High-grade Regional Metamorphism Research Articles

Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes

Determining the ages and duration of high-grade metamorphic episodes during orogenesis and concomitant melt extraction events is pivotal in reconstructing and quantifying the chemical and petrological evolution of the Earth's continental crust over time. Major and trace elements combined with multichronometric garnet data from the Pan-African Damara belt of Namibia place absolute time constraints on high-grade regional metamorphism and melting in the lower crust that led to the production of widespread granites emplaced in the upper crust. Some of these granites contain inherited peritectic garnet with inferred igneous rims that record their genesis and evolution from formation by partially melting in the deep crust to the emplacement of their host rock at shallower levels. Particular characteristics of these garnets are textural and chemical disequilibrium features such as resorption, local development of partly euhedral crystal faces and occurrence of quartz-feldspar stringers in the rim of the garnet and trace element abundances with sharp concentration gradients. These features document a previous melting event followed by garnet crystallization in an ascending melt. With the exception of the outermost rim, which records late-stage diffusional features, the garnet crystals have constant Mg and Mn concentrations. In contrast, Ca shows pronounced chemical zoning. Trace elements (HREE, LREE, Zr, Sr, Y) are depleted in the core and enriched in the rim region indicating disequilibrium conditions during garnet growth. A pseudosection calculated with the garnet rim composition and the composition of the host granite yields P-T conditions of 0.62 GPa and 760 °C for the formation of the rim. This result is consistent with a P-T estimate of 0.68 GPa and 760 °C derived from the conventional garnet-biotite thermobarometer. Regional metamorphic conditions were above the cordierite-andalusite stability field with estimated conditions of ~550-600 °C and 0.4–0.5 GPa. This P-T regime indicates that the garnet originally crystallized at deep crustal levels. Diffusion modelling suggests that the observed concentration gradient of Sm in garnet is consistent with a maximum duration of ca. 0.5–1.5 Ma for the inferred melting interval during peak metamorphism in the lower crust. High-precision multi-point Lu-Hf garnet-whole rock data yield dates of 522.0 ± 1.0 and 522.3 ± 0.9 Ma for two samples. Because of presumed short intervals between melting in the lower crust and intrusion of the granite, these dates are interpreted as the age of the melting event during high-grade metamorphism and are temporally indistinguishable from the crystallization interval. Thus, dehydration melting in the deeper crust during peak metamorphism produced peritectic garnet that was incorporated into the granitic melt followed by subsequent new growth of igneous garnet around peritectic cores during intrusion. Previously obtained Sm-Nd garnet whole rock ages are 20–25 Ma younger and likely represent cooling ages. This study shows that granites enriched in mafic components such as garnet and biotite may intrude as mixtures of melt and the peritectic assemblage produced by incongruent melting involving dehydration-melting of biotite in the lower crust. Melting episodes represent short-lived processes relative to the absolute duration of orogenic cycles.

Late Neoarchean high-grade regional metamorphism in the eastern North China Craton: New constraints from monazite dating in northern Liaoning

Late Neoarchean tectono-thermal events in the eastern North China Craton have been comprehensively characterized in terms of mineralogy, petrology, geochemistry, metamorphic evolution, geophysics and geochronology. The Late Neoarchean metamorphic event is debated to correlate with vertical and/or horizontal tectonism, potentially an indicator as initiation of plate tectonics. With a growing number of zircon geochronological investigations, the precise timing of the Late Neoarchean metamorphic event should be questioned due to the complexity of post-Archean multiple tectono-thermal overprints. Here we demonstrate the capabilities of LA–ICP–MS U–Pb and LA–MC–ICP–MS Sm–Nd isotope measurements applied to monazite endowed with LREE-enriched geochemical peculiarities as well as zircon U–Pb–Hf isotope, to reevaluate the Late Neoarchean metamorphic event in northern Liaoning, eastern North China Craton. Monazites within the biotite two-feldspar gneiss from the Qingyuan greenstone belt are Ce-monazite in composition, with negligible amounts of the huttonite and brabantite components. The internally homogeneous and anhedral rounded granular monazites show textural equilibration in contact with plagioclase, quartz, and K-feldspar, indicating that monazite is a product of metamorphism. The apparent ages and Eu/Eu* values of monazite analyses show a positive correlation, which confirms the continuous formation of monazite and coeval feldspar during the metamorphic evolution. These metamorphic monazites yield a weighted mean 207Pb/206Pb age of 2493 ± 8 Ma and an upper intercept age of 2496 ± 8 Ma, consistent with the metamorphic zircon dates of 2492–2468 Ma. These results, along with monazite Sm–Nd isochron of 2459 Ma, thus provide a critical clue to understand the Late Neoarchean high-grade regional metamorphism in the eastern North China Craton. As a consequence, the Qingyuan greenstone belt was only partially influenced by the Paleoproterozoic tectono-thermal events, not largely as previously thought by some workers.

Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare Metal Enrichment During High-Grade Regional Metamorphism. III: Fluid Inclusions as Potential Indicators for Anatectic Pegmatite Parent Melt Formation

ABSTRACT Fluid inclusions trapped in quartz, beryl, and cassiterite from a granitic pegmatite of the Austroalpine basement of the Eastern Alps in Northern Italy were analyzed using a cooling/heating stage and micro-Raman spectroscopy with the following aims: (1) to characterize the range in fluid compositions and fluid densities, (2) to calculate minimum pressures of fluid entrapment from density data and temperature estimates for pegmatite crystallization based on pegmatite mineral assemblages, and (3) to assess possible relations between the compositions of the fluids, in particular their N contents, and potential sources of pegmatite parent melts. The pegmatite intruded into Al-rich metapelitic gneisses during the Permian tectonometamorphic event (∼290 Ma) and was overprinted at amphibolite-eclogite-facies P-T conditions during Eoalpine (∼80–90 Ma) metamorphism. It is characterized by a major phase assemblage quartz + muscovite + paragonite + albite with minor garnet + beryl and accessory columbite-group phases + cassiterite + ixiolite/wodginite + chrysoberyl + cordierite + staurolite + apatite + wyllieite-group phases + zircon. Two types of fluids are preserved in the phases investigated: (1) complex CO2±N2±CH4-H2O-NaCl±MgCl2±CaCl2 fluids with highly variable proportions of the constituent species trapped in all three host phases. The compositions of these fluids range from CO2-rich (∼72–92 mol.%) to N2-rich (∼25–66 mol.%) with additional minor (∼3–8 mol.%) CH4 contents; and (2) H2O-rich fluids with variable salinity (∼3–23 mass%) trapped in beryl and quartz. Trapping of CO2-N2-CH4 fluids in cassiterite along interfaces between the cassiterite host and columbite-group phase inclusions indicates that these fluids were present during joint cassiterite-columbite-group phase crystallization when the pegmatite solidified. In the absence of clear textural evidence for a successive trapping of the two types of fluids, a possible explanation for their simultaneous presence is fluid immiscibility. Isochores for fluid inclusions with the highest densities observed in cassiterite and beryl yield trapping pressures of ∼9–10 kbar for 650–700 °C. By comparison, temperatures estimated for pegmatite crystallization based on the assemblage beryl + chrysoberyl + gahnite and the stability of albite + muscovite + quartz lie in the range ∼650–700 °C. As a potential source of N for the N2-rich fluids, biotite and muscovite from the metapelites of the pegmatite host rocks were identified. Infrared analysis yields NH4 contents of 1430–1772 μg/g in biotite and of 297–489 μg/g in muscovite from two representative gneiss samples. This finding provides further evidence for an anatectic origin of most, if not all, Permian pegmatites found throughout the Austroalpine basement of the Eastern Alps.

Field and zircon U–Pb geochronological evidence for the occurrence of Cambrian banded iron formations in the West Kunlun Orogenic Belt, China

