Youngest Zircon Age Research Articles

Age and emplacement of the Permian–Jurassic Menghai batholith, Western Yunnan, China

ABSTRACTThe Menghai batholith (Yunnan Province, China) is the southern extension of the ~370 km long Lincang granite body that syntectonically intruded the collisional zone between Gondwana (Baoshan block) and Laurasia (Simao block) terranes during closure of the Palaeo-Tethyan Ocean. Eight Menghai granodiorites were analysed across an ~45 km E–W transect from the pluton’s central region to eastern perimeter. Each rock was imaged in cathodoluminescence and geochemically analysed for major and trace elements. A minimum 30 zircons per sample were dated using laser ablation inductively coupled plasma–mass spectrometry. Samples are peraluminous to strongly peraluminous, magnesian, calcic or calc-alkalic granodiorites. Trace element suggest a high pressure (12–15 kbar) low clay source with >20–30% volume interaction with basalt. Crustal anatexis was likely related to post-collisional lithosphere delamination and upwelling of hot asthenosphere, forming large-volume melts. Zircon ages (207Pb–206Pb and 238U–206Pb) range from 3234 ± 42 to 171.7 ± 5.4 Ma (±2σ). Inherited zircon ages include the Palaeoarchaean–Neoarchaean (average 2938 ± 27 Ma, n = 8 ages), Lüliang (2254 ± 38 Ma, n = 7), Changcheng–Jixianian (1274 ± 47 Ma, n = 33), Qinbaikou (963 ± 29 Ma, n = 7), Nanhua (787 ± 24 Ma, n = 7), Sinian (595.4 ± 12.2 Ma, n = 14), Qilian (452.2 ± 8.7 Ma, n = 24) and Tienshan (358.9 ± 12.4 Ma, n = 5). The presence of these ages decrease from the batholith’s central portion (>50% ages) to eastern perimeter (2–16% ages), as the rocks appear progressively metamorphosed. The distribution of U/Th ratio suggests inherited zircons are Carboniferous (317.6 ± 5.7 Ma) and older and crystallization ages span the Permian to Early Jurassic. The average and youngest zircon age per sample decreases from the centre of the batholith to its eastern perimeter, from 226.8 ± 8.8 and 210.7 ± 3.3 to 211.8 ± 5.7 and 171.0 ± 5.4 Ma, respectively. If recorded by syntectonic zircon crystallization, collision and closure of a branch of the Palaeo-Tethyan Ocean occurred here over an ~100 million years time period from the Permian (281.0 ± 13.0 Ma) to Jurassic (171.5 ± 5.4 Ma).

U‐Th Zircon Dating by Laser Ablation Single Collector Inductively Coupled Plasma‐Mass Spectrometry (LA‐ICP‐MS)

Zircon crystals in the age range of ca. 10–300 ka can be dated by 230Th/238U (U‐Th) disequilibrium methods because of the strong fractionation between Th and U during crystallisation of zircon from melts. Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of nine commonly used reference zircons (at secular equilibrium) and a synthetic zircon indicates that corrections for abundance sensitivity and dizirconium trioxide molecular ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. When corrected for abundance sensitivity and interferences, mean activity ratios of (230Th)/(238U) for nine reference zircons analysed on five different days averaged 0.995 ± 0.023 (95% confidence weighted by data‐point uncertainty only, MSWD = 1.6; n = 9), consistent with their U‐Pb ages > 4 Ma that imply equilibrium for all intermediate daughter isotopes (including 230Th) within the 238U decay chain. U‐Th zircon ages generated by LA‐ICP‐MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th are potentially unreliable. To validate the applicability of LA‐ICP‐MS to this dating method, we acquired data from three late Quaternary volcanic units: the 41 ka Campanian Ignimbrite (plutonic clasts), the 161 ka Kos Plateau Tuff (juvenile clasts) and the 12 ka Puy de Dôme trachyte lava (all eruption ages by Ar/Ar, with zircon U‐Th ages being of equal or slightly older). A comparison of the corrected LA‐ICP‐MS results with previously published secondary ion mass spectrometry (SIMS) data for these rocks shows comparable ages with equivalent precision for LA‐ICP‐MS and SIMS, but much shorter analysis durations (~ 2 min vs. ~ 15 min) per spot with LA‐ICP‐MS and much simpler sample preparation. Previously undated zircons from the Yali eruption (Kos‐Nisyros volcanic centre, Greece) were analysed using this method. This yielded a large age spread (~ 45 to > 300 ka), suggesting significant antecryst recycling. The youngest zircon age (~ 45 ± 10 ka) provides a reasonable maximum estimate for the eruption age, in agreement with the previously published age using oxygen isotope stratigraphy (~ 31 ka).

