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1.
An exceptionally well-exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite-in, prehnite- and pumpellyite-out, hornblende-in, oligoclase-in, and actinolite-out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP-GS) transition and greenschist to amphibolite facies (GS-AM) transition. Across the GS-AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase-in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16 (3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29 (3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure-sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low-pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate- to high-pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP-GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP-GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite-bearing assemblages at low pressures and pumpellyite–actinolite-bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS-AM transition (average: ~1.8 wt% H2O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS-AM transition zone predicts more progressive dehydration of ~2 wt% H2O over a >50°C interval. Observations from the field suggest devolatilization across the PP-GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP-GS transition as observed in the Flin Flon sequence.  相似文献   

2.
The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite, Tibet, in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction. The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures. Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock (~10 m × 30 m), and they record wide variations in bulk composition, mineralogy, and texture. The mineral assemblages, from center to margin, vary from (1) relics of primary clinopyroxene (Cpxr) and primary amphibole (Ampr) + newly formed late amphibole (Act) + primary plagioclase (Plr) + clinozoisite + prehnite + albite + chlorite + titanite + ilmenite (R1 rodingite), through (2) relics of primary clinopyroxene (Cpxr) + newly formed late clinopyroxene (Cpxn) + primary and late amphiboles (Ampr + Act) + clinozoisite + prehnite + albite + chlorite + titanite (R2 rodingite), to (3) newly formed late clinopyroxene (Cpxn) and amphibole (Act) + clinozoisite + prehnite + albite + chlorite + titanite (R3 rodingite). As a result of the metasomatic process of rodingitization, the content of CaO in the whole rock chemical composition from R1 to R3 increases, SiO2 decreases, and Na2O + K2O is almost completely removed. Mass-balance diagrams show enrichments in large ion lithophile elements such as Rb, Cs, Ba, and Pb as well as Ni during rodingitization. The central part of the rodingitized intrusion (R1 rodingite) was only slightly affected by metasomatism. On the other hand, the contents of the rare earth elements (REEs), high field strength elements (HFSEs; e.g. Zr, Nb, Ta, Hf, and Y), and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization. Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H2O-saturated, CO2-rich environment. The estimated conditions of metamorphism were ~281–323 °C and 0.4–3.9 kbar, representing the subgreenschist facies. In this environment, REEs and HFSEs were soluble in the fluids and partly removed. Moreover, these prehnite rodingites formed in a progressively reducing and less alkaline environment, as indicated by decreases in f(O2) and bulk-rock Fe3+/Fe2+ ratios, and the records of fluid ΔpH from the center to the margin of the studied rodingitized intrusion.  相似文献   

3.
The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes  相似文献   

4.
Abstract Archaean greenstone belts are often cut by major shear zones, for example the Cadillac tectonic zone (CTZ) of the southern Abitibi region in north-western Quebec. At McWatters, the CTZ contains slices of metavolcanic units bounded by corridors of highly strained and altered rocks. Mineral assemblages of the metabasites record the metamorphic evolution of the CTZ.
The McWatters metabasalts and metagabbros have similar chemistry but different mineral assemblages consisting of variable amounts of actinolite, hornblende, chlorite, albite, epidote, quartz, carbonates, titanite, biotite, rutile, magnetite, ilmenite and sulphides. The different mineral assemblages, which coexist in a single tectonic slice, can be divided into three types, characterized by (A) presence of hornblende and actinolite, (B) presence of actinolite and epidote, and (C) absence of amphibole and epidote. Partial replacements indicate that these mineral assemblages are not in equilibrium. The hornblende of the least altered and deformed samples of the type A assemblage is a relict of a prograde metamorphic event, contemporaneous with the development of the main schistosity. The prograde conditions are estimated at P = 5 kbar, T = 475° C with low Pf . The more altered and deformed samples of the type C assemblage record a later retrograde metamorphic event. Conditions of the later event are estimated at P = 4 kbar, T = 400° C with higher Pf . Widespread calcite precipitation occurred during a later episode. The diversity of the mineral assemblages results from permeability variations along the high-strain zones of the CTZ.  相似文献   

