Zircon Morphology Research Articles

Tracing southern Gondwanan sedimentary paths: A case study of northern Namibian late Palaeozoic sedimentary rocks

AbstractThe northern Namibian Karoo‐aged successions are part of a Gondwana‐wide sedimentary system emerging at the Carboniferous–Permian boundary and existing for more than 50 Ma. The Karoo Supergroup sedimentary successions are of importance in understanding the evolution of the Karoo rift system. This study presents new whole‐rock geochemical data combined with detrital zircon morphology as well as U−Pb ages and Lu−Hf composition of late Palaeozoic siliciclastic rocks of the Namibian Huab Basin and Kunene area (south‐west Africa). Inferred by youngest detrital zircon U−Pb ages the Verbrande Berg Formation (lower Ecca Group) yields a Sakmarian to Asselian maximum depositional age, whereas the overlying Tsarabis Formation yields an Artinskian maximum depositional age. These ages coincide with the end of the Dwyka ice age and an overall warming and a contemporaneous evolution of the Karoo I rift system across southern Gondwana. The zircon age distribution of the investigated samples yields clusters ranging between ca 500 to 650 Ma (Cambrian–late Neoproterozoic), ca 950 to 1200 Ma (early Neoproterozoic–Mesoproterozoic) and ca 1800 to 1900 Ma (Palaeoproterozoic). Their rounded shapes characterize the zircon grains of the Kunene area and the lower Huab Basin section, whereas upper Huab Basin strata yield mostly unrounded grains. The rounded nature of zircon grains with a diverse U−Pb age spectrum putatively points towards sediment homogenization and multiple recycling stages during the deposition of the sediments and large catchment areas of the depositing rivers. As suggested by zircon grains with a low roundness value and a single Palaeoproterozoic age cluster, the upper Huab Basin successions were probably deposited under drier climatic conditions, small catchment areas and limited sedimentary homogenization. Therefore, the southern Gondwana sedimentary transport and homogenization system may change over time and is dependent on the climate prevailing during deposition. This study shows that the laws of detrital zircon are very complex and are yet to be explored.

Ultrahigh-temperature granites and a curious thermal eye in the post-collisional South Bohemian batholith of the Variscan orogenic belt (Europe)

Abstract Comprehensive zircon thermometry that takes into account zircon saturation temperatures, Ti-in-zircon measurements, and zircon morphologies and microstructures can provide key information on the thermal evolution of a granite batholith. The Variscan South Bohemian batholith (Germany, Austria, and Czech Republic) comprises a series of granitoid units that intruded between ca. 330 and ca. 300 Ma. We categorize the granitic rocks according to their emplacement temperature into very low temperature (T) (VLT; <750 °C), low T (LT; 750–800 °C), medium T (MT; 800–850 °C), high T (HT; 850–900 °C), and ultrahigh T (UHT; >900 °C). The first stage of batholith formation (ca. 330–325 Ma) is characterized by LT to MT melting of mainly metasedimentary sources driven by their isothermal exhumation. In turn, ca. 322 Ma HT and UHT granites in the southern half of the batholith reveal an ephemeral thermal anomaly in the subbatholithic crust, which is presumably linked to a hidden mafic intrusion. The HT and UHT granites are weakly peraluminous, high-K, I-type rocks. Although sharing some features with A-type granites such as high Zr and rare earth element contents, they differ from classical A-type granites in being magnesian, not enriched in Ga over Al, and having high Ba and Sr contents. A ring structure of ca. 317 Ma MT and/or LT plutons is observed around the HT and/or UHT granite complex and interpreted as an aftermath of the hotspot event. This study is an example of how deep-crustal hotspots, presumably caused by mantle magmatism, can significantly enhance the effects of decompressional crustal melting in a post-collisional setting.

Paleozoic evolution and heterogeneity of sediment provenance in the Permian Basin

Paleozoic sediment provenance of the Permian Basin remains controversial. This study presents new detrital zircon morphology, UPb ages, and Hf isotope data of eight Paleozoic sandstone samples, and integrates published zircon UPb data in the basin to illustrate the spatial and temporal changes of sediment provenance. The Cambrian sample has predominantly Mesoproterozoic (1500–1300 Ma) grains because of fluvial transport of basement detritus from the local Texas Arch. The Ordovician shoreface sandstone contains mostly rounded grains of Paleoproterozoic (1800–1600 Ma) age, mostly likely by longshore drift reworking detritus from the Transcontinental Arch. The Pennsylvanian sediment provenance shows heterogeneity. While samples from the Northwestern Shelf have both the Mesoproterozoic and Paleoproterozoic groups and low grain roundness, reflecting sediment delivery from basement rocks in the Ancestral Rocky Mountains, a sample in the Midland Basin contains a broad range of ages, including a major Neoproterozoic (800–500 Ma) group that were primarily from peri-Gondwana terranes. The difference suggests that there was limited sediment mixing in the Permian Basin. The similarity of zircon age distributions between the Midland Basin and Fort Worth Basin confirms synorogenic progradation of detritus from the Fort Worth Basin to the Eastern Shelf during the Pennsylvanian. During the early Permian Cisuralian epoch, the compiled UPb data show that the southern and eastern basin contains a significant fraction of Gondwanan detritus, and the amount reduced by the latest Cisuralian as the orogenesis waned. The middle Permian Guadalupian samples on the Northwestern Shelf contain well-rounded grains that are of mainly Grenvillian and Paleozoic ages, suggesting sediment recycling from Laurentia by wind and likely by river as well. The change of sediment provenance from the early to the middle Permian suggests that after the continental collision, Laurentian sediments dominated detritus in the Permian Basin.

