Absolute Dating of Permian Ash-Fall in the Rio Bonito Formation,Paraná Basin,Brazil

The Rio Bonito Formation in southern Paraná basin contains a set of tonsteins interbedded with coal-seams. These tonsteins are composed mainly of kaolinite with zircon, apatite and beta-quartz paramorphs as accessory minerals, and were interpreted as volcanic ashes deposited by ash falls over pits protected by barrier islands in a barrier-lagoon system. A U-Pb dating of zircons in the tonstein A, which furnished an age of 267.1 ± 3.4 Ma (Early Permian) confirming previous age-dates based on palynology and correlating them with one of the main periods of volcanic activity in the Gondwana.The source of the pyroclastic material was attributed to the early Permian Choiyoi magmatic arc in Argentina, developed during the Sanrafaelic orogeny, and with a main peak of volcanic activity between 260 and 272 Ma.     

Constraints on sedimentary ages of the Chuanlinggou Formation in the Ming Tombs,Beijing, North China Craton: LA-ICP-MS and SHRIMP U–Pb dating of detrital zircons

Detrital zircons in five sedimentary samples, MC1 to MC5, from the bottom of the Chuanlinggou Formation in the Ming Tombs District, Beijing, were dated with the LA-ICP-MS and SHRIMP U–Pb methods. Age spectra of the five samples show a major peak at 2500 Ma and a secondary peak at 2000 Ma, suggesting their provenances were mainly from the crystalline basement of the North China Craton and the Trans-North China Orogen. The youngest zircon has an age of 1673 ± 44 Ma, indicating that the Chuanlinggou Formation was deposited after this age. From sample MC4 to MC5, lithology changed from a clastic rock (fine-grained sandstone) to a carbonate rock (fine-grained dolomite), suggesting that the depositional basin became progressively deeper. The age spectrum of sample MC5 shows a major peak at 2500 Ma and a secondary peak at 2000 Ma. Sample MC4, which is stratigraphically lower than sample MC5, only had one peak at 2500 Ma. We conclude that there was a transgressive event when sediments represented by MC5 was deposited, and seawater carried ca. 2000 Ma clastic materials to the basin where the Chuanlinggou Formation was deposited, leading to the addition of ca. 2000 Ma detritus. Our research indicates that the source area for the sediments became more extensive with time. We conclude that the Chuanlinggou Formation in the Ming Tombs District was deposited in a low-energy mud flat sedimentary environment in the inter-supra tidal zone because it is mainly composed of silty mudstone and fine-grained sandstone with relatively simple sedimentary structures.     

Lithologic and structural controls on fluvial knickzones in basalts of the Paraná Basin,Brazil

Knickzones are common features along rivers on the basaltic plateaus of the Paraná Basin. According to current conceptual models, knickpoints are formed in massive basalts that have a high density of vertical joints. Vesicular–amygdaloidal basalts and those with horizontal joints tend to form reaches of low slope due to their lower resistance to erosion. However, field surveys revealed complexities in this general relationship. The research presented here sought to verify the controls on the genesis of knickzones in this type of geological environment. We studied a 61 km-long mixed bedrock–alluvial river. The longitudinal profile of the river was surveyed on a topographic map with 5 m contour intervals. Tectonic lineaments oriented transverse to the channel and longitudinal lineaments in which the river lies were identified from maps. A detailed field survey of the lithologic characteristics of the riverbed was also performed. The results show that knickzones may form in any litho-structural zone in the flood basalts. On the other hand, low slope zones are predominantly sculpted into vesicular–amygdaloidal basalts, which are less resistant to erosion. The fracture densities of vesicular–amygdaloidal basalts are similar in low slope zones and in knickzones (4.86 and 4.93 m/m², respectively). This indicates that knickzones in this type of basalt are not caused by higher resistance to erosion. Approximately 60% of the 18 knickzones identified are associated with tectonic lineaments, irrespective of the structural characteristics of the basalts. Vesicular–amygdaloidal basalt and/or basalt with horizontal joints allow the fastest knickzone migration and aid in the formation of convexities. Knickpoints in these basalts do not migrate, but erosion in the pools advances downstream and breaks the bedrock steps, thus increasing the slope. Massive basalt with vertical joints causes slower migration, and its presence at convexities indicates local uplift. Convex segments are only formed upstream of faults.     