Abundant bedded iron deposits have been discovered recently in the Taxkorgan Terrane, West Kunlun Orogenic Belt, China. Magnetite is the dominant iron mineral, which is interbedded with amphibolite-facies metamorphic silicate minerals, quartz, and calcite. Field observations revealed that the iron orebodies are hosted in the Bulunkuole Complex, which consists mainly of submarine volcanic and sedimentary sequences. The iron orebodies are typically in conformable contact with bimodal volcanic rocks. The nature of iron mineralization in this region is unclear, mainly because it appears to be similar to typical banded iron formations (BIFs), but is of inferred Cambrian age. We undertook detailed field investigations of this area and zircon U–Pb dating of meta-rhyolites from three iron deposits (i.e., the Zankan, Taaxi, and Ziluoyi deposits). Meta-rhyolites from the deposits have ages of 540.2 ± 2.4, 540.2 ± 2.4 to 515.2 ± 2.7, and 526.4 ± 2.5 to 508.8 ± 3.6 Ma, respectively, indicating that the iron orebodies can be subdivided into four groups, based on their formation ages (i.e., 540, 526, 515, and 508 Ma). These ages suggest that the iron orebodies are unique Cambrian BIFs, although the gangue minerals in these orebodies have been mainly converted into amphibolite-facies minerals, owing to the regional high-grade metamorphism. The close relationship between iron orebodies and bimodal volcanic rocks indicates the iron deposits are Algoma-type BIFs. Abundant pyrite in the iron orebodies from the Zankan deposit suggests that the basin in which these rocks precipitated was sulfidic and in a relatively shallow shelf environment. The lack of pyrite in ores from the Taaxi and Ziluoyi deposits indicates they precipitated in ferruginous basins with relatively deep seawater. We propose that intense submarine magmatism and local anoxic basins were the key factors that controlled the formation of these unique Cambrian BIFs in the Taxkorgan Terrane.

Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

Abstract Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of ∼5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation–inductively coupled plasma–single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or ductilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370–360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegmatites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370–355 Ma in age. All metapelitic paragneisses contain foliations and lineations that are synkinematic with retrograde garnet + K-feldspar → sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360–330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400–370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375–370 Ma, leading to advection of heat to the lower continental crust and attendant magmatic and metamorphic responses over the time span 370–355 Ma.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen, of which the Solonker suture, as a major regional suture, coincides closely with an early Permian paleobiogeographical boundary. This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane. Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian–Early Triassic, uncertainty remains because of a lack of typical collision-related features (e.g., high-grade regional metamorphism and well-developed fold–thrust structures) and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes (288–275 Ma) and Middle-Late Permian undeformed layered gabbros, strongly peraluminous granites, and I-type granites (265–254 Ma) in the Xinhure area along the northern margin of the North China Craton. The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton. In contrast, the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle–upper crust, respectively, triggered by asthenospheric upwelling. Therefore, a transition from an end-compressional to an extensional environment according to a transition from collision termination to post-collision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma. This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Regional, Contact Metamorphism, and Autometamorphism of the Olkhon Terrane (West Baikal Area)

The rocks of the Olkhon terrane experienced high-grade regional metamorphism reaching granulite facies. In addition to regional metamorphism, they were subjected to contact metamorphism and autometamorphism (mafic dikes). Three types of regional metamorphism have been distinguished. A terrane collage north of the Orso Zone revealed two stages of regional metamorphism: early-stage granulite metamorphism (Т = 750–900°С, Р = 8–9 kbar, around 500 Ma) and late-stage amphibolite and low–amphibolite facies (Т = 550–710°С, Р = 4–6 kbar, 460–470 Ma). The Orso microterrane separating the Krestovsky island-arc subterrane from a collage of other microterranes contains high-pressure mineral assemblages (up to 10 kbar), which are characterized by the presence of high-calcium garnet in metapelites. The Orso microterrane and Krestovsky subterrane are characterized by relatively high-pressure path of metamorphism. An extended zone of contact high-temperature metamorphism related to the emplacement of gabbro of the second phase of the Ustkrestovsky Complex has been mapped in the Krestovsky subterrane. The temperature of formation of two-pyroxene hornfels after metaporphyrites of the Birkhin volcanoplutonic association is estimated at 750–850°С. Beerbachites were formed by autometamorphism of subvolcanic mafic bodies, which compose the northern part of the Tazheran composite massif (syenite, Ne-syenite, subalkaline gabbro), dikes in the Birkhin gabbro massif, as well as dikes and separate blocks in marble melange. The temperature of autometamorphism during formation of beerbachites is estimated from two-pyroxene geothermometer as 700–1000°С.

Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare-Metal Enrichment During High-Grade Regional Metamorphism – I: Mineral Assemblages, Geochemical Characteristics, and Emplacement Ages

Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare-Metal Enrichment During High-Grade Regional Metamorphism – I: Mineral Assemblages, Geochemical Characteristics, and Emplacement Ages

Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare Metal Enrichment During High-Grade Regional Metamorphism. II: Pegmatite Staurolite As an Indicator of Anatectic Pegmatite Parent Melt Formation – A Field and Experimental Study

Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare Metal Enrichment During High-Grade Regional Metamorphism. II: Pegmatite Staurolite As an Indicator of Anatectic Pegmatite Parent Melt Formation – A Field and Experimental Study

