occur in contact with either quartz or albite. Manganian phengite (3.41–3.49 Si per 11 oxygen anions) is of the 3T type and contains 1.4–2.2 wt% of Mn2O3. At the known P–T conditions of high-pressure metamorphism on Syros (~1.4 GPa/ 470 °C), the mineral sub-assemblage braunite + quartz + calcite (former aragonite) suggests high oxygen fugacities relative to the HM buffer (+7 ≤ ?fO2 ≤ + 17) and relatively high CO2 fugacities. The exact origin of the conglomerate is not known, but it is assumed that the Fe–Mn-rich and the calcite-rich particles originated from different sources. Braunite has rather low contents of Cu (~0.19 wt%) and the concentrations of Co, Ni and Zn are less than 0.09 wt%. Hollandite shows even lower concentrations of these elements. Furthermore, the bulk-rock compositions of two samples are characterized by low contents of Cu, Co and Ni, suggesting a hydrothermal origin of the manganese ore. Most likely, these Fe–Mn–Si oxyhydroxide deposits consisted of ferrihydrite, todorokite, birnessite, amorphous silica (opal-A) and nontronite. Al/(Al + Fe + Mn) ratios of 0.355 and 0.600 suggest the presence of an aluminosilicate detrital component.  相似文献   

16.
Microstructures and quartz c-axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the Eureka Valley–Joshua Flat–Beer Creek composite pluton. Temperatures of deformation are estimated to be 740±50 °C based on a modified c-axis opening angle thermometer of Kruhl (J. Metamorph. Geol. 16 (1998) 142). In quartzite layers located closest (140 m) to the pluton-wall rock contact, flattened detrital grains are plastically deformed and partially recrystallized. The dominant recrystallization process is subgrain rotation (dislocation creep regime 2 of Hirth and Tullis (J. Struct. Geol. 14 (1992) 145)), although grain boundary migration (dislocation creep regime 3) is also evident. Complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact, and coincides with recrystallization taking place dominantly through grain boundary migration (regime 3). Within the quartzites, strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains.The c-axis fabrics indicate that a slip operated within the quartzites closest to the pluton-wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism [c] slip. The spatial inversion in microstructures and slip systems (apparent “high temperature” deformation and recrystallization further from the pluton-contact and apparent “low temperature” deformation and recrystallization closer to the pluton-contact) coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units. Marble and calc-silicate assemblages dominate close to the pluton-wall rock contact, whereas mixed quartzite and pelite assemblages are dominant further from the contact.We suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton. Decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc-silicate assemblages in the inner aureole quartzites may have produced high XCO2 (water absent) fluids during deformation. The presence of high XCO2 fluid is inferred from the prograde assemblage of quartz+calcite (and not wollastonite)+diopside±K-feldspar in the inner aureole quartzites. We suggest that it was these “dry” conditions that suppressed prism [c] slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization, which would normally be associated with lower deformation temperatures. In contrast, the prograde assemblage in the pelite-dominated outer part of the aureole is biotite+K-feldspar. These “wet” pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism [c] slip and regime 3 recrystallization. Because other variables could also have caused the spatial inversion of c-axis fabrics and recrystallization mechanisms, we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz. Our conclusions are based on a small number of samples, and therefore, the unusual development of crystal fabrics and microstructures in the aureole to the EJB pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks.  相似文献   

