Basic Granulites Research Articles

Investigation of rock genesis and geodynamic significance based on Sr–Yb granite classification and geochemical characteristics: A case study from Guangxi, China

The intrusive bodies in the study area are categorized into two intrusive units, γπK2 and ηoπK2, based on rock structure characteristics and intrusive contact relationships. The primary lithologies are granite porphyry and quartz monzonite porphyry. Through LA-ICP-MS zircon U–Pb dating, the weighted mean ages of zircon ^206Pb/^238U for these two units were determined to be 84.09 ± 0.86 Ma and 86.10 ± 0.76 Ma, respectively. According to geochemical characteristics and Sr–Yb granite classification, the quartz monzonite porphyry is classified as Zhejiang-Fujian type granite, and the granite porphyry as Nanling type granite. The study explores the geodynamic significance of granites in the area based on Sr–Yb granite classification, identifying the lower crust metamorphic facies as amphibolite facies. The source rocks are primarily basic granulites and felsic granulites, reflecting a thermally active lower crust with poorly developed fluids. Additionally, Sr–Yb granite classification aids in determining the crustal thickness of the lower crust, which is crucial for understanding continental geological evolution. This paper discusses the optimal locations and conditions for mineralizing fluid precipitation, noting that tungsten-tin mineralization is associated with Nanling type granites and that the mineralization process generally occurs later than the diagenetic process under conditions of low pressure and low oxygen fugacity. Tungsten-tin prospecting should focus on areas where the surrounding rock temperature ranges from 300 to 500 °C, with particular attention to the contact zones of sedimentary rocks or near structural fault zones.

Identifying Ultrahigh-Temperature Metamorphism in Basic Granulites

Abstract Identifying ultrahigh-temperature (UHT) metamorphism in basic granulites is challenged by the uncertainty of peak temperatures. Consequently, available temperature indicators are desiderated all along. In this study, we investigated low-pressure basic granulites from the Daqingshan, North China Craton and determined a UHT peak condition around 1000–1050 °C and 0.8–0.9 GPa. This peak condition was constrained by combining the AlT of 0.10–0.12 in clinopyroxene (Cpx) cores, maximum Ti of 0.36–0.38 in amphibole (Amp) cores and anorthite proportion (XAn) of 0.67–0.70 and 0.61–0.63 in plagioclase (Pl) cores/mantles analyzed in two selected samples. The P–T records obtained from several thermobarometers related to amphibole, plagioclase and/or clinopyroxene were also verified to be in agreement with the phase equilibria modeling results. Notably, AlT-in-Cpx is assessed to be a reliable temperature indicator in basic granulites with the Opx–Cpx–Pl–Amp assemblage, albeit could be influenced by bulk-rock compositions in some extent and has a temperature standard error of ~70 °C. Besides, the post-peak evolution was dominated by near-isobaric cooling at 1.94–1.93 Ga, to a fluid-absent solidus as high as ~940 °C. During the cooling process, plagioclase and amphibole grew together at the expense of Al-rich clinopyroxene and unsegregated melts, accompanied by the (rimward) decrease of AlT-in-Cpx, Ti-in-Amp and XAn-in-Pl. A pre-peak process is inferred to be decompressional heating. The UHT metamorphism in Daqingshan is interpreted to result from asthenosphere upwelling and thermal advection from regional mantle-derived magmatism during post-orogenic extension, in a newly developed back-arc-related environment.

U–Pb SHRIMP zircon dating and geochemistry of metapelites from the Shillong Meghalaya Gneissic Complex, NE India: Implications for nature of protolith and tectonic setting

The Sonapahar area of Shillong Meghalaya Gneissic Complex (SMGC) is an extension of the Central India Tectonic Zone within the Precambrian Indian shield. The study area mainly consists of metapelites, basic granulites, granitic gneisses and high-grade gneisses as metamorphic imprints. U–Pb SHRIMP zircon dating yields a late Paleoproterozoic age of 1672 ± 6 Ma, which is age of the metapelites protolith. The petrochronology and geochemistry of metapelites and granitic gneiss basement rocks have been studied to learn more about the SMGC's tectonic environment, involvement in the supercontinent cycle, crustal development and geodynamic context. In the NCKFMASHTO system, phase equilibria modelling of metapelites shows peak metamorphic stage at P-T conditions of 8.25 kbar/ 820°C. Cordierite is formed during retrograde metamorphism during the decompression phases. The negative anomalies in Nb, Sr and Ti indicate that crustal sources played a significant role in magmatic emplacement. Geochemical data from metapelites indicate that protoliths were trachy-andesitic in nature, generated during the syn-orogenic environment by subduction tectonism. The SMGC experienced significant tectonic activity throughout the Paleoproterozoic age when the subduction tectonic setting coincided with orogenic activity. The presence of Paleoproterozoic age in the study area, as well as Southwestern Australia and East Antarctica correlates the Sonapahar granulite with the Columbia supercontinent history.

