Exhumation Of Granulites Research Articles

Exhumation of an Archean Granulite Terrane by Paleoproterozoic Orogenesis: Evidence from the North China Craton

Abstract Granulites represent high-grade metamorphic rocks of the deep continental crust. The metamorphism and exhumation of granulites from Archean terranes provide insights into the crustal evolution of Archean cratons and shed light on the formation and reactivation of cratons. We present petrology, U–Pb geochronology (zircon, rutile, and titanite), and pressure–temperature (P–T) paths for metadiabase dikes in an Archean granulite terrane of the North China Craton. Garnet (Grt) coronae in the metadiabase dikes are developed between plagioclase (Pl) and clinopyroxene (Cpx) via the reaction Pligneous + Cpx → Pl1 + Grt ± quartz. The reaction proceeds inward within the plagioclase, progressively consuming Pligneous (XAn = 0.65–0.69) and leaving Ca-poor Pl1 (XAn = 0.46–0.53) as the residue. Geothermobarometry and P–T pseudosections suggest peak conditions for garnet formation at ~800°C and 10–13 kbar. During retrograde metamorphism, Grt broke down to Ca-rich Pl2 (XAn = 0.73–0.74), and ilmenite replaced rutile. Geothermobarometry and Zr-in-titanite temperatures constrain the P–T conditions of retrograde metamorphism at 700–750°C and 4.5–7.5 kbar. Zircon and titanite U–Pb geochronology shows that the protolith of the metadiabase dike was formed at 2.4 Ga and underwent granulite-facies metamorphism at 1.86 Ga. The intrusion of mafic dikes into the Archean granulite terrane indicates that the Archean basement was also heated and buried in the Paleoproterozoic. The metadiabase dikes and the hosting Archean basement underwent Paleoproterozoic granulite-facies metamorphism at a depth of ~40 km, followed by near-isothermal decompression and subsequent near-isobaric cooling (cooling rate of 1–3°C Myr−1) at depths of 15–25 km. Crustal shortening and thickening may have been caused by the underplating of the Khondalite series beneath the Archean basement during the amalgamation of supercontinent Columbia. The Paleoproterozoic orogeny induced a second generation of metamorphism of the Archean basement along the margin of the craton and drove the exhumation of the Archean granulite terranes to the middle crust (~15 km).

Melt- to Shear-Controlled Exhumation of Granulites in Granite–Gneiss Domes: Petrological Perspectives from Metapelite of the Neoarchean Ha-Tshanzi Structure, Central Zone, Limpopo Complex, South Africa

Abstract Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean–Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810–830 °C) exhumation from pressure of 10·0–10·2 kbar to 7·5–7·0 kbar during the Neoarchean event (2·68–2·62 Ga). The exhumation mediated by anatectic melt supports interpretation of the Ha-Tshanzi structure as a diapir-related granite–gneiss dome. During upwelling of the dome, the melt segregated into leucosome, while growth of cordierite sequestered water from the melt, assisting its crystallization at the end of the sub-isothermal decompression stage. As the rheology of the rock changed, melt-dominated deformation was transformed to solid-dominated ductile shear deformation. In contrast to the earlier sub-isothermal decompression P–T path, the gentler slope of ~75 °C kbar–1 of the decompression-cooling path marks the exhumation from pressures of ~7 kbar to pressures of 5–4·5 kbar and cooling to 600–550 °C. Dating of zircon, monazite and rutile shows that the Neoarchean evolution of the metapelite was strongly overprinted by the Paleoproterozoic event at c. 2·01 Ga. The results of the study highlight the significance of domed structures related to granitic diapirs in the exhumation of the Central Zone of the Limpopo Complex.

