High-grade Gneisses Research Articles

Sedimentary provenance and palaeoenvironment of the Baixo Araguaia Supergroup: constraints on the palaeogeographical evolution of the Araguaia Belt and assembly of West Gondwana

Abstract Provenance studies on metasedimentary rocks of the Baixo Araguaia Supergroup of the Brasiliano Araguaia Belt, central Brazil, yield 207 Pb/ 206 Pb zircon evaporation ages for detrital zircons from quartzites concentrated around 1000–1200 Ma and 2800–2900 Ma; Sm–Nd T DM model ages of schists and phyllites scatter around 1600–1700 Ma. Facies analysis of low-grade metasedimentary rocks from drill cores suggests a sedimentary environment of basin floor and lower- to upper-slope turbidites. Nearby sources are indicated by the textural and mineralogical immaturity; together with structural geological data indicating tectonic transport of the supracrustal pile towards the NW, this suggests probable provenance from the southeastern portion of the Araguaia Belt and not from the Amazonian Craton as usually believed. The Goiás Massif, Goiás Magmatic Arc, São Francisco Craton and Paranapanema block are considered to be the best candidates. They may have formed a larger continental mass during West Gondwana amalgamation, prior to their collision with the Amazonian Craton to form the Araguaia Belt. Final timing of this collision is constrained by c . 550 Ma syntectonic granites. Similar ages for high-grade gneisses in the Rokelide Belt suggest coeval collision and coetaneous metamorphism of the Araguaia and Rokelide belts, but more geological and geophysical data are required for a decisive correlation between these belts.

Thermo-tectonic evolution of the North Singhbhum Mobile Belt (eastern India): A view from the western part of the belt

The arcuate North Singhbhum Mobile Belt (NSMB) comprises multiply-deformed greenschist–amphibolite facies phyllites and schists flanking the centrally-located belt of the Dalma meta-igneous suite of rocks. The NSMB is separated from the Archaean Singhbhum Craton in the south, and the suite of granulite–amphibolite facies gneisses and foliated granites of the Chotanagpur Gneissic Complex to the north, by ductile shear zones. Three fabrics (S 1S, S 2S, and S 3S) formed due to D 1S, D 2S and D 3S deformation events and three metamorphic stages (M 1S: pre-S 2S; M 2S: syn-S 2S; and M 3S: syn/post-S 3S) were identified in phyllites and schists to the south of the Dalma meta-igneous in the NSMB. Reverse-sense transport along north-dipping imbricate thrusts (S 3S) emplaced the high-grade garnet–kyanite–staurolite schists centrally located in the orogen over low-grade muscovite–biotite–chlorite schists in the foreland. The M 1S stage (∼1.5 Ga; EPMA monazite date) in the high-grade gneisses characterized by high P/ T prograde metamorphism ( P > 10 kbar) was followed sequentially by post-loading prograde heating ( T max ∼650 °C) and exhumation – cooling at amphibolite facies (M 2S) and greenschist facies (M 3S: ∼1.3 Ga; EPMA monazite date) conditions. In the northern part of the NSMB, mica schists fringing the CGC document a northward increase in strain (sequential cleavage formation) and temperature (greenschist to amphibolite facies). The strain–temperature spatial gradient is correlated with crustal shortening and heating ( T ∼620 °C, P ∼7 kbar) synchronously with reverse-sense transport of schists along southerly-dipping thrust planes. The structural framework and thermal structure across the mobile belt is correlated with (a) collisional thickening of the orogen (>1.5 Ga), followed by convective removal of the lithospheric root resulting in post-burial heating and (b) syn-collision and exhumation of the orogen (∼1.3 Ga). The exhumation was caused by outward-directed extrusion of the crustal slices along imbricate foreland-vergent thrusts dipping towards the centre of the orogen.

