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1.
Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.  相似文献   

2.
The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.  相似文献   

3.
The late Mesozoic and Cenozoic metamorphic evolution of the western North American continental margin is recorded in a belt of homogeneous metapelitic rocks, the Kluane metamorphic assemblage (KMA), in the northern Coast Belt of Yukon Territory. A record of Late Cretaceous medium‐pressure and ‐temperature (c. 7 kbar, 500 °C) metamorphism, M1, is preserved in Ca‐rich garnet and Na‐rich plagioclase cores in rocks that were little affected by later events. M1 was synchronous with mylonitization and is attributed to accretion of the KMA to the ancient continental margin. Isothermal decompression during rapid uplift was followed by early Eocene emplacement of the Ruby Range Batholith (RRB), part of a magmatic arc produced by subduction of the Kula plate. The intrusion of the RRB led to a contact metamorphic overprint, M2, producing a 5–6 km wide aureole in which the grade ranges from subgarnet zone to garnet–cordierite–K‐feldspar zone. Pressure and temperature estimates for M2, calculated from mineral equilibria, are 3.5–4.5 kbar and 530–720 °C, generally consistent with the stability limits of the observed mineral assemblages. Comparison of mineral assemblages and P–T conditions in the KMA with those in the Mclaren Glacier metamorphic belt in Alaska does not support the correlation of the two metamorphic sequences. This weakens the hypothesis proposing 400 km of dextral slip along the Denali fault zone.  相似文献   

4.
Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P?T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P?T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.  相似文献   

5.
High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.  相似文献   

6.
The Madurai Block, southern India, lies between the Palghat-Cauvery and the Achankovil shear zones. The Karur area represents a portion of the granulite-facies terrain of the Madurai block. High-pressure (HP) and ultrahigh-temperature (UHT) mafic granulites have been found as enclaves within the gneisses. The peak assemblage (M1) consists of garnet, orthopyroxene, clinopyroxene, quartz, and plagioclase. Garnet breaking down during isothermal decompression is indicated by the development of pyroxene+plagioclase symplectites, which characterize the M2 stage of metamorphism. Late stage hornblende-plagioclase symplectites rimming garnet is related to the decompression-cooling M3 stage of metamorphism. Peak metamorphism M1 occurs at ~12 kbar pressure and temperatures in excess of 1,000°C. This was followed by a retrograde M2 stage when the mafic granulites suffered isothermal decompression to 6 kbar to 7 kbar at 800–900°C. At the terminal retrograde stage M3 solid-melt back reaction took place at 4.5–5.5 kbar and 650–700°C. The proposed clockwise P-T path implies that rocks from the study area could have resulted from thickened continental crust undergoing decompression. The SHRIMP data presented here from the Karur area provide evidence for a Neoproterozoic (521?±?8 Ma) metamorphic event in the Madurai block. The formation of symplectic assemblages during near isothermal decompression can be attributed to tectonic activity coinciding with the Pan-African phase of a global orogeny.  相似文献   

7.
The metamorphic evolution of micaschists in the north‐eastern part of the Saxothuringian Domain in the Central European Variscides is characterized by the early high‐pressure M1 assemblage with chloritoid in cores of large garnet porphyroblasts and a Grt–Chl–Phe–Qtz ± Pg M2 assemblage in the matrix. Minerals of the M1–M2 stage were overprinted by the low‐pressure M3 assemblage Ab–Chl–Ms–Qtz ± Ep. Samples with the best‐preserved M1–M2 mineralogy mostly appear in domains dominated by the earlier D1 deformation phase and are only weakly affected by subsequent D2 overprint. Thermodynamic modelling suggests that mineral assemblages record peak‐pressure conditions of ≥18–19 kbar at 460–520 °C (M1) followed by isothermal decompression 10.5–13.5 kbar (M2) and final decompression to <8.5 kbar and <480 °C (M3). The calculated peak P–T conditions indicate a high‐pressure/low‐temperature apparent thermal gradient of ～7–7.5 °C km?1. Laser ablation inductively coupled plasma mass spectrometry isotopic dating and electron microprobe chemical dating of monazite from the M1–M2 mineral assemblages give ages of 330 ± 10 and 328 ± 6 Ma, respectively, which are interpreted as the timing of a peak pressure to early decompression stage. The observed metamorphic record and timing of metamorphism in the studied metapelites show striking similarities with the evolution of the central and south‐western parts of the Saxothuringian Domain and suggest a common tectonic evolution along the entire eastern flank of the Saxothuringian Domain during the Devonian–Carboniferous periods.  相似文献   

