Rounded Zircon Research Articles

Tracing source and palaeo‐weathering conditions of quartzose sandstone in the Cretaceous Cambay Basin: A combined petrographical, geochemical and geochronological approach

Based on detrital monazite dating, mineral chemistry of tourmaline and rutile, bulk rock geochemistry, modal analysis of petrographic and palaeocurrent data, this study tracks the source of quartzose sandstone within the Cretaceous succession of the Cambay Basin primarily to rocks within the Aravalli‐Delhi Fold Belt. Monazite geochronology shows predominant peaks around 506 ± 11 Ma, 905 ± 12 Ma and a minor peak at 750 ± 20 Ma, 1485 ± 30 Ma relating sediment sources to the Aravalli‐Delhi orogeny and Pan‐African orogeny. The petrographic modal analysis of the moderate‐to‐well‐sorted Himmatnagar Sandstone indicates at least 91% quartz, with minor feldspar and rock fragments. The mineral chemistry of tourmalines in Himmatnagar Sandstone matches with Li‐ and Ca‐poor granitoid, pegmatites, metapelites and quartz tourmaline rocks. The trace element content of rutiles in sandstones corresponds to metapelitic sources. Detrital monazite dates and mineral chemistry of tourmalines and rutiles pinpoint the sediment source to the South Delhi Supergroup of rocks. The dominance of rounded zircons and tourmalines and abraded quartz overgrowth in the Himmatnagar Sandstone indicate the recycling of sediments, possibly from the Marwar Supergroup. Palaeoclimatic proxies based on mineralogy and geochemistry, particularly, the high value (93–97) of mineralogical index alteration, indicate intense chemical weathering of the source rocks under humid climatic conditions. Therefore, the combination of factors, including the quartzo‐feldspathic source rocks, intense chemical weathering under humid climatic conditions and the recycling of sediments facilitated the formation of quartzose sandstone within the Himmatngar Sandstone in the Cambay Basin.

Syn-Bushveld “granite sheets” associated with the Molopo Farms Complex intruding into Transvaal Supergroup strata in southern Botswana

Abstract LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.

Multimethod provenance analysis using detrital zircon and rutile U-Pb geochronology across Devonian basin systems in the Tasmanides of eastern Australia

The Devonian represents a period of transition for the accretionary orogens of the Australian Tasmanides where the largest and inboard Thomson Orogen became stabilised and orogenic processes became focused in the outboard and newly developing New England Orogen. The end of tectonic activity in the Thomson and the adjacent Delamerian and Lachlan orogens culminated with the development of several overlying intracratonic sedimentary basins. We examine the two largest cover basins, the Adavale and Darling, using stratigraphic logging, sandstone petrography, detrital zircon and rutile U-Pb geochronology to fingerprint sediment sources to test whether the basins were once connected and part of a much larger cover basin system. Sediment provenance in the Adavale Basin is characterised by (i) continuous input from a basement-derived Ordovician (∼480 Ma) igneous zircon source, (ii) reworking of metasedimentary basement rocks with a ‘Pacific-Gondwana’ age signature (iii) reworking of detrital rutile from Cambro-Ordovician sedimentary rocks in the Thomson Orogen ultimately sourced from the Musgrave Block and (iv) an addition of syn-depositional volcanic zircon from contemporary volcanism between ∼380 and 360 Ma. Sediment provenance of the Darling Basin is dominated by reworking of (meta)sedimentary basement, evident from large proportions of rounded zircons exhibiting a ‘Pacific-Gondwana’ age signature and detrital rutile with Peterman Orogeny ages. A much less significant age population of syn-depositional volcanic detrital zircons suggests input from contemporary volcanic sources which were more distal and extra-basinal.The comparison of sandstone compositions and detrital age information indicate both basins record similar provenance signals in terms of reworking of their respective hinterlands and receiving contributions from relatively distal syn-depositional volcanism. However, the comparison of sediment provenance proxies, suggests that the Adavale and Darling basins were not connected during the Devonian and that the basins are intracratonic or cover basins, recording the stabilisation of the Thomson and Lachlan orogens, respectively.