Petrogenesis of a nepheline syenite from parts of the Chotanagpur Granite Gneissic Complex: implications for Neoproterozoic crustal extension in the East Indian Shield

AbstractThe North Purulia Shear Zone that dissects the granulite basement of the Chotanagpur Granite Gneissic Complex of the East Indian Shield exposes a deformed and metamorphosed nepheline syenite. The studied ‘foid-monzosyenite’ shows high abundances of large ion lithophile elements and high field strength elements with low abundances of compatible elements. Trace-element signatures show negative U, Th, Zr, Ti and Pb and positive Sr, Ba and Eu anomalies with respect to the primitive mantle. The chondrite-normalized diagram shows strongly fractionated rare earth element patterns ((La/Lu)N ∼23–87). Geochemical fingerprints suggest that the basanitic protolith was formed by low-degree partial melting of garnet peridotite in the sub-continental lithospheric mantle. The enriched large ion lithophile, high field strength element and light rare earth element concentrations (relative to primitive mantle) can be explained by a mixed mantle source with components from a previously deformed alkaline rock/carbonatite. Geochemical data do not support any significant crustal contamination and suggest variable fractionation of clinopyroxene, ilmenite, titanite and apatite from the parental melt. Petrological data are consistent with the view that the nepheline syenite magma was emplaced in a rift setting with a minimum temperature of 800–900°C, low fO2 conditions (below the fayalite–magnetite–quartz buffer) at a mid-crustal depth between 950 and 900 Ma. The continental rift zone, however, did not lead to the formation of an open ocean basin. Subsequently, the studied rock and its basement was deformed and metamorphosed in a continent–continent collisional setting at ∼900 Ma. Combining information from the other Indian occurrences with this study, it is demonstrated that the deformed alkaline rocks and carbonatite are potentially valuable for tracing the birth and demise of the palaeo-supercontinents.

Anatectic origin and geological significance of the Neoarchean ~2.5 Ga charnockite in the Jining area, northern North China Craton

The Neoarchean charnockite of Yinshan Block has been a hot topic into understanding the Precambrian geology of the North China Craton. Although there is a broad consensus that the charnockite is usually related to granulite‐facies metamorphism, its petrogenesis and tectonics still remain controversial. We present a comparative study of the petrography, geochemistry, and geochronology of charnockite and intermediate–basic granulite in the Jining area along the northern margin of the North China Craton. Inclusions within hypersthene in charnockite are used to identify the peak granulite facies mineral assemblage, with the formation of charnockite being attributed to anatexis of the protolith associated with this granulite‐facies metamorphism. Anatexis occurred mainly during postpeak isothermal decompression, as inferred from the metamorphic evolution and pressure–temperature conditions (granulite and charnockite record peak pressure–temperature conditions of 750°C–1,000°C and 0.8–1.2 GPa and 850°C–975°C and 0.9–1.0 GPa, respectively) derived from analyses of metamorphic minerals using electron probe microanalysis, combined with previous zircon geochronology. Hypersthene in charnockite is interpreted to be a residual or peritectic mineral phase that was present during anatexis, and the charnockite is inferred to have been crystallized from a melt containing abundant residual minerals. Charnockite has similar geochemical characteristics to those of intermediate–basic granulite but differs in several key ways: (a) The charnockite is strongly depleted in the large‐ion lithophile element Cs, the heat‐producing elements U and Th, and the high‐field‐strength elements Nb, Ta, P, and Ti; (b) it is enriched in Sr; and (c) it exhibits both positive and negative Eu anomalies, which are characteristics of granites formed largely by in situ anatexis. Geochemical constraints indicate that the intermediate charnockite and retrograde metamorphic biotite–hornblende gneiss may both be the product of mantle‐derived magma in a magmatic subduction‐related back‐arc extensional environment. This setting provided a hot spot for metamorphic anatexis, with additional heat being provided by the upwelling of asthenospheric mantle. After the cessation of subduction by ~2,470 Ma, a magmatic arc extensional setting developed in the Jining area. The charnockite was involved in Paleoproterozoic orogenesis in the North China Craton and was strongly modified by a ~1,950‐Ma metamorphism event.

