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1.
The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving high Ca and Mn contents in garnet and AlVI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C and 589 °C ± 12 °C, assuming a H2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained. Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar, respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar. Received: 18 June 1997 / Accepted: 14 May 1998  相似文献   

2.
Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD=Δ (XCa,g) where KD is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low aO2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.  相似文献   

3.
In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM1 and ChRM2) were identified by their unblocking temperature spectra. The ChRM1 is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM2 by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal.In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (RPYR/MAG) and saturation magnetisation (SPYR/MAG). Over a stretch of 10 km the RPYR/MAG and SPYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.  相似文献   

4.
南大别地体中榴辉岩的围岩──石榴黑云片麻岩具有角闪岩相变质矿物组合,其变质温度为525℃,Sm-Nd矿物等时线年龄为(229±3)Ma.黑云母的K-Ar封闭温度为300℃,相应K-Ar年龄为(231±5)Ma.因此该片麻岩在230Ma左右从525℃迅速冷却到300℃以下。然而该区榴辉岩在印支期(221Ma)变质温度为700℃左右,直到134Ma才降至400-450℃。说明该区榴辉岩与该片麻岩具有不同的冷却史。它不支持榴辉岩是原地(in situ)成因的。  相似文献   

5.
Wood-and-Banno temperatures for the coexisting pyroxenes of equilibrated metamorphic rocks in the hornblende granulite subfaoies fall in the range 780–860° C. Minimum temperature estimates for granulites include 760–790 °C, from the dehydration of hornblende to an orthopyroxene assemblage, and about 800 °C, from other evidence. The pyroxene temperatures are generally consistent with these temperature estimates, and are certainly not too low or more than 50 ° too high. Pyroxene temperatures for the three subzones of Broken Hill granulites increase away from the orthopyroxene isograd and are sufficiently precise that the difference between the lowest and intermediate gubzones is statistically significant. Temperatures for pyroxenes in pyroxene-granulite subfacies rocks are greater than 860 °C. The internal consistency, precision and apparent accuracy of the Wood-and-Banno pyroxene geothermometer in the metamorphic temperature range make it an important tool.  相似文献   

6.
The fluid regime of medium-grade metapelitic systems was studied with mineral fugometers. It has been established that at least some of the rocks under consideration contain mineral assemblages equilibrated with the participation of fluids with an appreciably lowered mole fraction of the aqueous component (0.4–0.7). The data obtained are used to explain the systematic discrepancy between the temperature estimated with the garnet-biotite geothermometer and the program THERMOCALC. It has been suggested that the negative correlation between the temperature intervals of crystallization and the nucleation temperature of the studied garnets is related to the kinetic features of porphyroblast growth.  相似文献   

7.
The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken with widely used calibrations for the systems garnet-biotite, calcite-dolomite, and calcite-graphite. Temperatures that are obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using calcite-graphite thermometry. However, calcite-graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite. Application of calcite-graphite and garnet-biotite systems may yield temperatures up to 70 °C higher than calcite-dolomite in amphibolite facies rocks. Calcite-dolomite temperatures most closely approach those from calcite-graphite and garnet-biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae. However, some of these samples yield temperatures considerably lower than temperatures calculated from calcite-graphite and garnet-biotite thermometry, indicating that the calcite-dolomite thermometer may have been partially reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist facies) and Bancroft (upper amphibolite facies) range from 500 to 650 °C. These results place the chlorite-staurolite isograd at 540 °C, the kyanite-sillimanite isograd at 590 °C, and the sillimanite-K-feldspar isograd at 650 °C. Although each thermometer may have an absolute uncertainty of as much as ±50 °C, the 50 to 60 °C temperature differences between the isograds are probably accurate to 10 to 20 °C. An incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area. Simultaneous application of several systems allows one to recognize and overcome the inherent limitations of each thermometer. Received: 26 March 1997 / Accepted: 15 April 1998  相似文献   

