Zircon Ages Research Articles

Petrography, geochemistry and detrital zircon U–Pb dating of the Pliocene‐Pleistocene Dupi Tila Formation from the Lalmai Anticline, Bengal Basin: Regional tectonic implications

The Bengal Basin is a well‐known foreland basin that archives the tectonic evolution of the Himalayan and Indo‐Burman Ranges (IBR). The Dupi Tila Formation is the youngest among the stratigraphic successions of the Bengal Basin, which witnessed the last phase of the Himalayan uplift. However, there is still controversy regarding sediment provenances since the Himalayan and IBR are in the late Neogene exhumation stage. In this study, the petrographical and geochemical compositions of the Dupi Tila sandstones from the Lalmai Anticline of the Bengal Basin are examined to infer the source rocks, tectonic settings, palaeoclimate and weathering intensity in provenance areas. The detrital modes and geochemical discriminations, as well as Chondrite‐normalized rare earth elements patterns with enrichment in light rare earth elements and nearly flat heavy rare earth elements and negative Eu anomalies (average Eu/Eu* = 0.77) indicate dominantly felsic source rocks. The geochemical data also suggest that these source rocks in provenance areas were influenced by weak to moderate chemical weathering (chemical index of alteration: 65.64–75.62) and multiple recycling under warm and humid to semi‐humid climatic conditions. The detrital zircon ages show three dominant peaks at ca. 100, 500 and 1000 Ma with minor 1700 Ma. The amount of young zircon grains (<120 Ma) is noticeably higher (~18%) in this Lalmai outcrop compared to the Dupi Tila samples from the Sylhet Trough and Chittagong‐Tripura Fold Belt. Results from integrated zircon geochronology and geochemistry indicate that the detritus are likely derived from a recycled orogen provenance related mainly to active continental margin setting with subordinate influence from passive setting. Therefore, the regionally extensive Himalayan Orogen appears to be the primary sediment source, with additional mixing from the IBR and recycled sediments originating from the Gangdese batholith of the Trans‐Himalaya.

In situ U-Pb dating of Jurassic dinosaur bones from Sichuan Basin, South China

Abstract Direct dating of vertebrate fossils is difficult due to complex postburial diagenetic processes and the often low and heterogeneous concentration of radioisotopes (e.g., U) in fossilized bone material. Here, we demonstrate a novel approach to dating vertebrate fossils via laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of early diagenetic calcite cements precipitated within bone cavities of a Jurassic sauropod from the Sichuan Basin, South China. Early diagenetic calcite yielded a U-Pb age of 165.3 ± 3.6/5.6 Ma, in agreement with a maximum depositional age of 165.8 ± 1.0 Ma from detrital zircon ages of the surrounding rocks, suggesting that diagenesis occurred shortly after the death of the sauropod. This new age demonstrates that the best-known Jurassic large sauropod faunas in South China are much older than those in North America and Africa, suggesting the geographical isolation of South China. Authigenic apatite U-Pb dating on the fibrolamellar bones from the same sauropod gave a distinctly younger age, indicating more complex U-Pb system behavior, possibly due to U uptake by residual organic matter and recrystallization of apatite after early diagenesis. Our findings demonstrate that U-Pb dating of calcite cements within bone cavities has significant potential for constraining the burial age of vertebrate fossils, which could aid in constructing a more robust temporal framework for the radiation and evolution of vertebrates.

Devonian intrusions of southwestern New Brunswick, Canada: U-Pb geochronology, petrogenesis and genetic constraints on gold mineralization

