SHRIMP Zircon Dating Research Articles

Turbulent paleoenvironment linked to astronomical forcing during the Permian–Triassic transition

The Milankovitch cycles are essential for establishing high-resolution chronological frameworks, which can provide a reliable timescale for biological and environmental events. However, astronomical solutions for the Paleozoic are still imperfect. Moreover, global paleoenvironmental turbulence during the Permian-Triassic (PT) transition caused interference with most proxies reflecting astronomical cycles. This study discovered that the Sb/Cd series can reflect astronomical cycle signals through a time-series analysis of geochemical elemental data from cores around the Lopingian–Lower Triassic in the Lower Yangtze region. Combined with zircon SHRIMP dating of volcanic ashes, a reliable absolute astronomical timescale with a duration of 2.5 Myr was established from 100-kyr eccentricity-tuned results. Based on the coupling analysis of palaeoenvironmental proxies, this study evaluated the effect of astronomical forcing and volcanism on the paleoenvironment. The sea level change was influenced by obliquity during the Lopingian–Early Triassic. During the Wuchiapingian–Changhsingian (WC) and PT transitions, regional volcanic activities at South China and the Siberian Traps Large Igneous Province (STLIP) intervened the normal trend of a low-temperature arid climate dominated by the obliquity forcing, respectively. Thus, South China exhibited a hot humid climate during the WC and PT transitions. Conversely, the low-temperature climate in South China was significantly modulated by obliquity forcing during the middle Changhsingian. Besides the influences of intense igneous activities and related environmental effects, disruption the long-term stable low-temperature environment by the abnormal high-temperature around the PT transition was a tremendous challenge to the biological ecological adaptation, and thereby may have contributed to the end-Permian mass extinction.

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

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

Zircon SHRIMP U-Pb Dating and Significance From Weathered Residual Kaolin Deposits on the Northern Margin of the Qinzhou-Hangzhou Suture Zone, China

Weathered residual Kaolin deposits on the northern margin of the Qinzhou-Hangzhou Suture Zone are characterized by more feldspar-containing minerals and fewer dark minerals. These deposits have attracted increasing attention because of their high whiteness, low harmful impurity content, low depth of weathered mineralization, and ease for mining. The ore-forming rock masses mainly include granite (granite porphyry), quartz porphyry, eurite, haplite, and other acidic veins; the ore-bearing walls are mainly of Upper Proterozoic. Previous studies suggested that dyke rocks related to Kaolin deposits in this belt belong to the Mesozoic, but there is a lack of information regarding specific mineralization ages. In this study, two dyke wall-type weathered residual Kaolin deposits were selected from the northern margin of the Qinzhou-Hangzhou Suture Zone (Dongxicun and Yinbeidong, Jiangxi Province, China) for zircon SHRIMP dating. Two zircon U-Pb-based ages were obtained: 843 ± 10 Ma from Dongxicun granite veins that intrude into the Neoproterozoic Jiuling rock masses; 152.3 ± 1.8 Ma for Yinbeidong granite porphyry veins that intrude into the Neoproterozoic Anlelin Formation. We suggest that the dykes related to the Kaolin deposits were formed in the Neoproterozoic and late Jurassic. The Neoproterozoic dyke may be related to the convergence, formation, or pyrolysis of the Rodinia Supercontinent and the late Jurassic dyke may be related to the subduction of the Paleo-Pacific Plate. These results provide a chronological basis for the investigation and evaluation of weathered residual Kaolin deposits on the Qinzhou-Hangzhou Suture Zone.

