Zircon Ages Research Articles

Late Paleozoic sedimentary and tectonic evolution of the Linxi basin in the eastern Inner Mongolia, eastern Central Asian Orogenic Belt

The eastern Central Asian Orogenic Belt (CAOB) encompasses a vast region from northeast China to Inner Mongolia, where there were many Late Paleozoic sedimentary basins that can provide key constraints to tectonic evolution of the eastern CAOB. Analysis of sedimentary sequence and environment of eight measured sections from the Linxi basin, Inner Mongolia indicates that there were shallow marine clastic-carbonate sedimentary facies, delta and littoral facies and terrestrial facies during the Early, Middle and Late Permian, respectively, and no oceanic sedimentary facies was found. Sedimentary sequence from the initial clastic-carbonate rocks to marine volcanic rocks reveals a transgression process during 300–270 Ma, while clastic and carbonate rocks with conglomerates and plant fossils represent a regression process during 270–250 Ma. According to the detrital zircon (DZ) age distribution of twenty-two samples (1610 zircons), the northern and southern provenances of the Permian sedimentary strata have been recognized, respectively. These DZ samples express the dispersed polymodal age populations and are attributed to a divergent dominated tectonic setting, implying the Linxi basin is an intracontinental extension basin during the Permian. This is consistent with an extensional process revealed by the syn-depositional deformed structures, coarse clastic sediments and olistostrome successions in the Linxi basin during the Early to Middle Permian and also similar to the speculation that there was an extension setting based on studies of volcanic rocks and intrusions in the Linxi basin. Combined with previous studies on the West Ujimqin basin, the Late Carboniferous to Permian tectonic frame of the eastern Inner Mongolia expresses extensional tectonics characterized by alternating combination of basins and uplifts superimposed on a pre-existing Early-Middle Paleozoic orogenic lithosphere and Precambrian basement, rather than a single ocean basin of Paleo Asian Ocean (PAO), which provides constraints for understanding sedimentary and tectonic evolution of the eastern CAOB during the Late Paleozoic.

The onshore West Baram Delta deposits: Provenance and drainage in the Middle Miocene to Pliocene in NW Borneo and comparison to the Champion Delta

The Baram Delta province in NW Borneo forms a major hydrocarbon reservoir offshore northern Sarawak and Brunei. The delta sequence is thereby subdivided into the West Baram delta to the south and the Champion delta to the north. Onshore are the remains of the Neogene delta deposits exposed and provide the possibility to study the equivalent offshore successions in outcrop. This study focuses on the Neogene West Baram delta successions which were studied for sedimentological facies and provenance characteristics. The successions consist of the Lambir, Miri, Tukau, and the enigmatic southern Lambir/Belait-Sarawak formations. Deposition took place in various mixed-energy delta environments between the Langhian and early Pliocene. The sediments are all quartz-rich and heavy minerals are dominated by ultra-stable zircon, rutile and tourmaline. Dominant detrital zircon age clusters are in the Early Cretaceous and Permian-Triassic. Based on light mineral petrography, heavy mineral assemblages, and detrital zircon U–Pb geochronology, all formations are interpreted as derived from multi-recycled sources, likely the underlying Paleogene Rajang Group turbidites and the Oligocene to Lower Miocene Nyalau-Tatau delta deposits. Additionally, literature data of the Champion Delta and one sample from Labuan analysed for provenance in this study are used to demonstrate that the Champion Delta can be distinguished from the West Baram Delta by having higher chrome spinel and garnet contents and slightly different detrital zircon age populations. The Champion Delta deposits are interpreted as sourced by recycling of the Crocker Formation and older turbidites (e.g., Sapulut Formation) with potentially input from ultra-mafic basement rocks of Sabah.