Thick-skinned tectonics of the Achankovil Shear Zone, southern India, inferred from new deep seismic reflection image: Constraints on the East African Orogen

The East Gondwana was formed by the assembly of continental fragments, namely India, Madagascar, Sri Lanka, and Antarctica, with the evolution of 2000 km long East African Orogen around 550 Ma. Lack of knowledge on the subsurface structure of the orogen led to a wide range of evolutionary models. The present study attempts to resolve various models’ ambiguity by presenting new results of the first deep seismic reflection study across the Achankovil Shear Zone, Southern Granulite Terrain, India that was evolved during the East African Orogen. The seismic study is also the first of its kind among the east Gondwana fragments. The Common reflection surface stack images of the present study reveal hitherto unknown details along with a unique south-dipping reflection fabric extending from the surface to upper mantle depth. We interpret this fabric is formed due to subduction-accretion of Madurai and Trivandrum blocks during the East African Orogen and represents a suture zone. Two deep-seated shear zones, the Achankovil and Tenmalai were also developed at both ends of collision fabric with strike-slip characteristics. Orogenic collapse and post-orogenic extension are inferred from the subhorizontal lower-crustal reflection fabric, which is interpreted as an equilibrated younger Moho. The present study provided an opportunity to understand the formation and exhumation of granulites, which act as a window on the inaccessible lower continental crust. Gravity modelling is performed along the seismic profile. Integration of present results with the existing geological and geophysical data provided basic constraints on the structure and tectonics of east Gondwana fragments and the East African Orogen.

Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India)

The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW India, took place along NNW-SSE trending D2-shear zones. The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle-ductile-transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0 implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing. We interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with that of the Himalayas.

Cryptic regional magmatism in the southern Saharan Metacraton at 580 Ma

The Saharan Metacraton is largely covered by the Sahel and Sahara with only sparse bedrock exposure making it one of the most poorly studied cratons in the World. Consequently, detrital zircon geochronology is invaluable to discern the lithotectonic development of the Saharan Metacraton in the absence of outcrop. Detrital zircons from the northern Chari River, the largest river system of North-Central Africa, have ages between ~700 Ma and ~500 Ma with a small Cambrian to Ordovician (486 ± 11 Ma and 422 ± 11) population, a few Devonian to Middle Jurassic zircons (394 ± 9 Ma–159 ± 4) and a single Mesoproterozoic (959 ± 19 Ma) zircon. The absence of ancient detrital zircons suggests that the crust in the vicinity of Lake Chad is primarily juvenile. A significant (~17%) proportion of zircons yielded ages of ~580 Ma that are broadly coeval with nappe and wrench faulting, and granulite exhumation in the Central African and Trans-Saharan orogenic belts. Moreover, we identified a gabbro from the Guéra Massif of south-central Chad that yielded an in situ zircon weighted-mean 238U/206Pb age of 580 ± 6 Ma. The age of the gabbro is similar to the Kekem gabbro-norite complex (576 ± 4 Ma) in west Cameroon, inherited zircons from silicic rocks near Lake Fitri and Lake Chad, and the detrital zircons from the Chari River. Our results unequivocally demonstrate that there was magmatism at ~580 Ma and that it was contemporaneous with deformation along the Tcholliré-Banyo and Central Cameroon shear zones indicating there was a major regional tectonomagmatic episode at that time. We suggest the mafic magmatism was related to post-orogenic mantle upwelling that may have been responsible for or contributed to lithospheric delamination of the southern Saharan Metacraton.

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.

Monazite Dating of Prograde and Retrograde P–T–d paths in the Barrovian terrane of the Thaya window, Bohemian Massif

