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1.
. The granulite complex of Paderu, in the south central sector of the Eastern Ghats belt, India, consists of closely related pelitic granulites and peraluminous granitoids which could be linked via dehydration melting in pelitic and greywacke-like precursors. The pelitic granulites, including high-Mg-Al sapphirine granulites with early deformation microstructures, also record a high-temperature decompression from ~10 to ~8 kbar at ~1,000 °C, preceding isobaric cooling from above 900 to ~600 °C at 8 kbar. Highly magnesian biotite in the pelitic granulites, the presence of spinel in some of the granitoids, and granitoids of two distinct compositions, namely granite and quartz-monzonite, all suggest dehydration melting in highly magnesian pelitic and greywacke-like precursors. Moreover, high-temperature melting in highly magnesian pelitic precursors is indicated by the migmatitic spinel-bearing layers which, besides having significant abundance of quartz and feldspar, also contain aluminous orthopyroxene and cordierite. These melting reactions, occurring above 9 kbar, may constrain the prograde arm of the P-T trajectory. This and the high-temperature decompression constitute a clockwise P-T path. This clockwise P-T path is consistent with the tectonic model in which crustal thickening and granulite metamorphism in the Eastern Ghats belt is interpreted as the result of homogeneous shortening in a compressional setting.  相似文献   

2.
Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (XMg>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.  相似文献   

3.
A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.  相似文献   

4.
Evidence collated from different parts of the Eastern Ghats belt north of the Godavari rift (barring the “Western Charnockite Zone” ) indicates that this sector evolved through a series of compressive structures (F1 to F3), with prolific migmatization in quartzofeldspathic and metapelitic gneisses synchronous with F1 shortening, as was the syn-F1 emplacement of profuse megacrystic K-feldspar-bearing granitoid bodies. Thereafter, melt productivity of the rocks (synchronous withF 2– F3 folding) sharply decreased. Mineral parageneses stable in the S1, S2 and S3 fabrics indicate persistence of granulite facies conditions. P-T estimates on orthopyroxene + garnet + plagioclase + quartz assemblages anchored to recrystallized mosaic that overgrow all penetrative fabric elements in mafic granulites, granitoids and quartzofeldspathic gneisses are in the range of 900‡-950‡C and P≅ 8–9 kbar. This estimate is comparable to those retrieved from sapphirine-bearing paragenesis in Mg-Al metapelites that appear to be diachronous in relation to the fabric elements, and arguably disrupt the granoblastic mosaic. These facets in the northern sector of the orogenic belt are compatible with either a single cycle of tectonic events (i.e., F1, F2 and F3 in continuum), or temporally-separate thermo-tectonic events, with the peak of earlier metamorphism (pre- to syn-F1) at lower temperature (in the granulite facies) in comparison to the record of high post-F3-Tmax values. It is suggested on the basis of the above evidence that the late Proterozoic/Pan-African granulites in the Eastern Ghats belt north of the Godavari rift, are unlikely to be reworked equivalents of any older granulitic crust, such as the ∼1.6 Ga granulites south of the rift. Instead, the temporally disparate sectors may represent different crustal segments with unconnected pre-amalgamation tectonic history. However, if the ∼ 1.6 Ga granulites of the Western Charnockite Zone continue northwards across the rift, as suggested by recent isotope data, there are serious doubts as to the validity of a north-south division within the Eastern Ghats belt.  相似文献   

5.
The northern margin of the Eastern Ghats Mobile belt against the Singhbhum craton exposes granitic rocks with enclaves from both the high-grade and low-grade belts. A shear cleavage developed in the boundary region is also observed in these granitoids. Field features and petrography indicate syn-tectonic emplacement of these granitoids. Petrology-mineralogy and geochemistry indicate that some of the granitoids are derived from the high-grade protoliths by dehydration melting. Others could have been derived from low-grade protoliths. Moreover, microstructural signatures in these granitoids attest to their syn-collisional emplacement.  相似文献   

