Granulite Formation Research Articles

Coincidence of gabbro and granulite formation and their implication for Variscan HT metamorphism in the Moldanubian Zone (Bohemian Massif), example from the Kutná Hora Complex

Leucocratic metagabbro and amphibolite from a mafic–ultramafic body within migmatite and granulite in the Kutná Hora Complex were investigated. The mafic–ultramafic rocks show amphibolite facies metamorphism, but in the central part of the body some metagabbro preserves cumulus and intercumulus plagioclase, clinopyroxene and spinel. Spinel forms inclusions in both clinopyroxene and plagioclase and shows various degree of embayment structure, that was probably a result of reaction with melt during magmatic crystallization. In the metagabbro, garnet forms coronae around clinopyroxene at the contacts with plagioclase. Amphibolite contains garnet with prograde zoning and plagioclase. Phase relations of igneous and metamorphic minerals indicate that magmatic crystallization and subsequent metamorphism occurred as a result of isobaric cooling at a depth of 30–35km. U–Pb dating on zircon from leucogabbro yielded a Variscan age (337.7±2Ma) that is similar or close to the age of granulite facies metamorphism (ca 340Ma) in the Moldanubian Zone. Based on the calculated PT conditions and age data, both the mafic–ultramafic body and surrounding granulite shared the same exhumation path from their middle–lower crustal position at the end of Variscan orogeny. The coincidence of mafic–ultramafic intrusives and granulite–amphibolite facies metamorphism is explained by lithospheric upwelling beneath the Moldanubian Zone that occurred due to slab break-off during the final stages of subduction of the Moldanubian plate beneath the Teplá Barrandian Block. The model also addresses questions about the preservation of minerals and/or their compositions from the early metamorphic history of the rocks subjected to ultradeep subduction and subsequent granulite facies metamorphism.

The Study of Major Element Geochemistry of Migmatites in and Around Melur Region, Madurai District, Tamil Nadu, India

The study investigating the exposed Archean lower continental crust of the Southern Granulite Terrain, India, shield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of Granulites. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening. The Southern Granulite Terrain (SGT) in India is one of the largest exposed Precambrian deep continental crustal sections in the world consisting of multiply deformed Archean and Neoproterozoic high grade metamorphic and magmatic rock.

Origin and geodynamic significance of the early Mesozoic Weiya LP and HT granulites from the Chinese Eastern Tianshan

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U–Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic–pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granulites, and orthopyroxene–clinopyroxene-bearing granulites. Average P–T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600–650°C at 3.2–4.2kbar and peak metamorphism of 750–840°C at 2.9–6.3kbar, indicating a rather high geothermal gradient of ca. 60°C/km. Zircon U–Pb LA–ICP–MS dating revealed metamorphic ages between 244±1 to 237±3Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution.

Petrology and geochemistry of ultrahigh‐temperature granulites from the South Altay orogenic belt, northwestern China: Implications for metamorphic evolution and protolith composition

AbstractUltrahigh‐temperature (UHT) granulites in the South Altay orogenic belt of Northwestern China provide important clues for the lower crustal components and tectonic evolution of the Central Asian Orogenic Belt during the Paleozoic. In this paper, we studied whole‐rock geochemistry and mineral characteristics to understand the protolith and metamorphic evolution of the Altay UHT granulite. The Altay granulite shows negative discriminant function values (DF) of −9.27 to −3.95, indicating a sedimentary origin, probably an argillaceous rock. The peak metamorphic temperature–pressure conditions of 920–1010 °C and > 9 kbar were estimated from the geothermobarometry, together with the stability of spinel (low ZnO) + quartz and orthopyroxene (Al2O3 up to 9.2 wt.%) + sillimanite + quartz in the Altay UHT rock, indicate a UHT metamorphic condition has been achieved. Two stages of retrograde conditions are recognized in these rocks; the first is an isothermal decompression to approx. 750 °C at 5.2–5.8 kbar at the early stage, and the second is the cooling down to 520–550 °C at 4.8–5.2 kbar. Combined with previous study, the formation of the Altay UHT pelitic granulite with a clockwise retrograde P–T path is inferred to be related with collisional and accretional orogenic process between the Siberian and Kazakhstan–Junggar plates.