Monazite laser ablation–split-stream inductively coupled plasma–mass spectrometry (LASS) was used to date monazite in situ in Barrovian-type micaschists of the Moravian zone in the Thaya window, Bohemian Massif. Petrography and garnet zoning combined with pseudosection modelling show that rocks from staurolite–chlorite, staurolite, kyanite and kyanite–sillimanite zones record burial in the S1 fabric under a moderate geothermal gradient from 4–4·5 kbar and ∼530–540°C to 5 kbar and 570°C, 6–7 kbar and 600–640°C, 7·5–8 kbar and 630–650°C, and 8 kbar and 650°C, respectively. In the kyanite and kyanite–sillimanite zones, garnet rim chemistry and local syntectonic replacement of garnet by sillimanite–biotite aggregates point to re-equilibration at 5·5–6 kbar and 630–650°C in the S2 fabric. Heterogeneously developed retrograde shear zones (S3) are marked by widespread chloritization, but minor chlorite is present in the studied samples. Monazite abundance and size increase with metamorphic grade from 5 µm in the staurolite–chlorite zone to >100 µm in the kyanite and kyanite–sillimanite zones. Irrespective of the monazite-forming reaction, this is interpreted as the onset of limited prograde monazite growth at staurolite grade, and continued prograde monazite growth after the kyanite-in reaction, compatible with conditions of about 5·5 kbar and 570°C and 7·5 kbar and 630°C from pseudosection modelling. Monazite is zoned, showing embayments and sharp boundaries between zones, with low Y in the staurolite zone, high-Y cores and low-Y rims in the kyanite zone, and high-Y cores, a low-Y mantle and a high-Y rim in the sillimanite zone. The 207Pb-corrected 238U/206Pb ages from three samples range from 344 ± 7 to 330 ± 7 Ma, irrespective of metamorphic grade. The dates from monazite inclusions are interpreted as the ages of the staurolite- and kyanite-in reactions along the prograde path at 340 and 337 ± 7 Ma, respectively. The monazite in the matrix (and some inclusions) is interpreted as dating the prograde crystallization at (340–337) ± 7 Ma within the S1 fabric, and then being affected by recrystallization at or down to 332 ± 7 Ma in the S2 and S3 fabrics. The two groups of data, for 340–337 and 332 Ma, are significantly different when only their in-run uncertainties (±1–3 Myr) are compared and indicate a 9 ± 3 Myr period of monazite (re)crystallization. A systematic increase in heavy rare earth element (HREE) content with decreasing monazite age from 344 to 335 Ma is correlated with growth on the prograde P–T path; the drop in HREE of monazite at 335–328 Ma is assigned to recrystallization. The presence of chlorite even in the least retrogressed samples witnesses limited external fluid availability on the retrograde P–T path. Migration of this fluid was probably responsible for heterogeneous fluid-assisted recrystallization and resetting of original prograde monazite, even where included in garnet, staurolite or kyanite. It is suggested that the rocks passed the chlorite-in reaction on the retrograde path at 332 ± 7 Ma. The timing of burial in the Thaya window, a deformed part of the underthrust Brunia microcontinent, was coeval with exhumation of granulites and migmatites of the Moldanubian orogenic root at c. 340 Ma.

Fluid-assisted interaction of peraluminous metapelites with trondhjemitic magma within the Petronella shear-zone, Limpopo Complex, South Africa