ABSTRACT The Clarence Stream intrusion-related gold deposit is hosted within metasedimentary and metavolcanic sequences of the Ordovician Cookson and Silurian Mascarene groups that were intruded by several phases of the Late Silurian to Early Devonian Saint George Plutonic Suite. Based on geochemical and geochronological data, the intrusive rocks in the deposit are categorized into three distinct suites, including gabbroic, granitoid, and Whittier Ridge Granite (WRG). The gabbroic suite is the same as the East Branch Brook metagabbro intrusion, which exhibits a subalkaline mafic composition fractionated from a depleted mantle in a suprasubduction zone setting. The granitoid suite shows the same geochemical characteristics and age as the metaluminous to slightly peraluminous Magaguadavic Granite and its fractionated phases. The WRG suite reflects characteristics of calc-alkaline, metaluminous to peraluminous I-type granites in the area, such as Sorrel Ridge and Mount Douglas granitic plutons. In situ U-Pb zircon geochronology yielded an age of 419.5 ± 3.8 Ma for a monzodiorite from the gabbroic suite. Porphyritic granite and alkali granite from granitoid suite yielded U-Pb zircon age of 402.4 ± 1.8 Ma and 394.3 ± 3.8 Ma, respectively. Geochronologic data from the WRG suite reflect two different crystallization ages of 383.8 Ma (382.4 ± 3.1 to 385.1 ± 4) and 372.4 Ma (367.4 ± 1.3 to 377.1 ± 1.9). Several pieces of evidence demonstrate that the gold mineralizing events occurred after the emplacement of the East Branch Brook metagabbroic dykes and are linked to the various intrusive phases of the nearby syntectonic Early Devonian Magaguadavic Granite. The absence of gold and sulphide mineralization in felsic dykes related to the WRG confirms that this plutonic body formed adjacent to the gold zones after the main gold mineralizing event.

Detrital U[sbnd]Pb zircon geochronology, zircon Lu[sbnd]Hf and Sr[sbnd]Nd isotopic signatures of the Lopingian volcanic-ash-derived Nb-Zr-REY-Ga mineralized horizons from eastern Yunnan, SW China

The Lopingian altered volcanic-ash horizons in southwest China have attracted significant attention due to highly-elevated concentrations of critical element Nb-Zr-REY-Ga. Despite extensive investigation, the provenance of these distinctive horizons remains controversial. Are they a product of air-born volcaniclastic, erosional source sedimentation, or a complex mixture of multiple sources?This investigation performs a diverse array of mineralogical and isotopic geochemical analyses to investigate the provenance of these horizons and critical-element mineralization time. The results showed that the concordant ages of detrital zircons 207Pb/206Pb-206Pb/238U from five representative mineralized samples are 253.84 ± 2.60 Ma, 252.49 ± 2.39 Ma, 253.47 ± 2.99 Ma, 257.44 ± 2.01 Ma, and 243.94 ± 2.32 Ma, respectively. Importantly, this dating does not correspond to the stratigraphic position of the studied deposits. Instead, this data set indicates that the low-temperature hydrothermal alteration of the primary Nb-Zr-REY-Ga mineralized layers occurred in Late Permian-Middle Triassic times (260–240 Ma) immediately after volcanic ash deposition at 257–260 Ma. The Sr-Nd-Lu-Hf isotopic compositions, diagrams of both the Al2O3/TiO2 vs. Eu/Eu* and Zr/TiO2 vs. Nb/Y, and the trace element patterns of the zircons (Nb/Hf vs. Th/U, Hf/Th vs. Th/Nb) indicate that the mineralized horizons were derived from both ELIP alkaline rocks and arc-related intermediate-felsic magmatic rocks from the Paleo-Tethys realm. Furthermore, varying amounts of older (∼350 Ma and older) zircon grains defined by the dominant Late Permian-Middle Triassic zircons indicate that there was also some erosional-source input from the uplands surrounding the basin at the time of deposition.

Heavy mineral indicators of ridge subduction beneath the Bainaimiao arc in the southeastern Central Asian Orogenic Belt

