Minimum Depositional Age Research Articles

Sand ridges from the coastal zone of southern Thailand reflect late quaternary sea-level history and environmental conditions in Sundaland

At many times during the Quaternary, when the global sea level was much lower than today (at least 30 m), the Thai-Malay Peninsula was connected by land to the Indonesian archipelago. This now submerged region is known as Sundaland and played a key role in the expansion of flora, fauna, and humans during the Quaternary. However, information regarding past environmental conditions is sparse as post-glacial sea-level rise submerged and eroded most sedimentary archives. In this study, a combination of geomorphological mapping, investigating sedimentary features, and optically stimulated luminescence dating is used to decipher the evolution of sand ridges found in the coastal zone of the Nakhon Si Thammarat Province, southern Thailand. The two main ridges (eastern and western) are composed of moderately to poorly sorted medium to coarse, structureless massive sands. The eastern ridge deposited between ca. 8.3 ka and 0.16 ka and formed by coastal processes related to sea-level change and storm surge during the Holocene. Surprisingly, the western ridge yields a minimum depositional age of ca. 82 ka, and further deposition dates to different periods of the Late Pleistocene. We suggest that the initial formation of the western ridge possibly occurred during the Last Interglacial (Marine Isotope Stage 5e). During periods of low global sea level, when the region was far away from the coast, the ridge accumulated further sand, most likely due to aeolian activity. This would imply a rather limited vegetation cover in the interior of Sundaland and support previous assumptions of savannah-like conditions during global-scale glacial periods.

Using multiple methods to better constrain the depositional age of a meta-sedimentary succession: An example of the Laoling Group from the northeastern North China Craton

The youngest detrital zircon (DZ) or groups of zircons age is usually interpreted as the maximum depositional age (MDA) and the oldest metamorphic zircon since deposition is used to resolve the minimum depositional age of a meta-sedimentary succession. However, DZ grains with multi-cycle or fingerprinting proximal sources may cause the MDAs older than the true depositional age, whereas post-depositional resetting has the opposite effect. Here, we present DZ U-Pb, Lu-Hf isotopic and trace elemental data, supplemented by whole-rock geochemistry, from the greenschist-facies metamorphosed Laoling Group consisting of clastic and carbonate rocks in the northern margin of the Jiao-Liao-Ji belt (JLJB) in the North China Craton to investigate the use of multiple methods to constrain its true depositional age. The dominant late Neoarchean and middle Paleoproterozoic DZ age peaks from the meta-sandstones, meta-siltstone, meta-mudstone, and meta-carbonate rocks are interpreted to be sourced from the Southern Jilin Complex and the JLJB, respectively. The low Th/U ratios (<0.18) and Ce/Nd ratios (<2.5) of the 1.96–1.77 Ga metamorphic zircons, which is consistent with the monazite ages of 1.95–1.76 Ga, are used to filter the possibly altered detrital magmatic zircons and the source-normalized α-dose metrics bigger than ca. 0.8 imply a predominant first-cycle origin, therefore the true depositional age of the group is constrained at 2.08–2.03 Ga. Additionally, in consideration of the strike of the basin parallel to the boundary between the JLJB and the Longgang Block, two periods of regional extensional tectonism, bimodal provenance, and depositional age coeval with the second magmatic event in the JLJB, we suggest that the Laoling Group most likely deposited in a back-arc basin in the northern margin of the JLJB. Finally, the temporal changes of zircon Hf isotope and Eu/Eu* in zircon crustal thickness proxy, especially the coeval excursion towards more radiogenic Hf signals and crustal thinning at 2.17–2.13 Ga, is consistent with the back-arc basin opening and closure model rather than the intra-continental rift model.

Unravelling the histories of Proterozoic shales through in situ Rb-Sr dating &amp; trace element laser ablation analysis

Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post depositional overprints in Precambrian sequences are challenging. We demonstrate a new laserbased analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation– inductively coupled plasma–tandem mass spectrometry (LA-ICPMS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleoredox reconstructions. Shales from well UR5 yielded initial ⁸⁷Sr/⁸⁶Sr ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 ± 56 Ma) with REEY and initial ⁸⁷Sr/⁸⁶Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.