Uplift and exhumation in the Tianshan, western China: New insights from detrital zircon morphology and thermochronology

Uplift and exhumation in the Tianshan, western China: New insights from detrital zircon morphology and thermochronology

Age, provenance and geological significance of (meta)–sedimentary rocks in the Yitong–Gongzhuling area, NE China: Constraints from zircon U–Pb geochronology

The Permo–Triassic was a crucial interval for the Changchun–Yanji suture zone in NE China since it witnessed the late–stage tectonic evolution of the Paleo–Asian Ocean (PAO). Thus, provenance analysis of Triassic sedimentary rocks in this region can shed light on the eventual closure process of the PAO. In this study, we carried out petrological, geochemical, and detrital zircon U–Pb geochronological research on two (meta)–sedimentary rocks from the Yitong–Gongzhuling area located in the westernmost section of the Changchun–Yanji suture zone. LA–ICP–MS U–Pb ages of detrital zircons from the two samples indicate a middle Triassic maximum depositional age (~ 240 Ma). The two samples present a most significant Permo–Triassic (299–240 Ma) detrital zircon population that corresponds to the multi–phase subduction and collision–related magmatism along the Changchun–Yanji suture zone. In addition, a late Silurian–early Devonian (429–390 Ma) detrital zircon population is also present and presumably fed from a pre–existing accreted arc terrane along the northern margin of the North China Craton (NCC). In contrast to some previously reported Permo–Triassic sedimentary rocks in adjacent areas, our new results show an absence of ~ 2.5 and ~ 1.8 Ga ages characteristic of NCC magmatism or metamorphism. The results imply that the Yitong–Gongzhuling area probably has a higher relief than the adjacent area during the middle Triassic, preventing the deposition of materials from the NCC to the south. This is further evidenced by the proximal orogenic source, short–distance transportation, and rapid accumulation for the studied samples as indicated by the subhedral to anhedral and poor roundness morphology of detrital zircons. The inferred regional uplift was hypothesized to be caused by the continued transpression during the PAO final closure based on the widely distribute early–middle Triassic syn–collisional granitic rocks along the Changchun–Yanji suture zone.

Discovery of Early Paleozoic eclogite-facies metamorphic rocks in the western part of North Qinling Orogen and its geological significance