Gold Mineralisation in Chigargunta Area of the Kolar Schist Belt, South India- A Part of the Archean Greenstone Belt

Gold mineralisation is reported for the first time in ‘Champion gneiss’ (quartzo-feldspathic schist) a felsic unit,in the eastern sector of the Chigargunta area (Lat: 120 430300N, Long:780 150 000E) of the Kolar schist belt, South India, during 1979-80. Quartzo-feldspathic gneiss and hornblende schist are the predominant rock types of the area. These rocks are characterised by a strong pervasive foliation trending N50E –S50W to N200E - S200W with easterly dips of 70-85. In the northern part of the area the rocks are folded with the foliation which is axial planar. Lower to middle amphibolite facies metamorphism is widespread in the area. Gold mineralisation is localised along shear zones which are ductile to brittle in nature. These zones are parallel to subparallel to the trend of foliation in the host rock and are characterized by strong mylonitic fabric, profuse quartz venation and hydrothermal alteration. Pyrite and pyrrhotite are the dominant sulphides. Gold occurs in native form. This felsic hosted gold mineralisation termed as E-2 lode (for exploration purpose) is estimated to contain a reserve of 3.13 million tonnes averaging 4.7 g/t gold. Besides, this lode there are several loads ( E-1,E-3, etc) occur within mafic units as well as in felsic unit adjacent to E-2 lode with different reserves, within the schist belt. Epigenetic gold mineralisation shows a close temporal and spatial relationship to late Archean (2700-2500 m.y) crustal accretion, stabilisation and granulite formation in the South Indian Shield. The present work delineate, the nature of mineralization in felsic unitwithin the schist belt which was not considered as source rock of gold mineralization during the period and left untouched until the present work

Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 1. Petrology and pressure–temperature evolution

The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves of migmatitic metapelites. Reaction textures indicate two generations of mineral growth. Spinel–hercynite solid solution+quartz-bearing Fe–Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. The association of spinel and quartz, now separated by garnet and sillimanite coronas, suggests that the rocks underwent T>950°C peak metamorphism at P=6.5–7kbar, as constrained from P–T pseudosection. Coarse-grained orthopyroxene in garnet+cordierite-bearing metapelites shows the highest Al2O3 content up to 8.2wt.%, suggesting T=950–1000°C. Temperatures estimated from mesoperthite and plagioclase pairs and other geothermometers (T=900–1000°C) further support ultrahigh-temperature metamorphism in the Ongole domain. The absence of cordierite in retrograde mineral assemblages points to isobaric cooling, giving rise to a near isobaric heating–cooling trajectory. The second generation of garnet, occurring as thick overgrowths over coarse-grained garnet porphyroblasts, in metapelites and especially in charnoenderbites have higher grossular contents than the porphyroblasts. Thermobarometry and P–T pseudosections indicate that they formed at a higher pressure but lower temperature (ca. 780°C, 9.5kbar) during a second metamorphic event. Orthopyroxene+sillimanite±kyanite±spinel symplectites replacing cordierite is most likely formed during this second metamorphic event. The presence of sillimanite, kyanite as well as andalusite along with coarse-grained retrograde biotite in the same rock marks the last stage of the second metamorphic event characterized by isobaric cooling just below the aluminosilicate triple point. This further suggests near-isothermal decompression from higher pressures. UHT metamorphism at low pressures during the first metamorphic event is most likely caused by magma emplacement. The higher pressure but lower temperature conditions achieved during the second metamorphic event is due to crustal thickening during collision of continental blocks. Near-isothermal decompression to ca. 4kbar points to subsequent rapid exhumation of the over thickened crust. The two-stage evolutionary history of the Ongole domain fits well to the expected evolution of a magmatic arc, in which the first can be attributed to magmatic heat advection during arc growth and the second to crustal thickening during collision.