A new U–Pb zircon age dating and palynological data from a Lower Permian section of the southernmost Paraná Basin,Brazil: Biochronostratigraphical and geochronological implications for Gondwanan correlations

Index species useful for correlations with the International Stratigraphical Column are rare or absent in the Pennsylvanian–Permian strata of the Paraná Basin in Brazil, preventing accuracy in geochronologic assignments. Besides, absolute datings are very scarce in comparison with other Gondwana basins. This paper presents palynological data from an outcrop on the surroundings of the Candiota coal mine, southmost Brazil, from several levels of the Rio Bonito and Palermo formations. The presence of certain index species of spore–pollen allowed the recognition of two Permian palynozones: the Vittatina costabilis and the Lueckisporites virkkiae zones. Furthermore, U–Pb in zircons from a volcaniclastic level interbedded in the coal strata of the former unit was analyzed through LA-MC-ICP-MS method, providing a new absolute age dating of 281.4 ± 3.4 Ma (Cisuralian, Early Permian). This dating is assumed as the oldest occurrence of the L. virkkiae Zone in Paraná Basin, which contains index species that are widespread in other Gondwana basins. A well distributed surface boundary occurs in this section also, allowing local and regional correlations. These new biostratigraphical and geochronological data are integrated, in order to offer a deep analysis on the stratigraphical significance for correlations across the Occidental Gondwana.     

New U–Pb ages and lithochemical attributes of the Ediacaran Rio Doce magmatic arc,Araçuaí confined orogen,southeastern Brazil

The Araçuaí orogen of southeastern Brazil, together with its counterpart located in Africa, the West Congo belt, formed through closure of a gulf connected to the Adamastor Ocean by the end of the Ediacaran and beginning of the Cambrian. Convergence of the margins of the gulf led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma on a continental basement mostly composed of Rhyacian orthogneisses. The Rio Doce arc mainly consists of tonalite-granodiorite batholiths, generally crowded with mafic to dioritic enclaves, and minor gabbronorite-enderbite-charnockite plutons, suggesting mixing processes involving crustal and mantle sources. We investigate the basement, magma sources and emplacement ages of the Rio Doce arc. Our data suggest the arc comprises three main granitic rock groups: i) Opx-bearing rocks mostly of enderbite to charnockite composition; ii) enclave-rich tonalite-granodiorite (ETG); and iii) enclave-poor granite-granodiorite with minor tonalite (GT). The Opx-bearing rocks are magnesian, calc-alkalic to alkali-calcic and metaluminous. Together, the ETG and GT rock groups range in composition from tonalite to granite, are metaluminous to slightly peraluminous, show a predominantly medium- to high-K, expanded calc-alkaline signature, and other geochemical and isotopic attributes typical of a pre-collisional volcanic arc formed on a continental margin setting. Mineralogical, chemical, and geochronological data suggest the involvement of HT-melting of granulitic (H₂O-depleted) sources of Rhyacian age for the generation of Opx-bearing granitic rocks, additionally to magma mixing and fractional crystallization processes. In conclusion, the studied rock groups of the Rio Doce arc were likely formed by interactions of mantle and crustal processes, in an active continental margin setting. These processes involved ascent of mantle magmas that induced partial melting on the continental basement represented by the Rhyacian gneisses.     

The Early Cretaceous Jacuí Group,a newly discovered volcaniclastic–epiclastic accumulation at the top of the Paraná Basin,southern Brazil

The presence of volcaniclastic rocks related to the silicic magmatism within the Serra Geral Formation has been a matter of long-standing debate. In this paper, we present extensive documentation that supports the presence and abundance of these rocks in the Jacuí Group, a newly discovered volcaniclastic and epiclastic accumulation in southern Brazil. The Jacuí Group is composed of two interfingered stratigraphic units, the Volta Alegre and Tupanciretã formations, and it represents the uppermost stratigraphic unit of the Paraná Basin. The Volta Alegre Formation is primarily composed of resedimented volcaniclastic tuffites, the pyroclasts which were sourced from the Santa Maria subgroup of the Palmas-type of the Serra Geral Formation. The Tupanciretã Formation is composed of fluvial and aeolian deposits transported towards the north–northwest. Deposition of the Jacuí Group began in the Early Cretaceous (∼132 Ma) and was coeval with the acidic volcanism of the Santa Maria subgroup. This group was deposited in a probable interior sag basin that represents either the beginning of the extension in the inner part of the continent that subsequently migrated to the east or the far-field impact of extensional processes that preceded the break-up of Gondwana and the opening of the South Atlantic Ocean.     

The Vendian passive continental margin in the southern Siberian Craton: geochemical and isotopic (Sr,Sm–Nd) evidence and U–Pb dating of detrital zircons by the LA-ICP-MS method

Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.     

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

Doklady Earth Sciences - 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...     

