Undeformed Pegmatite Research Articles

Titanite links rare-element (meta-)pegmatite mineralization to Caledonian metamorphism

The metamorphic reworking of mineralized pegmatites during orogenesis remains unclear, making the genesis and the tectonomagmatic significance of pegmatite mineralization obscure. This study demonstrates the multiple utilities of titanite geochemistry in establishing the Caledonian metamorphic evolution of the world’s largest intra-plutonic Nb-Y-F pegmatites in the Paleoproterozoic host rocks in Tysfjord, Northern Norway. A combination of titanite-specific barometry, Zr-in-titanite thermobarometry, and titanite U-Pb geochronology yields peak metamorphism of ∼12 kbar and 730–750 °C at ∼410 Ma for the host rocks and the largest metamorphosed Paleoproterozoic pegmatite (meta-pegmatite) in the region.In relation to published U-Pb ages of 410–400 Ma for other regional meta-pegmatites, interpreted as Caledonian overprinting here, we argue that the Paleoproterozoic pegmatite protoliths have undergone metamorphism analogous to the host rocks, which was caused by allochthonous nappe stacking late in the Caledonian collisional orogeny (∼440–400 Ma). Published formation ages of ∼400–380 Ma for the regional undeformed pegmatites are 10–30 Ma younger than the peak metamorphism, supporting an anatectic link to the Caledonian post-collisional extension. The Caledonian orogeny led to strong shearing of preexisting pegmatite bodies and resetting of trace elements and U-Pb isotopic systems of the constituent minerals via mineral re-crystallization, inheritance of Pb isotopic components, and element diffusion. This cautions against the interpretation of meta-pegmatite-derived ages and geochemical information in tectonomagmatic terranes without detailed textural investigation.

Genesis of the Markam gneiss dome within the Songpan-Ganzi orogenic belt, eastern Tibetan Plateau

The Markam gneiss dome within the Songpan-Ganzi orogenic belt, eastern Tibetan Plateau, is cored by the Keryin-Taiyanghe granitoids and mantled by Upper Triassic, turbiditic, metasedimentary rocks, which have been intruded by numerous pegmatites and veins. The large-scale Markam detachment fault separates the granitoid core from the overlying Upper Triassic metasedimentary rocks. Microstructures and quartz fabrics from the Markam detachment fault show a top-to-the-south shearing sense and point to formation under high temperatures (>650 °C). New zircon UPb geochronological data from the Markam gneiss dome reveal two stages for the granitoid core at ca. 226–212 Ma and ca. 224–218 Ma. In addition, zircon UPb dating of syn-tectonically deformed pegmatites in the Markam detachment fault shows that the detachment fault was initiated at ca. 212–207 Ma. Later-stage, undeformed pegmatites (including spodumene-bearing varieties) that form sill- and dyke-networks have ages of ca. 200–190 Ma. Our data indicate that formation of the Markam gneiss dome was triggered by development of the Markam detachment fault after the emplacement of the granitoid core. And the following diapiric uplift of the granitoids continued the growth of the Markam gneiss dome, resulting in formation of a thermal domal structure, which further uplifted the magma to a more favorable metallogenic environment and controlled the occurrence of the ore veins. Thus, it indicates that the spodumene-bearing pegmatites emplaced under extensional and decompressional conditions during the Songpan-Ganzi exhumation orogeny.

Identification of the Neoarchean Jianping pyroxenite-mélange in the Central Orogenic Belt, North China Craton: A fore-arc accretional assemblage