Evolution of a Palaeoproterozoic giant magmatic dome in the Finnish Svecofennian; New insights from U–Pb geochronology

The Svecofennian domain comprises a c. 800km by 800km area in Finland and Sweden, mainly composed of Palaeoproterozoic granitoids, schists and gneisses. In the central part of the domain in western Finland, the Vaasa complex hosts a sequence of high-grade metamorphic rocks and granites formed by crustal anatexis during the culmination of the ∼1.9Ga Svecofennian orogeny. The Vaasa complex consists of a c. 8000km2 granitoid batholith surrounded by diatexitic and metatexitic migmatites as well as metasedimentary rocks. In order to enhance the perception of the crystallisation history, provenance, and maximum sedimentation age of the protoliths involved, nine granitoid-xenolith pairs and a palaeosome-neosome pair were collected from the main body of the Vaasa batholith and surrounding rocks, and zircon from them was U–Pb dated using the LA-MC-ICPMS method.The U–Pb zircon crystallisation ages of the granitoids are 1.88–1.87Ga, which is in accordance with the few previous results from the Vaasa region, and confirms a relatively swift crystallisation of the granite core throughout the Finnish part of the complex. The samples from the central part of the batholith have younger zircon ages than those from batholith margin. The inherited zircon age distribution pattern of the granitoids resembles that of the metasedimentary xenoliths. Our new comprehensive data, combined with whole-rock geochemistry from previous studies, render an in situ origin of the Vaasa granite probable. The maximum sedimentation age of the supracrustal precursors of the Vaasa complex is 1.92Ga. Some of the metasedimentary xenoliths from the Vaasa complex host zircon that is coeval with their host granitoids and register incipient melting. Based on these ∼1.88–1.86Ga zircon grains, the maximum sedimentation age would be deemed too young. Therefore, our data calls for caution when determining maximum sedimentation ages of high-grade metasedimentary rocks.

The U–Th–Pb systematics of zircon from the Bishop Tuff: A case study in challenges to high-precision Pb/U geochronology at the millennial scale

Dating zircon by U–Pb in the Pleistocene Bishop Tuff is challenging because the concentration of radiogenic Pb is low and the correction required for disequilibrium in the intermediate daughter products is large. The dates can be difficult to interpret, because their absolute precision is similar in magnitude to purported timescales of crystallization. Previous estimates of the duration of zircon crystallization span two orders of magnitude and appear to depend on the analytical approach. To reconcile the differing interpretations, we present new SIMS and ID-TIMS zircon Pb/U dates, including some crystals that are dated by both techniques. Both the SIMS and ID-TIMS dates have similar distributions as previous results. Normalized to a Th/Umelt=2.81, SIMS spot dates from interior regions of sectioned grains range from 769±31ka to 845±28ka (2σ) and ID-TIMS dates have a range from 760±7ka to 793±6ka. Individual crystals dated by both methods have an average difference of 31ka, with the SIMS dates being consistently older than the bulk crystal dates. This level of variability precludes the assignment of a geological significance to a mean (or weighted-mean) zircon date. A previous ID-TIMS investigation of single BT zircon that showed apparent statistically significant clustering of zircon ages was compromised by an incorrect treatment of the covariance structure of 230Th-corrected Pb/U data. We show approximate and exact methods for the correct treatment and demonstrate substantially more scatter in that data than previously recognized. Close scrutiny of available partition coefficient data show that they are not adequately precise for <±10ka corrections and even if determined precisely, evidence for disequilibrium uptake of U and Th implies that equilibrium partition coefficients may not be useful. Mismatches between the U–Th systematics of zircon and of glasses suggest that there are gaps in our understanding of the relationship between the two phases, lending uncertainty to the accuracy of disequilibrium corrections that use measured glass compositions. Given these uncertainties in the disequilibrium correction, caution is urged when correcting dates with nominal precisions that are better than ±10ka. Nevertheless, the youngest zircon ages very likely constrain the eruption age of the Bishop Tuff to <775ka, which conflicts with some estimates of the 40Ar–39Ar sanidine age of eruption.