Tracking C-O-H fluid-rock interactions in reworked UHT granulite: Tectonic evolution from ca. 990 Ma to ca. 500 Ma in orogenic interior of Eastern Ghats Belt, India

A suite of high-grade granulite rocks occurring close to one of the earliest reported ultra-high temperature rocks (“Paderu” rocks) from the central part of the Eastern Ghats Belt, India has been studied. Here we present a detailed pressure-temperature-time-fluid history of the suite of aluminous granulite and associated gneisses from a single exposure, which were injected by granitic aplite veins at a high angle at a ductile depth of the host gneissic rocks. Detailed studies of the sapphirine-spinel-quartz-bearing aluminous granulite reveal a peak metamorphic condition of ~1000 °C at ~8 kbar. Porphyroblastic garnet and quartz in the peak assemblage have mono-phase, high-density, CO2-rich primary fluid inclusions (~1 g/cm3). Orthopyroxene-garnet-sillimanite forming corona textures in the aluminous granulite and the presence of cordierite-K-feldspar-quartz intergrowths imply post-peak cooling and subsequent decompression down to ~6 kbar. Secondary fluid inclusions in quartz grains of the aluminous granulite have the signature of bi-phase CO2-H2O fluids with a comparatively low estimated density (0.8 g/cm3). The CO2 isochore plots suggest a pressure drop from 7 to 8 kbar to 4–5 kbar (simultaneous or after the cooling from 1000 °C to 800 °C). Texture-constrained monazite dating using U-Th-total Pb EPMA technique reveals that there are strong peaks at ~940 Ma, ~900 Ma, and ~ 800 Ma with several smaller and younger peaks. On the other hand, the UPb SHRIMP dating of zircon from the same rock reveals discordant ages with upper and lower intercepts at ~1700 Ma and ~ 990 Ma. Late pegmatite dykes and aplite veins were emplaced cutting across the dominant structural fabric of this litho-assemblage at a high angle. The asymmetric dragging of the foliation of the host leptynite possibly implies a shear-induced intrusion. The fluid inclusion study of the aplite reveals that the quartz grains contain numerous primary and pseudosecondary fluid inclusions (CO2 monophase to biphase). The calculated density is in the range of 0.71–0.82 g/cm3, suggesting the entrapment of carbonic fluid at <3 kbar. Monazite dating yields a strong single peak at 497 ± 4 Ma, one of the rare dates from the orogenic interior of the Eastern Ghats Belt. The studied suite of deep crustal granulites with aplite veins preserves a history of at least two events of fluid-rock interactions during its residence at the extremely hot deep crustal level until its exhumation to shallower levels where a magmatic fluid is trapped, through a time corridor between the Grenvillian and Kuunga orogenies.

Archean–Ediacaran evolution of the Campos Gerais Domain — A reworked margin of the São Francisco paleocontinent (SE Brazil): Constraints from metamafic–ultramafic rocks

The Campos Gerais Domain (CGD) in southeastern Brazil is an approximately 180 km × 35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton (SFC). The Archean–Paleoproterozoic evolution of the CGD — alongside its potential correlation with the SFC or other cratonic blocks in the region — is currently poorly-constrained. We present the results of systematic petrography, bulk-rock geochemistry, mineral chemistry and geochronology for a suite of scarcely studied mafic–ultramafic rocks from the CGD. We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic–ultramafic rocks throughout the CGD, and geochronological information for various lithotypes in the region. The CGD records a protracted Mesoarchean to Statherian (3.1–1.7 Ga) crustal evolution, which we interpret to share a common history with the southern SFC and their related reworked segments, suggesting that it is a westward extension of this cratonic terrain. The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide, and that is broadly coeval with a local suite of tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites. U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca. 2.96 Ga, revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent. Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite, as previously suggested, instead presenting features that point to formation in association with a continental arc. Coupled with a U-Pb (SHRIMP) crystallization age of ca. 2.13 Ga recorded by zircon grains from a metaultramafic rock, these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny, and with the Pouso Alegre/Amparo and São Vicente complexes. Finally, a U-Pb (SHRIMP) concordia age of ca. 590 Ma — obtained from metamorphic-textured zircon grains from a metaultramafic rock — points to a late metamorphic overprint related to upper amphibolite conditions, brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana’s assembly in the southern SFC, with later retrogression to greenschist-facies.