The principal problem concerning the evolution of Precambrian granulite complexes located between granite-greenstone cratons is their interaction with the underthrusted greenstone belts during exhumation from the lower crust. Besides evidence for pervasive and localized fluid fluxes arising from devolatilization of the greenschist and amphibolite-facies rocks in the course of their burial underneath the granulites, this interaction might also be expressed in formation of diverse magmas of granitic, trondhjemitic and granodioritic composition. We present results of a petrological, fluid inclusion and thermobarometric study of interaction between fluidized trondhjemitic magma and peraluminous metapelitic granulites associated with the regional high-grade Petronella shear zone located in the Southern Marginal Zone (SMZ) of the Limpopo Granulite Complex (South Africa). The hot (T∼1000°C) trondhjemitic magma, which, presumably, originated from partial melting of a basaltic (amphibolite) material at the base of the granulite complex or at the top of the underthrusted greenstone blocks, intruded granulites at P∼8.0–9.5kbar (24–28km depth) at 2.667±0.9Ga during the exhumation of the SMZ. The magma heated and assimilated orthopyroxene-cordierite metapelites and dragged them to a depth of 18–20km (6.3–6.5kbar). The magma was heterogeneously saturated with MgO, FeO, Al2O3 by the dissolved metapelites. This process provoked crystallization of several garnet generations from the trondhjemitic melt. Various mineral assemblages included in the different generations of garnet allowed application of TWQ method combined with PERPLE_X pseudo sections to trace sub-isobaric cooling of the magma from T∼900–600°C at 5.5–6.5kbar. The isobaric cooling also affected the associated metapelites. Fluid inclusions trapped in garnet and quartz in the trondhjemite show that the magma transported carbonic fluid with densities corresponding to the late stages of magma cooling (600–650°C and 5.5–6.5kbar). Carbonic fluids coexisted with aqueous-salt fluids (preserved as inclusions with salinity up to 20.58wt.% NaCl eq.). These low water activity fluids (aH2O<0.3) bearing Na, K and Ca salts, being exsolved from the magma on cooling and solidification, provoked formation of complex Na-gedrite+biotite+sillimanite+quartz±staurolite±plagioclase-bearing assemblages after cordierite in metapelites at temperatures 630–570°C and pressures 5.5–6.5kbar. These data provide evidence that hot trondhjemitic melts played a critical role in the exhumation of granulites onto the adjacent granite-greenstone craton. The trondhjemite transferred heat from the lower to the middle crust and transported large volumes of external aqueous-carbonic-salt fluids that participated in the rehydration of a significant portion of the SMZ.

Tectono-Metamorphic and Geochronologic Studies from Sandmata Complex, Northwest Indian Shield: Implications on Exhumation of Late-Palaeoproterozoic Granulites in an Archaean-early Palaeoproterozoic Granite-Gneiss Terrane

Abstract Several bodies of granulites comprising charnockite, charno-enderbite, pelitic and calc-silicate rocks occur within an assemblage of granite gneiss/granitoid, amphibolite and metasediments (henceforth described as banded gneisses) in the central part of the Aravalli Mountains, northwestern India. The combined rock assemblage was thought to constitute an Archaean basement (BGC-II) onto which the successive Proterozoic cover rocks were deposited. Recent field studies reveal the occurrence of several bodies of late-Palaeoproterozoic (1725 and 1621 Ma) granulites within the banded gneisses, which locally show evidence of migmatization at c. 1900 Ma coeval with the Aravalli Orogeny. We report single zircon ‘evaporation’ ages together with information from LA-ICP-MS U-Pb zircon datings to confirm an Archaean (2905 – ca. 2500 Ma) age for the banded gneisses hosting the granulites. The new geochronological data, therefore, suggest a polycyclic evolution for the BGC-II terrane for which the new term Sandmata Complex is proposed. The zircon ages suggest that the different rock formations in the Sandmata Complex are neither entirely Palaeoproterozoic in age, as claimed in some studies nor are they exclusively Archaean as was initially thought. Apart from distinct differences in the age of rocks, tectono-metamorphic breaks are observed in the field between the Archaean banded gneisses and the Palaeoproterozoic granulites. Collating the data on granulite ages with the known tectono-stratigraphic framework of the Aravalli Mountains, we conclude that the evolution and exhumation of granulites in the Sandmata Complex occurred during a tectono-magmatic/metamorphic event, which cannot be linked to known orogenic cycles that shaped this ancient mountain belt. We present some field and geochronologic evidence to elucidate the exhumation history and tectonic emplacement of the late Palaeoproterozoic, high P-T granulites into the Archaean banded gneisses. The granulite-facies metamorphism has been correlated with the thermal perturbation during the asymmetric opening of Delhi basins at around 1700 Ma.

