Granulite Belt Research Articles

Domainal fabric development, associated microstructures and P-T records attesting to polymetamorphism in a granulite complex of the Eastern Ghats granulite belt, India

The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation (D1 and D2) attended with foliation (fabric) development (S1 and S2). Two distinct metamorphic events at P-T conditions of ∼900°C at ∼9 kbar and ∼600°C at ∼6 kbar are correlated with D1 and D2 respectively. The reaction textures in S1-microdomains are interpreted to be the product of near isobaric cooling at ∼9 kbar from 950°C to 600°C, whereas those in the S2-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D1-M1 high P-T granulite event is interpreted to be Archean in age (ca. 3 Ga) on the basis of the isotopic data obtained from the charnockite suite of the area. The later relatively low P-T granulite facies event, attendant to D2-S2 is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt.

Infiltration metasomatism under high-pressure granulite-facies conditions based on orthopyroxene-sillimanite rocks in shear zones of the Lapland granulite belt

Infiltration metasomatism under high-pressure granulite-facies conditions based on orthopyroxene-sillimanite rocks in shear zones of the Lapland granulite belt

The thermal and geodynamic evolution of the Lapland granulite belt: Implications for thermal structure of the lower crust during granulite-facies metamorphism

The tectono-thermal history of the Lapland Granulite belt is considered within the framework of a combined superplume-plate tectonic model on the basis of new geochronological and petrological data. The high-grade metamorphic events were linked with plume-related extensional episodes at ∼ 2.44 Ga (M0), ∼ 2.0–1.95 Ga (M1), ∼ 1.95–1.92 Ga (M2) and 1.92–1.91 Ga (M3). The last (M4) high-grade event at ∼ 1.89–1.87 Ga corresponds to the final collision. P– T parameters of metamorphism during M1 event ranged from 960 to 860 °C and from 14.0 to 10.3 kbar. They decreased from the lower to the upper part of the belt during the M2 event from 860 °C at 12.4 kbar and to 800 °C at 5.8 kbar, during the M3 event from 770 °C at 10.7 kbar to 640 °C at 4.8 kbar, and during the M4 event from 650 °C at 8.4 kbar to 550 °C at 4.5 kbar. Upward displacement of a hot tectonic slice during the M4 stage caused inverted metamorphic zoning in the autochthonous rocks and a clockwise P– T evolution within the mélange zone at the base of LGB 1 1 Lapland Granulite Belt. . The internal thermal gradient within the lower crust during events M2 and M3 was 1.8–2.3 °C/km, and during the M4 event it fell to 4–5 °C/km. The agreement of thickness estimates of the granulite ensemble obtained from seismic and petrological data attests to the lithostatic nature of metamorphic pressures. The average thickness of the Lapland granulite slice was initially ∼ 21–23 km.

The role of radiogenic heat production in the thermal evolution of a Proterozoic granulite-facies orogenic belt: Eastern Ghats, Indian Shield

The Eastern Ghats Belt (EGB) is a deeply eroded Proterozoic orogenic belt juxtaposed against the Archaean Dharwar–Bastar–Singhbhum cratons in the Indian Shield. The tectono-metamorphic history of this belt is broadly similar to the Grenvillian orogenic belts. The EGB exposes dominantly the granulite-facies rocks such as charnockites, khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites. The granulites are intruded by small plutons of granites, syenites and anorthosites. The northern and southern parts of the belt (NEGB and SEGB) have distinct crustal histories, but 1600–1000–550 Ma old tectono-metamorphic events are common to both the belts. In this research, we attempt to understand the present-day and past thermal state of this orogen on the basis of our radioelement (K, U and Th) measurements at 562 sites using in-situ gamma-ray spectrometry, with other geological and geophysical constraints. The dominant rock types of the NEGB and SEGB have similar radioelement abundances (>3% K, 3 ppm U and 30 ppm Th) and heat production. Heat production of charnockites, gneisses, khondalites and intermediate granulites of the NEGB is 2.9, 2.8, 2.9 and 1.1 μW m−3, respectively. In the SEGB, they are 2.7, 2.5, 2.8 and 0.6 μW m−3, respectively, and the mafic granulites are the lowest in heat production (0.3 μW m−3). On the basis of the heat production data and crustal petrologic models, we show that the crustal contribution of the NEGB and SEGB is 54 and 45 mW m−2, respectively, which are broadly in good agreement with the available surface heat flow data. Thermal models of the present-day EGB crust indicate that the temperature at the Moho is ∼550 °C. Radioelements and heat production of the Eastern Ghats granulites are higher than the other granulite belts in the Indian shield. The crustal radiogenic heat contribution and Moho temperatures of the EGB are higher than the adjoining Archaean cratons. This could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the EGB and the adjoining cratons during a collisional orogenic event at about 550 Ma ago.

