Syntectonic Granitoids Research Articles

Petrological, geochemical and geodynamic evolution of the Wadi Al-Baroud granitoids, north Arabian-Nubian shield, Egypt

The Wadi Al-Baroud area, in Egypt's Eastern Desert, exposes Neoproterozoic rocks of the Arabian-Nubian Shield (ANS), including both syntectonic granitoids (granodiorite and tonalite) and post-collisional granites. We present field work, petrographic study, mineral compositions, and whole-rock geochemistry results from these granitoids and discuss their petrogenesis, magmatic sources, evolution, and tectonic significance. The syntectonic granitoids show subduction affinity and an anomalous steep trend of K-enrichment that suggests assimilation of a granitic component during their evolution. The post-collisional granites form two plutons, on opposite sides of Wadi Al-Baroud, named here the Ras Baroud pluton (RBP) and the Abu Hawis pluton (AHP). They intruded the syntectonic granitoids with sharp intrusive contacts. The post-collisional plutons are devoid of mafic enclaves and are cut by very few dikes. They dominantly consist of biotite monzogranite that grades into muscovite monzogranite. The latter lithology hosts Nb-Ta oxide minerals (columbite, tantalite, and wodginite) displaying a variety of textural and compositional features. The cores are primary columbite-(Mn), whereas rims are overgrown or partly replaced by tantalite-(Fe) and wodginite due to late interactions with highly fractionated residual melt. The highly-evolved AHP and RBP granites are typical of the post-collisional granitoids of the ANS, including high concentrations of rare earth elements (REE), Ta, Hf, Nb, Zr, Y, and Rb; elevated ratios of Ga/Al; and low contents of Sr, CaO, and MgO. Their geochemistry suggests that the parental magma of both plutons formed from an I-type tonalitic source rock that underwent partial melting during the thermal disturbance that followed a lithospheric delamination event during the post-collisional stage of the East African Orogeny. The variations in major oxide and trace element contents among individual samples of the AHP and the RBP cannot be explained as a liquid line of descent due to fractional crystallization; rather we interpret them as sampling variable proportions of an evolved liquid and the solid crystals in equilibrium with that liquid.

Geochemical Characteristics of the Syn-Tectonic Granitoids along Wadi El-Sheikh Area, South Sinai, Egypt.

Geochemical Characteristics of the Syn-Tectonic Granitoids along Wadi El-Sheikh Area, South Sinai, Egypt.

Tectonometamorphic evolution and U–Pb dating of the high-grade Hammar Domain (Southern Ethiopian Shield); implications for the East-African Orogeny

Based on new U–Pb zircon data, field structural analysis and a detailed petrological study of the plutonic and high-grade metamorphic rocks of the Hammar Domain (Southern Ethiopian Shield) overall geodynamic scenario was inferred, bringing broad implications for the East-African Orogeny. The structural evolution of the Hammar Domain can be summarized into four phases, D1 to D4, resulting in (a) relict compositional banding (S1), (b) flat-lying migmatite foliation (S2) defining the primary contacts of granulites and migmatites, (c) superimposed steeply dipping N–S trending compressional foliation (S3) due to regional ~E–W oriented compression and (d) later ~NW–SE trending left-lateral transpressive fabric (S4). New geochronological data point to long-lasting orogenic convergence forming the East-African Orogeny which resulted in two main geodynamic events: (a) Late Tonian to late Cryogenian episode (ca. 770 to 650 Ma) where large volcanic arc construction as the source of rock photoliths (dated at ca. 770 Ma) was followed by crustal accretion and flat-lying fabrics origin (D2 stage), intense migmatization and HT-MP metamorphism (T: 700–850 °C and P: 0.7–0.9 GPa) at depths of ~25–35 km (dated at ca. 720 and 715 Ma). (b) Late Cryogenian to early Ediacaran episode (ca. 650 to 620 Ma) as the key era of continental collision leading to the Greater Gondwana assembly. An early ~E–W oriented compression (D3 stage) resulted in ~N–S trending fabrics that have been continuously changed to the left-lateral transpression (D4 stage) forming ~NW–SE oriented foliations. The time-scale of D3 and D4 events is inferred by syn-tectonic granitoid intrusions yielded at ca. 648 Ma and ca. 630 Ma respectively. Furthermore, the syn- to post-tectonic leucogranite dike, dated at ca. 630 Ma, marks the upper limit for the ductile or brittle-ductile deformation and regional metamorphic events.

