Geochronological Constraints Research Articles

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

Geochronological constraints on Cryogenian ice ages: Zircon U Pb ages from a shelf section in South China

The Gondwanide deformation along the southwestern border of the Río de la Plata Craton: Geochemical and geochronological constraints on ductile shear zones from the Ventania System basement, Argentina

This work deals with the study of Gondwanide ductile shear zones developed in Neoproterozoic–middle Cambrian basement rocks of the Ventania System and possibly related to the reactivation of the Sierra de la Ventana Shear Zone. This reactivation was caused by the northeastward migration of the Gondwanide deformation from the North Patagonian Massif. The crustal shortening related to the Gondwanide Orogeny gave rise to thrusting of the Ventania System basement over Paleoproterozoic rocks of the Río de la Plata Craton. This work focuses on the study of muscovite ± quartz phyllonites of the Ventania System basement and quartz ± muscovite veins crosscutting basement and Paleozoic cover rocks of the system. Structural and magnetic surveys were performed in order to characterize the Gondwanide structures of the basement, while petrographic, X-ray diffraction, geochemical, and geochronological studies were carried out in order to investigate the nature, P-T conditions, element mobility, and age of these shear zone-related rocks. In the Sauce Chico Inlier, mylonitized basement rocks crop out along the western edge of the system, near the basement–Paleozoic cover interface, where the main structures are top-to-NNE reverse ductile shear zones with related phyllonites. Ductile shearing under greenschist-grade conditions (125–340 MPa and 300–400 °C) promoted major and trace element mobility. Phyllonites are the result of extreme hydrolysis of feldspars from the acidic igneous rocks of the basement, producing muscovite and releasing aqueous SiO2 later redeposited as quartz veins. These phyllonites developed in connection with reactivated contacts between basement units and also the basement–Paleozoic cover interface. New Rb–Sr quartz-muscovite and Ar/Ar muscovite ages for phyllonites and a quartz vein indicate a protracted tectono-metamorphic history mainly restricted to the Cisuralian (ca. 287 Ma), comprising regional folding and metamorphism, shear zone activity, vein-type mineralization, and syntectonic deposition of the Tunas Formation. Subsequent reactivations during the Lopingian (ca. 256 Ma) and probably in Late Triassic times (ca. 227 Ma) resulted from localized shearing along mylonitic belts.

Archean crustal generation and Neoproterozoic partial melting in the Ivindo basement, NW Congo craton, Republic of Congo: Petrology, geochemistry and zircon U–Pb geochronology constraints

We present systematic petrographic descriptions, geochemical and geochronological data from Souanké, Bomalinga, Elogo and Zoula areas in the Ivindo basement to understand the evolution of NW Congo Craton, Republic of Congo. Hornblende tonalite, biotite granodiorite, usually with hornblende-bearing enclaves, and granitic leucosome veins are ubiquitous in all areas. Specifically, coarse-grained muscovite-garnet-bearing pegmatite cross-cutting ancient mafic tonalite and felsic granodiorite in the Souanké area. Overall, three well-defined chemical patterns are described: (1) low SiO2 (57–63 wt%), high MgO + FeO (9–19 wt%) and metaluminous composition with intermediate LREE/HREE ratios and slightly positive Eu anomaly for tonalite, (2) middle SiO2 (67–73 wt%), MgO + FeO (7–1.5 wt%) and peraluminous composition with intermediate LREE/HREE ratios for granodiorite and (3) high SiO2 (72–75 wt%), low MgO + FeO (∼<1 wt%) and peraluminous character with low LREE/HREE ratios (flat patterns) and negative Eu anomaly in granitic veins and pegmatites. Regardless of the study area, the hornblende tonalites present similar ages (2884 ± 2.7, 2875 ± 5.6 and 2888 ± 3.2 Ma) that are near coeval with the ages presented by biotite granodiorites (2896 ± 8.9, 2880 ± 5.9 and 2875 ± 6.4 Ma), while granitic leucosomes have slightly younger ages (2850 ± 7.3, 2870 ± 8.2 and 2858 ± 7.2 Ma). Lastly, garnet-bearing pegmatite presents a Neoproterozoic/Cambrian transition age of 540 ± 1.6 Ma. Different enrichment LREE/HREE ratios combined with negative Nb–Ta and Ti anomalies may suggest a subduction-like setting for the generation of near coeval Mesoarchean tonalite-granodioritic crust. Whereas field relationships and flat REE patterns in the muscovite-garnet-bearing pegmatite indicate a younger ∼540 Ma crustal melting in the NW Congo Craton likely related to West Gondwana assembly. Further, these results confirm that the ∼2.9 Ga represents a significant period of continental reoworking within a magmatic arc setting.