U-Pb zircon SHRIMP dating of a protracted magmatic setting and its volcanic emplacement: Insights from the felsic volcanic rocks hosting the sulphide ore of the giant Aljustrel deposit, Iberian Pyrite Belt

• A protracted (~50 Ma) magmatic activity is suggested. • Upper Devonian subvolcanic activity (~373–365 Ma) took place before main volcanism . • Early Carboniferous long-lasting volcanism was identified (353–359 Ma) • Data suggests diachronism of sulphide deposition between different sub-basins. • Pb loss effects are attributed to the Variscan deformation and metamorphism . A geochronological study using SHRIMP U-Pb analysis of zircon grains has been conducted to date felsic volcanic rocks hosting the six massive sulphide deposits of the giant Aljustrel mining district in the Iberian Pyrite Belt. A multiple method age calculation approach was used to validate and ponder calculated Concordia ages (emplacement and inherited), which included weighted average, probability density peak(s), Tuff Zirc and Unmix functions. This approach was particularly useful to interpret the wide continuous single U-Pb ages (320–405 Ma) recorded in the Aljustrel volcanic rocks. The volcanic pile (>250 m) that hosts the Aljustrel deposits was emplaced between 359 and 353 Ma. Upper Devonian inheritance, representing subvolcanic activity, is well-represented in the volcanic rocks of Aljustrel (373–365 Ma). Older Devonian inherited zircon ages at 405 Ma, 388 Ma and 380 Ma were retrieved, hypothetically representing deep plutonism or other melting episodes, which suggests a long-lasting (~50 Ma) magmatic activity in the Aljustrel district. Older pre-Devonian inherited ages, uppermost Silurian and early to late Cambrian, and post-emplacement ages (~330–345 Ma) were also detected, with the latter reflecting Pb loss most likely driven by the main Variscan orogenic event. Maximum ages obtained for the volcanic rocks in the different deposits open the possibility that the last pulses of volcanic activity and subsequent deposition of the massive sulphides were diachronic in the different Aljustrel sub-basins. Additionally, results imply that, contrary to previously assumed, Gavião and São João-Moinho deposits are probably not the same ore lens disrupted by tardi-Variscan faults. This opens new opportunities for mining exploration and targeting in the Aljustrel district and points out the importance of high-resolution geochronological studies in mining and brownfield areas.

The First U–Pb SHRIMP Dating of Zircons from Capitanian (Middle Permian) Deposits of the Omolon Massif (Northeast Russia)

The U–Pb dating of zircons from two samples taken in the stratotype sections of the Middle Permian Gizhiga Formation in the Omolon Massif has been carried out for the first time. Weighted average ages of 266 ± 2 and 265 ± 3 Ma taking the error into account are consistent with the Capitanian age calculated previously by paleontological determination. The observed detrital zircon population has made it possible to identify a few source areas in the Omolon Basin. Pre-Permian zircons are related to erosion of basement deposits of the Omolon Massif and the Middle Paleozoic volcanic rocks of the Kedon Group, while the Permian zircon population is due to the tuff supply from the Okhotsk-Taigonos volcanic arc.

Age of the Archaean Murchison Belt and mineralisation, South Africa

Abstract The east-northeast-trending Murchison-Thabazimbi Lineament in northern South Africa is one of the world’s most important structures for its control on world-class mineral deposits, Proterozoic sedimentary basins and giant igneous intrusions. The deepest exposed Archaean parts of the lineament are the Murchison Belt. Bounded by granitoids, the belt comprises greenschist to amphibolite facies volcano-sedimentary strata with isoclinal folds and the 7 km thick meta-igneous Rooiwater Complex. The Rooiwater Complex is intruded by a northern regional granitoid dated at 2 929 ± 7 Ma by SHRIMP U-Pb on zircons. Using field relationships, published isotopic age data and new SHRIMP zircon dates we confirm the age Rooiwater Complex at 2 965 Ma, showing it to be contemporaneous with the Archaean volcanic and sedimentary formations, the meta-igneous Complex being the lower sequence in a ~2 980 to 2 960 Ma island arc. Despite being implicated as a source of gold for the world’s largest natural accumulation of gold in the Witwatersrand Basin, the absolute age of Sb-Au mineralisation in the Murchison Belt is poorly constrained. We have utilised SHRIMP U-Pb geochronology to date monazites from a Sb-Au ore sample from the granitoid-hosted Malati Pump orebody and determine ages for two different generations of monazite, both associated with ore minerals. The older age of 2 832 ± 23 Ma is from a minority of grains and is interpreted to date the primary Sb-Au mineralisation, about 120 Ma after belt formation. This age predates, or is possibly synchronous with, sedimentation of the upper-Witwatersrand Central Rand Group. The younger age of 1 968 ± 17 Ma from a majority of monazite grains is unrelated in time to known events and interpreted here as a cryptic hydrothermal reworking of the Sb-Au ores in this deposit.