THE CONTRIBUTION OF REMOTE SENSING AND AEROMAGNETISM TO GOLD PROSPECTING: THE CASE OF THE MEIGANGA ZONE, CAMEROON

In order to optimize gold prospecting in the Meiganga zone located in the Adamaoua region of Cameroon, aeromagnetic and remote sensing prospecting was carried out in the eastern and southern parts. The remote sensing approach on a Landsat 8 OLI/TIRS image highlighted areas of maximum gold concentration. Thus, ferric ion bearing minerals are located in the North-West, silicate minerals bearing ferrous ions are in the Centre while clay minerals are in the North-East and East. The principal component analysis revealed important structural information. The PCA Spatial Map (PC1, PC2, PC3) showed the plutonic formations composed of anatexis and anatexis granites, vegetation cover (at the date of image acquisition: February 22, 2019), areas of permanent water circulation or accumulation, and metamorphic and sedimentary formations namely gneisses, quartzites, schists and superficial clay formations. A Landsat SRTM (Shuttle Radar Topography Mission) image was also used to enhance the lineaments through the Sobel filter to highlight the geomorphological (cliffs, valleys, ...) and topographic (river network, ridge and drainage segment) structures. The aeromagnetic approach was also important. The study of the modified magnetic field (CM) showed 4 ranges: very high, high, medium and low. The Total Magnetic Anomalies (TMI) of the area are subdivided into 2 ranges large positive anomalies (221.1-103.0 nT) located in the lower part of NE-SW orientation, small positive anomalies (103.0-(-)89.7 nT) located in the upper part of NE-SW orientation. The reduced total magnetic anomaly at the equator shows a fairly similar distribution to the total magnetic anomaly with the large positive anomalies in almost the entire lower part. Superimposed on the geological map, Neoproterozoic pre- to syn-tectonic granitoids (C) are superimposed on the large positive anomalies and Neoproterozoic conglomerates, quartzites, sedimentary shales and volcanosedimentary rocks (A) and Neoproterozoic syn-tectonic granitoids (B) are superimposed on the large and small positive anomalies. The grid of the reduced residual equatorial anomaly (ARRE) confirms that the local geology is strongly magnetic (gneiss and quartzite). The filters of the derivatives allowed to establish a map of magnetic lineaments of major orientation N045° and minor orientation N130°. The horizontal gadient superimposed on the local maxima showed the presence of deep structures oriented NE-SW. The analytical signal superimposed on the local maxima highlights the metamorphic basement consisting of rocks with strong magnetism. The application of Euler deconvolution localizes the depth of the sources of linear anomalies.

Restoring original composition of hydrothermally altered Archean metavolcanic rocks of the Carajás Mineral Province (Brazil): Geodynamic implications for the transition from lid to mobile tectonics

One of the most debated issues about the Carajás Mineral Province in Brazil is the tectonic setting that prevailed at the time of the major Neoarchean (ca. 2.7 Ga) volcanic event, with views ranging from subduction- to rift-related. During this event, thick lava flows of mainly basaltic and basaltic-andesitic composition extruded at the Earth's surface, cooled and underwent significant alteration by hydrothermal fluids rising through deep-seated crustal faults. Archean settings have been inferred for hydrothermally altered volcanic rocks using as a proxy their whole-rock compositions in discrimination diagrams for modern geotectonics. Despite its debatable application to Archean volcanic sites, the same working approach is used in reconsidering the geodynamic view about the Carajás volcanic event. We use an artificial neural networks-based method to perform a full mass balance calculation on a large compiled database of metavolcanic rocks from Carajás. Predicted precursors reproduce a linear and limited FeO*/MgO increase with SiO2 typical of modern calc-alkaline arc environment. Moreover, the identified least-altered rocks show that they may petrogenetically involve subducted water-bearing sediments. But contrary to what is expected for such a volcanic arc environment, amphibole and magnetite fractionations have not been observed in the precursors. Discrimination diagrams using incompatible trace elements were unable to rule out the subduction hypothesis so other geological and geochemical evidence from the granitoid record of Carajás was taken into consideration. In combination with the Archean granitoid evolution of CMP, we conclude that crustal contamination associated with continental rifting appears to be the most likely process for inducing the calc-alkaline affinity of the Carajás Neoarchean metavolcanic rocks. This calc-alkaline affinity in least-altered volcanic rocks appears to be best reproduced by petrogenetic modelling involving wall-rock assimilation and melting of the Carajás basement (TTG and other K-rich granitoids) by parental magmas derived from depleted mantle source, prior to their fractional crystallization. This volcanic event was coeval with the intrusion of A-type syntectonic granitoids at the transition from a stagnant lid to mobile plate tectonic regime associated with localized translithospheric transpressional rift.