Concurrence of Late Triassic lithostratigraphic, radioisotopic, and biostratigraphic data support a Carnian age for the Popo Agie Formation (Chugwater Group), Wyoming, USA

The Upper Triassic Popo Agie Formation of the Chugwater Group of Wyoming, northeastern Utah, and northwestern Colorado has been an enigmatic unit since its definition and is commonly excluded from large-scale studies of continental Upper Triassic strata in the western USA. Lithostratigraphic correlations of Popo Agie Formation outcrops are documented from west-central Wyoming through northeastern Utah and northwestern Colorado, which demonstrates the presence of the Popo Agie Formation throughout this region. Unique detrital zircon age distributions have led previous workers to hypothesize a paleodrainage connecting basal units of the Dockum in west Texas and eastern New Mexico, USA, with the Gartra Grit, a basal unit of the Popo Agie in northeastern Utah. Biostratigraphically informative taxa such as parasuchid phytosaurs in the absence of leptosuchmorph phytosaurs support an assignment of the Gartra Grit and Popo Agie Formation to the Otischalkian estimated holochronozone. We present the first detrital zircon age distributions for the Popo Agie Formation. Multidimensional scaling analysis of zircon populations shows that the Popo Agie samples are similar to other Upper Triassic units surrounding the Ancestral Uncompahgre Highlands, Central Colorado Trough, and the Ancestral Front Range. Additionally, we present the first maximum depositional ages (youngest population) for the Popo Agie Formation at a location where the top of the ochre unit is no older than 225 ± 4 Ma, and the upper purple to ochre transition is no older than 230 ± 5 Ma. By leveraging existing biostratigraphy, regional lithostratigraphy, and new radioisotopic ages we temporally constrain the Popo Agie Formation, enabling us to integrate the upper Chugwater Group, Chinle Formation, and Dockum Group strata into a testable Late Triassic chronostratigraphic framework for the western USA. The consilience of data support a Carnian age for the majority (if not entirety) of the Popo Agie Formation, making this—and equivalent strata in the Dockum Group (i.e., Camp Springs Conglomerate, and strata of the Tecolotito and Los Esteros members of the Santa Rosa Formation)—among the oldest continental Late Triassic stratigraphic units in the western USA.

U–Pb zircon ages from tuffaceous beds in the Terreneuvian to Cambrian Series 2 sections of Avalonian southern New Brunswick, Canada: new constraints on chronostratigraphic correlations and the Cambrian time scale

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

U–Pb zircon ages from tuffaceous beds in the Terreneuvian to Cambrian Series 2 sections of Avalonian southern New Brunswick, Canada: new constraints on chronostratigraphic correlations and the Cambrian time scale

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

In the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution:•Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology.•Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest.•We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories.

The Late Carboniferous Mafic–Ultramafic Complex Induced by Slab Breakoff in Eastern North Tianshan, Central Asian Orogenic Belt

The Late Carboniferous to Early Permian is a critical period of the Chinese Tianshan, witnessing the tectonic transition from subduction to post-collisional extension during the final amalgamation of the Central Asian Orogenic Belt (CAOB). The late Carboniferous Mozbaysay mafic–ultramafic complex in the Qijiaojing–Balikun area, eastern North Tianshan, provides important clues for revealing the nature and timing of this tectonic transition. The Mozbaysay complex comprises mainly hornblende gabbros and lherzolites. LA-ICP-MS U-Pb zircon ages of hornblende gabbro yielded a weighted mean age of 306 ± 1.9 Ma for this complex. These mafic–ultramafic rocks have high contents of MgO (up to 30 wt.%), Cr (up to 2493 ppm), and Ni (up to 1041 ppm), but low contents of SiO2 (40.34–47.70 wt.%). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Th/Yb and Ba/Nb ratios imply the mantle sources could have been metasomatized by slab–mantle interaction with aqueous fluids from dehydration of the subducted slab. Thus, these mafic–ultramafic rocks were most likely produced by partial melting of the asthenospheric and lithospheric mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous mafic–ultramafic rocks was triggered by the decompression-induced influx of asthenospheric heat and melting through a slab window during post-collisional slab breakoff. Combined with geological data, the petrogenetic links of the Late Carboniferous mafic–ultramafic rocks in eastern North Tianshan to slab breakoff suggest that the tectonic transition from convergence to post-collision most likely initiated in situ at ca. 306 Ma and lasted to ca. 300 Ma.

