Granulite Belt Research Articles

Stress, rheological structure and earthquakes in the POLAR profile in the northern Fennoscandian Shield

We model stress fields and potential areas of deformation, use differences to analyse the possible variations in the modelling results and investigate the relations between the earthquakes and our modelling results in the POLAR profile in the northern Fennoscandian Shield. Calculated stress fields show rather uniform distribution, with significant deviations concentrated in rheologically weaker regions, mainly in the middle crust. Significant areas of deformation are also connected to these weaker regions, in the crust and also in the mantle. These regions control whether decoupled or coupled mechanical conditions exist. Further analysis shows that major differences between the stress fields calculated with dry and wet rheologies are found in the lower part of the middle crust extending from the Lapland Granulite Belt area towards the northeast. Similarly, differences between the models due to the varying heat production and the depth of the thermal lithosphere–asthenosphere boundary concentrate in rheologically weak crustal regions and in the mantle. These differences display importance of the rheological composition and thermal structure in rheological modelling. Our rheological model for reverse stress regime with typically used values of 0.35 for pore fluid factor and 0.75 for friction coefficient seems to result in too strong crust in order to explain satisfactorily the earthquakes focal depths in the POLAR profile. Most of the earthquakes can be explained by reverse stress regime if high pore pressure conditions exist. However, for the deepest events, it seems that also changes both in stress regime and in friction coefficients are needed. Rheologically, weak middle crust in the central and north-eastern parts of the POLAR could explain the absence of deep earthquakes in these parts of the profile.

Sequence of geologic events in the central and northeastern parts of the Lapland Granulite Belt: Isotopic geochemical zircon data and results of geological-petrological studies

Geological and petrological studies of rocks were conducted to reproduce the succession of endogenic processes in the central and northeastern parts of the Lapland Granulite Belt along the course of the Lotta River. Mineralogical-geochemical data on zircon from these rocks (the anatomy of zircon crystals seen in BSE and CL imagery; U, Th, Hf, and Y concentrations; and REE patterns) and their crystallization temperatures (calculated using the Ti concentration) make it possible to correlate the newly obtained geochronologic data with the sequence of metamorphic events inferred from petrological data. Three major episodes of enderbite-forming processes are distinguished: (i) early episode at 1.99 Ga, (ii) main episode at 1.940–1.925 Ga; and (iii) late episode at 1.89 Ga. The early medium-pressure metamorphic episode is dated at 1.97–1.96 Ga. The diatexis of the quartz-feldspar granulites with the derivation of garnet plagioclase leucogranites took place at 1.917–1.909 Ga.

The age of the protolith of metamorphic rocks in the southeastern part of the Lapland granulite belt, southern Kola Peninsula: Correlation with the Belomorian mobile belt in the context of the problem of Archean eclogites

U-Pb zircon isotopic data on rocks from the Kandalaksha-Umba zone of the Lapland granulite belt in the Por’ya Bay area constrain the age of the protolith of the apodacite (apotonalite) Opx-Bt granulite gneisses at 2799 ± 4 Ma, and the age of the apogabbronorite Grt-Opx-Cpx-Hbl crystalline schists at 2315 ± 23 Ma. The U-Pb sphene age of the magmatic crystallization of the postmetamorphic granodiorites is 1901 ± 5 Ma. The zircon yields the U-Pb age of the contamination of xenogenic zircons, which were captured during the dissolution of xenoliths of the host Grt-Opx-Cpx-Hbl crystalline schists in granodiorite melt. The comparison of the most important attributes of the endogenic histories of the adjacent Lapland Granulite and Belomorian Mobile belts testifies to their similar evolutionary histories: (1) the protolith age of the acid Opx-Bt granulites of the Lapland Belt (2799 ± 4 Ma) coincides with the protolith age of acid gneisses in the Belomorian Belt (2890-2690 Ma); (2) the ages of the gabbronorite protolith of Grt-Opx-Cpx-Hbl granulites in the Lapland Belt (2315 ± 23 Ma) and gabbro-anorthosite in the Kolvitsa Massif (2462-2423 Ma) are close to the protolith age of eclogitized gabbronorites in the Belomorian coronite suite (2.46–2.36 Ga); (3) the age of granulite metamorphism of acid and mafic rocks in the Lapland Belt is 1912–1925 Ma, and the age of eclogite metamorphism of gneisses and metabasites in the Belomorian Belt is approximately 1.9 Ga, i.e., their metamorphism took place in Svecofennian time; (4) the peak pressure of granulite metamorphism in the Lapland Belt was 9–11 kbar at a temperature of 800–850°C, whereas the peak metamorphic parameters of eclogite metamorphism in the Belomorian Belt were 10–12 kbar and 640–700°C. This means that the metamorphic complexes of the Lapland and Belomorian belts had the same Mezo- and Neoarchean protoliths hosting bodies of Paleoproterozoic gabbroids and were completely formed largely by a single cycle of Svecofennian high-pressure zonal metamorphism within a temperature range from the lowest grade of the eclogite to the granulite facies.