Role of granite intrusions for the formation of ring structures in granulite complexes: Examples from the Limpopo belt, South Africa

Ring structures are cylindrical or sheath folds with concentrically distributed beds, often with granites in the cores. This paper reports structural, petrographic, and petrological evidence for four such structures from the Central Zone of the Limpopo complex, which were formed during granulite exhumation in Neoarchean time (event D2/M2). It was demonstrated that the orientation of linear and planar elements in the rocks are practically identical and independent of the position within the Central Zone. All existing measurements (hundreds for the four structures) project within the same fields in stereograms. The fold axes plunge SW at an angle of ∼40° within the whole area of the Central Zone of the Limpopo complex. This implies that the rocks were metamorphosed and deformed during event D2/M2, which is typical of the Neoarchean stage of the development of the Limpopo granulite complex. Local mineral equilibria and fluid inclusions were studied in a series of key rocks, and P-T paths were derived for them. A gravitation mechanism was substantiated for the ascent of granulites and accompanying granite bodies. The structure of ring complexes was evaluated on the basis of various erosion sections. It was shown that stocklike granite bodies occur at the base of each ring structure. Petrochemical and structural data were used to demonstrate that the granites (2627 Ma) had been derived by the complete or partial melting of the lower parts of the section of Neoarchean (2651 Ma) country rocks. The upwelling of a less dense granite magma synchronously with the exhumation resulted in the helical squeezing of the overlying gneisses. This led to the concentric arrangement of beds and development of a ring structure, a sheath fold containing a granite core in some sections. A preliminary numeric 2D model is considered for the ascent of a granite diapir accompanied by the downwelling of colder and denser country rocks. A better understanding of this process can be gained by 3D numerical simulation.

Timing and role of the Maranhão River Thrust in the evolution of the Neoproterozoic Brasília Belt and Tocantins Province, central Brazil

A low-angle thrust fault places high-PT granulites (hangingwall) of the Internal Zone of the Neoproterozoic Brasília Belt (Tocantins Province, central Brazil) in contact with a lower-grade footwall (External Zone) comprised of nappes of distal passive margin- and back-arc basin-related supracrustals. The footwall units were emplaced at ∼ 750 Ma onto proximal sedimentary rocks (Paranoá Group) of the São Francisco paleo-continent passive margin. The high-PT belt is comprised of 645–630 Ma granulite-facies paragneiss and orthogneiss, and mafic–ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at ∼ 750 Ma. These complexes occur within lower-grade metasedimentary rocks in the hangingwall of the Maranhão River Thrust, which forms the Internal Zone–External Zone boundary fault to the north of the Pirineus Zone of High Strain. Detailed lithostructural studies carried out in Maranhão River Thrust hangingwall and footwall metasedimentary rocks between the Niquelândia and Barro Alto complexes, and also to the east of these, indicate the same lithotypes and Sm–Nd isotopic signatures, and the same D 1– D 2 progressive deformation and greenschist-facies metamorphism. Additionally, footwall metasedimentary rocks exclusively display a post- D 2 deformation indicating that the Maranhão River Thrust propagated through upper crustal rocks of the Paranoá Group relatively late during the tectonic evolution of the belt. Fault propagation was a consequence of intraplate underthrusting during granulite exhumation. The results allow for a better tectonic understanding of the Brasília Belt and the Tocantins Province, as well as explaining the presence of the Pirineus Zone of High Strain.

Crustal structure and tectonics of the northern part of the Southern Granulite Terrane, India

Deep seismic reflection studies investigating the exposed Archean lower continental crust of the Southern Granulite Terrane, India, yield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of granulites. Seismic reflection images along the Kuppam–Bhavani profile reveal a band of reflections that dip southward from 10.5 to 15.0 s two-way-time (TWT), across a distance of 50 km. The bottom of these reflections beneath the Dharwar craton is interpreted as the Moho. Further south, another reflection band dipping northward is observed. These bands of reflectivity constitute a divergent reflection fabric that converges at the Moho boundary observed at the Mettur shear zone. Reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks, which we infer resulted in crustal thickening and the formation of granulites. Anomalous gravity and magnetic signatures are also observed across the Mettur shear zone. The gravity model derived from the Bouguer gravity data corroborates seismic results. The tectonic regime and seismic reflection profiles are combined in a 3-D representation that illustrates our evidence for paleo-subduction at a collision zone. The structural dissimilarities and geophysical anomalies suggest that the Mettur shear zone is a suture between the Dharwar craton in the north and another crustal block in the south. This study contributes significantly to our understanding of the operation of Archean plate tectonics, here inferred to involve collision and subduction. Furthermore, it provides an important link between the Gondwanaland and global granulite evolution occurring throughout the late Archean.