The multistage tectonic evolution of the northeastern Carajás Province, Amazonian Craton, Brazil: Revealing complex structural patterns

Structural data collected in the Carajás Province region led to a new interpretation of the southeastern Amazonian Craton geotectonic evolution. The purpose of this article is to present a new evolutionary proposal for the region. A detailed analysis of the several extensional-compressional cycles that overprinted each other from the Archean to the Neoproterozoic-Cambrian is presented. At about 2.87–2.83 Ga, collisional processes led to the formation of a stable crustal substrate that supported the installation of an extensional basin at 2.76–2.70 Ga and the deposition of the Itacaiúnas Supergroup shallow marine volcanosedimentary sequences, together with contemporary bimodal plutonism. Paleoproterozoic arc magmatism in the Bacajá Domain was followed by collision with the Carajás Province between 2.09 and 2.06 Ga, resulting in expressive tectonic thickening and low to high grade regional metamorphism, and in the deposition of the Águas Claras Formation. A second Paleoproterozoic orogenic event affected the Carajás Province, which resulted in oblique tectonism and regional counterclockwise rotation of previous associations, followed by late to post-orogenic sedimentation and 1.88 Ga anorogenic alkaline A-type magmatism. The eastern Carajás Province margin was extensionally reactivated during the Neoproterozoic, in a rifting event, followed by tectonic inversion during the Ediacaran/Cambrian.

Structural evolution of the Paleoproterozoic Trans-North China Orogen: Evidence from the Xiaoqinling region, central China

High-grade metamorphic rocks of Archean to Paleoproterozoic age, collectively called the Taihua Complex, are well exposed in the Xiaoqinling region in the western part of the southern segment of the Trans-North China Orogen (TNCO), central China. However, the evolution of orogenic events in this part of the TNCO remains unclear. Here we present structural and zircon U–Pb data to constrain the structural evolution of the rocks in the Xiaoqinling region. The first phase of deformation (D1) produced penetrative, ductile fabrics and localized ductile shear belts, and it was accompanied by a regional high-grade metamorphism. The D1 structures were the result of WNW-directed thrusting movements that have been dated at 1863–1840 Ma, and the microstructures suggest temperatures of 600–650 °C, which correspond to the conditions of exhumation of these orogenic rocks. These data support the ca. 1.85 Ga collision model for the Western and Eastern blocks that involves the SE-directed subduction of the Western Block. The second phase of deformation (D2) produced localized E–W-striking normal shear belts, and the D2 structures provide reliable evidence for N–S extension during the late Paleoproterozoic. Intense magmatic activity accompanied this period of extension. Zircon U–Pb dating constrains the extension to the period between 1829 and 1703 Ma. Post-orogenic extension in the TNCO accounts for D2 and the associated magmatism, and orogen-parallel extension is indicated by the structures. Thus, the Paleoproterozoic structural evolution of the rocks in the Xiaoqinling region of the TNCO records orogenic processes from exhumation to post-orogenic extension.

Ediacaran (∼620 Ma) high-grade regional metamorphism in the northern Arabian Nubian Shield: U-Th-Pb monazite ages of the Elat schist

Ediacaran times witnessed a hemisphere-scale orogenesis, forming the extensive Pan-African mountain ranges and resulting in the final assembly of the Gondwana supercontinent. The Elat metamorphic association (S Israel), located at the northernmost tip of the Arabian Nubian Shield (ANS), comprises amphibolite-facies pelitic and psammitic schists and orthogneisses and was presumably shaped by Ediacaran East African orogenesis. However, the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are inconclusive and a major emphasis was previously given to pre-Ediacaran island-arc related tectonics. Here U-Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat association.In-situ analysis of metamorphic monazites by LASS-ICPMS (Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry) yields two concordant age clusters at ∼710 and 620Ma, recording two regional metamorphic events. The corresponding REE patterns of the dated monazite grains indicate that garnet porphyroblast growth took place only during the younger event (M2). Likewise, the regional south-dipping penetrative foliation (S2), common to the Elat schist and the surrounding orthogneisses, formed during the Ediacaran event (M2). The peak M2 staurolite-garnet-sillimanite assemblage was overprinted by retrograde andalusite and cordierite during decompression, followed by the intrusion of andesitic dykes that were immediately metamorphosed at greenschist-facies conditions. This metamorphic P-T-d-t path corresponds to the collision of the already accreted arc terranes of the ANS with the Saharan Metacraton, as also suggested by the large geochronological database of 610–580Ma post collisional granitoid batholiths throughout NE Africa and Arabia. The Cryogenian M1 metamorphism therefore records arc-arc collision during major convergence forming the proto-ANS to the south.