Late-orogenic Sveconorwegian massif anorthosite in the Jotun Nappe Complex, SW Norway, and causes of repeated AMCG magmatism along the Baltoscandian margin

The age and tectonometamorphic history of massif anorthosite in the Jotun Nappe Complex, SW Norway, were investigated by zircon and titanite U–Pb ID-TIMS. The anorthosite contains sparse zircons showing complex U–Pb systematics reflecting events dated at 965 ± 4 and 913 ± 2 Ma, and a pronounced Caledonian metamorphic overprint. The oldest age is interpreted as the protolith age of the massif anorthosite. We propose that the Jotun anorthosite is related to 970–960 Ma magmatism in the Western Gneiss Region and coeval, orogen-perpendicular extension. Conversely, a 930 Ma high-grade metamorphic event in the Jotun Nappe Complex and the related Lindas Nappe is likely related to formation of the autochthonous ca. 930 Ma Rogaland anorthosite complex. We suggest that the two late- to post-orogenic AMCG events reflect two instances of lithospheric foundering below the orogen separated by ca. 20–30 my. The 913 ± 2 Ma metamorphic episode appears to date a heating event restricted to the outermost edge of the Western Gneiss Region. Leucosome formation in high-grade gneisses geographically close to the Jotun anorthosite is dated at 892 ± 4 Ma and suggested to reflect CO2-rich (?) fluid flux along shear zones.

Evolution of polycyclic basement complexes in the Araçuaí Orogen, based on U–Pb SHRIMP data: Implications for Brazil–Africa links in Paleoproterozoic time

Paleoproterozoic basement of the Araçuaí Orogen is composed of amphibolite and granulite facies orthogneiss units known as Mantiqueira and Juiz de Fora complexes. Six U–Pb SHRIMP crystallization ages for the Mantiqueira Complex range from 2137 ± 19 to 2041 ± 7 Ma. Studied samples are characterized by the abundance of inherited Archean zircon grains, T DM model ages ranging from 2.9 to 3.2 Ga, and strongly negative ɛ Nd(t) (−9 to −13). Gneiss protholits were mainly generated by partial melting of older continental material, and the Mantiqueira Complex is related to active margin and syn-collisional magmatism. High-grade gneiss samples of the Juiz de Fora Complex yields crystallization ages of 2119 ± 16 and 2084 ± 13, and inherited zircon grains are absent. This plutonic unit much probably evolved within an oceanic magmatic arc setting, or on a very stretched continental crust. The Mantiqueira and Juiz de Fora complexes, together with the Kimezian basement of the West Congo Belt, were parts of a Paleoproterozoic orogenic system disrupted and deeply reworked during the evolution of the Araçuaí–West Congo Orogen. Neoproterozoic metamorphic overprint is dated at ca. 590–574 Ma.

Geotectonic framework of the Blueschist Unit on Anglesey–Lleyn, UK, and its role in the development of a Neoproterozoic accretionary orogen

A 560–550 Ma, 5 km × 25 km Blueschist Unit extends NE–SW on the island of Anglesey, Wales, UK, and continues to the southwest for more than 70 km along the northern coast of the Lleyn Peninsula. It has the shape of a shallow-dipping slab or sheet up to a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest. Today the top boundary of the slab is commonly a thrust that was originally a low-angle normal fault at its top in the subduction zone; above it is an accretionary complex and a unit of high-grade gneisses. The bottom boundary is today a thrust (that was originally a thrust), below which is a unit of low-grade or unmetamorphosed rocks belonging to an olistostrome-type accretionary complex. This thrust was originally at the bottom of the slab when it was exhumed in the subduction zone. The sandwiched assemblage was created by the tectonic insertion of the blueschist between the other units. Folding, particularly during late doming of the tectonic sandwich, rotated the top normal-fault boundary into a thrust. Later, high-angle, NE–SW-trending normal faults displaced the tectonic stratigraphy in horst and graben. A 200 km long and 200 km wide, 680–450 Ma calc-alkaline volcano-plutonic belt extends southeastwards from the blueschist belt via northern Wales to central England, suggesting that the subduction of oceanic lithosphere was to the southeast, and this is consistent with the extrusion of the thin high-pressure slab into the accretionary complex to the northwest. This took place on the western accretionary margin of Avalonia. Finally, we outline the key steps involved in unravelling the structure of a Pacific-type accretionary complex.