Application of anisotropy of magnetic susceptibility (AMS) in understanding regional deformation, fabric development and granite emplacement: examples from Indian cratons

Abstract In this paper the authors review various applications of analysing fabric in granites from Indian cratons using anisotropy of magnetic susceptibility (AMS). First the general importance of AMS in identifying the internal fabric in massive granitoids devoid of visible foliations/lineations is highlighted. Subsequently, three important applications of AMS in granitoids are discussed. (a) The case of Godhra Granite (southern parts of the Aravalli Mountain Belt) is presented as an example of the robustness of AMS in working out the time relationship between emplacement/fabric development and regional deformation by integrating field, microstructural and magnetic data. (b) AMS orientation data from Chakradharpur Granitoid (eastern India) are compared with field-based information from the vicinity of the Singhbhum Shear Zone to highlight the use of AMS in kinematic analysis and vorticity quantification of syntectonic granitoids. (c) Magnetic fabric orientations from the Mulgund Granite (Dharwar Craton) are presented to document the application of AMS in recognizing superposed deformation in granitoids. Moreover, AMS data from Mulgund Granite are also compared with data from another pluton of similar age ( c. 2.5 Ga) from the Dharwar Craton (Koppal Granitoid; syenitic composition). This highlights the use of AMS from granitoids of similar absolute ages in constraining the age of regional superposed deformation.

Structural analysis combined with new geothermobarometric and geochronological results of the Além Paraíba shear zone, between Três Rios and Bananal, Ribeira Orogen, SE Brazil

The Além Paraíba Shear Zone (APSZ) is composed of subvertical mylonites trending ENE/WSW with subhorizontal stretching lineation indicative of dextral movement. In the western part of the area mapped in this paper the shear zone, defined as an elongated body of mylonites, dies out. This constrains the possible displacement along the zone. The mylonites are of relatively high metamorphic grade, product of deformed orthogneisses, granulites, paragneisses and associated syn-tectonic granitoids. These rocks reached granulite facies conditions at temperatures and pressures in excess of 850 °C and 8 kbar. Geothermobarometric results indicate that the progressive ductile deformation related to the shear zone occurred under amphibolite facies conditions at temperatures of 714 ± 23 °C to 562 ± 38 °C and pressures between 7.1 ± 0.8 kbar and 4.3 ± 1.1 kbar. Zircon U-Pb dates by LA-ICPMS were obtained from three samples, one from a mylonitic orthogneiss with age 583.6 ± 7.1 Ma, one from a mylonitic paragneiss with age 591.5 ± 6.1 Ma, and one from a pegmatitic vein that cuts the mylonites obliquely and yields an age of 597.7 ± 9.4 Ma. Monazite crystals from the mylonitic orthogneiss were also analyzed by LA-ICPMS U-Pb, resulting in a spread of concordant dates ranging from 611 to 553 Ma. Taking these data together, we estimate the age of peak metamorphism to be between 595 and 590 Ma. The age of the monazite crystals suggests continuous crystallization and/or re-equilibration after the metamorphic peak, probably catalyzed by the activity of the shear zone and/or fluid migration under amphibolite facies conditions. Since the APSZ affects a 535 Ma granite, we argue that the shear zone was probably active in the time span between 580 and 530 Ma, after metamorphic peak conditions but still at relatively high temperatures. The temporal coincidence with the main collision in the Ribeira Orogen supports the idea that the shear zone is the result of oblique collision.

Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland

The Saglek Block of Labrador comprises Eoarchean to Neoarchean lithologies, metamorphosed at high temperature at ca. 2.7 Ga. Here, we investigate the gneisses of Tigigakyuk Inlet, previously identified as the locality exposing the most ancient rocks in the Saglek Block. New geochronological and geochemical results reveal a multistage history. Precise magmatic emplacement ages of 3.75–3.71 Ga refine the age of the Uivak Gneiss. Zircon rims and neoblastic grains with low Th/U record metamorphism at ca. 3.6 and 2.8–2.7 Ga. Magmatism between these tectono-metamorphic events is recorded by the presence of meta-mafic dykes in the gneisses, gabbroic enclaves in ca. 2.7 Ga syn-tectonic granitoids, as well as by a ca. 3.56 Ga age for monzonitic gneiss in which metamorphic zircon is present as xenocrysts. Felsic (TTG) magmatism between ca. 3.75 Ga and 3.71 Ga, as well as metamorphism at both ca. 3.6 Ga and 2.8–2.7 Ga, is also recognised in the Itsaq Gneiss Complex of south West Greenland, and is restricted to the Færingehavn Terrane. Our new data enable a more rigorous correlation between these formerly conjugate parts of the North Atlantic Craton.