Discerning Permian orogenic metamorphism from other tectonothermal events (Mesoproterozoic to Alpine, contact to orogenic or extensional) in the concealed basement of the Basque-Cantabrian Basin (northern Spain)

Recently discovered Albian submarine diatremes sample tectonites from the Basque-Cantabrian Basin concealed basement. Garnet-sillimanite gneiss xenoliths provide petrologic, petrofabric, and radiometric evidence on a so-far unidentified early Permian (ca. 280–275 Ma) high-grade regional metamorphism. It is interpreted as a syntectonic event recorded at middle to lower crustal settings and related to the Variscan orogeny. This metamorphism is 25–30 m.y. younger than the Variscan high-grade regional metamorphism preserved in the axial zone of the Pyrenees and 15–30 m.y. younger than low-pressure plutonometamorphism documented in the core of the Ibero-Armorican arc. By contrast, it is 15–20 m.y. older than Variscan regional and contact metamorphism unraveled in basement outcrops around the Basque-Cantabrian Basin; it can be thoroughly discriminated from Proterozoic high-grade metamorphism preserved in lower crustal segments of the submerged north Iberian continental margin, and from low-pressure metamorphism related to mid-Cretaceous hyperextension predating the Alpine orogeny. The early Permian metamorphism reveals protracted and diachronic Variscan tectonomagmatic activity in the core of the Ibero-Armorican arc due to orocline buckling processes triggering lithosphere delamination, asthenosphere upwelling, and eventually heat and mass transfer through the continental crust.

The Sveconorwegian Pegmatite Province – Thousands of Pegmatites Without Parental Granites

Abstract The Late-Proterozoic Sveconorwegian pegmatite province in southern Norway and southwest Sweden hosts seven rare-element pegmatite districts with more than 5000 rare-element pegmatites. Most of these pegmatites with Niobium-Yttrium-Fluorine (NYF) signature are not related to a parental granite, but instead occur in areas of high-grade metamorphism and are the result of migmatization and local melt collection. There are three groups of pegmatites: (1) rare-element pegmatites related to H P -H T high-grade metamorphism associated with the assembly of the Sveconorwegian orogen; (2) rare-element pegmatites related to post-orogenic extension with L P -H T granulites; and (3) rare-element pegmatites related to granite magmatism during post-orogenic extension. The pegmatite formation principally comprises four periods restricted to certain tectono-metamorphic domains: (I) 1094–1060 Ma (Bamble sector); (II) 1041–1030 Ma (Idefjord terrane); (III) 992–984 Ma (Idefjord terrane, Rogaland-Hardangervidda-Telemark sector); and (IV) 922–901 Ma (Rogaland-Hardangervidda-Telemark and Bamble sectors). The observed relationships between pegmatite formation and regional high-grade metamorphism reveal that the majority of Sveconorwegian pegmatites are formed by anatexis, either by crustal stacking during different stages of continental/terrane collision (H P metamorphism) (periods I to III), or by mafic magma underplating (H T metamorphism) during orogenic extension (period IV). In several provinces that have been affected both by early H P metamorphism during continental collision and by late H T metamorphism during crustal extension, there may occur several generations of pegmatites that show mineralogical and geochemical affinity, even though they formed during several different periods. In addition, the results imply that the majority of Sveconorwegian NYF pegmatites are not necessarily formed in an anorogenic setting in relation to A-type magmatism, but in compressional or extensional orogenic settings unrelated to pluton-scale magmatism. In light of this, the genetic criteria of the pegmatite family classification scheme [NYF versus Lithium-Cesium-Tantalum (LCT)] will have to be re-evaluated.