西秦岭地处中国中央造山系东西转换衔接部位，随着东秦岭以及祁连、柴北缘和东昆仑早古生代高压-超高压变质岩石的陆续发现和深入研究，西秦岭造山带变质作用研究吸引了大家的普遍关注。本文在大范围野外地质调查基础上，在北秦岭造山带西段天水南部的秦岭岩群长英质片麻岩中发现了一套石榴子石斜长角闪岩（榴闪岩），并开展了详细的锆石形貌和内部结构、微区微量元素和U-Th-Pb同位素研究。CL图像显示榴闪岩锆石普遍具有核-幔-边或核-边结构，部分存在原岩残留锆石。定年结果得到榴闪岩原岩残留锆石年龄为710±52Ma，同时得到497±3Ma、452±3Ma和423±7Ma三期变质年龄。其中497±3Ma变质年龄来自锆石核部并显示出轻稀土亏损、重稀土平坦且没有明显负Eu异常的稀土配分曲线特征，表明该时期的矿物组合中有石榴子石但没有斜长石，与榴辉岩相变质锆石特征一致；452±3Ma变质年龄来自锆石幔部或边部，对应测点重稀土元素的分异加大，并出现弱的负Eu异常，说明此时石榴子石被消耗且出现少量斜长石；423±7Ma变质年龄来自锆石的最边部，对应稀土配分曲线表现出更明显的负Eu异常和更大的重稀土元素分异特征，指示此时岩石中石榴子石含量更少，斜长石含量更多。上述结果表明该榴闪岩可能经历了~500Ma的榴辉岩相变质作用，并在~450Ma和~420Ma叠加了两期退变质改造。天水地区榴闪岩无论是其野外产状，还是变质锆石的形貌和内部结构、稀土配分曲线特征及其所记录的原岩和三期变质年龄都与北秦岭造山带东段大陆俯冲型高压-超高压变质榴辉岩近乎一致，指示早古生代时期，北秦岭造山带西段与东段经历了相似的大陆（深）俯冲和折返过程，它们共同构成一条统一的早古生代高压-超高压变质岩带。;The western part of North Qinling Orogen (NQO) is located at the conjunction area of several Paleozoic high pressure-ultrahigh pressure (HP-UHP) belts in the central China continent, the North Qinling UHP belt in the east and the North Qaidam and the East Kunlun UHP belts in the west, thus its metamorphism has drawn broad scientific interest. We document here an integrated study of zircon morphology, microstructure, in-situ trace element compositions and U-Th-Pb dating on a newly discovered garnet amphibolite in southern Tianshui, the western part of NQO. The garnet amphibolite occurs as a lensoid block in the felsic gneiss of the Qinling Complex, and mainly consists of amphibolite (50%~60%), garnet (10%~15%), quartz (~10%), plagioclase (5%~10%), muscovite (~5%) and ilmenite (~3%). Zircons from the garnet amphibolite display obvious core-mantle-rim or core-rim structures with a few protolith zircon relicts. LA-ICP-MS U-Pb dating yielded a protolith age of 710±52Ma and three metamorphic ages of 497±3Ma, 452±3Ma and 423±7Ma. The 497±3Ma metamorphic cores display flat heavy rare earth elements (HREE) patterns without obvious Eu anomalies, indicating they were formed in presence of garnet but absence of plagioclase under eclogite-facies condition; The 452±3Ma metamorphic mantles or rims show shallow HREE slopes with moderately negative Eu anomalies, indicating that these zircons grew accompanied by initial appearance of plagioclase and decomposition of garnet; The 423±7Ma metamorphic rims exhibit steep HREE patterns and marked negative Eu anomalies, indicating massive crystallization of plagioclase and decomposition of garnet during their formation. These results suggest that the garnet amphibolite probably experienced eclogite-facies metamorphism at~500Ma and two stages retrograde overprint at ~450Ma and ~420Ma. The occurrence, zircon microstructures, protolith and multiple metamorphic ages of the garnet amphibolite from Tianshui area are basically consistent with the continental-type HP-UHP rocks of the eastern part of NQO. Therefore, we suggest that both western and eastern part of NQO experienced similar continental deep subduction and exhumation processes and thus they form a united Early Paleozoic HP-UHP metamorphic belt.

Implications for sedimentary transport processes in southwestern Africa: a combined zircon morphology and age study including extensive geochronology databases

Extensive morphological and age studies on more than 4600 detrital zircon grains recovered from modern sands of Namibia reveal complex mechanisms of sediment transport. These data are further supplemented by a zircon age database containing more than 100,000 single grain analyses from the entire southern Africa and allow for hypothesising of a large Southern Namibian Sediment Vortex located between the Damara Orogen and the Orange River in southern Namibia. The results of this study also allow assuming a modified model of the Orange River sand highway, whose origin is likely located further south than previously expected. Moreover, studied samples from other parts of Namibia give first insights into sediment movements towards the interior of the continent and highlight the potential impact of very little spatial variations of erosion rates. Finally, this study points out the huge potential of detrital zircon morphology and large geo-databases as an easy-to-use additional tool for provenance analysis.

Genesis and age of Pb−Zn mineralization from the Ningi-Burra ring complex, North Central Nigeria: Constraints from zircon morphology, U−Pb dating and Lu−Hf isotopes

The Ningi-Burra Pb−Zn mineralization forms part of the Nigerian Sn-Nb ± (Pb-Zn-Mo)-rich anorogenic alkaline province, also known as the Nigerian Younger Granite Complexes. Six distinct E-W trending magmatic centres have been identified in the Ningi-Burra complex, but the Pb−Zn mineralization is confined to highly evolved biotite granite in Koluki (centre 6). In this paper, a combination of geochronological and geochemical data of magmatic and hydrothermally-altered zircon from the Ningi-Burra complex are used to constrain magmatic and hydrothermal processes that facilitated the formation of Pb−Zn mineralization. Zircon trace element and Hf isotopes (εHf(t) = −5.3 to −10.3) indicate that the parental magma of these ore-bearing granites were largely derived from lower crustal sources with contributions from the upper mantle. The hydrothermal alteration that accompanied the Pb−Zn mineralization include silicification, chloritization and limited sodic metasomatism. Zircon U/Pb dating reveal that magma emplacement and subsequent hydrothermal process in Ningi-Burra complex occurred at (~192 Ma) and (<191 Ma), respectively. Zircon grains from unmineralized and hydrothermally altered rock suites have similar 176Hf/177Hf but distinct 176Lu/177Hf and 176Yb/177Hf ratios which suggest a magmatic source for the hydrothermal fluids responsible for the formation of the mineralization. This study highlights the application of zircon trace element and U-Th-Hf isotopes as robust tools for constraining magmatic and mineralization process that facilitate the formation of economically significant mineralization in highly differentiated granites.