Recurrent high grade metamorphism recording a 300 Ma long Proterozoic crustal evolution in the western part of the East European Craton

The Palaeoproterozoic lower crust, forming several belts and domains, is a major component of the crystalline basement within the large region to the southeast of the Baltic Sea in Belarus, Lithuania and Poland. Four stages of high grade metamorphism have been determined in the Western Lithuanian Granulite domain (WLG) and Belarus–Podlasie Granulite belt (BPG), the western East European Craton (EEC). We have carried out P–T studies, secondary ion mass-spectrometry (SIMS) zircon- and electron probe (EPMA) — monazite dating of peak metamorphism. The first stage occurred at 1.81–1.79Ga under 800–900°C and 8–10kbar and was related to both accretionary and postcollisional tectonics in the South Baltic region, whereas the stages at 1.73–1.68Ga (700–800°C, 6–7kbar), 1.62–1.58Ga (700°C, 4–5kbar), and 1.52–1.50Ga (900°C, c. 10kbar) can be attributed to extensional intracratonic regimes. The 1.81–1.79Ga stage was connected both to the major Sarmatia–Fennoscandia collision and the eastward accretion, which led to the formation of Baltica (East-European Craton) during the assembly of the Columbia (Nuna) supercontinent. The later high grade events associated with intracratonic extensions and magmatism may be distal manifestations of accretionary processes along the long-lived common Laurentia–Baltica margin. The 1.52–1.50Ga metamorphism was associated with extensive anorthosite–mangerite–charnockite–granite magmatism in already consolidated crust.

Rare earth element and gold exploration in glaciated terrain: example from the Mäkärä area, northern Finland

The Mäkärä Au-rare earth element (REE) prospect area is located in the Tana Belt, south of the 1.9 Ga Lapland Granulite Belt, in northern Finland. The Belt has prominent lanthanum (La) and yttrium (Y) anomalies in regional till and bedrock geochemical data. High Y indicates enrichment of heavy REE in the bedrock. At Mäkärä, promising narrow Au-hematite-quartz veins occur in connection with tensional fractures in the great shear zone. Under the latest ice divide of the last glaciation, subglacial erosion was weak and glacial transport distance short. High Au contents in saprolite and till together with deep weathering have a strong positive correlation with the positive electromagnetic anomalies caused by the sulphidic gneisses. The highest La and Y contents in till correlate well with the Th maxima of airborne-radiometric datasets. Locally, the <0.06 mm till size fraction contains up to 0.4% REE. A recent exploration project in Mäkärä revealed a 13 m wide Au-hematite-quartz vein with a mean of 3 ppm Au and 0.04–0.1% REE in kaolinitic saprolite derived from arkosic gneiss. The elevated REE content resembles that of ionic adsorption clays in China. Typical REE-rich minerals are monazite, rhabdophane, xenotime and kaolinite. Till geochemistry proved useful in REE exploration.

Structural framework for the emplacement of the Bolangir anorthosite massif in the Eastern Ghats Granulite Belt, India: implications for post-Rodinia pre-Gondwana tectonics

Massif type anorthosites at Bolangir, eastern India are emplaced at the vicinity of the proto-Indian craton—Eastern Ghats Granulite belt contact. Micro- and meso-structural evidences indicate that the emplacement of the anorthosite pluton and the adjoining granitoids was syn-tectonic with respect to the D3 deformation phase (950–1,000 Ma) in the host gneiss. Anisotropy of magnetic susceptibility confirms that magnetic fabrics within anorthosite were dominantly developed during D3 deformation. Emplacement of felsic melts in the N-S trending dilatant shear zones in the granitoids, Fe-Ti-Zr-REE rich melt bands along N-S trending shear zones and localized N-S magnetic foliation in anorthosite near the Fe-Ti-Zr-REE rich melt bands indicate change in the stress field from NNW-SSE (D3) to E-W (D4). Available geochronological and paleogeographic data coupled with the structural analyses of the intrusive and the host gneiss indicate that the emplacement of massif type anorthosite in the EGP is not related to the accretion of Eastern Ghats Granulite Belt over proto-Indian continent during late Neoproterozoic.