Field Relationships and Geochemical Constraints on the Emplacement of the Jinchuan Intrusion and its Ni-Cu-PGE Sulfide Deposit, Gansu, China

Field mapping and petrological-geochemical investigation of the Jinchuan intrusion in north-central China clarifies how the intrusion was emplaced and provides a new model that explains how its large and rich Ni-Cu-platinoid deposits may have formed. The intrusion was emplaced into high-grade gneisses and marbles along a disconformity at the base of an overlying cover sequence, indicating that it was emplaced as a sill, not a near-vertical dike, as previously proposed. After emplacement the intrusion was rotated to its present orientation and deformed and metamorphosed under greenschist-facies conditions. Relative enrichment of incompatible trace elements coupled with negative U-Th and Nb-Ta anomalies in all samples from the intrusion provide evidence that the parental magma assimilated granitoid rocks in the lower crust. The presence of abundant marble xenoliths, now decarbonatized to diopside-rich skarns, and chemical indices such as high CaO/SiO2, indicate that the magma assimilated carbonate on reaching its present site. This contamination may be linked to the formation of the Ni-Cu platinoid ores. We propose that the assimilation of carbonate-rich fluids increased the oxygen fugacity of the magma and led to the segregation of metal-rich sulfides.

A major high-strain zone in the Lewisian Complex in the Loch Torridon area, NW Scotland: insights into deep crustal deformation

Abstract The Lewisian Complex is an Archaean-Proterozoic high-grade gneiss region with widespread amphibolite-facies fabrics. These fabrics have, on the 100 km scale, a subhorizontal enveloping surface and are interpreted as forming in gently dipping crustal-scale shear zones and steeper lateral ramp structures. The Scourie dyke swarm, of Proterozoic age, runs NW-SE subvertically outside such zones, but is transformed into subhorizontal concordant amphibolite sheets within them. The terms ‘Scourian’ and ‘Laxfordian’ are used to describe pre-dyke and post-dyke fabrics and events. In the southern mainland Lewisian at Loch Torridon, unmodified Scourian gneisses in the north pass southwards into lozenges bounded by anastomosing zones of Laxfordian deformation, and eventually to a region consisting entirely of Laxfordian fabrics. This geometry is compatible with a major Laxfordian shear zone that dips northeastwards beneath a Scourian hanging wall. Detailed maps show that structures on the metre to kilometre scale do not relate in a simple way to the crustal shear zone model for four reasons. First, many amphibolite-facies fabrics in the quartzo-feldspathic gneisses predate the dykes (they relate to a late Scourian or ‘Inverian’ event), so the dykes cannot be used to infer overall geometry and movement sense. A few structures along the Loch Roag Line, the most northeasterly high-strain belt in the Loch Torridon inliers, may indicate syntectonic dyke emplacement. Second, local shear with opposing senses is present in both the pre-dyke and post-dyke (Laxfordian) deformation. Third, foliation deflections on a variety of scales relate more to varying shear planes than to the deflection caused by increasing simple shear strain, and cannot be used to infer shear sense. Fourth, there is pure-shear stretching within the gneisses that masks the effects of simple shear. These structures can be incorporated into a crustal-scale shear zone model, but are not diagnostic of it. The scatter of foliation planes with a relatively constant lineation is a feature of other crustal-scale high-strain zones (e.g. Canisp, Nordre Strømfjord), although different explanations are available. Other high-strain zones (e.g. the Laxford Front) show a spatially varying lineation pattern, but this could relate more to a varying shear direction than to deflections of linear features into a single movement direction. The geometry of classic simple shear is not versatile enough to explain the kilometre-scale geometry of such high-strain zone, although it may work well in local subareas.

Archaean and Palaeoproterozoic gneisses reworked during a Neoproterozoic (Pan-African) high-grade event in the Mozambique belt of East Africa: Structural relationships and zircon ages from the Kidatu area, central Tanzania