As one of the major subduction-accretionary systems in the southeastern Central Asian Orogenic Belt, the Bainaimiao arc presents a valuable opportunity for investigating the subduction–accretion process of accretionary orogens. It is located adjacent to the northern margin of the North China Craton and developed arc-related deposits during the Silurian–Devonian period. In this paper, we employed heavy mineral geochemistry and U–Pb ages of zircon and rutile, to determine the provenance of the Silurian–Devonian deposits and reconstruct the paleogeography of the source area. The stratigraphic units involved include the Xuniwusu Formation of Early Silurian and the Xibiehe Formation of Early Devonian.Both the Xuniwusu and Xibiehe formations exhibit heavy mineral assemblages dominated by zircons and tourmalines. Nevertheless, the Xibiehe Formation contains Cr-spinels and garnets which are absent in the Xuniwusu Formation. Geochemical analyses of the detrital Cr-spinels exhibit characteristics of a supra-subduction zone ophiolite. The garnets originated from middle- to high-grade metamorphic rocks, as well as mafic and felsic rocks. The tourmalines in both formations primarily indicate a metasedimentary rock source, with a small percentage of Li-poor granitoid species in the Xuniwusu Formation. The U–Pb age spectra of both detrital zircon and rutile in the Xuniwusu Formation show multiple peaks. The age spectra consist of Precambrian ages, indicative of a recycled origin, as well as Paleozoic ages, suggesting a magmatic origin. In contrast, the U–Pb age spectra of zircon and rutile in the Xibiehe Formation display a unimodal age peak at ca. 455 and 490 Ma, respectively. These differences suggest a provenance shift from the Xuniwusu to Xibiehe formations. The Xuniwusu Formation was sourced from the Bainaimiao arc itself, whereas the Xibiehe Formation has a mixed origin of the Bainaimiao arc and Ondor Sum accretionary complex. The provenance change correlates with the unconformity between the two formations, suggesting a forearc uplift event. In combination with the previous research on the Tulinkai ophiolite, it can be inferred that the forearc uplift was triggered by ridge subduction of the Paleo-Asian Ocean plate at ca. 450–430 Ma. This research provided a new perspective on the ridge subduction of the oceanic slab in fossil orogens.

Structure and age of the Rassokha granitoid massif (Arga-Tas Ridge, Eastern Yakutia)

The Rassokha (Rassoshina) granitoid massif is located at the junction of the Rassokha and Argatass terranes, which are located in the western part of the Kolyma–Omolon microcontinent (superterrane), within the Verkhoyansk–Kolyma fold belt, extend in a northwesterly direction and border in the southwest with the Omulevka terrane of the passive continental margin, and in the north-the east is blocked by Cenozoic deposits of the Ozhogino depression. The massif is mainly composed of light gray, creamy massive and trachytoid porphyritic coarse- and medium-grained moderately alkaline leucogranites and alaskites. The age of the Rassokha massif and its ore mineralization was determined by a complex of isotope-geochronological methods such as U-Pb dating of zircon and Re-Os dating of molybdenite. U-Pb zircon age of the Rassokha massif is 165±0.7 Ma (n=38). The results of U-Pb dating of zircon indicate that the introduction of rocks of the Rassokha massif (alaskites, leukogranites and aplites) occurred in a relatively narrow time interval about 165 million years ago. The obtained values of the age of sulfide mineralization (179±11 million years), obtained by the Re-Os system in molybdenite isolated from alaskites, turned out to be older than the values of the U-Pb age of zircon. Most likely, some increase in the values of the Re-Os age reflects the heterogeneous isotopic composition of the captured osmium. The time of granitoids formation correlates with the early stages of Uyadino-Yasachenskiy volcanic belt formation, and most likely the massif represents intrusive part of volcanogenic belt complexes. Taking into account the geological position of the massif in the zone of the Argatass thrust, it is possible to assume the formation of granitoids in an environment of transform interaction.

Cretaceous Fore‐Arc Basin and Its Provenance in Sakhalin, Far East Russia: U‐Pb Ages of Detrital Zircons From the Yezo Group

ABSTRACTIn order to reconstruct the pre‐Japan Sea configuration of the Cretaceous arc‐trench system, we measured U‐Pb ages of detrital zircons in the five sandstone samples from the Yezo Group in Sakhalin using LA‐ICP‐MS. A sandstone sample from the lowermost horizon (Ai Formation) contains abundant Early Cretaceous (110–100 Ma) zircons with minor amounts of pre‐Jurassic ones. Four sandstones from the uppermost (Krasnoyarka Formation) are enriched dominantly in Late Cretaceous (100–66 Ma) grains. The YC1σ ages of these samples are generally consistent with the previous fossil ages; nonetheless, a possible age extension of the Ai Fm up possibly into the Cenomanian was newly suggested. The stepwise younging in peak age of zircons according to the stratigraphy suggests the continuous replacement of arc granitoid exposures in the Cretaceous magmatic arc in the provenance likely developed on the continent side. This secular trend in zircon age spectra of the Yezo Group in Sakhalin is properly correlated with that in Hokkaido and also with that of coeval sandstones in southwest Japan, suggesting a monotonous sedimentation in the fore‐arc basin of the Late Cretaceous arc‐trench system developed along the Pacific margin, from Sakhalin to southwest Japan for ca. 2500 km in length. Its spatial dimension corresponds to that of the modern representative fore‐arc basin in the Java‐Sumatra region.