Zircon provenance of Quaternary cover beds using U–Pb dating: Regional differences in the Southwestern USA

AbstractU–Pb dating of detrital zircons (DZ) in Quaternary slope deposits (cover beds) and examination of the surface structure of single zircon grains may enable reconstruction of the provenance of their aeolian components and help to reconstruct sedimentary palaeo‐transportation cascades. We distinguish several layers of slope deposits (cover beds) using a palaeosol‐supported sequence‐stratigraphic approach. In a pilot study, we demonstrate the usability of using end‐member‐modelled analyses of grain sizes to identify the aeolian matter, multi‐dimensional scaling and density functions (probability density functions (PDF) and Kernel density estimations (KDE)) of the U–Pb data, and the grain morphology of DZ, and show that the age and appearance variation of DZ in the Great Basin differ remarkably from those of the Colorado Plateau.All samples contain aeolian matter. Density functions of the DZ ages show similarities within each of the two regions. The Great Basin samples are dominated by late Cretaceous and Paleogene zircons, which are assumed to derive from volcanism. In contrast, the Colorado Plateau samples are almost free of these contributions. Presumably, the difference is due to palaeolakes, which were frequent in the Great Basin only, whose sediments contain a similar spectrum of DZ ages. Zircon morphology indicates that most grains have a history of aeolian transportation, whereas the grains assumed to be of volcanic origin often do not show similar transportation marks; this indicates that their transportation was dominated by the eruptive process but fewer steps of a transportation cascade. Multidimensional scaling of age data defines clusters of samples with similar DZ ages and – compared with published ages from possible sources – allows first interpretations of provenance.Quaternary aeolian sediments have not been studied in, and the applied mix of methods has not been applied to, the southwestern USA before. We demonstrate their feasibility for this study area.

Complex Characteristic of Zircon from Granitoids of the Verkhneurmiysky Massif (Amur Region)

The study presents a complex characteristic of zircon from the Verkhneurmiysky intrusive series with Li-F granites. A wide range of morphological and chemical properties of zircon allowed us to obtain new information on the formation and alteration of zircon from biotite and zinnwaldite granitoids and to determine its features, which contribute to the correct definition of Li-F granites formed directly before the tin mineralization. The reviled trends of zircon morphology and composition evolution in the Verkhneurmiysky granites series are: the high-temperature morphotypes are followed by low-temperature ones with more complicated internal structure with secondary alteration zones, mineral inclusions, pores, and cracks; the increasing concentration of volatile (H2O, F), large ion lithophile (Cs, Sr), high field strength (Hf, Nb) and rare-earth elements with decreasing crystallization temperatures and the determining role of the fluid phase (predominantly, F) in the trace element accumulation. The composition of zircon cores in biotite and zinnwaldite granites is very similar. However, the zircon rims from zinnwaldite granites are much more enriched in trace elements compared to those from biotite granites. The first study of zircon from the Verkhneurmiysky granitoids provides new data on the formation and alteration conditions of granitoids, including zinnwaldite ones.

Tectonic exhumation of the Central Alps recorded by detrital zircon in the Molasse Basin, Switzerland

Abstract. Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin in Switzerland are well studied, yet they lack robust geochronologic and geochemical analysis of detrital zircon for provenance tracing purposes. Here, we present detrital zircon U–Pb ages coupled with rare-earth and trace element geochemistry to provide insights into the sedimentary provenance and to elucidate the tectonic activity of the central Alpine Orogen from the late Eocene to mid Miocene. Between 35 and 22.5 ± 1 Ma, the detrital zircon U–Pb age signatures are dominated by age groups of 300–370, 380–490, and 500–710 Ma, with minor Proterozoic age contributions. In contrast, from 21 Ma to ∼ 13.5 Ma (youngest preserved sediments), the detrital zircon U–Pb age signatures were dominated by a 252–300 Ma age group, with a secondary abundance of the 380–490 Ma age group and only minor contributions of the 500–710 Ma age group. The Eo-Oligocene provenance signatures are consistent with interpretations that initial basin deposition primarily recorded unroofing of the Austroalpine orogenic lid and lesser contributions from underlying Penninic units (including the Lepontine dome), containing reworked detritus from Variscan, Caledonian–Sardic, Cadomian, and Pan-African orogenic cycles. In contrast, the dominant 252–300 Ma age group from early Miocene foreland deposits is indicative of the exhumation of Variscan-aged crystalline rocks from the Lepontine dome basement units. Noticeable is the lack of Alpine-aged detrital zircon in all samples with the exception of one late Eocene sample, which reflects Alpine volcanism linked to incipient continent–continent collision. In addition, detrital zircon rare-earth and trace element data, coupled with zircon morphology and U∕Th ratios, point to primarily igneous and rare metamorphic sources. The observed switch from Austroalpine to Penninic detrital provenance in the Molasse Basin at ∼ 21 Ma appears to mark the onset of synorogenic extension of the Central Alps. Synorogenic extension accommodated by the Simplon fault zone promoted updoming and exhumation the Penninic crystalline core of the Alpine Orogen. The lack of Alpine detrital zircon U–Pb ages in all Oligo-Miocene strata corroborate the interpretations that between ∼ 25 and 15 Ma, the exposed bedrock in the Lepontine dome comprised greenschist-facies rocks only, where temperatures were too low for allowing zircon rims to grow, and that the Molasse Basin drainage network did not access the prominent Alpine-age Periadriatic intrusions located in the area surrounding the Periadriatic Line.