Late neoproterozoic igneous complexes of the western Baikal-Muya Belt: Formation stages

The paper presents new geological, geochemical, and isotopic data on igneous rocks from a thoroughly studied area in the western Baikal-Muya Belt, which is a representative segment of the Neoproterozoic framework of the Siberian Craton. Three rock associations are distinguished in the studied area: granulite-enderbite-charnockite and ultramafic-mafic complexes followed by the latest tonalite-plagiogranitegranite series corresponding to adakite in geochemical characteristics. Tonalites and granites intrude the metamorphic and gabbroic rocks of the Tonky Mys Point, as well as Slyudyanka and Kurlinka intrusions. The tonalites yielded a U-Pb zircon age of 595 ± 5 Ma. The geochronological and geological information indicate that no later than a few tens of Ma after granulite formation they were transferred to the upper lithosphere level. The Sm-Nd isotopic data show that juvenile material occurs in rocks of granitoid series (ɛNd(t) = 3.2–7.1). Ophiolites, island-arc series, eclogites, and molasse sequences have been reviewed as indicators of Neoproterozoic geodynamic settings that existed in the Baikal-Muya Belt. The implications of spatially associated granulites and ultramafic-mafic intrusions, as well as granitoids with adakitic geochemical characteristics for paleogeodynamic reconstructions of the western Baikal-Muya Belt, are discussed together with other structural elements of the Central Asian Belt adjoining the Siberian Platform in the south.

Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 2. LA-ICP-MS zircon dating and texturally controlled in-situ monazite dating

To quantify the Mesoproterozoic pressure–temperature–time evolution of the granulite-facies Ongole domain, a deep crustal Proterozoic magmatic arc in the Eastern Ghats Belt, LA-ICP-MS U–Pb zircon dating and in-situ electron microprobe U–Th–total Pb monazite dating are integrated with the petrologic observations. Zircon grains in the metapelites often preserve oscillatory-zoned, sometimes partly resorbed cores yielding a spread of 207Pb/206Pb ages between ca. 2700 and ca. 1750Ma, interpreted to reflect the age of detrital grains from the surrounding crustal blocks. Metamorphic zircon overgrowths and rims yield Palaeoproterozoic concordia ages from about 1625 to 1600Ma, indicating the timing and duration of an ultrahigh-temperature (UHT) metamorphic event.Texturally controlled in-situ monazite dating however reveals two metamorphic events separated by 60 to 80Ma. In metapelites, the monazite inclusions in garnet and cores of some matrix monazite yield weighted mean ages of ca. 1610Ma, similar to the concordia ages from zircon and indicate the timing of the UHT metamorphism. The monazite grains in the matrix, growing along with a second generation of garnet and in late-stage symplectites are highly recrystallized and yield weighted mean ages of ca. 1540Ma, indicating the timing of a second metamorphism at higher pressures but lower temperatures. The weighted mean ages obtained from metapelites in the adjoining Vinjamuru domain are the same as those obtained from the Ongole domain. The chemical differences between the two generations of monazite (Th, Y and HREE contents) support the above interpretation of monazite growth under different conditions and at different times. Monazite in the charnoenderbites records the second metamorphic event but only rarely the earlier UHT event. In addition, they show evidence for later imprints of ductile to brittle deformation between ca. 1450 and 1360Ma, which may correlate with Mesoproterozoic crustal extension and alkaline plutonism along the western boundary of the Eastern Ghats Belt. Though the Ongole domain was not involved in major crustal reworking during the Neoproterozoic, monazites do record minor disturbances at ca. 730Ma and ca. 510Ma. Hence the combination of zircon and monazite dating unveils the sequence of tectonothermal events in the Ongole domain in a hitherto unknown exactness: UHT metamorphism at ca. 1610Ma due to magmatic heat advection, subsequently, 60 to 80Ma later, a second metamorphic event at medium-pressure and lower-temperature due to the collision of the arc with the Indian continent, and finally minor disturbances due to rifting between ca. 1450 and 1360Ma.