SIMS Pb-Pb dating of phosphates in the Proterozoic strata of SE North China Craton: Constraints on eukaryote evolution

Meso-Neoproterozoic sedimentary successions in the southeastern margin of North China Craton contain a wealth of fossil and chemostratigraphic information, which aids in our understanding of early multicellular life and environment coevolution. However, a lack of reliable age constraints means the timing of these fossil occurrences and the corresponding evolutionary tempo of multicellular life remain disputed. This study aims to provide new age constraints by means of authigenic xenotime and monazite geochronology, coupled with morphological observation and mineral chemical analyses. Morphological observation and mineral chemistry allows differentiation of authigenic xenotime and monazite from detrital origins. Authigenic xenotime from the Shijia Formation, upper part of Meso-Neoproterozoic sedimentary successions, gave a SIMS Pb-Pb age of 919 ± 23 Ma, whereas authigenic monazite from the Xinxing and Liulaobei formations, lower part of Meso-Neoproterozoic sedimentary successions, produced SIMS Pb-Pb ages of 1086 ± 17 Ma and 1086 ± 120 Ma, respectively, all of which are interpreted as minimum depositional ages. The new ages suggest the crown group multicellular eukaryotes have highly evolved and diversified prior to 1.0 Ga, and confirm the use of Trachyhystrichosphaera aimika as an index fossil for the Mesoproterozoic-Neoproterozoic transitional period.

Unraveling the histories of Proterozoic shales through in situ Rb-Sr dating and trace element laser ablation analysis

Abstract Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post-depositional overprints in Precambrian sequences are challenging. We demonstrate a new laser-based analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation–inductively coupled plasma–tandem mass spectrometry (LA-ICP-MS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleo-redox reconstructions. Shales from well UR5 yielded initial 87Sr/86Sr ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 ±56 Ma) with REEY and initial 87Sr/86Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.

Cretaceous forearc basin remnant sediments in the eastern Yarlung Zangbo suture zone, near the eastern Himalaya syntaxis

Low-grade metamorphosed Cretaceous sedimentary records of forearc basin systems in the eastern part of the Transhimalaya (Gangdese) arc system have not been reported yet, impairing understanding of the Mesozoic-Cenozoic convergent margin processes along the southern margin of Asia. This study incorporates observations from petrography, stratigraphy, sedimentology, and analyses of detrital-zircon geochronology and bulk-rock Sr-Nd isotope for a series of low-grade metamorphosed sedimentary rocks referred to as the Lilong succession in the eastern Yarlung Zangbo suture zone, east of Langxian, in southeastern Tibet, China. Field observations reveal that this unit has three different lithological associations (I, II, and III) including gravel-coarse sandy inner-middle submarine fan facies, sandy-muddy middle-outer submarine fan facies, and silty-muddy outer submarine fan and/or deep sea plain facies moving higher in the succession. Detrital-zircon geochronology indicates a maximum depositional age of ~110 Ma and the absence of younger zircons that are dominant in the Gangdese batholith north of the basin suggests the minimum depositional age ~ 85 Ma, comprising an interval of the Albian-Santonian age. Volcanic and granitic clasts in sandstones/conglomerates, εNd(0) values (−0.09 and 4.6) and 87Sr/86Sr ratios (0.704105–0.704770) of slates, and detrital zircon age spectra (mainly 100–140 Ma) demonstrate provenance from the central-northern and southern Lhasa Terrane. In addition, there is evidence for recycling of cherty beds of the Chongdui Formation. Similarities between provenance, lithofacies, and tectonic setting lead us to interpret that the newly identified remnant unit correlates to the Ngamring Formation in the Xigaze forearc basin. When taking into account the lower Bomi Group and Nyingchi complex, we propose that the Lilong succession and the metamorphosed composite in the eastern Himalayan syntaxis are counterpart remnants of the eastern Xigaze forearc basin system, which are different from the two forearc basin systems in the western Himalaya.

Diagenetic and Epigenetic Mineralizing Events in the Kalahari Copperbelt, Botswana: Evidence from Re-Os Sulfide Dating and U-Th-Pb Xenotime Geochronology