Mineralogical and geochemical characteristics of mudstones in the Jersovec chert deposit

Jersovec chert deposit is a part of the transitional zone between the internal and the external Dinarides. Tethyan cherts are often interlaid with fine-grained material, as is also the case in Jersovec, where chert beds interchange with several thin, up to 10 cm thick layers of fine-grained sedimentary rocks. The source of this material is often questionable and interpreted to be terrigenous, volcanogenic or even combination of both. In order to determine origin and depositional environment of fine-grained material, detailed mineralogical and geochemical analyses were performed. Fine-grained sedimentary rocks were characterised by X-ray diffraction (XRD), scanning electronmicroscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and inductively coupled plasma emission spectroscopy (ICPES) and mass spectroscopy (ICP-MS). All samples are mineralogically very similar but differ in mineral ratios. They contain quartz and clay minerals, predominantly illite/muscovite and chlorite group minerals. SEM/EDS analysis additionally revealed zircon, monazite, Ti-oxide (probably rutile) and iron oxides/hydroxides in all samples, whereas chromite, ilmenite, xenotime, apatite and baryte were found only in some of them. The average sizes of accessory minerals range from 3-20 µm. Two samples differ from the others by their brown colour and higher Fe2O3 and lower SiO2 contents. All other samples are green. Chemical analysis showed that they consist mostly of SiO2 (46.3-69.3 %), Al2O3 (15-24.1 %) and minor contents of K2O (2.8-3.4 %), Fe2O3 (2.1-8.4 %), MgO (1.6-2.3 %), TiO2 (0.5-0.7 %), CaO (0.3-1.4 %), Na2O (0.1 %) and P2O5 (up to 0.5 %). Position of all samples in the Zr/TiO2 vs. (V+Ni+Cr)/ Al2O3 diagram points to terrigenous origin and Sedimentation on the Continental margin. According to the chemical weathering indices (CIA and CIW) material has been subjected to the intense and long weathering, supported also by the index of compositional variability (ICV), which corresponds to the compositionally mature material, and by rounded zircon and ilmenite grains.

U–Pb zircon and biostratigraphic data of high-pressure/low-temperature metamorphic rocks of the Talea Ori: tracking the Paleotethys suture in central Crete, Greece

Inherited deformation microfabrics of detrital quartz grains and U–Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite–Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U–Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite–Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U–Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U–Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta)conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite).

History of the West African Neoproterozoic Ocean: Key to the geotectonic history of circum-Atlantic Peri-Gondwana (Adrar Souttouf Massif, Moroccan Sahara)

Bordered by the Archaean basement of the Reguibat Shield to the E and S, the Adrar Souttouf Massif is located in the southern regions of the Moroccan Sahara and represents the northern part of the Mauritanide belt. The central areas of this massif comprise the Dayet Lawda and Sebkha Matallah units that are mainly composed of mafic rocks. Ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology, U-Th-Pb, and Lu-Hf isotope composition. Additionally, 155 apatite grains from six samples were also dated by the U-Th-Pb method. Mostly well rounded zircon grains commonly showed bright cathodoluminescent (CL) overgrowth domains and leaching zones, indicating metamorphic overprint around unaffected oscillatory zoned core areas. All samples but one yielded two significant zircon age populations: ~605 Ma and ~634 Ma with some inherited grains in the range of ~650–740 Ma and very scarcely up to ~1190 Ma. For six samples the Hf isotope composition suggests a major contribution of Neoproterozoic juvenile magmas from depleted mantle source. The zircon Hf data for the remaining samples points to a predominant recycling of older Archaean to Palaeoproterozoic crust. Based on the geochemical composition of the rocks, an island arc setting is assumed at the periphery of the West African Craton close to the Cryogenian-Ediacaran boundary. Subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 Ma. A minor Variscan overprint could be demonstrated for only one sample. This case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the West African Craton for palaeogeographic and geodynamic reconstructions including the peri-Gondwanan terranes during the Late Neoproterozoic.