U–Pb SHRIMP zircon ages of Ediacaran-Cambrian granitic bodies in central Brazil: Implications for the tectonic evolution of the Araguaia belt

The small granitic plutons of the Ramal do Lontra, Presidente Kennedy, Barrolândia and Santa Luzia are emplaced into the Estrondo Group metasedimentary rocks of the Neoproterozoic Araguaia Belt. They have similar petrographic and compositional characteristics and are coeval with the medium-grade regional metamorphism of the belt. New U–Pb SHRIMP zircon data yielded quite robust ages. These results fall within the experimental error, and the integrated Concordia age of 542 ± 3 Ma was chosen as representative of their magmatic crystallization. A relevant number of inherited grains are present, showing overgrowth rims with high-U content, formed during the later stage of magmatic crystallization. LA-ICP-MS U–Pb dating on detrital zircon grains from a metasedimentary rock of the Estrondo Group yielded several Paleoproterozoic ages and a few Neoarchean ages, but the major peak in the histogram was between 1000 and 1200 Ma. The youngest grains indicate an Ediacaran age near 600 Ma, which is considered a reasonable upper limit for the deposition of the Estrondo Group. The Araguaia Belt started as an intracontinental system, segmenting the Amazonian Craton, formed by rifting during the breakup of Rodinia and was after occupied by a sedimentary basin which evolved to form an oceanic lithosphere. In the Ediacaran, the huge continental collisions that formed the West Gondwana Orogen most likely caused its remote activation, producing tectonic inversion with vergence to the west. Granitic plutons were emplaced in the last stages of the evolution of the Araguaia Belt, during the Brasiliano-PanAfrican orogenesis, at the Ediacaran-Cambrian boundary.

Provenance of Tanjero and Red Bed clastic sedimentary rocks revealed by detrital zircon SHRIMP dating, Kurdistan region, NE Iraq: Constraints on ocean closure and unroofing of Neo-Tethyan allochthons

Closure of Neo-Tethys between the Late Cretaceous and Miocene caused obduction of intra-Neo-Tethys volcanic arc allochthons onto the northern margin of the Arabian-Nubian Shield. This loading led to downwarping of the shield forming a flexural basin in which the Tanjero Formation and later Red Beds accumulated. Analysis of the detrital zircon populations in these sedimentary units provides additional evidence supporting the petrographic data for the presence and unroofing of the allochthons. Both the Tanjero Formation and Red Beds show evidence of initial derivation of detritus from the Nubian-Arabian Shield together with minor Paleozoic detritus from Egypt, Turkey and/or Iran. The youngest detrital zircon grains in the Tanjero Formation are of Paleocene age and probably represent northern derivation from the Sanandaj-Sirjan Zone. In contrast, the upper Miocene Red Beds contain detrital zircon of Eocene age that confirms the petrographic evidence for derivation from, and hence emplacement of, the lower allochthon of intra-Neo-Tethys Walash-Naopurdan rocks.