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part II: Isotopic dating

The migmatitic metapelites of the Lapland granulite belt (LGB) in the NE part of the Fennoscandian Shield represent an arc-related greywacke basin metamorphosed in the granulite facies. Detrital zircons from migmatitic metapelites are derived from 1.94 etrital zircons from migmatitic metapelites are derived from 1.94 zircons from migmatitic metapelites are derived from 1.94 derived from 1.94 − 2.9 Ga old acid source rocks (U-Pb SIMS ages). The clustering of detrital zircon ages between 1.97 and 2.2 Ga is problematic, because abundant felsic crust of this age is absent in the shield. The metasediments are characterized by Sm-Nd model ages of ca. 2.3 Ga. A younger, 1905 − 1880 Ma population of homogeneous zircons was formed during regional metamorphism. The peak high-grade metamorphism took place at ~1900 Ma and the latest chronological record from subsequent decompression and cooling phase is from ca. 1870 Ma. The norite-enderbite series of the LGB represents arc-magmas, which were intruded into the metasediments at ~1920 − 1905 Ma ago according to zircon U-Pb ages and were probably an important heat source for metamorphism. Older, zoned zircon grains in a quartz norite vein, initial e Nd values of 0 to +1 and the continuous spectrum of LILE enrichment in the enderbite-series probably reflect assimilation of metasediments by magmas. Monazite U-Pb ages of migmatitic metasediments in the range 1906 − 1910 ± 3 Ma overlap the late stage of enderbite intrusion and growth of early metamorphic zircons. Garnetwhole rock Sm-Nd ages from leucosomes in the range 1880 − 1886 ± 7 Ma are concurrent with the growth of the youngest metamorphic zircons and probably indicate the crystallization of leucosomes or the influence of a fluid liberated from them. Isotopic and pet rologic data reveal that the evolution of Lapland Granulite belt from the erosion of source rocks to the generation of a sedimentary basin, its burial, metamorphism and exhumation took place within ca. 60 Ma.

Geodynamic cycles and geodynamic systems of various ranks: Their relationships and evolution in the Earth’s history