8.
I. A. Tararin 《Petrology》2008,16(2):193-209
Geological, mineralogical, and geothermobarometric data testify that the regional metamorphism of the terrigenous protolith of the Kolpakovskaya Series, which composes the stratigraphic basement of the Kamchatka Median Crystalline Massif, corresponded to the kyanite mineral subfacies of the amphibolite facies at temperatures of 560–660°C and pressures of 5.9–6.9 kbar. This metamorphism predetermined wide kyanite development in high-Al garnet-biotite plagiogneisses. The younger granitization and migmatization of the plagiogneisses took place at a decrease in the pressure (depth), as follows from the textures of kyanite reaction replacement by andalusite in both the metamorphic rocks and the vein synmetamorphic granitoids and pegmatites. The temperature of the granitization and migmatization processes in the plagiogneisses was estimated at 620–650°C, and the pressure was evaluated at 1.9–3.0 kbar. Acid leaching that accompanied the granitization and migmatization processes resulted in the intense replacement of biotite by sillimanite (fibrolite) and, to a lesser degree, muscovite in the metamorphic and vein magmatic rocks. The highest temperature orthopyroxene-cordierite-biotite-orthoclase-plagioclase-quartz mineral assemblages were determined to have been formed in the Kolpakovskaya Series at a temperature of 830–840°C not by the regional metamorphism but in contact aureoles around gabbro-granitoid intrusions of the Lavkinskii intrusive complex of Oligocene-Miocene age in garnet-biotite and kyanite-garnet-biotite plagiogneisses of the amphibolite facies and cannot thus be regarded as evidence of an early granulite stage in the metamorphism of these rocks.  相似文献   

9.
Metamorphic temperatures and pressures of the Archean Miyun Group were determined from orthopyroxene-clinopyroxene, garnet-clinopyroxene, garnet-biotite and δO Q 18 -δO Mt 18 geothermometers and orthopyroxene barometer. The results show that the temperature in the first metamorphic stage of the Miyun Group is 820°+50°C and the pressure about 10 kb, which suggests that the granulite facies occurs under moderate pressures with a geothermal gradient of 22°–25°C/km. The corresponding burial depth is about 35 km. The temperature prevailing during the second metamorphic stage is in the range 650°–700°C, indicating a moderate condition between granulite and high grade amphibolite facies. Oxygen isotope data also show that the temperature of later superimposed regression metamorphism of high green schist facies in this area may be within the range of 470°–560°C.  相似文献   

10.
Garnet-biotite (-cordierite) phase relations in high-grade gneisses of the south coast of Western Australia reflect at least two metamorphic episodes. Chemical uniformity of the interiors of garnet and cordierite grains suggest thorough equilibration during a major phase of metamorphism. Narrow Mg-depleted rims on garnet grain boundaries in contact with biotite or cordierite, and complementary Mg-enriched rims on contiguous cordierites are the result of subsequent retrograde re-equilibration. The absence of reaction zoning in biotites suggests more complete retrograde modification of this mineral.Comparison between granulite and amphibolite facies garnet-biotite pairs shows that Mn contents of both minerals are higher, and Ti contents of the biotites are lower, in the lower-grade rocks. These differences, although not entirely unrelated to grade, are more directly controlled by variations in host rock chemistry and modal amounts of garnet and biotite.Partitioning of Mg, Fe2+ and Mn between garnet and biotite is fairly uniform, with no clear differences between granulite and amphibolite facies pairs. Application of the Mg-Fe2+ distributions to the geothermometers devised by Perchuk, Thompson, and Goldman & Albee yields variable T estimates of 600–680°C, 580–780°C, and 475–715°C respectively, for the main metamorphism. These estimates are low compared with the T indicated for the granulite facies rocks by other evidence (i.e. > 750°C at 5 kb PT). The Mg-Fe2+ distributions between contiguous garnet-biotite rims suggest that retrograde re-equilibration occurred at least 20–140°C below the T of the main metamorphism.  相似文献   