Abstract The ages of sedimentation and copper-silver mineralization in the late Meso- to Neoproterozoic Kalahari Copperbelt in Botswana, an economically significant copper province, have previously been poorly constrained within an ~600 m.y. period that spans the Neoproterozoic from the assembly and breakup of Rodinia to the assembly of Gondwana. Rhenium-osmium geochronology of molybdenite and copper sulfide minerals and U-Th-Pb laser ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analysis of xenotime grains are utilized to provide absolute and relative age data on the host rocks and mineralizing events within the Ghanzi Ridge region of the Kalahari Copperbelt. The data reveal a prolonged history of events, which is partially comparable with depositional and mineralizing events in the neighboring Central African Copperbelt. Abundant disseminated molybdenite is located within a shale layer near the base of the Proterozoic D’Kar Formation at the Northeast Mango Two deposit. Unusual molybdenite textures suggest organic matter may have been a precursor. Two molybdenite separates from a small calcite-molybdenite stringer in a wall-rock fragment that is enclosed within an epigenetic quartz-calcite-chalcopyrite vein with ill-defined and mismatched margins yielded Re-Os ages of 981 ± 3 and 981 ± 7 Ma. These ages indicate an early hydrothermal mineralizing event in the basin. A xenotime inclusion intergrown with molybdenite and chalcopyrite within the epigenetic vein yielded a younger U-Th-Pb age of 538 ± 8 Ma, suggesting two mineralizing events are preserved in a complex 6-cm-wide vein. Based on vein texture and alteration, the ages represent an ~981 Ma calcite-molybdenite mineralization event overprinted by an ~538 Ma quartz-chalcopyrite-molybdenite mineralization event, perhaps during reopening of the vein. Re-Os and U-Th-Pb geochronology were utilized at the Zone 6 deposit on minerals associated with a hydrothermal quartz-calcite-chalcocite-idaite-bornite vein. Several authigenic xenotime grains that occur along the margin of the vein yielded three concordant U-Th-Pb ages that indicate xenotime growth at ~950 to 925 Ma while other xenotime grains in a similar position yielded mostly discordant data, suggesting disturbance of the isotopic system in the xenotime grains. A coprecipitated chalcocite-idaite mixture within the hydrothermal vein produced an Re-Os age of 549.0 ± 11.2 Ma. Re-Os analysis obtained from a coprecipitated molybdenite-bornite mixture at the Northeast Fold deposit yielded an age of 515.9 ± 2 Ma. Together, the earliest Neoproterozoic Re-Os molybdenite and U-Th-Pb xenotime ages provide both a minimum depositional age constraint for the lowermost D’Kar Formation and clear evidence that diagenetic hydrothermal mineralizing events took place within the Ghanzi basin. The timing of this mineralizing event corresponds with a poorly documented regional thermal event that affected the northern margin of the Kalahari craton during the final stages of the assembly of Rodinia at ~980 Ma. The lower to middle Ghanzi Group of the Kalahari Copperbelt is at least 100 m.y. older than the host rocks within the neighboring Central African Copperbelt, which are associated with the breakup of Rodinia. The latest Neoproterozoic to Cambrian Re-Os and U-Th-Pb ages indicate that hydrothermal copper-silver mineralizing events occurred during the Pan-African (~600–480 Ma) fold-thrust evolution of the Ghanzi-Chobe zone and were broadly synchronous with widespread epigenetic hydrothermal copper-cobalt mineralizing events in the adjacent Central African Copperbelt.

Paleo- to Mesoproterozoic magmatic and tectonic evolution of the southwestern Yangtze Block, south China: New constraints from ca. 1.7–1.5 Ga mafic rocks in the Huili-Dongchuan area

Paleo- to early Mesoproterozoic rocks in the Yangtze Block are extremely sparse and are predominately exposed along the southwestern margin. Their petrogenesis and tectonic significance remain unclear, resulting in a limited understanding of the tectonic evolution of the southwestern Yangtze Block during this period. Therefore, we report here detailed geochronological and geochemical analyses of newly discovered Paleo- to early Mesoproterozoic mafic intrusive rocks in the Huili-Dongchuan area, southwestern Yangtze Block and interpret their petrogenesis and tectonic significance. Secondary ion mass spectroscopy (SIMS) zircon and baddeleyite U-Pb dating yielded crystallization ages of ca. 1716–1714 Ma for gabbros and diabases (referred to as the 1.72–1.71 Ga group), ca. 1705–1700 Ma for gabbros and a diabase-porphyrite (the 1.70 Ga group), and ca. 1519–1504 Ma for a gabbro and a diabase (the 1.52–1.50 Ga group), which effectively constrain the minimum depositional age of the Tong'an and Dongchuan groups that they intrude to 1.72 Ga. The three groups (or episodes) of intrusive rocks differ from each other in their chemical compositions and isotopic characteristics, most likely because of varying degrees of melting in a convective upper mantle (probably the asthenosphere), which had been metasomatized by melts with enriched components input by a plume from the lower mantle. Modeling calculations suggest that fractional crystallization (FC) and assimilation and fractional crystallization (AFC) were the most important processes involved in modifying the chemical compositions of these rocks. These magmatic activities are interpreted to have been related to mantle plumes and continental rifting that occurred in the Huili-Dongchuan area near the southwestern margin of the Yangtze Block.