Potential sources of Ediacaran strata of Iberia: a review

Advances in stratigraphy, geochemistry and U-Pb detrital zircon geochronology from Ediacaran strata of Iberia allow for the improved characterisation of crustal growth in the North Gondwana active margin. The formation of Cadomian magmatic arcs and associated back-arc basins that took place in the North Gondwana active margin was a long-term process. Iberia has been placed in the Cadomian belt in currently accepted palaeogeography reconstructions at c. 570–560 Ma, based on the characteristics of Ediacaran strata. The Ediacaran strata of Iberia with outstanding geochemical homogeneity are distributed across three zones of Iberia: (1) Narcea Slates in the Cantabrian and West Asturian Leonese zones (maximum depositional age of c. 600 and 553 Ma); (2) Schist–Greywacke Complex (Lower Series) in the Central Iberian Zone (maximum depositional age of c. 578 to 550 Ma); and (3) Série Negra in the Ossa-Morena Zone (maximum depositional age of c. 590–545 Ma). Pre-Cryogenian detrital zircons found in the Ediacaran strata of Iberia seem to be related to distal sources distributed across three main areas of North Gondwana inland. The oldest detrital zircons probably derive from distal sources such as the West African craton, the Trans-Saharan belt and the Arabian–Nubian Shield, in view of the increase in distance from sedimentary basins. The West African craton is the most likely source for Archean and Palaeoproterozoic detrital zircons, while the Trans-Saharan belt and the Arabian–Nubian shield could provide a source for Tonian and Mesoproterozoic grains. The youngest zircon ages (c. 630–545 Ma), which make up the dominant population in the Ediacaran strata of Iberia, are probably derived from proximal sources as would be the Cadomian magmatic arc system, not excluding the contribution of the Pan-African orogen.

Provenance of metasedimentary rocks of the Western Pernambuco-Alagoas Domain: Contribution to understand the crustal evolution of southern Borborema Province

The Borborema Province is a complex neoproterozoic orogen in northeastern Brazil, made of a mosaic of fault-bounded terrains and several metassedimentary sequences. In the present work, new zircon U–Pb provenance data for metasedimentary rocks in the Western Pernambuco-Alagoas Domain, southern part of the Province, are reported. Detrital zircon ages range from Archean to Neoproterozoic. Three samples of the Cabrobó Complex were investigated: (i) sillimanite-kyanite-garnet-biotite schist, which presented mostly Ediacaran and Cryogenian detrital zircon ages (youngest zircon at ca. 554 Ma) indicating erosion of neoproterozoic sources, (ii) garnet-biotite schist, which has a dominant Tonian/Stenian population, a less abundant Cryogenian (youngest zircon age at ca. 643 Ma) as well as Paleoproterozoic and Archean zircon grains, and (iii) tourmaline-muscovite quartzite, which contains detrital zircon varying in age between ca. 2.08 Ga and 1.57 Ga, and an abundant population close to the Meso/Paleoproterozoic boundary, possibly associated with the erosion of rocks formed during the Statherian taphrogenesis, known in the central part of the São Francisco Craton as well as in other areas of the Borborema Province. Two samples of the Riacho Seco Metasedimentary Complex were also investigated: (i) a biotite schist with a dominant population presenting ages mostly between 2.3 and 2.7 Ga (youngest zircon age at ca. 2023 Ma) and (ii) a magnetite-biotite-muscovite quartzite, having detrital zircon grains with ages ranging between ca. 1.9 and 2.7 Ga. The sedimentary rocks of the Riacho Seco Complex may have their origin related to the erosion of sources within the São Francisco Craton. The data for the Riacho Seco metasedimentary rocks, however, are not conclusive with respect to the depositional age of the original sedimentary rocks. The sequence might represent exposure of an old (Paleoproterozoic) sedimentary pile or, alternatively, it comprises a neoproterozoic passive margin sequence, with the original sediments derived from the erosion of the cratonic areas to the south.

The final collision of the CAOB: Constraint from the zircon U–Pb dating of the Linxi Formation, Inner Mongolia

The Linxi Formation occupies an extensive area in the eastern Inner Mongolia in the Central Asian Orogenic Belt (CAOB). The Linxi Formation is composed of slate, siltstone, sandstone and plant, lamellibranch microfossils in the associated strata. Major and trace element data (including REE) for sandstones from the formation indicate that these rocks have a greywacke protolith and have been deposited during a strong tectonic activity. LA-ICPMS U–Pb dating of detrital zircons yield ages of 1801 to 238 Ma for four samples from the Linxi Formation. 425–585 Ma, together with the ∼500 Ma age for the metamorphism event previously determined for Northeast China, indicates that their provenance is the metamorphic rocks of Pan-African age that have a tectonic affinity to NE China. A few older zircons with U–Pb ages at 1689–1801 Ma, 1307–1414 Ma, 593–978 Ma are also present, revealing the Neoproterozoic history of NE China. The youngest population shows a peak at ca. 252 Ma, suggesting that the main deposition of the Linxi Formation was at late Permain. Moreover, the ca. 250 Ma zircon grains of all four samples yield weighted mean 206Pb/238U ages of 250 ± 3 Ma, 248 ± 3 Ma, 249 ± 3 Ma, and 250 ± 2 Ma, respectively. These ages, together with the youngest zircon age in the sample ZJB-28 (ca. 238 Ma), suggest that the deposition of the Linxi Formation extended to the early Triassic. Combining with previous results, we suggest that the final collision of the Central Asian Orogenic Belt (CAOB) in the southern of Linxi Formation, which located in the Solonker–Xra Moron–Changchun suture, and the timing for final collision should be at early Triassic.

