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1.
The Neoproterozoic granite of Gabal Abu Diab, central Eastern Desert of Egypt, comprises mainly garnet-bearing granite and alkali feldspar granite intruded into calc-alkaline granodiorite–tonalite and metagabbro–diorite complexes. The garnet-bearing granite is composed mainly of plagioclase, K-feldspar, quartz, garnet and primary muscovite ± biotite. The presence of garnet and primary muscovite of Abu-Diab granite suggests its highly fractionated character. Geochemically, the garnet-bearing granite is highly fractionated as indicated from the high contents of SiO2 (74.85–77.5%), alkalis (8.27 to 9.2%, Na2O+K2O) and the trace elements association: Ga, Zn, Zr, Nb and Y. This granite is depleted in CaO, MgO, P2O5, Sr and Ba. The alumina saturation (Shand Index, molar ratio A/CNK) of 1.0 to 1.1 indicates the weak peraluminous nature of this garnet-bearing granite. The geochemical characteristics of the Abu Diab garnet-bearing granite are consistent with either the average I-type or A-type granite and also suggest post-orogenic or anorogenic setting. A fluid inclusions study reveals the presence of three fluid generations trapped into the studied granite. The earlier is a complex CO2–H2O fluid trapped in primary fluid inclusions with CO2 contents >?60 vol.%. These inclusions were probably trapped at minimum temperature >?400°C and minimum pressure >?2 kb. The second is immiscible water–CO2 fluid trapped in secondary and/or pseudo-secondary inclusions. The trapping conditions were estimated at temperature between 400°C and 170°C and pressure between 900 and 2000 bar. The latest fluid is low-salinity aqueous fluid trapped in secondary two-phase and mono-phase inclusions. The trapping conditions were estimated at temperature between 90°C and 160°C and pressure <?900 bar. The origin of the early fluid generation is magmatic fluid while the second and third fluids are of hydrothermal and meteoric origin, respectively.  相似文献   

2.
The a–x relations recently presented in White et al. ( 2014 , Journal of Metamorphic Geology, 32, 261–286) are extended to include MnO. This provides a set of internally consistent a–x relations for metapelitic rocks in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn‐bearing minerals were estimated using the micro‐? approach of Powell et al. ( 2014 , Journal of Metamorphic Geology, 32, 245–260). Then the Mn‐end‐member thermodynamic properties were calibrated using a database of co‐existing minerals involving literature data from rocks and from experiments on natural materials. Mn‐end‐members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end‐member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet‐bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet‐in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P–T range outside its predicted Mn‐free P–T range.  相似文献   

3.
The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M1 overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.  相似文献   

4.
Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–O2–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.  相似文献   

5.
Interpretations based on quantitative phase diagrams in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3–SiO2–H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).  相似文献   

6.
Eclogite of high‐pressure low‐temperature origin occurs within the Great Serpentine belt of New South Wales. The presence of glaucophane‐bearing rocks and other medium to high‐pressure assemblages associated with the belt is similar in many respects to the Californian and Oregon occurrences. The chemical composition of the eclogite is characterized by low K2O values comparable to many oceanic tholeiites, although one analysis is nepheline‐normative. Ti‐Zr‐Y ratios also show affinities to ocean‐floor basalts.

The garnet contains approximately 30% grossular and is strongly zoned from almandine (Alm 56%, Py 9%) at the core towards pyrope (Alm 44%, Py 27%) at the margin. Sodic augite contains 30–33% Jd, 4–7% Ac, and 72–74% Di+He.

Distribution of Fe and Mg between co‐existing garnet and pyroxene would suggest an increasing temperature during eclogite crystallization with a possible range from 290°C to 600°C and a minimum pressure of 7–12 kb.  相似文献   

7.
The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80–77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2(71.68%–72.47%), K2O(4.73%–5.54%), total alkali(K2O + Na2O = 8.21%–8.53%), K2O/Na2O ratios(1.36–1.94) and low P2O5(0.13%–0.17%), with A/CNK of 1.025–1.055; enriched in U, Th, and K, depleted in Ba, Nb, Sr, Ti, P and Eu. They are highly fractionated, slightly peraluminous I-type granite. The two samples of the Kejie pluton give a large variation of εHf(t) values(-5.04 to 1.96) and Hf isotope crustal model ages of 1.16–1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock(801°C–823°C) show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.  相似文献   

8.
Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (XGrs = 0.25–0.29), but lower pyrope‐content (XPrp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (XCaGrt = 0.18–0.21) and symplectite clinopyroxene (XNaCpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (XMg = 0.86–0.87) and spinel (XMg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.  相似文献   