The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U–Pb ages of zircons from recent river sands

The southern Congo Craton is widely overlain by Meso- to Cenozoic sediments of the northern Kalahari Basin, which hamper any correlation of basement units. The latter are represented by the Archaean Angola and Kasai Blocks, while the southern cratonic margin is framed by several Meso- to Neoproterozoic orogenic belts. For provenance analysis of the sedimentary cover and reconstruction of the main zircon-forming events, we studied zircons from recent sediments of the largest rivers at the southern margin of the Congo Craton. U–Pb zircon ages suggest a major amount of the sediments to originate from E Lufilian and Kibaran Belts, while input from the S Damara Belt seems to increase to the W. Ages related to the Angola Block were only noticed in the westernmost parts of the working area, which is not in accordance with the SE-trending drainage pattern, proposed to have been onset during the Cretaceous. Thus, it is assumed that the Meso- to Cenozoic sedimentary cover extended further west than today prior to the Mesozoic to Neogene uplift of the Angola Block and that subsequent erosion exhumed the basement stepwise from west to east. A recurrent destabilisation of the southern margin of the Congo Craton at ~2.7, 1.9, 1.0 and 0.6 Ga is supposed to be represented by major peaks in the age distribution pattern of the total amount of concordant zircons. This is in accordance with similar studies in adjacent areas. Additionally, the obtained data fit well to several hypothesised major events during the supercontinent cycle.     

The Taihua group on the southern margin of the North China craton: further insights from U–Pb ages and Hf isotope compositions of zircons

The "Taihua Group" is a collective term for a series of old terranes scattered along the southern margin of the North China Craton. The timing of formation and thermal overprinting of the Taihua Group have long been contentious, and its relationship with the Qinling orogenic belt has been unclear. In this study, new data from integrated in-situ U–Pb dating and Hf isotope analysis of zircons from an amphibolite (from the Xiong’ershan terrane) and a biotite gneiss (from the Lantian-Xiaoqinling terrane) indicate that the Upper Taihua Group formed during the Paleoproterozoic (2.3–2.5 Ga) and thus was originally part of the southern edge of North China Craton, detached during the Mesozoic Qinling orogeny and displaced about 100 km north from its original location. This suggests that the Taihua Group became part of the tectonic terrane associated with the Qinling orogeny and now forms part of the overthrust basement section of the Qinling belt. Before the Qinling orogeny, the Taihua Group was metamorphosed at 2.1 Ga. The initial Hf-isotope compositions of zircons, together with positive εNd(t) values for the whole-rocks, imply that the original magmas were derived from a juvenile source with some assimilation of an Archean crustal component.     

Results of U–Pb dating of zircons from wehrlite of the platinum-bearing Feklistov massif (Shantar Archipelago,Russia)

The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.     

Sources of clastic material of Cretaceous–Paleogene deposits in the southwestern part of the Amur–Zeya Basin: results of dating U–Pb detrital zircons

The results of our study indicate that at the Late Cretaceous–Paleogene boundary in the southeastern part of the Amur–Zeya Basin, the sedimentation conditions changed drastically, namely, the change of provenance areas of debris. In the Maastrichtian, the clastic material was mainly transported from the Bureya–Jiamusi Superterrane and the volcanic–plutonic belts of Khingan–Okhotsk and East Sikhote–Alin located to the east: sedimentation occurred simultaneously with magmatic activity. During the Danian Stage, the major source of debris to the southern part of the basin was located to the south of the young mountain system of the Lesser Khingan (the uplifted part of the basement of the Songliao Block).     

Geochemical and Sr–Nd–Pb isotopic insight into the low-Ti basalts from southern Paraná Igneous Province,Brazil: the role of crustal contamination