11.
Empirical geothermometer dealing with Ti solubility in the Fe‐Mg biotites was originally proposed for biotites in graphitic, peraluminous metapelites containing ilmenite or rutile that equilibrated roughly at 4–6 kbar. Given that biotites are abundant in the porphyry copper systems, this geothermometer has frequently been used for the determination of magmatic–hydrothermal temperatures in the porphyry copper systems. Common associations of porphyry copper deposits (PCDs), that is, low Al content of biotite, biotite chloritization (causes the biotite to become more magnesian and to lose Ti), and biotite formation by amphibole replacement, as well as disequilibrium, local equilibrium, or re‐equilibration of biotites, especially through potassic alteration, may provide significant uncertainty in the temperatures estimated a by Ti‐in‐biotite geothermometer. In addition, besides the calibration range of thermometer for pressure (400–600 MPa), the temperatures of major sulfide precipitation in PCDs (>~400°C) does not fit with the temperature range of thermometer calibration (480–800°C). Worth noting, as confirmed by fluid inclusion data in the Sarkuh PCD, regardless of presence of mineralogical requirements, obtained temperatures of sulfide mineralization using Ti in biotite thermometer could be overestimated. This may be due to the difference between general conditions of sulfide mineralization and calibration range of Ti in the biotite thermometer for pressure and temperature, as well as the metaluminous nature of biotites in PCDs.  相似文献   

12.
在对胶北荆山群麻粒岩相富铝岩石中石榴石、黑云母的成分环带进行深入研究基础上,选取不同粒径、与不同矿物相邻的石榴石、黑云母各微区点成分,利用石榴石-黑云母温度计分别进行了温度估算。确定在黑云母含量较高的岩石(V_(Grt)/V_(Bt)≤1)中,利用大颗粒石榴石(d≥1500μm)晶体核部(或靠近长英质矿物一侧的晶体幔部)成分与基质中远离石榴石等镁铁矿物处于长英质矿物之间的黑云母核部成分配合。通过石榴石-黑云母温度计可以获得相当可信的变质峰期温度。但是对于黑云母含量极低的岩石(V_(Grt)/V_(Bt)≥6),由于黑云母的成分普遍遭到了强烈改造。使得温度估算结果异常偏低,因此不适合采用石榴石-黑云母温度计估算峰期温度。同一岩石中,采用不同的相邻石榴石-黑云母矿物对晶体边缘成分获得的温度值差异较大,反映它们在峰期后发生Fe-Mg交换反应并达到封闭温度平衡状态的程度不同,因此利用石榴石-黑云母温度计难以获得准确的封闭温度。通过热力学计算,建立了一个新的石榴石-黑云母温度计公式。确定胶北荆山群所经历的变质峰期温度为720～770℃,峰期后最低相对封闭温度为480～500℃。  相似文献   

13.
The influences of temperature and environmental hypoxia on the growth rates of two California anadromous fishes, white sturgeon (Acipenser transmontanus) and striped bass (Morone saxatilis) were examined. Fish (0.5–0.6 g initial weight) were fedad libitum rations ofArtemia in flow-through aquaria regulated for temperature (15, 20, and 25°C) and oxygen tension (130 and 90 torr Po 2). Growth of sturgeon was significantly greater at 20 °C compared with 15 °C, but there was no difference between 20 and 25 °C. Striped bass growth increased with each 5° increment of temeprature elevation to 3.2% body weight per d at 25 °C, the fastest growth rate measured. The temperature of maximum growth reflected the temperature of the native estuarine rearing area. Environmental hypoxia (90 torr Po 2) reduced growth of sturgeon within each temperature level, whereas striped bass growth was reduced by hypoxia only at the upper two temperatures. Sturgeon were much more active in the growth chambers than striped bass. Sturgeon activity increased with each 5 °C temperature increase under normoxia and hypoxia, except at 25 °C (hypoxia) where activity was insignificantly different from that at 20 °C (hypoxia).  相似文献   