A Novel Approach to Identifying Mantle‐Equilibrated Zircon by Using Trace Element Chemistry

AbstractOne of the requirements for inferring mantle redox states via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry is to select mantle‐equilibrated zircon derived from melts that have not interacted with the hydrosphere nor assimilated exogenous materials. Traditional protocols for identifying mantle‐equilibrated zircon require a comprehensive examination of whole rock geochemistry in addition to zircon morphology, texture, and trace element and isotopic measurements. This study proposes a simple filter scheme for selecting mantle‐equilibrated zircon using trace element systematics alone (P, Ti, Y, Nb, rare earth elements [REE], Hf, Th, and U) to provide the geological context of zircon from three perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and (3) zircon saturation context. The filter scheme presented here, comprising of 13 criteria, is established based on detailed classifications of 2,173 zircon analyses from 30 independent references and is shown to distinguish between nonmagmatic zircon, magmatic zircon with significant inclusions and/or sourced from highly enriched source melts, and mantle‐equilibrated zircon. The filter scheme is validated with known, well‐characterized mantle‐equilibrated Jack Hills zircon (n = 53) and derives an average mantle redox state of (sd) circa 4,400 Ma, in agreement with previously published results. Further application of the filter scheme to zircon dataset (n = 76) from three metasedimentary rocks successfully reveals the mantle‐equilibrated zircon in the dataset, and implies a mantle redox state of (sd) circa 2,950 Ma. This filter scheme can be applied to studies of out‐of‐context detrital and/or xenocrystic zircon in the future.

Geochemical discrimination of igneous zircon in the Gawler Craton, South Australia

Zircon from spatially and temporally distinct igneous rock units across the Gawler Craton, Australia, show subtle differences in trace and rare earth element ratios. These igneous suites range in composition from granite, rhyolite to gabbro and originate from the ca 2450 Ma Sleaford Complex, the ca 1850 Ma Donington Suite, the ca 1633–1608 Ma St Peter Suite, the ca 1595–1587 Ma Gawler Range Volcanics and the ca 1595–1575 Ma Hiltaba Suite granites. The geochemical characterisation of zircon is carried out on relatively unaltered samples in order to establish primary signatures and to understand the processes controlling the geochemical signatures of the igneous suites. A range of zircon morphologies and internal textures that are preserved in these igneous suites infer the geological history of the host rock, including metamorphism and multiple episodes of zircon saturation suggesting a complex magma evolution. Despite differences in whole-rock geochemistry between igneous suites, zircon largely overlaps geochemically. However, subtle differences in Th/U, U/Hf and (Nb + Ta)/P ratios are evident between the Donington Suite, Gawler Range Volcanics and Hiltaba Suite. This is likely due to the tectonic setting, composition of the crystallising magma and timing of zircon crystallisation in the magma evolution. The subtle geochemical differences in zircon show the potential of using these ratios to aid in provenance studies with large populations of zircon that are of similar age. However, as the dataset is limited and focused only on unaltered samples, further work needs to be undertaken to look at the altered counterparts of these igneous suites. KEY POINTS Zircon recovered from unaltered samples of spatially and temporally distinct igneous suites in the Gawler Craton is used here. Th/U, U/Hf and (Nb + Ta)/P ratios of zircon can discriminate the Donington Suite, Gawler Range Volcanics and Hiltaba Suite. Zircon morphologies, internal textures and geochemical characteristics are a value aid to provenance studies.