Depositional age, provenance and tectonic setting of the Proterozoic Ruyang Group, southern margin of the North China Craton

Late Paleoproterozoic rifting and accompanying magmatism and sedimentation, which occurred widely in the North China Craton, are interpreted to be related to the breakup of the supercontinent Columbia. The Ruyang Group, overlying the Xiong’er Group in the southern margin of the craton, is a typical fluvial-marine sedimentary sequence mainly composed of sandstone with minor shale and dolostone. Sandstones from the Ruyang Group exhibit high contents of SiO2 (86.8–98.7wt.%) but relatively low contents of Al2O3, K2O, Na2O, TiO2 and MgO, typical of mature siliceous clastic rocks. Trace elements of these rocks indicate that the sequence was deposited in a passive continental margin environment and that the sedimentary detritus was derived from a Paleoproterozoic silicic-intermediate source, similar to other Paleo-Mesoproterozoic sedimentary sequences in the region.Detrital zircons from three sandstone samples of the Ruyang Group yield 207Pb/206Pb ages ranging from 1.70Ga to 2.70Ga with two pronounced peaks at ca. 2.10Ga and 1.86Ga. A weighted mean age at 1744±22Ma from nine young zircon ages between 1.80Ga and 1.70Ga can constrain the maximum depositional age of the Ruyang Group. Most detrital zircons exhibit negative ɛHf(t) values ranging from −15.6 to 0 and old TDMC ages ranging from 2.50Ga to 3.60Ga, implying reworking of Archean crust. A close match between the U–Pb ages and TDMC(Hf) ages, as well as the positive ɛHf(t) values for some detrital zircons, indicate a contribution of juvenile materials. The detrital zircon grains have TDMC(Hf) ages clustering between 3.00Ga and 2.60Ga with a peak at around 2.80–2.70Ga, indicating a major period of crustal growth at this time.

Age constraints on late Mesozoic lithospheric extension and origin of bimodal volcanic rocks from the Hailar basin, NE China

Following the amalgamation of the late Mesoproterozoic–Carboniferous Central Asian Orogenic Belt with the Siberian and North China cratons, NE China underwent late Mesozoic lithospheric extension and widespread formation of volcano-sedimentary basins. We report U–Pb zircon ages and geochemical data for mafic and felsic volcanic rocks from the Hailar basin, located about 1000km north of Beijing. Zircon populations of six felsic rock samples analyzed by laser ablation ICP-MS yielded similar U–Pb age spectra ranging from 158 to 125Ma. The youngest zircon ages are interpreted as time of magma eruption and the xenocrystic zircon-age spectra as evidence for a protracted melting of lower crust due to the underplating of mantle-derived magmas during lithospheric extension. The volcanic assemblage has a bimodal composition comprising geochemically evolved trachybasalts and felsic volcanic rocks of I- and subordinate A-type compositions. The mafic volcanic rocks have negative Nb-anomalies, high Th/Nb and Ce/Pb ratios, low initial εNd values of +0.4 to +3.4, and radiogenic Pb and Sr isotopes all interpreted as evidence for the melting of passively upwelling asthenosphere and lithospheric mantle previously modified by plate subduction. The xenocrystic zircon ages and chemical/isotopic data of the felsic rocks support an origin from juvenile crustal protoliths: the data of I-type felsic rocks are consistent with the melting of underplated mafic protoliths and those of the A-type rhyolites support the melting of a crustal source with a composition similar to the I-type felsites with apatite controlling their Nb anomaly. The evidence for the persistent melting of a subduction-modified mantle in NE China is in agreement with a model of an extending coupled upper mantle–crust system due to a retreating Paleo-Pacific trench.