New SIMS U–Pb zircon age on the macroscopic multicellular eukaryotes from the early Mesoproterozoic Gaoyuzhuang Formation, North China

Abstract Decimetre-scale carbonaceous macrofossils from the Mesoproterozoic Gaoyuzhuang Formation in the Yanshan Range are known as the current oldest unambiguous evidence of macroscopic multicellular eukaryotes. Here, we reported a new SIMS zircon age of 1588.8 ± 6.5 Ma from a volcanic tuff in the Qianxi County of Hebei Province, about 11 m above the macrofossil’s horizon. This new age provides a direct age constraint on the macroscopic eukaryotic fossils from the Gaoyuzhuang Formation. It indicates that macroscopic life with the moderate diversity and certain morphological complexity had already evolved at the beginning of the Mesoproterozoic, and implies a possibility of discovering macroscopic eukaryotes in earlier rocks. This study also calls for a stratigraphic framework to integrate biological and environmental studies in different regions for a better understanding of the evolution of multicellular organisms and environmental change during this important period.

ИСТОЧНИКИ И ПАЛЕОГЕОДИНАМИЧЕСКАЯ ОБСТАНОВКА НАКОПЛЕНИЯ ТЕРРИГЕННЫХ ОТЛОЖЕНИЙ НИЖНЕКЕМБРИЙСКОЙ БЫСТРИНСКОЙ СВИТЫ АРГУНСКОГО МАССИВА

The article presents the results of mineralogical, geochemical, isotopic, and geochemical (Sm-Nd) studies on siltstones from the upper Bystraya subformation of the Argun continental massif, as well as U-Pb dating of detrital zircon grains from them. The main goal of the research was to identify the main sources of clastic material, reconstruct the paleogeodynamic environment of accumulation, and constrain the lower age limit for the formation of terrigenous rocks of the upper Bystraya subformation. The mineralogical composition of the studied rocks indicates their accumulation in an environment associated with subduction processes. The trace element composition of siltstones from the subformation suggests the presence of rocks of mixed composition in the source area in terms of their silica content. Based on Sm-Nd isotopic and geochemical data, it was established that the main sources of clastic material for siltstones of the stratigraphic unit were Paleoproterozoic rocks and (or) younger rocks whose formation is associated with the reworking of the Paleoproterozoic continental crust. Most of the zircons from the studied sample of siltstones of the upper Bystraya subformation are Neo- and Paleoproterozoic in age. Their sources are probably Neo- and Paleoproterozoic igneous rocks widespread within the Argun massif in China. The age of the youngest zircons (556 ± 9 and 566 ± 10 Ma) isolated from siltstones of the upper Bystraya subformation was used to constrain the lower limit of their accumulation in the middle of the Ediacaran.

The Age and Origin of the Rocks of the West Azov Group (Lozuvatka Antiform, the Ukrainian Shield)

Geochronological studies of amphibole-biotite gneisses of the Verkhnii Tokmak Unit and amphibolites and Banded Iron Formation (BIF) of the Kaiinkulak Unit of the West-Azov Group in the Lypovets area of the Lozuvate antiform in the West Azov block of the Ukrainian Shield were carried out. A multi-stage history of the formation of amphibole-biotite gneisses has been established. In terms of chemical composition they correspond to quartz diorites. Rare earth elements are differentiated — (La/YbN = 7.54), with a significant negative europium anomaly — Eu/Eu* = 0.66. The REE distribution indicates that the initial melt for quartz diorites was produced in a crustal magmatic source. In zircon crystals from amphibole-biotite gneisses, relics (cores) of older zircon and mantles that surround them were distinguished. Relict zircon is coarse-zoned, cracked, and its mantles have a thin-zoned structure. A small amount of transparent non-zonal zircon crystals was also found. Based on the results of zircon dating by the LA-ICP-MS method, it was determined that the age of the relict zircon is 3.16 Ga. The thin-zonal mantles have an age of 2.9-2.8 Ga, which probably corresponds to the time of crystallization of quartz diorites. Non-zonal transparent zircon with an age of 2.1 Ga corresponds to the time of dynamometamorphism (collision). BIF of the Lypovets deposit in the West Azov iron ore province, which belongs to the Kaiinkulak Unit of the West Azov Group, has a high Ni/Fe×10–4 = 0.5-2.1 ratio, typical for the Archean BIFs of the Algoma type. This deposit is confined to an Archean greenstone belt in a synclinal structure composed of metamorphosed basaltic komatiites and sedimentary rocks. Thus, the Verkhnii Tokmak Unit comprises Mesoarchean quartz diorites of crustal genesis, while the Kaiinkulak Unit probably represents heavily metamorphosed rocks of an Archean greenstone belt.