The peridotite-pyroxenite sequence of Rocca d'Argimonia (Ivrea-Verbano Zone, Italy): Evidence for reactive melt flow and slow cooling in the lowermost continental crust

The present study investigates the origin of a ∼ 400 m thick peridotite-pyroxenite sequence, locally known as Rocca d'Argimonia, which is encased within the lowest levels of the ∼8 km thick Mafic Complex, a gabbro-dioritic batholith from the lower continental crust section of the Ivrea-Verbano Zone (Southern Alps). The broad purpose of this work is to elucidate the evolution of mantle-derived magmas emplaced in the lowermost continental crust. We also wish to provide new geochronological constraints on the intrusion and the cooling evolution of the Lower Mafic Complex, which are still mostly unknown.Zircon grains separated from two gabbronorites enclosing the peridotite-pyroxenite sequence document a whole reset of the original UPb isotopic system at 286 ± 2 Ma. A Rocca d'Argimonia peridotite provided a consistent age of 296 ± 19 Ma, based on a LuHf clinopyroxene-amphibole alignment (initial εHf = +7.9). These results are interpreted to date the early cooling evolution of the Lower Mafic Complex. The clinopyroxene-amphibole peridotite pair also gave a RbSr alignment corresponding to an age of 250 ± 16 Ma (initial 87Sr/86Sr = 0.7045), which is reconciled with the subsequent, slow cooling evolution of the Lower Mafic Complex.The Rocca d'Argimonia peridotites (dunites to harzburgites and lherzolites) have whole-rock εNd(286 Ma) and εHf(286 Ma) values ranging from +0.4 to −1.1 and from +8.8 to +3.4, respectively, and 87Sr/86Sr(286 Ma) ratios varying from 0.7048 to 0.7060. The peridotites are crosscut by gabbronorite dykes and associated with olivine-free orthopyroxene-dominated pyroxenites. The gabbronorite dykes and the pyroxenites share the Nd-Hf-Sr signature of the peridotites. The gabbronorites enclosing the peridotite-pyroxenite sequence differ in the relatively enriched Nd-Hf-Sr isotopic fingerprint (εNd(286 Ma) = −4.5 to −5.6, εHf(286 Ma) = +0.2 to −2.5, 87Sr/86Sr(286 Ma) = 0.7073 to 0.7086). Remarkably, pyroxenes from the pyroxenites typically contain higher Cr2O3 than the peridotite counterparts (e.g., 0.23–0.37 wt% Cr2O3 in peridotite orthopyroxenes and 0.43–0.50 wt% Cr2O3 in pyroxenite orthopyroxenes).We propose that the peridotite-pyroxenite sequence formed by reactive melt percolation through an olivine-rich spinel-bearing matrix. In particular, the relatively high Cr2O3 content of the pyroxenitic pyroxenes is attributed to redistribution of Cr released by the breakdown of accessory spinel that was originally present in the olivine-dominated matrix, in response to a high melt/solid ratio. The melt residual after the crystallization of the pyroxenites continued to percolate the olivine-dominated matrix, thereby forming spinel-bearing lherzolites, harzburgites and dunites with the progression of the melt migration process. The chemically most primitive dunite records Nd-Hf-Sr isotopic disequilibrium conditions, thereby providing further information about the process of reactive melt flow. It is inferred that the original olivine-dominated matrix crystallized from a mantle magma having a slightly enriched Nd-Hf-Sr isotopic signature, whereas the percolating melts overall had a relatively depleted Nd-Hf-Sr isotopic fingerprint, despite being variably contaminated by the continental crust.

