Sapphirine-bearing Granulites Research Articles

Occurrence of sapphirine-bearing granulites from Kothuru, Eastern Ghats Mobile Belt: implications on ultra-high temperature metamorphism

Occurrence of sapphirine-bearing granulites from Kothuru, Eastern Ghats Mobile Belt: implications on ultra-high temperature metamorphism

UHT Metamorphism Peaking Above 1100 °C with Slow Cooling: Insights from Pelitic Granulites in the Jining Complex, North China Craton

Abstract The peak temperature and duration of ultrahigh-temperature (UHT) metamorphism are critical to identify and understand its tectonic environment. The UHT metamorphism of the Jining complex in the Khondalite Belt, North China Craton is controversial on the peak temperature, time and tectonic setting. A representative sapphirine-bearing granulite sample is selected from the classic Tianpishan outcrop for addressing the metamorphic evolution and timing. The rock is markedly heterogeneous on centimetre scale and can be divided into melanocratic domains rich in sillimanite (MD-s) or rich in orthopyroxene (MD-o), and leucocratic domains (LD). On the basis of detailed petrographic analyses and phase equilibria modelling using THERMOCALC, all three types of domains record peak temperatures of 1120–1140 °C and a series of post-peak cooling stages at 0·8–0·9 GPa to the fluid-absent solidus (∼890 °C), followed by sub-solidus decompression. The peak temperature for MD-s is constrained by the coexistence of sillimanite-I + sapphirine + spinel + quartz, where sillimanite-I contains densely exsolved aciculae of hematite, yielding reintegrated Fe2O3 contents up to 2·1–2·3 wt %. The post-peak cooling evolution involves the sequential appearance of K-feldspar, sillimanite-II + garnet, orthopyroxene and biotite, where sillimanite-II is exsolution-free and contains variable Fe2O3 contents of 1·3–1·8 wt %. The peak temperature for MD-o is constrained by the sapphirine + orthopyroxene assemblage, where orthopyroxene has a maximum AlIV of 0·22 (Al2O3 = 9·5 wt %) in the core. The cooling evolution involves the sequential appearance of K-feldspar, garnet and biotite, and the decreasing AlIV (0·22→0·17) from core to rim in orthopyroxene. The peak temperature for LD is constrained by the inferred K-feldspar-absent assemblage and the maximum anorthite content of 0·11 in K-feldspar. The cooling evolution involves the crystallization of segregated melts, exsolution of supra-solvus ternary feldspars and growth of biotite. The Al in orthopyroxene, Fe2O3 in sillimanite and anorthite in K-feldspar are good indicators for constraining extreme UHT conditions although they depend differently on bulk-rock compositions. In-situ SHRIMP U–Pb dating of metamorphic zircon indicates that the UHT metamorphism may have occurred at >1·94 Ga and the cooling under UHT conditions lasted over 40 Ma. The extreme UHT metamorphism in the Jining complex is interpreted to be triggered by the advective heating of intraplate hyperthermal mafic magmas together with a plume-related hot mantle upwelling, following an orogenic crustal thickening event.

Extreme thermal metamorphism associated with Gondwana assembly: Evidence from sapphirine-bearing granulites of Rajapalayam, southern India

Abstract The Madurai block is the largest composite crustal block in the Southern Granulite terrane of India, where granulite-facies rocks metamorphosed at ultrahigh-temperature (UHT) conditions occur in several localities. Here, we investigated UHT rocks from Rajapalayam, in the southern domain of the Southern Granulite terrane, using integrated thermobarometry and in situ monazite geochronology to precisely constrain the nature and timing of this extreme metamorphism and its implications for regional tectonics. Conventional thermobarometry and petrological phase equilibrium modeling reveal prograde pressure-temperature (P-T) conditions at 0.75–1.2 GPa and <900 °C, followed by peak/postpeak UHT metamorphism at 0.72–0.82 GPa and 1025–1050 °C, and retrograde reequilibration at 0.72–0.80 GPa and 875–895 °C. The granulites thus record a clockwise P-T path defining geothermal gradients of 1200–1500 °C/GPa at peak metamorphism, indicating the presence of an extreme thermal perturbation in the middle to lower continental crust. In situ monazite dating indicates prograde metamorphism at 607–585 Ma, peak metamorphism at 546–543 Ma, and retrograde cooling and exhumation at 539–483 Ma. As such, the entire tectonothermal cycle was complete within ∼120 m.y., although temperatures exceeding 900 °C were likely sustained for at least 30 m.y. Such extreme thermal events preserved in geological terranes worldwide are commonly associated with lithospheric extension, although our data show that prolonged heating can occur during continental convergence instead, supporting inferences made by thermomechanical models. Thus, supercontinent formation may act as a driver for spatially distributed UHT tectonometamorphism, as shown by the episodic records in geological history. The age of peak metamorphism constrained here was synchronous with UHT metamorphism in other localities in the Southern Granulite terrane, Sri Lanka, Madagascar, and Antarctica, indicating their correlation with the final amalgamation of eastern Gondwana at ca. 550 Ma.