The previously stated ideas of hierarchical geodynamic cyclicity [42] and geodynamics of hierarchically subordinate geospheres [13] are compared in detail. The convective geodynamic system of the first rank (GS-1) that functions throughout the mantle and crust beneath the entire surface of the Earth corresponds to the geodynamic cycle of the first rank (GC-1, or the Wilson cycle). The geodynamic system of the second rank (GS-2) that embraces the mantle and crust only beneath oceans corresponds to the geodynamic cycle of the second rank (GC-2, or the Bertrand cycle). The geodynamic system of the third rank (GS-3) functioning in the tectonosphere (asthenosphere + lithosphere) in zones of elevated heat flow (spreading, subduction, and collision zones) is brought into the line with the geodynamic cycle of the third rank (GC-3, or the Stille cycle). The geodynamic system of the fourth rank (GS-4) that embraces the sedimentary cover of mobile belts corresponds to the geodynamic cycle of the fourth rank (GC-4) (the phase cycle of increasing and decreasing intensity of folding and thrusting). This hierarchy controlled by internal endogenic factors, above all, by the heat flow from the Earth’s core and internal sources within the mantle, is supplemented by the geodynamic system of the zeroth rank (GS-0) that embraces the entire Earth and that is controlled by external rotational factors, primarily, the tidal effect of the Moon. The GS-0 is characterized by interference of the permanent westward and meridional (southward and northward, alternately) continental drift in frames of the zeroth geodynamic cycle (GC-0) twice as long as the Wilson cycle (GC-1). An attempt is made to connect cyclicity of various ranks with periodic excitation and waning of convection in a geosphere of the respective rank. The convective geospheres progressively grew downward in the course of geologic history. Only the GS-3 functioned in the Archean, embracing tectonosphere and creating greenstone belts around the gray-gneiss islands with gradual accretion of these belts and the formation of the granite-greenstone continent (Pangea-0). In the Paleoproterozoic, the process spread over the entire upper mantle with switching Rayleigh-Benard polygonal convection expressed in pure form as granulite belts along the polygon perimeters that bounded the protoplatform blocks. The contemporaneous limited convection in the lower mantle (GS-1) led to some divergence of these blocks and formation of minor oceans and their subsequent closure, resulting in the formation of Pangea-1. This tendency developed further in the Mesoproterozoic and completed with the formation of Pangea-2 (Rodinia). Afterward, in the Neogean, the cyclic-hierarchical geodynamics started to work in full as described above.

Petrology, fluid inclusions and metamorphic history of Bhopalpatnam granulites, central India

The Bhopalpatnam granulite belt (BGB) flanks the Pranhita–Godavari (PG) rift basin with ages ranging from 1.6 to 1.9 Ga, representing a major Mesoproterozoic thermal regime in central India. We present here new results on the metamorphic P–T conditions from garnet-bearing enderbitic charnockite and pelitic gneisses, and document the fluid regimes from detailed fluid inclusions studies in corundum and quartz in corundum-bearing gneiss and garnet–sillimanite–rutile gneiss, respectively, from this granulite belt. A peak pressure–temperature window of 8–9.5 kbar and 720–800 °C is defined by phase equilibrium considerations on garnet–clinopyroxene–plagioclase–quartz assemblages in garnet–bearing charnockite. The garnet–plagioclase–sillimanite–quartz assemblages in pelitic gneiss yield slightly lower P–T estimates of 660–720 °C at 4.7–6.5 kbar. Primary fluid inclusions in corundum comprise monophase carbonic inclusions with melting temperatures varying from −56.6 to −57.4 °C, indicating that the dominant part of the trapped fluid is pure CO 2. The inclusions homogenize into the liquid phase at temperatures in the range of −18.5 to −5.9 °C (1.024–0.961 g/cm 3). Fluid inclusions in quartz from garnet–sillimanite–rutile gneiss correspond to two categories; Group I (primary) and Group II (pseudosecondary). Both the groups show melting temperatures in the range of −55.9 to −62.9 °C, suggesting the presence of traces of other volatiles (CH 4/N 2) in addition to CO 2. The homogenization temperature of Group I inclusions ranges from −46.3 to −0.1 °C, the extreme low homogenizations indicating very high-density (up to 1.140 g/cm 3) for the carbonic fluid. Group II inclusions show higher homogenization temperatures of up to 16.2 °C. The estimated CO 2 isochore for primary inclusions in corundum and quartz intersects the peak P–T condition of the BGB derived from mineral phase equilibria. We therefore, infer that CO 2 was the dominant fluid species that was trapped at or near the peak metamorphic conditions in BGB. The metamorphic and fluid regimes, as well as the exhumation history of this granulite belt are thus constrained from a combination of petrologic and fluid inclusion studies.