Apatite fission-track thermochronology and magmatism-mineralization geochronology constraints on the tectono-thermal history of the Xiadian gold deposit, Jiaodong Peninsula, eastern China

胶东是全球唯一已知前寒武纪变质地体内的晚中生代巨型金成矿系统,其成矿机制独具特色,属胶东型金矿床成因新类型;深入剖析该成矿系统的形成、变化与保存过程对丰富和完善金成矿理论具有重要意义。夏甸超大型金矿床位于胶东招平金矿带中南段,赋存于晚侏罗世玲珑花岗岩和早白垩世郭家岭花岗岩中,是一典型的胶东型金矿床;该矿床成岩-成矿时代已有精确限定,但其精细演化过程与成矿后变化-保存尚缺乏有效约束,是开展矿床形成与隆升剥蚀研究的理想选择。因此,本文通过成岩-成矿年代学与磷灰石裂变径迹热年代学综合约束该矿床的构造-热历史,揭示其主控因素,提高对该成矿系统的认识,为深部找矿潜力评价提供支撑。本次获取的锆石U-Pb定年结果显示玲珑花岗岩和郭家岭花岗岩分别于151.7&#177;2.1Ma(2<i>&#963;</i>)和125.8&#177;1.6Ma(2<i>&#963;</i>)侵位。区域已有黑云母和白云母⁴⁰Ar/³⁹Ar年龄表明玲珑岩体侵位后缓慢冷却,并发生韧性变形;郭家岭花岗岩则经历了同岩浆韧性变形和快速冷却;约于124Ma两者均冷却至350&#177;50℃,区域发生韧脆性变形转换。夏甸金矿床与载金矿物共生的热液绢云母⁴⁰Ar/³⁹Ar和热液独居石U-Pb年龄表明,该区于约120Ma发生大规模脆性构造活动和金成矿作用。本文利用采自夏甸金矿床-652m中段532巷道中包括主控矿断层上、下盘经历不同变形程度的7件岩/矿石样品进行磷灰石裂变径迹(AFT)分析。除遭受异常热事件影响的主断裂上盘次级断层断层泥样品外,其余6件样品AFT中值年龄在32.5&#177;1.7Ma(1<i>&#963;</i>)至23.6&#177;2.9Ma(1<i>&#963;</i>)之间,其径迹长度平均值低于14μm(范围为12.0&#177;0.3μm~13.8&#177;0.3μm)、为单峰分布、并略具负偏斜特征,表明该矿床大致在32~23Ma单调缓慢通过磷灰石裂变径迹部分退火带(125~60℃)。综合区域已有热年代学资料与上述锆石U-Pb、AFT数据,重建了夏甸金矿床自赋矿玲珑和郭家岭花岗岩侵位以来的构造-热演化历史,计算出自120Ma金成矿至今该矿床平均冷却和剥蚀速率分别为1.8℃/Myr和0.059km/Myr,总剥蚀量约为7km,对比前人对胶西北地区整体剥蚀程度与深部成矿潜力的定量评估以及目前~1.5km的钻孔最大见矿深度,推断其深部找矿潜力良好。

The Sukari Gold Deposit, Egypt: Geochemical and Geochronological Constraints on the Ore Genesis and Implications for Regional Exploration