Implications of Geochemistry in support of Palaeo-Proterozoic Tectonothermal Evolution of Bhopalpatnam Granulite Belt, Bastar Craton, Central India

Abstract: Bhopalpatnam Granulite Belt which occur along SW margin of Bastar Craton and NE shoulder of Pranhita-Godavari Rift comprise of charnockite (enderbitic variety), garnet-sillimanite-biotite gneiss, quartzo-feldspathic gneiss and corundum bearing aluminous gneiss. High La/Yb ratio, low Eu anomaly (Eu/Eu*=1.0), high LREE/HREE ratio with uniform REE pattern, high La/Sc ratio (0.53-6.43), high Th/Sc ratio (0.03-2.56), low Ni (5.52-20.95), low Cr (31.05-117.05) and uniform Zr/Hf distribution pattern indicate a Proterozoic character. Distribution pattern of K2O, Na2O and CaO in ternary diagram show quartz–monzonite–granodiorite trend for the bulk rocks indicating that the bulk rock composition is close to TTG of early Archaean, which might have supplied the sediments for the rocks of Bhopalpatnam Granulite Belt. Geochemical and mineralogical evidence indicate an argillaceous protolith for garnet – sillimanite - biotite gneiss and corundum bearing aluminous gneiss, whereas an arkosic protolith for quartzo-feldspathic gneiss. The geochemical signatures also suggest an active continental margin setting for the rocks of Bhopalpatnam Granulite Belt with prominent Nb and Ta anomaly favouring a subduction environment between Bastar Craton and East Dharwar Craton. This is in conformity with the finding of the earlier workers suggesting a clockwise P-T path based on the combined fluid inclusion and mineral phase equilibria. The LILE geochemistry of charnockite suggests a bi-phase evolution. High LREE/HREE ratio portrays a highly evolved nature of the charnockitic melt generated through partial melting of the continental crust at the final stage of the granulite facies metamorphism during collision between Bastar and East Dharwar Cratons.

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt, India: implications for Neoarchean-Paleoproterozoic events in southern India

High Mg-Al granulite occurs as enclave within granite gneisses at Karimnagar, southern India, and it contains coarse granoblastic aggregates of orthopyroxene and sapphirine with minor amount of cordierite, spinel and phlogopite. An important chemical characteristic of these minerals is their extremely high MgO content and the high Cr2O3 in sapphirine and spinel. Textural analysis shows sapphirine + orthopyroxene + cordierite as the peak-metamorphic assemblage that possibly evolved though the breakdown of a spinel-bearing assemblage. Cation exchange geothermometers involving orthopyroxene, sapphirine and spinel yield temperatures of 600–800 °C with a maximum of 860 °C implying an event of high temperature (HT) metamorphism. Pseudosection analysis in the FeO–MgO–Al2O3–SiO2 chemical system shows the stability of the peak- assemblage below 6.2 kbar. Subsequently, the rock underwent hydration and cooling with the appearance of phlogopite in the assemblage. Chromium enrichment is possibly inherited from the protolith and its presence presumably stabilized sapphirine and spinel below their high-temperature stability field. The recorded Rb–Sr age of ca. 2,500 Ma in host granite gneiss marks the upper age limit of HT metamorphism. Presence of patchy, lobate grains as well as veinlets of uraninite and brannerite is also a characteristic feature of the rock. Uranium mineralization took place during the post peak metamorphic stage, sulfide mineralization represented by tiny grains and veinlets of pyrite, millerite and pentlandite coincided with, and outlasted the uranium mineralization. The U–Th–Pb chemical ages of uraninite grains suggest ca. 2,200 ± 12 Ma for the age of uranium mineralization in the granulite. Based on the field relations, it is surmised that the granulite metamorphism in the study area is older than ca. 2,500 Ma and is comparable with an event in the other parts of Eastern Dharwar Craton. It can be conceived as a widespread event in southern India.