8.
A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine ＋ quartz assemblage from two localities （Tonagh Island （TI） and Priestley Peak （PP）） in the Napier Complex,East Antarctica,provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean.The reaction microstructures associated with sapphirine ＋ quartz vary among the samples,probably suggesting different tectonic conditions during the metamorphic evolution.Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage,but now separated by corona of Grt ＋ Sil ＋ Opx suggesting near isobaric cooling after the peak metamorphism,whereas the Spr ＋ Qtz ＋ Sil ＋ Crd ＋ Spl assemblage replaces garnet in PP sample suggesting post-peak decompression.The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr ＋ Qtz stability is lowered down to 930 ℃ due to small Fe3＋ contents in the rocks （mole Fe2O3/（FeO ＋ Fe2O3） =0.02）.The TI sample yields a peak p-T range of 950-1100 ℃ and 7.5-11 kbar,followed by cooling toward a retrograde stage of 800-950 ℃ and 8-10 kbar,possibly along a counterclockwise p-T path.In contrast,the peak condition of the PP sample shows 1000-1050 ℃ and ＞12 kbar,which was followed by the formation ofSpr ＋ Qtz corona around garnet at 930-970 ℃ and 6.7-7.7 kbar,suggesting decompression possibly along a clockwise p-T trajectory.Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks.The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.  相似文献   

9.
High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time-deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re-equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re-equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) re-equilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to ‘Andean-type’ underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a ‘Cascadia-type’ active margin, which corresponds to a supra-subduction tectonic switching paradigm.  相似文献   

10.
Northward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure (UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a?1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.  相似文献   

11.
Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. This P–T evolution is consistent with low to medium aH2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north of In Ouzzal.  相似文献   

12.
The Chinese Altai orogen formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), and the study on the metamorphism will provide novel and robust constraints on its tectonic evolution. In this study, we investigate our newly recognized garnet–orthopyroxene–cordierite granulites at Wuqiagou area in the southern Chinese Altai. Detailed petrographic study and P–T estimates suggest four distinct metamorphic stages of mineral assemblages: (1) pre–peak (M1) stage containing the spinel–cordierite–bearing association or biotite–plagioclase–quartz–bearing inclusion–phase assemblage, with P–T conditions of 3.0–4.0 kbar/700–750 °C; (2) peak ultrahigh–temperature (UHT) (M2) stage represented by relatively coarse–grained garnet–orthopyroxene–cordierite–bearing porphyroblastic assemblage, with high–Al2O3 contents (up to ∼8.7 wt%) in orthopyroxene and P–T conditions of ∼8.0 kbar/∼980 °C; (3) post–peak high–temperature granulite facies (M3) stage consisted of orthopyroxene–cordierite and cordierite–quartz corona assemblages, formed during cooling and moderate decompression; and (4) post–peak upper amphibolite facies (M4) stage represented by retrograde biotite–plagioclase–quartz intergrowths. These four discrete metamorphic stages define an anticlockwise P–T path involving a post–peak moderate decompression followed by nearly isobaric cooling process. LA–ICP–MS U–Pb age dating results of metamorphic zircons for UHT samples show two weighted mean ages of ∼390 Ma and ∼280 Ma. We propose that the M1 stage might occur in the middle Devonian, whereas the near–peak UHT stage probably occurred in the early Permian. The Permian UHT metamorphism was further supported by the monazite U–Th–Pb dating results (287.9 ± 2.1 Ma), reflecting a prominent HT–UHT reworking event in the late Paleozoic. We proposed that the Permian UHT reworking event in the southern Chinese Altai probably occurred in a post–orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of deep–derived magma as a result of the Tarim mantle plume activity.  相似文献   