6.
《Gondwana Research》2000,3(1):105-117
Metapelitic migmatites in the Pangidi Granulite Complex, Eastern Ghats belt preserve a range of high-grade mineral assemblages that vary with bulk composition. Reaction textures preserved in the migmatites indicate that the crystallized granitic melt was formed in situ by biotite dehydration-melting reactions. Decompression occurred during an episode of partial melting and melt crystallization, and was synchronous with exhumation of migmatites along shear zones. Retrograde reaction textures and the calculated positions of the mineral reactions and discrete P-T points obtained by thermobarometry reflect ∼3 kbar (11 km) of exhumation from near peak P-T conditions of ∼9 kbar and 950°C.Decompression has been traditionally ascribed to be the cause of dehydration melting in many orogens, but the bulk of melting reactions in Pangidi were ultimately driven by elevated temperatures. Simple pressure release during exhumation did not force melting reactions, instead as the present study reveals, it is more likely that the presence of melt triggered deformation in the migmatites and facilitated exhumation.  相似文献   

7.
Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na2O-K2O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.  相似文献   

8.
Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.  相似文献   

9.
Summary Two assemblages have been identified in spinel-bearing metapelites from Rayagada, in the Eastern Ghats Belt, India: the first, an iron-rich assemblage, is characterized by iron-rich spinel and garnet; the second, iron-poor assemblage, by iron-poor spinel and garnet, together with cordierite and hematite. Garnet and sillimanite coronas around spinel show elongation parallel to the dominant fabric in the iron-rich assemblage but not in the iron-poor one. Both textures suggest the corona forming retrograde reaction spl + qtz = grt + sil. The P-T conditions for these corona-forming reactions are estimated at 950°C and 8.7-9.0 kbar for the iron-rich, and 800°C and 8 kbar for the iron-poor assemblage. Thermobarometric results and observed phase relations, using the model system FeO-MgO-Al2O3-SiO2 (FMAS) indicate near-isobaric cooling from 950° to 800°C and subsequent decompression from 8 to 6.5 kbar. The corona-forming event in the iron-rich assemblage has been correlated with the dominant second phase of deformation (D2), but that in the iron-poor assemblage apparently postdates D2.
Der retrograde P-T Pfad Spinell führender Metapelite in Rayagada, östliche Ghats, Indien
Zusammenfassung Zwei unterschiedliche Spinell-führende Mineralvergesellschaftungen in Metapeliten werden aus Rayagada in den östlichen Ghats Indiens beschrieben. Die erste, eine eisenreiche, ist durch Fe-reichen Spinell und Granat, die zweite eisenarme, durch Fe armen Spinell, Granat, Cordierit und Hämatit charakterisiert. Granat und Sillimanit Coronas um Spinell sind nur in der eisenreichen Vergesellschaftung parallel zum dominierenden metamorphen Gefüge elongiert, nicht aber in der eisenarmen. Beide Texturen belegen die retrograde Reaktion Spl + Qtz = Grt + Sil. Die P-T Bedingungen der Koronareaktionen in der Fe-reichen Assoziation werden mit ca. 950°C und 8.7-9.0 kbar, die in der Fe-armen mit ca. 800°C und 8 kbar abgeschätzt. Die Ergebnisse der Geothermobarometrie und die beobachteten Phasenbeziehungen im Modellsysem FeO-MgO-Al2O3-SiO2 (FMAS) belegen eine mehr oder weniger isobare Abkühlung von ca. 950 bis 800°C mit anschließender Dekompression von ca. 8 bis 6.5 kbar. Die koronabildende Raktion in der eisenreichen Vergesellschaftung wird mit der dominierenden zweiten (D2) Deformationsphase korreliert, überdauert diese aber in den Fe-armen Gesteinen.

With 5 Figures  相似文献   

10.
Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO2-N2 fluids (5–30 mol. % N2) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.  相似文献   

11.
Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO2–N2 inclusions (0.69–1.06 g cm?3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N2 contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO2 liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (XH2O≤0.7), which was masked by mechanical separation of the CO2- and H2O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.  相似文献   

12.
Stock-like granite plutons in the Eastern Ghats belt and their host granulites exhibit similar solid state fabric. Both mineralogically and chemically these plutons are peraluminous and granitic in composition with S-type granite affinities. The granite plutons are product of a variety of mica dehydration melting reactions, as evident from their (1) compositional plots in the (FeO + MgO) ?? Na2O.Al2O3 ?? K2O.Al2O3 (MNK) pseudoternary diagram and (2) trace element contents. Muscovite and/or biotite breakdown reaction with or without plagioclase are the characteristic melting reactions. Restitic signature is evident in the host metapelitic granulites, locally known as khondalites (Qtz-Kfs-Grt-Sil-Fe-Ti oxides-bearing gneisses). The compositional variability of the associated khondalites can be a result of (1) original compositional variation and (2) they represent different stages of restites. Trace element monitoring following restite separation model suggests that these granite plutons are largely saturated equilibrium melts. The granites are chemically discriminated as syn-collisional. The plutons could also be the product of segregation during the regional exhumation of the Eastern Ghats terrain.  相似文献   