Sapphirine-bearing granulites of the anabar shield

The results of a detailed study of the mineral and chemical composition, geothermobarometry, and U-Pb isotope dating of zircon of high-aluminous sapphirine-bearing crystals of shales associated with biotite-garnet-sillimanite gneisses, hypersthene-two feldspar and high-potassium hypersthene orthogneisses, Ti-rich and subalkaline metabasites belonging to the Kilegirian Formation of the Daldyn series of the Anabar shield. Unlike the known granulites of the Daldin and Upper Ananbar series, the rocks of this association are enriched in K, Rb, Ba, Th, light REEs, differing in elevated (La/Yb)n. The results of thermobarometry yielded the P–Tparameters of formation and evolution of sapphirine-bearing granulites in the Anabar shield with peak values of UHT metamorphism in the range of T= 920–1000°C at P= 9–11 kbar. Isotope-geochronological data indicate a polymetamorphic evolution of these rocks. Detrital zircon cores in the center of crystals yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later, superimposed metamorphic transformations of the detrital zircon formed rims dated to 2.4, 2.3, 2.2, and 1.8 Ga. The timing of formation of aluminous metasedimentary and associated metamagmatic rocks can be estimated in the interval from 2.5 to 2.4 billion years. A potential provenance source of the detrital zircons could be hypersthene plagiogneisses and metabasites of the Daldyn series with a premetamorphic age no less than 3.3 Ga, and products of their metamorphism of about 2.7 Ga old, and possibly not yet detected in the region Na-K granites with an age of about 2.6–2.5 billion years, known on other shields and enriched with radioactive (K, Th, U) and rare elements. An additional source of clastic material in the formation of aluminous sediments could be the associated two feldspar magmatic rocks of rhyolite composition, having the same model age with them.

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

ABSTRACTThe Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.

Metamorfismo de temperatura ultra-alta de granulitos com safirina de 2,0 Ga do Bloco Itabuna-Salvador-Curaçá, Bahia, Brasil

A área de pesquisa se situa no Cráton do São Francisco, no sul da Bahia, Brasil. Nessa região, no segmento crustal Bloco Itabuna-Salvador-Curaçá (BISC), ocorrem rochas supracrustais associadas a gnaisses tonalíticos/trondhjemíticos a monzoníticos e rochas máficas subordinadas, de idades arqueana e paleoproterozoica, todos fortemente deformados e reequilibrados na fácies granulito em 2,07 a 2,08 Ga. As rochas supracrustais incluem granulitos com safirina constituídos por quartzo, ortopiroxênio, silimanita, granada, mesopertita, plagioclásio, biotita e cordierita. Com base nos arranjos microestruturais entre os minerais constituintes dos granulitos com safirina, a paragênese de pico metamórfico é Grt1 + Opx1 + Bt1 + Qtz + Sil1 + Spr1 + Mp. Microestruturas de reações entre minerais, como coroas de simplectitos ao redor de porfiroblastos, permitiram identificar as seguintes paragêneses retrógradas, interpretadas como formadas durante a ascensão tectônica dessas rochas: Grt1 ± Qtz = Opx2 + Sil2, Grt1 = Opx2 + Spr2 (+ Sil2); Opx1 + Sil1 + Qtz = Crd e Grt1 + Qtz = Opx3 + Crd. Por sua vez, simplectitos de Bt2 + Qtz formados pela reação Opx1-2 + Mp + H2O = Bt2 + Qtz estão relacionados à evolução retrógrada durante resfriamento. Cálculos termobarométricos realizados com aplicação de termobarômetros clássicos e método de multiequilíbrios (THERMOCALC, por exemplo) indicaram P = 7-11 kbar e T ≥ 900°C para os granulitos com safirina. Um dos mecanismos associados à elevada temperatura está relacionado à adição de material radioativo com a presença de magmas basálticos durante a formação do BISC. A cristalização fracionada desses magmas produziu intrusões tonalíticas que datam de aproximadamente de 2,15 a 2,13 Ga, próximo do pico do metamorfismo de 2,07 a 2,08 Ga.