Early metamorphic evolution and exhumation of felsic high-pressure granulites from the north-western Bohemian Massif

Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close to 12 kbar and temperatures of more than 800 °C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to pressures of 13–14 kbar only at relatively low temperatures of 620 °C and, thus, to considerable, nearly isobaric heating before the exhumation of the granulites started at 800 °C or somewhat higher temperature.

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

AbstractAt the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west‐oriented compression and vertical extrusion of the high‐grade rocks into higher crustal levels. The high‐pressure mineral assemblage Grt‐Ky‐Kfs‐Pl‐Qtz‐Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high‐grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite‐bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high‐grade rocks with a cold upper‐crustal lid.

Metamorphic evolution and zircon geochronology of early Proterozoic granulites in the Aravalli Mountains of northwestern India

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss–amphibolite–metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the ‘country rocks’ led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates Tmax&gt;900°C and Pmax∼7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschist-facies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende–biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb–Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma.

Single stage, late Archaean exhumation of granulites in the Northern Marginal Zone, Limpopo Belt, Zimbabwe, and relevance to gold mineralization at Renco mine

Detailed field mapping and geochronological studies in the Renco mine area of the Northern Marginal Zone of the Limpopo Belt allow two distinct tectonic events to be distinguished in the late Archaean to Palaeoproterozoic. In the first event, granulite-facies metamorphism (~2720 to ~2590 Ma) was followed by the formation of pervasive steep fabrics accommodating north-northwest shortening, and a network of amphibolite-facies shear zones. Enderbitic rocks were intruded over the interval ~2739 to ~2571 Ma, and granites were intruded over a largely overlapping period of ~2654 to ~2517 Ma. The intrusion of syn-tectonic granites (~2600 to ~2500 Ma) allows the convergent tectonics to be dated as late Archaean. The second event produced deformation zones dominated by cataclasis at maximum temperatures of 400°C, and may have involved localized pseudotachylite generation. These deformation zones have maximum lengths and widths of several hundred metres and 2m respectively: they are separated along and across strike by several kilometres, and did not accommodate significant crustal displacement. They probably formed in the second event at ~2000 Ma. The granulites were therefore exhumed in the first event, following directly on granulite-facies metamorphism, implying that a substantial crustal thickness was attained in the late Archaean. Amphibolite facies gold mineralization at Renco mine probably occurred at this time, followed by Palaeoproterozoic remobilization.

Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of the granulite-facies Kanskiy complex bordered by the lower-grade Yeniseyskiy and Yukseevskiy complexes. Samples of metapelite of the Kanskiy complex typically show characteristic garnet-forming reaction textures and near-isobaric cooling P–T paths. An important new result of this study concerns the difference in shape of the P–T paths from different parts of the Kanskiy granulite complex: metapelites collected ∼8 km from the boundary with the Yeniseyskiy complex followed a linear path with dP/dT ≈ 0·006 kbar/°C; metapelites collected ∼3 km from this boundary reveal a kinked P–T path with an interval of burial cooling (dP/dT ≈ −0·006 kbar/°C). The difference in the shape of the P–T paths is supported by the chemical zoning of garnet studied in the second group of samples. A mechanism of buoyant exhumation of granulite is suggested by comparison with the results of numerical modelling, which indicate that such a diversity of P–T paths may result from a transient disturbance of the thermal structure by rapid differential movement of material from different crustal levels. To arrive at a correct tectonic interpretation, the whole assemblage of interrelated P–T paths of metamorphic rocks collected from different localities within the same complex must be studied.

Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.

Two-dimensional numerical modeling of pressure–temperature-time paths for the exhumation of some granulite facies terrains in the Precambrian

Pressure ( P)–temperature ( T) paths accurately record movement of metamorphic rocks within the Earth’s crust. 1D geodynamic modeling of P– T-time paths of regional metamorphism have explained many important features of the P– T evolution of metamorphic rocks. Further progress may be achieved using 2D numerical geodynamic modeling. Different types of P– T paths obtained for some granulites allow their 2D numerical modeling in terms of a theory of gravitational redistribution of material within the Earth’s crust. This modeling was done using the finite differences method for a Newtonian incompressible fluid. The P– T paths obtained geothermobarometrically for metapelites from the Limpopo granulite complex were compared with the P– T-time paths calculated numerically by monitoring the movement of different ‘samples’ in the thermal and gravitational field. The results of this monitoring support a hypothesis of isobaric cooling of granulites during the thermal interaction between rising hot granulites and sinking cool cratonic rocks. Effective viscosities of the rocks were gradually adjusted during the modeling by using the shape of the P– T paths. The calculated average viscosity of 10 19 Pa s for the granulites, supports an idea that the lower crust is a weak layer between upper crust (10 21 Pa s) and the Upper Mantle. The average rate of exhumation of granulites during the gravitational redistribution process is estimated at about 2.5 mm per year. This suggests exhumation of granulites from depths of about 30 km during approximately 10 Myr. The results of numerical modeling successfully explain the presence of two types of P– T paths observed in metapelites of the Limpopo granulite complex emplaced between the >3 Ga Kaapvaal and Zimbabwe cratons at ∼2.65 Ga.

U–Pb constraints for the plutonic and tectonometamorphic evolution of Lake Melville terrane, Labrador and implications for basement reworking in the northeastern Grenville Province

Along the northeastern margin of the Grenville Province in Canada, crust of Labradorian age (1.71–1.62 Ga) was variably reworked during northward (present day coordinates) propagation of the 1.20–1.00 Ga Grenvillian Orogen. To provide constraints on the nature of Grenvillian reworking, we describe previously unreported field relationships and present ten new U–Pb ages from a transect across Lake Melville terrane (LMT), which forms the leading edge of the Allochthonous Polycyclic Belt in the study area. Exhumation of granulites on the northern margin of LMT, along the Rigolet thrust zone, occurred during the interval 1617–1610 Ma and was therefore a late Labradorian feature. Approximately 120 m.y. after the Labradorian Orogeny, during the interval 1489–1474 Ma, dioritic to granitic plutons inferred to be related to Pinwarian (1.51–1.47 Ga) magmatism, were emplaced in northern LMT. This, together with a revised U–Pb baddeleyite and zircon age of 1472+27/–21 Ma for the Michael gabbro, suggests that Pinwarian ensialic arc magmatism was coeval with the emplacement of the Michael and Shabogamo gabbro dyke swarms. The data also indicate that Pinwarian magmatism was not limited to the Pinware terrane, but occurred throughout the entire Grenville Province in eastern Labrador, implying that the region was already accreted to the Laurentian margin prior to the Grenvillian Orogeny. The principal phase of Grenvillian deformation and uppermost-amphibolite facies metamorphism in LMT occurred between 1.08 and 1.05 Ga and appears to have been focused between the Rigolet shear zone and the anorthosite-dominated Mealy Mountains terrane. Grenvillian amphibolite facies reactivation of the Rigolet shear zone was limited to its upper structural levels only, with the central and lower levels preserving Labradorian fabrics and granulite facies assemblages. High-grade Grenvillian deformation within LMT was dominated by tight to isoclinal folding and belt-parallel transposition, minor thrusting, followed by localized strike–slip shearing at the northeastern and southwestern margins.