Geochemistry and U–Pb zircon geochronology of S-type granites in the Karagwe Ankolean Belt, northwestern Tanzania: Implications for rare-metals mineralization

The Mesoproterozoic Karagwe Ankolean Belt in the Central Eastern Africa region hosts suites of granites that intruded supracrustal rocks and are associated with Sn–W mineralization. In northwestern Tanzania, the granites are S-type, peraluminous and calc-alkalic to alkalic-calcic in composition. Geochemical analyses of the granites reveal the following concentrations: SiO2 (71–77 wt%), Al2O3 (11–15 wt%), K2O (3–7 wt%), MgO (0.06–0.86 wt%), TiO2 (0.07–0.36 wt%) and Fe2O3T (0.71–2.40 wt%). The SiO2 contents correlate negatively with MgO, CaO and K2O and positively correlate with Nb, Ta, Zr, Sr, Ba, Hf, Th and the REE (Eu, Ce and Sm). Primitive mantle normalized spider plots show elevated LILE against depletions in Ba, Nb, Ti and La, suggesting inherited arc-magmatic signatures from the protolith. Chondrite-normalized REE spider plots show elevated LREE and relatively flat MREE and HREE as indicated by (La/Yb)cn = 2–9 and negative Eu* anomalies (0.05–0.24). Modelling of nearby Uha group sedimentary rocks partial melts, supports fractional crystallization and upper crustal contamination for generation of the KAB granites. Trace elemental ratios of Y/Nb, Nb/Ta, Zr/Hf and P205 suggest the granites are weakly mineralized to barren in Sn–W.The U–Pb zircon ages point to two successive and overlapping periods: from 1418 ± 15 Ma to 1395 ± 6 Ma (mean = 1406 Ma) and from 1385 ± 8 Ma to 1376 ± 11 Ma (mean = 1381 Ma). Inheritance U–Pb dates of xenocryst zircons, comprise early to late Paleoproterozoic ages ranging from 2376 ± 30 Ma, 2000 ± 27 Ma to 1901 ± 39 Ma, and from 1795 ± 7 Ma to 1738 ± 7 Ma, implying the participation of the Paleoproterozoic sedimentary rocks in the petrogenesis of the granites. A zircon rim overgrowth age of 1015 ± 26 Ma is interpreted to date a metamorphic/anatexis event that coincided with the c.1000 Ma amalgamation of the supercontinent Rodinia and the emplacement of Sn–W enriched granites. The ages of S-type granite magmatism (1418–1376 Ma) obtained from this study are restricted to the Mesoproterozoic within the widespread bimodal magmatism event c.1375 Ma.

Evolving Permian magmatism associated with arc migration in the southern Beishan Orogenic Collage, Central Asian Orogenic Belt, NW China

Abstract The Beishan Orogen contains the youngest suture in the Central Asian Orogenic Belt and is vital to understanding its terminal collision history and closure of the last vestiges of the Palaeo-Asian Ocean. Multidisciplinary evidence is provided for a close temporal and spatial linkage between the Liuyuan Complex back-arc ophiolite and the Ganquan Complex arc rocks. The Ganquan Complex mainly comprises felsic volcanic and volcaniclastic rocks and is divided into a lower and upper sequence, both of which exhibit arc-like geochemical signatures and have magmatic zircons characterized by mantle-like δ 18 O values. Their U–Pb zircon ages range between 295 and 286 Ma and between 283 and 281 Ma, with corresponding ε Hf values varying from −4.2 to +1.9 and from +1.3 to +7.6, respectively, indicating a more juvenile magma source over time. The increase of juvenile isotopic signatures in the Ganquan Complex rocks is attributed to arc-trench migration, which progressively removed the arc from influences of an old arc basement contaminant and led to back-arc spreading and formation of the Liuyuan ophiolite. Cessation of the multi-phase Ganquan arc system at c. 280 Ma coincided with the accretion of the continental Baidunzi Complex in the south.

Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona

Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.