Sapphirine-bearing granulites from the Tongbai orogen, China: Petrology, phase equilibria, zircon U-Pb geochronology and implications for Paleozoic ultrahigh temperature metamorphism

Abstract We report here for the first time the occurrence of sapphirine-bearing granulites within the Qinling Group of the Qinling-Tongbai orogen and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions. We document the UHT indicator of sapphirine and spinel in a mafic granulite consisting of orthopyroxene, biotite, plagioclase, amphibole and rutile/ilmenite. The ferromagnesian minerals in the sapphirine-bearing granulite have high X Mg [Mg/(Mg + Fe)] (orthopyroxene X Mg = 0.84–0.95; biotite X Mg = 0.81; amphibole X Mg = 0.87–0.96). The phase equilibria modeling demonstrates that the early spinel-bearing assemblage is stable at 923–950 °C and 6.7–8.9 kbar, and the peak assemblage of Opx + Pl + Spr/Spl + Amp + Bt + Ilm (+ melt) defines a field at 922–947 °C and 8.4–10.2 kbar. Rutiles have variable Zr concentrations but mostly cluster at ca. 1,500 and 3400 ppm. Zr-in-rutile geothermometry yielded high temperatures of up to 890–940 °C. Zircon U–Pb dating of the granulite constrains the timing of the immediate post-peak and retrograde metamorphic stages as 429 ± 7 Ma and 412 ± 4 Ma, respectively. The UHT metamorphism, together with extensive occurrence of coeval magmatic suites suggests that the Tongbai orogen experienced a Paleozoic Andean-type orogeny probably derived from mid-oceanic ridge subduction of the Qinling Ocean.

New finding of sillmanite in sapphirine-bearing granulites from Pedapalli, NE part of the Eastern Dharwar Craton, India

We report the occurrence of sillimanite in sapphirine-bearing granulites near Pedapalli town in the north eastern part of the Eastern Dharwar Craton (EDC). The sapphirine-bearing granulites of the Pedapalli area display evidence of metamorphic reactions involved in the formation of diverse mineral assemblages documented in different types of reaction textures. The common stable assemblage contains sapphirine-spinel-orthopyroxene-cordieriteplagioclage-corundum-sillimanite. The P-T evolution of these sapphirine granulites has been constrained by using the TWEEQU program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 °C, and pressure ca. 8 kbar. The assemblage reported in this study has important bearing on the high-temperature metamorphism and exhumation history of the EDC.

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740 °C and 6.5–7.5 kbar in Oligocene times (ca 30 Ma). Metamorphic conditions in the charnockites and granulites (>920 °C for 8.5–9.5 kbar) largely exceed these regional metamorphic conditions and are dated at 282–260 Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf complex.

