Deformation correlations,stress field switches and evolution of an orogenic intersection: The Pan-African Kaoko-Damara orogenic junction,Namibia

Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing,and switches between different deformation events.Horizontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution,and~135° during Damara Belt evolution.At most stages,stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata,Congo,and Kalahari Cratons.Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma,involving E-W directed shortening,progressing through different transpressional states with ~45° rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt,and subsequently evolved through ~90°rotation of the stress field to NE-SW shortening at ~512-508 Ma.Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE.Progressive stress field rotation was punctuated by rapid and significant switches at ~530-525 Ma,~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System,most plausibly from arc obduction along the orogenic margin of Gondwana(Ross-Delamerian Orogen).(3)This imposed stress field established a N-S extension direction exploited by decompression melts,switch to vertical shortening,and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella,Dronning Maud Land,East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Deciphering the metamorphic architecture and magmatic patterns of large hot orogens: Insights from the central Grenville Province

Widespread exposure of the orogenic core in the Mesoproterozoic Grenville Province, provides opportunity to assess high-T processes in large hot orogens. The Manicouagan–Escoumins transect in the central Grenville cuts through mostly granulite-facies deep to shallow crustal levels of the orogenic hinterland, and of the structurally lower Parautochthonous belt, that contain aluminous rocks with evidence of anatexis at different crustal depths. Melt-related microstructures, imaged at different scales, in conjunction with P–T pseudosections, were instrumental for assessing P–T patterns. However, the data also highlight limitations relevant to the interpretation of aluminous granulites in general, such as the difficulty to obtain peak metamorphic T, and the unreliability of Ca in garnet as a P–T indicator in apatite-bearing rocks. Monazite ages show that the mid-P portion of the hinterland records the oldest and longest duration granulite-facies metamorphism, associated to crustal thickening and spreading of the orogen at 1.08–1.04 Ga. In contrast, the high-P portion records younger metamorphic ages (1.05–1.02 Ga) decreasing upwards, and linked to local thickening behind the orogenic front at that time. ~1.04 Ga ages are also reported at higher, low-P crustal levels and are consistent with younging and shorter duration of metamorphism at the structural top. Short-lived and youngest (1.0–0.98 Ga) metamorphism in the Parautochthonous belt marks the final propagation of the orogen towards its foreland. The hinterland of the central Grenville is also characterized by diverse syn-orogenic and mostly high-T magmatism suggesting that the mantle was an important heat contributor to the metamorphism. The metamorphic domains of the hinterland along the transect are bounded by long-lived deformation zones, underpinned by 1.1–1.0 Ga magmatic bodies, that likely represent reactivated crustal scars inherited from the SE Laurentian margin on which the Grenville orogen was built.     

The P–T–t paths of high-grade gneisses,Kaoko Belt,Namibia: Constraints from mineral data,U–Pb allanite and monazite and Sm–Nd/Lu–Hf garnet ages and garnet ion probe data

The Damara Orogeny is a late Neoproterozoic to Cambrian (ca. 570–480 Ma) intracratonic event that affected the Kaoko Belt, the inland branch of the Damara orogen and the Gariep Belt in Namibia and South Africa. This study focuses on the Pan-African evolution of part of the Kaoko Belt between the Puros shear zone and the Village mylonite zone which consists of Mesoproterozoic migmatitic para- and orthogneisses with minor granulite and amphibolite. Pseudosection modeling combined with thermobarometric calculations indicate that the para- and orthogneisses equilibrated at about 670–800 °C and ca. 0.6–0.8 GPa. Some garnets display a pronounced bell-shaped Ca, HREE, Y and Sr zoning, flat zoning profiles of Mn and Fe and concave upward concentration profiles of Sm and Nd. Pressure–temperature estimates obtained on these garnets reveal similar temperatures of 700–750 °C but slightly higher pressures of ca. 0.9 GPa. The preservation of distinct major and trace element zoning in garnet and the existence of broadly similar (near prograde) Sm–Nd and Lu–Hf garnet–whole rock ages of ca. 525 Ma obtained on the same sample indicate an extremely fast cooling path. Retrograde conditions persisted until ca. 490 Ma indicating a slow, late stage near isobaric cooling path. The resulting clockwise P–T–t path is consistent with crustal thickening through continent–continent collision followed by post-collisional extension and suggests that the upper amphibolite to granulite facies terrain of the central Kaoko Belt formed initially in a metamorphic field gradient of ca. 25–35 °C km^− 1 at moderately high pressures.     