This study presents new zircon ages and Sm–Nd whole-rock isotopic compositions for high-grade gneisses from the Udzungwa Mountain area in the central part of the Mozambique belt, Tanzania. The study area comprises a succession of layered granulite-facies para- and orthogneisses, mostly retrograded to amphibolite-facies. The original intrusive contacts became obscured or severely modified during non-coaxial ductile deformation, and extensive shearing occurred during retrogression. Structures reflecting the early deformational history were mostly obscured when the rocks were transported into the lower crust as documented by severe flattening. Only the fragmented gneisses in the eastern part of the area testify to a brittle regime. Structures in narrow low strain zones that predate the currently observed layering are preserved in rootless isoclinal folds and boudins. Magmatic and detrital zircons from tonalitic to felsic orthogneisses and a metapelite sample were dated using the U–Pb and Pb–Pb evaporation methods and SHRIMP II. Cathodoluminiscence images reveal ubiquitous xenocrystic cores, rimmed by clear, unzoned overgrowth due to high-grade metamorphism. Discordant U–Pb data therefore reflect core-rim relationships, and it was not always possible to obtain precise crystallisation ages. The analyses reveal Neoarchaean, Palaeoproterozoic and Neoproterozoic protolith ages. Nd isotopic systematics yielded strongly negative ε Nd( t) -values and Neoarchaean to Palaeoproterozoic model ages, even for gneisses emplaced in the Neoproterozoic. The trace element distribution suggests upper crustal derivation of the gneisses. Therefore, our study provides evidence that recycling of older crust played a major role during the evolution of the Kidatu area. Neoarchaean rocks are interpreted to represent fragments of the Tanzania craton. Our results, together with those of earlier workers, lead to the conclusion that the central part of the Mozambique belt mainly consists of ancient crustal remnants that were reworked during the Neoproterozoic Pan-African orogeny.

The geodynamic evolution of the Southern European Variscides: constraints from the U/Pb geochronology and geochemistry of the lower Palaeozoic magmatic-sedimentary sequences of Sardinia (Italy)

The high-grade metamorphic complex of northern Sardinia consists of a strongly deformed sequence of migmatitic ortho- and paragneisses interlayered with minor amphibolites preserving relic eclogite parageneses. The protolith ages and geochemical characteristics of selected gneiss samples were determined, providing new constraints for reconstructing the Palaeozoic geodynamic evolution of this sector of the Variscan chain. The orthogneisses are metaluminous to peraluminous calcalkaline granitoids with crustal Sr and Nd isotopic signatures. One orthogneiss from the high-grade zone and one metavolcanite from the volcanic belt in southern Sardinia were dated by LAM-ICPMS (and SHRIMP) zircon geochronology. The inferred emplacement ages of the two samples are 469 ± 3.7 and 464 ± 1 Ma, respectively. The analysed paragneisses are mainly metawackes with subordinate metapelites and rare metamarls. Three paragneiss samples were dated: zircon ages scatter between 3 Ga and about 320 Ma, with a first main cluster from 480 to 450 Ma, and a second one from about 650 to 550. Variscan zircon ages are rare and mostly limited to thin rims and overgrowths on older grains. These data indicate that the high-grade complex principally consists of middle Ordovician orthogneisses associated with a thick metasedimentary sequence characterised by a maximum age of deposition between 480 and 450 Ma. The association of nearly coeval felsic-mafic magmatic rocks with immature siliciclastic sedimentary sequences points to a back-arc setting in the north Gondwana margin during the Early Palaeozoic. The Variscan metamorphic evolution recorded by the high-grade gneisses (Ky-bearing felsic gneisses and mafic eclogites) testifies to the transformation of the Late Ordovician–Devonian passive continental margin into an active margin in the Devonian–Early Carboniferous.

First evidence of eclogite facies metamorphism in the Shackleton Range, Antarctica: Trace of a suture between East and West Gondwana?

The Shackleton Range in East Antarctica is part of the Pan-African collisional orogen related to the convergence between East and West Gondwana. In the northern Shackleton Range, alpine-type ultramafic rocks occur as lenses in high-grade gneisses. These include peridotite and pyroxenite that contain garnet and/or spinel in addition to olivine, orthopyroxene, and clinopyroxene. Thermobarometric results are in the range of 20–23 kbar and 710–810 °C, corresponding to a peak metamorphic gradient of ∼11 °C/km, typical of subduction-zone metamorphism. This finding is the first evidence of eclogite facies metamorphism in the Shackleton Range and, to our knowledge, the first documented eclogite facies ultramafic rocks in a Pan-African orogen. The ultramafic rocks are clear evidence of a suture zone within the northern Shackleton Range, which probably marks the convergence site between East and West Gondwana.