9.
Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H2O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.  相似文献   

10.
The Chakradharpur Granite—Gneiss complex (CKPG) is exposed as an elliptical body within the arcuate metamorphic belt sandwiched between the Singhbhum Granite in the south and the Chotonagpur Granite—Gneiss to the north. It consists of an older bimodal suite of grey gneiss and amphibolites, intruded by a younger unit of pegmatitic granite. The bimodal suite represents the basement to the enveloping metasediments.The average major-element chemistry of the grey gneiss conforms to the definition of trondhjemite and includes both low-Al2O3 and high-Al2O3 types. The amphibolites can be grouped into a low-MgO and a high-MgO type. Rocks of the younger unit range in composition from granodiorite to quartz monzonite. The two granitic units also differ significantly in their Rb, Sr and Ba contents, and in the REE distribution pattern. The grey gneiss shows a highly fractionated REE pattern and a distinct positive Eu anomaly, with Eu/Eu* values increasing with increase in SiO2 %. In samples of the younger granite, the REE pattern is less fractionated, with higher HREE abundance relative to the grey gneiss and usually shows a negative Eu anomaly. The two types of REE patterns in amphibolites are interpreted to represent the two broad groups identified on the basis of major element chemistry.On the basis of chemical data, a two-stage partial melting model for the genesis of grey gneiss is suggested, involving separation of hornblende and varying amounts of plagioclase in the residual phase. Varying amounts of plagioclase in the residuum result in the wide range of Al2O3 content of the partial melt from which the trondhjemites crystallised. Residual hornblende produces the highly fractionated REE pattern, but a relatively higher HREE content of the trondhjemites compared to those produced by separation of garnet in the residual phase. The high level of Ba together with moderate levels of Sr in the trondhjemites can also be explained by plagioclase in the residue, whose effectiveness in partitioning Ba compared to Sr is lower. Of the two groups of amphibolites, the low-MgO type shows relative depletion of LREE compared to the high-MgO type. It contains varying amounts of plagioclase and is tentatively suggested to represent the residue. The other group, with a slightly fractionated REE pattern (CeN/ YbN = 2.01), is generally considered to represent the source material for the trondhjemites. This may have been generated by 5–15% partial melting of mantle peridotites, containing higher concentrations of LIL elements than those which produced average Precambrian tholeiites. This first phase of partial melting resulted in the slightly fractionated REE pattern of these amphibolites. Derivation of the younger granitic unit from the trondhjemites can be ruled out on the basis of REE data alone. REE data suggest partial melting of metasediments to be the origin of these rocks. It is also possible that deeply buried volcanic rocks, similar to calc-alkaline components of greenstone belts, are the parent for this component.  相似文献   

11.
Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica...  相似文献   

12.
Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals.The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO2, Al2O3 and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al2O3 and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.  相似文献   

13.
The tin‐ and tantalum‐bearing pegmatites of the Bynoe area are located in the western Pine Creek Geosyncline. They are emplaced within psammopelitic rocks in the contact aureole of the Two Sisters Granite. The latter is a Palaeoproterozoic, fractionated, granite with S‐type characteristics and comprises a syn‐ to late‐orogenic, variably foliated, medium‐grained biotite granite and a post‐orogenic, coarse‐grained biotite‐muscovite granite. The pegmatites comprise a border zone of fine grained muscovite + quartz followed inward by a wall zone of coarse grained muscovite + quartz which is in turn followed by an intermediate zone of quartz + feldspar + muscovite. A core zone of massive quartz is present in some occurrences. Feldspars in the intermediate zone are almost completely altered to kaolinite. This zone contains the bulk of cassiterite, tantalite and columbite mineralization. Fluid inclusions in pegmatitic quartz indicate that early Type A (CO2 + H2O ± CH4) inclusions were trapped at the H2O‐CO2 solvus at P～100 MPa, T～300°C (range 240–328°C) and salinity ～6 wt% eq NaCl. Pressure‐salinity corrected temperatures on Type B (H2O + ～20% vapour), C (H2O + < 15% vapour) and D (H2O + halite + vapour) inclusions also fall within the range of Type A inclusions. Oxygen and hydrogen isotope data show that kaolin was either formed in isotopic equilibrium with meteoric waters or subsequent to its formation, from hydrothermal fluid, underwent isotopic exchange with meteoric waters. Fluid inclusion waters from core zone quartz show enrichment in deuterium suggesting metamorphic influence. Isotope values on muscovite are consistent with a magmatic origin. It is suggested that the pegmatites were derived from the post‐orogenic phase of the Two Sisters Granite. Precipitation of cassiterite took place at about 300°C from an aqueous fluid largely as a result of increase in pH due to feldspar alteration.  相似文献   