Oblique collision and evolution of large-scale transcurrent shear zones in the Kaoko belt,NW Namibia

Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision was governed by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting in the development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phase of deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Western and Central zones. During this second phase, extensive partial melting and intrusion of ∼550 Ma granitic bodies occurred in the high-grade Western zone. In the Central zone, the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the Puros shear zone, and to the development of kilometre-scale folds in the more distal parts. Strain partitioning into transcurrent deformation along the Puros shear zone and NE–SW oriented shortening in the Central zone is consistent with a sinistral transpressional regime during the second phase of deformation. Transcurrent deformation continued during cooling of the entire belt, giving rise to the localized low-temperature Village Mylonite Zone that separates a segment of elevated Mesoproterozoic basement from the rest of the Western zone in which only Pan-African ages have so far been observed. The data suggest that the boundary between the Western and Central Kaoko zones represents a modified thrust zone controlling the tectonic evolution of the Pan-African Kaoko belt.     

The transpressional connection between Dom Feliciano and Kaoko Belts at 580�C550?Ma

A new U?CPb SHRIMP age of 551?±?4?Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580?C550?Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550?C530?Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.     

Ultrahigh-temperature metamorphism and anticlockwise P–T–t path of Paleozoic granulites from north Qinling-Tongbai orogen,Central China

Mafic and semi-pelitic granulites from the Qinling-Tongbai orogen in central China preserve petrological evidence and mineral paragenesis suggesting four distinct stages of metamorphic evolution. The prograde history (M₁) is recorded by the occurrence of cordierite, orthopyroxene and biotite inclusions in garnet porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by cooling with minor decompression (M₃), which formed symplectites and coronitic textures. The greenschist facies retrograde metamorphic assemblage (M₄) is represented by hydrous minerals replacing minerals of the M₂ and M₃ assemblages. We present LA-ICPMS zircon U-Pb data which show ages of 432 ± 4 Ma for the peak metamorphism and 403 to 426 Ma for the retrograde stage. Microstructural analysis, P–T pseudosections, and mineral isopleths in conjunction with the zircon U-Pb ages define an anticlockwise P–T–t path. The P–T estimates for peak metamorphic conditions of 880–920 °C and 8.0–10 kbar suggest that these rocks witnessed extreme crustal metamorphism under ultrahigh-temperature conditions. The anticlockwise trajectory reported in this study is comparable with similar P–T paths recorded from subduction–collision settings, and correlate the Tongbai granulites to hot orogens developed within a Paleozoic collisional suture. We propose a ridge subduction and slab window setting to explain the formation of the Tongbai orogen, in a convergent plate setting associated with the northward subduction of the Paleo-Tethyan Qinling Ocean.     

A record of ultrahigh temperature metamorphism in the Dabie orogen during Triassic continental collision

Ultrahigh temperature (UHT) metamorphism is traditionally recognized by the development of characteristic mineral associations in Mg–Al-rich metapelitic rocks. However, recognition of UHT metamorphism in non-supracrustal rocks is more difficult. UHT metamorphic conditions are recorded by a migmatite from the North Dabie Terrane (NDT) of the Dabie orogen, east China. The migmatite is composed of intercalated layers of melanosome and K-feldspar-rich leucosome. Zircon grains in the migmatite have a core–rim structure comprising a metamorphic core and an anatectic rim. The metamorphic cores have low U contents (mainly <657 ppm) and low Th/U ratios (<0.2), and are depleted in heavy rare earth element (HREE). The metamorphic domains yield concordant ²⁰⁶Pb/²³⁸U ages ranging from 205.1 ± 4.8 Ma to 248.0 ± 4.1 Ma with a weighted mean of 217.7 ± 4.3 Ma (n = 20, MSWD = 4.2). They contain a granulite-facies inclusion assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile. Conventional geobarometry and Ti-in-zircon thermometry constrain P–T conditions to approximately 11–12 kbar and 900–950 °C, suggesting UHT metamorphism. The discovery of Triassic UHT metamorphism in the Dabie orogen, which was previously best known for ultrahigh pressure metamorphism, provides new insights into the thermal structure and geodynamics of the orogeny during continental collision. The anatectic rims of zircon grains have relatively high U contents and low Th/U ratios (<0.14), and are enriched in HREE. They yield concordant ²⁰⁶Pb/²³⁸U ages of 133.6 ± 1.1 Ma to 156.4 ± 2.2 Ma, indicating that anatexis occurred during post-collisional collapse of the Dabie orogen.     

Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

Interpretation of reaction microstructures may provide constraints on the P–T path followed by rocks, with implications for the geodynamic evolution. Sapphirine generally occurs in diverse microstructures in ultrahigh-temperature (UHT) Mg–Al-rich granulites. Understanding multi-stage sapphirine formation processes and the resultant P–T path may provide insights into the cause of UHT metamorphism, which is otherwise under broad debate. Here, we investigate samples of UHT granulite containing two distinct types of sapphirine from the Dongpo locality in the Khondalite Belt, North China Craton, with the aim of understanding the processes of sapphirine formation and the metamorphic evolution of the host rocks. Petrographic observations show that early sapphirine, which occurs as coronas on spinel and as single porphyroblasts, formed together with biotite, sillimanite, and inclusion-rich garnet. Late symplectitic sapphirine along with fine-grained plagioclase and spinel plus plagioclase symplectites, formed by consumption of sillimanite, biotite, and garnet. Three pseudosections based on the bulk compositions of microdomains inferred to reflect spatially restricted equilibrium suggest that the rocks record near isobaric cooling (IBC) from ~980 to 830ºC at ~0.9 GPa for early sapphirine formation, and decompression and heating to ≤0.7 GPa and ~900ºC for late sapphirine formation. Our study in combination with other metamorphic P–T and age information reveals the common occurrence of IBC paths and long duration (c. 1.93 to 1.86 Ga) regional UHT metamorphism in the Khondalite Belt, North China Craton. Locally, this is followed by decompression–heating paths at c. 1.86 Ga. The Palaeoproterozoic UHT metamorphism with long-lived IBC path in the Khondalite Belt, North China Craton supports large hot orogen model in the amalgamation of this part in the supercontinent Nuna.     

530 Ma syntectonic syenites and granites in NW Namibia — Their relation with collision along the junction of the Damara and Kaoko belts

The Lower Ugab and Goantagab structural domains are located at the junction between the N–S trending Kaoko and the E–W trending Damara belts (NW Namibia), where Neoproterozoic metavolcano-sedimentary sequences were intruded by several syenitic/granitic plutons. We present here new U–Pb ages on zircon grains from the Voetspoor and Doros plutons. Together with petrological, geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of Gondwana amalgamation.The plutons are composed of three main rock types: hornblende quartz-syenite, syenodiorite and biotite granite. The two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures. The biotite granite occurs in the SW parts of the intrusions and clearly cuts the syenitic rocks. Although the plutons are mainly isotropic, the structures around them demonstrate that their intrusion occurred during a second deformation phase (D2) with a component of sinistral solid state rotation with respect to the wall rocks in response to D2 transpression. Four samples were dated using U–Pb SHRIMP methodology in single zircon grains. A hornblende monzodiorite from the Voetspoor pluton yielded an age of 534 ± 4.5 Ma. A hornblende monzonite from the Doros pluton produced an age of 528 ± 5 Ma. The biotite granite facies was sampled in the Doros intrusion and yielded an age of 530 ± 4.5 Ma. In addition, a granitic vein folded by D2 close to the northeastern contact of the Doros pluton with the encasing phyllites (Amis River Formation) was also dated, yielding an age of 533 ± 6 Ma. The data show that all granite–syenite from Doros and Voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522 Ma within the errors. D1–D2 deformational phases took place under greenschist facies (biotite zone) conditions and during D3 the metamorphic grade was slightly lower. We interpret that the plutons are coeval to peak metamorphism of the region (530–520 Ma) and that D2 and D3 sinistral transpressional phases are due to collision in the Damara Belt. The E–W compressional event and second metamorphic episode in the Kaoko Belt occurred between 580 and 560 Ma and are apparently unrelated to the thermo-tectonic evolution described here, although D1 might be partially related to this event. The sinistral transpressional D2 phase resulted probably from the position of the area considered at the junction between the belts, and not in the frontal Damara collision further to the east. This new interpretation is consistent with the Ar–Ar ages for the region (about 500 Ma), interpreted to reflect cooling of the orogen. The enrichment in LREE, K, Rb, Ba and Sr, and depletion in Nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination. We interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab-breakoff during collision between Kalahari and Congo cratons.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Petrology and thermobarometry of mafic granulites and migmatites from the Chafalote Metamorphic Suite: New insights into the Neoproterozoic P–T evolution of the Uruguayan—Sul-Rio-Grandense shield