Integration of remote sensing, geochemical and field data in the Qena-Safaga shear zone: Implications for structural evolution of the Eastern Desert, Egypt

The Qena-Safaga shear zone (QSSZ) represents a significant structural characteristic in the Eastern Desert of Egypt. Remote Sensing, field and geochemical data were utilized in the present study. The results revealed that the QSSZ dominated by metamorphic complex (MC) that intruded by syn-tectonic granitoids. The low angle thrust fault brings calc-alkaline metavolcanics to overlie MC and its association. Subsequently, the area is dissected by strike-slip faults and the small elongated basins of Hammamat sediments of Precambrian were accumulated. The MC intruded by late-to post-tectonic granites (LPG) and Dokhan Volcanics which comprise felsic varieties forming distinctive columnar joints. Remote sensing analysis and field data revealed that major sub-vertical conspicuous strike-slip faults (SSF) including sinistral NW-SE and dextral ca. E-W shaped the study area. Various shear zones that accompanying the SSF are running NW-SE, NE-SW, E-W, N-S and ENE-WSW. The obtained shear sense presented a multiphase of deformation on each trend. i.e., the predominant NW-SE strike-slip fault trend started with sinistral displacement and is reactivated during later events to be right (dextral) strike slip cutting with dextral displacement the E-W trending faults; while NE-SW movements are cut by both the N-S and NNW - SSE trends. Remote sensing data revealed that the NW-SE direction that dominated the area is associated with hydrothermal alteration processes. This allowed modifying the major and trace elements of the highly deformed rocks that showed depletion in SiO2 and enrichments in Fe2O3, MnO, Al2O3, TiO2, Na2O, K2O, Cu, Zn and Pb contents. The geochemical signatures of major and trace elements revealed two types of granites including I-type calc-alkaline granites (late-to post-tectonic) that formed during an extensional regime. However, syn-tectonic granitoids are related to subduction-related environment.

The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion?

In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5Ga) Betul Belt and the Neoproterozoic (∼1.0Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MSU-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2Ga and 0.95Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0Ga and 0.95Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

Mesoproterozoic base metal sulphide deposits in the Namaqua Sector of the Namaqua-Natal Metamorphic Province, South Africa: a review