14.
Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (XMg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of XMg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.  相似文献   

15.
Mineral textures, coupled with thermodynamic modelling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) model system, of mid‐amphibolite facies metapelites from the Georgetown Inlier, northeast Australia, point to the recording of two separate and unrelated metamorphic events. The first occurred contemporaneously with Palaeo‐ to Mesoproterozoic orogenesis and involved heating and burial to temperatures and pressures of approximately 600–650 °C and 6.0–7.0 kbar. Textural evidence for the up‐temperature (and pressure) prograde part of this path is inferred from the inclusion of garnet in biotite and staurolite. The second metamorphic event resulted in a low‐pressure thermal overprint that is equated with the advective addition of heat to the terrane via the emplacement of the Forsayth Batholith (c. 1550 Ma). This event is inferred from subsequent growth of andalusite and randomly orientated fibrolitic sillimanite after garnet, biotite and staurolite. This two stage metamorphic evolution, when coupled with a number of other distinct geological characteristics, suggests that the Georgetown Inlier is dissimilar to the other Australian Palaeoproterozoic terranes with which it is commonly correlated.  相似文献   

16.
Petrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India‐Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite‐grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction‐related granite magmatism before the India‐Asia collision, and a kyanite‐grade overprint of sillimanite‐grade rocks with peak P–T conditions of ～7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India‐Asia collision. A kyanite‐grade event observed in the Baltoro region with similar peak P–T conditions (～7.4–8.0 kbar, ～640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite‐grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (～30 °C km?1) and grew during a significant increase in pressure of ～2.6 kbar over a temperature increase of ～100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over‐accretion.  相似文献   

17.
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.  相似文献   

18.
Metapelitic residual enclaves in the Neogene Volcanic Province of SE Spain are residues left after melt extraction. Glass (quenched melt) of granitic composition occurs as inclusions in most minerals and as intergranular pockets. The most common enclave types show one stage of garnet growth that is interpreted to have occurred at the same time as glass production. Some of these show a well‐developed foliation outlined by fibrolite, biotite, graphite and glass, which wraps around elongate garnet crystals that have aspect ratios up to 10:1. Based on microstructures and chemistry, the garnet within these rocks shows clear core and mantle structure. The core has an average composition of Alm76–Prp08–Sps14–Grs03 and contains primary inclusions of biotite and melt, trapped during garnet growth. A thin (c. 100 μm), irregular mantle overgrows the garnet core, enclosing oriented fibrolite inclusions in strain caps, and biotite in strain shadows. In places, the overgrowths form skeletal elongated arms, which extend parallel to the foliation. The garnet mantle contains less Mn and higher XMg, but both core and mantle display flat Mn profiles, the contact being a sharp break. Ternary feldspar and Grt–Bt thermometry yield temperatures in the range 800–900 °C, with no systematic differences among the different microstructural domains of elliptical garnet. Based on the observed intracrystalline microstructures, the high amount of melt extraction in the rock by flattening component strain and the chemical zoning of garnet, the formation of elliptical garnet is modelled by a multistage sequence. This involves pressure solution and reprecipitation of the core, followed by post‐kinematic, partly mimetic growth of the garnet mantle.  相似文献   

19.
Prograde P–T–t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P–T trajectories, M1–2 and M3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M1) consist of garnet+omphacite+barroisite/Mg‐pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M2 assemblages of garnet+omphacite+barroisite+phengite+rutile±quartz. The inclusion‐rich inner part of garnet porphyroblasts preserves a bell‐shaped Mn profile of the M1, whereas the inclusion‐poor outer part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions estimated from two independent geothermobarometers: the garnet–clinopyroxene–phengite (~25 ± 3 kbar and 660 ± 100°C) and Zr‐in‐rutile (~650–700°C at 22–27 kbar). The second segment (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine‐grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high‐resolution ion microprobe U–Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (<2 mm/year) for cold subduction regime (~5–10°C/km), suggesting that an exhumation stage intervened between two prograde segments of P–T path. Thus, the P–T–t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.  相似文献   

20.
Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S2 assemblages with garnet + staurolite and post-S2 assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S2 in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S2 regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.  相似文献   

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