5.
Brecciated mafic+ultramafic plutonic rocks of the East Taiwan Ophiolite occur as detritus and slide blocks in the Pliocene Lichi Mélange. These plutonic rocks have been subjected to two stages of post-magmatic recrystallization: (I) pre-brecciation ridge-type metamorphism attended by high-grade greenschist and rare amphibolite facies physical conditions; and (II) later off-axis metamorphism under zeolite to lowest greenschist facies conditions that postdated brecciation, submarine talus accumulation and deposition of associated pelagic sediments. The effects of the earlier ridge metamorphism are the main concern of this paper. (I) Dominant antigorite together with chlorite and talc in some ultramafics suggests that these rocks recrystallized at T>350 ° C. The primary compositions of gabbroic calcic plagioclase have been modified from An 45–70 to An 13–38, and the igneous clinopyroxenes and hornblendes partly replaced by actinolite+chlorite. Stable mineral assemblages in the metagabbros are thus ∼oligoclase+actinolite+chlorite±very rare epidote+sphene, and intermediate plagioclase +actinolite+chlorite+sphene. Amphibolites are less common and consist of more calcic plagioclase (An 25–49)+hornblende. The presence of assemblages transitional between greenschist and amphibolite facies for basaltic compositions is suggestive of very low-pressure thermal metamorphism such as would be appropriate to the crustal portions of an oceanic spreading center. (II) The occurrence of vein albite+actinolite+ chlorite near the base of the brecciated plutonic sequence and vein prehnite+laumontite in the upper part suggests that the brecciated plutonic rocks were later feebly retrograded under conditions of the greenschist and zeolite facies respectively-probably some distance removed from the thermal regime of a mid-oceanic ridge. The East Taiwan Ophiolite probably represents the western termination of the Philippine Sea lithospheric plate. Portions of this oceanic crust and underlying mantle were incorporated in the Lichi Mélange of the Coastal Range of eastern Taiwan as a consequence of antithetic faulting and erosion. This process evidently accompanied east-directed underflow of the Asiatic (South China Sea) plate.  相似文献   

6.
ABSTRACT Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event.  相似文献   

7.
An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with NaB from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between NaB= 0.92 and NaB= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.  相似文献   

8.
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   

9.
Calcsilicate granulites of probable Middle Proterozoic age ( c .1000–1100  Ma) in the vicinity of Battye Glacier, northern Prince Charles Mountains, East Antarctica, contain prograde metamorphic assemblages comprising various combinations of wollastonite, scapolite, clinopyroxene, An-rich plagioclase, calcite, quartz, titanite and, rarely, orthoclase, ilmenite, phlogopite and graphite. Comparison of the prograde assemblages with calculated and experimentally determined phase relations in the simple CaO–Al2O3–SiO2–CO2–H2O system suggests peak metamorphism at ≥835 °C in the presence (in wollastonite-bearing assemblages at least) of a CO2-bearing fluid ( X CO≥0.3) at a probable pressure of 6–7  kbar.
Well-preserved retrograde reaction textures represent: (1) breakdown of scapolite to anorthite+calcite±quartz; (2) formation of grossular–andradite garnet and, locally, (3) epidote, both principally by reactions involving scapolite breakdown products and clinopyroxene; (4) local coupled replacement of clinopyroxene and ilmenite by hornblende and titanite, respectively; and finally (5) local sericitization of prograde and retrograde plagioclase. These retrograde reactions are interpreted to be the result of cooling and variable infiltration by H2O-rich fluids, possibly derived from crystallizing pegmatitic intrusions and segregations that may be partial melts, which are common throughout the area.  相似文献   

10.
Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32–35 mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼700–800°C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼300°C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro.  相似文献   