Mélanges are widespread in accretionary and collisional orogens at convergent plate margins. We document a Neoarchean mélange belt exposed in the Jianping Complex, North China Craton (NCC). The mélange belt contains a structurally chaotic tectonic mixture of paragneiss, metapelites mixed with exotic tectonic blocks of mafic and ultramafic rocks including pyroxenites, metagabbros, metadiabases, metabasalts, and TTG gneisses. The blocks have formation ages of ca. 2.53–2.51 Ga and all units in the mélange have been intruded by undeformed pegmatite dykes (ca. 2.49 Ga). Geochemical studies show that the pyroxenites in the mélange have high contents of MgO (15.84–20.87%), Mg# (68–80), Ni (211–446 ppm) and Cr (963–2491 ppm). They are characterized by slightly negative Eu anomalies, enrichment in light rare earth elements (LREE) and pronounced negative high field strength elements (HFSE) anomalies on chondrite- and primitive mantle- normalized diagrams. We suggest that the pyroxenites are cumulates derived from an arc-related lithospheric mantle source. The pyroxenites in the mélange are suggested to have formed in a subduction-related fore-arc environment whose mantle source was metasomatized by fluids derived from the subducted slab. The recognition of the Neoarchean Jianping pyroxenite-mélange and regional tectonic analysis of the NCC indicate that the mélange belt in the Jianping Complex is one segment of a 2.5 Ga suture zone that crosses the NCC, stitching a forearc segment of an oceanic arc terrane in the Central Orogenic Belt (COB) with the Eastern Block. The Jianping pyroxenite-mélange is therefore correlative with the Zunhua, Zanhuang, and Dengfeng mélanges in more southern segments of the Central Orogenic Belt, defining a Neoarchean suture that extends for approximately 1600 km across the craton. The geochemistry of the arc-derived magmas, and the tectonic association with a subduction-related mélange is similar in petrology, chemistry, structural style, and size to modern-day forearc settings, negating any calls for non-plate tectonic settings for arc-like magmas in the Archean.

Depicting the 3D geometry of ore bodies using implicit lithological modeling: An example from the Horto-Baratinha iron deposit, Guanhães block, MG

Abstract The definition of geological domains is essential for mineral exploration and is traditionally done manually, section by section, in a laborious, time consuming and subjective process, to construct a 3D model. The technique referred to as implicit modeling is an option to model deposits in a fast and practical manner. This article presents the lithological model of the Horto-Baratinha iron deposit, located in the Antonio Dias municipality, central-eastern part of Minas Gerais state, constructed using the Leapfrog Geo® software. The deposit is situated at the Guanhaes tectonic block that is a basement domain of the Neoproterozoic Aracuai Orogenic Belt. The deposit is comprised of itabirite-hosted high-grade iron bodies (>60 wt.% Fe) associated with polydeformed mica schists, amphibole schist with a Statherian maximum deposition age, enclosed by Statherian granitoids of the Borrachudos Suite and crosscut by undeformed pegmatite dikes crystallized during Late Ediacaran-Cambrian. The sequence was affected by progressive multiple deformation phases, associated with the collisional stage of the Brasiliano Orogeny (630-560 Ma), generating a dome and basin mesoscale refolding shape. The model points the proximity between high-grade bodies and pegmatite indicating the possibility that the intrusions contribute to the iron mineralization.

SHRIMP zircon U-Pb ages and tectonic implications of igneous events in the Ereendavaa metamorphic terrane in NE Mongolia

The Ereendavaa metamorphic terrane in NE Mongolia has long been considered as a Pre-Altaid block or a Precambrian cratonic terrane with a Paleoproterozoic basement overlain by Neoproterozoic-Cambrian rocks, but the idea has not been supported by any isotopic dating. Sensitive high resolution ion microprobe (SHRIMP) zircon U-Pb dating on gneisses, amphibolite and schists (mylonites) of the Ereendavaa terrane suggests that the terrane mainly formed during Early Paleozoic (495–464Ma) and Late Paleozoic-Early Mesozoic (295–172Ma). A minor amount of Precambrian rocks might have been involved in the formation of the protoliths of these rocks, as shown by Precambrian inherited zircons (1796–794Ma). The new age data also suggest that the Ereendavaa terrane experienced at least two periods of magmatism: (1) Early Paleozoic (495–464Ma) and (2) Late Paleozoic-Early Mesozoic (295–172Ma), which are probably produced by the subduction of the Paleo Asian Ocean in the south and the subduction of the Mongol-Okhotsk Ocean in the north, respectively. The mylonitized granite (172Ma) and undeformed pegmatite (163Ma) are interpreted to be syn- and post-kinematic products. The new age data constrain the closure of the Mongol-Okhotsk Ocean at mid-Jurassic.

