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1.
《Gondwana Research》2015,27(3-4):888-906
The Ongole Domain in the southern Eastern Ghats Belt of India formed during the final stages of Columbia amalgamation at ca. 1600 Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U–Pb–Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen.Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200–2100, 2080–2010, 1980–1920, 1850 and 1750 Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550 Ma). Combined U–Pb ages and Lu–Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57 Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing εHf values between − 2 and − 12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68 and 1.60 Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63 Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68 and 1.59 Ga.The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).  相似文献   

2.
《Gondwana Research》2009,15(4):663-674
The presence of 1.52–1.50 Ga charnockites from the anorthosite–mangerite–charnockite–granite (AMCG) Mazury complex in southern Lithuania and NE Poland, in the western East European Craton (EEC) is revealed by secondary ion mass-spectrometry (SIMS) and EPMA geochronology. Early 1.85–1.82 Ga charnockites are related to major orogeny in the region whereas the newly studied charnockites intrude the already consolidated crust. The 1.52–1.50 Ga charnockite magmatism (SIMS data on zircon) was followed by high-grade metamorphism (EPMA data on monazite), which strongly affected the surrounding rocks. The 1.85–1.81 Ga zircon cores in Lazdijai and 1.81 Ga monazite domains in the Lanowicze charnockites represent the protolith age of a volcanic island arc. The 1.52–1.50 Ga charnockite magmatism and metamorphism are likely related to the distal, Danopolonian, orogeny further to the west, at the margin of Baltica. The c.1.52–1.50 Ga AMCG magmatism and metamorphism in the western EEC as well as the paired accretionary-rapakivi suites in Amazonia, may be the inboard manifestations of the same early Mesoproterozoic orogeny associated with the juxtaposition of Amazonia and Baltica during the amalgamation of the supercontinent Columbia.  相似文献   

3.
The Panrimalai area constitutes part of the granulite-facies rocks of the Madurai block in the Southern Granulite Terrain (SGT), India. Garnet-bearing mafic granulites in Panrimalai occur as small enclaves within charnockite. The common stable assemblage during peak metamorphism contains hornblende, garnet, orthopyroxene, clinopyroxene, quartz and plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene–plagioclase and hornblende–plagioclase symplectites characterize the subsequent stages of metamorphism. Application of multi-equilibrium calculation procedures for mineral core compositions of the early assemblage yields near peak conditions at  ≥ 900 °C at 9 kbar. These estimates are the highest yet reported in mafic granulites from the Madurai block. The post-peak P–T path is constructed for the mafic granulites based on observed microstructural relations and thermobarometric results is characterized by a steep clockwise decompressional P–T segment from  ≥ 9 to  < 4.5 kbar. Constraints from model Nd ages provide evidence for Paleoproterozoic magmatism restricted to the Madurai block in the Southern Granulite Terrain. The early part of the crustal evolution of the Panrimalai granulites could be coeval with the Paleoproterozoic event. Subsequent development of symplectitic assemblages via near-isothermal decompression can be ascribed to a distinctly later tectonic event. Available U–Pb and Sm–Nd mineral dates suggest a widespread Pan-African tectonothermal event in the SGT. Given the general recognition of ultrahigh-temperature (UHT) and isothermal decompression (ITD) in Pan-African age metamorphism in the East-African–Antarctic Orogen (EAAO) , the Panrimalai UHT history is considered to be part of this record.  相似文献   