This paper reports a study of the metamorphic evolution of pelitic, semi-pelitic migmatites and mafic granulites of the Chafalote Metamorphic Suite (CMS), Uruguay, which represents the southernmost exposures of high-grade metamorphic rocks in the Dom Feliciano Belt, Uruguain—Sul-Rio-Grandense shield, South America. This belt is one of the Brasiliano orogens that crop out along the Brazilian and Uruguayan Atlantic margin, and the CMS is one of several disconnected segments of supracrustal rock in a dominantly granitic terrain. Petrological evidence from CMS mafic granulites and semi-pelitic migmatites indicates four distinct metamorphic assemblages. The early prograde assemblage (M₁) is preserved only as inclusions in porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by near-isothermal decompression (M₃), which resulted in symplectites and coronitic textures in the mafic granulites and compositional zoning of Ca in garnet (decreasing rimwards) and plagioclase (increasing rimwards) in the semi-pelitic migmatites. The retrograde metamorphic assemblage (M₄) is represented by hydration reaction textures replacing minerals of the M₂ and M₃ assemblages. Average P–T calculations using the program THERMOCALC and conventional thermobarometric methods yield peak-metamorphic (M₂) P–T conditions of 7–10 kbar and 830–950 °C, near-decompressional (M₃) P–T conditions of 4.8–5.5 kbar and 788–830 °C and M₄ retrograde P–T conditions of 3–6 kbar and 600–750 °C. The calculated P–T path for the CMS rocks is ‘clockwise’ and incorporates a near-isothermal decompression segment followed by minor cooling, consistent with a history of crustal thickening followed by extensional collapse at ca. 650–600 Ma. The metamorphism recorded by rocks of this crustal segment may be correlated with 650 Ma metamorphism in the Coastal Terrane of the Kaoko Belt in Namibia, being the first unequivocal match between South America and Africa provided by crystalline rocks south of the Congo Craton.     

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan,Sanandaj–Sirjan zone,Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D₁. The main mylonitization event was during the D₂ deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D₃ folding and related mylonitization. We documented the systematic changes in the orientations of D₂ linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.     

Evidence of 1.7- to 1.8-Ga Collisional Arc in the Kaoko Belt, NW Namibia

Detailed structural and isotopic analyses in the Hoanib and Ugab River Valleys indicate the existence of an exotic 1.7- to 1.8-Ga terrane in the Pan-African Kaoko Belt. This crustal block, called as Mudorib Complex, is imbricated between autochthonous and para-autochthonous rocks of Congo Craton, Kaoko Basin, and Western Kaoko Batholith units during the main tectono-thermal phase of Kaoko Belt collision around 580?Ma, involving the Rio de La Plata, Congo and Kahalari paleoplates. This terrain is positioned between the 1.9-Ga Pruwes Complex units of SW edge of the Congo Craton and the 0.58- to 0.55-Ga Amspoort Suite granitoids of the Western Kaoko Batholith. It is coincident with a regional positive aeromagnetic anomaly trending from NNW in the Ugab region to the Namibia-Angola border. Internally, Mudorib Complex consists in 1.73- to 1.81-Ga tonalitic?Ctrondhjemitic?Cdioritic?Cgranodioritic sequence of gneisses associated with cogenetic gabbroic and anothositic-gneisses in the core zone of this Pan-African structure. Field relationship and U?CPb zircon and Sm?CNd whole-rock isotope data combined with geochemical information suggest the existence of two rock associations in the Mudorib Complex, namely late Paleoproterozoic tonalitic?Ctrondhjemitic?Cdioritic-gneisses with island-arc affinity and tholeiitic metabasites of juvenile origin, showing Nd model age of 1.73?C2.17?Ga and ??Nd(t) of ?2.05?C+4.3. This 1.8- to 1.7-Ga complex is also intruded by granitic dykes formed at 1.49?C1.50?Ga with Nd model age of 1.75?C2.34?Ga during stable tectonic conditions. In addition to widespread Pan-African tectono-metamorphic events, a secondary metamorphic event of ~1.3?Ga is also recognized in the Mudorib rocks, which may be associated with accretion process of the complex to the Paleoproterozoic to Archean nucleus of the Kaoko Belt in the Hoanib River Valley.     

Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt,Bahia, Brazil

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.     

Metamorphic record of collision and collapse in the Ediacaran‐Cambrian Araçuaí orogen,SE‐Brazil: Insights from P–T pseudosections and monazite dating

The Araçuaí orogen is the Brazilian counterpart of the Araçuaí‐West Congo orogenic system (AWCO), a component of the Ediacaran‐Cambrian orogenic network formed during the amalgamation of West Gondwana. The northwestern portion of the Araçuaí orogen is dominated by a succession of metasedimentary rocks made up of Meso‐ to Neoproterozoic rift, passive margin and syn‐orogenic sequences, locally intruded by post‐collisional granites. These sequences are involved in three distinct tectonic units, which from west to east are: the southern Espinhaço fold‐thrust system (SE‐thrust system), the normal‐sense Chapada Acauã shear zone (CASZ) and the Salinas synclinorium. Three deformation phases were documented in the region. The first two phases (D₁ and D₂) are characterized by contractional structures and represent the collisional development stage of the orogen. The third phase (D₃) is extensional and currently viewed as a manifestation of orogenic collapse of the system. The distribution of the metamorphic mineral assemblages in the region characterizes two metamorphic domains. The M‐Domain I on the west, encompassing the SE‐thrust system and the CASZ, is marked by a syn‐collisional (syn‐D₁) Barrovian‐type metamorphism with P–T conditions increasing eastwards and reaching ~8.5 kbar at ~650°C between 575 and 565 Ma. The M‐Domain II comprises the Salinas synclinorium in the hangingwall of the CASZ, and besides the greenschist facies syn‐collisional metamorphism, records mainly a Buchan‐type metamorphic event, which took place under 3–5.5 kbar and up to 640°C at c. 530 Ma. The northwestern Araçuaí orogen exhibits, thus, a paired metamorphic pattern, in which the Barrovian and Buchan‐type metamorphic domains are juxtaposed by a normal‐sense shear zone. Lithospheric thinning during the extensional collapse of the orogen promoted ascent of the geotherms and melt generation. A large volume of granites was emplaced in the high grade and anatectic core of the orogen during this stage, and heat advected from these intrusions caused the development of Buchan facies series over a relatively large area. Renewed granite plutonism, hydrothermal activities followed by progressive cooling affected the system between 530 and 490 Ma.     

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.     

Thermal history of a Late Mesoproterozoic paired metamorphic belt (?) during Rodinia assembly: New insight from medium-pressure granulites from the Aravalli-Delhi Mobile Belt,Northwestern India

In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt(ADMB), NW India using a suite of supracrustal and metaigneous granulites from the Pilwa-Chinwali granulite terrain at the north-western margin of the ADMB. Using metamorphic reaction textures, mineral chemistry, metamorphic reaction history, geothermobarometric computations and electron microprobe dating of monazite in 5 samples of pelitic granulite, leptynite gneiss, enderbite and charnockite, we have deduced a medium-pressure granulite facies metamorphism(P between 4.9 and 6.8 kbar, T 760-815℃) along a heating-cooling, counterclockwise P-T path between 1.09 and 1.01 Ga. When collated with published metamorphic and chronological constraints and geological settings of the adjoining crustal domains of the ADMB, these findings provide new insights into the developments of two tectonic domains of contrasting thermal gradients at ca. 1.0 Ga, consistent with metamorphic transformations in tectonically thickened middle-lower crustal sections during continental collision to continental subduction and in the root zones of spatially adjacent island arc, as part of the Rodinia supercontinent assembly event.     