Volcanism Pacing Slumping Gravity Flow Deposits during the Late Carboniferous in the Southern Margin of the Junggar Basin, China

AbstractDeep‐water gravity depositional processes and evolution in arc systems have become topics of intense research focus in recent years. This study discusses the co‐evolution of volcanism and deep‐water gravity flow deposits at the southern margin of the Junggar Basin, based on petrology, geochronology and geochemical analyses. The results show that a massive collapse of unstable sediments from the slope was triggered by volcanism, resulting in the formation of slumping gravity flows. The occurrence of volcanic beds in the slump deposits confirm that synchronous volcanism likely affected sediment instability, triggering gravity flows. The Th/Yb, Ta/Yb and Th/Ta elemental ratios, U‐Pb ages of detrital zircons and paleocurrent directions indicate that the North Tianshan (NTS) island arc represents the provenance of the Qianxia Formation. Moreover, statistical data on the pyroclastic components in the gravity flow deposits reveal an intensity index of volcanism, indicating that volcanism is strongly related to gravity flow deposits, especially in terms of the type and distribution of the deposits. A model for volcanically‐triggered deep‐water gravity flow deposits is established, in order to provide a more in‐depth understanding of the co‐evolution of volcanism and gravity flow deposits within the depositional setting of the late Paleozoic NTS oceanic subduction margin in the Junggar Basin.

Refining the understanding of Cretaceous subduction zone processes in the central Indonesian region: The Bantimala Complex (SW Sulawesi) revisited

A prominent occurrence of Cretaceous metamorphic rocks in the central Indonesian region is the Bantimala Complex, SW Sulawesi, where an eclogite- and blueschist-bearing sequence is part of an accretionary wedge with a complex internal structure. Zircon U–Pb dating of metamafic blocks indicate a significant protolith age range (c. 369–185 Ma) for the HP/LT rock suite. The HP/LT rocks include lawsonite-free and lawsonite-bearing samples. Lawsonite in eclogite is of retrograde origin. Assuming fast exhumation, multipoint Rb–Sr age data (c. 132–125 Ma) are interpreted to broadly approximate the time of peak HP/LT metamorphism. Existing and new K–Ar and Rb–Sr dates can be subdivided into two groups (c. 137–130 Ma and c. 126–113 Ma). Samples with post-eclogitic lawsonite growth belong to the older age group and all lawsonite-free samples are in the younger age group. The mineralogical and age diversity of various blocks indicates detachment of individual fragments from the subducting plate at different depths, followed by return flow in the subduction channel along different P–T–t trajectories. The role of serpentinite during exhumation remains ambiguous. Field observations offer little support for the suggestion that most HP/LT blocks were originally transported in a serpentinite matrix. Subduction-related magmatism is recorded by a U–Pb zircon age of 109.2 ± 1.1 Ma for a quartz dioritic mélange block. Detrital zircon U–Pb data from a sandstone indicate that tectonic stacking of the Bantimala Complex occurred only after partial erosion of already exposed Cretaceous basement.

Provenance changes of the Yangtze River Delta sediments since ∼3.6 Ma: Evidence from heavy mineral assemblages and detrital zircon U–Pb ages spectra

The Yangtze River drains from the eastern Tibetan Plateau and provides insights into the evolution of the Asian monsoon and uplift of the Tibetan Plateau. The age and evolution of the Yangtze River have been investigated, yet consensus remains lacking. This study aimed to investigate the sediment source changes of the Yangtze Delta since approximately 3.6 Ma. A new sediment core (YZQK2, 300.36 m) was subjected to high-resolution magnetostratigraphic, zircon U–Pb age spectral, and heavy mineral assemblage analyses. Magnetostratigraphy showed that 79.65, 171.22, and 293.4 m reflected the Matuyama–Brunhes, Gaussian–Matuyama, and Gilbert–Gaussian boundaries, respectively. The bottom age of the core was approximately 3.60 Ma. Between 3.60 and 3.44 Ma, heavy mineral assemblies were characterized as high reductive heavy minerals, pyrite, and siderite with the dominant peak of the detrital zircon U–Pb ages being 400–450 Ma. The sediments were primarily fed by tributaries draining into the middle Yangtze River, which were the Han River oriented in the Tongbai-Dabieshan orogenic belt and Xiang River and Gan River oriented in the Cathaysia Block. Between 3.44 and 2.79 Ma, the reductive heavy minerals, pyrite, and siderite disappeared, while garnet, apatite, sphene, and ilmenite showed a sharp increase. The dominant peak of detrital zircon U–Pb ages exhibited an extremely narrow range of 100–150 Ma. The sediments originated from the rock basin in the lower Yangtze River, the dominant river system, and were sourced locally or regionally from the adjacent Cretaceous granite plutons and volcanic rocks. The zircon age spectrum similar to the mainstream of the Yangtze River, containing amphibole, pyroxene, and magnetite from the upper to the middle and lower reaches of Yangtze River, has formed the Yangtze drainage pattern since 2.79 Ma. The sediment in the middle Yangtze reached the Yangtze Delta no later than 3.6 Ma, while the integration of the upper Yangtze into the Yangtze Delta occurred no earlier than 2.79 Ma. Our results provide novel insights into the established timeline of the Yangtze River.