AbstractThe Sukari gold deposit (&amp;gt;15 Moz Au) in the Eastern Desert of Egypt is hosted by a deformed granitoid stock (Sukari tonalite-trondhjemite intrusion) and mainly occurs as a network of crosscutting sulfide-bearing quartz (± carbonate) veins and intensely sulfidized-silicified-sericitized wall rock. Emplacement of the Sukari intrusion into a tectonized Neoproterozoic accretionary complex was controlled by a system of NE- to NNE-trending oblique faults that are related to a deep-seated positive flower structure. A robust genetic model has been hampered by the poorly understood relationships between gold mineralization and host rocks. In this study, zircon U-Pb ages of three samples from the Sukari intrusion define a crystallization age of ~695 ± 2 Ma. In contrast, hydrothermal sericite from the ore zone yields an 40Ar/39Ar age of ~625 ± 3 Ma, which coincides with the onset of major sinistral transpression in the region.Features including sigmoidal morphology of gold quartz veins and abundant subhorizontal tension gashes alongside widespread brecciation and recrystallization suggest that quartz veining occurred during renewed shortening and exhumation through the brittle-ductile transition. Petrographic and micro-X-ray fluorescence (µXRF) studies indicate that disseminated gold and sulfides, commonly associated with sericite and carbonate alteration, are mostly confined to stylolitic bands in the quartz veins. Oscillatory and sector zoning patterns, irregular As-rich bands, and truncations between early- and late-genetic pyrites reflect variations in temperature and mechanism of ore deposition, demonstrated by variable As/S and Co/Ni ratios in the different pyrite generations. Laser ablation-inductively coupled plasma-mass spectrometry analysis pinpoints the covariance of gold and arsenic contents in pyrite, but free milling gold inclusions in microfractures consistently have a mercury-bearing electrum composition, depicting different ore formation stages.Ore fluids with δ34SH2s values of –1.9 to –3.0‰, modeled from gold-associated pyrite and arsenopyrite assemblages with nearly identical δ34S values, suggest a likely single source of sulfur. Alternatively, multisourced sulfur could have extensively mixed and equilibrated by fluid reaction with carbonaceous wall rock. Gold deposition was triggered by abrupt changes in fluid pH and fO2. As an implication for future exploration, sites of maximized strain gradients adjacent to granitoid bodies along extensive transpression zones in the district could be highly prospective targets, particularly where imprinted by sericite-carbonate ± graphite alteration.

Genesis of the Zhongshangou Au-Te deposit linked to alkaline magmatism at the northern margin of the North China Block: Evidence from sulfides Re-Os geochronology

The northern margin of North China Block (NCB) is a world-class gold metallogenic province with >1000 t gold, where has undergone multiphase tecto-magmatic overprinting since the Archean. Yet the genesis of the mineralization is still in doubt due to poor geochronological constraints using methods such as Ar-Ar and Rb-Sr dating. The Zhongshangou Au-Te deposit is located in the central part of the NCB gold metallogenic province, where it is structurally controlled and hosted by an amphibolite quartz monzonite phase of the Shuiquangou Alkaline Complex (SAC). Hydrothermal alteration at the deposit is characterised by an intense K-silicate assemblage with variable amounts of Fe-Mn carbonate, which is commonly observed in alkaline plutons associated with Au-Te deposits.Pyrite and chalcopyrite associated with the Au-Te mineralization contain very low common Os and yield model ages ranging from ca. 384 to 373 Ma, with a precise Re-Os isochron age of 379 ± 5 Ma (2σ, MSWD = 0.92). Analyses of molybdenite associated with the Au-Te mineralization yield precise model ages ranging from ca. 379 to 375 Ma, with a Re-Os isochron age of 379 ± 4 Ma (2σ, MSWD = 0.3). The Re-Os geochronology thus provides a reliable constraint of ca. 379 Ma for the mineralization, which is coeval with the syenite in the SAC. The Re-Os geochronology of the pyrite and chalcopyrite also yield a very low initial 187Os/188Os value of 0.22 ± 0.11, which indicates a significant source from the mantle material. Our evidence coupled with previous published data in the literature show that the Zhongshangou deposit is an alkaline pluton-hosted epithermal system related to the emplacement of the SAC in an extensional setting. The Re-Os geochronology using the low-level highly radiogenic sulfides has constrained the timing of the gold metallogeny at the northern margin of NCB and has opened new perspectives for gold exploration in the metallogenic province.

Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m3 in size); Group 1: 1100–1030 Ma and Group 2: 930–890 Ma. All pegmatites except those from the Østfold area crystallized significantly earlier or later than adjacent granites. The Tørdal granite, yielding an age of 946 ± 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite- and biotite-amphibole gneisses and amphibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating.Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt.Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville–Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville–Sveconorwegian orogenic belt were diachronous.We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

Implications of new geochronological constraints on the Aberfeldy stratiform barite deposits, Scotland, for the depositional continuity and global correlation of the Neoproterozoic Dalradian Supergroup

The Dalradian Supergroup of largely clastic metasediments in Scotland and Ireland is a well-known record of Neoproterozoic to Cambrian sedimentation on the Laurentian margin. Its apparent long duration, from ∼ 800 to ∼ 500 Ma, has encouraged a search for tectonic breaks and unconformities within the sequence. One suggestion is that a break in sedimentation occurred within the upper part of the Argyll Group, specifically in the Easdale Subgroup. Here we report dates for demonstrably synsedimentary barite mineralization hosted by the Easdale Subgroup Ben Eagach Schist Formation. Bedded pyrite from the Foss and the Ben Eagach–Duntanlich barite deposits, stratigraphically < 100 m apart, yields Re-Os ages of 604.0 ± 7.2 Ma and 612.1 ± 18.6 Ma respectively with two-sigma overlap. These ages are similar to the previously constrained 601 ± 4 Ma Tayvallich Volcanic Formation lying 2–5 km stratigraphically above. The new ages suggest continuous sedimentation through this period and, in combination with existing time markers, increase the likelihood that the basal-Argyll, Port Askaig Formation diamictite represents the Marinoan glaciation rather than the Sturtian glacial episode which may not be represented in the Dalradian sequence. As such, we urge caution in using the Dalradian in global compilations of Neoproterozoic history until further geochronological constraints are obtained for the Cryogenian-Tonian parts of the stratigraphy, which is now a high priority.

New age constraints on the MIS 9 – MIS 5.3 marine terraces of the Pontine Plain (central Italy) and implications for global sea levels

New constraints were set on the age of marine deposits in the Pontine Plain and of the related sea level indicators on the Tyrrhenian Sea coast of central Italy by twelve new 40Ar/39Ar dates on detrital sanidine from these deposits. By combining a new geomorphologic analysis and previous morpho-pedo-stratigraphic studies with these geochronological constraints we reconstructed the geometry of four marine terraces and correlated these with the highstands during the marine isotopic stages (MIS) 9.3, 7.5, 5.5 and 5.3.Results point to a progressive tilting of the terraces, the elevation increasing from the SE to the NW due to differential tectonic uplift that occurred over the last 300 ka. We identified a MIS 9 sea level at 30 - 25 m asl in the northwestern sector, whereas the MIS 7.5 sea level reached a maximum of 24 m asl in the NW and descended to 18 m asl in the central sector. Moderate tilting affected the MIS 5.5 sea level, with an elevation of 12 to 9.5 m asl in between the Anzio and Circeo headlands. Finally, an undeformed MIS 5.3 sea level at ca. 3 m asl is indicated throughout this coastal reach, confirming previous data suggesting a much higher absolute sea level during this highstand with respect to the δ18O-derived predicted level.

Overview of age constraints for gold mineralization in central and western Newfoundland and new 40Ar/39Ar ages for muscovite from selected auriferous zones