Shale and pyrite Re-Os ages from the Hornby Bay and Amundsen basins provide new chronological markers for Mesoproterozoic stratigraphic successions of northern Canada

Here we present Re-Os geochronological data from two carbonaceous shale units from the Hornby Bay and Amundsen basins that provide important chronological markers for Mesoproterozoic stratigraphic successions within northern Canada. Shale from the basal Escape Rapids Formation yields a robust depositional age of 1067.3 ± 13.5 Ma for the lowermost Shaler Supergroup of Amundsen basin with a remarkably unradiogenic initial 187Os/188Os value (0.15), implying that it derived from weathering of juvenile source material, such as basalt. Sedimentological features, indicative of large basin conditions, suggest deposition within an extensive epeiric sea that was intermittently mixed with waters from an exterior ocean. This age, and the 720 Ma age of the uppermost Shaler Supergroup, constrain deposition of the ∼4 km-thick succession to a period of approximately 350 myr, recording sedimentation through a complete supercontinent cycle - amalgamation through to break-up of Rodinia. Further, it indicates that the Amundsen basin formed at the same time as the Borden, and Fury and Hecla basins on northern Baffin Island, and the Thule basin in northwestern Greenland, and that these basins were perhaps connected as part of a much larger basin situated in the interior of Rodinia. Diagenetic pyrite from sandstone dykelets in carbonaceous shales of the Fort Confidence Formation yielded a Re-Os model age of 1438 ± 8 Ma, providing a minimum age for deposition of the lower Dismal Lakes Group of the Hornby Bay basin. This age strengthens proposed regional correlation of the Hornby Bay basin with the large intracontinental Athabasca and Thelon basins, located on the Rae craton to the southeast, and is evidence for a Laurentia-wide marine flooding episode ca. 1500 Ma. Both new ages place important constraints on newly discovered eukaryotic microfossils from the Shaler Supergroup and Dismal Lakes Group and calibration points for critical chemostratigraphic data.

Spatiotemporal constraints on the western Cauaburi Belt tectonics – northwestern Amazon Craton, Brazil

ABSTRACT The study of Precambrian tectonics is an intricate task, demanding a holistic approach to take an accurate view of geologic history. Here we attempt to set up the overview of an Orosirian-Statherian continental arc in the northwestern Amazon Craton, the Cauaburi Belt. Field relations, petrological, geochemical, and geochronological data of calc-alkaline metagranitoids, high-grade metasedimentary rocks, and peraluminous metasyenogranites yielded new interpretations about its tectonic evolution. The record of intense and episodic calc-alkaline magmatism between ca. 1805 and 1759 Ma, assign to the Cauaburi Complex, with field evidence of magma mixing and lodgement in a compressional tectonic regime, suggest a continental arc as the tectonic setting. During the magmatic lulls, deformation and metamorphism increase, culminating with the partial fusion of the high-grade metasedimentary rocks of the Taiuaçu-Cauera Complex. This succession composes the precursor paleobasin, presenting maximum and minimum depositional ages of 1819 and 1794 Ma, and was a relevant source for the calc-alkaline magmatism. The geochronological record of a low-volume production of peraluminous crustal rocks (Igarapé Tocandira Granite), with crystallization age at ca. 1791 Ma and metamorphic age at ca. 1771 Ma, also highlights the time and effect of the magmatic lulls. The Cauaburi Belt structural framework follows a general E-W to ENE-WSW trend, expressed in the Paleoproterozoic rocks as syn-magmatic and solid-state deformation fabrics. Assuming that Columbia Supercontinent nucleus assembled in the late Orosirian, we interpret that this coeval continental arc developed at its border and followed a north-south shortening direction, at current orientation.