Prolactine et cortisol : valeur prédictive chez les polytraumatisés

The Inner Mongolia Suture Zone marks the Paleoproterozoic collisional boundary between the Yinshan Block and Ordos Block in the North China Craton. The ultrahigh-temperature (UHT) orogen developed within this collisional suture reflects the extreme thermal conditions associated with the tectonic processes during the incorporation of the North China Craton within the Columbia supercontinent. Here we report the results from a systematic petrological and phase equilibria modeling study of sapphirine-bearing granulites from Daqingshan, a region within the central domain of the Inner Mongolia Suture Zone. The sapphirine–spinel–garnet–sillimanite–biotite-bearing granulites in Daqingshan occur typically as dark lenses or layers within spinel–garnet–sillimanite–biotite-bearing gneisses, and are associated with mafic granulites carrying orthopyroxene + clinopyroxene + plagioclase + calcic amphibole. The sapphirine in these rocks is magnesian (XMg = 0.73–0.79) and occurs in various microstructural associations such as: (1) matrix coarse-grain mineral in plagioclase aggregates, (2) subidioblastic mineral mantling spinel or in direct contact with spinel, (3) fine-grained phase in sillimanite, and (4) vermicular symplectitic aggregates with plagioclase. Garnet + plagioclase + sapphirine + sillimanite + ilmenite + inferred melt is considered to represent the peak assemblage in these rocks. We apply systematic phase equilibria analysis in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) to constrain the peak P–T conditions of the Daqingshan Mg–Al granulites. Our pseudosection analysis defines the stability of the peak assemblage at T > 970 °C at P > 11 kbar. The peak temperature derived from pseudosection analysis is broadly consistent with the temperature estimate from conventional thermometry of sapphirine–spinel pairs (ca. 940 °C). The extreme metamorphism recorded from the Mg–Al granulites from Daqingshan is also consistent with the occurrence of dry orthopyroxene + clinopyroxene + plagioclase, and orthopyroxene + pargasite + plagioclase in the associated mafic granulites, although conventional thermometry yields lower temperatures from these rocks (830–840 °C) marking the post-peak retrograde stage. Our data confirm UHT conditions in Daqingshan and indicate that the extreme metamorphism is of regional extent within the Inner Mongolia Suture Zone, in agreement with the model of ultra-hot metamorphic orogen developed within a Paleoproterozoic subduction–collision system.

Ultrahigh-temperature metamorphism in Daqingshan, Inner Mongolia Suture Zone, North China Craton

The Inner Mongolia Suture Zone marks the Paleoproterozoic collisional boundary between the Yinshan Block and Ordos Block in the North China Craton. The ultrahigh-temperature (UHT) orogen developed within this collisional suture reflects the extreme thermal conditions associated with the tectonic processes during the incorporation of the North China Craton within the Columbia supercontinent. Here we report the results from a systematic petrological and phase equilibria modeling study of sapphirine-bearing granulites from Daqingshan, a region within the central domain of the Inner Mongolia Suture Zone. The sapphirine–spinel–garnet–sillimanite–biotite-bearing granulites in Daqingshan occur typically as dark lenses or layers within spinel–garnet–sillimanite–biotite-bearing gneisses, and are associated with mafic granulites carrying orthopyroxene + clinopyroxene + plagioclase + calcic amphibole. The sapphirine in these rocks is magnesian ( X Mg = 0.73–0.79) and occurs in various microstructural associations such as: (1) matrix coarse-grain mineral in plagioclase aggregates, (2) subidioblastic mineral mantling spinel or in direct contact with spinel, (3) fine-grained phase in sillimanite, and (4) vermicular symplectitic aggregates with plagioclase. Garnet + plagioclase + sapphirine + sillimanite + ilmenite + inferred melt is considered to represent the peak assemblage in these rocks. We apply systematic phase equilibria analysis in the system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O–TiO 2–Fe 2O 3 (NCKFMASHTO) to constrain the peak P–T conditions of the Daqingshan Mg–Al granulites. Our pseudosection analysis defines the stability of the peak assemblage at T > 970 °C at P > 11 kbar. The peak temperature derived from pseudosection analysis is broadly consistent with the temperature estimate from conventional thermometry of sapphirine–spinel pairs (ca. 940 °C). The extreme metamorphism recorded from the Mg–Al granulites from Daqingshan is also consistent with the occurrence of dry orthopyroxene + clinopyroxene + plagioclase, and orthopyroxene + pargasite + plagioclase in the associated mafic granulites, although conventional thermometry yields lower temperatures from these rocks (830–840 °C) marking the post-peak retrograde stage. Our data confirm UHT conditions in Daqingshan and indicate that the extreme metamorphism is of regional extent within the Inner Mongolia Suture Zone, in agreement with the model of ultra-hot metamorphic orogen developed within a Paleoproterozoic subduction–collision system.

