Cenomanian-? early Turonian minimum age of the Chubut Group,Argentina: SHRIMP U–Pb geochronology

Four new SHRIMP U–Pb zircon ages older than 93 Ma from samples of the two uppermost formations accumulated in two different depocenters (Golfo de San Jorge and Cañadón Asfalto basins) of the Chubut Group in central Argentinean Patagonia, establish a pre-late Cenomanian-? early Turonian age for the group. It also confirms a coeval and comparable evolution of the two depocenters, where distal pyroclastic material was deposited together with fluvial and lacustrine facies.     

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.     

A new U–Pb zircon age and a volcanogenic model for the early Permian Chemnitz Fossil Forest

International Journal of Earth Sciences - The Chemnitz Fossil Forest depicts one of the most completely preserved forest ecosystems in late Paleozoic Northern Hemisphere of tropical Pangaea. Fossil...     

Combined zircon and cassiterite U–Pb dating of the Piaotang granite-related tungsten–tin deposit,southern Jiangxi tungsten district,China

The large low-grade Piaotang W–Sn deposit in the southern Jiangxi tungsten district of the eastern Nanling Range, South China, is related to a hidden granite pluton of Jurassic age. The magmatic-hydrothermal system displays a zonation from an inner greisen zone to quartz veins and to peripheral veinlets/stringers (Five-floor zonation model). Most mineralization is in quartz veins with wolframite > cassiterite. The hidden granite pluton in underground exposures comprises three intrusive units, i.e. biotite granite, two-mica granite and muscovite granite. The latter unit is spatially associated with the W–Sn deposit.Combined LA-MC-ICP-MS U–Pb dating of igneous zircon and LA-ICP-MS U–Pb dating of hydrothermal cassiterite are used to constrain the timing of granitic magmatism and hydrothermal mineralization. Zircon from the three granite units has a weighted average ²⁰⁶Pb/²³⁸U age of 159.8 ± 0.3 Ma (2 σ, MSWD = 0.3). The cathodoluminescence (CL) textures indicate that some of the cassiterite crystals from the wolframite-cassiterite quartz vein system have growth zonations, i.e. zone I in the core and zone II in the rim. Dating on cassiterite (zone II) yields a weighted average ²⁰⁶Pb/²³⁸U age of 159.5 ± 1.5 Ma (2 σ, MSWD = 0.4), i.e. the magmatic and hydrothermal systems are synchronous. This confirms the classical model of granite-related tin–tungsten mineralization, and is against the view of a broader time gap of >6 Myr between granite magmatism and W–Sn mineralization which has been previously proposed for the southern Jiangxi tungsten district. The elevated trace element concentrations of Zr, U, Nb, Ta, W and Ti suggest that cassiterite (zone II) formed in a high-temperature quartz vein system related to the Piaotang granite pluton.     

SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit,southeastern Mt Isa Block,Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.     

U–Pb SHRIMP II age and origin of zircon from lhertzolite of the bug Paleoarchean complex,Ukrainian Shield

Complex study of the U–Pb and Lu–Hf systems of zircon from a lhertzolite lens of Archean gneiss enderbites of the Bug complex, Ukrainian Shield, showed that ultramafic magma was contaminated by the material of the country gneiss enderbites. The age of the zircons of 2.81 ± 0.05 Ga corresponds to the period of ultramafic magmatism within the Bug complex. Previously, this peak of endogenic activity was considered the stage of manifestation of metamorphism and magmatism of mafic composition.     

First SHRIMP U–Pb zircon dating of granulites from the Kontum massif (Vietnam) and tectonothermal implications

The Kontum massif in Central Vietnam represents the largest continuous exposure of crystalline basement of the Indochina craton. The central Kontum massif is chiefly made of orthopyroxene granulites (enderbite, charnockite) and associated rocks of the Kannack complex. Mineral assemblages and geothermobarometric studies have shown that the Kannack complex has severely metamorphosed under granulite facies corresponding to P–T conditions of 800–850°C and 8±1 kbars. Twenty-three SHRIMP II U–Pb analyses of eighteen zircon grains separated from a granulite sample of the Kannack complex yield ca 254 Ma, and one analysis gives ca 1400 Ma concordant age for a zoned zircon core. This result shows that granulites of the Kannack complex in the Kontum massif have formed from a high-grade granulite facies tectonothermal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from ∼850°C at ca 254 Ma to ∼300°C at 242 Ma, with an average cooling rate of ∼45°C/Ma.     

Sedimentological characteristics and new detrital zircon SHRIMP U–Pb ages of the Babulu Formation in the Fohorem area,Timor-Leste

Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin.     