Lithogeochemical and Nd-Sr isotope data of the orthogranulites of the Juiz de Fora complex, SE-Brazil: insights from a hidden Rhyacian Orogen within the Ribeira belt

Abstract New petrography, geochemistry and Sm-Nd and Sr data from the orthogranulites of the Juiz de Fora complex in southern Rio de Janeiro State and compiled information provide insights on the petrogenetic and tectonic evolution. The complex comprises several geochemical groups including mafic orthogranulites (three tholeiitic and one alkaline) and felsic orthogranulites (three calc-alkaline and one tholeiitic/low K calc-alkaline). New geochemical and isotope data, combined with available U-Pb data suggest a long evolutionary history from the Paleoproterozoic to the Neoproterozoic. The oldest magmatic episode produced juvenile to slightly contaminated arc-type Rhyacian rocks, as well as granitic rocks related to collision or post-collision episodes. Altogether these rocks integrate part of a dismembered Rhyacian orogen within the Ribeira belt. Few TDM Nd model ages yielded 2.75 to 2.58 Ga, suggesting minor Archean contribution for magma genesis. Some of the basic granulites’ bodies yield Meso- to Neoproterozoic TDM Nd model ages, which may refer to an extensional magmatism. Orthogranulites present granulite facies paragenesis, related to the youngest tectonic episode in the Juiz de Fora Complex (Brasiliano Orogeny). The new data are potentially important for Paleoproterozoic reconstruction models, due to the predominantly juvenile character of the Juiz de Fora complex, as similarly worldwide.

Formation Conditions of Basic Granulites and High-Alumina Gneiss of the Baikal–Muya Belt (Northern Baikal Area)

The Kichera zone of the Baikal–Muya Belt consists of alternating tectonic plates with rocks of different metamorphic facies. Garnet–cordierite–sillimanite gneiss from the tectonic fragment in granite–gneiss of the Baikal massif with an age of 755 ± 15 Ma from the Goremyka plate and two-pyroxene schist of the granulite complex with an age of 617 ± 5 Ma from the Boguchan plate were studied. Thermobarometric studies of these key metamorphic rocks were carried out using the avPT (THERMOCALC) and TWEEQU (TWQ 2.01) methods. The P–T parameters estimated for the cordierite⎯sillimanite gneiss of the Goremyka plate correspond to the boundary between the amphibolite and granulite facies. Granulites of the Boguchan plate belong to the HT–LP type. Exhumation of metamorphic rocks could be caused by extension upon the evolution of the Late Baikal rifting.

Element Mobility and Behaviour of Zircon during HT Metasomatism of Ferroan Basic Granulite at Ayyarmalai, South India: Evidence for Polyphase Neoarchaean Crustal Growth and Multiple Metamorphism in the Northeastern Madurai Province

An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P–T estimates of ∼800 °C, 8 kbar for the granulite-facies metamorphism (M1) and ∼730 °C, 7 kbar for the high-grade metasomatism (M2). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ∼2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ∼2·6 Ga: high-grade metamorphism M1; ∼2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ∼2·48 Ga: high-grade metamorphism and anatexis M2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M3, ∼610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.

REE distribution in zircon from reference rocks of the Arctic region: Evidence from study by the LA-ICP-MS method

The results of the LA-ICP-MS analysis of the concentrations of REEs, U, Th, and Hf in zircon from Paleo- and Neoarchean reference rocks of the Kola region (garnet–amphibole gneiss, basic and acid granulites, and granite pegmatite) are reported. A new methodology of the study of accessory zircons has been validated and modified. The accuracy of the results is confirmed by analysis of standard zircons Temora 1 and 91 500 and by comparison with the data obtained in other laboratories.