New SHRIMP U-Pb zircon ages of the metapelitic granulites from NW of Madurai, Southern India

Zircon cathodoluminescent imaging and SHRIMP U-Pb dating were carried out for metapelitic rocks (sapphirine-bearing granulites and garnet-cordierite gneisses) from the NW of Madurai, Southern India. The cathodoluminescence images reveal the complex, inhomogeneous internal structure having irregular-shaped core and overgrowths. Zircon grains have obliterated oscillatory zoning. SHRIMP U-Pb chronological results yield ages of 550±15 Ma and 530±50 Ma as a time of metamorphic overprint, and the age of 2509±12 Ma and 2509±30 Ma corresponding to a timing of protolith formation for sapphirine-bearing granulites and garnet-cordierite gneisses respectively. Zircon ages reflect that continental crust in the NW of Madurai region resulted from the recycling of Archaean protolith of an igneous origin similar to the preserved crust in the southern part of Dharwar craton. The present SHRIMP U-Pb zircon ages are in close agreement with earlier published Nd isotopic data which suggest an extended precrustal history of their protoliths. The abraded zircon grains indicate multiple recycling and repeated metamorphism that has ultimately resulted in present day continental crust exposed in Madurai region. These SHRIMP U-Pb zircon ages from metapelitic UHT granulites are also significant to understanding the architecture of the SGT during the amalgamation of Gondwana in Neoproterozoic time.

Chemical U-Th-Pb dating of monazite by 3D-Micro X-ray fluorescence analysis with synchrotron radiation

A confocal set-up for three-dimensional (3D) micro X-ray fluorescence (micro-XRF) was used at the mySpot beamline at BESSY II, which allows compositional depth profiling for various applications. We present results obtained with a confocal 3D micro-XRF set-up for chemical age dating using the U, Th and Pb concentrations of monazite within rock thin sections. The probing volume was determined to be approximately 21 × 21 × 24 μm 3 for W- L α using an excitation energy of 19 keV. The relative detection limits particularly for Pb are below 10 ppm (for counting times of 1000 s). Therefore, this 3D micro-XRF set-up is suitable for dating of minerals with low Pb concentrations as long as all Pb is radiogenic, allowing spatial resolution comparable to ion microprobe or laser ablation techniques. The set-up was tested on monazites that are well characterized by isotopic techniques and have a wide range of ages, varying from 20 Ma to 1.82 Ga. Reference materials (GM3, F6, 3345) can be reproduced within error. The spread in the ages of all points determined by 3D micro-XRF is within 8 % of the isotopic reference value. The average 3D micro-XRF dates reproduce the reference ages with discrepancies between 10 and 13 Ma which translate to deviations of 1–4 %. Younger monazite samples from the European Alps (Baceno2, Bi9801) show slightly older 3D micro-XRF dates than those determined by isotopic techniques and table-top micro-XRF on whole grains. We interpret this to be related to age heterogeneities smaller than the spatial resolution and/or contributions by common Pb, which is a limitation for this technique. The method was then applied to monazites of unknown age from sapphirine-bearing granulites originating from the Gruf Complex (Alps, N-Italy). The 3D micro-XRF mean date was determined as 33 ± 4.4 Ma (2 σ) and was verified by ID-TIMS techniques. This age can be reliably interpreted as the age of the high-temperature (H T ) event because the monazites are included in and intergrown with H T minerals (sapphirine, high-Al orthopyroxene).