Geochemistry of zircons from ultrametapmorphic granitoids in junction zone of Aldan Shield and Dzhugdzhur-Stanovoi Fold Region

The geochemistry of zircons from autochthonous granite gneiss (Lc1) anatectic (Lc3–4) and injection (Lc5) leucosomes has been studied. Neoarchean prismatic zircon grains with cores that reveal oscillatory zoning and are overgrown by a couple of rims have been seen to occur in Lc3–4. The prismatic grains are occasionally modified into isometric grains with block structure by Paleoproterozoic secondary alteration, which is accompanied by the depletion in HREE, Y, Nb, U; enrichment in Ti, Li, LREE; increasing Th: U ratio and Ce anomaly; and decreasing Eu anomaly. The Paleoproterozoic alteration is related to the low-temperature amphibolite-facies metamorphism followed by partial melting. The Neoarchean prismatic zircons were formed under the conditions of high-temperature amphibolite-facies ultrametamorphism at a temperature of ∼700°C. Judging by the higher Ce/Ce* ratio, the metamorphic rounded zircons were formed at a higher oxygen fugacity as compared with ultrametamorphic zircons from Lc1 and Lc3–4. Specific variation trends of trace element concentrations in prismatic L1 and L3–4 zircons, occasionally with opposite directions, emphasize their different origin. The former are products of metasomatic granitization completed by selective melting with appearance of dispersed melt drops, while the latter are products of anatexis in the open system and by lit-par-lit migmatization. Prismatic zircons L5 are characterized by rhythmic zoning in the core surrounded by rims. The concordant U-Pb age of rims is 129 Ma; the 206Pb/238U age of cores varies from 2213 to 147 Ma. The appreciable enrichment (by a factor of 2–13) of zircons in all minor elements from the core to the rims is caused by the effect of residual postmagmatic fluid, which not only altered zircons, but also facilitated the recrystallization of granite into a pegmatoid variety.

Lower-Crustal Xenoliths from Jurassic Kimberlite Diatremes, Upper Michigan (USA): Evidence for Proterozoic Orogenesis and Plume Magmatism in the Lower Crust of the Southern Superior Province

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/204Pb vs 207Pb/204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/204Pb vs 206Pb/204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a unique peraluminous composition. It has the lowest eNd and eHf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3–1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.

Chlorine-rich fluid or melt activity during granulite facies metamorphism in the Late Proterozoic to Cambrian continental collision zone—An example from the Sør Rondane Mountains, East Antarctica

In the granulite facies rocks, the role of low H2O activity fluids is still unclear. The importance of understanding its role is gradually being recognized, and it enables us to help understand the process of granulite formation. Attention has been specifically focused on Cl-rich fluids and high-salinity alkaline fluids in the geochemical modification of continental crust. We have investigated the field distribution of Cl-rich biotite in the pelitic gneisses of the Sør Rondane Mountains, East Antarctica where Late Proterozoic to Cambrian granulites are widely exposed. Among 27 samples studied, a garnet-biotite-sillimanite gneiss from Balchenfjella was selected as best suited sample to constrain the P-T-t conditions of Cl-rich fluid or melt activity. This gneiss contains garnet porphyroblasts that have a P-rich core with oscillatory zoning in P. The garnet core includes Cl-poor biotite and fluorapatite. This core has been partially resorbed and discontinuously overgrown by a P-poor rim, in which Cl-rich biotite and chlorapatite are included. Coarse-grained, rounded zircon grains are exclusively included in the rim of the garnet porphyroblast and are also present in the matrix. This mode of occurrence suggests that the Cl-rich biotite and chlorapatite, together with coarse-grained zircon were formed almost simultaneously. The P-T conditions of Cl-rich biotite entrapment in the garnet rim are estimated to be ca. 800°C and 0.8GPa. In comparison, peak metamorphic condition is ca. 850°C and 1.1GPa. These pieces of observation suggest that Cl-rich fluid or melt infiltrated at the core-rim boundary of garnet. In the case of fluid infiltration, the fHCl/fH2O ratio of the fluid in equilibrium with Cl-rich biotite and chlorapatite in the garnet rim are estimated to be ten times larger than that in equilibrium with Cl-poor biotite and fluorapatite in the matrix and the garnet core. The LA-ICP-MS U-Pb dating of the coarse-grained zircon included in the garnet rim gave concordia age of 603±14Ma. Therefore, Cl-rich fluid or melt infiltration took place close to the metamorphic peak condition of ca. 800°C and 0.8GPa at 603±14Ma. The field distribution of Cl-rich fluid or melt activity is somewhat linear in the Sør Rondane Mountains. High Cl-activity is located near the large scale ductile shear zones, suggesting its relation to high-strain zones. Regional distribution of high-grade Cl-rich fluid or melt activity in the Sør Rondane Mountains implies that it is one of the major phenomena in the continental collision processes.