Finding of corundum-bearing rocks in the Lapland granulite belt

Corundum-bearing rocks are described for the first time in the Kandalaksha structure of the Lapland granulite belt. Corundum is confined to rocks of two types: metagabbro‒anorthosites constituting lenses among metaanarthosites of the Kandalaksha massif and basic granulites. Corundum crystals (up to 200 μm long) occur in plagioclase and garnet and differ from each other depending on the host mineral, which serves as evidence against their xenogenic nature. Some corundum crystals exhibit an axial zone, which may indicate their crystallization from the gaseous phase. Corundum-bearing rocks are accompanied by piclogites (pyroxene‒garnet varieties with olivine). Piclogites and their minerals (clinopyroxene, garnet) are characterized by a positive Eu anomaly, which implies rock reworking by fluids during corundum formation, when deep-seated complexes were subjected to exhumation.

Mineralogy and Mineral Chemistry of Placer Deposit around Jhatipodar, Odisha

Abstract: The heavy mineral placer sands of Jhatipodar deposit, Odisha, India extends in NE-SW parallel to the coast over a width of 750-980m. A total of 195 samples were collected vertically in a grid pattern from 46 bore holes in 5 transects from beach and dunes covering an area of 75 hectares. The total heavy minerals (THM) content of the samples vary from 3.01% to 44.38%. The heavy minerals as observed from field and laboratory investigations are ilmenite, leucoxene, garnet, sillimanite, zircon, rutile, monazite, magnetite and corundum together with hornblende, diopside, tourmaline, sphene, apatite and epidote. The heavy minerals show similar pattern of grain size distribution from beach to dunes and are dominantly in the size of 90 - 350 μm with a peak distribution in -180, +125 μm. A bulk sample of dunal sands analysed 20.48% THM, contain 7.85% ilmenite, 7.65% garnet, 4.09% sillimanite, 0.20% zircon, 0.18% rutile and 0.13% monazite. The mineral grains are well liberated, rounded to sub-rounded and mostly free from surface coatings. The ilmenites contain 47.87-56.59% TiO2 with an average of 51.76% TiO2. The high TiO2 content (9.86-16.30%) in exsolved hematite is due to Fe2O3-FeTiO3 solid solution. Compared to ilmenite, leucoxene has higher amounts of Ti, Al, Si, Cr and lower amounts of Fe and Mn. The Ti/(Ti+Fe) in ilmenite varies between 0.46% and 0.55% while that in leucoxene has a range from 0.64% to 0.96%. The provenance rocks for this deposit are the khondalites, quartzites, calcsilicate granulites, granitic gneisses, charnockite, basic granulite, anorthosite and pegmatites of the Eastern Ghats Mobile Belt in the catchment areas of Rushikulya river. This paper deals with the distribution of heavy minerals in beaches and dunes, mineralogical association and chemistry of different minerals to evaluate the economic potentiality of Jhatipodar placer deposit and to decipher the amenability of different heavy minerals for mineral processing.

Granodiorites of the South Mountain Batholith (Nova Scotia, Canada) derived by partial melting of Avalonia granulite rocks beneath the Meguma terrane: Implications for the heat source of the Late Devonian granites of the Northern Appalachians

The Late Devonian South Mountain Batholith (SMB) of Nova Scotia is the largest batholith of the northern Appalachians. The peraluminous granitic rocks range from biotite granodiorite to leucogranite. Samples collected from a drill core of the Scrag Lake granodioritic pluton of the western SMB are chemically homogeneous from the surface to a depth of ~1425m. The homogeneous composition implies that the granodiorite was derived from a relatively homogeneous source and that country rock assimilation was not an important source for the parental magma. Equilibrium partial melt modeling of underlying sub-Meguma granulite rocks indicates that they are the primary source rocks of the granodiorites. We suggest that mantle-derived magmas intruded the lower crust and induce large-scale melting of the granulite basement rocks to produce the granodiorites. Fractional crystallization of the granodiorites plus assimilation of Meguma Supergroup metasediments likely produces the silica-rich rocks of the SMB. The cause of mantle melting is uncertain however it may be related to the transitioning of the northern Appalachians from a position above the deep mantle Pacific large low shear velocity province (LLSVP) to a higher shear velocity region of the mantle.