4.
The enigmatic Arequipa Massif of southwestern Peru is an outcrop of Andean basement that underwent Grenville-age metamorphism, and as such it is important for the better constraint of Laurentia–Amazonia ties in Rodinia reconstruction models. U–Pb SHRIMP zircon dating has yielded new evidence on the evolution of the Massif between Middle Paleoproterozoic and Early Paleozoic. The oldest rock-forming events occurred in major orogenic events between ca. 1.79 and 2.1 Ga (Orosirian to Rhyacian), involving early magmatism (1.89–2.1 Ga, presumably emplaced through partly Archaean continental crust), sedimentation of a thick sequence of terrigenous sediments, UHT metamorphism at ca. 1.87 Ga, and late felsic magmatism at ca. 1.79 Ga. The Atico sedimentary basin developed in the Late-Mesoproterozoic and detrital zircons were fed from a source area similar to the high-grade Paleoproterozoic basement, but also from an unknown source that provided Mesoproterozoic zircons of 1200–1600 Ma. The Grenville-age metamorphism was of low-P type; it both reworked the Paleoproterozoic rocks and also affected the Atico sedimentary rocks. Metamorphism was diachronous: ca. 1040 Ma in the Quilca and Camaná areas and in the San Juán Marcona domain, 940 ± 6 Ma in the Mollendo area, and between 1000 and 850 Ma in the Atico domain. These metamorphic domains are probably tectonically juxtaposed. Comparison with coeval Grenvillian processes in Laurentia and in southern Amazonia raises the possibility that Grenvillian metamorphism in the Arequipa Massif resulted from extension and not from collision. The Arequipa Massif experienced Ordovician–Silurian magmatism at ca. 465 Ma, including anorthosites formerly considered to be Grenvillian, and high-T metamorphism deep within the magmatic arc. Focused retrogression along shear zones or unconformities took place between 430 and 440 Ma.  相似文献   

5.
This paper investigates the age, P–T conditions and kinematics of Karakorum Fault (KF) zone rocks in the NW part of the Himalaya–Karakorum belt. Granulite to greenschist facies assemblages were developed within the KF zone during strike-slip shearing. The granulites were formed at high temperature (800 °C, 5.5 kbar), were subsequently retromorphosed into the amphibolite facies (700–750 °C, 4–5 kbar) and the greenschist facies (350–400 °C, 3–4 kbar). The Tangtse granite emplaced syn-kinematically at the contact between a LT and the HT granulite facies. Intrusion occurred during the juxtaposition of the two units under amphibolite conditions. Microstructures observed within the Tangtse granite exhibit a syn-magmatic dextral S–C fabric. Compiled U–Pb and Ar–Ar data show that in the central KF segment, granulite facies metamorphism occurred at a minimum age of 32 Ma, subsequent amphibolite facies metamorphism at 20–18 Ma. Further shearing under amphibolite facies (650–500 °C) was recorded at 13.6 ± 0.9 Ma, and greenschist-facies mica growth at 11 Ma. These data give further constrains to the age of initiation and depth of the Karakorum Fault. The granulite-facies conditions suggest that the KF, accommodating the lateral extrusion of Tibet, could be at least a crustal or even a Lithosphere-scale shear zone comparable to other peri-Himalayan faults.  相似文献   

6.
The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite–sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P–T range of 10–12 kbar at 700 °C to 900 °C. The zircon data yield weighted mean 207Pb/206Pb ages of 3229 ± 80 Ma for metagabbro, 3168 ± 25 Ma for the charnockite, and 3181 ± 20 Ma for the mafic granulite. Ages ranging from 3248 ± 28 Ma to 3506 ± 26 Ma were obtained from zircons in the kyanite/sillimanite bearing metapelite, 3335 ± 44 Ma from khondalite, 3135 ± 14 Ma from garnet biotite gneiss, 3145 ± 17 Ma to 3292 ± 57 Ma from quartz mica schist and 3153 ± 15 Ma to 3252 ± 36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2 Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0 Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu–Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent “Ur”.  相似文献   

7.
The Salvador–Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny (c. 2.0 Ga). Well preserved in enclaves of silica-undersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene1 + spinel1 + biotite1). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel–cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900–950 °C at 7.0–8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P–T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirine-bearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08–2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.  相似文献   

8.
Different continental collision belts show contrasting metamorphic trend along their length, including the distribution of extreme metamorphism; i.e., ultrahigh-pressure (>100 km depth) and ultrahigh-temperature (900–1150 °C) metamorphisms. However, no previous study has succeeded in explaining these trends. The present study investigates the main factors that control the metamorphic trends along collision belts, with reference to the Dabie–Hongseong collision belt between the North and South China blocks and the Himalayan collision belt between the Indian and Asian blocks. In the Dabie–Hongseong collision belt, collision began in the east before 245 Ma and propagated westward until ca. 220 Ma. In the eastern part of the belt, the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism; however, ultrahigh-pressure metamorphism occurred in the western part of the belt. Slab break-off also migrated from east to west, with a westward increase in the depth of break-off (from ca. 10 kbar in the west to ca. 35 kbar in the east). These lateral trends along the belt resulted in a westward change from ultrahigh-temperature (915–1160 °C, 9.0–10.6 kbar) to high-pressure (835–860 °C, 17.0–20.9 kbar) and finally ultrahigh-pressure metamorphism (680–880 °C, 30–40 kbar). In the Himalayan collision belt, collision started from the west at 50 Ma and propagated eastward. The amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism in the west, but not in the east. Slab break-off started in the west at ca. 46 Ma and propagated eastward, with an eastward decrease in the depth of slab break-off from 27–29 to 17–18 kbar. Consequently, the metamorphic trend along the belt changes eastward from ultrahigh-pressure (690–750 °C, 27–29 kbar) to high-pressure and finally high-pressure granulite facies metamorphism (890 °C, 17–18 kbar). The differences in metamorphic trend between the Dabie–Hongseong and Himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break-off along each belt.  相似文献   