ПЕРВЫЕ ДАННЫЕ О ПОЗДНЕМЕЛОВЫХ ТЕРРИГЕННЫХ ОТЛОЖЕНИЯХ ВЕРХНЕЙ ЧАСТИ РАЗРЕЗА ЖУРАВЛЕВСКО-АМУРСКОГО ТЕРРЕЙНА В НИЖНЕМ ПРИАМУРЬЕ (ПО РЕЗУЛЬТАТАМ U/PB ДАТИРОВАНИЯ ЦИРКОНА)

The study was performed on the age and composition of little-studied Cretaceous terrigenous deposits of lower Priamurie, which are part of the Zhuravlevka-Amur terrane and are referred to the Berriasian-Valanginian Komsomolsk Series. U/Pb dating of detrital zircons yielded reliable dates of 99 and 90 Ma for the Gorin and Pioneer formations, respectively. Thus, for the first time, a Late Cretaceous age of sedimentation was identified for the sedimentary complex within the Sikhote-Alin orogenic belt based on U/Pb dating of detrital zircons. In terms of composition and detrital zircon age distribution patterns, the studied rocks are similar to the sediments of the upper Hauterivian-Albian part of the Zhuravlevka-Amur terrane section, and not the Berriasian-Valanginian. In rocks of the Pioneer Formation, the youngest population of zircon with an age of about 90 Ma accounts for more than 40 % of all the dated grains. Andesites of the Bolba Formation and granitoids of the Nizhneamurskii Complex, which are distributed nearby are probably the main source of clastic material of this age. Felsic rocks of the Albian-Cenomanian igneous province of Pacific Asia were probably the source of detritus for sediments of the Gorin Formation of about 99 Ma. The issue of inconsistency between the Late Cretaceous age obtained by us and the paleontologically established Berriasian-Valanginian stratigraphic age of the formations requires additional research. New data on the Late Cretaceous age of sediments of the Zhuravlevka-Amur pull-apart continental margin basin invite further investigations to better understand the geologic evolution of the terrane and the Sikhote-Alin orogenic belt as a whole.

The Late Cretaceous metamorphic rocks of the Akrotiri and Vari subunits on Tinos and Syros, Cyclades, Greece: field observations, geochemistry and geochronology

Abstract The relationship of the Late Cretaceous amphibolite-facies Akrotiri and Vari subunits on the Greek islands of Tinos and Syros to similar occurrences in the Aegean is not fully understood, but a correlation with the Asterousia Crystalline Complex on Crete and corresponding rocks in the Upper Cycladic Unit on other islands of the archipelago is a plausible interpretation. There is currently no clear evidence that the Akrotiri subunit represents a fragment of the metamorphic sole of the nearby Tsiknias ophiolite, as there are differences in field appearance, geochemistry, metamorphic grade and retrograde overprint. Felsic rocks from amphibolite-gneiss sequences of the Akrotiri and Vari subunits are interpreted as reworked igneous rocks with minor admixture of terrigenous material. The Triassic U–Pb zircon age (c. 240 Ma) of such a gneiss from Syros indicates the magmatic crystallization age of the original source rocks. A similar interpretation is suggested for the studied Akrotiri gneiss, but is less clear due to the not fully clarified significance of the presumed Pb loss thought to be responsible for the considerable U–Pb age range of the dated zircons (c. 256–114 Ma). The Vari gneiss sensu strictu is closely associated with metadioritic rocks containing a Triassic zircon population (c. 238 Ma). The protolith is probably plutonic, but a mixture of volcanic and plutonic detritus cannot be excluded. Riebeckitic amphiboles occur in all rock types of the Akrotiri subunit, indicating late overprinting at elevated pressures, which is not known from similar occurrences in the southern Aegean.