New Data on the Age of Igneous Complexes of the Alazey–Oloy Foldbelt (Western Chukotka)

New data on the age of the intrusive and subvolcanic rocks of the Oloy zone in Chukotka hosting commercially important Au–polymetallic and Cu–porphyry ore occurrences of the Burgakhchan area are reported. The age of the intrusive rocks of the Nichan and Vukney massifs, as well as the volcanic rocks of the Zybkin Formation in the Nenkan and Vukmaam structural–facies zones of Chukotka is determined reliably for the first time. U–Pb SHRIMP dating of zircons from three monzodiorite samples from the Nichan massif and three samples from the Vukney massif related to the Ekdygkych Complex provides evidence for their Berriasian–Valanginian age with mid-weighted values from 143 to 139 ± 2 Ma. The ages obtained correlate with the age of the porphyric diorite and monzonite of the Ekdygkych massif on the Peschanka deposit and indicate the high commercial significance of the studied intrusive complexes. The ore-bearing rocks intrude the volcanogenic–sedimentary rocks of the Zybkin Formation with a U–Pb zircon age of 146.5 ± 3 Ma.

Geochemistry and SHRIMP U‐Pb Zircon Dating of Mafic Rocks North of Zunhua City, Eastern Hebei, North China Craton: Paleoproterozoic Gabbro rather than Neoarchean Ophiolite

Abundant mafic‐ultramafic blocks and dikes occur in the area north of Zunhua City, eastern Hebei Province, and were previously suggested to be part of a late Archean ophiolitic assemblage. We employed SHRIMP zircon dating and a geochemical study on these mafic and surrounding rocks to test the ophiolite hypothesis. The SHRIMP data suggest that three metagabbro samples were metamorphosed at ∼1.8 Ga. Numerous ∼2.5 Ga zircons display strong oscillatory zoning, characteristic of zircons from granitoid rocks but not from gabbro, so we suggest that these are xenocrystic grains. The age of these xenocrystic zircons and their metamorphic rims suggests that these mafic blocks formed in Paleoproterozoic. The surrounding gneiss of intermediate composition also contains 2.5 Ga zircons with oscillatory zoning and 1.8 Ga metamorphic rims. Fractionated REE patterns and Nb, Ta, Zr, Hf negative anomalies to variable extent were observed in the mafic blocks and surrounding rocks, also supporting a significant difference in the chemistry of ophiolitic rocks. Our data suggest that many mafic blocks in northern Zunhua are not part of a late Archean ophiolite complex but part of a tectonically dismembered Paleoproterozoic intrusive gabbro complex. This study shows that late Paleoproterozoic metamorphism occurred in the western part of eastern Hebei Province.

Zircon U–Pb geochronology and geochemistry of the Cerro Colorado porphyry copper deposit, northern Chile

Porphyry Copper Deposits (PCDs) dominantly form in convergent plate margins genetically related to volcanic arcs. However, their temporal relationships with active volcanism are less clear. Active volcanic activity is often considered to impede the formation of PCDs and some workers have concluded that PCD formation must take place at times of volcanic quiescence or at a waning stage of volcanism. However, several studies have documented evidence suggesting syn-eruption PCD formation. A combination of radiometric dating, geochemistry studies and observations of rock cores from the Cerro Colorado PCD, a BHP Billiton property located in N. Chile, provides evidence compatible with syn-volcanic PCD formation within a volcanic edifice.Textural observations at both hand specimen and microscopic scales combined with new LA-ICPMS and SHRIMP dating of zircons suggest the intrusive history of the Cerro Colorado area can be separated into two main units: a porphyritic tonalite ca. 53.5 Ma and a porphyritic quartz monzonite ca. 50 Ma. There is also evidence for crystallisation at ca. 60 Ma, but a corresponding distinct lithology was not identified. A quartz porphyry unit previously identified as the youngest intrusive body is reinterpreted as an altered equivalent of the monzonite. The youngest intrusive units are represented by porphyritic granodiorite with an age 51–50 Ma. These units only show minor metal mineralisation suggesting that the main phase of mineralisation in the Cerro Colorado mine occurred over a few million years, bracketed by the two main pulses of igneous intrusion, which are consistent with documented Ar–Ar ages. Re–Os ages reported by previous studies suggest an earlier phase of mineralisation at ca. 56–54 Ma. U–Pb dating of zircon from a volcanic-related breccia that cuts the host rocks of the Cerro Colorado Mine reveals a phase of volcanic activity at ca. 57 Ma, after formation of the regional Cretaceous Cerro Empexa Formation. This age range overlaps the ages derived for the intrusive porphyry units suggesting a close temporal relationship between volcanism and the ore-forming subvolcanic intrusions in Cerro Colorado. A link between active volcanism and mineralisation is reinforced by the ultrafine-grained matrix and the presence of strong brecciation associated with the monzonite-quartz porphyry stock suggesting decompression and volatile release have taken place during emplacement.