13.
The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.  相似文献   

14.
The Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton. The dominant rock types in the Fuping Complex are high‐grade tonalitic–trondhjemitic–granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks. The petrological evidence from the mafic granulites indicates three stages of metamorphic evolution. The M1 stage is represented by garnet porphyroblasts and matrix plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende. Orthopyroxene+plagioclase symplectites and clinopyroxene+plagioclase±orthopyroxene coronas formed in response to decompression during M2 following the peak metamorphism at M1. Hornblende+plagioclase symplectites formed as a result of further isobaric cooling and retrograde metamorphism during M3. The P–T estimates using TWQ thermobarometry are: 900–950 °C and 8.0–8.5 kbar for the peak assemblage (M1), based on the core compositions of garnet, matrix pyroxene and plagioclase; 700–800 °C and 6.0–7.0 kbar for the pyroxene+plagioclase symplectites or coronas (M2); and 550–650 °C and 5.3–6.3 kbar for the hornblende+plagioclase symplectites (M3), based on garnet rim and corresponding symplectic mineral compositions. These P–T estimates define a clockwise P–T path involving near‐isothermal decompression for the Fuping Complex, similar to the P–T path estimated for the metapelitic gneisses. The inferred P–T path suggests that the Fuping Complex underwent initial crustal thickening, subsequent exhumation, and finally cooling and retrogression. This tectonothermal path is similar to P–T paths inferred for the Wutai and Hengshan Complexes and other tectonic units in the central zone of the North China craton, but different from anti‐clockwise P–T paths estimated for the basement rocks in the eastern and western zones of the craton. Based on lithological, structural, metamorphic and geochronological data, the eastern and western zones of the craton are considered to represent two different Archean to Paleoproterozoic continental blocks that amalgamated along the central zone at the end of Paleoproterozoic. The P–T paths of the Fuping Complex and other tectonic units in the central zone record the collision between the eastern and western zones that led to the final assembly of the North China craton at c. 1800 Ma.  相似文献   

15.
Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.  相似文献   

16.
LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ～6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.  相似文献   

17.
Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The  相似文献   

18.
High‐pressure (HP) metagreywacke from the Namche Barwa Complex, Eastern Himalayan Syntaxis (EHS), consists of garnet, biotite, plagioclase, quartz, rutile and ilmenite with or without K‐feldspar, sillimanite, cordierite, spinel and orthopyroxene. Two types of metagreywacke are recognized: medium‐temperature (MT) and high‐temperature (HT) types. Garnet in the MT metagreywacke shows significant growth zoning and contains lower MgO than the weakly zoned garnet in the HT metagreywacke. Petrographic observations and phase equilibria modelling for four representative samples indicate that both types of metagreywacke experienced clockwise P–T paths subdivided into three stages: stage I is the pre‐peak prograde to pressure peak (Pmax) stage characterized by progressive increase in P–T conditions. The Pmax conditions are estimated using the garnet composition with maximum CaO, being 12.5–13.5 kbar and 685–725 °C for the MT metagreywacke, and 15–16 kbar and 825–835 °C for the HT one. Stage II is the post‐Pmax decompression with heating or near‐isothermal to Tmax stage and the Tmax conditions, constrained using the garnet compositions with maximum MgO, are 11 kbar and 760 °C for the MT metagreywacke, and ~12 kbar and 830–845 °C for the HT one. The modelled mineral assemblages at Tmax are garnet + biotite + K‐feldspar + rutile + plagioclase ± ilmenite in the presence of melt for both types of metagreywacke, consistent with the petrographic observations. Stage III is the post‐Tmax retrograde metamorphism, characterized by decompression and cooling. The modelling suggests that the melts with high Na/K ratios (1.7–5.2) have been produced during stages I and II, which could be responsible for the formation of sodium‐rich leucogranites. This study and previous results indicate that the Higher Himalayan Crystallines in the EHS consist of MT–HP and HT–HP metamorphic units separated by a speculated tectonic contact. Petrological and structural discontinuities within the EHS cannot be easily interpreted with ‘tectonic aneurysm’ model.  相似文献   

19.
New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.  相似文献   

20.
Eclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (XSps 0.80–0.24, XAlm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (XSps 0.13–0.24, XAlm 0.36–0.45), aegirine‐augite/omphacite (XJd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (XJd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (XJd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).  相似文献   

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