9.
The Rondonian-San Ignacio Province (1.56–1.30 Ga) is a composite orogen created through successive accretion of arcs, ocean basin closure and final oblique microcontinent–continent collision. The effects of the collision are well preserved mostly in the Paraguá Terrane (Bolivia and Mato Grosso regions) and in the Alto Guaporé Belt and the Rio Negro-Juruena Province (Rondônia region), considering that the province was affected by later collision-related deformation and metamorphism during the Sunsás Orogeny (1.25–1.00 Ga). The Rondonian-San Ignacio Province comprises: (1) the Jauru Terrane (1.78–1.42 Ga) that hosts Paleoproterozoic basement (1.78–1.72 Ga), and the Cachoeirinha (1.56–1.52 Ga) and the Santa Helena (1.48–1.42 Ga) accretionary orogens, both developed in an Andean-type magmatic arc; (2) the Paraguá Terrane (1.74–1.32 Ga) that hosts pre-San Ignacio units (>1640 Ma: Chiquitania Gneiss Complex, San Ignacio Schist Group and Lomas Manechis Granulitic Complex) and the Pensamiento Granitoid Complex (1.37–1.34 Ga) developed in an Andean-type magmatic arc; (3) the Rio Alegre Terrane (1.51–1.38 Ga) that includes units generated in a mid-ocean ridge and an intra-oceanic magmatic arc environments; and (4) the Alto Guaporé Belt (<1.42–1.34 Ga) that hosts units developed in passive marginal basin and intra-oceanic arc settings. The collisional stage (1.34–1.32 Ga) is characterized by deformation, high-grade metamorphism, and partial melting during the metamorphic peak, which affected primarily the Chiquitania Gneiss Complex and Lomas Manechis Granulitic Complex in the Paraguá Terrane, and the Colorado Complex and the Nova Mamoré Metamorphic Suite in the Alto Guaporé Belt. The Paraguá Block is here considered as a crustal fragment probably displaced from its Rio Negro-Juruena crustal counterpart between 1.50 and 1.40 Ga. This period is characterized by extensive A-type and intra-plate granite magmatism represented by the Rio Crespo Intrusive Suite (ca. 1.50 Ga), Santo Antonio Intrusive Suite (1.40–1.36 Ga), and the Teotônio Intrusive Suite (1.38 Ga). Magmatism of these types also occur at the end of the Rondonian-San Ignacio Orogeny, and are represented by the Alto Candeias Intrusive Suite (1.34–1.36 Ga), and the São Lourenço-Caripunas Intrusive Suite (1.31–1.30 Ga). The cratonization of the province occurred between 1.30 and 1.25 Ga.  相似文献   

10.
The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.  相似文献   

11.
A major Mesoproterozoic paleo-plate boundary in the southwestern Amazonian Craton, the Guaporé Suture Zone, is investigated by U–Pb zircon geochronology, Sr–Nd isotope geochemistry and aeromagnetic data. This suture zone is constituted dominantly by ophiolitic mafic–ultramafic rocks of the Trincheira Complex, and minor proportion of tonalites of the Rio Galera and São Felipe complexes, Colorado Complex, amphibolites of the Rio Alegre Terrane and syn- to late-kinematic mafic to felsic plutonic rocks. The ophiolitic Trincheira Complex formed during an accretionary phase from 1470 to 1430 Ma and was overprinted by upper amphibolite–granulite facies metamorphism during the collisional phase of the Ectasian followed by syntectonic emplacement of gabbro and granite plutons (1350–1340 Ma). The ophiolites were intruded by syntectonic tonalitic–plagiogranitic plutons ca. 1435 Ma. Mafic–ultramafic rocks of the Trincheira ophiolites show moderate to highly positive initial epsilon Nd (t = 1.46 Ga) values (+2.6 to +8.8) and very low initial 87Sr/86Sr ratio (0.7013–0.7033). It is suggested that these magmas originated from a depleted mantle source in an island-arc–back-arc setting. The identification of a fossil ophiolite in the Guaporé Suture Zone early as 1470–1435 Ma and later collisional phase, as late as 1350 Ma, marks the impingement of the proto-Amazonian Craton against the Paragua Block, before the formation of the Rodinia supercontinent. The results provide important insights into the geodynamic history of the SW Amazonian Craton, with evidence for both accretionary orogen and subduction of oceanic lithosphere in the Mesoproterozoic, and provide information that allows other workers to evaluate the configuration of supercontinents.  相似文献   