Proterozoic granulite formation driven by mafic magmatism: An example from the Fraser Range Metamorphics, Western Australia

Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293Ma. Peak metamorphic conditions at c. 1290Ma were c. 850°C at pressures of 7–9kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290Ma, essentially coincident with the emplacement of mafic rocks at 1292Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400°C) occurred at 1260Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234±17Ma.

Chlorine-rich fluid or melt activity during granulite facies metamorphism in the Late Proterozoic to Cambrian continental collision zone—An example from the Sør Rondane Mountains, East Antarctica

In the granulite facies rocks, the role of low H2O activity fluids is still unclear. The importance of understanding its role is gradually being recognized, and it enables us to help understand the process of granulite formation. Attention has been specifically focused on Cl-rich fluids and high-salinity alkaline fluids in the geochemical modification of continental crust. We have investigated the field distribution of Cl-rich biotite in the pelitic gneisses of the Sør Rondane Mountains, East Antarctica where Late Proterozoic to Cambrian granulites are widely exposed. Among 27 samples studied, a garnet-biotite-sillimanite gneiss from Balchenfjella was selected as best suited sample to constrain the P-T-t conditions of Cl-rich fluid or melt activity. This gneiss contains garnet porphyroblasts that have a P-rich core with oscillatory zoning in P. The garnet core includes Cl-poor biotite and fluorapatite. This core has been partially resorbed and discontinuously overgrown by a P-poor rim, in which Cl-rich biotite and chlorapatite are included. Coarse-grained, rounded zircon grains are exclusively included in the rim of the garnet porphyroblast and are also present in the matrix. This mode of occurrence suggests that the Cl-rich biotite and chlorapatite, together with coarse-grained zircon were formed almost simultaneously. The P-T conditions of Cl-rich biotite entrapment in the garnet rim are estimated to be ca. 800°C and 0.8GPa. In comparison, peak metamorphic condition is ca. 850°C and 1.1GPa. These pieces of observation suggest that Cl-rich fluid or melt infiltrated at the core-rim boundary of garnet. In the case of fluid infiltration, the fHCl/fH2O ratio of the fluid in equilibrium with Cl-rich biotite and chlorapatite in the garnet rim are estimated to be ten times larger than that in equilibrium with Cl-poor biotite and fluorapatite in the matrix and the garnet core. The LA-ICP-MS U-Pb dating of the coarse-grained zircon included in the garnet rim gave concordia age of 603±14Ma. Therefore, Cl-rich fluid or melt infiltration took place close to the metamorphic peak condition of ca. 800°C and 0.8GPa at 603±14Ma. The field distribution of Cl-rich fluid or melt activity is somewhat linear in the Sør Rondane Mountains. High Cl-activity is located near the large scale ductile shear zones, suggesting its relation to high-strain zones. Regional distribution of high-grade Cl-rich fluid or melt activity in the Sør Rondane Mountains implies that it is one of the major phenomena in the continental collision processes.

Granulite formation in a Gondwana fragment: petrology and mineral equilibrium modeling of incipient charnockite from Mavadi, southern India

We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Grt + Opx + Ilm + Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs + Qtz + Pl + Bt + Grt + Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T > 900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H2O (M(H2O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H2O) = 0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H2O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.