Petrography, geochemistry and regional significance of crystalline klippen in the Garhwal Lesser Himalaya, India

Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal–Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350°C) and continued till lowering of temperature and deformation led to the mylonitization. Geochemically, these granitic gneisses (UG, DP and BMG) exhibit similar composition. Features such as high molecular A/CNK value (>1), presence of normative corundum and absence of normative diopside, enhanced Rb/Sr, Rb/Zr ratios, enrichment of Th and containing rounded zircons support their crustally-derived S-type granitic nature. The linear plot in major oxides is interpreted in terms of fractional crystallization processes. Mantle normalized multi-element spider diagram of UG illustrates depletion of Ba, Nb, Sr, P and Ti and enrichment of Th and show similarities with DP and BMG. Similarities were observed in lithology, petrographic characters and chemical composition of UG, DP, BMG and Ulleri augen gneisses (Nepal). Comparison with the rocks of Higher Himalayan crystallines (≈Vaikrita), suggests that these rocks (UG) are not transported from Higher Himalaya as understood earlier. This study however proposes that, these gneissic bodies represent an older basement occurring as a tectonic sliver which emplaced within the cover sequence as wedges at different structural levels. This is a regional phenomena observed throughout the Lesser Himalayan region.

Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China: Implications for the crustal evolution of the Cathaysia Block

U–Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6–1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ± 21 Ma, 1857 ± 29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield ε Hf( t) values between −8.8 and +3.7 and T DM model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94–1.28), characterized by high SiO 2 (68–72 wt.%), Al 2O 3 (14–15 wt.%) and low CaO, MgO, FeO t , TiO 2 and P 2O 5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks. Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO 2, and are characterized by high CaO/Al 2O 3 (4.1–5.3), low Al 2O 3/TiO 2 ratios (8.7–9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb) N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity. Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.

The first finding of native platinum in Cretaceous-Paleogene boundary clay, the gams section, Eastern Alps, Austria

For the first time, native platinum was found by microprobe analysis in the course of detailed studies of a transitional clay bed in the Cretaceous-Paleogene boundary, the Gams section, Eastern Alps, Austria. The analysis revealed no impurities. The morphology of platinum grains and its occurrence on the surface of a rounded zircon grain point to platinum syndeposition. It is concluded that native platinum formation was related to the action of volcanic aerosol, resulting from mantle plume activity 65 Ma BP.