The Mesoproterozoic Namaqua Sector of the east-west trending Namaqua-Natal Metamorphic Province (NNMP) located in the Northern Cape Province is underlain by supracrustal rocks and abundant felsic intrusive rocks that have been subjected to medium to high grade metamorphism and polyphase deformation. It is the host to world class base metal sulphide resources that have been mined for more than a century. Most of these mines are no longer in production such that the Cu-Pb-Zn-Ag mines in the Aggeneys-Gamsberg Ore District (A-GOD), with a total output of 1.8 million ton (Mt) per annum, are the only current significant producers. Development of the supergiant Gamsberg zinc deposit commenced in 2015 and will add a further 4 Mt per annum (250 000 t of zinc) to the output with the mine eventually becoming one of the larger zinc producers in the world. The Namaqua Sector of the NNMP has been subdivided into several lithologically distinct, structural discontinuity bounded Subprovinces and Terranes, each with its characteristic base metal sulphide deposits. The most extensive and best mineralized of these is the Bushmanland Subprovince where the ~1300 Ma stratabound polymetallic Broken Hill-type meta-SEDEX deposits contain total resources exceeding 500 Mt of Zn-Pb-Cu-Ag ore. Further east much smaller and subeconomic stratabound deposits occur at Putsberg (Cu-Zn-Pb-Ag) and Adjoining Geelvloer (Zn-Cu-Pb-Ag-Au). Magmatic ore deposits include minor magnetite-copper skarn/iron-oxide-copper-gold (IOCG) deposits associated with syntectonic granitoids (~1200 Ma) in the east and small syntectonic gabbronorite hosted Ni-Cu-(Co) occurrences at Hondekloof (~1170 Ma) to the southwest. The mined-out Okiep Copper District in the west hosts an abundance of small, syn- to late-tectonic mafic cupriferous intrusions (~1030 Ma), which collectively contributed some 100 Mt of ore yielding ~2 Mt of blister copper. The Areachap Terrane in the far eastern part of the Namaqua sector of the NNMP has a number of stratabound volcanogenic massive sulphide (VMS) Zn-Cu (Ag-Au) deposits of variable tonnage and hosted by intermediate to mafic meta-volcanic and meta-sedimentary rocks (~1300 Ma). The largest of these, the world class Prieska deposit (47 Mt) has been mined out. The remainder of the deposits are small (1.5 to 9 Mt), of low grade and uneconomic. The Terrane has been intruded by a suite of poorly mineralized mafic to ultramafic rocks (~1180 Ma) of which the high tonnage (114 Mt) but low grade Jacomynspan Ni-Cu deposit, is the most significant. The Richtersveld and Gordonia Subprovinces are poorly mineralized whereas the Kaaien Terrane has no Mesoproterozoic mineralization of any significance. The Gordonia Subprovince hosts the small magmatic Oup Ni-Cu-(Co) deposit (~1150 Ma) and is the only massive sulphide occurrence in the Subprovince. Both domains however, have minerals deposits associated with the regionally extensive post-tectonic Namaqualand pegmatite belt (~1000 Ma) which is a resource of beryl, tantalite, wolframite, cassiterite, micas, and feldspar. The Mesoproterozoic stratabound deposits of the Namaqua Sector of the NNMP are all of similar age and terrane bounded. Their different characteristics are the result of relative position within a subduction related plate tectonic setting. The siliciclastic hosted deposits of the Aggeneys-Gamsberg Ore District are interpreted to have formed in a continental sub-basin whereas the more chemogenic-volcanogenic hosts of the Putsberg and Adjoining Geelvloer deposits have the lithological characteristics associated with a back arc basin. The VMS deposits of the Areachap Terrane are linked to a mature island arc and were located more proximal to the subduction zone. The younger magmatic, mainly Ni-Cu and Cu deposits are syn- to post-tectonic, formed over a timespan of ~150 Ma, do not appear to be terrane specific and are considered to have been generated from basaltic magma during partial melting of subcontinental lithospheric mantle. Localization of intrusive suites can be the result of crustal thinning or proximity to major zones of dislocation such as the Pofadder Shear Zone and Boven Rugzeer Shear zone. The exploration success within the Namaqua Sector of the NNMP has been significant and effectively all known deposits, apart from those of the Okiep Copper District, were discovered using basic airborne and ground geophysical methods, soil geochemistry, lithological mapping and limited remote sensing during the 1970’s and early 1980’s. Future exploration should focus on the use of advanced airborne, ground and down-the-hole geophysics supported by remote sensing, soil and lithogeochemistry, the use of heavy mineral tracers and a workable geological ore deposit model. Over the last 35 years, the area has shown a decrease in the number of operating mines and commodity production, in particular copper. Although several areas show exploration potential, the Aggeneys-Gamsberg Ore District offers the best brown fields target whereas initiation of mining of the known smaller deposits is dependent on the revival of the currently depressed commodity demand and price.

Rapid high-resolution U–Pb LA-Q-ICPMS age mapping of zircon

206Pb–238U LA-ICPMS image map of a complex, polyphase zircon produced using 7 micron rasters.

Refined geological history of the polyphase plutonometamorphic complex in the Thala Hills area (Enderby Land, East Antarctica) from zircon SHRIMP dating and implications for Neoproterozoic amalgamation of Gondwanaland

Abstract The Thala Hills area occupies a key position in Gondwanaland reconstructions near the India–Sri Lanka–Antarctica junction. We present U–Pb zircon isotopic age determinations from SHRIMP II obtained on four granite gneiss samples. Three high-temperature tectonomagmatic episodes may be distinguished in the study area at c. 980–970, c. 780–720 and c. 545–530 Ma. The c. 980–970 Ma event corresponds to the Rayner Structural Episode that affected East Antarctica, including the Sør Rondane Mountains to the west and Kemp Land to the east. The c. 780–720 Ma episode included two events at approximately 780 Ma (high-grade anatexis) and 720 Ma (syntectonic granitoid emplacement), and was roughly coeval with tectonomagmatic events in Dronning Maud Land of East Antarctica, as well as in other Gondwanaland regions, such as Madagascar, Sri Lanka and eastern Africa. The c. 780–720 Ma episode may be correlated with the East African Orogeny. These correlations argue for a similar geological evolution and conjugate position for Dronning Maud Land and Enderby Land despite a postulated Cambrian Lützow–Holm Bay suture separating them. The Cambrian ( c. 545–530 Ma) episode was manifested by high-grade anatexis that confirms previously identified late Neoproterozoic–Cambrian tectonothermal activities in the study area. Supplementary material: Trace element compositions of zircons and whole-rock chemical compositions obtained by previous studies are available at https://doi.org/10.6084/m9.figshare.c.3738332