14.
The degree of graphitization of carbonaceous material (CM) has been widely used as an indicator of metamorphic grade. Previous work has demonstrated that peak metamorphic temperature (T) of regional metamorphic rocks can be estimated by an area ratio (R2) of peaks recognized in Raman spectra of CM. The applicability of this method to low‐pressure (<3 kbar) contact metamorphism was tested using Raman spectroscopic analyses of samples from two contact‐metamorphic aureoles in Japan (Daimonji and Kasuga areas). A suitable measurement procedure allows the dependence of the geothermometer on sample type (thin section, chip) and incident angle of laser beam relative to the c‐axes of CM to be tested. Two important general results are: (i) in addition to standard thin sections, chips are also suitable for spectral analysis; and (ii) the incident angle of the laser beam does not significantly affect the temperature estimation, i.e. spectral measurements for the geothermometer can be carried out irrespective of the crystallographic orientation. A laser wavelength of 532 nm was used in this study compared with 514.5 nm in an independent previous study. A comparison shows that the use of a 532‐nm laser results in a slightly, but systematically larger R2 ratio than that of a 514.5‐nm laser. Taking this effect into account, our results show that there is a slight but distinct difference between the R2–T correlations shown by contact and regional metamorphic rocks: the former are slightly better‐crystallized (have slightly lower R2 values) than the latter at the same temperature. This difference is interpreted as due to the degree of associated deformation. Despite the slight difference, the results of this study coincide within the estimated errors of ±50 °C with those of the previously proposed Raman CM geothermometer, thus demonstrating the applicability of this method to contact metamorphism. To facilitate more precise temperature estimates in regions of contact metamorphism, a new calibration for analyses using a 532‐nm laser is derived. Another important observation is that the R2 ratio of metamorphosed CM in pelitic and psammitic rocks is highly heterogeneous with respect to a single sample. To obtain a reliable temperature estimate, the average R2 value must be determined by using a substantial number of measurements (usually N > 50) that adequately reflects the range of sample heterogeneity. Using this procedure (with 532‐nm laser) and adapting our new calibration, the errors of the Raman CM geothermometer for contact metamorphic rocks decrease to ～±30 °C.  相似文献   

15.
Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (Tcalc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (Vgarnet/Vbiotite) and the cooling rate. For garnets with radii of 1 mm and Vgarnet/Vbiotite<1, Tcalc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For Vgarnet/Vbiotite= 1 and 4 and a cooling rate of 10° C/Ma, Tcalc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher Tcalc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below Tcalc of the garnet core, provided that Vgarnet/Vbiotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (Tmax > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.  相似文献   

16.
Four new formulations of the garnet-clinopyroxene geothermometer (Ellis and Green 1979; Ganguly 1979; Saxena 1979; Dahl 1980) have been evaluated in the Adirondacks and five other granulite terranes using results from 94 mineral pairs. The Saxena and Ganguly formulations give temperatures that are generally 100–150° C above those constrained by phase equilibria and other independent thermometry while the empirical calibration of Dahl gives widely scattered, erratic results. Despite some scatter in the data, the Ellis and Green calibration appears to be more accurate and precise than the others and is the most useful garnet-clinopyroxene thermometer currently available for quantitative thermometry in granulites. All four formulations are sensitive to large variations (>70–80° C) in temperature suggesting that problems with accuracy and precision can be improved with further refinement of model-based aspects of the thermometers.  相似文献   

17.
A precise olivine-augite Mg-Fe-exchange geothermometer   总被引：4，自引：1，他引：4  
 Olivine and augite that were experimentally equilibrated in the temperature interval 1175–1080°C at 1 bar in natural basaltic and andesitic bulk compositions are used to calibrate an Mg-Fe2+ cation-exchange geothermometer. Within its temperature interval of experimental calibration, and over a broad range in olivine Mg/Fe ratio, the geothermometer has a standard error of ±6°C. In compositionally simpler synthetic systems, the same calibration retrieves appropriate experimental temperatures up to at least 1250°C. In application to intermediate and felsic volcanic rocks erupted at ∼1080 –800°C (below the range of experimental calibration), calculated olivine-augite temperatures are in good agreement with Fe-Ti oxide thermometry in the same samples. These results encourage confidence in the olivine-augite geothermometer over at least the 800–1250°C interval at low pressures. Sparse experimental data up to 1250°C at higher pressures for olivine + augite in the assemblage olivine + plagioclase + augite ± pigeonite or orthopyroxene suggest that the low-pressure calibration recovers experimental temperatures without systematic bias to pressures of 10 kbar. Examples illustrate applications to determining igneous equilibration temperatures in holocrystalline extrusive and intrusive rocks, and to estimating intratelluric H2O content dissolved in magmas. Received: 24 February 1995 / Accepted: 1 March 1996  相似文献   