Detrital zircon U–Pb and Hf analyses of Silurian–Devonian sediments in the Sukhothai Terrane, northern Thailand: implications for the Mid-Paleozoic arc belt

The Sukhothai Terrane in northern Thailand consists of continental basement and a Permo-Triassic magmatic arc related to the subduction of the main Palaeo-Tethys Ocean. The Donchai Group represents the oldest sedimentary sequence of the Sukhothai Terrane and consists mainly of meta-sandstone, quartzo-feldspathic schist, phyllite and silty slate. We present new detrital zircon U–Pb geochronology, Lu–Hf isotope data and geochemical data for the sedimentary rocks of the Donchai Group to elucidate the depositional age, provenance and nature of the group. The youngest detrital zircon ages (433–403 Ma) suggest that the Donchai Group accumulated between 433 and 368 Ma, revealing Silurian–Devonian arc magmatic rocks on the western margin of the Sukhothai Terrane. Sediments of the Donchai Group were sourced from both the continental basement and a Silurian–Early Devonian magmatic arc, suggesting a depositional setting on the continental slope of a back-arc basin along the western flank of the Sukhothai Terrane. The Silurian–Devonian arc belt in SW China probably extends to the Chiang Rai region to the west of the Sukhothai Terrane, northern Thailand, revealing the northwards subduction of the Proto-Tethys Ocean along the western Simao and Sukhothai margin during the mid-Paleozoic. The inferred arc and back-arc configuration of the Proto-Tethys Ocean in northern Thailand is comparable with that recently established in Yunnan, SW China. Supplementary material: Detrital zircon U–Pb ages and trace element analyses, zircon in-situ Lu–Hf isotopic compositions and whole-rock geochemical data are available at https://doi.org/10.6084/m9.figshare.c.6858373

U–Pb Geochronology of the Baguio Area, a Major Mining District in Northern Luzon (Philippines)

ABSTRACTThe Baguio District, located in northern Luzon (the Philippines), is one of the major gold and copper mining districts in the Philippines and SE Asia. U–Pb zircon ages for igneous rocks and for detrital zircons extracted from sediment samples of the principal formations of the Baguio District reveal major magmatic pulses at ca. 1, 3, 16, and 20 Ma. The oldest zircon dated in this study has an age of 27.3 ± 1.6 (2σ) indicating earlier activity which has, however, left only a small imprint on the zircon inventory of the sediments. The absence of older zircons suggests that northern Luzon formed as an intra‐oceanic arc. Zircon Hf isotope data suggests that the magmas, from which the zircons crystallized, were derived from time‐integrated highly depleted mantle sources, compatible with an intra‐oceanic origin of the island. The results of the U–Pb dating of zircons mostly confirm previous estimates of the ages of the rock units. Mid‐Miocene ages obtained for samples thought to represent the Pugo Formation, the lowermost unit, which is thought to be Cretaceous‐Eocene in age, most likely date magmatic rocks that intruded the Pugo Formation.

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.

The Cerro Guacha Caldera complex, SW Bolivia: A long-lived, multicyclic, resurgent caldera complex in the Altiplano-Puna Volcanic Complex of the Central Andes