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology

The Palaeoproterozoic Lapland granulite belt was juxtaposed between Archaean and Proterozoic terrains in the NE part of the Fennoscandian Shield concurrently with the accretion of Svecofennian arc complexes at ~1.9 Ga. The belt consists mainly of aluminous migmatitic metagreywackes. Abundant noritic to enderbitic magmas were intruded concordantly into the metasediments and were probably an important heat source for metamorphism, which took place during the crystallization of the magmas. This is supported by structural and contact relations of metasediments and igneous rocks, and by the lack progressive metamorphic reaction textures in the igneous rock series. The peak of metamorphism took place above the dehydration melting temperature of the biotite-sillimanite-plagioclase-quartz assemblage at 750−850°C and 5−8.5 kbar which lead to formation of a restitic palaeosome and peraluminous granitic melt in metapelites. Subsequently, the rocks were decompressed and cooled below the wet melting temperature of pelitic rocks (650°C) under the stability fi eld of andalusite coexisting with potassium feldspar (2−3 kbar). Cooling was accompanied by the crystallization of the neosomes, often carrying aluminium-rich phases. Postmetamorphic duplexing of the LGB is clearly seen in the distribution of calculated PT conditions.

Lu–Hf systematics of the ultra-high temperature Napier Metamorphic Complex in Antarctica: Evidence for the early Archean differentiation of Earth's mantle

The Napier Complex of the East Antarctic Craton comprises some of the oldest rocks on Earth (∼ 3.8 billion years old), overprinted by an ultra-high temperature (UHT) metamorphic event near the Archean–Proterozoic boundary. Garnet, orthopyroxene, sapphirine, osumilite, rutile and a whole rock representing a fully equilibrated assemblage from this UHT granulite belt have yielded a Lu–Hf isochron age of 2403 ± 43 Ma, the first ever determined on a UHT mineral assemblage. Preservation of the UHT mineral assemblage in the rock analyzed, without any significant retrogression, suggests rapid cooling with closure likely to have occurred for the Lu–Hf system at post-peak UHT conditions near a temperature of ∼ 800 °C. This mineral–whole rock isochron yields an initial 176Hf/ 177Hf ratio corresponding to an ε Hf value of − 14 ± 1, acquired during UHT metamorphism. Such a low value demonstrates that overall UHT granulites evolved in a low Lu/Hf environment, probably formed when the rocks were first extracted from a highly depleted mantle. Zircon ε Hf values we have measured “see through” the UHT metamorphism and show that the source materials for the magmas that formed the Napier Complex were extremely depleted (> + 5.6 ε Hf at 3.85 Ga) relative to the chondritic uniform reservoir (CHUR). These results also suggest significant depletion of the early Archean mantle, in agreement with the early differentiation of the Earth that the latest core formation models require.

Age and geochemical constraints for partial melting of granulites in Estonia

The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.

The Lapland-Kola orogen: Palaeoproterozoic collision and accretion of the northern Fennoscandian lithosphere

Abstract A tectonic model is proposed for the Palaeoproterozoic Lapland-Kola orogen (LKO) in the northern Fennoscandian Shield. Although long regarded as an Archaean craton, integrated geological, geochemical and geophysical observations show that the Lapland-Kola orogen is a Palaeoproterozoic collisional belt containing both Archaean terranes and an important component of juvenile Palaeoproterozoic crust. Rifting, from 2.5 to 2.1 Ga, began under the influence of a mantle plume (> 1000 km diameter), related to the break-up of the Kenorland supercontinent. Two linear suture zones within the orogeriic core mark the sites of continental separation, ocean formation and closure. One of these is identified as a belt of 1.98-1.91 Ga juvenile crust of both arc magmatic and sedimentary origin, marked by the Lapland Granulite, Umba and Tersk terranes. Palaeomagnetic data and ancient sedimentary detritus within these terranes suggest limited oceanic separation. Collision of juvenile terranes with the surrounding Archaean took place mainly between 1.93 and 1.91 Ga, resulting in a Himalayan-scale mountain belt, manifest by a thick-skinned region of high- P granulite-facies metamorphism, including the classical Lapland Granulite Belt and a broad zone of compressional deformation extending southwards into the Belomorian Mobile Belt. Protracted cooling and exhumation, possibly related to the buttressing effect of surrounding lithosphere, culminated in the intrusion of 1.80-1.77 Ga post-tectonic granites.