Magmatic records from the Karakoram terrane: U–Pb zircon ages from granites and modern sediments in the Nubra Valley, NW Himalaya

The Karakoram terrane is a sliver of peri-Gondwana continental crust preserved within the Karakoram and Saltoro ranges in Ladakh, India. It became a part of Eurasia during the Jurassic, and until Late Cretaceous its southern margin was an active continental margin represented by the Karakoram (Arc) Batholith. Its magmatism is thought to be extinguished during Late Cretaceous by collision with the Shyok ophiolite, however, geochronological data are limited. The Karakoram terrane is a key element in the geodynamics of Himalaya and more studies are needed to improve geological understanding and tectonic evolution of the southern margin of Eurasia pre- and post- final continental collision with the Indian continent. We present petrographic, geochemical, and geochronological observations for anatectic magmatism which formed granitic intrusions within the terrane at c. 14 Ma and 12 Ma. Results from detrital zircon dating of modern sediments from the Karakoram terrane reveal three distinct age populations at c. 108, 82, and 13 Ma which correspond to known geological events. Current geochronological data for the Karakoram terrane indicate presence of two major magmatic episodes, the first occurred during Jurassic–Cretaceous and was related to subduction processes, and second, occurred during the Miocene and was related to the anatexis of the Karakoram lithosphere upon the terminal continental collision. The intervening magmatic events are still uncharacterised.

Detrital Zircon Geochronology and Provenance of Metasedimentary Rocks from the Susong Complex Zone in the Dabie Orogen

AbstractThe Susong complex zone is a relatively low‐grade metamorphic unit located in the southern part of the Dabie orogen and preserves a variety of metasedimentary rocks, mostly with epidote–amphibolite facies. However, their depositional age, provenance and tectonic setting of sedimentary protolith remain controversial due to the lack of fossils, precise dating and integrated geochemical investigation. This study has conducted whole‐rock elemental, and zircon U‐Pb SHRIMP dating and Lu‐Hf isotope analyses on three types of representative metasedimentary rocks including garnet‐bearing mica‐quartz schist, graphite‐muscovite‐quartz schist and dolomitic marble from the Susong complex zone. The U‐Pb SHRIMP dating data indicate that the sedimentary protolith of these rocks have the maximum depositional ages of less than 840–750 Ma and have various sedimentary provenances. In combination with zircon Lu‐Hf isotope compositions, the provenance of the metasedimentary rocks in the region is for the first time documented to be mainly derived from four groups of magmatic rocks formed at ∼2.5 Ga, ∼2.0 Ga, ∼1.4 Ga and ∼0.8 Ga in response to four episodes of igneous activity in the northeastern margin of the Yangtze Block. The conventional geothermobarometry combined with rare metamorphic zircon ages indicate that the studied rocks underwent the Late Triassic continental subduction‐related metamorphism with peak epidote–amphibolite facies conditions at P = 0.34–0.91 GPa and T = 427–532°C. In addition, results of the elemental contents (i.e., La, Ce, Th and ΣREE) and ratios (i.e., Eu/Eu* and LaN/YbN) suggest that the protoliths of the metasedimentary rocks were mainly shales, wackes and limestones, most probably related to the Rodinia supercontinent rifting along the northern margin of the Yangtze Block during the Neoproterozoic.