Introduction to a virtual special issue on crustal melting

This virtual special issue represents a collection of papers concerning Crustal Melting selected by the Editor from those published on various aspects of this theme in the JMG since 1982. The papers are grouped into sequences that address topics that have been prominent in the JMG during the last 30 years concerning the origin, evolution and tectonic role of migmatites and migmatitic granulites in crustal evolution. These topics are: Open‐ and closed‐system processes in the formation of migmatites and migmatitic granulites; thermobarometry, P–T paths, phase equilibria modelling and retrograde processes in formerly melt‐bearing rocks; geochronology in partially melted rocks; and, microstructures, deformation and tectonics of melt bearing rocks.About one‐third of the papers published in the JMG since its inception concern the origin, evolution and tectonic significance of migmatites and migmatitic granulites in crustal evolution, including the first special issue published by the JMG concerning ‘Studies in the genesis and deformation of migmatites’ edited by Tracy &amp; Day (1988; Volume 6, Issue 4, Pages 385‐543). Three subsequent Special Issues of the JMG include papers relevant to the theme of this virtual special issue; they are ‘Metamorphic processes: a celebration of the career contribution of Ron Vernon’, ‘Processes in granulite metamorphism’ and ‘Granulites, partial melting and the rheology of the lower crust’, edited by Brown &amp; Clarke (2002; Volume 20, Issue 1, Pages 1‐213), Brown &amp; White (2008; Volume 26, Issue 2, Pages 121‐299) and Brown et al. (2011; Volume 29, Issue 1, Pages 1‐166), respectively.The selection of papers in this virtual special issue is by no means comprehensive, but it is intended as a representative selection of what has been published in the JMG over 30 years to give the reader a broad overview of crustal melting. Furthermore, although many papers address more than one topic, each is included only once and has been placed within the most appropriate section.

Phase equilibrium modeling of incipient charnockite formation in NCKFMASHTO and MnNCKFMASHTO systems: A case study from Rajapalaiyam, Madurai Block, southern India

Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust. Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block, southern India, and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature (UHT) metamorphism have been reported. Proximal to the UHT rocks are patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm) occurring within Opx-free Grt-Bt gneiss (Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt) which we report in this study. The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of ∼820 °C and ∼9 kbar. Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T (∼760 °C and ∼7.5 kbar), which is consistent with the results obtained from geothermobarometry (710–760 °C, 6.7–7.5 kbar), but significantly lower than the peak temperatures (>1000 °C) recorded from the UHT rocks in this locality, suggesting that charnockitization is a post-peak event. The modeling of T versus molar H2O content in the rock (M(H2O)) demonstrates that the Opx-bearing assemblage in charnockite and Opx-free assemblage in Grt-Bt gneiss are both stable at M(H2O) = 0.3 mol%–0.6 mol%, and there is no significant difference in water activity between the two domains. Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid. T-XFe3+ (=Fe2O3/(FeO + Fe2O3) in mole) pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene; Opx is stable at XFe3+ <0.03 in charnockite, while Opx-free assemblage in Grt-Bt gneiss is stabilized at XFe3+ >0.12. Such low oxygen fugacity conditions of XFe3+ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid (e.g., H2O + CH4) during the retrograde stage.

Strain localization, granulite formation and geodynamic setting of ‘hot orogens’: a case study from the Eastern Ghats Province, India

The Eastern Ghats Province underwent major orogenic events in the Neoproterozoic–Cambrian period: 980–930, 900–650 and 550–500 Ma. At each time interval, deformation occurred synchronous with high‐grade metamorphism. The first event was characterized by distributed strain throughout the entire province, while strain and thermal anomalies during the later orogenies were confined to the province boundaries. In the first case, pre‐orogenic rifting caused ultrahigh‐temperature metamorphism at the base of the crust, and was closely followed by crustal shortening related to the collision of the granulite belt with the Indian craton. The thermal anomaly persisting from the rifting event, followed by the thermal relaxation associated with crustal shortening led to prolific melt production and transfer of heat producing elements (HPE) into the middle and upper parts of the thick post‐collisional crust. Heating and associated melting caused rheological weakening of the crustal section, and explains why strain was distributed across the entire thermally perturbed zone. Erosion removed a substantial portion of the HPE‐rich upper crust, and deposited the detritus in cratonic sedimentary basins to the west. Subsequently, the rheologically stronger Eastern Ghats Province crust could effectively transmit stresses and concentrate them along pre‐existing zones of weakness near the province boundaries. Progressive thrusting in these domains led to consistent loading of the footwall, even as it underwent thermal relaxation. Since both strain and heating were associated with the footwall of pre‐existing discontinuities, the ‘hot orogen’ so formed was also spatially restricted. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Tectonothermal evolution of the Banded Gneissic Complex in central Rajasthan, NW India: Present status and correlation