Crustal structure and tectonic model of northeastern Baltica, based on deep seismic and potential field data

Abstract The Early-Mid-Palaeozoic successions of the Pechora Basin along continent Baltica's northeastern margin unconformably overlie Neoproterozoic complexes of the Timanide orogen; further west, Palaeozoic platform strata cover the East European Craton basement. The latter is exposed in the northern parts of the Fennoscandian (Baltic) Shield and is also present beneath the Neoproterozoic (perhaps also Mesoproterozoic) and younger successions of the Mezen Basin. Geophysical data help define the crustal and upper mantle structures of this part of northwestern Russia, from the Kola Peninsula to the Uralide orogen. Within the Shield, seismic refraction and near-vertical reflection profiling allow the terranes of Archaean and Palaeoproterozoic age, defined at the surface (the Murmansk, Kola-Liinakhamary, Central Kola, Keivy and Belomorian terranes, and the Pechenga-Imandra-Varzuga and Lapland-Kolvitsa Granulite Belts constituting the Lapland-Kola orogen, and the West Karelian, Central Karelian and Vodlozer terranes forming the Karelian province), to be traced into the deeper crust, promoting interpretation of various early Precambrian compressional and extensional regimes. Thrusting, transcurrent and extensional faulting have been identified in a crust that has been partly thickened during late Archaean and Palaeoproterozoic collision and locally extended both in back-arc basin settings and during syn-post-collisional collapse. The main Fennoscandian terranes can be traced southeastwards beneath the Mezen Basin based on gravity and magnetic data. Seismic profiling shows that the Phanerozoic successions of this basin overlie intracratonic rifts of Neoproterozoic and possibly Mesoproterozoic age. The rifts are composed of a number of half-graben, the details of which are well defined by vibroseis reflection profiling. During the development of Mezen Basin rifts, a Neoproterozoic passive margin was established in the vicinity of what is now the Timan Range. Late Neoproterozoic (Vendian) orogeny emplaced slope-rise turbidite-dominated successions southwestwards onto the platform. Towards the hinterland of the Timanides (also referred to as Baikalides), beneath the Phanerozoic Pechora Basin, a combination of potential field and seismic surveys, both shallow reflection and deep refraction profiling, have allowed analysis of the crust and uppermost mantle. Neoproterozoic magmatic arcs, Grenville-age(?) microcontinents and possible oceanic domains are inferred to exist beneath the early Palaeozoic unconformity. A vast region of Neoproterozoic accretion reaches from the western Urals, northwards beneath the Pechora Basin and the eastern Barents Shelf.

Rb–Sr and Sm–Nd dating of different metamorphic events from the Sausar Mobile Belt, central India: implications for Proterozoic crustal evolution

Sr and Nd isotopic studies of various rock suites and their mineral separates in and around the Sausar Mobile belt, Central India, have revealed that (1) the last amphibolite facies metamorphism (≡main Sausar metamorphism) occurred during 800–900 Ma and has more or less reset the Rb–Sr isotopic clock in this whole terrain whereas older Sm–Nd isotopic signatures were mostly unaffected; (2) the main Sausar Group of rocks is bounded both in the North and the South by two granulite belts of different crustal histories at least up to ∼1100 Ma; (3) the Northern granulite belt experienced peak metamorphism during pre 1100 Ma, followed by steep decompression and subsequent cooling around 1100 Ma; (4) peak metamorphism in the Southern granulite belt occurred at ∼2672 Ma. This was followed by emplacement of a vast basic igneous body (represented as metagabbro) and subsequent cooling around ∼1400 Ma. These two granulite belts were juxtaposed with each other during the Neo-Mesoproterozoic which is correlatable with high pressure metamorphism in the northern granulite belt. This tectonic movement might have been triggered by Mesoproterozoic rifting of the southern Bastar craton, resulting in basic magmatism in the southern granulite belt. Mafic dykes were emplaced during subsequent readjustment of the crust and amphibolite facies reactivation (∼main Sausar event) of granulites in the whole terrain. Lithological and tectonic arrangements, metamorphic history along with the present geochronological data from CITZ point towards an evolutionary history similar to the Grenville Province of Canada.