Petrogenesis and geochemistry of circa 2.5 Ga granitoids in the Zanhuang Massif: Implications for magmatic source and Neoarchean metamorphism of the North China Craton

The tectonic framework of the North China Craton (NCC) during late Archean to early Paleoproterozoic (circa 2.5Ga) is still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. Circa 2.5Ga magmatic and metamorphic activities are widely spread throughout the NCC, which can be used as an efficient target to better understand the tectonic evolution at this period. In this study, based on a detailed field, structural, geochemical, geochronological and Sm–Nd isotopic study, we focus our work on the Haozhuang granitoids in the Zanhuang Massif located at the eastern margin of the Central Orogenic Belt of the NCC. The granitoids mainly include undeformed pegmatite and granodiorite. One pegmatite and two granodiorite samples yield zircon 207Pb/206Pb ages of 2513±29Ma, 2511±36Ma and 2528±18Ma, respectively. The granodiorites show metaluminous and shoshonitic to high-K calc-alkaline series characteristics with A-type granite affinity. The circa 2.5Ga granodiorites have highly negative εNd(t) values (−29.22~−33.12) and TDM model ages between 2671Ma and 3151Ma. This work shows clearly, from whole-rock major and trace elements and Sm–Nd isotopic studies, that the Haozhuang granodiorites were derived from partial melting of old and thickened TTG crust rather than mantle sources, and formed in a subduction-related tectonic setting. With geochemical comparison studies to other similar-aged granitic rocks in the Zanhuang Massif, we suggest that these granitic rocks possibly have a certain correlation during the magma evolution. Coupled with our previous geochemical and isotopic studies on circa 2.5Ga mafic dike swarms, we propose that the similar-aged granitic rocks and mafic dike swarms were produced by an east-dipping subduction polarity reversal event following an arc–continent collision between the Fuping/Wutai island arc and Eastern Block of the NCC above a west-dipping slab. The east-dipping subduction resulted in partial melting of the enriched lithospheric mantle, firstly forming the circa 2.5Ga mafic dikes inducing the widespread circa 2.5Ga metamorphism in the Central Orogenic Belt and the Eastern Block, and then the parental magma of these mafic dike swarms heated the old and thickened TTG crust causing the formation of similar-aged granitic rocks at a later stage.

Evidence for a far-traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 + 7/ − 10 Ma and ~570 Ma, with metamorphic zircon growth at 23–29 Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475 + 7/ − 3 and ~660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile “thrust sheets,” present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.

Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites, South Marginal Zone, Limpopo Belt, South Africa

AbstractThe metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta‐ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P–T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P–T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 ± 13 Ma. Peak metamorphic conditions of 852.5 ± 7.5 °C and 11.1 ± 1.3 kbar were attained at 2713 ± 8 Ma. The clockwise P–T path is characterized by heating in the sillimanite field along a P–T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near‐isothermal decompression at peak temperature and near‐isobaric cooling at ~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post‐date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of ~0.17 cm yr−1. Peak metamorphism was followed by an isothermal decompression to 787.5 ± 32.5 °C and 6.7 ± 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640 °C prior to 2680 ± 6 Ma. This age for the end of the high‐grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 ± 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P–T path, the burial of the rocks to in excess of 30 km depth and the post‐peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and that the SMZ was metamorphosed as a consequence of continental collision along the northern margin of the Kaapvaal craton at c. 2700 Ma.

Structural, petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region, Norway

AbstractEvidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P(ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in theWGRdefines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. TheREEpatterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in theWGRbefore or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H2O‐rich fluid phase at great depth to facilitate H2O‐present partial melting.

TERTIARY PEGMATITE DIKES OF THE CENTRAL ALPS

The largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E–W direction and 15 km in a N–S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the beryl-columbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N–S to NNE–SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed pegmatites from this area may contain miarolitic pockets. There is no systematic difference in the mineral assemblage between the two structural types of pegmatites. However, the chemistry of pegmatite minerals, especially of garnet, in addition to field data suggests that the dikes of the Codera and Bodengo areas represent two distinct generations of pegmatites. Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time span between 29 and 25 Ma (and possibly as young as 20 Ma), with the youngest dikes postdating the ductile deformations of the Alpine nappes. The present work presents a first comprehensive field description and geochemical – mineralogical characterization of the Alpine pegmatite field of the Central Alps.