Noise directivity and group velocity tomography in a region with small velocity contrasts: the northern Baltic shield

Ambient noise tomography (ANT) is widely used to image strong velocity variations within the upper crust. Using careful processing, we obtained a 3-D model of shear velocities in the upper crust beneath northern Finland, where the lateral velocity variations are less than 3 per cent. As part of the tomography, the noise field is analysed. It is strongly heterogeneous but the signal-to-noise ratio is sufficient to obtain stable dispersion curves for all profile azimuths. Our results show that the directions of dominant noise sources of Rayleigh and Love waves are the same, but the amplitude distribution with azimuth is different for the two types of waves. More intriguingly, the high frequency Love waves are dominated by a mixture of higher modes rather than the fundamental mode. The reconstructed 3-D model shows the Lapland Granulite Belt as a high velocity body with a limit at surface in excellent agreement with geological observations at surface. Following this interface at depth, our results are compatible with previous studies suggesting an Archean north oriented subduction.

On oriented ilmenite needles in garnet porphyroblasts from deep crustal granulites: Implications for fluid evolution and cooling history

Garnet porphyroblasts from a litho-assemblage containing aluminous granulite and quartzofeldspathic gneisses of the Eastern Ghats granulite belt, India contain nanometer- to micrometer-thick ilmenite needles oriented crystallographically. Petrographic and chemical analyses reveal that garnet was formed by dehydration melting reaction(s) of titaniferous biotite in an oxidized condition. Elevated oxygen fugacity might have promoted enrichment of Ti-bearing andradite component of garnet porphyroblasts formed during pre- to peak metamorphic condition in appropriate bulk chemistry. During the post-peak cooling history, Ti-bearing components in garnet decomposed to rhombohedral oxide solid solution (ilmenite–hematite). Detailed transmission electron microscopic study of the host garnet and ilmenite solid solution indicates that though there is an overall parallelism of (011) plane of host garnet and (011) plane of ilmenite, structural coherence between the two phases was progressively lost during growth from thin to thick needles. Appropriate cooling rate from high-temperature peak metamorphic condition arguably promoted growth of ilmenite solid solution through reaction–exsolution process within garnet porphyroblasts.

Geochronology and geochemistry of the enderbite series in the Lapland Granulite Belt: generation, tectonic setting, and correlation of the belt1NORDSIM Publication No. 322.

The Lapland Granulite Belt of the northeastern Fennoscandian Shield is dominated by blastomylonitic migmatitic metagreywackes and a deformed, structurally conformable norite–enderbite series. The belt contains minor granodiorites and one major anorthosite massif and is crosscut by post-tectonic granite plutons and diabase dykes. The major and trace element composition of the enderbite series is similar to that of arc-related igneous rocks. Magmatically zoned zircon yield U–Pb secondary ion mass spectrometry ages of 1.91–1.92 Ga, while homogeneous metamorphic zircon domains are aged 1.88–1.90 Ga. Major enderbite bodies reveal only a few older inherited, xenocrystic zircon cores or grains, mainly Archaean in age, while a tonalite body and a narrow quartz norite vein have abundant inherited zircons comparable with the detrital zircon population in the metagreywackes. The intrusive series has slightly positive initial εNdvalues, while the tonalite body has a slightly negative initial εNd, consistent with the occurrences of inherited zircon grains. The major intrusive series, interpreted to have been emplaced in an arc environment, has juvenile character and assimilated only a minor amount of older crustal material during ascent, intrusion, and cooling. Narrow veins and more evolved bodies had more interaction with the metasediments and other crustal rocks. The enderbite series of the Lapland Granulite Belt may be correlated with arc-related plutonism in the Nagssugtoqidian and Torngat orogens, but metamorphic and structural evolution took place considerably later in Greenland and the northeastern Canadian Shield, with the exception of the Inglefield Belt of northwestern Greenland.