A Late Ordovician U–Pb age for the Tromsø Nappe eclogites,Uppermost Allochthon of the Scandinavian Caledonides

The Scandinavian Caledonides contain the record of several high-pressure events reflecting distinct episodes of collision and subduction in the course of the global Caledonian plate reorganization process. In this study, the timing and speed of one of these events in the Tromsø Nappe of the Uppermost Allochthon are detailed using multiple U–Pb geochronometers. This unit contains eclogites, the largest of which forms a whole mountain top, whereas many others occur as smaller lenses enclosed within a metamorphosed supracrustal sequence. A minimum age for the sedimentation is provided by a zircon age of 493 +5/-2 Ma for an eclogitized felsic intrusion. Formation of the eclogite, at pressures reaching 2.8 GPa, occurred at 452.1±1.7 Ma as evidenced by U–Pb in eclogitic zircon. Similar ages of 451–450 Ma are also provided by high-Al titanite in eclogite and titanite in leucosome veins, the latter of which was formed by partial melting during the exhumation of the eclogite. An age of 449 Ma for a rutile porphyroblast in another vein further confirms the rapidity of this high-pressure process. Matrix rutiles in two other eclogites yielded ages of 436 Ma and younger, probably indicating partial resetting during a subsequent metamorphic overprint. Lead isotopic compositions with high ²⁰⁷Pb/ ²⁰⁴Pb ratios are indicative of old crustal sources, thus supporting the previously proposed notion that the Uppermost Allochthon was derived from the Neoproterozoic margin of Laurentia.     

Evaluating igneous sources of the Taveyannaz formation in the Central Alps by detrital zircon U–Pb age dating and geochemistry

Late Palaeogene syn-tectonic volcanic products have been found in the Northern Alpine foreland basin and in the South Alpine hemipelagic basin. The source of abundant volcanic fragments is still in debate. We analyzed the geochronology and geochemistry of detrital zircons, and evaluated their temporal and genetic relationships with potential volcanic sources. The study shows that the detrital zircon U–Pb age patterns have two major age groups: a dominance (ca. 90%) of pre-Alpine zircons was found, as commonly observed in other Alpine flysch formations. These zircons apparently derived from erosion of the early Alpine nappe stack in South Alpine and Austroalpine units. Furthermore, a few Neo-Alpine zircons (ca. 10%) have ages ranging from Late Eocene to Early Oligocene (~ 41–29 Ma). Both source materials were mixed during long riverine transport to the basin margins before being re-deposited by gravity flows. These Palaeogene ages match with the activity of Peri-Adriatic magmatism, including the Biella volcanic suite as well as the Northern Adamello and Bergell intrusions. The values of REE and ¹⁷⁶Hf/¹⁷⁷Hf_(t) ratios of the Alpine detrital zircons are in line with the magmatic signatures. We observe an in time and space variable supply of syn-sedimentary zircons. From late Middle Eocene to Late Eocene, basin influx into the South Alpine and Glarus (A) basins from the Northern Adamello source is documented. At about 34 Ma, a complete reorganisation is recorded by (1) input of Bergell sources into the later Glarus (B) basin, and (2) the coeval volcaniclastic supply of the Haute-Savoie basin from the Biella magmatic system. The Adamello source vanished in the foreland basin. The marked modification of the basin sources at ~ 34 Ma is interpreted to be initiated by a northwestern shift of the early Alpine drainage divide into the position of the modern Insubric Line.     

Late Triassic–Late Cretaceous accretion/subduction in the Taiwan region along the eastern margin of South China – evidence from zircon SHRIMP dating

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma Talun metagranite, the oldest granitic intrusion ever reported in the Taiwan region and along the eastern coast area of South China. In spite of a large age uncertainty, the occurrence of this metagranite is not consistent with the apparent younging trend of Jurassic‐Cretaceous igneous activity toward the coastline in South China, and should be taken into consideration by future studies.     

U–Pb zircon age of the base of the Ediacaran System at the southern margin of the Qinling Orogen

Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive δ¹³C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deep-water basinal cap carbonate section with strong negative δ¹³C values in the southern margin of the Qinling Orogen, Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma (1σ, MSWD_CE = 0.31, Probability_CE = 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides a geochronological constraint for the Ediacaran successions in the Qinling Orogen.     

Petrology and U–Pb zircon dating of coesite-bearing metapelite from the Kebuerte Valley,western Tianshan,China

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + coesite + phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.     

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc-alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U–Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic–volcanoclastic succession belonging to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in conjunction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean-type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle–Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.     