The Columbia connection in North China

AbstractThe Archaean and Proterozoic geology, structure and metamorphism of the North China Craton (NCC) reveal that the amalgamated Eastern and Western blocks of the craton collided as a single entity with the Columbia supercontinent at 1.93 Ga, and that the Northern Hebei orogen correlates with the Transamazonian-Eburnian belts of Africa and South America, and the Svecofennian of Baltica. This metamorphic belt preserves evidence for extreme crustal metamorphism with diagnostic ultrahigh-temperature (UHT) assemblages such as sapphirine+quartz, spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature perthites which record temperatures exceeding 1000 °C and pressure above 12 kbar. The metamorphic P–T trajectory is characterized by initial isobaric cooling followed by steep isothermal decompression, defining an overall anticlockwise exhumation history. Electron probe monazite geochronology and precise SHRIMP zircon dating of sapphirine-bearing granulites constrain the timing of the UHT event at c. 1.92 Ga, suggesting that the UHT metamorphism coincided with collisional orogenesis as the North China Craton joined the Columbia supercontinent amalgam along the Northern Hebei orogen during the Palaeoproterozoic. Fluid inclusion studies in the UHT rocks provide evidence for the involvement of synmetamorphic pure CO2, linking the thermal anomaly and fluid flux to underplated mafic magmas during asthenospheric upwelling. Evidence for probable-plume related mafic magmatism is also provided by the extensive mafic dyke swarms cutting the region and elsewhere within the NCC, with geochemical characters testifying to emplacement within rifts that opened up during the extensional collapse of the orogen subsequent to the collisional event. Recognition of the Palaeoproterozoic Columbia suture in the Northern Hebei orogen represents a major paradigm shift, as one popular group of models for the NCC suggests that the Palaeoproterozoic suture resides in an older orogen, the Central Orogenic belt. However, either model for the location of the suture is able to explain metamorphic P–T–t data for crustal thickening at 1.85 Ga, whereas only the model for a Late Archaean collision in the Central Orogenic belt and a Palaeoproterozoic collision in the Northern Hebei orogen can explain the structural, sedimentological, geochronological, and petrological data. The Central Orogenic belt contains several hundred fragments of a c. 2.505 Ga ophiolite suite, a contemporaneous 2.5–2.4 Ga foreland basin deposited on 2.7–2.5 Ga passive margin sediments on the Eastern Block, and contains rare evidence for c. 2.5 Ga granulite facies metamorphism that was largely overprinted by 1.92–1.85 Ga high-grade assemblages. East-directed 2.5 Ga fold-thrust structures are overprinted by 1.92–1.85 Ga south-directed thrusts associated with large-scale thickening of the craton, succeeded by strike-slip shear zones that slice the orogen into numerous fault-bounded terranes that preserve different levels of exhumation.

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 (orthopyroxene 1 + spinel 1 + biotite 1). 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.

A new occurrence of sapphirine-bearing granulite from Podur, Andhra Pradesh, India

We report here on a new occurrence of sapphirine bearing granulites in the high-grade granite gneisses from Podur. Podur is situated in the north-eastern part of the Eastern Dharwar Craton about 150 km NNE of Hyderabad, India. The common stable assemblage contains cordierite-biotite-sapphirine-spinel-corundum-K-feldspar. Interpretation of multiphase reaction textures in conjunction with mineral chemical data and topology in the (Fe,Mg)O-(Al2O3-K2O)-(SiO2-6K2O) system is consistent with the following reactions: \({\rm biotite} + {\rm corundum} = {\rm spinel} + {\rm K\hbox{-}feldspar} + {\rm H_2O}\)\({\rm phlogopite\ (in\ biotite)} + {\rm corundum} = {\rm eastonite\ (in\ biotite)} + {\rm spinel} + {\rm cordierite}\)\({\rm spinel} + {\rm cordierite} = {{\rm sapphirine}_{2:2:1}}^{*}\)\({\rm cordierite} + {\rm biotite} + {\rm corundum} = {{\rm sapphirine}_{7:9:3}}^{*} + {\rm K\hbox{-}feldspar} + {\rm H_2O}\)\({\rm sapphirine}_{7:9:3} + {\rm biotite} = {\rm cordierite} + {\rm spinel} + {\rm K\hbox{-}feldspar} + {\rm H_2O}\)\({\rm sapphirine} = {\rm cordierite} + {\rm spinel} + {\rm corundum}\)

The Late Permian to Triassic Hongseong-Odesan Collision Belt in South Korea, and Its Tectonic Correlation with China and Japan

We propose a new correlation of Middle Triassic (ca. 230 Ma) eclogites from the Hongseong area of the southwest Gyeonggi Massif, South Korea, with the Dabie-Sulu collision belt of China. Late Permian (ca. 257 Ma) mangerites that intrude the Odesan area in the eastern part of the Gyeonggi Massif show geochemical characteristics of collisional tectonic settings, implying that the Hongseong collision belt extends to the Odesan area. In the Higo terrain of southernmost Japan, sapphirine-bearing granulites and related high-temperature metamorphic rocks reveal ca. 245 Ma ultrahigh-temperature (UHT) metamorphic conditions. This metamorphism is well matched with the 245 ± 10 Ma UHT metamorphism estimated for spinel granulite in the Odesan area, suggesting that the Dabie-Sulu collision zone continues through the Hongseong-Odesan belt into the Higo area and that Paleozoic subduction complexes in southwest Japan represent an eastern extension of the Dabie-Sulu collision belt. The Paleozoic subduction complexes in Japan continue further to the Yanji belt, a Carboniferous and Permian subduction complex along the northeastern boundary of the North China block. These data indicate that Phanerozoic subduction along the margin of the North China block and the collision between the North and South China blocks contributed to formation of the Dabie-Sulu-Hongseong-Odesan-Higo-Yanji belt. P-T estimations reveal that there was a decrease in geothermal gradients and an increase of exhumation rates from east to west along the belt during collision. These gradients resulted in preservation of UHT metamorphism in the Odesan area in Korea and the Higo area in Japan, high-pressure (HP) metamorphism in the Hongseong belt, and ultra high-pressure (UHP) metamorphism in the Dabie-Sulu belt.