Detrital zircon sedimentary provenance ages for the Eoarchaean Isua supracrustal belt southern West Greenland: Juxtaposition of an imbricated ca. 3700 Ma juvenile arc against an older complex with 3920–3760 Ma components

The amphibolite facies Eoarchaean Isua supracrustal belt (northern part of the Nuuk region, southern West Greenland) is dominated by strongly deformed metabasalts, with chert, banded iron formation, felsic volcanic and volcano-sedimentary rocks and minor gabbro and sedimentary carbonates. It comprises a suture zone between a northern terrane formed at ca. 3700Ma and a southern one formed at ca. 3800Ma. At the junction between these two terranes is a strongly tectonised, thin unit of metachert, BIF and carbonate-bearing rocks with minor detrital components, named the dividing sedimentary unit. Away from the belt, the northern terrane is dominated by ca. 3700Ma tonalites and the southern one by ca. 3800Ma tonalites.The Isua supracrustal belt chemical metasedimentary rocks give low yields of small (commonly ≤50μm long), prismatic, oscillatory-zoned zircons with less common, somewhat rounded zircons of volcanic and/or detrital origin. The most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons. The ages of these zircons are distinct in different parts of the belt. In the northern terrane, most have ages of 3710–3690Ma, with a few slightly older <3750Ma grains. In the southern 3800Ma terrane, a fuchsite-quartzite has 3900–3810Ma zircons. Three samples of the dividing sedimentary unit along the whole length of the belt yielded both 3920–3880Ma (in two samples) and ca. 3750Ma detrital zircons (in all three samples). The part of the ca. 3700Ma northern terrane in the Isua supracrustal belt is interpreted as lithologically diverse and imbricated assemblage of ca. 3700Ma arc-related volcanic rocks (boninitic, island arc tholeiite-picrite and andesite-dacite). In the associated metasedimentary rocks, the lack of >3750Ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800Ma crust. In contrast, the up to 3940Ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca. 3800Ma terrane, although dominated by arc-related rocks, might contain older (pre-3800Ma) crust. We propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks, and then acted as a décollement when the northern terrane ca. 3700Ma arc complex collided with the southern terrane by ca. 3660Ma (from previous age constraints). Thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca. 3700 and 3800Ma terranes in the Isua area.Some thin siliceous metasedimentary rocks along post-assembly mylonite zones in the Isua supracrustal belt yield some <3660Ma zircons. A tentative interpretation of these siliceous rocks (requiring confirmation) is that they could be younger, post-collisional sedimentary rocks, which were incorporated as tectonic lenses along some of the post-terrane-assembly <3660Ma mylonites which partition the collisional assemblage.

U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: Constraints on the age of volcanism and metamorphism

Zircons from a felsic volcanic rock in the Toija Formation and a synvolcanic gabbro intrusion in the Salittu Formation within the Orijarvi area were dated by U-Th-Pb SIMS in order to provide depositional constraints on these formations. Zircon crystals from the felsic rock preserve a two-stage crystallisation history with zoned core domains and homogeneous rim domains. Inner domains yield a 1878 ± 4 Ma concordia age, interpreted to determine the crystallisation of this rock. Rims yield a 1815 ± 3 Ma concordia age interpreted to determine the regional metamorphism. Small rounded zircon grains from the Salittu gabbro, located within the Jyly shear zone, yield a concordia age of 1792 ± 5 Ma. We interpret the grain textures to suggest that they recrystallised from inherited zircon seeds during the heat and fluid flow into the shear zone. Although no direct ages for the Salittu Formation have been recovered, field relationships imply that it was deposited between 1878 − 1875 Ma.

Zircon Compositions of Lower Mesozoic Redbeds of the Tethyan Margins, West-Central Mediterranean Area