12.
We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~ 900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~ 1.77 Ga) and two-pyroxene granulites (~ 1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~ 1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to + 6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~ 1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).  相似文献   

13.
《Gondwana Research》2014,26(4):1535-1553
The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45 Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50 Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45 Ga and then underwent high-grade metamorphism at 2.45–2.40 Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly < 0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly < 0.1), and record ages of 1.95–1.90 Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39 Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40 Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.  相似文献   

14.
We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons.  相似文献   

15.
The Bajgan Complex, one of the basement constituents of the arc massif in Iranian Makran forms a rugged, deeply incised terrain. The complex consists of pelitic schists with minor psammitic and basic schists, calc silicate rocks, amphibolites, marbles, metavolcanosediments, mafic and felsic intrusives as well as ultramafic rocks. Metapelitic rocks show an amphibolite facies regional metamorphism and contain garnet, biotite, white mica, quartz, albite ± rutile ± apatite. Thermobarometry of garnet schist yields pressure of more than 9 kbar and temperatures between 560 and 675 °C. The geothermal gradient obtained for the peak of regional metamorphism is 19 °C/km, corresponding to a depth of ca. 31 km. Replacement of garnet by chlorite and epidote suggest greenschist facies metamorphism due to a decrease in temperature and pressure through exhumation and retrograde metamorphism (370–450 °C and 3–6 kbar). The metapelitic rocks followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal decline following tectonic thickening. The formation of medium-pressure metamorphic rocks is related to presence of active subduction of the Neotethys Oceanic lithosphere beneath Eurasia in the Makran.  相似文献   

16.
This study documents the metamorphic evolution of mafic granulites from the Eastern Hebei Complex in the Eastern Block of the North China Craton. Mafic granulites from Eastern Hebei occur as boudins or enclaves within Neoarchean high-grade TTG gneisses. Petrographic observations reveal three characteristic metamorphic mineral assemblages in the mafic granulites: the pre-peak hornblende + plagioclase + ilmenite + quartz + sphene assemblage (M1) existing as mineral inclusions within coarse-grained peak assemblage (M2) represented by garnet + clinopyroxene + orthopyroxene + plagioclase + hornblende + ilmenite + quartz, and post-peak assemblage (M3) marked by garnet + quartz ± ilmenite symplectites surrounding the peak pyroxene and plagioclase. Based on pseudosection modeling calculated in the NCFMASHTO model system using the program THERMOCALC, P–T conditions of the pre-peak (M1), peak (M2) and post-peak (M3) assemblages are constrained at 600–715 °C/6.0 kbar or below, 860–900 °C/9.6–10.3 kbar, and 790–810 °C/9.6–10.4 kbar, respectively. These P–T estimates, combined with their mineral compositions and reaction relations, define an anticlockwise P–T path incorporating isobaric cooling subsequent to the peak medium-pressure granulite-facies metamorphism for the mafic granulites from Eastern Hebei. Such an anticlockwise P–T path suggests that the end-Neoarchean metamorphism of the Eastern Hebei Complex correlated closely with underplating and intrusion of voluminous mantle-derived magmas. In conjunction with other geological considerations, a mantle-plume model is favored to interpret the Neoarchean tectonothermal evolution of the Eastern Hebei Complex and other metamorphic complexes in the Eastern Block. The prograde amphibolite-facies metamorphism (M1) was initiated due to the upwelling of the relatively cooler mantle plume head, followed by the peak medium-pressure granulite-facies metamorphism (M2) as triggered by the uprising hotter plume “tail”, and finally when plume activity ceased, the heated metamorphic crust experienced nearly isobaric cooling (M3).  相似文献   