Reconstruction of the early Mesozoic plate margin of Gondwana by U–Pb ages of detrital zircons from northern Victoria Land, Antarctica

Abstract Detrital zircons of eight sandstone samples from the Triassic–Early Jurassic Section Peak Formation (Victoria Group, Beacon Supergroup) in northern Victoria Land, Antarctica, were investigated by U–Pb LA–ICPMS dating. The basin was flanked by the East Antarctic craton, and by a magmatic arc at the palaeo-Pacific margin of Gondwana. It accommodated sandstones ranging from quartzo-feldspathic to volcaniclastic in composition. The detrital zircon age spectra yield pronounced concentrations at c. 190–250 Ma, 500–700 Ma and 800–1200 Ma. The proportion of Triassic–Early Jurassic zircons increases from base to top of the formation, and correlates positively with the abundance of detrital volcanic rock fragments. The youngest zircon ages are close to the stratigraphic age of each sample, indicating contemporaneous magmatic activity along the active margin of Gondwana. Igneous rocks that formed during the Ross Orogeny ( c. 470–545 Ma) were a minor source only, suggesting that the Ross Orogen became progressively covered by sediments as the basin expanded. Pan-African ( c. 500–700 Ma) and Grenville ( c. 800–1200 Ma) age zircons may have been derived from crustal sources currently covered beneath the polar ice sheet, although recycling from Cambro-Ordovician units provides an alternative explanation. Supplementary material: The exact results of the age analyses of all samples are presented in Tables A1 to A8 at www.geolsoc.org.uk/SUP18624

경기육괴 북서부 영흥도-선재도-대부도에 분포하는 변성퇴적암 내 쇄설성 저어콘의 SHRIMP U-Pb 연대

경기육괴의 북서부에 위치한 영흥도-선재도-대부도는 지구조적 중요성에도 불구하고 지질 및 지질연대 자료가 거의 없는 지역이다. 우리는 이 지역에 산출하는 변성퇴적암의 암상과 쇄설성 저어콘의 U-Pb 연대를 밝히는 연구를 수행하였다. 연구 지역의 변성퇴적암은 대부분 규암-변성사암 및 규암과 편암의 호층대로 이루어져 있으며, 규암 역을 포함하는 석회질 역질 사암, 석회질 편암 그리고 변성이질암이 소규모로 산출한다. 변성이질암에는 석류석 반상변정이 두드러지게 산출하며, 변성이질암 내의 석영맥에서는 남정석 또는 홍주석이 백운모와 함께 산출한다. 두개의 변성사암 시료에서 분석한 쇄설성 저어콘들은 신시생대(~2.5 Ga), 고원생대(~2.0-1.5 Ga), 신원생대(~1.1-0.7 Ga), 초기 고생대(~560-400Ma) 연대가 우세하게 분포하며, 가장 젊은 연령의 군집은 ~420 Ma에서 나타난다. 이러한 결과는 분석된 변성사암이 실루리아기 이후, 아마도 데본기에 퇴적되었음을 시사하며, 태안층에서 보고된 쇄설성 저어콘의 연령분포와 유사하다. 이와 함께 임진강대 변성퇴적암에서도 유사한 쇄설성 저어콘의 연령분포와 바로비안형 변성상이 보고되었다. 따라서, 경기육괴 서부에 산출하는 태안층은 임진강대의 서남 연장부에 해당될 것으로 판단된다. We investigated the various lithologies and zircon U-Pb ages of metasedimentary rocks from the Yeongheung-Seonjae-Daebu Islands, western Gyeonggi Massif, whose geologic and geochronologic features are poorly constrained in spite of their significance for tectonic interpretation. Major lithology consists of quartzites or meta-sandstones commonly alternating with semi-pelitic schists, together with lesser amounts of calcareous sandstones with matrix-supported quartzite clasts, calcareous schists, and pelitic schists. Pelitic schists uncommonly contain large porphyroblasts of garnet as well as quartz veins with large crystals of muscovite and andalusite or kyanite. SHRIMP U-Pb ages of detrital zircons from two analyzed metasandstones define four age populations: Neoarchean (~2.5 Ga), Paleoproterozoic (~2.0-1.5 Ga), Neoproterozoic (~1.1-0.7 Ga), and Early Paleozoic (~560-400 Ma). The youngest zircon ages are clustered at ~420 Ma. These results suggest that the deposition of meta-sandstones took place after the Silurian, possibly during the Devonian, and are analogous to those of the Taean Formation reported from the western part of the Gyeonggi Massif. Moreover, The age distribution patterns of detrital zircons and the Barrovian-type metamorphic facies of pelitic schists are similar to those reported from the Imjingang belt, suggesting that the Taean Formation likely corresponds to southwestward extension of the Imjingang Belt.