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.     

Implications of U–Pb SHRIMP zircon data on the age and evolution of the Felbertal tungsten deposit (Tauern Window, Austria)

U–Pb SHRIMP analyses of zircons from various lithologies and ore bodies of the Felbertal scheelite deposit (western and eastern ore field) and neighbouring areas allow the reconstruction of the pre-Alpine magmatic and metamorphic processes responsible for the tungsten mineralization. The ore deposit belongs to the Magmatic Rock Formation, which is tectonically squeezed between the Habach Phyllite Formation and the Basal Schist Formation (all members of the Habach Group). In both the eastern and western ore field, the pre-mineralization geological processes are marked by the emplacement of basalts (547±27?Ma). Ensialic back-arc extension provided pathways for gabbroic and pyroxenitic melts as well as normal "I-type" granitoids (minimum crystallization age of 529±18?Ma). The rock assemblage forms a magmatic arc on an approximately 2?Ga continental Gondwana (?) margin. Post-emplacement tectonism and metamorphism have converted the basalts to fine-grained amphibolites, the gabbroic and pyroxenitic rocks to coarse-grained amphibolites and hornblendites, and the granitoids to leucocratic orthogneisses, respectively. Tungsten mineralization is intimately related to small patches and dikes of differentiated granitoids in the eastern ore field and the K2 ore body in the western ore field. The granitic melts have supposedly been generated by ongoing differentiation of calcalkaline magmas. They cut the older lithologies and intruded along the same pathways as the earlier melts. Fluids have been carried up along a major line in the eastern ore field. They caused the formation of an elongate ore body with a scheelite-quartz stockwork zone (scheelite-bearing quartz veinlets and veins) and an overlying, likewise elongate, 900-m-long, scheelite-rich quartzite lens. In the western ore field, accompanying fluids produced the K2 ore body. In this ore body, an eruption breccia occurs above a mineralized quartzite. The breccia (younger than 529±18?Ma) contains mineralized quartzite clasts as well as barren fine-grained amphibolite clasts and leucocratic orthogneiss-clasts that are similar to the surrounding host rock equivalents. The quartzite, which represents the main mineralization stage of the K2 ore body, is unsuitable for dating. However, the scheelite-rich quartzite lens of the eastern ore field is probably coeval. This lens locally lies on top of a differentiated and strongly mineralized gneiss. The crystallization age of this gneiss is 529±17?Ma, and marks the peak of tungsten input in the eastern ore field. Small, differentiated granitic dikes, which cut both the K2 eruption breccia and the K2 quartzite in the western ore field, contain only minor scheelite and mark a decrease in mineralization at 519±14?Ma. Thus, a period between 530 and 520?Ma and a setting between magmatic arc and (ensialic) back-arc may properly explain the likely scenario for the primary tungsten input (stage-1 scheelite) by differentiated granitic melts of calcalkaline character. Surprisingly, a second stage-2 scheelite formation was induced in the western ore field by a Variscan granite intrusion (K1–K3 gneiss; 336±19?Ma), the emplacement time of which is pre-dated by a cross-cutting dacitic dike of 340±5?Ma. This mineralization, which occurs in small quartz veins and within a quartz aureole atop the intrusion as well as an even younger mineralization in shear zones (yellowish-fluorescent stage-2 scheelite porphyroblasts), is bracketed between 355?Ma (the upper age limit of the K1–K3 gneiss precursor) and 335?Ma (the lower age limit of the dacitic dike, which is stage-2 scheelite free). Supposedly, long-lasting Variscan (amphibolite facies) metamorphic conditions till 282±2?Ma extended the scheelite remobilization. They caused a further dispersion of scheelite and induced the growth of individual grains and of rims around older grains (bluish-fluorescent stage-3 scheelite). The Alpine metamorphism of lower amphibolite to upper greenschist facies conditions caused a further, minor scheelite remobilization, especially along some faults and quartz veins, including sparse, but large, whitish-bluish-fluorescent crystals (stage-4 scheelite).     