Initial cyclostratigraphy of the middle Nama Group (Schwarzrand Subgroup) in southern Namibia

The Ediacaran Period includes critical evolutionary trends of early complex life as well as climatic variations associated with ocean oxygenation, glaciations, and carbon cycling, which are still poorly understood in terms of astronomical climate forcing. The middle Nama Group in southern Namibia was deposited during the late Ediacaran in mainly shallow marine environments within a foreland basin and consists of hierarchically arranged depositional sequences. Here, we test a possible astronomical origin of these sedimentary variations by developing an initial cyclostratigraphic framework based on satellite images integrated with recently published high-precision U-Pb zircon ages. Regular sedimentary alternations occur dominantly on scales of several tens of meters, accompanied by smaller- and larger-scale variations, and are correlatable over distances of ∼50 km. Throughout the studied succession, 35 to 39 alternations are recognized on this dominant scale, which have an average duration of ∼120–180 k.y. This duration corresponds well with the period of short eccentricity (∼100 k.y.), given the likely presence of hiatuses, or alternatively, the period of obliquity amplitude modulation (∼173 k.y.), which would imply no time is missing on this scale. The dominant alternations are consistent with previously identified medium-scale sequences in this succession, which have been interpreted to record fluctuations in relative or eustatic sea level. We hypothesize that astronomically-forced fluctuations in eustatic sea level modulated deposition of the middle Nama Group. Geochemical studies suggested a relation between fossil distribution, redox variability and sea level, implying that astronomical forcing may have played a role in the distribution of early complex life.

Episodic water-fluxed anatexis recorded by migmatites from the Paleozoic Wuyi-Yunkai Orogeny in South China

The Wuyi-Yunkai Orogeny (WYO) in South China occurred during the assembly of Gondwana, and has abundant granites and migmatites which have been mostly interpreted as products of dehydration anatexis. However, the early Paleozoic migmatites in the Cathaysia Block, investigated in this study, instead document water-fluxed anatexis during the orogeny. The quartzo-feldspathic migmatites are moderately peraluminous, and have ca. 700 Ma and ca. 1800 Ma inherited zircon sub-populations, indicating that they were derived from Middle Neoproterozoic paragneisses. Titanite is common, while other anhydrous peritectic minerals such as garnet and pyroxene are absent in the migmatites and their leucosomes. Both mineral assemblages and microstructures indicate that the rocks were melted through reactions that include quartz + plagioclase + biotite + H2O = titanite + opaque + melt, K-feldspar + plagioclase + quartz + H2O = melt, and K-feldspar + biotite + quartz + H2O = melt. U-Pb ages of the newly-formed zircon and titanite display at least four age populations, recording episodic melting mainly at ca. 466, 437, 425 and 412 Ma, with evidence of local anatexis extending back to at least 480 Ma. Multiple stages of anatexis with short time intervals may have resulted from water-present melting due to episodic fluid influx. Zircon εHf(t) values are negative (-5 to -15) in the 700 Ma and 1800 Ma inherited zircons to nearly zero (0 to -5) in 470–400 Ma newly-formed zircons, and may have resulted from dissolution of the Lu-rich apatite during water-fluxed anatexis. The newly-formed zircons also have high δ18O values (9–12 ‰). These lines of evidence suggest that fluid flux-influenced melting was induced by metamorphic fluids that were released from polyphase metamorphism during crustal thickening and extension in the WYO, and channelized through regional shear zones. This highlights the extent and significance of water-fluxed anatexis and its role in the differentiation of the continental crust during orogeny.

Locating the Yangtze Block in Nuna: Constraints from age and isotopic data from Paleoproterozoic sedimentary rocks in the Phan Si Pan Zone, northwest Vietnam

New U-Pb ages and Hf isotopic data for detrital zircons from the late Paleoproterozoic sedimentary rocks (Suoi Chieng Formation) in the Phan Si Pan Zone of northwest Vietnam constrain their depositional ages, provenance, and tectonic setting. The weighted average age of the youngest coherent zircon population is 1769 ± 31 Ma (MSWD = 1.3, n = 8), which provides a maximum depositional age for the Suoi Chieng Formation. The detrital zircons obtained from six Paleoproterozoic samples exhibit various ages, showing multiple magmatic and metamorphic events. The detrital zircon age pattern shows that magmatic activities in the source areas occurred at 2650 Ma, 2350–2150 Ma, and 2050 Ma, as well as minor activities at 2850 Ma, 1850 Ma, and 1780 Ma. Additionally, a prominent metamorphic age peak is observed around ca. 1850 Ma, while a few metamorphic zircons exhibit ages ranging from 2600 to 2500 Ma and 2100 Ma. This pattern indicates that the primary source of these zircons is the local basement rocks of the Phan Si Pan Zone and SW Yangtze. However, contributions from other adjoining cratonic blocks are also possible and these likely included the Rae craton of North Laurentia. Our new data verifies and expands earlier results, showing that the SW Yangtze and north Yangtze have distinct Archean-Paleoproterozoic crustal evolutionary histories prior to their assembly in the late Paleoproterozoic. Regional correlations based on detrital zircon spectra, stratigraphic and tectono-thermal histories show strong similarities between the SW Yangtze and the Rae craton of north Laurentia throughout the Paleoproterozoic, suggesting the proximity of the two blocks before or during the formation of the Nuna supercontinent. We further suggest that the north Yangtze adjoined southern Siberia in the Archean to late Paleoproterozoic as supported by their identical tectono-thermal histories and detrital zircon age spectra. In combination with published data, we propose that the Yangtze Block was located between north Laurentia and south Siberia during the Nuna assembly.