This contribution presents new 40Ar/39Ar laser step-heating data for muscovite associated with four significant orogenic gold-mineralized zones in central and western Newfoundland to build upon existing geochronological constraints for orogenic gold mineralization across much of the Newfoundland Appalachians. Additionally, we provide the first 40Ar/39Ar laser step-heating data for detrital muscovite from gold-mineralized sandstone of the Wigwam Formation (Botwood Group) at the Moosehead gold prospect. Most known gold zones on the isIand occur in proximity to reactivated crustal-scale faults and related structures, and are commonly localized within competent, rigid lithological units (e.g., granitoid rocks); although, some vein systems cut less competent, clastic sedimentary rock sequences. Host rocks range in age from Neoproterozoic to Devonian and may extend into the Carboniferous. Robust Pragian to Emsian (ca. 410–375 Ma) rutile, muscovite and zircon ages constrain the timing of gold mineralization in parts of the Exploits Subzone, whereas less precise age estimates for gold mineralization in the Notre Dame Subzone and Laurentian margin range from Wenlock to Emsian (ca. 433–375 Ma). Collectively, the geochronological data suggest that fluid-pressure cycling associated with gold mineralization in central and western Newfoundland occurred during progressive Siluro-Devonian tectonics associated with the waning stages of the Salinic orogenic cycle and spanning the Acadian and Neoacadian orogenic cycles. Multiple, polyphase, overlapping orogenic events allowed for repeated, gold mineralizing fluid flow events, particularly in proximity to long-lived, reactivated crustal-scale fault zones.

Paleo‐denudation rates suggest variations in runoff drove aggradation during last glacial cycle, Crete, Greece

AbstractFluvial aggradation and incision are often linked to Quaternary climate cycles, but it usually remains unclear whether variations in runoff or sediment supply or both drive channel response to climate variability. Here we quantify sediment supply with paleo‐denudation rates and provide geochronological constraints on aggradation and incision from the Sfakia and Elafonisi alluvial‐fan sequences in Crete, Greece. We report seven optically stimulated luminescence and ten radiocarbon ages, eight10Be and eight36Cl denudation rates from modern channel and terrace sediments. For five samples,10Be and36Cl were measured on the same sample by measuring10Be on chert and36Cl on calcite. Results indicate relatively steady denudation rates throughout the past 80 kyr, but the aggradation and incision history indicates a link with climate shifts. At the Elafonisi fan, we identify four periods of aggradation coinciding with Marine Isotope Stages (MIS) 2, 4, 5a/b, and likely 6, and three periods of incision coinciding with MIS 1, 3, and likely 5e. At the Sfakia fan, rapid aggradation occurred during MIS 2 and 4, followed by incision during MIS 1. Nearby climate and vegetation records show that MIS 2, 4, and 6 stadials were characterized by cold and dry climates with sparse vegetation, whereas forest cover and more humid conditions prevailed during MIS 1, 3, and 5. Our data thus suggest that past changes in climate had little effect on landscape‐wide denudation rates but exerted a strong control on the aggradation–incision behaviour of alluvial channels on Crete. During glacial stages, we attribute aggradation to hillslope sediment release promoted by reduced vegetation cover and decreased runoff; conversely, incision occurred during relatively warm and wet stages due to increased runoff. In this landscape, past hydroclimate variations outcompeted changes in sediment supply as the primary driver of alluvial deposition and incision.

Opening of the Algerian Basin: Petrological, geochemical and geochronological constraints from the Yaddene Complex (Lesser Kabylia, Northeastern Algeria)

The Yaddene complex, located in Lesser Kabylia (northeastern Algeria), is composed of Tertiary mafic and ultramafic rocks that outcrop within syn-rift sedimentary rocks of the Algerian marginal basin, in the Maghrebide belt of North Africa. Petrographic observations show that the Yaddene complex is composed mainly of two distinct lithologies: (1) a layered quartz-bearing gabbro at the bottom, which consists mainly of plagioclase (An93), clinopyroxene (altered to actinolite) and rare interstitial quartz; and (2) layered plagioclase-bearing lherzolites composed of olivine (Fo86-88) orthopyroxene (En86–87), clinopyroxene (diopside and augite), rare plagioclase (anorthite) and amphibole of pargasitic composition.The trace element concentrations in pyroxenes and olivine of the Yadenne complex show a depletion in light rare earth elements (LREE) similar to basalts of the Mid-Ocean Ridge (N-MORB), which reflects the depletion of the mantle source. In addition, high #Cr in spinel reflects a high degree of partial melting of the mantle source. On the other hand, slight enrichment in Large Ion Lithophile Elements (Cs, Rb, Ba), weak but significant negative Nb, Ta, Zr, Hf and Ti anomalies, and positive Pb spikes are observed in both whole-rocks and minerals. These patterns are best explained by interaction between parental mantle and fluids and/or siliceous melts derived from a subducting oceanic slab. The tectonic discrimination diagrams of minerals and whole rocks indicate that the most likely geodynamic setting for emplacement of the Yaddene Complex is a back arc basin environment.In situ U-Pb dating of euhedral zircons and titanite from a quartz-bearing gabbro yields an age of 18.97 ± 0.12 Ma (2σ), which is interpreted as the crystallization age of magmatic zircons and the gabbroic magma. Zircons from a plagioclase-bearing lherzolite gave a consistent age of 19.05 ± 0.39 Ma (2σ), which is attributed to zircon growth during metamorphic/metasomatic processes coeval to gabbroic magma emplacement. The mafic/ultramafic rocks of the Yaddene complex are intercalated within fine-grained sediments of the syn-rift “Oligo-Miocene Kabyle” and the obtained early Burdigalian ages are interpreted as the early stages of an extensional regime that ultimately led to the opening of the oceanic Algerian Basin.