11.
The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K-feldspar, white mica, calcite, and sulphide and oxide minerals.
Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite-out isograd separates the PrP and PrA zones, and the prehnite-out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite-bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO-rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.
The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H2O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.  相似文献   

12.
Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.  相似文献   

13.
Abstract The North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low-grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite-scolecite-smectite, (2) heulandite-stilbite-smectite, (3) laumontitechlorite-albite, (4) laumontite-chlorite-albite ± prehnite ± pumpellyite and (5) epidote-chlorite-albite ± actinolite zone. In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca-rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed-layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed-layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior. The above features suggest that the overall pattern is one of burial-type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern. Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low-grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions.  相似文献   

14.
Most titanomagnetite in the Khibina alkaline igneous complex, sampled through 500 m of a vertical cross-section, is represented by Ti-rich varieties. The ulvöspinel component is most commonly around 55 mol%, rarely reaching up to 80 mol%.

We calculated an fO2–T diagram for magnetite + ilmenite + titanite + clinopyroxene + nepheline + alkali feldspar and magnetite + titanite+ clinopyroxene + nepheline + alkali feldspar phase assemblages at a hedenbergite activity of 0.2. The diagram shows that magnetites with 55 mol% of ulvöspinel crystallized at oxygen fugacities just slightly below the quartz–fayalite–magnetite buffer. More Ti-rich varieties crystallized at higher temperatures and slightly lower ΔQMF values, whereas more Ti-poor magnetites crystallized at or below about 650 °C.

Under the redox conditions estimated for the apatite-bearing intrusion of the Khibina complex (close to the QFM buffer), substantial quantities of methane may only form during cooling below 400 °C in equilibrium with magma. However, even at higher orthomagmatic temperatures and redox conditions corresponding to ΔQMF = 0, the hydrogen content in the early magmatic stage is not negligible. This hydrogen present in the gas phase at magmatic temperatures may migrate to colder parts of a solidifying magma chamber and trigger Fischer-Tropsch-type reactions there. We propose therefore, that methane in peralkaline systems may form in three distinct stages: orthomagmatic and late-magmatic in equilibrium with a melt and — due to Fischer-Tropsch-type reactions — post-magmatic in equilibrium with a local mineral assemblage.  相似文献   


15.
Titanite (sphene, CaTiSiO5) is sensitive to changes in temperature, oxygen and water fugacity, and fluid composition. In order to understand formation processes and the nature of hydrothermal fluids, various types of titanite from Cu ores at the Baogutu reduced porphyry Cu deposit were chosen for detailed study. Magmatic titanite is associated with biotite, plagioclase and K-feldspar, whereas hydrothermal titanite occurs with K-feldspar, chlorite, actinolite and calcite. The formation of hydrothermal titanite was related to hydration of igneous minerals under high fH2O, whereas the widespread replacement of ilmenite by titanite (without magnetite) indicates a relatively low oxygen fugacity. Magmatic titanite has low Al, high Fe, Y, Sn, Zr, Nb and REE contents, relative to hydrothermal titanite. On the basis of the Zr-in-titanite and Al-in-chlorite geothermometers, formation temperatures for magmatic and hydrothermal titanite are estimated to be 687–739 °C and 250–670 °C, respectively. The gradual decrease in REE, Y and Sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of REE-bearing minerals. Magmatic and hydrothermal titanites have similar chondrite-normalized REE patterns with negative Eu anomalies and relatively flat HREE. Randomly selected titanites have Nd isotopic compositions similar to the host rocks. Thus, both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source.  相似文献   