Morphological and geochemical features of zircon from intrusive rocks of El Sela area, Eastern Desert, Egypt

The intrusive rocks in El Sela area can be arranged from the oldest to the youngest into: two-mica granite and postgranitic dikes which include microgranite, dolerite and bostonite dikes. Zircon is the most abundant accessory mineral. Zircon morphology and geochemical features are good indicators for evolution of rocks. The aim of the work is to determine the morphology, internal structure and chemical composition of zircon to identify the difference of zircon in various intrusive rocks. Results show that morphologically, zircon in the two-mica granite is euhedral coarse- grained with zonation. It is represented by crystals up to 125 µm and corresponds to S10 and P2. Zircon in post-granitic dikes exhibit irregular forms. Geochemically, zircon crystals have higher ZrO2 values in the core whereas HfO2 , UO2 , ThO2 increase at the peripheries of zoned crystals of the two-mica granite. Zircon of two-mica granite contains high HfO2 , UO2 , ThO2 and CaO contents but low Sc2 O3 content. HfO2 is not detected in zircon of microgranite. TiO2 in zircon of two-mica granite and bostonite dikes is under detection limits. REEs are not recorded in zircon of the studied intrusive rocks

Provenance analysis of Devonian peripheral foreland basins in SW Gondwana, case of the Gualilán Group, Precordillera Argentina

We carried out a sedimentary provenance analysis of the Devonian Gualilan Group, Central Precordillera of western Argentina, focusing on the siliciclastic record of the Talacasto (Lower Devonian) and the Punta Negra (Lower-Middle Devonian) formations. Provenance is determined based on petrography, heavy minerals, whole-rock geochemistry, Sm–Nd isotopes, and U–Pb detrital zircon dating. Sandstones are composed of quartz, feldspar, and metamorphic and plutonic lithoclasts. The Gualilan Group underwent moderate weathering indicated by CIA values and Th/U ratios below 76 and 4.9, respectively. Th/Sc, Zr/Sc, Cr/V and La/Th ratios, negative Eu-anomalies and REE patterns point to felsic source compositions. The heavy minerals and zircon morphologies indicate that high-grade metamorphic and igneous rocks provided the bulk of detritus. TDM ages between 1.40 and 1.46 Ga and eNd(t) values ranging from − 9.21 to − 11.13 constrain the signature of the sources. U–Pb detrital zircon ages for the Talacasto Formation are equally distributed between the Famatinian (Late Cambrian–Late Devonian), Pampean–Brazilian (Neoproterozoic–Early Cambrian) and Grenvillian-Sunsas (Mesoproterozoic) orogenic cycles. Detrital zircon ages of the Punta Negra Formation were dominantly supplied from Mesoproterozoic rocks, linked to the Grenvillian-Sunsas orogeny. The data indicate a provenance from rocks located eastwards of the basin represented by the basement of the Pampean Ranges. The peripheral foreland basin developed during the post-collisional regime that followed the accretion of Cuyania against Gondwana. Whereas eastern and western Pampean Ranges provided detritus to the Talacasto Formation, the Punta Negra Formation was sourced by the western Pampean Ranges, implying exhumation and erosion of such basement during the Devonian.

Zircon formation in mafic and felsic rocks of the Bushveld Complex, South Africa: Constraints from composition, zoning, Th/U ratios, morphology, and modelling

Zircon is a potential petrogenetic indicator that can be used to derive physicochemical conditions during magma crystallization. In this study, such conditions are obtained from zircon of both felsic and mafic rocks of the Bushveld Complex (BC), which are characterized by a wide range in bulk-rock Zr contents (4–552 ppm). For that, information from bulk-rock compositions, petrography, zircon trace element data and morphologies are combined with results of thermodynamic modelling using the software packages rhyolite-MELTS and Perple_X. In felsic rocks (Lebowa Granite, Rashoop Granophyre), zircon is formed in rutile-free assemblages together with olivine-clinopyroxene-amphibole-biotite-ilmenite-titanite-apatite after ≥20% fractional crystallization at 761–935 °C (mean: 860 °C), based on Ti-in-zircon thermometry using aTiO2 = 0.3, in agreement with independent geothermometers and modelling results. The resulting zircon populations show {100}- and {101}-dominated morphologies as well as high ƩREE (mean: 651 ppm) and low Ti contents (mean: 9.5 ppm), and only minor zoning in Th/U (0.3–0.8) and Nb/Ta (1.2–4.4). Identical zircon characteristics in gabbros and diorites of the Upper Zone within the Rustenburg Layered Suite (RLS) suggest admixing of felsic melts during ingression of mafic parental magmas. In contrast, zircons in mafic rocks of the lower RLS (Basal Ultramafic Sequence to lower Main Zone) show significantly lower ƩREE (mean: 324 ppm), commonly higher Ti contents (8–60 ppm), as well as large variations in Ti, U, Th contents, Th/U (0.2–24) and Nb/Ta ratios (0.15–18) as well as in zircon morphology. These zircons mostly occur in rutile-bearing intercumulus domains associated with orthopyroxene-biotite-amphibole-plagioclase-quartz-rutile and are formed at 690–962 °C (mean: 835 °C; aTiO2 = 1.0) based on Ti-in-zircon thermometry. These temperatures are in good agreement with zircon morphologies, but mostly higher than those obtained by rhyolite-MELTS modelling, suggesting zircon growth at <810 °C after >75% fractional crystallization of high Mg andesitic (B1) and tholeiitic parental magma (B2). These lower temperatures perhaps result from oversimplified modelling parameters or may reflect variable mixing of parental magma with evolved resident magma. Zircon populations in rutile-bearing mafic rocks of the lower RLS reveal two distinct zoning trends: an early trend at high Ti (>20 ppm), characterized by increasing Th/U (0.5 to 18) at decreasing U (175 to 10 ppm) from core to rim and a reverse trend at lower Ti contents. Both trends require zircon formation in intercumulus melt pockets at high zircon/melt ratios. The high-Ti trend can be explained by Rayleigh-like fractionation due to zircon growth together with rutile, both having highly different partition coefficients for U ≫ Th. The low-Ti trend results from zircon growth after onset of the biotite-in reaction, causing breakdown of previously formed rutile, thereby releasing U but no Th. The absence of pronounced Th/U zoning of zircon in felsic rocks reflects zircon growth in less fractionated melts, resulting in ThU fractionation compensated by coeval crystallization of abundant rock-forming minerals, all being highly incompatible for Th and U.