13.
High Mg-Al spinel-sapphirine granulites, orthopyroxene-bearing quartzofeldspathic granulites, two pyroxene-bearing mafic granulites and metapelitic gneisses are exposed around Paderu, Eastern Ghats Belt. Geothermobarometry in orthopyroxene-bearing quartzofeldspathic granulites and mafic granulites indicate near isobaric cooling through 90°C from ca. 720°C to 630°C, at 8.0 kbar. However, signatures of ultrahigh temperature metamorphism are recorded from the mineralogy and reaction textures in the high Mg-Al granulites. Mineral reactions deduced in this work, when combined with others described by Lalet al (1987) from the same area and plotted in an appropriate petrogenetic grid in the system FMASO indicate an ACW path comprising a high dT/dP prograde arm reaching Pmax − Tmax = 9.5 kbar, ∼ 1000°C, followed by near-isobaric cooling down to 9 kbar, 900°C and subsequent decompressive reworking.  相似文献   

14.
Textural relations, thermobarometry and petrogenetic grid considerations in the syn-tectonic granitoid massif and the enveloping metasedimentary gneisses at Salur are consistent with a counter-clockwise P–T –t path for the rocks. The low-P/high-T prograde sector is documented by the pre- to syn-D1 cordierite±orthopyroxene±garnet±spinel–bearing metatexite leucosomes in metapelites. Heating and loading of the rocks (syn- to post-D1) resulted in the formation of garnet+orthopyroxene± cordierite-bearing diatexites, and decomposition of cordierite in metatexite leucosomes to orthopyroxene+sillimanite+biotite+quartz symplectites. Near-peak temperature, 850 °C at 8.0 kbar, was reached syn- to post-D2. Post-peak cooling resulted in the stabilization of coronal grossular and anorthite+calcite symplectites at the expense of scapolite+wollastonite+calcite assemblages in calc-silicate gneisses, and the resetting of cation exchange temperatures at 700–750 °C. Near-isothermal decompression at c. 700–750 °C is manifested by the decomposition of garnet porphyroblasts in the granitoid gneisses to plagioclase+orthopyroxene/ilmenite/biotite two-phase coronas and restabilization of cordierite at garnet margins in metapelites. Subsequent low-P, near-isobaric cooling led to the overprinting of granulite facies assemblages by muscovite+calcite assemblages, and further resetting of cation exchange thermometers to lower temperatures c. 600 °C. The tectonothermal evolution of the Salur gneiss complex vis-a-vis the Eastern Ghats Belt is therefore consistent with high degrees of lower crustal melting, followed by prograde heating of the cover rocks due to magma invasion synchronous with crustal compression, and finally thermal relaxation over a protracted period punctuated by tectonic/erosional denudation of the thickened crust.  相似文献   

15.
Massif type anorthosites at Bolangir, eastern India are emplaced at the vicinity of the proto-Indian craton—Eastern Ghats Granulite belt contact. Micro- and meso-structural evidences indicate that the emplacement of the anorthosite pluton and the adjoining granitoids was syn-tectonic with respect to the D3 deformation phase (950–1,000 Ma) in the host gneiss. Anisotropy of magnetic susceptibility confirms that magnetic fabrics within anorthosite were dominantly developed during D3 deformation. Emplacement of felsic melts in the N-S trending dilatant shear zones in the granitoids, Fe-Ti-Zr-REE rich melt bands along N-S trending shear zones and localized N-S magnetic foliation in anorthosite near the Fe-Ti-Zr-REE rich melt bands indicate change in the stress field from NNW-SSE (D3) to E-W (D4). Available geochronological and paleogeographic data coupled with the structural analyses of the intrusive and the host gneiss indicate that the emplacement of massif type anorthosite in the EGP is not related to the accretion of Eastern Ghats Granulite Belt over proto-Indian continent during late Neoproterozoic.  相似文献   