Provenance, tectonics and palaeoclimate of Proterozoic Chandarpur sandstones, Chattisgarh basin: A petrographic view

Sandstones of early Neoproterozoic Chandarpur Group, Chattisgarh Supergroup, central India display progressive change towards greater textural and mineralogical maturity from base to top of the succession. The clay-silt matrix decreases, sorting of sand grains improves, frequency of rounded grains increases, monocrystalline quartz content increases with concomitant decrease in polycrystalline quartz, feldspar and rock fragments. The trend of variations in different mineralogical and textural attributes, however, exhibits inflections at different stratigraphic levels. The sandstones of the basal Lohardih Formation are alluvial fan deposits, characterized by high matrix and feldspar content, iron-oxide impregnated highly angular grains and poor sorting. Petrographic properties collectively indicate that the sandstones were derived from a weathered granitic crust under a humid climatic condition. Abundance of well rounded grains within the alluvial fan and overlying braided fluvial deposit indicates prolonged wind action during episodes of high aridity. The shallow marine deposit overlying the fluvial deposits in the upper part of the Lohardih Formation exhibits bed-to-bed variation in the frequency of angular grains, feldspar content and overall maturity suggesting environmentally controlled segregation of sediments. The abrupt appearance of coarse-grained immature sandstones with concomitant reappearance of iron-oxide impregnated/altered feldspar grains in the upper part of the shelf deposits of the Chaporadih Formation point to a phase of tectonic uplift that possibly triggered a regression. Continued regression and peneplanation heralded the deposition of supermature medium-grained purple quartzarenite of the upper shoreface Kansapathar Formation in the uppermost part of the Chandarpur succession under a hot desertic climatic condition. The provenance analysis revealed that the Chandarpur clastics were derived from granites and granite-gneisses of a continental block tectonic provenance. Petrographic studies further indicate that high grade metamorphic rocks did not make any perceptible contribution to the Chandarpur system. The Eastern Ghats Granulite Belt apparently did not emerge till the early Neoproterozoic.

Metacarbonate Rocks (Calciphyres) of the Lapland–Kolvitsa Granulite Belt, Baltic Shield

Geological, petrochemical, and geochemical data are presented on metacarbonate rocks (calciphyres) of the Lapland—Kolvitsa granulite belt, Baltic Shield. The normative mineral composition of source rocks of the studied calciphyres was first reconstructed. High contents of some indicator elements (Fe, Mn, Cr, Co, P, Pb, and others) suggest that material supplied to the paleobasin was derived from diverse rocks (ultramafic, mafic, intermediate, and felsic). Contents and correlations of some trace elements (Sr, Li, F, Ba, and others) indicate that primary sediments formed under humid-semihumid paleoclimate in a fresh-water paleobasin (lagoon) characterized by occasional increase in salinity.

High-temperature crustal scale shear zone at the western margin of the Eastern Ghats granulite belt, India: implications for rapid exhumation

A mylonitic shear zone occurs along the western margin of the Eastern Ghats Granulite belt, India. In the northern sector, it separates metapelitic and mafic granulites from the granite gneisses of the Bastar craton, while in the southern sector, it separates charnockitic gneisses and mafic granulites from the granite gneisses of the Bastar craton. This boundary shear zone is characterized by a mylonitic foliation and a stretching lineation. Microstructures characterize high-grade conditions and quartz c-axis patterns show strong point maxima close to the long axes of the strain ellipsoids, indicating dominant prism [c] slip in quartz, commonly interpreted in terms of ductile deformation at high-temperatures. Quartz c-axis patterns in the recrystallized and unrecrystallized domains also imply grain boundary migration as the principal mechanism of dynamic recrystallization at high temperature conditions. A rapid exhumation during shearing in this marginal part of the convergent Eastern Ghats orogen is indicated by decompression reaction textures and retrograde hydration reactions, presumably due to hydrous fluid ingress, in the shear zone rocks.