U–Pb geochronology of the syn-orogenic Knaben molybdenum deposits, Sveconorwegian Orogen, Norway

AbstractPaired isotope dilution – thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS) zircon U–Pb data elucidate geochronological relations in the historically important Knaben molybdenum mining district, Sveconorwegian Orogen, south Norway. This polyphase district providedc. 8.5 Mt of ore with a grade of 0.2%. It consists of mineralized quartz veins, silica-rich gneiss, pegmatites and aplites associated with a heterogeneous, locally sulphide-bearing, amphibolites facies gneiss called Knaben Gneiss, and hosted in a regional-scale monotonous, commonly weakly foliated, granitic gneiss. An augen gneiss at the Knaben I deposit yields a 1257±6 Ma magmatic zircon age, dating the pre-Sveconorwegian protolith of the Knaben Gneiss. Mineralized and non-mineralized granitic gneiss samples at the Knaben II and Kvina deposits contain some 1488–1164 Ma inherited zircon and yield consistent intrusion ages of 1032±4, 1034±6 and 1036±6 Ma. This age links magmatism in the district to the regional 1050–1020 Ma Sirdal I-type granite suite, corresponding to voluminous crustal melting during the Sveconorwegian orogeny. A high-U, low-Th/U zircon rim is present in all samples. It defines several age clusters between 1039±6 and 1009±7 Ma, peaking atc. 1016 Ma and overlapping with a monazite age of 1013±5 Ma. The rim records protracted hydrothermal activity, which started during the main magmatic event and outlasted it. This process was coeval with regional high-grade Sveconorwegian metamorphism. Molybdenum deposition probably started during this event when silica-rich mineralizing fluids or hydrous magmas were released from granite magma batches. An analogy between the Knaben district and shallow, short-lived porphyry Mo deposits is inappropriate.

Generation of magnesian, high-K alkali-calcic granites and granodiorites from amphibolitic continental crust in the Damara orogen, Namibia

The Pan-African Damara orogen of Namibia is characterized by large-scale granitoid intrusions. Two plutons in the Northern Central Zone (NCZ) of the Damara orogen within the Okombahe district have U–Pb zircon ages of 576.2 ± 5.7 Ma and 570.9 ± 4.9 Ma that predate the time of high grade regional metamorphism which occurred between 540 and 480 Ma. The intrusive rocks are magnesian high-K alkali-calcic granodiorites to granites, are enriched in HFSE and REE, and have undergone only a limited degree of fractional crystallization, and do not contain xenoliths of local country rocks. Initial isotope compositions are unevolved with 87Sr/86Sr between 0.704 and 0.706 and initial εNd ranging from − 1.9 to − 3.9. Lead isotopes are radiogenic (206Pb/204Pb: 18.32 to 18.61, 207Pb/204Pb: 15.61 to 15.69 and 208Pb/204Pb: 37.87 to 39.29) with variable 207Pb/204Pb ratios at almost constant 206Pb/204Pb and 208Pb/204Pb ratios, suggesting a derivation from ancient sources with comparatively high U/Pb but low Th/Pb ratios. The limited variations in Sr, Nd and Pb isotopes were not caused by crustal contamination or magma mixing, but instead reflect source heterogeneities. Strontium and Nd isotope compositions suggest mafic lithologies similar to amphibolites from the Kalahari Craton basement as potential sources. A comparison with amphibolite melting experiments confirms the possible derivation of the granodiorites from an amphibolitic source. Calculated maximum zircon saturation temperatures at insignificant amounts of inherited zircon, indicate intrusion temperatures of up to 900 °C. Apatite saturation temperatures are higher, up to ca. 950 °C. Pressures of 5 to 10 kbar are determined through Qz-Ab-Or systematics and are interpreted as minimum pressures at the site of melting suggesting that the granodiorites/granites represent high temperature partial melts generated in the lower crust. Although there are some compositional similarities with granites generated in subduction zones, radiogenic Pb isotope ratios and high δ18O values suggest that reprocessed amphibolitic rocks are more likely sources.

Syn-orogenic high-temperature crustal melting: Geochronological and Nd–Sr–Pb isotope constraints from basement-derived granites (Central Damara Orogen, Namibia)