ABSTRACT

The south hinge of the Torres Syncline in southernmost Brazil hosts a volcanic succession of pahoehoe and rubbly Gramado-type lavas belonging to the ~132 Ma Paraná–Etendeka Igneous Province. We evaluate the geochemical and Sr–Nd–Pb isotopic variations using local-scale stratigraphy in order to discuss the petrogenesis of lava flows in a single magma type and to estimate the role of crustal contamination and the potential contaminants involved. The geochemical and isotopic variations along the lava pile are not systematic, implying that the magma chamber could have undergone successive replenishments of basaltic magma. The process of crustal assimilation explains the high and widespread initial Sr isotopic ratios at 0.707798–0.715751 and the very low ε_Nd at ?8.36 to ?5.41, with associated Pb isotopic variations (18.42 < ²⁰⁶Pb/²⁰⁴Pb < 18.86; 15.65 < ²⁰⁷Pb/²⁰⁴Pb < 15.71; 38.62 < ²⁰⁸Pb/²⁰⁴Pb < 39.37). The magmatic evolution of the SCSH and LJ lava flows begins with the storage of mafic liquids during a short period in the shallow-level magma chamber, which allowed the magma ascent with composition of olivine basalts. The continuous fractional crystallization within the magma chamber coupled with variable assimilation degrees of distinct contaminants with Palaeoproterozoic and Neoproterozoic ages, in addition to significant contribution of magma recharge, led to magma ascent with basaltic andesite composition that displays at surface a simple pahoehoe morphology. The continuous magma recharge in the magma chamber coupled with higher assimilation degree allowed the formation of basaltic andesite lavas with more contaminated isotopic signatures that exhibit rubbly morphology at the surface. Differentiation process of liquids coupled with the highest assimilation degrees of distinct contaminants during longer time in a shallow-level magma chamber, which is distinct from that where SCSH and LJ magmas have been stored, led to the formation of andesites of the Morro da Cruz section that exhibit the most contaminated isotopic signatures.     

U–Pb ages of detrital zircons within the Inthanon Zone of the Paleo-Tethyan subduction zone,northern Thailand: New constraints on accretionary age and arc activity

U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from the Inthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated with Paleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngest detrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14 and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast, Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a Middle Triassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derived from a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was active during sedimentation. The data presented within this study provide information on the development of the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneath the Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arc magmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presence of conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of the Sukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Triassic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after the deposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatus in Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic, coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presence of abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived from discrete accretionary units in an original accretionary prism that was located along the western margin of the Sukhothai Arc.     

Zircon U–Pb age constraints on the Paleoproterozoic sedimentary basement of the Ediacaran Porongos Group,Sul-Riograndense Shield,southern Brazil

Thick quartzites record significant information on cratonic environments during long geological periods. The capacity to resist weathering and deformation turn the quartzite covers especially useful in the provenance studies of Precambrian basins. Provenance of 194 detrital zircon grains from two samples of thick quartzite cover on the Paleoproterozoic Encantadas Complex displays mostly Paleoproterozoic (95%) and minor Archean (5%) sources. The results indicate that sediments were derived from the La Plata Craton with the maximum depositional age at 2.03 Ga possibly up to 1.7 Ga. In comparison, the adjacent Porongos Group has provenance data of 61 detrital zircon grains indicating mostly Mesoproterozoic (69%), subordinately Paleoproterozoic (26%) and minor Archean ages (5%). Considering previous published data, the Porongos Group is Ediacaran in age and probably chronocorrelated with sedimentary basins from the Tandilia Belt (Argentina). Therefore, the quartzite cover and the Porongos Group require distinct evolution in time and in tectonic environment.     

Zircon U–Pb ages of rocks from the Rio Apa Cratonic Terrane (Mato Grosso do Sul,Brazil): New insights for its connection with the Amazonian Craton in pre-Gondwana times

This paper presents 14 zircon U–Pb determinations (SHRIMP and LA-MC-ICP-MS) for key geological units from the Rio Apa Cratonic Terrane (RACT), which is considered the southernmost exposed part of the Amazonian Craton in southwestern Brazil. The zircon U–Pb ages and geological data indicate that the RACT was formed by the accretion of magmatic arcs in a continental margin active from 1950 to 1720 Ma. The RACT is composed of three major terranes (Western, Eastern and Southeastern Terranes) with distinct evolution histories. The Western Terrane presents orthogneisses and granites formed at ~ 1950–1940 Ma and subduction-related granites and volcanic rocks formed at 1900–1880 Ma and 1840–1830 Ma. These basement rocks are covered by a greenschist facies metavolcano-sedimentary succession (Rio Naicata Formation) with basal volcanic rocks formed at 1813 ± 18 Ma. A gabbronoritic dyke of the Rio Perdido Suite hosted by the Rio Naitaca Formation yields an age of 1589 ± 44 Ma. The Eastern and Southeastern Terranes present deformed leucogranites generated within the intervals 1780–1720 Ma and 1810–1790 Ma, respectively. Both terranes are covered by a metavolcano-sedimentary succession (Alto Tererê Formation) dominated by Barrovian-type amphibolite facies metamorphic assemblages, suggestive of a collisional event. Available ⁴⁰Ar–³⁹Ar data (hornblende, muscovite and biotite) indicate that the proto-RACT evolved to a collisional orogen between 1310 and 1270 Ma and behaved as a cratonic mass after 1270 Ma, preceding the assembly of Rodinia. The available data allow us to interpret the RACT as a part of the Ventuari–Tapajós Province of the Amazonian Craton, which was fragmented and dispersed as a microcontinent. It was subsequently reincorporated into the SW Amazonian Craton, along the Sunsás Belt, as an allochthonous terrane. In a global perspective, the tectono-magmatic events of the RACT are consistent with a long-lived accretionary orogen possibly related to an active margin of Columbia.     