16.
在中亚造山带东部构造带中发现角闪变质辉绿岩,岩相学特征表明具非典型辉绿结构,显示变余辉绿结构及变余嵌晶含长结构.主要矿物为斜长石及角闪石,次要矿物为单斜辉石和阳起石.全岩岩石化学分析结果：SiO2含量为49.15%～52.96%、Na2O+K2O含量为2.71%～5.07%、CaO含量为7.07%～10.40%、Al2O3含量为14.77%～17.31%、TFe0含量为8.80%～12.27%、MgO含量为6.45%～9.57%;在火山岩TAS分类图落入玄武岩区,属于基性岩范围;说明研究区具有非典型辉绿结构的岩石命名为角闪变质辉绿岩的正确性.角闪变质辉绿岩经历了退变质作用：基性斜长石（拉长石）退变质为中性斜长石（中长石）,进一步退变质为钠长石或钠更长石;单斜辉石退变质为角闪石,角闪石退变质为阳起石.  相似文献   

17.
The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe2O3 (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe2O3‐saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe2+‐Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.  相似文献   

18.
A new petrogenetic grid for low-grade metabasites   总被引：7，自引：0，他引：7  
Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na2O-CaO-MgO-Al2O3-SiO2-H2O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.  相似文献   

19.
A new discovery of lawsonite eclogite is presented from the Lancône glaucophanites within the Schistes Lustrés nappe at Défilé du Lancône in Alpine Corsica. The fine‐grained eclogitized pillow lava and inter‐pillow matrix are extremely fresh, showing very little evidence of retrograde alteration. Peak assemblages in both the massive pillows and weakly foliated inter‐pillow matrix consist of zoned idiomorphic Mg‐poor (<0.8 wt% MgO) garnet + omphacite + lawsonite + chlorite + titanite. A local overprint by the lower grade assemblage glaucophane + albite with partial resorption of omphacite and garnet is locally observed. Garnet porphyroblasts in the massive pillows are Mn rich, and show a regular prograde growth‐type zoning with a Mn‐rich core. In the inter‐pillow matrix garnet is less manganiferous, and shows a mutual variation in Ca and Fe with Fe enrichment toward the rim. Some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites. Matrix omphacite in both rock types is zoned with a rimward increase in XJd, locally with cores of relict augite. Numerous inclusions of clinopyroxene, lawsonite, chlorite and titanite are encapsulated within garnet in both rock types, and albite, quartz and hornblende are also found included in garnet from the inter‐pillow matrix. Inclusions of clinopyroxene commonly have augitic cores and omphacitic rims. The inter‐pillow matrix contains cross‐cutting omphacite‐rich veinlets with zoned omphacite, Si‐rich phengite (Si = 3.54 apfu), ferroglaucophane, actinolite and hematite. These veinlets are seen fracturing idiomorphic garnet, apparently without any secondary effects. Pseudosections of matrix compositions for the massive pillows, the inter‐pillow matrix and the cross‐cutting veinlets indicate similar P–T conditions with maximum pressures of 1.9–2.6 GPa at temperatures of 335–420 °C. The inclusion suite found in garnet from the inter‐pillow matrix apparently formed at pressures below 0.6–0.7 GPa. Retrogression during initial decompression of the studied rocks is only very local. Late veinlets of albite + glaucophane, without breakdown of lawsonite, indicate that the rocks remained in a cold environment during exhumation, resulting in a hairpin‐shaped P–T path.  相似文献   

20.
通过对澜沧江杂岩带小黑江-上允地区蓝片岩的岩相学、地球化学、成因矿物学以及相平衡模拟的综合研究,阐述蓝片岩的原岩以及变质演化过程。地球化学分析结果显示,蓝片岩具有一致的稀土元素配分模式,具弱Eu正或负异常,稀土元素和微量元素特征与OIB相似,其原岩可能为OIB型玄武岩。详细矿物学研究表明,本区蓝片岩记录了俯冲峰期蓝片岩相变质和峰期后绿片岩相变质两个变质阶段,其矿物组合分别为蓝闪石+钠长石+多硅白云母+绿泥石+绿帘石和蓝闪石+钠长石±阳起石+绿泥石+绿帘石。通过Na_2O-Ca O-Fe O-MgO-Al_2O_3-SiO_2-H_2O-O体系相平衡计算,得到两个阶段的压力范围分别约为0.95 GPa和0.40 GPa。  相似文献   

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