Generation of Granitic Plutons during Crustal Orogenesis: An Example from the Eastern Ghats Granulite Belt

Abstract Stock-like granite plutons in the Eastern Ghats belt and their host granulites exhibit similar solid state fabric. Both mineralogically and chemically these plutons are peraluminous and granitic in composition with S-type granite affinities. The granite plutons are product of a variety of mica dehydration melting reactions, as evident from their (1) compositional plots in the (FeO + MgO) - Na2O.Al2O3 – K2O.Al2O3(MNK) pseudoternary diagram and (2) trace element contents. Muscovite and/or biotite breakdown reaction with or without plagioclase are the characteristic melting reactions. Restitic signature is evident in the host metapelitic granulites, locally known as khondalites (Qtz-Kfs-Grt-Sil-Fe-Ti oxides-bearing gneisses). The compositional variability of the associated khondalites can be a result of (1) original compositional variation and (2) they represent different stages of restites. Trace element monitoring following restite separation model suggests that these granite plutons are largely saturated equilibrium melts. The granites are chemically discriminated as syn-collisional. The plutons could also be the product of segregation during the regional exhumation of the Eastern Ghats terrain.

Crustal age domains and metamorphic reworking of the deep crust in Northern-Central Tanzania: a U/Pb zircon and monazite age study

Geochronological data are presented from Northern Tanzania, where deep-crustal terranes of different age are exposed. Stacking of these terranes was diachronous with one peak around 640 Ma, defined as East African Orogeny, and final consolidation at 550–580 Ma, that is defined as Kuunga Orogeny. This later event is predominant in the Western Granulite Belt of northern Tanzania and related to thrusting onto the Tanzanian Craton. The Tanzania Craton itself experienced a polycyclic history; age domains around 2.64 Ga prevail in the studied samples. There is no evidence of the Paleoproterozoic Usagaran Belt in northern Tanzania. Here the gneisses contain relicts of reworked Archean basement and are therefore considered part of the Western Granulites. Inliers of the Western Granulites are also found in the cores of marble antiforms that are part of the upper, sedimentary sequence of the Eastern Granulites. Those inliers formed during the Kuungan orogenic phase when the Eastern Granulites have taken their final position and were folded together with the Western Granulites.

Thermodynamic conditions of metasomatism in high-temperature and high-pressure shear zones (Kandalaksha-Umba Zone, Kola Peninsula)

874 The Kandalaksha–Umba zone on the Kola Penin sula is the southeastern fragment of the Lapland gran ulite belt (Fig. 1), which is known for high pressure and high grade metamorphism. It was the study of the Lapland belt that allowed P. Eskola to distinguish the granulite facies in the general system of the mineral facies [1]. P. Eskola was the first to discover orthopy roxene–sillimanite rocks in Lapland granulites, which provide the main evidence for high pressure. M.D. Kry lova and L.A. Priyatkina [2] originally described such rocks in the Por’ya Guba area, and O.A. Belyaev [3] and S.A. Bushmin et al. [4] identified them as the products of acid leaching and related iron–magne sium metasomatism. N.E. Kozlova et al. [5] per formed microstructural investigations of the associa tions of orthopyroxene–sillimanite rocks and demon strated the paragenetic character and age links to the stage of shear deformations. Investigations of the con ditions of granulite metamorphism [6–9] demon strated wide variations of pressure from 7 to 12 kbar in the zone considered.

Investigations of garnets from polymetamorphic rocks of the Lapland Granulite Belt of the Kandalaksha Region

Introduction: The Lapland Granulite Belt is placed on the Kandalaksha region (Kola Peninsula, Russia). The rocks of this Belt are composed mainly of amphibolites and granulites. Materials and methods: The research were focused on the garnets from the amphibolite and granulite rocks of Lapland Granulite Belt. The petrological methods like polarizing microscopy (PM), SEM-EDS, XRD for powdered samples and single crystal diffraction were used together with IR and Mossbauer spectroscopy and REE analysis by ion–microprobe. Results: It was found that the garnets from studied amphibolite and granulite rocks could be classified to pyralspite group without hydrogarnets components, so they were formed in high metamorphic facies. Conclusions: The joint geological observations and results of the performed experiments suggest that the garnets were subject of a blastesy, i.e. there were formed in long lasting metamorphic processes of low dynamics, except of those garnets from tectonic zones, found in the vicinity of mineral veins.