Paleoproterozoic granulites from the Xinghe graphite mine, North China Craton: Geology, zircon U–Pb geochronology and implications for the timing of deformation, mineralization and metamorphism

The Xinghe graphite deposit is located in the north-central part of the North China Craton (NCC) where Archean gray gneiss, metasedimentary rocks (khondalites) and mafic (high-pressure) granulites constitute the dominant rock types. In this study, we report results of detailed geological and structural mapping around the Xinghe graphite mine. We identify three stages of deformation (D1–D3) and the graphite mineralization is correlated with D1 or between D1 and D2. The first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity (S2) of D2, was constrained at ca. 1.95–1.90Ga by U–Pb age of zircon grains from a mafic high-pressure granulite. The second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing, when garnet broke down to plagioclase+pyroxene. This stage is dated as ca 1.85Ga, from U–Pb analysis of zircons in leucosome from a migmatitic gneiss. The third stage of deformation is represented by large scale folding, which probably formed between 1.85 and 1.80Ga, as constrained from the ca. 1.80Ga emplacement age of an undeformed pegmatite that cuts across the host metasediments. Prior to the high-pressure granulite metamorphism, voluminous ca. 2.15–2.0Ga granites were emplaced within the khondalites. The graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites. The protolith of the mafic high-pressure granulite dike that intruded the khondalite at ca. 2.0Ga was also metamorphosed at 1.95–1.90Ga, and is part of the exploited graphite ore body. We therefore constrain the timing of graphite mineralization to be between ca. 1.95 and 1.85Ga. Our geological and geochronological data indicate that all the rock units in the Xinghe graphite mine were subjected to high-pressure granulite facies metamorphism at ca. 1.95–1.90Ga.

Geology of the Monapo Klippe, NE Mozambique and its significance for assembly of central Gondwana

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35×40km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637±5Ma and metamorphism of the Metachéria Complex at 634±8Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Constraints on the timing of deformation, magmatism and metamorphism in the Dalradian of NE Scotland

Synopsis The Portsoy Gabbro was emplaced into Argyll Group Dalradian metasediments as part of a suite of mafic intrusives that outcrop around the low-P/high-T Buchan metamorphic domain in NE Scotland. This paper presents new field data together with geobarometry and calculated pseudosections for pelites and metabasites, and geochronological data from the Portsoy Shear Zone, on the western boundary of the Buchan domain, which constrain the relative and absolute timing of deformation, metamorphism and magmatism. Textural and cross-cutting relationships indicate the D2 event, magmatism and Buchan metamorphism overlapped. Pelitic schists containing all three Al 2 SiO 5 polymorphs occur structurally beneath the Portsoy Gabbro. The sequence of aluminosilicate growth in these rocks is andalusite–sillimanite–kyanite, implying an anti-clockwise P–T–t path. This study indicates that the andalusite to sillimanite transformation took place during D2, synchronous with the intrusion of the Portsoy Gabbro, and was followed by kyanite growth after D2. The late growth of kyanite indicates that, at least initially, cooling was not associated with uplift. Magmatic zircons from the Portsoy Gabbro yield a U–Pb age of 471.3 ± 0.6 Ma. A U–Pb zircon age from an undeformed pegmatite that cuts the S2 foliation in the gabbro indicates D2 shearing had largely terminated by ~ 471 Ma. We demonstrate, using a metabasite pseudosection, that the titanite formed during deformation as the gabbros cooled. The cooling rate for the gabbro from 1000°C to 620°C was rapid – at least 49°C Ma −1 – and, allowing that the Portsoy Shear Zone was not an uplift structure, was caused by a change in the thermal regime.

Magmatic and metamorphic development of an early to mid-Paleozoic continental margin arc in the southernmost Central Asian Orogenic Belt, Inner Mongolia, China