Typologic study and chemistry of zircon populations from Middle Triassic-Lower Liassic arenitic continental redbeds of the Internal Domains successions of the Mediterranean chains, from the Gibraltar Arc to the Calabria-Peloritani Arc, allow assessment of whether this type of study may be used to obtain more comprehensive inferences on the sedimentary evolution of ancient clastic rocks. Calabria-Peloritani Arc zircons are primarily euhedral, with subordinate rounded and subrounded grains. Rifian Maghrebides samples contain abundant euhedral zircons, with minor rounded and ovoid zircons. Betic Cordillera zircons are rounded or subrounded, with minor euhedral elongate grains; rounded zircons of these samples cannot be typologically classified. Electron-microprobe analyses (Si, Zr, Hf, P, and HREE) indicate homogeneous compositions, suggestive of common source areas characterized by granitoid rocks, whereas typology suggests the existence of two distinct groups. The first group includes arenites from the Calabria-Peloritani Arc and the Rifian Maghrebides. The second group includes arenites from the Betic Cordillera. Differences between the two zircon groups may be related to recycling effects, in agreement with a geochemical study of Triassic to lowermost Jurassic mudrock redbeds. Recycling likely was more significant for the Betic Cordillera samples. These results suggest that zircon typology and mudrock geochemistry can be useful tools for provenance, especially in complex sedimentary contexts where arenite-mudrock couplets occur.

U-Pb Zircon and Sm-Nd isotopic study of the huangtuling granulite, dabie-sulu belt, China: Implication for the paleoproterozoic tectonic history of the yangtze craton

SIMS data demonstrate Archean (2.9 to 2.7 Ga) and Paleoproterozoic (1991 ± 43 Ma) ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from Huangtuling, Dabie-Sulu orogenic belt, China. The granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the Yangtze Craton. The rounded zircon cores show oscillatory zoning and have Th/U ratios (0.17 − 0.78) systematically higher than those of the unzoned luminescent rims (0.005 − 0.03). The age of the detrital zircon cores and whole rock Nd isotope model age suggest that the Huangtuling granulite was possibly derived from a provenance dominated by Archean rocks similar to the Kongling Complex exposed 200 km in the southwest. Multiple phases of metamorphism have been identified for the granulites, with the dominant granulite-facies mineral assemblage, defining an early clock-wise metamorphic P-T path. Orthopyroxene, plagioclase and whole-rock material define a linear trend in a ^147^Sm/^144^Nd − ^143^Nd/^144^Nd diagram, corresponding to an age of 1926 ± 146 Ma. This result and the U-Pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the Yangtze Craton at ∼2.0 Ga, inviting further study on whether the Yangtze Craton was involved in the assembly of the recently-proposed Paleo-Mesoproterozoic supercontinent Columbia.

Zircon geochronology and Sr, Nd, Pb isotope geochemistry of granitoids from Bayuda Desert and Sabaloka (Sudan): Evidence for a Bayudian event (920–900 Ma) preceding the Pan-African orogenic cycle (860–590 Ma) at the eastern boundary of the Saharan Metacraton