Petrology and U–Th–Pb geochronology of the sapphirine–quartz-bearing metapelites from Rajapalayam, Madurai Block, Southern India: Evidence for polyphase Neoproterozoic high-grade metamorphism

Strongly migmatitic sapphirine-bearing granulites occur as dm- to m-sized enclaves in garnetiferous leucogranites in the southeastern part of the Madurai Block (MB), the largest section of Neoproterozoic crust in the Southern Granulite Terrain of India (SGT). Early mineral assemblages (biotite–sillimanite–quartz, garnet–biotite–quartz) break down via successive dehydration melting reactions to high- T and high- P phase assemblages (e.g. garnet-melt, orthopyroxene–sillimanite-melt, orthopyroxene–cordierite-melt). Melt-consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxene–sillimanite, orthopyroxene–sapphirine and/or orthopyroxene–cordierite as well as orthopyroxene–sillimanite assemblages to cordierite–biotite–quartz symplectites. The occurrence of UHT metamorphism is indicated by the presence of small inclusions of sapphirine–quartz intergrowths in garnet. This reaction sequence is interpreted in a KFMASH partial petrogenetic grid taking both melt-producing and melt-consuming reactions into account which yields a clockwise heating–cooling–decompression P– T path culminating at 8–11 kbar and 1000–1100 °C. In-situ U–Th–Pb monazite dating has been performed to more precisely constrain the ages given by chemical electron-microprobe on monazite in various textural positions. Monazite inclusions in garnet often yield a bimodal distribution of early (950–850 Ma) and late Neoproterozoic (600–500 Ma) ages which is in good agreement with the results of U–Pb and EPMA zircon dating in other places of the southeastern Madurai Block. By contrast, monazite inclusions in other mafic minerals (orthopyroxene, cordierite, biotite) or as interstitial phase exclusively yield ages between 550 and 480 Ma. The early Neoproterozoic population is interpreted to reflect the timing of migmatisation and peak UHT metamorphism, whereas the late Neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the Madurai Block.

U-Th-total Pb dating of monazite from orthogneisses and their ultra-high temperature metapelitic enclaves: implications for the multistage tectonic evolution of the Madurai Block, southern India

Electron microprobe dating of monazite was performed on enclaves of sapphirine-bearing granulites and their host granites from the southeastern part of the Madurai Block in India. The granulites are strongly migmatitic and comprise quartzofeldspathic leucosome veins and restitic domains of sapphirine, garnet, orthopyroxene, sillimanite and cordierite. Inclusions of sapphirine-quartz intergrowths in garnet testify to an UHT stage of metamorphism which was followed by a stage of decompression during which garnet was replaced by coronitic opx-sill intergrowths. Monazite inclusions in garnet, orthopyroxene and quartz often yield a bimodal age distribution of Mid- to early Neoproterozoic (1500-850 Ma) ages in the cores which are rimmed by late Neoproterozoic age domains (630-580 Ma). Monazites in the granite and the charnockite exclusively yield Pan-African values. The occurrence of sapphirine-quartz intergrowths and the early Neoproterozoic age population in the same petrographical position infers that the crystalline basement of the Madurai Block already experienced a stage of UHT metamorphism prior to the Pan-African orogenic event.

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at ∼ 2420 Ma, while those from Mettanganam show ∼ 2300 Ma peak. Younger zircons with peak ages of ∼ 2100 and ∼ 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with ∼ 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at ∼ 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma. While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.