Detrital zircon U–Pb analysis indicate a provenance shift on the Neogene Barreiras formation, atlantic coast of Brazil

The Neogene Barreiras Formation is composed of continental to shallow marine sedimentary rocks that fringe the Brazilian Atlantic coast. Due to the large outcropping extent, various open questions about the evolution of this unit are still under debate. We present new U–Pb detrital zircon age spectra of four samples from the southern Espírito Santo state (Marataízes section) and two samples from the southern Bahia state (Arraial section). In both of the studied outcrops, a basal succession composed of coarse-grained cross-bedded sandstone, conglomerate and arkose with mudstone and claystone intercalations is superposed by an upper succession composed of massive sandstone, conglomerate and wacke. The U–Pb detrital zircon age spectra of all samples are dominated by Cambrian and Neoproterozoic grains. However, while the lower portion of both sections is dominated by Ediacaran-Cambrian grains, their upper portions also present significant Cryogenian-Tonian and sparse Paleoproterozoic (mainly Rhyacian and Statherian) and Neoarchean grains. This marks an important provenance shift within the Barreiras Formation, with a simple, local provenance sourced from the G1 to G5 granite supersuites of the Brasiliano Araçuaí Orogen for the basal layers contrasting with exposure of older basement rocks and Neoproterozoic anorogenic suites during deposition of the superior layers. We interpret the lower coarse sandstone and arkose-rich succession as deposited in an alluvial setting, with mud/claystones related to local flooding plains. Tectonic movements related to the evolution of the Atlantic passive margin, as described in previous works, caused a renewed uplift of the basement areas, exposing and denuding older rocks. Then, the upper layers deposited through debris flows covering the first succession. Further geochronological data is needed to test if the erosional event responsible for exposure of older basement rocks and sourcing of the upper layers would correspond to the Tortonian Sea level fall, related to growth of the Antarctic ice sheet to its present size. In this case, deposition of the lower succession would have occurred during the early Miocene, and the upper succession during the Pliocene. Our results shed light in the complex evolution of the sedimentary rocks generally grouped under the Barreiras Formation, and suggest important tectonic and climatic shifts during the Neogene evolution of the Brazilian Atlantic coast.

Characterization and Geological Meaning of the Crystalline Basement Occurrence in the Unaí Region, Minas Gerais State (Central Brasilia Belt)

Granitoid rocks petrographically and compositionally characterized as tonalite, monzonite granodiorite and granite were observed in restricted areas in the central portion of the external zone of the Brasilia Belt. These outcrops of peraluminous rocks were interpreted as the sialic basement of the region, associated with paleogeographic highs prior to the deposition of the Proterozoic sedimentary cover. U-Pb zircon age of 2.14 Ga and geochemical analysis are compatible with other Paleoproterozoic bedrock areas observed mainly in the north Brasilia Belt. The existence of a basement bulkhead conditioned the Neoproterozoic deformation, causing the inflection of regional structures from NNW to N60-70W and over again to the NNW regional trend. The absence of contact metamorphism along the adjacent supracrustal rocks, regional foliation attitude that is different from that observed in the granitoids (tonalite, granotiorite, monzonite and granite) and the older age, show that these rocks represent basement windows and not younger intrusive bodies, as previously interpreted. The rocks studied in this research are correlated to the Aurumina Suite, which represents the main basement rock set in the North Brasília Belt.