A review of the pressure–temperature–time evolution of the Limpopo Belt: Constraints for a tectonic model

Published literature argues that the Limpopo Belt can be subdivided into three zones, each with a distinctive geological character and tectono-metamorphic fingerprint. There are currently two contrasting schools of thought regarding the tectono-metamorphic evolution of the CZ. One camp argues that geochronological, structural and prograde pressure–temperature (P–T) evidence collectively indicate that the CZ underwent tectono-metamorphism at ca. 2.0 Ga which followed a clockwise P–T evolution during a transpressive orogeny that was initiated by the collision of the Kaapvaal and Zimbabwe cratons. Deformation and metamorphism consistent with this scenario are observed in the southern part of the NMZ but are curiously absent from the whole of the SMZ. The opposing view argues that the peak metamorphism associated with the collision of the Kaapvaal and Zimbabwe cratons occurred at ca. 2.6 Ga and the later metamorphic event is an overprint associated with reactivation along Archean shear zones. Post-peak-metamorphic conditions, which at present cannot be convincingly related to either a ca. 2.6 or 2.0 Ga event in the CZ reveal contrasting retrograde paths implying either near-isothermal decompression and isobaric cooling associated with a ‘pop-up’ style of exhumation or steady decompression–cooling linked to exhumation controlled by erosion. Recent data argue that the prograde evolution of the ca. 2.0 Ga event is characterised by isobaric heating prior to decompression–cooling. Contrasting P–T paths indicate that either different units exist within the CZ that underwent different P–T evolutions or that some P–T work is erroneous due to the application of equilibrium thermobarometry to mineral assemblages that are not in equilibrium. The morphology of the P–T path(s) for the ca. 2.6–2.52 Ga event are also a matter of dispute. Some workers have postulated an anticlockwise P–T evolution during this period whilst others regard this metamorphic event as following a clockwise evolution. Granitoid magmatism is broadly contemporaneous in all three zones at ca. 2.7–2.5 suggesting a possible causal geodynamic link. P–T contrasts between and within the respective zones prevent, at present, the construction of a coherent and inter-related tectonic model that can account for all of the available evidence. Detailed and fully-integrated petrological and geochronological studies are required to produce reliable P–T–t paths that may resolve some of these pertinent issues.     

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

A single metapelitic sample from the Verbaard locality, near Messina was investigated in order to construct a P–T path and moreover, highlight pertinent contradictions in the current P–T database. Interpretations based on P–T pseudosections, garnet isopleth thermobarometry and mineral mode/isopleth modelling indicate that the mineral assemblages, textures and zonations developed in the metapelite formed along a single clockwise P–T path. The metamorphic evolution is characterized by an early high-pressure phase at 10–11 kbar/800 °C, followed by a simultaneous pressure decrease and temperature increase to ∼8/850 °C and subsequent retrogression via decompression-cooling to 4–5 kbar at T < 650 °C. Growth zoning in garnet provides evidence for an earlier, prograde history, however, as potential melt-loss was not accounted for this must be deemed speculative. The results of this study agree entirely with that of [Zeh, A., Klemd, R., Buhlmann, S., Barton, J.M. 2004. Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa); constraints from quantitative phase diagrams and geotectonic implications. Journal of Metamorphic Geology 22, 79–95], who adopted a similar approach to thermobarometry i.e. pseudosections. The results are, however, inconsistent with recent publications that argue for a twofold, metamorphic history defined by two decompression-cooling paths (DC1 ∼2.6 Ga and DC2 ∼2.0 Ga) that are separated by an isobaric heating path (∼2.0 Ga). The disparity in the results obtained from different workers can be explained by an examination of the thermobarometric methods employed. The methodology employed to derive the twofold, polymetamorphic P–T path appears to be erroneous. At present, the most reliable and robust method for determining P–T paths is the pseudosection approach to thermobarometry. Future modelling of Limpopo Belt granulites should adopt this strategy and ensure potential melt-loss is taken into account. Alternatively, this potential problem can be avoided altogether by investigating rocks of mafic composition.