Measuring ∼50 × 30 km, the Cerro Guacha Caldera Complex (CGCC) is a polycyclic, nested caldera complex within the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Previous work had established that CGCC was built in three stages. Catastrophic supereruptions of the 1300 km3 (DRE - Dense Rock Equivalent) Guacha ignimbrite at ca. 5.65 Ma and the 800 km3 (DRE) Tara ignimbrite at ca. 3.49 Ma resulted in two nested caldera collapses that define the first two stages (G1C and G2C) of the CGCC. The third stage, G3C, initiated ∼1.8 Ma with the eruption of the Puripica Chico ignimbrite and associated lavas. The only previous detailed systematic study of the CGCC was of the G2C (Grocke et al., 2017a, 2017b). In this work we provide new field, petrochemical and geochronological data on the first and third cycles that now allow us to present the first comprehensive account of the eruption history, as well as the magmatic and volcanological evolution of the CGCC as a long-lived caldera complex.Erupted compositions during the G1C and G3C cycles are dominantly high-K, calc-alkaline dacites and minor andesites and these fall within the andesite to rhyolite compositional array previously defined for the G2C. In the CGCC as a whole, dacitic compositions account for over 90% of the ∼2200 km3 of magma erupted. These data reveal a consistency of magma composition, storage and evolution throughout the >4 Ma history of the CGCC.New UPb in zircon ages reveal distinct zircon age distributions that indicate that each caldera cycle represented new magmatic cycles that accumulated 10's to 100's of thousands of years prior to eruption. We find zircon antecrysts in all eruptive stages which we interpret as assimilation of non-erupted progenitors of earlier magmatic cycles. Occassional zircon xenocrysts and cores are interpreted as assimilation of upper crustal basement lithologies, supporting previous interpretations of upper crustal open system magmatic evolution for the G2C to the entire CGCC.Both the G1C and G2C calderas are west-hinged trap-door style collapses. Of these, the 5.65 Ma G1C caldera measures ∼50 km × 30 km, whereas the 3.49 Ma G2C is 30 × 20 km and is nested within the G1C. Resurgent uplift in both the calderas reveals at least a kilometer thickness of intracaldera ignimbrite. Outflow ignimbrites from the CGCC extend 100 km N-S and 40 km E-W around CGCC and were topographically limited. Intracaldera ignimbrite volume dominates outflow volumes by a ratio of ∼5:1. Repose periods between successive stages are now calculated to be 2.1 (G1C to G2C) and 1.8 (G2C to G3C) Myr respectively.The evolution of the CGCC parallels other APVC calderas and is closely linked to the history of the APVC flare-up. The two supereruptions at 5.65 Ma (Guacha Ignimbrite) and 3.49 Ma (Tara Ignimbrite) coincide with the peak of the flare-up and its eruptive and magmatic tempo correlate with the regional tempo. The eruptive and magmatic histories connote a composite granodioritic subcaldera pluton of significant extent accumulated in the upper crust beneath the CGCC.

Early–middle Jurassic source to sink evolution and its tectonic significance in the northeastern Sichuan Basin

This paper discusses the sedimentary environment and source supply in the sedimentary area and their coupling relationship through a detailed description of the deposits from the Ziliujing Formation to Shaximiao Formation of the Lower–Middle Jurassic on the Tieshan section in the Dazhou City, northeastern Sichuan Province, through sedimentary characterization, determination of the paleocurrent direction, analysis of heavy minerals, and detrital zircon U–Pb dating. The results show that the Zhenzhuchong Member is sufficiently supplied with detrital sediments and is dominantly composed of fluvial-delta deposits. The Dongyuemiao Member—the first member of the Lianggaoshan Formation—is dominated by lacustrine deposits, with the detrital supply increased initially in the early Lianggaoshan. The second member of the Lianggaoshan Formation suggests a significant increase in detrital supply, with shrunken lake basin and changed paleocurrent direction. The Shaximiao Formation reveals a complete disappearance of the lake basin in the northern Sichuan Basin. The comprehensive analysis on source supply indicates that the change in source property is apparently coupled with the change in the sedimentary environment, both controlled by orogenesis around the basin. It is inferred from the zircon age distribution that the changes in the sedimentary environment and source supply during the late Ziliujing period and the middle and late Lianggaoshan period resulted from the uplifting of the Micangshan Mountain, Dabashan Mountain, and Qinling Mountain, respectively.

Late Tonian (ca. 780–750 Ma) bimodal magmatism in the Ogcheon Metamorphic Belt, Korea, at the eastern margin of North China Craton

Neoproterozoic rift-related rocks of the North China Craton (NCC), including the Korean Peninsula, provide crucial information for global supercontinent reconstruction during the Rodinia breakup and Gondwana assembly. Here, we report not only zircon and baddeleyite U-Pb ages but also zircon Hf isotopic and bulk rock compositions of amphibolites in the Ogcheon Metamorphic Belt (OMB), Korea. Zircon occurs as either primary phase or secondary one replacing igneous baddeleyite. Primary zircons analyzed from three amphibolites yielded the weighted mean 206Pb/238U ages of 755 ± 5 Ma (n = 19), 772 ± 3 Ma (n = 9), and 779 ± 13 Ma (n = 12), respectively; this result is corroborated by the baddeleyite age of ∼760–745 Ma. Secondary zircons define discordia lines with lower intercepts corresponding to the Jurassic or Cretaceous age of granites emplaced adjacent to the amphibolite. In conjunction with the occurrence of coeval (ca. 762–745 Ma) metatrachytes, our result suggests that late Tonian bimodal magmatism has lasted for ∼30 m.yr. in the OMB. Moreover, positive εHf(t) values (4.9–21.4) of zircon in amphibolites indicate a depleted mantle source of mafic magma, whereas trace element compositions attest to an intraplate setting for the crystallization. A compilation of Nb-Th and Zr–Zr/Y data suggests that the intraplate setting is also pertinent to ca. 950–750 Ma mafic rocks from the Gyeonggi Marginal Belt, Korea, as well as the NCC. Such a tectonomagmatic correlation is corroborated by detrital zircon age spectra of metasedimentary rocks in the OMB equivalent to those of early Tonian rocks in the NCC. In marked contrast to tectonic scenarios conjecturing a linkage between the OMB and the Nanhua rift, South China Craton (SCC), we conclude that the Ogcheon Belt provides rare geologic-geochronologic evidence for late Tonian breakup of the Rodinia supercontinent at the eastern NCC margin.