17.
Studies of gneisses from the Yenisei regional shear zone (YRSZ) provide the first evidence for Mesoproterozoic tectonic events in the geologic history of the South Yenisei Ridge and allowed the recognition of several stages of deformation and metamorphism spanning from Late Paleoproterozoic to Vendian. The first stage (~ 1.73 Ga), corresponding to the period of granulite-amphibolite metamorphism at P = 5.9 kbar and T = 635 °C, marks the final amalgamation of the Siberian craton to the Paleo-Mesoproterozoic Nuna supercontinent. During the second stage, corresponding to a hypothesized breakup of Nuna as a result of crustal extension, these rocks underwent Mesoproterozoic dynamic metamorphism (P = 7.4 kbar and T = 660 °C) with three peaks at 1.54, 1.38, and 1.25 Ga and the formation of high-pressure blastomylonite rocks in shear zones. Late-stage deformations during the Mesoproterozoic tectonic activity in the region, related to the Grenville-age collision processes and assembly of Rodinia, took place at 1.17-1.03 Ga. The latest pulse of dynamic metamorphism (615–600 Ma) marks the final stage of the Neoproterozoic evolution of the Yenisei Ridge, which is associated with the accretion of island-arc terranes to the western margin of the Siberian craton. The overall duration of identified tectonothermal processes within the South Yenisei Ridge during the Riphean (~ 650 Ma) is correlated with the duration of geodynamic cycles in the supercontinent evolution. A similar succession and style of tectonothermal events in the history of both the southern and the northern parts of the Yenisei Ridge suggest that they evolved synchronously within a single structure over a prolonged time span (1385–600 Ma). New data on coeavl events identified on the western margin of the Siberian craton contradict the hypothesis of a mantle activity lull (from 1.75 to 0.7 Ga) on the southwestern margins of the Siberian craton during the Precambrian. The synchronous sequence and similar style of tectonic events on the periphery of the large Precambrian Laurentia, Baltica, and Siberia cratons suggest their spatial proximity over a prolonged time span (1550–600 Ma). The above conclusion is consistent with the results of modern paleomagnetic reconstructions suggesting that these cratons represented the cores of Nuna and Rodinia within the above time interval.  相似文献   

18.
Detailed mapping, coupled with geochronological and geochemical investigations, has revealed the presence of a 1917–1913 Ma gabbro–monzodiorite–monzonite suite along the southeast margin of the Hearne Craton in northern Saskatchewan, Canada. The predominantly plutonic suite is also characterised by 1915 Ma old trachyandesitic subvolcanic and volcaniclastic inclusions. The rocks are hornblende–epidote–titanite ± augite bearing and collectively termed the Porter Bay Complex. The plutonic rocks cut the 2569 Ma Lueaza River granitoid suite, a component of the Hearne Craton and are themselves intruded by 1859 Ma pegmatitic diorite, 1856 Ma layered gabbro-anorthosite, and 1853 Ma quartz-diorite belonging to the Wathaman Batholith, one of the world's largest Paleoproterozoic Andean-type continental arcs. Wholerock major element geochemistry characterises the Porter Bay Complex as calc-alkalic to alkali-calcic, metaluminous and variable from ferroan to magnesian. Trace element concentrations are characterised by negative high field strength element anomalies, suggesting emplacement along a destructive plate margin. The geochemical signatures of the Wathaman Batholith and the Porter Bay Complex are largely identical. The geographic location, map relationships, and geochronological, geochemical and petrographic constraints are consistent with the Porter Bay Complex having formed in a subduction-related continental arc setting. The southeastern margin of the Hearne Craton was therefore a long-lived active continental margin with two separate periods of continental arc magmatism between 1.92–1.91 Ga and 1.86–1.85 Ga.  相似文献   