17.
The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the western province, Sulawesi, Indonesia. The gold mineralization is hosted by the meta‐sedimentary rocks intercalated with the meta‐volcanic and volcaniclastic rocks of the Latimojong Metamorphic Complex. The ores are approximately three meters thick, consisting of veins, stockwork, and breccias. The veins can be classified into three stages, namely, early, main, and late stages, and gold mineralization is related to the main stage. The mineral assemblage of the matrix of breccia and the veins are both composed of quartz, carbonate (mainly ankerite), and albite. High‐grade gold ores in the Salu Bulo prospect are accompanied by intense alteration, such as carbonatization, albitization, silicification, and sulfidation along the main stage veins and breccia. Alteration mineral assemblage includes ankerite ± calcite, quartz, albite, and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral that is spatially related to native gold and electrum (<2–42 μm in size). It is more abundant as dissemination in the altered host rocks than those in veins. This suggests that water–rock interaction played a role to precipitate pyrite and Au in the Salu Bulo prospect. The Au contents of intensely altered host rocks and ores have positive correlations with Ag, Ni, Mo, and Na. Fluid inclusions in the veins of the main stage and the matrix of breccia are mainly two‐phase liquid‐rich inclusions with minor two‐phase, vapor‐rich, and single‐phase liquid or vapor inclusions. CO2 and N2 gases are detected in the fluid inclusions by Laser Raman microspectrometry. Fluid boiling probably occurred when the fluid was trapped at approximately 120–190 m below the paleo water table. δ18OSMOW values of fluid, +5.8 and +7.6‰, calculated from δ18OSMOW of quartz from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ34SCDT of pyrite narrowly ranges from ?2.0 to +3.4‰, suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the host rocks. It formed at a relatively shallow depth from fluids with low to moderate salinity (3.0–8.5 wt% NaCl equiv.). The temperature and pressure of ore formation range from 190 to 210°C and 1.2 to 1.9 MPa, respectively.  相似文献   

18.
Calculated phase equilibria among the minerals sodic amphibole, calcic amphibole, garnet, chloritoid, talc, chlorite, paragonite, margarite, omphacite, plagioclase, carpholite, zoisite/clinozoisite, lawsonite, pyrophyllite, kyanite, sillimanite, quartz and H2O are presented for the model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH), which is relevant for many greenschist, blueschist, amphibolite and eclogite facies rocks. Using the activity-composition relationships for multicomponent amphiboles constrained by Will and Powell (1992), equilibria containing coexisting calcic and sodic amphiboles could be determined. The blueschist–greenschist transition reaction in the NCFMASH system, for example, is defined by the univariant reaction sodic amphibole + zoisite = calcic amphibole + chlorite + paragonite + plagioclase (+ quartz + H2O) occurring between approximately 420 and 450 °C at 9.5 to 10 kbar. The calculated petrogenetic grid is a valuable tool for reconstructing the PT-evolution of metabasic rocks. This is shown for rocks from the island of Samos, Greece. On the basis of mineral and whole rock analyses, PT-pseudosections were calculated and, together with the observed mineral assemblages and reaction textures, are used to reconstruct PT-paths. For rocks from northern Samos, pseudomorphs after lawsonite preserved in garnet, the assemblage sodic amphibole-garnet-paragonite-chlorite-zoisite-quartz and the retrograde appearance of albitic plagioclase and the formation of calcic amphibole around sodic amphibole constrain a clockwise PT-path that reaches its thermal maximum at some 520 °C and 19 kbar. The derived PT-trajectory indicates cooling during exhumation of the rocks and is similar to paths for rocks from the western part of the Attic-Cycladic crystalline complex. Rocks from eastern Samos indicate lower pressures and are probably related to high-pressure rocks from the Menderes Massif in western Turkey. Received: 8 July 1997 / Accepted: 11 February 1998  相似文献   

19.
Metabasaltic rocks in the Klamath Mountains of California with ‘komatiitic’ major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentrations preserve relict igneous trends and suggest that the rocks are not komatitic basalts, but immature arc rocks and within-plate alkalic lavas. Correlation of ‘excess’ MgO with the volume per cent hornblende (±clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions. Early submarine alteration produced regional δ18O values of +10±1.5%° and shifts in Al2O3, Na2O, and K2O concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350–550°C, c. 3 kbar) southward to prehnite–actinolite facies (200–400°C, ≤3 kbar), but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclase + microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these phases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearly stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatible with P–T conditions in a magmatic arc. Later contact metamorphism at 2–2.9±0.5 kbar and at peak temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3–4–km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, muscovite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatism during contact metamorphism produced further increases in bulk-rock δ18OSMOW of as much as +6%°. Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metasomatism and the presence of magnesian cumulus phases.  相似文献   

20.
Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H2O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H2O and Na2O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.  相似文献   

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