High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province

High-precision U-Pb ages have been obtained for high-grade gneisses, late-kinematic to postkinematic granitic plutons, and a crosscutting mafic dike of the Archean Minnesota River Valley tectonic subprovince, at the southern ramparts of the Superior craton of North America. The antiquity of the Minnesota River Valley terranes is confirmed by a high-precision U-Pb zircon age of 3422 ± 2 Ma for a tonalitic phase of the Morton Gneiss. Voluminous, late-kinematic monzogranites of the Benson (Ortonville granite) and Morton (Sacred Heart granite) blocks yield identical crystallization ages of 2603 ± 1 Ma, illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the Minnesota River Valley terranes. Postkinematic, 2591 ± 2 Ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the Minnesota River Valley subprovince. Monazite growth from 2609 to 2595 Ma in granulitic paragneisses of the Benson and Montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism. Neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the Mesoarchean continental Minnesota River Valley terranes to the southern margin of the Superior Province, along the western Great Lakes tectonic zone. Subsequent Paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion, expressed in the Minnesota River Valley by major WNW-trending tholeiitic diabase dikes dated at 2067 ± 1 Ma, only slightly younger than the structurally and geochemically similar 2077 ± 4 Ma Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada.

Metamorphic evolution of the Maud Belt: P– T– t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P– T– t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M 1) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P– T– t path for the Pan-African orogeny. Peak metamorphic (M 2b) conditions recorded by most rocks in the area ( T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M 2a). The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M 1; 1090–1030 Ma) and again during the Pan-African orogeny (M 2a, M 2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M 2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.

Buckle folding and deep-crustal shearing of high-grade gneisses at the junction of two major high-strain zones, Central Gneiss Belt, Grenville Province, Ontario

Low-plunging, transport-parallel F3 folds are common at all scales in the Central Gneiss Belt of the Grenville Province, but few of these folds are sheath folds. Where the D1–D2 Parry Sound shear zone intersects the D3 Shawanaga shear zone (SSZ) at a high angle, F3 folds formed at several scales (centimetre to greater than outcrop scale) in layered D1–D2 "straight" gneisses. At the start of their evolution, the F3 folds formed just beyond the SSZ with hinges near orthogonal to the D3 shear direction and with typical buckle features, e.g., wavelengths vary with layer thickness, and hinges are discontinuous and bifurcate. The buckle folds evolved within the SSZ by rotation of hinges towards the shear direction. Even though hinges initiated at a high angle to the shear direction, sheath folds were not produced. In addition to tightening the buckles, the ductile reorientation produced thin–thick (extended–shortened) limb pairs and very straight, ridge-like fold hinges and removed small folds from the extended limbs of larger folds. Such features may serve as criteria to distinguish transport-parallel folds that initiated in layering at high angles to the shear direction from those formed in layers containing the shear direction. A general shear parallel to the SSZ can reproduce several features inferred to mark stages in the progressive reorientation of the folds; the pure shear component of the general shear is inferred to have had a positive stretch direction down the dip of the shear zone, at a high angle to the transport (simple shear) direction. The interplay of buckling and shearing in the study area is, plausibly, the expression of deformation at the upper boundary of a channel-like flow that succeeded initial crustal thickening.

Chronological constraints on the pre-orogenic history, burial and exhumation of deep-seated rocks along the eastern margin of the Variscan Orogen, Bohemian Massif, Czech Republic

Key lithological units of the high-grade eastern margin of the Bohemian Massif were dated using the U-Pb and Pb-Pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the Variscan orogenic root. The protolith ages for metagranitoids, orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from Cambro-Ordovician to early Devonian times, probably related to discontinuous intracontinental rifting of Neoproterozoic crust. Our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted Neoproterozoic crust, preserved farther to the east as the Brunia microcontinent. Zircon ages for felsic granulites, high-grade gneisses of the lower crust and of a syn-convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 Ma. Exhumation of the lower crust to mid-crustal levels was a localized process that occurred at ∼340 Ma and was associated with crustal-scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin. Both types of exhumation mechanisms were driven by deep-level wedging (indentation) of the easterly Brunia continent, followed by deposition of heavy minerals and pebbles derived from high-pressure rocks in the adjacent foreland basin. Final orogenic development was characterized by NE-SW dextral transpressive shearing parallel to the Brunia margin as well as dextral trans tension associated with activity along the Elbe lithospheric fault. These processes affected the marginal parts of. the orogenic root and were accompanied by 330 to 325 Ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries. Rotation of the assembled orogenic belt, accompanied by lithospheric faulting driven by westerly subduction roll-back, may be the most plausible model to explain late deformation of the orogenic root.