New results of U–Pb SHRIMP dating of zircons from upper Wuchiapingian (Upper Permian) deposits in northeastern Russia

The first results are presented for U-Pb SHRIMP-II dating of zircons from the upper part of the Khivachian regional horizon (stage) of the Regional Stratigraphic Scale (RSS) of the Permian in northeastern Russia. The obtained isotope age of 255 ± 2 Ma is close to that of the present boundary between the Wuchiapingian and Changhsingian stages of the Permian system in the International Stratigraphic Scale (254.1 Ma). Based on the distribution of bivalves—Intomodesma spp. and Claraioides aff. primitivus (Yin)—in the sections considered, their relations to the stratigraphic positions of the samples considered and dated formerly, and in view of the interregional correlation of recent δ13Сorg data for clayey rocks, one may assume with certainty that most of the regional zone of Intomodesma costatum corresponds to the upper part of the Wuchiapingian stage. Here, the Changhsingian stage in northeastern Asia complies only with the uppermost part of this zone within the I. postevenicum subzone and, partially, of Otoceras layers within the Otoceras concavum zone.

2.09 Ga old eclogites in the Eburnian-Transamazonian orogen of southern Cameroon: Significance for Palaeoproterozoic plate tectonics

Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18–20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12–14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.

Combined garnet and zircon geochronology of the ultra-high temperature metamorphism: Constraints on the rise of the Orlica-Śnieżnik Dome, NE Bohemian Massif, SW Poland

Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9±1.2 and 348.3±2.0Ma recording peak 2.5GPa pressure and 950°C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9±3.4Ma. Ti-in-zircon thermometry indicates crystallization at 810–860°C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6Ma.All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337Ma.The estimated PTt path documents the ca. 10Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347 - >342Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at >342–337Ma which corresponds to a fast, nearly isobaric cooling.

The first data on U–Pb (SHRIMP) dating of zircon from metamorphic rocks of the crystalline base of the Taigonos-Paren Uplift (southern margins of the Omolon Massif)

U–Pb SHRIMP dating of zircons from metamorphic rocks of the crystalline basement of the Omolon-Taigonos area (northeastern margins of Asia) has been used to determine the periods of early endogenic activity of the crust. These periods correlate to the time of formation of the magmatic rock protolith (3.2–3.3 Ga), the stage of regional metamorphism (2.6–2.8 Ga), and the superimposed granitization. The resulting data can be used to reconstruct the history of development of the crust in the Precambrian.

Late Permian volcanic dykes in the crystalline basement of the Považský Inovec Mts. (Western Carpathians): U–Th–Pb zircon SHRIMP and monazite chemical dating

AbstractThis paper presents geochronological data for the volcanic dykes located in the northern Považský Inovec Mts. The dykes are up to 5 m thick and tens to hundreds of metres long. They comprise variously inclined and oriented lenses, composed of strongly altered grey-green alkali basalts. Their age was variously interpreted and discussed in the past. Dykes were emplaced into the Tatricum metamorphic rocks, mostly consisting of mica schists and gneisses of the Variscan (early Carboniferous) age. Two different methods, zircon SHRIMP and monazite chemical dating, were applied to determine the age of these dykes. U-Pb SHRIMP dating of magmatic zircons yielded the concordia age of 260.2 ± 1.4 Ma. The Th-U-Pb monazite dating of the same dyke gave the CHIME age of 259 ± 3Ma. Both ages confirm the magmatic crystallization at the boundary of the latest Middle Permian to the Late Permian. Dyke emplacement was coeval with development of the Late Paleozoic sedimentary basin known in the northern Považský Inovec Mts. and could be correlated with other pre-Mesozoic Tethyan regions especially in the Southern Alps.