Avalonian arc-to-platform transition in southeastern New England: U-Pb geochronology and stratigraphy of Ediacaran Cambridge “argillite,” Boston Basin, Massachusetts, USA

High-precision CA-TIMS 206Pb/238U zircon dates clarify the age and tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by ∼50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 ± 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 ± 0.20 Ma ash bed from the Mystic Quarry in Somerville, Massachusetts lies near the top of an approximately 5350 m thick, dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960 tunnels and from mapping logs obtained from tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 ± 0.16 Ma aplite sill intruding argillite ∼800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing strata of the lower Cambrian Weymouth Formation located south of Boston that overlies the Cambridge Formation without obvious break in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous after 584 Ma, the depositional interval would exceed 40 million years. An estimated ∼20 Ma depositional hiatus seems more likely because the base of the Cambridge Formation appears to define a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595 to 584 Ma Roxbury Conglomerate and the 597 to 593 Ma Lynn-Mattapan Volcanic Complex. Cambridge deposition set in once arc activity in more northerly “West” Avalonian terranes extending through Atlantic Canada to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic volcanic textures and thin basaltic flows with soft sediment contacts are present in argillite of the City Tunnel Extension, and whole rock major element and REE compositions reveal mixed terrigenous and volcanic components deposited under marine conditions throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560 to 550 Ma Coldbrook Group in New Brunswick9s Caledonia terrane.

Tourmaline 40Ar/39Ar geochronology and thermochronology: Example from Hadean-zircon-bearing siliciclastic metasedimentary rocks from the Yilgarn Craton

We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study.

Ferricretes at Burringurrah (Mount Augustus), Western Australia: Proof of lateral derivation

Surficial iron-rich deposits (ferricretes) occur on and around Burringurrah (Mount Augustus), a huge, remote inselberg in the Gascoyne district of Western Australia. Oriented NW-SE, Burringurrah is approximately 14 km long, 5 km wide, rises to 1105 m above sea level and stands some 700 m above the surrounding plain. The majority of the ferricretes occupy bedrock valley bottom floors on the southern side of the inselberg at Edney Springs, Flintstone Creek, and on the Edney Trail where they have formed on unweathered quartzose Mount Augustus Sandstone, which only contains a few percent Fe2O3. At ‘The Pound’, a boulder ferricrete overlies deeply weathered igneous bedrock, unroofed by erosion of the overlying Mount Augustus Sandstone, exposing the unconformity between the weathered granitic bedrock and the Mount Augustus Sandstone in the core of a broad, asymmetrical and doubly plunging anticline. On the northern, steeper, side of the mountain, vermiform and conglomeratic ferricretes are sparsely exposed in drainage lines. Ferricrete mineralogy is dominated by goethite with only small amounts of hematite. Oxygen isotope inferred ages of weathered monzogranite produced a top-down weathering front extending up to the Neogene at an elevation coincident with the ferricretes within bedrock valley bottoms. Beyond the mountain, the age of ferricreted sandy alluvium was established by thermoluminescence polymineral fine grain dating. These produced a minimum depositional age of >37.6 ± 2.6 ka and a second finite age of 139 ± 44 ka, suggesting ferricrete formation here may have begun during the last interglacial period and continued throughout the late Pleistocene. Formation of ferricretes on top of iron-poor quartzites is unusual. It reflects not only the absolute accumulation of iron oxides derived from the Mount Augustus Sandstone but possibly the additional contributions of iron-bearing aeolian dust from the surrounding plains subsequently washed down drainage lines to accumulate in swampy depressions at the foot of the inselberg. Ferricrete formation was initially simple, with iron in solution impregnating sandy sediment and organic host materials, sporadically impacted by bioturbation by termites. Thin-section petrography reveals an interesting form of in-situ, self-brecciation of quartz and feldspar clasts, which are fractured and infilled with iron oxides and newly precipitated clay which further promotes fragmentation. The mode of ferricrete formation described from Burringurrah is relevant to the laterite-ferricrete debate, for here it is most unlikely that the zones of iron enrichment developed in situ and vertically above the iron-poor quartzite basement as in the sense of the regional ‘classic laterite profile’.