INCLUSIONS OF RUTHENIAN RUTILE AND TITANIAN RuO2 IN ISOFERROPLATINUM NUGGETS FROM FINNISH LAPLAND

Rutile, ruthenian rutile and titanian ruthenium dioxide inclusions were found in isoferroplatinum nuggets obtained as by-products of gold sluicing in the historical gold-rush area of Finnish Lapland. Analytical data suggest that an extensive solid-solution exists between the end-member compositions TiO 2 and RuO 2 . Ruthenian phases occur almost exclusively with laurite inclusions in the nuggets, in some cases as inclusions and intergrowths in laurite, suggesting the primary nature of the ruthenian phases. The TiO 2 phase is considered to be rutile rather than anatase or brookite, because there seems to be an extensive solid-solution between the TiO 2 and RuO 2 phases, and RuO 2 is known to be isostructural with rutile. The abundant mafic and ultramafic metamorphosed intrusive and extrusive rocks occurring southwest of sluicing sites could represent source rocks of the isoferroplatinum nuggets. After the Cenozoic weathering event, the nuggets drifted in the Holocene ice sheets to their present position in till overlying the migmatitic to enderbitic bedrock of the Lapland granulite belt.

Sapphirine granulites from Panasapattu, Eastern Ghats belt, India: Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

We report equilibrium sapphirine + quartz assemblage in biotite–orthopyroxene–garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030–1050 °C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet–orthopyroxene–plagioclase–K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt–Sill–Qtz assemblage, and constrain the prograde stage of the P– T path. The core domains of orthopyroxene porphyroblasts have up to wt(Al 2O 3) 9.6%, which suggest that the temperatures reached up to 1150 °C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet–orthopyroxene thermobarometery, which yields a P– T range of 1012–960 °C and 9.4 kbar. The P– T phase topologies computed using isochemical sections calculated in the model system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These P– T results underestimate ‘peak’ conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the P– T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin.

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.

U–Pb zircon chronology of the Pangidi–Kondapalle layered intrusion, Eastern Ghats belt, India: Constraints on Mesoproterozoic arc magmatism in a convergent margin setting

The Pangidi–Kondapalle layered intrusion is emplaced within high-grade supracrustal rocks in the southern sector of the Eastern Ghats Granulite Belt proximal to the south-eastern periphery of the Indian plate. This layered complex consists mainly of anorthosites and gabbronorites with pyroxenite cumulates containing orthopyroxenites and chromitites in a regressive intrusive sequence. Here we report laser ablation ICP-MS data and REE geochemistry of zircons from three co-spatial anorthositic bodies in this complex. Our results show mean 207Pb/206Pb ages of 1634±18Ma (MSWD=0.55) for the anorthosite from Pangidi. Two anorthosite samples from Kondapalle yield mean207Pb/206Pb ages of 1628±36Ma (MSWD=0.54) and 1693±110Ma (MSWD=5.5). The zircons from all the anorthosite samples show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common melt source. The prominent Mesoproterozoic ages revealed from our U–Pb dating of zircons from these layered complexes suggest subduction-related magmatic underplating in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic supercontinent.

Transition between “Archaean-type” and “modern-type” tectonics: Insights from the Finnish Lapland Granulite Belt

This paper discusses the exhumation history of Palaeoproterozoic high-grade units from the Lapland Granulite Belt (northern Finland) through a reappraisal of previous structural and metamorphic data and a new structural analysis. The Lapland Granulite Belt is located between two cratons (Inari Craton and South Lapland Craton). It is marked by a major thrusting event associated with the development of an inverse metamorphic gradient within the underlying Tanaelv and Karasjok Belts. The review of published metamorphic data combined with our new structural analysis suggests that the exhumation history of the Lapland Granulite Belt was accommodated by the development of coeval basal thrusting and early normal-sense shear-zones, and later by distributed erosion. We propose also that the early exhumation stages of the Lapland granulite unit were associated with heating of both the overlying Inari Craton and the underlying South Lapland Craton. The occurrence of coeval thrusts and normal-sense shear-zones combined with the important role of distributed erosion during the final exhumation stages suggest that this Palaeoproterozoic collisional belt may represent an intermediate stage between ultra-hot accretionary orogens involving weak lithospheres and modern orogens involving stiff lithospheres (mixed-hot orogen of Chardon et al., 2009). Our analysis emphasizes that the transition between ancient-type orogens and modern-type ones was gradual in space and time, being firstly dependent on the thermal and rheological state of the sub-continental lithospheric mantle of the lithospheres involved in compression.