In this study, we make a critical synthesis of the available magmatic, metamorphic and geochronological data, particularly from the Banded Gneissic Complex of central Rajasthan in northwestern India. Based on well-constrained database, we demonstrate that the Precambrian crust of this segment of Peninsular India records a protracted sedimentational, tectono-magmatic and tectono-metamorphic history over a period of >1Ga from c. 2.2–2.1Ga to 0.95–0.88Ga that can be correlated with the accretion and dispersal of two supercontinents, namely Columbia and Rodinia. Following the stability of the Archean craton at ∼2.5Ga, the Aravalli basin opened up by ∼2.2–2.1Ga on a tonalite–trondhjemite–granodiorite–amphibolite basement, and closed at ∼1.8–1.9Ga due to westward subduction of the Aravalli crust during the Aravalli orogeny. This can be correlated with the global accretionary process associated with the formation of Columbia. A-type granite magmatism in the North-Delhi Fold Belt at 1.7Ga and the formation of medium-pressure granulites, synchronous with mafic and felsic magmatism with or without orthopyroxene at ∼1.74–1.72Ga in the Sandmata Metamorphic Complex (SMC) are related to a switching of tectonic style from subduction to extension or from subduction to a cycle of back-arc extension and back-arc closure. The supracrustal and metaigneous granulites of the SMC and the A-type granites of the North-Delhi Fold Belt acted as basement for the opening up of two Mesoproterozoic sedimentary basins: the Delhi Basin to the west and the Mangalwar Basin to the east. These basins were closed with the onset of a continent–continent collisional orogeny at ∼0.95–0.88Ga (cf. Grenvillian orogeny), producing an extensive, amalgamated crustal domain, involving the inverted Delhi and Mangalwar basins, their reworked basement rocks and the cratonic domains lying to the east and the west of these basins. Crustal amalgamations and stitching of cratons/microcontinents at c. 1.0Ga in northwestern India are also correlatable with collisional tectonics in other Proterozoic mobile belts of central, eastern and southeastern India, and constitute an important part on the growth and the assembly of the Greater Indian Landmass in Rodinia.

Petrology and tectonic significance of the early Mesozoic granulite xenoliths from the eastern Inner Mongolia, China

Granulite xenoliths are found in the early Mesozoic diorite intrusions from Chifeng and Ningcheng areas, eastern Inner Mongolia. The granulites are granoblastic and weakly gneissic with mineral assemblage of hypersthene, diopside, plagioclase and minor biotite, amphibole and ilmenite. Some samples contain the intergrowth composed of labradorite and vermicular hypersthene, and some coarse-grained plagioclases of andesine and labradorite composition occasionally develop bytownite rims with vermicular hypersthene, indicating a possible presence of garnet. Presence of blastogabbroic texture and hypersthene with diopside exsolution lamellae in some samples suggests that the protolith of the granulite is norite or gabbro. On the basis of metamorphic research and thermobaric calculation, the evolution of the granulite xenoliths is summarized into the following stages: (1) Isobaric cooling of underplated noritic or gabbroic magma in the lower crust led to the formation of probable garnet-bearing medium-high pressure granulite. (2) These higher pressure granulites were adiabatically uplifted to upper crust by dioritic magma and transformed to low pressure two-pyroxene granulite during an isothermal decompression. (3) The two-pyroxene granulite underwent retrograde metamorphism of different degrees during an isobaric cooling process as a result of crystallization and cooling of the dioritic magma. The pyroxenite-dominated cumulates and the medium-high pressure granulites may have rejuvenated the lower crust during the early Mesozoic.