Drainage response to Arabia–Eurasia collision: Insights from provenance examination of the Cyprian Kythrea flysch (Eastern Mediterranean Basin)

AbstractThe Cenozoic geodynamics of the north‐eastern Mediterranean Basin have been dominated by the subduction of the African Plate under Eurasia. A trench‐parallel crustal‐scale thrust system (Misis–Kyrenia Thrust System) dissects the southern margin of the overriding plate and forms the structural grain and surface expression of northern Cyprus. Late Eocene to Miocene flysch of the Kythrea (Değirmenlik) Group is exposed throughout northern Cyprus, both at the hanging‐wall and foot‐wall of the thrust system, permitting access to an extensive Cenozoic sedimentary record of the basin. We report the results of a combined examination of detrital zircon and rutile U–Pb geochronology (572 concordant ages), coupled with Th/U ratios, Hf isotopic data and quantitative assessment of grain morphology of detrital zircon from four formations (5 samples) from the Kythrea flysch. These data provide a line of independent evidence for the existence of two different sediment transportation systems that discharged detritus into the basin between the late Eocene and late Miocene. Unique characteristics of each transport system are defined and a sediment unmixing calculation is demonstrated and explained. The first system transported almost exclusively North Gondwana‐type, Precambrian‐aged detrital zircon sourced from siliciclastic rock units in southern Anatolia. A different drainage system is revealed by the middle to late Miocene flysch sequence that is dominated by Late Cretaceous–Cenozoic‐aged detrital zircon, whose age range is consistent with the magmatic episodicity of southeast Anatolia, along the Arabia–Eurasia suture zone. Deposition of these late Miocene strata took place thereupon closure of the Tethyan Seaway and African–Eurasian faunal exchange, and overlap in time with a pronounced uplift of eastern Anatolia. Our analytical data indicate the onset of prominent suture‐parallel sediment transport from the collision zone of south‐eastern Anatolia into the Kyrenia Range of northern Cyprus, marking the drainage response to the continental collision between Arabia and Eurasia.

Detrital zircon constraints on late Paleozoic tectonism of the Bogda region (NW China) in the southern Central Asian Orogenic Belt

The Chinese North Tianshan (CNTS) in the southern part of the Central Asian Orogenic Belt (CAOB) has undergone multistage accretion-collision processes during Paleozoic time, which remain controversial. This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U–Pb dating and Lu–Hf isotopic analyses of detrital zircons. New detrital zircon U–Pb ages (N ​= ​519) from seven samples range from 261 ​± ​4 ​Ma to 2827 ​± ​32 ​Ma. The most prominent age peak is at 313 ​Ma and subordinate ages vary from 441 ​Ma to 601 ​Ma, with some Precambrian detrital zircon ages (~7%) lasting from 694 ​Ma to 1024 ​Ma. The youngest age components in each sample yielded weighted mean ages ranging from 272 ​± ​9 ​Ma to 288 ​± ​5 ​Ma, representing the maximum depositional ages. These and literature data indicate that some previously-assumed “Carboniferous” strata in the Bogda area were deposited in the Early Permian, including the Qijiaojing, Julideneng, Shaleisaierke, Yangbulake, Shamaershayi, Liushugou, Qijiagou, and Aoertu formations. The low maturity of the sandstones, zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS. The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc. Zircon ɛHf(t) values have increased since ~408 ​Ma, probably reflecting a tectonic transition from regional compression to extension. This event might correspond to the opening of the Bogda intra-arc/back arc rift basin, possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean. A decrease of zircon ɛHf(t) values at ~300 ​Ma was likely caused by the cessation of oceanic subduction and subsequent collision, which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous.