Differentiating Shawinigan and Ottawan orogenesis in the Central Adirondacks

The rocks at Chimney Mountain provide a rare glimpse into primary intrusive relations and exceptionally well-preserved pre-Shawinigan metasedimentary rocks in the Adirondack Highlands despite a strong Ottawan thermal overprint. A near vertical contact between granite (ca. 1172 Ma) and a shallowly dipping and structurally intact sequence of quartzose to calc-silicate metasedimentary rocks is exposed on the southern flank of Chimney Mountain in the Central Adirondacks. The contact is marked by foliation truncation and a metasomatic aureole with randomly orientated porphyroblasts of enstatite rimmed by anthophyllite (max. 5 cm) and phlogopite (max. 2 cm), and a zone of granular, quartz-rich rock. The granite is non- to weakly foliated and has a shallow, north-plunging, mineral lineation as do the metasedimentary rocks. Zircons separated from a diopside-bearing quartzite (82% SiO2; 0.75 m thick) are of variable size (up to 400 μm), equant, and contain, on average, >1000 ppm uranium. Scanning electron microscope investigation indicates that there is little variation in a uniformly dark cathodoluminescence response, no discernible cores or rims, few inclusions, and partially faceted to round morphologies. Zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP II) ages of 1042 ± 4 Ma and 1073 ± 15 Ma are coincident with Ottawan metamorphic ages from the Adirondack Highlands. Zircons from the intrusive granite yield large cores with typical anorthosite-mangerite-charnockite-granite (AMCG) ages (ca. 1171.6 ± 6.3 Ma) and sparse, thin younger rims (ca. 1060–1090 Ma) readily distinguishable by cathodoluminescence. Despite the younger zircon ages, the metasedimentary rocks and their fabric must predate the crosscutting granite. The thermal effect of the Ottawan event was likely enhanced by volatile fluxing and resulted in recrystallization and resetting of zircons in the metasedimentary rocks. However, it had limited effects on zircons in the granite and produced only thin metamorphic rims emphasizing the importance of local geochemical conditions to the response of zircon to metamorphism. Elzevirian or Shawinigan fabrics are preserved as the dominant foliation; the lineation and folding is likely late (post-1170 Ma) Shawinigan or Ottawan (ca. 1050 Ma). Titanites from the same metasedimentary sequence yield a range of 238U/206Pb ages from 969 to 1077 Ma, with a maximum probability age of 1035 Ma, similar to other titanites in the Adirondack Highlands. Ottawan paleotemperatures, estimated by zirconium in titanite thermometry, range from 787 to 818 °C.

Early Cretaceous Gangdese retroarc foreland basin evolution in the Selin Co basin, central Tibet: evidence from sedimentology and detrital zircon geochronology

Abstract The Selin Co basin in the northern Lhasa terrane includes more than 3000 m of upward coarsening Lower Cretaceous strata, and the sedimentary sequence from the flysch to the molasse indicates the evolution of a foreland basin. Petrographic analysis shows that sandstones are rich in volcanic and sedimentary lithics and most of them fall into recycled orogen and magmatic arc. Uranium–lead (U–Pb) ages were determined for 435 detrital zircons from the Lower Cretaceous strata in the Selin Co basin. Relative probability of detrital zircon ages shows the Eshaerbu Formation was rich in zircon grains with the age of 125–140 and 160–180 Ma, and the Duoni Formation was dominated by one main age cluster of 125–150 Ma. Analysis of the potential provenances suggests the Early Cretaceous zircon grains were primarily derived from the Gangdese magmatic arc to the south. The youngest zircon ages in the lowermost exposure of the Eshaerbru Formation are c . 130 Ma, providing a maximum depositional age of sediments in the Selin Co basin. Collectively, our studies, together with previously documented Cretaceous thrusting in the Lhasa terrane, suggest the Lower Cretaceous Selin Co basin was deposited in a retroarc foreland basin. From 145–90 Ma, a retroarc foreland basin was presumed to develop in the Lhasa terrane, migrating from the south to the north. Crustal thickening, likely associated with the evolution of the retroarc foreland basin, was speculated to start in the Early Cretaceous in the Lhasa terrane.