16.
An intrusive granitoid pluton into TTG-Dharwar Supergroup greenstone sequence is being reported for the first time from the Dharwar Foreland region. Based on field and petrographic characteristics, these granitoids are classified as - quartz-monzodiorites and granites. Occasional mafic bodies of dioritic-granodioritic composition with size ranging from small microgranular magmatic enclaves to bodies of several centimeters are common in these granitoids.The granitoids are devoid of any crystal-plastic fabric as well as high-strain characteristics. The textural (CSD) studies indicate that the quartz-monzodiorites are derived from magma mixing whereas the granites are derived from equilibrium crystallization of the magma derived from the reworking of quartz-monzodiorites. The P-T estimates indicate that the quartz-monzodiorites were crystallized at higher temperature (>950 °C) and pressure (3.09–4.36 kbar) conditions in a reducing environment at mid-crustal levels. However, the granites indicate lower temperature (<750 °C) and pressure (0.89–1.88 kbar) conditions of crystallization in an oxidizing environment at shallow-crustal levels. The bulk rock chemical characteristics indicate that the quartz-monzodiorites were derived from the melt generated by the mixing of two melts - a melt derived from the differentiation of sanukitoids senso lato (s.l.) and a melt derived from the partial melting of TTG. On the other hand, reworking of the hot crystallizing quartz-monzodiorite due to its rapid upliftment to shallow crustal levels resulted in a decompression melting which gave rise to granitic melts.The relative age of the Dharwad granitoids is estimated to be ∼2580–2560 Ma and unlike the other older granitoids (> 2.61 Ga) reported from the northern part of the Shimoga greenstone belt, the studied granitoids marks the final stage of cratonization in the Foreland region.  相似文献   

17.
The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation (D1 and D2) attended with foliation (fabric) development (S1 and S2). Two distinct metamorphic events at P-T conditions of ∼900°C at ∼9 kbar and ∼600°C at ∼6 kbar are correlated with D1 and D2 respectively. The reaction textures in S1-microdomains are interpreted to be the product of near isobaric cooling at ∼9 kbar from 950°C to 600°C, whereas those in the S2-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D1-M1 high P-T granulite event is interpreted to be Archean in age (ca. 3 Ga) on the basis of the isotopic data obtained from the charnockite suite of the area. The later relatively low P-T granulite facies event, attendant to D2-S2 is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt.  相似文献   

18.
To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P–T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T evolution is characterized by late-stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.  相似文献   

19.
New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.  相似文献   

20.
中亚萨亚克大型铜矿田矽卡岩型铜成矿作用的年代学制约   总被引：8，自引：5，他引：3  
萨亚克铜矿田是中亚成矿域巴尔喀什成矿带唯一的以矽卡岩型铜矿化为主的大型铜矿床,产在哈萨克斯坦的萨亚克复向斜内。铜矿田包括了几个在空间上相对独立的矽卡岩型铜矿床、斑岩型铜钼网状脉矿床和一系列石英脉型矿脉,构成了斑岩型和矽卡岩型两个端元形成的成矿系列,但以矽卡岩型为主。铜矿田花岗岩类岩石的锆石SHRIMPU-Pb定年,给出了两期与矽卡岩型铜成矿作用有关的深成岩浆作用的时代,早期的闪长岩结晶年龄为335±2Ma,晚期的花岗闪长岩结晶年龄为308±10Ma,反映了两期矽卡岩型铜成矿作用的年龄。结合成矿元素分析,认为早期335±2Ma为主要成矿期年龄,而308±10Ma为次要成矿期年龄。区域成矿时代的分析表明,主矽卡岩型成矿期先于区域的斑岩型铜成矿作用而发生,次要的矽卡岩型成矿期略晚于区域斑岩铜成矿期。花岗岩类岩石中的角闪石、黑云母、钾长石40Ar/39Ar测年结果进一步限定了矽卡岩型铜成矿作用的时代,分别给出冷却年龄为286.6±6.7Ma、306.6±2.9Ma和257±11Ma。4个花岗岩类岩石中的磷灰石裂变径迹测年和热历史模拟给出年龄范围为85.4±4.5Ma至66.9±4.1Ma,说明了矽卡岩型萨亚克铜矿田的剥露作用主要发生在晚白垩世晚期。本文花岗岩类岩石的U-Pb、40Ar/39Ar和裂变径迹热年代学研究,揭示了萨亚克铜矿田从深成的岩浆侵入活动、成矿作用、区域冷却到剥露作用的全过程。  相似文献   

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