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

High-pressure (HP) granulites are commonly regarded as indicators of plate convergence and collision following the subduction of oceanic or continental crust. In this study we report the discovery of Silurian HP basic granulites from Central Qiangtang on the Tibetan Plateau. Detailed petrology and geochronology reveal a three-stage metamorphic history based on inclusions, reaction textures, and garnet zoning patterns. Peak metamorphism at 830–860 °C and 1.15–1.45 GPa (M1) is defined by high-Ca garnet cores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions at ca. 810–830 °C and 0.65–0.85 GPa (M2). Kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 590–650 °C and 0.62–0.82 GPa (M3). These results help define a sequential P–T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. Identification of mineral inclusion assemblages in zircons dated by U–Pb SHRIMP and LA–ICP–MS reveals peak HP metamorphism at ca. 427–422 Ma, subsequent near-isothermal decompression with associated retrograde reactions at ca. 392–389 Ma, and continued cooling at ca. 360 Ma. The P–T–t path of HP basic granulites reflects collision followed by extensional exhumation during early Paleozoic orogenesis. The present results indicate the occurrence of a collisional event along the northern margin of Indo-Australian Gondwana during the Silurian. Renewed Gondwana-directed subduction and subsequent collision probably led to the opening of the Paleo-Tethys Ocean.

Petrology and Geochemistry of Metapelites and Basic Granulites from Sonapahar Region of Shillong Meghalaya Gneissic Complex, North East India

Abstract The geochemistry of the metapelites and basic granulite of Sonapahar has been studied in order to assess the nature of the protolith and their likely tectonic environment. Metapelites contains SiO2 (av. 62.16wt %), K2O+Na2O (av. 4.57 wt%), Al2O3 (av. 16.69 wt%), MgO (av. 5.31 wt%), Mg# av. 0.60 and low TiO2 (av. 0.68 wt%) with low Y (av. 23.48 ppm) and Yb (av. 1.6 ppm) which point to their derivation from andesite source. Metapelites show prominent negative europium anomaly (EuN/Eu* = 0.38-0.54) and fractionated LREE to HREE pattern (LaN/LuN) 17.23 to 56.69. Basic granulites demonstrate low content of SiO2 (av50.95 wt%), Al2O3 (av13.67 wt%), K2O (av 0.58 wt%) and enriched in Fe2 O3t (av 11.42 wt%), MgO (av 7.29 wt%), CaO (av 10.60 wt %), Na2 O (av 2.26 wt%), Mg# av. 0.56 and range from calc-alkaline to tholeiitic in nature. The geochemistry of the metapelites and basic granulites advocate that metapelites are derived from arc related andesitic source due to subduction whereas protoliths of basic granulites was originated in rift related volcanism through diapiric movement of hot rising mantle derived basaltic magmas.

Textural and Compositional Relationships of Rutile and Chromite in Sittampundi Anorthosite Complex, Tamil Nadu

Abstract The anorthosite complexes and related rock types of the Indian Precambrian shield are primarily associated with either cratons or mobile belts. They were metamorphosed under amphibolites to granulite facies conditions. The major rock types are chromite-bearing meta-anorthosites, amphibolites, basic granulites, pink granites and gneisses. This study was carried out on chromitite bearing samples from Sittampundi layered anorthosite complex, Tamil Nadu, to evaluate the textural and compositional relationships of rutile and chromite. The pristine composition of the chromites is still preserved inspite of intense metamorphic and tectonic process. The rutiles are differentiated into isolated, clustered and exsolved rutile grains.

Zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotopes from the metamorphic basement in the Wuhe Complex: Implications for Neoarchean active continental margin along the southeastern North China Craton and constraints on the petrogenesis of Mesozoic granitoids

We report zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope data from mafic granulites and garnet amphibolites of the Wuhe Complex in the southeastern margin of the North China Craton (NCC). In combination with previous data, our results demonstrate that these rocks represent fragments of the ancient lower crust, and have features similar to those of the granulite basement in the northern margin of the NCC. A detailed evaluation of the Pb isotope data shows that Pb isotopes cannot effectively distinguish the role of the Yangtze Craton basement from that of the NCC basement with regard to the source and generation of magmas, at least for southeastern NCC. The age data suggest that the protoliths of the granulites or amphibolites in the Wuhe Complex were most likely generated in Neoarchean and that these rocks were subjected to Paleoproterozoic (1.8–1.9 Ga) high-pressure granulite facies metamorphism. This study also shows that the Precambrian metamorphic basement in the southeastern margin of the NCC might have formed in a tectonic setting characterized by a late Neoarchean active continental margin.