Silurian granulite-facies metamorphism, and coeval magmatism and crustal growth in the Tongbai orogen, central China

It is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth. The Tongbai orogen is one of the particular areas in the Qinling–Tongbai–Dabie–Sulu orogen that contains both granulites and associated arc magmatic rocks, and thus is an optimal place to address this issue. However, the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained. In situ LA (MC)-ICPMS U–Pb dating, and trace element and Lu–Hf isotope analyses were undertaken on zircon grains from gneiss, granulite, gabbro and granodiorite samples. Most zircons in three granulite samples and a gneiss sample have core–rim structures. The cores have oval or rounded shapes, oscillatory zones, high Th/U ratios and variable U–Pb ages, suggesting a detrital genesis. The youngest group of the detrital zircons yields a weighted mean 206Pb/238U age of 450±5Ma, which is interpreted as the maximum depositional age of their protoliths. A major age cluster is ca. 450–490Ma, corresponding to prolonged arc magmatism in the northern Tongbai and Qinling orogens. Another group of ages range from 660 to 950Ma, implying that the Qinling Group of the Tongbai orogen belongs to the South China Block (SCB). Metamorphic zircons in the granulites have weak or no zoning, low Th/U ratios and REE contents, negative Eu anomalies, and relatively flat HREE patterns, which indicate that they formed contemporaneously with garnet and plagioclase, and thus under granulite-facies metamorphic conditions. Their weighted mean age of 424±4Ma is taken as the best estimated age of the granulite-facies metamorphism. Zircons in a leucosome sample have significant negative Eu anomalies and relatively flat HREE patterns with a formation age of 428±4Ma, indicating partial melting coeval with the granulite-facies metamorphism. Metamorphic zircons in the gneiss show apparently negative Eu anomalies, and variable Th/U ratios and HREE contents. They yield a weighted mean 206Pb/238U age of 438±4Ma, which was suggested to record the timing of the prograde metamorphism. Zircon grains in a gabbro and a granodiorite yield U–Pb ages of 432±4 and 424±4Ma, respectively, which are interpreted as their formation ages and identical to the ages of the granulite-facies metamorphism. They have positive εHf (t) values of up to 5.1, showing clear evidence for crustal growth during their formation. The appearance of the Silurian granulite-facies metamorphism and magmatism in the Tongbai orogen provides a good paradigm of coeval granulite-facies metamorphism, magmatism and crustal growth in an arc-continent collision orogenic belt.

Metamorphic P-T evolution of granulites in the central Ribeira Fold Belt, SE Brazil

Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 ± 50 °C; P = 8 ± 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T ≈ 500 °C and P ≈ 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites.

Finding of high-pressure mafic granulites in the Amdo basement, central Tibet

High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement, central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet, which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860–920°C and 1.46–1.56 GPa, which retrogressed from post-peak phase at 820–890°C and 0.88–1.15 GPa to amphibolite facies at 550–670°C and 0.52–0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ∼50 km and then were rapidly exhumed to a shallow crustal level (∼20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic, which is a crucial step to the tectonic evolution of the Tibetan Plateau.

Transpressive dextral shear in the Italva-Itaperuna section, Northern State of Rio de Janeiro, Brazil

Structural analysis carried out on a segment of the Neoproterozoic Ribeira Belt, southeastern Brazil, show that it represents part of the transpressive dextral orogen related to the Central Mantiqueira Province. NNE-trending and steeply dipping regional mylonitic belts form anastomosed geometry, and describe a map-scale, S-C-like structure that is characterized by their deflection towards NE near the Além Paraíba Lineament. Lithological and structural control related to deformation partition were responsible for the formation of felsic mylonitic granulites with S-type granites lenses developed in ductile shear zones, alternated with less deformed intermediate to basic granulites associated with charnockites. The dextral shear sense indicators are consistent with transpressive deformation in the region and are common especially at the border of the main shear zones. The presence of S-type leucogranite may lead to variations of linear and planar relationships, which result in local extension zones. These elements are consistent with oblique continental collision considering the São Francisco Craton as a stable block.