Evidence for ca. 2046 Ma high-grade metamorphism in Paleoproterozoic metasedimentary rocks of the northern Borborema Province, NE Brazil: constraints from U-Pb (LA-ICP-MS) zircon ages

This study presents LA-ICP-MS U-Pb detrital zircon ages on a migmatitic paragneiss of the Jaguaretama Complex, northern Borborema Province, NE-Brazil. This metasedimentary rock comprises a stromatic migmatite with alternating layers of metapelite, metapsammite, and garnet-tourmaline-bearing leucosome. The paleosome displays a well-developed schistosity, with fine to medium-grained textures, and locally garnet porphyroblasts. Backscattered electron images show that many of the zircon grains display metamorphic overgrowths (recrystallization) and cores with igneous oscillatory zoning. The detrital zircon grains show 207Pb/206Pb ages clustering from 2.51 to 2.12 Ga (with 90% conc.), evidencing that the igneous sources were generated mostly during the Transamazonian/Eburnean orogeny. The age of 2137±9 Ma of the youngest and most concordant detrital zircon (core) is interpreted as the maximum depositional age of the protolith. The youngest and most concordant (conc. 99%) 207Pb/206Pb apparent age obtained in a metamorphic zircon overgrowth was 2046±6 Ma, which is interpreted as the age of high-grade metamorphism. This age of metamorphism is similar to other records of Paleoproterozoic high-grade metamorphic events in the Borborema Province and other cratonic domains (e.g. Sao Francisco and West Africa cratons). This age also represents the minimum depositional age of the protolith. Therefore, the depositional age of the studied supracrustal rock may be bracketed between ca. 2.14 and 2.05 Ga.

Age and provenance of the Chaung Magyi Group, Yeywa Dome, Myanmar, based on U-Pb dating of detrital zircons

The unfossiliferous Chaung Magyi Group of the Northern Shan Plateau of Myanmar, comprises metasedimentary rocks, in particular greywacke, slates, phyllites and schists, of inferred Proterozoic age that unconformably underlie the late Cambrian Pangyun Formation. To address the problem of the youngest stratigraphic extent of the group, four samples of the Chaung Magyi Group from the Yeywa Dome, East of Kyaukse, Mandalay Division, were collected for U-Pb dating of detrital zircons. Based on the youngest detrital zircons present in sandstone samples from the western Yeywa Dome area, at least the upper part of the Chaung Magyi Group is of latest Proterozoic age and possibly extends to the late Cambrian (~500 Ma). Comparison of detrital zircon age-frequency histogram distributions show only slight differences in distributions of detrital zircons in the Cambrian of NW Thailand and the Chaung Magyi Group samples (Sibumasu Terrane), with a concentration of ages between 1300 Ma and 820 Ma, with two major peaks at 1086 Ma and 1022 Ma. A younger Cryogenian age peak at 647 Ma is also observed. A spread of older concordant ages ranges from 3093 Ma to 1406 Ma. The minimum depositional age for the Chaung Magyi Group of the west of the Bawdwin Mine (982 Ma) based on ages of cross-cutting dykes (reported in Mitchell, 2018), and the maximum depositional age for Yeywa Dome (~600–500 Ma) based on detrital zircons suggest the age difference (~>400 my) is too great for the Chaung Magyi Group to be considered a single group, the dyke ages require re-investigation.

Contemporaneous formation of vein-hosted and stratabound gold mineralization at the world-class Dead Bullock Soak mining camp, Australia

We investigate the relative and absolute timing associated with the formation of the world-class Dead Bullock Soak (DBS) mining camp. The study area is situated in the Paleoproterozoic Granites-Tanami Gold Province in the Northern Territory, Australia, and consists of five orogenic gold deposits, including the Callie deposit with a resource of 14.2 Moz Au. At DBS, the ore is hosted in Paleoproterozoic metasedimentary rocks and consists of two contrasting styles of mineralization, vein-hosted and stratabound. The vein-hosted mineralization consists of visible gold in quartz, chlorite, carbonate veins preferentially hosted in finely laminated, decarbonized siltstones. In contrast, the stratabound mineralization consists of free gold associated with arsenopyrite concentrated in bedding-parallel sulfide-rich layers, hosted in iron-rich siltstones. This study combines structural and petrographic analysis with detailed U–Pb geochronological data on hydrothermal and metamorphic xenotime and monazite to constrain the relative and isotopic timing of ore formation in the DBS mining camp. The results of this study show that both mineralization styles developed within a series of NE-trending and SE-dipping vein corridors or structural pathways, associated with the development of axial planar fabric related to small-scale F2 folds. U–Pb geochronology shows that both mineralization styles formed at c. 1805 Ma, approximately 35 m.y. after peak metamorphism at c. 1840 Ma. A post-mineralization hydrothermal event is locally remobilizing gold and yields an age of c. 1790 Ma, which provides a minimum age for gold deposition.

Early Permian strike-slip basin formation and felsic volcanism in the Manning Group, southern New England Orogen, eastern Australia

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U–Pb (SHRIMP) age of 295.6 ± 4.6 Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.