Early Neoarchean Magmatic and Paleoproterozoic Metamorphic Events in the Northern North China Craton: SHRIMP Zircon Dating and Hf Isotopes of Archean Rocks from the Miyun Area, Beijing

The Miyun area of Beijing is located in the northern part of the North China Craton (NCC) and includes a variety of Archean granitoids and metamorphic rocks. Magmatic domains in zircon from a tonalite reveal Early Neoarchean (2752 +/- 7 Ma) ages show a small range in (Hf)(t) from 3.1 to 7.4 and t(DM1)(Hf) from 2742 to 2823 Ma, similar to their U-Pb ages, indicating derivation from a depleted mantle source only a short time prior to crystallization. SHRIMP zircon ages of granite, gneiss, amphibolite and hornblendite in the Miyun area reveal restricted emplacement ages from 2594 to 2496 Ma. They also record metamorphic events at ca. 2.50 Ga, 2.44 Ga and 1.82 Ga, showing a similar evolutionary history to the widely distributed Late Neoarchean rocks in the NCC. Positive (Hf)(t) values of 1.5 to 5.9, with model ages younger than 3.0 Ga for magmatic zircon domains from these Late Neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern NCC during the Neoarchean. Late Paleoproterozoic metamorphism developed widely in the NCC, not only in the Trans-North China Orogen, but also in areas of Eastern and Western Blocks, which suggest that the late Paleoproterozoic was the assembly of different micro-continents, which resulted in the final consolidation to form the NCC, and related to the development of the Paleo-Mesoproterozoic Columbia or Nuna supercontinent.

Origin and age of coeval gabbros and leucogranites in the northern subprovince of the borborema province, NE Brazil

The Paleoproterozoic Serrinha – Pedro Velho Complex comprises orthogneisses and migmatites, exposed in the south part of the Archean São José do Campestre Massif, Rio Grande do Norte domain of the Borborema Province, NE, Brazil. During the Ediacaran, the Serrinha - Pedro Velho Complex underwent metamorphism at HT/LP conditions in a tectonic setting dominated by dextral transpressive deformation, dated at ∼575Ma. Crustal melting generated various plutons and dykes of leucogranites with composition ranging from syeno-to monzogranites. Mafic rocks, including gabbros, norites and diorites also occur as dykes and small plutons intruded into the Serrinha - Pedro Velho Complex. Features of mixing and mingling between mafic and leucogranitic magmas were recorded locally. The granites and mafic rocks show field features and geochemical signatures of extension related magmatism. U-Pb SHRIMP zircon data yielded similar Concordia ages for felsic (582 ± 5 Ma) and mafic (588 ± 6 Ma) rocks. The leucogranites have strongly negative εNd (580 Ma) values (−19.8 to −24.3) and Paleoproterozoic to Archean Nd TDM model ages (2.2–2.6 Ga), similar to those recorded in the orthogneisses of the Serrinha - Pedro Velho Complex. The mafic rocks show slightly higher εNd (580 Ma) values (−13.09 to −19.63). The leucogranites were probably generated by partial melting of a source similar to the Serrinha Pedro Velho orthogneisses. The mafic rocks are MgO- rich, and the less evolved ones show picritic basaltic composition, similar to mafic rocks from continental flood basalt provinces. We suggest that melting of Paleoproterozoic ponds of picrite (plume related?) generated the mafic magmas studied that, subsequently, evolved by olivine and pyroxene fractionation at great depth and underwent contamination with crustal melts.In the studied area other Ediacaran extension related mafic rocks do occur. They have geochemical and isotopic signatures distinct from those recorded in Riachão mafic rocks. A Paleoproterozoic enriched mantle has been proposed as the source of these mafic rocks. The data suggest that extension and mafic magmatism favoured extensive melting of the Serrinha – Pedro Velho Complex at 582–588 Ma ago. A similar aged event is also recorded in southeastern Ghana, and interpreted as syn-orogenic extension related to the collisional orogeny at the margin of the West African craton.