The Bainaimiao arc in Inner Mongolia, China, comprises a weakly metamorphosed volcani-sedimentary sequence and a low-P/T metamorphic complex. In this study we present SHRIMP zircon ages and geochemical data to document the temporal and genetic relationships between these two tectonic units. Zircons from a rhyolite and two dacites (high-K calc-alkaline) of the volcani-sedimentary sequence yielded 206Pb/238U ages of 474±7Ma, 453±7Ma and 436±9Ma respectively, that we interpret as recording the timing of three volcanic arc episodes. Rocks from the low-P/T metamorphic complex yielded zircon ages of 462±11Ma for a sillimanite gneiss and 437±5Ma for a plagioclase–hornblende gneiss, reflecting two distinct anatectic events. Also from the low-P/T complex, the protolith age of a metadiorite is 438±2Ma. A diorite from a weakly deformed diorite–granodiorite pluton in the low-P/T complex has a zircon age of 419±10Ma that correlates in time with regional collisional magmatism. An undeformed pegmatite dike cutting the low-P/T metamorphic complex has an age of 411±8Ma, which postdates collision and provides an upper limit for the termination of orogeny. Accordingly, we conclude that the volcani-sedimentary sequence and the low-P/T metamorphic complex evolved coherently in the early to mid-Paleozoic and formed an integral continental margin arc in the southernmost Central Asian Orogenic Belt.

Episodic mantle melting-crustal reworking in the late Neoarchean of the northwestern North China Craton: Zircon ages of magmatic and metamorphic rocks from the Yinshan Block

The Yinshan Block in the northwestern North China Craton comprises late Neoarchean greenstones, high-grade metamorphic rocks (granulite, charnockite, enderbite and amphibole gneiss), diorites (sanukitoid and Mg-adakite) and granitoids (predominantly trondhjemite-tonalite). A greenstone terrane comprises a volcanic sequence of predominant basalt and dacite, with minor andesite. Zircons from a high-Mg andesite and an andesite have weighted mean 207Pb/206Pb ages of 2533±5Ma and 2510±7Ma, respectively, which reflect the timing of two distinct volcanic episodes. These two either predated or postdated a major volcanic episode that formed a basalt-dacite bimodal suite during 2515±10 (basalt) to 2516±10Ma (dacite). Magmatic zircons from the high-grade metamorphic rocks yielded weighted mean 207Pb/206Pb ages of 2544±5Ma for a mafic granulite, 2521±6Ma for a garnetiferous gneiss, 2535±5Ma for a charnockite, and 2526±5Ma for an amphibole gneiss. These ages reflect the (minimum) emplacement time of rocks that make up the protoliths of the high-grade metamorphic complex. The metamorphic ages of the high-grade complex were determined at 2515±18Ma for mafic granulite and 2503±12Ma for garnetiferous gneiss. Zircons from a high-Mg diorite (i.e. sanukitoid) have a weighted mean age of 2520±6Ma, which is significantly younger than the 2556±14Ma age of a magnesian diorite (Mg-adakite) from the same diorite massif. These ages reflect distinctive magmatic pulses that generated the diorite massif. A tonalite has a zircon age of 2516±7Ma, indistinguishable from the 2520±9Ma age of a trondhjemite from the same trondhjemite/tonalite-dominated batholith. A granitic compositional layer in the 2516±7Ma tonalite has zircon ages of 2515±6Ma (high-U zircon domains) and 2512±22Ma (low-U zircon domains), which reflect an intracrustal melting event. The presence of an older crustal remnant (ca. >2600Ma) prior to the formation of the Yinshan Block is indicated by a 2594±4Ma leucogranite xenolith in the diorite massif and by the widespread presence of ca. 2583–3485Ma (mostly, ca. 2700Ma) zircon xenocrysts in a variety of volcanic and plutonic rocks. An undeformed pegmatite dike in high-Mg andesite has an emplacement age of 2508±17Ma (zircon). An undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502±6Ma, similar to that of the pegmatite dike, and representing the latest Archean magmatic event in the Yinshan Block. These ages of undeformed granitoid dikes delimit the termination of thermo-tectonic development of the Yinshan Block. Based on these new ages and published geological and geochronological data, we conclude that the Yinshan Block is a typical Neoarchean cratonic block, not a segment of the so-called “Paleoproterozoic Inner Mongolia-North Hebei Orogen”. Episodic mantle melting and resultant crustal reworking account for the thermo-tectonic development of the Yinshan Block in a brief period (ca. 2556–2502Ma) in the late Neoarchean, and based on their regional chronological correlations we argue for their craton-scale significance.