Detrital zircons U–Pb SHRIMP ages and provenance of La Modesta Formation,Patagonia Argentina

This paper summarizes the geology of the Paleozoic La Modesta Formation in Patagonia, Argentina, and presents new SHRIMP U–Pb dating of detrital zircons from muscovite-chlorite schist and tourmalinite. Also complementary geochemical and lead isotopic data are presented, indicating that the protoliths were formed from upper crustal rocks by the contribution of a large input from recycled (or felsic) sources. The maximum age of sedimentation of La Modesta Formation is about 446 ± 6 Ma. The basin closure (or eventually a paleocurrent shift) occurs at Lower Devonian before the exhumation of the Middle-Devonian granitoids of the Rio Deseado Complex (Deseado Massif). Many of the detrital zircons are igneous and record Ordovician ages, with a prominent Lower Ordovician-age peak at approximately 473 Ma. Most favourable candidates to provide the younger zircons in the basin would Ordovician granites of the Rio Deseado Complex (Deseado Massif) and Punta Sierra Plutonic Complex (Somun Cura Massif). Older zircons have peaks of different importance (including Brasiliano and Grenvillian ages) between 530 and 700, 750–1500, 1750–2000 and 2550–2700 Ma. La Modesta Formation is also a potential area of materials (detrital zircon) to the basin where the rocks of the Eastern Andean Metamorphic Complex and equivalent formations of the Andean region were generated.     

U–Pb zircon ages,geochemical and Sr–Nd–Pb isotopic constraints on the dating and origin of intrusive complexes in the Sulu orogen,eastern China

TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.7083 to 0.7093, low ?_Nd(t) values from ?14.6 to ? 19.2, ²⁰⁶Pb/²⁰⁴Pb = 16.65–17.18, ²⁰⁷Pb/²⁰⁴Pb = 15.33–15.54, and ²⁰⁸Pb/²⁰⁴Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.     

Permian–Carboniferous arc magmatism in southern Mexico: U–Pb dating,trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

Undeformed felsic to mafic igneous rocks, dated by U–Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive εHf values (+3.8–+8.5) and depleted mantle model ages (using a mean crustal value of ¹⁷⁶Lu/¹⁷⁷Hf = 0.015) (T _DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative εHf values (?1 to ?14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous–Permian magmatic arc that extended from southern North America to Central America.     

U–Pb ages and Lu–Hf isotopes of detrital zircons from the southern Qinling Orogen: Implications for Precambrian to Phanerozoic tectonics in central China

The Qinling Orogen, central China, was constructed during the Mesozoic collision between the North China and Yangtze continental plates. The orogen includes four tectonic units, from north to south, the Huaxiong Block (reactivated southern margin of the North China Craton), North Qinling Accretion Belt, South Qinling Fold Belt (or block) and Songpan Fold Belt, evolved from the northernmost Paleo-Tethys Ocean separating the Gondwana and Laurentia supercontinents. Here we employ detrital zircons from the Early Cretaceous alluvial sediments within the Qinling Orogen to trace the tectonic evolution of the orogen. The U–Pb ages of the detrital zircon grains from the Early Cretaceous Donghe Group sediments in the South Qinling Fold Belt cluster around 2600–2300 Ma, 2050–1800 Ma, 1200–700 Ma, 650–400 Ma and 350–200 Ma, corresponding to the global Kenorland, Columbia, Rodinia, Gondwana and Pangaea supercontinent events, respectively. The distributions of ages and εHf(t) values of zircon grains show that the Donghe Group sediments have a complex source comprising components mainly recycled from the North Qinling Accretion Belt and the North China Craton, suggesting that the South Qinling Fold Belt was a part of the united Qinling–North China continental plate, rather than an isolated microcontinent, during the Devonian–Triassic. The youngest age peak of 350–200 Ma reflects the magmatic event related to subduction and termination of the Mian-Lue oceanic plate, followed by the collision between the Yangtze Craton and the united Qinling–North China continent that came into existence at the Triassic–Jurassic transition. The interval of 208–145 Ma between the sedimentation of the Early Cretaceous Donghe Group and the youngest age of detrital zircons was coeval with the post-subduction collision between the Yangtze and the North China continental plates in Jurassic.