Pan-African Tectonism in the Western Maud Belt: P–T–t Path for High-grade Gneisses in the H.U. Sverdrupfjella, East Antarctica

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic–Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U–Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar–Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P–T–t evolution than previously assumed. A c. 540Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of PanAfrican age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet–omphacite assemblage within strainprotected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T ¼ 687–758 � C, P ¼ 9� 4–11� 3kbar) were attained subsequent to decompression from P >12� 9kbar. By analogy with limited U–Pb single zircon age data and on circumstantial textural grounds, this earlier eclogitefacies metamorphism is ascribed to subduction and accretion around 565Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of postorogenic granite at c. 480Ma. The recognition of extensive PanAfrican tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140Ma volcanic arc and 1072 � 10Ma granite.

Rutile geochemistry and its potential use in quantitative provenance studies

Rutile is among the most stable detrital minerals in sedimentary systems. Information contained in rutile is therefore of prime importance, especially in the study of mature sediments, where most diagnostic minerals are no longer stable. In contrast to zircon, rutile provides information about the last metamorphic cycle as rutile is not stable at greenschist facies conditions. Several known geochemical characteristics of rutile can be used to retrace provenance. The lithology of source rocks can be determined using Nb and Cr contents in rutile, because the most important source rocks for rutile, metapelites and metabasites, imprint a distinct Nb and Cr signature in rutiles. Since Zr in rutile, coexisting with zircon and quartz, is extremely temperature dependent, this relationship can be used as a geothermometer. Metapelites always contain zircon and quartz, thus the Nb and Cr signatures of metapelites indicate rutiles that can be used for thermometry. The result is effectively a single-mineral geothermometer, which is to our knowledge the first of its kind in provenance studies. Several other trace elements are variably enriched in rutile, but the processes creating these variations are so far not understood. In a case study, Al, Si, V, Cr, Fe, Zr, Nb and W contents in rutiles were obtained by electron microprobe from three sediment samples from Upstate New York. A Pleistocene glacial sand, whose source was granulite-facies rocks of the southern Adirondacks, has detrital rutile geochemical signatures which are consistent with the local Geology; a predominantly metapelitic source with a minor metabasitic contribution. Calculated temperatures for the metapelitic rutiles from the glacial sand are consistent with a predominantly granulite-facies source. The two other samples are from Paleozoic clastic wedges deposited in the foreland of the Taconian and Acadian orogenies. Here several geochemical patterns of detrital rutiles are comparable to rutiles derived from the Adirondacks, implying that rutiles eroded from the Taconian and Acadian orogens were originally derived from similar high grade gneiss terranes, like those found in the Adirondacks. The preferred tectonic scenario calls for an accretionary wedge where eroded Grenville province sediments accumulated, which were later recycled during the Taconian and Acadian orogenies.

Zircon typology in high-grade gneiss terrains – a case study from central Dronning Maud Land (Antarctica)

Zircon typology in high-grade gneiss terrains – a case study from central Dronning Maud Land (Antarctica)

Geochemistry of metavolcanics from the Neoproterozoic Tuludimtu orogenic belt, western Ethiopia