Changing tectonic settings through time: Indiscriminate use of geochemical discriminant diagrams

No one method should be used to identify ancient tectonic settings, and this is especially true for rocks older than 2.5Ga, when Earth was changing from stagnant lid to plate tectonics. The most useful application of geochemical discriminant diagrams should be to constrain mantle source compositions, mantle heterogeneity, the effects of degree of melting, mixing of melts, and crustal contamination. It is only the convergence of evidence from greenstones that includes rock association, stratigraphic variation, sediment types and provenances, information from syntectonic granitoids, and geochemistry of all rock types that we can make progress in understanding evolving tectonic settings on planet Earth.

Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310-295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.

Distinct sources for syntectonic Variscan granitoids: Insights from the Aguiar da Beira region, Central Portugal

The Variscan syntectonic granitoid plutons from the Aguiar da Beira region (central Portugal) were emplaced into metasediments of Late Proterozoic–Early Cambrian age during the last Variscan ductile tectonic event (D3), which is related to dextral and sinistral shearing.The older intrusion, with a U–Pb ID–TIMS zircon age of 321.8±2.0Ma, consists of porphyritic biotite granodiorite–granite with transitional I–S type geochemical signature, relatively low 87Sr/86Sr322 ratios (0.7070–0.7074), εNd322 values of −3.9 to −4.6 and whole-rock and zircon δ18O values of 10.6‰ and 8.0‰, respectively. By contrast, the younger intrusion is an S-type muscovite–biotite leucogranite, emplaced at 317.0±1.1Ma, showing more radiogenic 87Sr/86Sr317=0.7104–0.7146, lower εNd317 values of −7.7 to −8.7 and higher δ18O-wr=11.3‰ and δ18O-zr=9.5‰. The combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite–granite, involving the anatexis of lower crustal metaigneous protoliths, and possible hybridization with mantle-derived magmas. A shallower origin, at mid crustal levels, from pure crustal derivation, through moderate degrees of partial melting of Proterozoic–Cambrian metasediments is proposed instead for the muscovite–biotite leucogranite.

Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures–Tanneron Massif (SE France)

Abstract Synthesis of structural, petrological and geochronological data for the Maures–Tanneron Massif and its integration in the framework of adjacent massifs (i.e. Sardinia and Corsica) has allowed us to propose a new model of evolution for the southern Variscan belt. After Siluro-Devonian subduction associated with high-pressure–low-temperature (HP/LT) metamorphism M 0 ( c. 10–15 °C km −1 ) and subsequent Carboniferous nappes stacking, the belt underwent strong reworking related to back-thrusting. Nappes stacking and back-thrusting were associated with typical Barrovian metamorphism M 1 ( c. 20–30 °C km −1 ) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M 2 ( c. 40–60 °C km −1 ) and M 3 ( c. 60–80 °C km −1 ) during 330–300 Ma in the internal part of the belt. Progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root. Continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold-dome structures. The mass transfer is accommodated by orogen-parallel transpressive shearing synchronous with M 3 during Late Carboniferous time. The orogenic wedge is characterized by two main tectono-metamorphic units decoupled by a major shear belt: an Internal Zone with migmatites and syntectonic granitoids, where HP relicts have been exhumed, and an External Zone that escaped the late HT event and preserved precious structures.

Petrography and zircon morphology of syntectonic granitoids rocks of Hafafit area, South Eastern Desert (Egypt).

The study deals with the petrography of the granitoid rocks intruding the five (A to E) domes of Hafafit Culmination. The morphological characteristics of zircon crystals are also examined, in order to elucidate the genesis of zircon and their host Hafafit granitoids. The granitoid rocks are tonalites, granodiorites and quartz diorites, all exhibiting gneissic texture. The zircon crystals from all the rock types intruding each one of the domes are examined in detail for colour, inclusions, crystal length, crystal width, elongation ratio, crystal form and habit, degree of roundness and corrosion, and are distinguished in groups on the basis of the previous characteristics. Two generations of zircons have been recognized; the first generation with magmatic features and the second one with post magmatic features. The first generation of zircon is attributed to the subduction, magmatic and deformation events while the second generation of zircon is attributed to metamorphism.