Hannuoba intermediate-mafic granulite xenoliths revisited: Assessment of a Mesozoic underplating model

The intermediate-mafic granulite xenoliths from Tertiary Hannuoba alkali basalts at the northern margin of the North China Craton have been commonly considered to be formed by Mesozoic basaltic underplating due to their abundant Mesozoic zircons and compositionally higher Mg# (molecular Mg/(Mg+Fe)) than rocks of the adjourning Archean granulite terrain. However, such a Mesozoic underplating model cannot account for the presence of Precambrian zircons in the xenoliths and their evolved Sr–Nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle. Instead, their Sr–Nd isotopic compositions resemble those of the granulite terrain and of the Mesozoic low Mg# granitoids. Here newly acquired data and published data are combined to assess the Mesozoic basaltic underplating model and to discuss the origin of the Hannuoba intermediate-mafic granulite xenoliths in terms of their relationships with the Mesozoic magmatism and the granulite terrain. Zircons from a mafic granulite xenolith (JN0724), which has geochemical and Sr–Nd isotopic compositions indistinguishable from other mafic granulite xenoliths, yield ∼2.5Ga U–Pb ages, providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust. Combined data reveals that the pyroxene-rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the Mesozoic low Mg# granitoids. Hf isotope analysis on Mesozoic zircons from the Hannuoba feldspar-rich granulite xenoliths demonstrates that the zircons crystallized from Mesozoic granitic magmas that were derived from ancient lower crust rather than from Mesozoic underplated magmas. The proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas. It is concluded that most of the Hannuoba intermediate-mafic granulite xenoliths and the Mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition. It provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust. Our data imply that whether younger zircon ages in granulite xenoliths from many other Archean cratons register post-Archean basaltic underplating could be tested by integrated analysis of U–Pb age and Hf isotopes.

Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm–Nd and zircon U–Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm–Nd ages of 126.1±2.0 and 122.6±2.0 Ma date crystal growth. Integration of the garnet ages with U–Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150 Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134 Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126 Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123 Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95 Ma. Phase diagram sections constrain orthopyroxene assemblages to < 0.6 GPa @ 650 °C, peak garnet granulite facies metamorphic conditions to 680–815 °C @ 1.1 – 1.4 GPa, and a P– T path with a P increase of ≥ 0.5 GPa. These sections are compatible with water contents ≥ 0.28 wt.%, local dehydration during garnet granulite metamorphism, and < 0.3 GPa P increases during garnet growth. Results demonstrate the utility of integrated U–Pb zircon and Sm–Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126 Ma, that loading occurred at a rate of ca. 0.06 GPa/m.y., and that garnet granulite metamorphism lasted 3–7 m.y. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3 m.y.

Provenance and tectonic setting of Paleoproterozoic metasedimentary rocks along the eastern margin of Hearne craton: Constraints from SHRIMP geochronology, Wollaston Group, Saskatchewan, Canada

Single detrital zircon grains from various parts of the Wollaston Group, a Paleoproterozoic metasedimentary succession deposited along the southeastern margin of the Hearne Province, northern Saskatchewan, Canada, were analyzed by SHRIMP U–Pb geochronological techniques. Zircon analyses are mostly concordant and yield ages ranging from ca. 2800 to 1780 Ma, although distinct age populations were detected in all samples. The stratigraphically oldest sample (Geoch 4) is dominated by a bimodal distribution of zircon ages (ca. 1.90 and 2.4–2.6 Ga), which is similar to that preserved in the sample (Geoch 2) from the middle portion of the Wollaston Group. The stratigraphically youngest sample (Geoch 9) contains ca. 2.1 Ga zircons, as well as zircons with the same ages as observed in Geoch 4 and Geoch 2. Zircon ages older than 2450 Ma appear to be consistent with the age of the Hearne Province basement, suggesting that part of the sedimentary detritus was locally derived. Zircons with ages in the 2430–2350 Ma range, found in all samples, may have been derived from a more distant source, such as Rae Province rocks that were affected by the recently identified Arrowsmith orogeny. Significant amounts of 1920–1880 Ma zircon grains are found in all samples; these are interpreted to represent sedimentary detritus derived from juvenile volcanic terranes. Zircons younger than 1860 Ma are interpreted to be the product of post-Wollaston Group thermal overprinting. Our data, together with field relationships and geochemical data, suggest that most of the preserved Wollaston Group was deposited in a back-arc to foreland basin environment. It received detritus from both Archean continental crust to the west and a juvenile continental magmatic arc, likely located to the east, as the youngest zircon ages are not consistent with the age of Taltson Orogen rocks to the west.

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa Area

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ∼400 to 500 Ma, ∼120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma. These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ∼120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40 - 70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Ultrasonic resonant inspection of PM parts: W.Hansen, U.Schlengermann. (Kraut-kramer GmbH, Germany.)