Age and kinematics of ductile deformation in the Cerro Durazno area, NW Argentina: Significance for orogenic processes operating at the western margin of Gondwana during Ordovician – Silurian times

The Cerro Durazno Pluton belongs to a suite of Paleozoic granitoid intrusions in NW-Argentina, that are central for understanding the tectonic setting of the western margin of Gondwana in Ordovician and Silurian times. The pluton and its host rocks were tectonically overprinted by metamorphic mineral shape fabrics formed under middle greenschist-facies metamorphic conditions and associated with the nearby Agua Rosada Shear Zone. Kinematic analysis of the shear zone based on the geometric relationship between individual segments of the shear plane and principal axes of mineral fabric ellipsoids indicates reverse-sense of shear with a minor component of left-lateral displacement. This is compatible with the kinematics of other ductile deformation zones in this area, collectively forming a network, which accomplished orogen-parallel extension in addition to vertical thickening. Using the Rb–Sr isotopic system, an undeformed pegmatite dike of the Cerro Durazno Pluton was dated at 455.8 ± 3.6 Ma and mineral fabrics of the Agua Rosada Shear Zone formed at middle greenschist-facies metamorphism gave deformation ages of 437.0 ± 3.8 Ma and ⩽428.4 ± 4.5 Ma. Thus, tectonic overprint at low metamorphic grade occurred about 20–30 Ma after terminal magmatism in the Cerro Durazno area. Our data from the Cerro Durazno area and regional considerations suggest that the western margin of Gondwana was characterized by orogen-parallel extension in addition to crustal thickening as well as episodes of magmatism and ductile deformation that varied greatly in time and space.

The Edison magnetite deposits in the context of pre-, syn-, and post-orogenic metallogenesis in the Grenville Highlands of New Jersey

Most of the magnetite deposits of the eastern Grenville Province have been described as precipitates from hydrothermal solutions derived from metamorphic processes, from early, late, or post-Grenville granites, or as metasediments. Granites host many of these deposits, but others, including the historic Edison iron mines of New Jersey, are hosted by potassium-feldspar gneiss, commonly interpreted as meta-arkose. Our new geochemical data indicate, however, that the protolith of the potassium-feldspar gneiss is rhyolite, not arkose. Supporting evidence includes (i) the absence of an underlying potassic provenance for arkosic sediment, (ii) potassium and sodium contents among host rocks that exceed the range of arkose but are consistent with A-type rhyolite, and (iii) a close chemical resemblance of the potassium-feldspar gneiss to an A-type granite (Byram granite) that is closely associated in time and space. As the ore zone through the Edison mines is approached, the K/Na ratio of the host rocks undergoes a distinct increase, consistent with extensive diagenetic alteration of rhyolitic pyroclastics in a hypersaline environment. This alteration provided a local ligand source for subsequent hydrothermal iron mineralization derived from the nearby pre-orogenic Byram granite. These iron concentrations were then remobilized and recrystallized during subsequent Grenville metamorphism. Although some of the magnetite deposits of the New Jersey Highlands display evidence of post-orogenic replacement and an association with undeformed pegmatites, the banded magnetite ore and related pegmatites of the Edison mines are conformable to the foliation of the host rock and are interpreted as metamorphic products of pre-orogenic, granite-derived, hydrothermal iron concentrates.

160 Ma of magmatic/hydrothermal and metamorphic activity in the Gällivare area: Re–Os dating of molybdenite and U–Pb dating of titanite from the Aitik Cu–Au–Ag deposit, northern Sweden