Results of zircon geochronology and Sr, Nd and Pb isotope investigations carried out on granitoid rocks from the boundary of the Saharan Metacraton with the Arabian–Nubian Shield (ANS) reveal a protracted, partly pre-Pan-African geodynamic evolution and the existence of crustal terranes of different ancestries. (Meta)granitoids and gneisses in north-central Bayuda Desert record a 920–900 Ma orogenic event, hitherto unrecognized in northeastern Africa. This early Neoproterozoic Bayudian event is restricted to a pre-Pan-African crustal terrane which appears only slightly affected by Pan-African tectogenesis and deformation, named here the Rahaba–Absol terrane. Within this terrane, amphibolite-facies metamorphism occurred at 921 ± 10 Ma in the El Melagi muscovite–biotite gneiss of the Rahaba Series, probably during a collisional phase; followed by the intrusion of the Absol medium-K granite–granodiorite pluton discordantly emplaced at 900 ± 9 Ma into high-grade schists and amphibolites of the metavolcanosedimentary Absol Series. Nd T DM model ages (2040–2430 Ma) and ages of rounded zircon cores (1060, 1980, 2540 and 2675 Ma) of the El Melagi gneiss indicate a predominantly late Archaean to Palaeoproterozoic source region and suggest a latest Mesoproterozoic to early Neoproterozoic depositional age of its pelitic precursor sediment. The post-collisional Absol pluton has isotope characteristics ( ɛ Nd 900 of −0.3 to −4.3, T DM model ages: 1300–1830 Ma, SrIR 900 of 0.7028–0.7055, 207Pb/ 204Pb of 15.66–15.81) indicating assimilation of old pre-Neoproterozoic crust. The Rahaba–Absol terrane is part of the Saharan Metacraton and is in tectonic contact with the high-grade metamorphic Kurmut terrane of eastern Bayuda Desert. The Kurmut terrane has juvenile Neoproterozoic isotope characteristics, its granitoids record only Pan-African orogenic events and it is therefore considered a part of the Arabian–Nubian Shield. Within the Kurmut terrane the Dam Et Tor medium-K epidote–biotite gneiss gave a concordant zircon age of 858 ± 9 Ma, interpreted as the crystallization age of the volcanic/subvolcanic precursor of the gneiss. Isotopic data of the Dam Et Tor gneiss ( ɛ Nd 858 of 5.7–6.1, T DM model ages: 860–900 Ma, SrIR 858 of 0.7026–0.7028, 207Pb/ 204Pb of 15.46) indicate a depleted mantle source and an oceanic arc magmatic environment. The age of the amphibolite-facies metamorphism and deformation of the Dam Et Tor gneiss and its enclosing metavolcanosedimentary Kurmut Series is best approached by a Sm–Nd isochron age (806 ± 19 Ma; Küster and Liégeois [Küster, D., Liégeois, J.P., 2001. Sr, Nd isotopes and geochemistry of the Bayuda Desert high-grade metamorphic basement (Sudan): an early Pan-African oceanic convergent margin, not the edge of the East Saharan ghost craton? Prec. Res., 109, 1–23]). Both the Rahaba–Absol and the Kurmut terrane of the Bayuda Desert are intruded by post-collisional high-K granitoid magmatism of late Pan-African age, with no intermediate events recorded. The An Ithnein pluton that intrudes the pre-Pan-African Rahaba–Absol terrane is dated at 597 ± 4 Ma. The isotope characteristics of this pluton ( ɛ Nd 600 of −1.5 to 2.4, T DM model ages: 910–1190 Ma, SrIR 600 of 0.7011–0.7038, 207Pb/ 204Pb of 15.59–15.69) and that of the Nabati pluton intruding the Kurmut terrane ( ɛ Nd 600 of 0.1–4.5, T DM model ages: 750–1050 Ma, SrIR 600 of 0.7032–0.7036, 207Pb/ 204Pb of 15.56–15.60) indicate a derivation from a juvenile crustal source or the mantle with limited assimilation of older crust. Post-collisional late Pan-African granite plutons at Sabaloka, south of Bayuda Desert, are distinguished into high-K and shoshonitic suites. The Banjedid high-K pluton was emplaced at 605 ± 4 Ma, while the Babados shoshonitic pluton intruded at 591 ± 5 Ma. Magmatic zircons from both plutons have rims with low Th/U ratios, suggesting a HT metamorphic event contemporaneous with magma emplacement. Both granitoid suites have isotopic characteristics ( ɛ Nd 600 of −8.3 to −2.1, T DM model ages: 1160–1560 Ma, SrIR 600 of 0.7029–0.7069, 207Pb/ 204Pb of 15.72–15.81) indicating the assimilation of older pre-Neoproterozoic crust. Sabaloka is thus clearly identified as a part of the Saharan Metacraton but is distinguished from the Rahaba–Absol terrane by a strong Pan-African high-grade metamorphic imprint [Kröner, A., Stern, R.J., Dawoud, A.S., Compston, W., Reischmann, T., 1987. The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan. Earth Planet. Sci. Lett. 85, 91–104] indicating the likely existence of a discrete Sabaloka terrane.