The 200 km wide Tuludimtu Belt of Western Ethiopia is one of a series of N-S trending Neoproterozoic orogenic belts of the East African Orogenic Province in southeastern Sudan and western and southern Ethiopia. The Tuludimtu Belt consists of deformed greenschist facies metasediments and metavolcanics, flanked to the east and west by gneissic terranes, all of which are invaded by pre-, syn- and post-tectonic intrusives ranging from ultramafic to felsic in composition. The Tuludimtu Belt has been subdivided into five lithotectonic domains, from east to west, the Didesa, Kemashi, Dengi, Sirkole and Daka Domains. On the basis of lithological associations, the three domains in the core of the belt, the Kemashi, Dengi and Sirkole Domains, are respectively interpreted to represent an ophiolitic terrane of oceanic crustal origin, a volcanic arc, and a fold-thrust terrane composed of interleaved thrust sheets of gneissic basement and cover strata. The Didesa and Daka Domains are composed of moderate- to high-grade gneisses, possibly representing the basement forelands to the belt. Major and trace element geochemistry for metavolcanics of predominantly basaltic and andesitic composition from the Kemashi, Dengi and Didesa Domains, are presented, and a preliminary analysis of the data undertaken. Two suites of metavolcanics are clearly differentiated by the discrimination diagrams. The samples from all three domains are predominantly tholeiitic, subalkaline basalts and andesites. Bivariate plots reveal weak negative correlations of SiO 2 and Al 2O 3 with MgO, and a positive correlation of CaO with MgO, indicating that magmas evolved by fractionation of clinopyroxene, orthopyroxene and olivine from the parent liquids. Trace element spider diagrams show a tight coherence of patterns for the Dengi and Sirkole samples, suggestive that alteration did not accompany metamorphic processes to any great extent, and a slight depletion of HFS elements possibly due to fractionation processes. On the other hand samples from the Kemashi Domain exhibit a wide variation in patterns, with large ranges in concentrations, particularly in the LFS elements, suggestive of significant alteration during metamorphism. Enrichment of Th relative to Nb, and an overall enrichment of LFS elements over HFS elements in the Kemashi samples, are characteristics of ocean floor and back-arc magmas. A Mullen plot shows the Dengi/Sirkole samples plotting predominantly within the calc-alkaline basalt field, whereas the Kemashi samples have a much wider spread, with no interpretation possible. However, a Ti–Zr–Y tectonic discrimination plot shows the Kemashi samples falling into the field of ocean floor basalts, whilst confirming the calc-alkaline basaltic character of the Dengi and Sirkole rocks. Thus the Kemashi Domain volcanics are interpreted to represent oceanic crustal material formed in a spreading environment, whilst the volcanics of the Dengi and Sirkole Domains developed in a convergent setting. Coherence of the Dengi and Sirkole samples suggests that the Sirkole Domain formed by thrusting of the Dengi Volcanic Arc over its foreland leading to imbrication of the Dengi volcanosedimentary sequence with basement gneisses.

Pervasive rehydration of granulites during exhumation ? an example from the Pulur complex, Eastern Pontides, Turkey

Summary High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotitedehydration melting at � 820 � C and 0.7–0.8 GPa, melt segregation and nearisothermal decompression to 0.4–0.5 GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between � 400 and 230 � C and fluid pressures between � 0.3 and 0.1 GPa. Metamorphic peak conditions are dated at 331–327 Ma, while hydrothermal retrogression occurred significantly later at 315– 310 Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low f O2, very low XCO2 (<0.002), variable activities of Ca 2þ ,K þ ,N a þ and H þ and relatively high activities of H2S and CH4. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite þ calcite þ quartz or (iii) K-feldspar þ calcite þ quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO2.

The role of xenoliths and flow segregation in the genesis and evolution of the Paleoproterozoic Itapema Granite, a crustally derived magma of shoshonitic affinity from southern Brazil

The Paleoproterozoic Itapema Granite (IG) is a large granitic body that intrudes orthogneisses and amphibolites, altogether representing part of the basement for Neoproterozoic granitic magmatism in southern Brazil. It is composed of fine- to medium-grained hornblende-biotite monzogranites to granodiorites that host abundant country-rock xenoliths and irregularly distributed mafic aggregates. Interaction between magmatic flow and low flow-velocity zones related to xenolith-rich portions resulted in flow segregation and mineral fractionation as the main mechanisms that led to compositional and textural diversity, expressed also in geochemical trends. The geometry and distribution of flow foliation suggest that the Itapema Granite is a subhorizontal, sheet-like intrusion and that its emplacement was controlled by thrusting under upper-amphibolite facies conditions. Many geochemical characteristics of the Itapema Granite are similar to those of shoshonitic granitoids. However, lower alkali contents, lack of basic or intermediate associated rocks and intimate temporal association with high-grade gneisses and migmatites suggest that it was crustally derived. Its origin is compatible with melting of amphibolitic orthogneisses derived from continental tholeiitic or shoshonitic sequences under high geothermal gradient conditions, where temperatures reached at least 850 °C.