Major and trace element and Nd, Sr and Pb isotope data from c. 550Ma-old gray granites and c. 510Ma-old red leucogranites of the high-grade central part of the Damara orogen (Namibia) indicate a dominantly deep crustal origin. Moderately peraluminous gray granites are isotopically evolved (initial εNd: c. −17) and were likely derived from meta-igneous sources with late Archean to Paleoproterozoic crustal residence ages. Based on a comparison with experimental results, the granites were derived by partial melting of a granodioritic biotite gneiss at c. 900–950°C and less than 10kbar. Slightly peraluminous red leucogranites are also isotopically evolved (initial εNd: −15 to −18) but have undergone extensive crystal fractionation coupled with minor contamination of mid crustal meta-pelitic material. Major and trace element data do not support closed-system fractional crystallization processes for all samples, however, some chemical features underline the importance of crystal fractionation processes especially for the leucogranites. Isotope data do not support mixing of different crust-derived melts or assimilation of crustal rocks by a mafic magma on a large scale. For the gray granites, unradiogenic Pb isotope compositions with substantial variation in 207Pb/204Pb at almost constant 206Pb/204Pb, strongly negative εNd values and moderately radiogenic Sr isotope compositions argue for an undepleted nature of the source. High Rb/Sr ratios of the red leucogranites permit a comparison with the gray granites but similar initial εNd values indicate that the source of these granites is not fundamentally different to the source of the gray granites. The most acceptable model for both granite types involves partial melting of meta-igneous basement rocks of Archean to Proterozoic age. The consistency of the chemical data with a crustal anatectic origin and the observation that the gray granites intruded before the first peak of high-grade regional metamorphism suggests that they intruded simultaneously with crustal thickening. The red leucogranites are interpreted to be a result of crustal melting during the main peak of regional metamorphism. The heating events that promoted melting of fertile deep-crustal rocks might have been caused by the inferred high heat productivity of heat-producing radioactive elements (Th, U, K) together with crustal thickening during the main periods of orogeny.

The Feiran–Solaf metamorphic complex, Sinai, Egypt: Geochronological and geochemical constraints on its evolution

Ion-microprobe U–Th–Pb geochronological data are presented for fourteen samples from the Feiran–Solaf metamorphic complex in the Sinai Peninsula, Egypt. These data reveal a succession of magmatic and metamorphic events from ca. 1000Ma to <600Ma. The oldest age of ∼1.0Ga is obtained from zircon in Solaf zone metapsammites, representing the age of a locally restricted acidic igneous source which eroded and was deposited as arkose sometime after ∼975±10Ma. A similar ∼1.0Ga age from granodioritic biotite gneisses is interpreted as dating its crystallization. A biotite gneiss sample bordering the Solaf granodioritic gneiss yields an age of 792±7Ma, suggesting at least two phases of arc development over ca. 200Ma. While detrital zircon from migmatitic metapelites indicates input of material as old as ∼1.0Ga, the probable maximum sedimentation age for their immature pelitic protolith, which was likely deposited in an active continental marginal basin within a continental arc, is 656±6Ma. These metapelites were metamorphosed at 603±6Ma.Leucosome and mesosome samples from migmatitic biotite-hornblende gneisses of the Feiran zone yield polyphase zircon with ca. 700Ma cores interpreted as dating igneous crystallization, while overgrowths of 627±7Ma and 592±10Ma define the time of high-grade metamorphism. A biotite-hornblende gneiss sample from the Feiran-zone contains zircon that defines a protolith crystallization age at 623±6Ma, with rim overgrowths defining a metamorphic age of 587±7Ma. Zircon from the diorite body separating the Feiran and Solaf zones yields crystallization ages of 617±4Ma and 602±4Ma for samples collected from the periphery and core, respectively, while zircon from a late-tectonic granodiorite sample intruding the metapsammitic gneisses yields a crystallization age of 589±6Ma.High spatial resolution age dating from the Feiran–Solaf metamorphic complex and intruding magmatic rocks thus reveals four episodes of arc magmatism at 799–785Ma, 713–692Ma, 629–613Ma, and 606–583Ma. The timing of regional high-grade metamorphism is constrained by the zircon rims in the migmatitic metapelites and migmatized biotite-hornblende gneisses which record two distinct events at 627±7Ma and 592±10Ma, interpreted to represent peak and isothermal decompressional metamorphic events, synchronous with heat flow from concurrent young magmatism.

High grade metamorphism of sedimentary rocks during Palaeozoic rift basin formation in central Australia