Mafic dyke remnants in the Lewisian Complex of the Outer Hebrides, NW scotland: a geochemical record of continental break-up and re-assembly

In the predominantly late Archaean Lewisian Complex of NW Scotland, the Palaeoproterozoic (2.4–2.0 Ga) Scourie dykes have been used to discriminate between late Archaean events, and ca.1.7 Ga Laxfordian reworking. On the Outer Hebrides metabasite dykes intrude the Lewisian Complex, and were previously assumed to be correlatives of the Scourie dykes of the Scottish mainland. New geochemical data allow the Outer Hebrides metabasites to be divided into two groups. Dykes intruded into Archaean Gneisses north and south of the ca. 1.9 Ga South Harris Complex (SHC) are identical, and have depleted geochemical signatures. They are linked with an early episode of rifting (ca. 2 Ga) related to the fragmentation of the Lewisian crust and eventual formation of ocean crust. Later subduction of this ocean crust generated the ca. 1.9 Ga arc related rocks of the SHC, which contain a second group of subduction-related metabasites that are different from the dykes in the adjacent Archaean gneisses. Closure of this ocean and subsequent Laxfordian continent-arc-continent collision at ca. 1.7 Ga, sutured the rifted Archaean fragments back together. The northeast boundary of the SHC has previously been interpreted as a terrane boundary. However, the new dyke data demonstrates that any terrane boundary actually lies within the SHC. The northeast segment of the SHC, the Langavat Belt, contains dykes chemically identical to those intruding the Archaean gneisses to the northeast. The southwest segment of the SHC, the Harris Granulite Belt, contains arc related plutons and metabasites with arc signatures, which are distinct from mafic dykes in the adjacent Archaean gneisses and Langavat Belt. Consequently, much of the Langavat Belt is unrelated to the arc complex. Data from the Outer Hebrides dykes casts doubt on their correlation with the Scourie dykes, and demonstrate that the Outer Hebrides dykes have a closer affinity with the Palaeoproterozoic Kangâmiut dykes of Greenland, than to the Scourie dykes.

Geochronology and Tectonic Evolution of Karimnagar and Bhopalpatnam Granulite Belts, Central India

The Karimnagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42±0.08 Ga and 2.47°0.03 Ga. Rims of monazite show mean age of 2.21±0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO 2* diagram delineating a clear isochron with age of 1.59±0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.

Metasedimentary Rocks of the Lapland–Kolvitsa Granulite Belt of the Baltic Shield: Primary Mineral Composition and Petrogeochemistry

The normative mineral composition is reported on source rocks of metasediments from the granulite belt of the Baltic Shield. The primary composition, CIA index, and position of data points of studied rocks in discriminant diagrams indicate that a significant part of the studied rocks formed from immature sediments (graywackes and subgraywackes). The material supplied to sedimentation paleobasins was obtained from different (ultrabasic, basic, intermediate, and acid) rocks. The paleobasin was characterized by organic activity and reducing environment in the bottom layer. Correlation found between some elements, e.g., (Rb, Ba, Pb)–K; Sr–(Na, Ca), and so on, is also typical of Phanerozoic deposits. The possible contents of OM (Corg) and U were reconstructed in source rocks of metasediments of the Lapland–Kolvitsa granulite belt of the Baltic Shield.

Outlines of the Precambrian basement of Estonia

The crystalline basement of Estonia can be subdivided into two major geological units - North Estonian amphibolite and South Estonian granulite complexes. The amphibolite facies terrain consists of a sequence of metavolcanics and -sediments, which resemble those of the island arc sequences in southern Finland. The South Estonian granulites, probably belonging to the Belarus- Baltic Granulitic Belt, show the peak metamorphism conditions of about 800'C and 5-6 kbar. U-Pb isotopic data demonstrate that the Estonian basement has formed during the Palaeoproterozoic, 1.9-1.8 Ga ago and is composed of magmatic, volcanic, and sedimentary components across major structural zones. The granulite metamorphism postdates these ages (1 .79-1.'73 Ga). The youngest igneous rocks in the basement belong to the Fennoscandian Palaeo-Mesoproterozoic Rapakivi Province and include the composite Riga batholith, as well as at least five minor stock-like porphyritic K-granite plutons (ca. 1.6 Ga).