Depositional history and provenance of cratonic “Purana” basins in southern India: A multipronged geochronology approach to the Proterozoic Kaladgi and Bhima basins

Peninsular India is a collage of Archaean cratonic domains separated by Proterozoic mobile belts. A number of cratonic basins, known as “Purana basins” in the Indian literature, formed in different parts of the Indian Peninsula during extensional tectonic events, from Paleoproterozoic through Neoproterozoic times. In this contribution, we present a diversity of new geochronological data for different units within the Kaladgi and the Bhima basins, which overlie the western and eastern Dharwar cratons, respectively. The new geochronology data are discussed in terms of depositional history and provenance of these poorly understood Proterozoic intracratonic basins. For the Kaladgi Group, a U–Pb baddeleyite age of 1,861 ± 4 Ma obtained for a dolerite dyke intruding the Yendigere Formation is used to constrain the minimum age of deposition of the lower Kaladgi Group. This result demonstrates that this part of the succession is comparable in age to the Papaghni Group of the Cuddapah Basin, heralding onset of Purana sedimentation at ~1,900 Ma. The detrital zircon populations from the clastic rocks of the Kaladgi and Bhima basins show unique and distinct age patterns indicating different source of sediments for these two basins. Palaeocurrent analysis indicates a change in provenance from south or southeast to west or northwest between the Kaladgi and Bhima clastic sedimentation. New U–Th–Pb and Rb–Sr radiometric dates of limestones and glauconite‐bearing sandstones of the Bhima Group (Bhima Basin) and the Badami Group (Kaladgi Basin) indicate deposition at around 800–900 Ma, suggesting contemporaneity for the two successions. Thus, the unconformity between the Kaladgi Group and the overlying Badami Group represents a time gap of up to 1,000 Myr. These new results demonstrate the complex multistage burial and unroofing history of the Archaean Dharwar Craton throughout the Proterozoic, with important implications for exploration of metal deposits and diamonds in Peninsular India.

The Neoproterozoic southern passive margin of the São Francisco craton: Insights on the pre-amalgamation of West Gondwana from U-Pb and Hf-Nd isotopes

Sedimentary basins are, in many cases, the only preserved record of the geological processes active through geological time. These deposits preserve age, isotopic and geochemical signatures of the source rocks, providing insights into tectonic setting. In this context, we investigate the lithostratigraphy, provenance and tectonic environment of the Carrancas Group, a meta-sedimentary sequence characterised by a basal psammitic unit (Lower Unit) passing up into interbedded psammo-pelites (Intermediate Unit), which are overlain by a further psammitic unit (Upper Unit). This succession, along with the Canastra, Paranoá and Vazante groups (Brasília orogen), comprises a mature passive margin developed at the western-southern margin of the São Francisco craton during the Neoproterozoic. Detrital zircon U-Pb ages and Nd-Hf isotope data enable revised stratigraphic correlations across the basin, highlighting variations in the source of detritus. Deposition began after 0.92 Ga, with the sedimentation of the Lower Unit, with input of juvenile ∼2.1 Ga detritus (εNdt = −0.1 to +3.2) derived from the São Francisco craton. A marine transgression characterises the transition to the Intermediate Unit, which gradually covered over juvenile ∼2.1 Ga terrains and rendered them unavailable as possible source rocks for detrital zircons. Evolved Archaean-Palaeoproterozoic basement rocks, the Espinhaço Supergroup and evolved Neoproterozoic granitoids, all belonging to the São Francisco craton, acted as sources for the Intermediate and Upper units (U-Pb = 2.7 Ga to 0.9 Ga; εNdt = −10.4 to +7.1). The Goiás massif might also have acted as a source, providing juvenile Mesoproterozoic detritus. The sediments of the Upper Unit register a significant shift in source, with incoming of juvenile Mesoproterozoic and crustal 1.9 – 1.7 Ga detritus. This change in the ages of the detrital material may be related to exhumation of the Espinhaço Supergroup during the evolution of the passive continental margin basin. The whole-rock Nd TDM model ages become progressively younger from the base to the top of the sequence (Nd TDM = 2.6–1.8 Ga). The collisional events between Amazonian and São Francisco cratons and the Paranapanema block set the minimum age of deposition along this passive continental margin at around 0.67 Ga.

U–Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton

AbstractSupracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb &gt; 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb &lt; 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a 207Pb/206Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.