Milankovitch cycles recorded by the Late Permian volcanic ash layers in southwestern China

The volcanic activity during the Cenozoic is thought to be forced by Earth's orbit, as it exhibits the Milankovitch cycle. However, the pattern of volcanic activity before the Cenozoic is still unclear. In this study, we studied the volcanic ash-bearing layers preserved in Upper Permian in Southwestern China and analyzed the cycle of volcanic activity from Emeishan Large Igneous Province (ELIP) during the waning stage of Emeishan mantle plume activity. Mineralogy, geochemistry, and gamma-ray (GR) data were used for high-resolution identification of volcanic ash-bearing layers. Al2O3/TiO2, aluminum normalized Th and Sn, and positive anomalies GR data can reflect volcanic ash input. The 206Pb/238U weighted age of zircon ranges from 256.00 ± 2.00 Ma to 251.47 ± 1.29 Ma, indicating synsedimentary volcanic ash. We apply cyclostratigraphic analysis to convert data from the depth domain to the time domain, and then utilize the multitaper method to analyze the cycle of volcanic activity of ELIP. Spectral analysis results reveal that the volcanic activity has long eccentricity (405-kyr), obliquity (∼33-kyr), and precession (∼20-kyr) periods. We propose the astronomical orbital forcing in volcanic activity during the Late Permian, and we have discovered volcanic activity cycles on a larger timescale of astronomical orbital periods (405-kyr long eccentricity). Volcanic activity was significantly correlated with sea level and climate change. During periods of low sea levels, the hot climate causes rapid sea level change, leading to rapid changes in crustal stress and strong volcanic activity. The absolute sea level controls volcanic activity, and the high sea levels suppress volcanic activity.

Rising mollusk bivalves from the ashes: Geologic, biostratigraphic and evolutionary implications from tuff data in the Permian Corumbataí Formation, Paraná Basin, Brazil

During the Permian, the southwestern Gondwana margin experienced episodes of intense volcanism tied to the Choiyoi magmatism. Several ash-fall horizons related to this magmatic event are preserved within the Late Paleozoic lake-sea sedimentary record of the Paraná Basin. Furthermore, near the Permian-Triassic boundary, a meteor fell in the northern border of the basin, generating unshocked zircon grains preserved in a stratigraphic marker called the Porangaba Bed. In this study, a new U-Pb zircon age of 265.17 ± 3.2 Ma is presented for a volcanic tuff in the upper part of the Corumbataí Formation, near the stratigraphic interval of the Porangaba Bed. In addition, Lu-Hf isotopic compositions of the zircon grains, whole-rock geochemistry, and atmospheric circulation data indicate the affinity of the ash-fall horizon with the Frontal Cordillera source. The combination of these new and previous U-Pb ages allowed us to draw a more realistic scenario of the evolution of bivalve mollusk of the Corumbataí Formation and to improve the use of these macrofossils in the biostratigraphy of the Permian succession of the Paraná Basin. In this context, we established the limits of the Pinzonella neotropica-Jacquesia brasiliensis Association Biozone between 262.38 ± 5.7 Ma and 257.5 ± 2.2 Ma. Hence, in the north and central Paraná Basin the local resolution of this biozone is of ∼5 Ma. Moreover, the range of this biozone indicates that the endemic bivalves survived the end-Guadalupian (ca. 259 Ma) extinction event unscathed.