Petrografia de arenitos e minerais pesados de depósitos cretáceos (Grupo Itapecuru), Bacia de São Luís-Grajaú, norte do Brasil

Petrography, heavy minerals and quartz analyses, palaeocurrent data revaled the sedimentary story and potential source areas of sandstones of the Itapecuru Group in the southern margin of the Sao Luis-Grajau Basin. The studied deposits consist of quartzarenites with heavy mineral assemblage consisting of tourmaline, zircon, staurolite, rutile and kyanite with a wide variety of forms, mechanical surface textures, and colors. The predominance of rounded zircon and tourmaline grains suggests a strong contribution from recycled sediments from sedimentary and metassedimentary rocks. Quartz grains exhibit irregular forms, monocrystaline and polycrystalline types and surface textures similar to the heavy minerals grains. The modal composition in a Q-F-Lt plot suggests their origin from continental block and recycled orogen. The potential sources were mainly sedimentary rocks, as well as medium -to high-grade metamorphic and granitic rocks. The studied profile was subdivided into four stratigraphic levels A, B, C and D, through palaeocurrent data and RuZi index, which indicate change in source areas. Palaeocurrent pattern suggests that the Zone A, at the base of the succession, were sourced from the northern and northeastern regions, including the Sao Luis Craton, Gurupi Belt, and northwestern portion of the Borborema Province. Sediments of the zones B, C and D were supplied from the southern, southwestern and southeastern regions, including the Amazonian Craton, Araguaia Belt, and Borborema Province.

Sensitive high‐resolution ion microprobe U‐Pb ages of the Latest Oligocene‐Early Miocene rift‐related Hidaka high‐temperature metamorphism in Hokkaido, northern Japan

Abstract In order to define the timing of granulite facies metamorphism, sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206Pb/ 238U ages (n = 31) with low Th/U ratios, mostly &lt;0.1). The internal structure of zircons and their SHRIMP U‐Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene‐Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene‐Early Miocene Hidaka high‐temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back‐arc rifting of the Kuril and Japan basins.

Timing of crust formation, deposition of supracrustal sequences, and Transamazonian and Brasiliano metamorphism in the East Pernambuco belt (Borborema Province, NE Brazil): Implications for western Gondwana assembly

The main structural feature of the central domain of Borborema Province (NE Brazil) is a network of dextral and sinistral shear zones. These shear zones rework an older, regionally developed, flat-lying foliation in orthogneisses and supracrustal belts, which in the East Pernambuco belt was formed under amphibolite facies conditions. This study reports LA-ICP-MS U–Pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre-transcurrent tectonothermal evolution in the Eastern Pernambuco domain. Ages of 2125±7 and 2044±5Ma in a mafic layer of banded orthogneiss are interpreted as the age of the protolith of the orthogneiss and of high-grade Transamazonian metamorphism, respectively. The latter age is consistent with the occurrence of low Th/U, metamorphic zircon xenocrysts, dated at 2041±15Ma, in the leucosome of a migmatitic paragneiss. A granitic orthogneiss dated at 1991±5Ma reflects late to post-Transamazonian magmatic event. A similar age (1972±8Ma) was found in rounded zircon grains from a leucocratic layer of banded orthogneiss. Ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the Archean to Neoproterozoic, with peak ages at ca. 2220, 2060–1940, 1200–1150 and 870–760Ma. Detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665Ma. Magmatic zircons with the age of 626±15Ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the Brasiliano high-temperature metamorphism. A lower intercept age of 619±36Ma from a deformed granodiorite dated at 2097±5Ma and the crystallization age of 625±24Ma of the felsic layer of banded orthogneiss also confirm the late Neoproterozoic metamorphism. These results show that the present fabric in basement and supracrustal rocks was produced during the Brasiliano orogeny.Paleoproterozoic ages reported in this study are similar to those found in other sectors of the Borborema Province, the Cameroon and Nigeria provinces, and the São Francisco/Congo craton. They show the importance of the Transamazonian/Eburnean event and suggest that these tectonic units may have been part of a larger, single continental landmass. Likewise, similarities in post-Transamazonian metamorphic and magmatic events in the Borborema, Nigeria and Cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the Atlantic Ocean.