18.
The revised titanium-in-zircon geothermometer was applied to Paleoproterozoic ultrahigh-temperature (UHT) granulites at Tuguiwula, Inner Mongolia, North China Craton. The Tuguiwula granulites contain diagnostic UHT mineral assemblages such as sapphirine + quartz and high alumina orthopyroxene + sillimanite + quartz, suggesting formation under temperatures of ca. 1,000°C and pressures of up to 10 kbar. Here, we report detailed petrographic studies and ICP-MS data on titanium concentration in zircons associated with the UHT assemblages. The zircons associated with sapphirine–spinel–sillimanite–magnetite assemblages have the highest Ti concentration of up to 57 ppm, yielding a temperature of 941°C, and suggesting that the growth of zircons occurred under ultrahigh-temperature conditions. The maximum temperatures obtained by the revised Ti-in-zircon geothermometer is lower than the equilibrium temperature of sapphirine + quartz, indicating an interval of cooling history of the granulites from UHT condition to ca. 940°C. Many of the zircons have Ti concentrations ranging from 10 to 33 ppm, indicating their growth or recrystallization under lower temperatures of ca. 745–870°C. These zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz, and orthopyroxene–cordierite symplectite around garnet. Previous geochronological study on the zircons of the Tuguiwula UHT granulites gave a mean U–Pb SHRIMP age of 1.92 Ga. However, based on the Ti-in-zircon geothermometer results reported in this work, and considering the relatively slow thermal relaxation of these rocks, we infer that the timing of peak UHT metamorphism in the Tuguiwula area could be slightly older than 1.92 Ga.  相似文献   

19.
A Report on a Biotite-Calcic Hornblende Geothermometer   总被引：1，自引：0，他引：1  
This paper presents a biotite-calcic hornblende geothermometer which was empirically calibrated based on the gamet-biotite geothermometer and the gamet-plagioclase-hornblende-quartz geobarometer, in the ranges of 560-800℃ (T) and 0.26-1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe-Mg-AlVI-Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe-Mg binary solid solution. The resulting thermometer may rebuild the input garnet-biotite temperatures well within an uncertainty of ±50℃. Errors of ±0.2 GPa for input pressure, along with analytical errors of ?% for the relevant mineral compositions, may lead to a random error of ±16℃ for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assume  相似文献   

20.
The Triassic Dehnow pluton of NE Iran is a garnet-bearing I-type calc-alkaline metaluminous diorite-tonalite-granodiorite intrusion. The parental magma formed as the result of partial melting of intermediate to felsic rocks in the lower crust. Petrological and geochemical evidence, which indicates a magmatic origin for the garnet, includes: large size (~10–20 mm) of crystals, absence of reaction rims, a distinct composition from garnet in adjacent metapelitic rocks, and similarity in the composition of mineral inclusions (biotite, hornblende) in the garnet and in the matrix. Absence of garnet-bearing enclaves in the pluton and lack of sillimanite (fibrolite) and cordierite inclusions in magmatic garnet suggests that the garnet was not produced by assimilation of meta-sedimentary country rocks. Also, the δ18O values of garnet in the pluton (8.3–8.7‰) are significantly lower than δ18O values of garnet in the metapelitic rocks (12.5–13.1‰). Amphibole-plagioclase and garnet-biotite thermometers indicate crystallization temperatures of 708°C and 790°C, respectively. A temperature of 692°C obtained by quartz-garnet oxygen isotope thermometry points to a closure temperature for oxygen diffusion in garnet. The composition of epidote (Xep) and garnet (Xadr) indicates ~800°C for the crystallization temperature of these minerals. Elevated andradite content in the rims of garnet suggests that oxygen fugacity increased during crystallization.  相似文献   

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