New U–Pb SHRIMP zircon age for the Schurwedraai alkali granite: Implications for pre-impact development of the Vredefort Dome and extent of Bushveld magmatism,South Africa

The Schurwedraai alkali granite is one of a number of prominent ultramafic-mafic and felsic intrusions in the Neoarchaean to Palaeoproterozoic sub-vertical supracrustal collar rocks of the Vredefort Dome, South Africa. The alkali granite intruded the Neoarchaean Witwatersrand Supergroup and has a peralkaline to peraluminous composition. A new zircon SHRIMP crystallization age of 2052 ± 14 Ma for the Schurwedraai alkali granite places it statistically before the Vredefort impact event at 2023 ± 4 Ma and within the accepted emplacement interval of 2050–2060 Ma of the Bushveld magmatic event. The presence of the alkali granite and associated small ultramafic-mafic intrusions in the Vredefort collar rocks extends the southern extremity of Bushveld-related intrusions to some 120 km south of Johannesburg and about 150 km south of the current outcrop area of the Bushveld Complex. The combined effect of these ultramafic-mafic and felsic bodies may have contributed to a pronouncedly steep pre-impact geothermal gradient in the Vredefort area, and to the amphibolite-grade metamorphism observed in the supracrustal collar rocks of the Vredefort Dome.     

New insights into peri-Gondwana paleogeography and the Gondwana super-fan system from detrital zircon U–Pb ages

We present a synopsis of detrital zircon U–Pb ages of sandstones from North Africa and neighboring Israel and Jordan, which allows us to identify zones with characteristic sediment provenance along the northern Gondwana margin (in present-day coordinates) in Cambrian–Ordovician times, and helps us to unravel the peri-Gondwana jigsaw puzzle. A special feature of the early Paleozoic cover sequence of North Africa is the eastward increase of 1.1–0.95 Ga detrital zircons, which become ubiquitous in the early Paleozoic sandstones of the Saharan Metacraton. Detrital zircons aged about 2.7–2.5, 2.15–1.75 and 0.75–0.53 Ga are also present. Early Paleozoic sandstones with similar provenance are known from peri-Gondwana terranes in the Eastern and Western Mediterranean and from NW Iberia. These terranes need not be transported from western Gondwana (Amazonia) as suggested previously. They were likely located to the north of the Saharan Metacraton during the early Paleozoic before they rifted off from Gondwana. Furthermore, we recognize an increase, as stratigraphic ages get younger, of ca. 1.0 Ga detrital zircons at some point between the Late Cambrian and late Middle Ordovician. We speculate that this might be linked to far-field tectonics and regional uplift in central Gondwana related to plate-tectonic reorganization along the Gondwana margin, leading to erosion of ca. 1.0 Ga basement and country rocks of the Transgondwanan supermountain and fluvial dispersal of detritus toward the Gondwana margin.     

The Pacific Gondwana margin in the late Neoproterozoic–early Paleozoic: Detrital zircon U–Pb ages from metasediments in northwest Argentina reveal their maximum age,provenance and tectonic setting

U–Pb detrital zircon ages are reported from Puncoviscana Formation (late Neoproterozoic–Early Cambrian) and Mesón Group (Late Cambrian) greywackes of northwest Argentina, to constrain provenance and depositional environment.The new data are combined with previously-published detrital zircon ages, to show that Puncoviscana Formation age patterns contain two broad groups: late Mesoproterozoic–early Neoproterozoic (1150–850 Ma) and late Neoproterozoic–Early Cambrian (650–520 Ma); with their relative proportions varying inversely with youngest component age. The 1150–850 Ma age components are dominant in greywackes with oldest late Neoproterozoic components > 600 Ma. The former diminish considerably when late Neoproteozoic components become dominant and younger, to 520 Ma. A northernmost greywacke sample from Purmamarca, Jujuy, is distinctive: whilst its zircon age pattern partly resembles other Puncoviscana Formation samples, it contains no Cambrian–late Neoproterozoic ages, the youngest ages being early Neoproterozoic. This may reflect an early, Neoproterozoic, passive-margin depocentre for the Formation, or an older (early Neoproterozoic) succession within it, which may predate the Brasiliano orogeny in Brazil. The youngest age components, c. 520 Ma, in a greywacke from Rancagua (Cachi, Salta province), dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late Early Cambrian depocentre. Detrital zircon age patterns of the Mesón Group (Lizoite Formation) have major Cambrian–latest Neoproterozoic components resembling those of the Puncoviscana Formation, but its Mesoproterozoic component is diminished, and there are no significant age components of this age. Small youngest components at c. 500 Ma suggest a maximum Late Cambrian stratigraphic age. The Puncoviscana Formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised, extensive late Mesoproterozoic orogen (with minor Paleoproterozoic and Archean precursors), and a more variable late Neoproterozoic orogen containing an evolving sequence of less extensive subcomponents. A direct relationship with the Brazilian Shield is suggested; with sediment supplies originating within active-margin orogens of the interior and collisional orogens at the suture between African and South American cratons, but ultimate deposition in passive-margin environments of western Gondwanaland.