Host rocks to the Aitik Cu–Au–Ag deposit in northern Sweden are strongly altered and deformed Early Proterozoic mica(-amphibole) schists and gneisses. The deposit is characterised by numerous mineralisation styles, vein and alteration types. Four samples were selected for Re–Os molybdenite dating and 12 samples for U–Pb titanite dating in order to elucidate the magmatic/hydrothermal and metamorphic history following primary ore deposition in the Aitik Cu–Au–Ag deposit. Samples represent dyke, vein and alteration assemblages from the ore zone, hanging wall and footwall to the deposit. Re–Os dating of molybdenite from deformed barite and quartz veins yielded ages of 1,876±10 Ma and 1,848±8 Ma, respectively. A deformed pegmatite dyke yielded a Re–Os age of 1,848±6 Ma, and an undeformed pegmatite dyke an age of 1,728±7 Ma. U–Pb dating of titanite from a diversity of alteration mineral associations defines a range in ages between 1,750 and 1,805 Ma with a peak at ca. 1,780 Ma. The ages obtained, together with previous data, bracket a 160-Ma (1,890–1,730 Ma) time span encompassing several generations of magmatism, prograde to peak metamorphism, and post-peak cooling; events resulting in the redistribution and addition of metals to the deposit. This multi-stage evolution of the Aitik ore body suggests that the deposit was affected by several thermal events that ultimately produced a complex ore body. The Re–Os and U–Pb ages correlate well with published regional Re–Os and U–Pb age clusters, which have been tied to major magmatic, hydrothermal, and metamorphic events. Primary ore deposition at ca. 1,890 Ma in connection with intrusion of Haparanda granitoids was followed by at least four subsequent episodes of metamorphism and magmatism. Early metamorphism at 1,888–1,872 Ma overlapping with Haparanda (1,890–1,880 Ma) and Perthite-monzonite (1,880–1,870 Ma) magmatism clearly affected the Aitik area, as well as late metamorphism and Lina magmatism at 1,810–1,774 Ma and TIB1 magmatism at 1,800 Ma. The 1,848 Ma Re–Os ages obtained from molybdenite in a quartz vein and pegmatite dyke suggests that the 1,850 Ma magmatism recorded in parts of northern Norrbotten also affected the Aitik area.

Constraining Sinistral Shearing in NW Ireland: A Precise U–Pb Zircon Crystallisation Age for the Ox Mountains Granodiorite

The age of the Ox Mountains Granodiorite has proved controversial in the past. Two published Rb‐Sr whole rock isochrons suggest ages of 487 ± 6 Ma and 500 ± 18 Ma, respectively (� 87 Rb = 0.0139 Ga ‐1 ), whereas Rb‐Sr muscovite‐feldspar isochrons from undeformed pegmatites associated with the intrusion have yielded c. 400Ma ages. The syn-kinematic nature of the Ox Mountains Granodiorite has been clearly demonstrated by several workers, with the pluton being emplaced as a series of sheets in a major transpressive sinistral shear zone. Knowledge of the age of crystallisation of the Ox Mountains Granodiorite would thus constrain the timing of strike-slip movement along this shear zone, which forms part of a major Caledonide structure known as the Fair Head ‐ Clew Bay Line. A U‐Pb zircon age (conventional isotope dilution TIMS) of 412.3 ± 0.8 Ma from a tonalitic facies of the pluton is presented here. The age confirms that emplacement was broadly contemporaneous with the majority of other volumetrically important Irish granite

Emplacement of polygeneration pegmatites in relation to Sveco-Norwegian contractional tectonics: examples from southern Norway

Recent studies have demonstrated that there is a strong link between sites of concentrated magmatism and crustal deformation zones. Magmatism is often described as being syn-kinematic where one or more increments of intrusion punctuate deformation with successive generations of injections being progressively deformed. Pegmatite formation in the Mesoproterozoic of south Norway has been considered as post-kinematic in nature relative to Sveconorwegian (Grenvillian) deformation (1.13 Ga to ∼0.85 Ga) during accretion of the SW margin of Baltica. This paper presents structural observations from mapping of some pegmatites in the Bamble Terrane of south Norway demonstrating that the pegmatites are structurally related to Sveconorwegian fold geometries associated with peak metamorphism at approximately 1.14 Ga and are kinematically related to overthrust geometries associated with the initial overthrusting phase of the Porsgrunn-Kristiansand Fault when the Bamble Terrane docked with the underlying Telemark Terrane. Local fold geometries suggest a strain regime where the maximum contractional axis is sub-horizontal and oriented NW-SE consistent with the regional kinematics. The pegmatites display a systematic deformation pattern consistent with deformation in the isoclinal fold limbs. Pegmatites are also deformed into asymmetric antiformal folds above reverse-shear structures and are also cut by these structures thus showing a thrust geometry. The undeformed pegmatites are emplaced in sub-horizontal fractures and suggests sub-horizontal contractional axis related to overthrusting which is identical to the strain regime of the folding. The evidence suggests that the pegmatites are syn-tectonic and were injected into reverse shear-related fractures. Deformation was progressive and incremental with longer periods of ductile deformation at low strain rate punctuated by shorter periods of fracturing and pegmatite injection at high strain rate. This work therefore implies an intimate spatial and temporal relationship between deformation and magmatism during crustal accretion on the western margin of Fennoscandia.