Oceanic Zircon as a Petrogenetic Indicator

Abstract —We present results of a study of the morphology, internal structure, and chemical composition of oceanic zircon, which show that zircon is a sensitive indicator of tectonic and physicochemical processes occurring in the lower oceanic crust. Crystallization of magmatic zircon in gabbroids is not an instantaneous process; it proceeds in the course of differentiation of parental melts in the tectonically active mid-ocean ridge (MOR) setting. The main geochemical indicator of crystallization differentiation during magma cooling is an increase in Hf content toward the zircon grain edge. Zoning is also observed in magmatic zircons from oceanic plagiogranites (OPG), but it is weaker, apparently because of the narrower temperature range of zircon crystallization in these rocks. The OPG zircons are depleted in REE as compared with the least altered magmatic zircons of gabbro, which is probably due to the formation of OPG during the partial melting of gabbro with the participation of concentrated water–salt fluid, a derivate of seawater, and due to the co-crystallization of zircon and apatite. High-temperature hydrothermal processes within slow-spreading MORs lead to a partial or complete recrystallization of zircon as a result of dissolution/redeposition. A significantly reduced cerium anomaly and the presence of microinclusions of xenotime, uranium and thorium oxides or silicates, and, sometimes, baddeleyite in zircon alteration zones indicate a reducing type and high alkalinity of the hydrothermal fluid. The fluid, a derivate of seawater, acquires these features during circulation near the axial zone of ridges as a result of phase separation in the system H2O–NaCl and interaction of the fluid with abyssal peridotites of oceanic core complexes. The estimated solubility of zircon in basic melts indicates that even near-solidus crystallization of zircon is highly unlikely in anhydrous basaltic melts but is possible in differentiates of deep-seated hydrous basic magmas. The Ti-in-Zrn geothermometer must be used with caution, because variations in the Ti content in zircon might be controlled not only by temperature but also by other factors, especially when mineral inclusions in zircon testify to a drastic change in its growth (dissolution) conditions. A geothermometer based on the distribution of Zr and Hf between zircon and the host rock has several advantages over indicators of the crystallization temperature of magmatic zircon that are based on the zircon saturation index and the content of Ti in zircon. It does not depend on the composition of melt and on the correct estimates of the SiO2 and TiO2 activity. In addition, reconstruction of the Zr and Hf fractionation trends during crystallization of zircon from granitoid melts makes it possible to evaluate the temperature of separation of more differentiated melt fractions.

Evidence of mingling between contrasting magmas in the Ribeirão do Óleo Pluton, Coastal Terrane and the tectonic implications on the Ribeira Belt, Brazil

Gneiss–migmatitic rocks and granites between the Cubatao and Itariri shear zones characterize the Mongagua Complex-Coastal Terrane. Three major units are defined: (1) the Itariri Suite (IS) composed of monzogranites, granodiorites, tonalites, and locally gneiss–migmatitic rocks; (2) the Areado pluton composed of monzogranite and locally tonalite; and (3) the Ribeirao do Oleo Pluton mainly composed of monzogranites, which is the subject of this article. Combined field observations, petrography, Sr–Nd–Hf isotopes, U–Pb zircon data, and preliminary geochemical studies are presented for the Ribeirao do Oleo Pluton (ROP), intrusive in granite-gneisses of the IS. It is a plutonic association of weakly peraluminous, medium-to-high-K biotite monzogranites that surround microgranular intermediate enclaves, which are products of magma mingling. Different from the most examples of magma mingling described in the literature, the ROP mingling did not result from the intrusion of hot mafic magma into colder host felsic magma. Conversely, the emplacement of the monzogranite started most likely during the medium-to-final stage of crystallization of the quartz microdiorite. A detailed study of LA-ICP-MS U–Pb zircon ages, including zircon morphology and internal structure, was useful to determine more precisely the emplacement sequence and the relation between the units of the ROP. Intermediate and acid facies have Paleoproterozoic and Neoproterozoic inheritances, moreover a mixture of xenocrysts, antecrysts, and autocrysts, indicative of a complex evolution of the ROP. Several magma crystallization pulses are recorded at 655–624 Ma, 627–601 Ma, and 613–586 Ma. The negative eHf(t) ranging from − 17.26 to − 12.51 in monzogranites, eNd(t) values of − 10.66 to − 7.83 at t = 600 Ma in both facies, 87Sr/86Sr of ca. 0.708 for the monzogranite and 0.706 for the quartz microdiorite indicate a strong crustal contribution in the genesis of intermediate and acid magmas of this pluton.