The 3.98–3.63 Ga zircons as indicators of major processes operating in the ancient continental crust of the east Antarctic shield (Enderby Land)

This study presents new results on zircons from the enderbite-charnockite rocks of Enderby Land, East Antarctica. U-Pb age of 3981 ± 8 Ma (SIMS SHRIMP II), which was first obtained for a protolith of massive enderbites from Aker Peaks, eastern Napier Mts, suggests that the existence of sialic crust in the study area at 4 Ga. Although there was only one magmatic zircon (of 150 grains analyzed) in the study area known with the oldest age, its significance cannot be overestimated, since it may indirectly evidence the existence of an Early Archean crustal block with a minimum age of 4 Ga, which extends for over 300 km across Enderby Land from its western to eastern part. Based on the U-Pb systematics, REE and trace element distributions in zircons from charnockite and enderbite gneisses, high-aluminous gneisses, and basic granulites, we first revealed that an early high-temperature metamorphic event accompanied by the emplacement of granodiorite intrusions occurred in the vicinity of Aker Peaks at 3620–3630 Ma. Although the 2850–3050 metamorphic overprints are clearly observed in some other areas of Enderby Land and are widely considered to be of critical importance on a regional scale, their metamorphic signatures are apparently absent from the U-Pb systematics of the studied zircon, thus suggesting the presence of similar old zircons in the study area. At the same time, all samples in this study record a 2480–2550 Ma granulite-amphibolite facies overprint represented as new zircon growths or recrystallization of earlier phases.

Palaeomagnetism of Precambrian dyke swarms in the North China Shield: The ∼1.8 Ga LIP event and crustal consolidation in late Palaeoproterozoic times

The North China Shield (NCS) is cut by a laterally-extensive dyke swarm emplaced at 1.78–1.76 Ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by ∼1.85 Ga. Palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with NE to N declinations and rare opposite polarity. Dykes with NE shallow magnetic declination (A1, D/ I = 36/−1°) recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations (A2, D/ I = 8/2°) in lower grade metamorphic terranes to the south. Contact tests indicate a primary cooling-related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable. Petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at ∼1.85 Ga. A temporal migration A2 → A1 is implied by the higher crustal level and earlier acquisition of the former, and the deeper source and later acquisition of the latter. A third population of dyke magnetisations (A3, D/ I = 18/43°) is distributed towards steeper inclinations and close to the Mesozoic-Recent palaeofield. These are either partial or complete overprints of A1–A2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra, or are much younger dykes of Mesozoic-Tertiary age. A minority fourth (later Precambrian but presently undated) dual polarity population has a magnetisation (11 dykes, D/ I = 108/7°) with contact tests indicating a primary cooling-related origin. The ∼1.78–1.76 Ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major Large Igneous Province (LIP)-related events. The new definition of a ∼1.83–1.76 Ga APW swathe from the North China Shield permits a comparison with other shields and yields a constraint to continental configurations during the late Palaeoproterozoic. A quasi-integral reconstruction of Palaeopangaea is tested here and supported by conformity of predominantly of uplift-related palaeopoles from the ∼1.90–1.70 Ga tectono-thermal belts and from SW → NE trending APW implied by the distribution of poles from the ∼1.80 Ga igneous suites including the LIP events. This trend incorporates the A2 → A1 migration and the granulite terrane cooling polar swathe from North China. The reconstruction indicates that continental crust consolidated in Palaeoproterozoic times by accretion of ∼2.3–1.7 Ga orogenic belts around a hemispheric and crescent-shape core already established by Late Archaean times. The North China Shield is interpreted to have bordered the western cratonic margin of the Indian Shield in a proximity supported by correlation of geological features and suggested by a number of previous workers. The Central Orogenic Zone of the North China shield characterised by tectono-thermal activity prior to ∼1.85 Ga was then contiguous with a comparable zone running through the centre of the Indian Shield and continuing into the Capricorn Belt of Western Australia. The ∼1.78–1.76 Ga dykes in North China continue into dyke swarms in the South India Shield and may have been sourced in a plume-related LIP focussed near the continental margin in the Xiong’er Aulacogen.

Crustal accretion and metamorphism of Mesoarchean granulites in Palghat-Cauvery Shear Zone, Southern India

This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛTCHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛTCHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.

Constraints on the crustal structure of the internal Variscan Belt in SW Europe: A magnetotelluric transect along the eastern part of Central Iberian Zone, Iberian Massif

[1] The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex decollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).