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm–Nd and zircon U–Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm–Nd ages of 126.1±2.0 and 122.6±2.0 Ma date crystal growth. Integration of the garnet ages with U–Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150 Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134 Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126 Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123 Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95 Ma. Phase diagram sections constrain orthopyroxene assemblages to < 0.6 GPa @ 650 °C, peak garnet granulite facies metamorphic conditions to 680–815 °C @ 1.1 – 1.4 GPa, and a P–T path with a P increase of ≥ 0.5 GPa. These sections are compatible with water contents ≥ 0.28 wt.%, local dehydration during garnet granulite metamorphism, and < 0.3 GPa P increases during garnet growth. Results demonstrate the utility of integrated U–Pb zircon and Sm–Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126 Ma, that loading occurred at a rate of ca. 0.06 GPa/m.y., and that garnet granulite metamorphism lasted 3–7 m.y. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3 m.y.

Single-zircon Pb evaporation geochronology constrains basement-cover relationships in the lower Hecla Hoek Complex of northern Ny Friesland, Svalbard

The Hecla Hoek Complex, a ∼ 18-km-thick succession in the Svalbard Caledonian Eastern Terrane, has been interpreted to be a more or less continuous volcano-sedimentary sequence. Amphibolite-facies rocks in the lower parts appear to grade conformably up into almost unmetamorphosed Neoproterozoic and Early Paleozoic strata in the middle and upper parts of the succession. However, recent work in the Ny Friesland transpressional orogen has indicated that the lower part of this Hecla Hoek Complex is composed of at least three thrust sheets. Each thrust sheet is made up of Paleoproterozoic ( ∼ 1750 Ma) granites, locally intruded into older metasedimentary rocks, and overlain by younger sedimentary cover, mainly composed of psammites and quartzites, locally with conglomerates. This isotopic study, focuses on basement-cover relationships in one of the thrust sheets in northern Ny Friesland, where a granitic unit (the Instrumentberget Granitic Gneiss) is overlain by conglomerates and quartzites (Polhem Formation). Ages obtained using Pb evaporation and conventional UPb techniques on both the granitic gneisses and granitic boulders in overlying conglomerates, yield similar ages of ∼ 1740 Ma, suggesting that the latter are derived from the former. Additional Pb evaporation analyses on detrital zircons from two quartzite samples higher up in the Polhem stratigraphy have given information about other source ages. One of these quartzitec samples, from northern Ny Friesland, is dominated by detrital zircon ages of 1870–2040 Ma; however, this sample also confirms the existence of a 1740-Ma source. Another quartzite within the same formation, collected 70 km farther south in Ny Friesland and from a higher stratigraphic level, has provided detrital zircon ages of 1320–2710 Ma; the youngest zircon age in this sample constrains the maximum age for the deposition of the Polhem Formation. These sediments may be as young as Early Paleozoic, but are thought to be Mesoproterozoic, because of the intercalation of abundant amphibolites, which are notable for their absence in the eastern Ny Friesland Neoproterozoic and younger succession. The younger age (< 1740 Ma) of some of the zircons in the Polhem sediments strongly support the recent mapping and structural interpretation that the lower part of the Hecla Hoek Complex is a tectonostratigraphic succession composed of several thrust sheets.

Late thermal events based on zircon fission track ages in northeastern New South Wales and southeastern Queensland: Links to Sydney Basin seismicity?

Zircon, concentrated from basaltic terrains in northeastern New South Wales and southeastern Queensland, reveals some unexpectedly young fission track peaks. Between 2 to 13 Ma in age, these peaks are younger than known Tertiary basaltic ages from these regions which match older fission track peaks. Analysis of the fission track data suggests that the young dates are probably not reset ages due to recent heating events such as bush fires, but more likely mark thermal resetting by later volcanic eruptions. The young ages decrease southwards from Queensland through northern New South Wales and trend toward seismic zones within the Sydney Basin in the Newcastle, Blue Mountains and Illawarra regions. A model based on northward motion of the Australian plate over a hot asthenospheric source (0.75° latitude/Ma over 12 Ma)) predicts the positions of most young zircon ages to within ± 70 km in latitude when projected from seismic sites at Newcastle and Bowral‐Robertson. A minor hot spot source is proposed, some 250 km across, which triggered isolated basaltic and zircon‐bearing eruptions every few million years and now underlies the southern Sydney Basin. This would bring Sydney Basin seismicity into line with other seismic zones known at present hot spot positions across southeastern Australia and the Tasman Sea. It raises questions concerning activation of local seismicity, potential for volcanic risk and distribution of young uplift in the Sydney region. Similar studies are needed to test other puzzling seismic zones (e.g. the Dalton‐Gunning Zone).