19.
《Gondwana Research》2014,25(2):630-648
High-pressure kyanite–K-feldspar granulites in the Běstvina granulite body, which belongs to the Variscan orogenic root in the Bohemian Massif, preserve muscovite, rutile and kyanite inclusions in garnet. High-Ti muscovite (Ti = 0.09–0.20 p.f.u., Si = 0.21–3.24 p.f.u.) included in garnet is associated with quartz and is in crystallographic continuity with biotite, interpreted in terms of exsolution from an original less-dioctahedral higher-Ti muscovite. The assemblage garnet–kyanite–antiperthite–perthite–quartz–rutile and the mineral compositions indicate a peak of metamorphism at about 900 °C and 17–21 kbar, based on P–T pseudosection modeling, ternary-feldspar and Zr-in-rutile thermometry. The matrix assemblage garnet–kyanite–plagioclase-K-feldspar–quartz–rutile–ilmenite and garnet rim compositions at contact with feldspars and quartz indicate the end of overall equilibration in the presence of melt at 12–14 kbar and 820–840 °C. Embayments of biotite and plagioclase locally replacing garnet, and connected with modification of garnet composition, may indicate sites of last isolated melt or diffusion of H2O from that melt down to 10 kbar and 800 °C. Zircon with uniform cathodoluminescence (CL) pattern is present as rims around cores with faint oscillatory zoning, or as entire rounded grains. These zircons gave a cluster of ages at 359 ± 4 Ma, interpreted as the age of metamorphism. Zircon ages from the cores with common faint oscillatory zoning range from 500 to 398 Ma, and are interpreted as magmatic grains variably reset during metamorphism. Two older ages obtained on cores of 620 ± 18 Ma probably represent an inherited zircon component. Molar isopleths of zircon along the P–T path in pseudosections suggest that crystallization of metamorphic zircon occurred during decompression and cooling from 17 to 21 kbar and 900 °C to 12–14 kbar and 820–840 °C. The inferred P–T path and the age of metamorphism are discussed in the framework of a geodynamic model that considers the granulites to be a part of a subducted plate that failed to continue to subduct and was spread below the upper plate.  相似文献   

20.
The Turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation/reactivation events of the NW–SE Pitangui Shear Zone (PSZ). The lodes are hosted in a Late Archaean sedimentary sequence on top of a strongly deformed (mafic-dominated) metavolcanic pile metamorphosed under 3.5–4 kbar and 540–610 °C; the association forms the upper part of a lithostratigraphic succession (Pitangui Group) that overlies an older TTG gneissic basement. According to field evidence and petrographic observations, the ore-forming process is polyphasic, starting at the time when the PSZ crossed the metamorphic quartz + staurolite + biotite + almadine + hornblende isograde; the main evolving stages, however, mostly took place throughout the metamorphic retrogression path. Fluid inclusion microthermometry also shows that metamorphogenic aqueous–carbonic solutions (initially with ≈ 16–20 eq. wt.% NaCl and circulating at approximately 4 kbar and 550 °C) were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1–2 kb and 300–350 °C. These boiling events, which were triggered by depressurization, were contemporaneous with gold (and later sulfide) deposition, preceding a late stage of hydrothermal activity under lower PT conditions (< 1 kbar and ≈ 130–230 °C). To constrain both the mineralization age and the source/pathways of the ore-forming fluids in the Turmalina deposit, a multi-system isotope (Pb–Pb, Rb–Sr and Sm–Nd) study was carried out using various whole-rock and mineral samples. The main results are as follows: (i) the onset of the ore-forming process took place at ca. 2.2–2.1 Ga; (ii) the critical timing for gold formation was confined to ca. 2–1.9 Ga; and (iii) the late hydrothermal influxes occurred after 1.75 Ga. Therefore, the ore-forming process can be envisaged as a result of successive physical–chemical processes that took place during two major, long-lasting (≈ 250 Ma) periods under initial cooling rates of approximately 1 °C/Ma and, after ca. 2–1.9 Ga, approximately 2.5 °C/Ma. Moreover, the rejuvenation episodes at ca. 2–1.9 Ga and ca. 1.75 Ga probably indicate reactivation events in the PSZ, which triggered new fluid inflows into the system and revitalized the ore-forming process. That was the case for fluids that circulated deeply through both the older basement rocks and the mafic volcanic pile, allowing either metal enrichment from multistage leaching processes of various reservoirs or possible U/Th decoupling during ore remobilization. The Turmalina ore-system lifetime is consequently confined to the Rhyacian period (Paleoproterozoic), which is compatible with the age constraints presented by other studies regarding the fold-thrust belt's development (ca. 2.125 Ga) and orogenic collapse (ca. 2.095 Ga).  相似文献   

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