Exhumation of middle and lower crustal rocks during the 450–320Ma intraplate Alice Springs Orogeny in central Australia provides an opportunity to examine the deep burial of sedimentary successions leading to regional high-grade metamorphism. SIMS zircon U–Pb geochronology shows that high-grade metasedimentary units recording lower crustal pressures share a depositional history with unmetamorphosed sedimentary successions in surrounding sedimentary basins. These surrounding basins constitute parts of a large and formerly contiguous intraplate basin that covered much of Neoproterozoic to early Palaeozoic Australia. Within the highly metamorphosed Harts Range Group, metamorphic zircon growth at 480–460Ma records mid-to-lower crustal (~0.9–1.0GPa) metamorphism. Similarities in detrital zircon age spectra between the Harts Range Group and Late Neoproterozoic–Cambrian sequences in the surrounding Amadeus and Georgina basins imply that the Harts Range Group is a highly metamorphosed equivalent of the same successions. Maximum depositional ages for parts of the Harts Range Group are as low as ~520–500Ma indicating that burial to depths approaching 30km occurred ~20–40Ma after deposition. Palaeogeographic reconstructions based on well-preserved sedimentary records indicate that throughout the Cambro–Ordovician central Australia was covered by a shallow, gently subsiding epicratonic marine basin, and provide a context for the deep burial of the Harts Range Group. Sedimentation and burial coincided with voluminous mafic magmatism that is absent from the surrounding unmetamorphosed basinal successions, suggesting that the Harts Range Group accumulated in a localised sub-basin associated with sufficient lithospheric extension to generate mantle partial melting. The presently preserved axial extent of this sub-basin is >200km. Its width has been modified by subsequent shortening associated with the Alice Springs Orogeny, but must have been >80km. Seismic reflection data suggest that the Harts Range Group is preserved within an inverted crustal-scale half graben structure, lending further support to the notion that it accumulated in a discrete sub-basin. Based on palaeogeographic constraints we suggest that burial of the Harts Range Group to lower crustal depths occurred primarily via sediment loading in an exceptionally deep Late Cambrian to Early Ordovician intraplate rift basin. High-temperature Ordovician deformation within the Harts Range Group formed a regional low angle foliation associated with ongoing mafic magmatism that was coeval with deepening of the overlying marine basin, suggesting that metamorphism of the Harts Range Group was associated with ongoing extension. The resulting lower crustal metamorphic terrain is therefore interpreted to represent high-temperature deformation in the lower levels of a deep sedimentary basin during continued basin development. If this model is correct, it indicates that regional-scale moderate- to high-pressure metamorphism of supracrustal rocks need not necessarily reflect compressional thickening of the crust, an assumption commonly made in studies of many metamorphic terrains that lack a palaeogeographic context.

Zircon U–Pb ages and geochemistry of the Qinglong volcano-sedimentary rock series in Eastern Hebei: Implication for ∼2500 Ma intra-continental rifting in the North China Craton

Voluminous Archean rocks (including orthogneisses and supracrustal rocks) are exposed in Eastern Hebei, northern part of the North China Craton (NCC), and nearly all of these Archean rocks have experienced high-grade metamorphism of granulite-facies or upper amphibolite facies from 2.54Ga to 2.52Ga. The Qinglong area of the eastern part of Hebei Province has a set of low-grade (greenschist- to lower amphibolite-facies) supracrustal rocks, e.g., the Qinglong volcano-sedimentary rock series (QVRS), unconformably overlying on the high-grade metamorphosed Qianxi Complex. Zircons yield a weighted mean 207Pb/206Pb age of 2511±12Ma meta-felsic volcanic rocks and 2503±13Ma meta-basic volcanic rocks. These ages indicate that the QVRS formed during the period from 2511Ma to 2503Ma. The meta-felsic volcanic rocks display low MgO (0.96–2.67wt.%), Cr (50.90–87.10ppm), and Ni (2.78–6.18ppm) contents, and low Nb/Ta ratios (13.56–14.54), and high light rare earth elements (LREEs) ((La/Yb)n=14.75–22.04), which are interpreted to have been produced by the partial melting of the lower crust. The meta-basic volcanic rocks (low-grade amphibolites) have abundance levels from 3.86wt.% to 7.74wt.% of MgO, and from 46.11wt.% to 51.99wt.% of SiO2. They are enriched in LREEs and depleted in high field strength elements (HFSEs) with positive ɛNd(t) values (+2.6 to +2.9). The volcanic rocks in the QVRS show bimodal compositions. The ɛHf(t) (t=2511Ma) values of amphibolite range from −4.7 to +5.7, and their single-stage Hf model ages vary from 2622Ma to 3030Ma (peak at 2835Ma). For the meta-felsic volcanic rocks, the ɛHf(t) values range from −3.2 to +5 with two-stage model ages changing from 2683Ma to 3180Ma (peak at 2872Ma). The QVRS is proposed to have formed during an intra-continent rift at 2500Ma, after the regional high-grade metamorphism from 2.52Ga to 2.54Ga. The QVRS probably formed during an intra-continental rift, as a part of the late Neoarchean cover of the NCC.