Terrestrial records of deglaciation events during terminations V and IV in the central Apennines (Italy) and insights on deglacial mechanisms

40Ar/39Ar geochronology constraints to aggradational phases and grain size variations show that the two large gravel beds occurring in the sedimentary filling of the Liri fluvial-lacustrine basin (central Italy) recorded the occurrence of deglaciation events synchronous within uncertainties with global meltwater pulses at ca. 450 and 350 ka. In particular, we find a precise match between the ages of gravel deposition and the occurrence of moderate sea-level rise events which anticipate those more marked during the glacial termination V and IV in the Red Sea relative sea level curve, as already verified by data from the Tiber River catchment basin. Such correspondence suggests that gravel deposition is facilitated by melting of Apennine mountain range glaciers, which provide the water transport energy and a surplus of clastic input to the rivers draining the mountain regions and flowing into the Tyrrhenian Sea. Therefore, the thick gravel beds intercalated in the sedimentary filling of the catchment basins of the major rivers in central Italy may be regarded as an equivalent proxy of large deglaciation events, similar to the ice-rafted debris in northern Atlantic. Consistent with this hypothesis, we also show the close correspondence between the occurrence of particularly mild (warmer) minima of the mean summer insolation at 65° N and these early aggradational phases, as well as with other anomalous early sea-level rises occurring c. 750 ka and 540 ka at the onset of glacial termination VIII and VI, and 40 ka at the onset of the so-called Heinrich events.

Geochronological constraints of high-grade metasedimentary rocks of the Italva and Costeiro basins: Reconstructing the Outer Magmatic Arc System of the ribeira belt, SE Brazil

U-Pb provenance studies of the magmatic arc-related basins of the Ribeira Orogen are essential to unravel the depositional history and the paleogeography prior to the diachronic collision that has resulted in Neoproterozoic to Cambrian Western Gondwana assembly. This work brings new geology and LA-ICPMS data from metasedimentary units and felsic to mafic magmatic rocks that compose the Outer Magmatic Arc System (OMAS) in the Ribeira segment of the Araçuaí-Ribeira Orogenic system (AROS), contributing to unravel the construction of the West-Gondwana supercontinent. The OMAS evolution started in Tonian time (860–790 Ma) as the juvenile intra-oceanic Serra da Prata arc and evolved into the immature continental Rio Negro arc during the Cryogenian-Ediacaran times (650–620 Ma). The long-term subduction revealed the coeval stages development of arc-related basins. In the first Tonian stage, basin development is associated with the Italva group, divided into two sequences representing both fore and back-arc settings. The forearc sequence (Euclidelândia unit) comprises meta-psammitic sedimentary rocks with a single provenance source from the SP arc (peaks at 812 Ma and 828 Ma). The back-arc sequence (São Joaquim unit) consists of marbles with MORB-like interlayered amphibolites (ca. 831 Ma) bracketing the maximum age of carbonatic platform deposition. On the other hand, the younger Cryogenian to Early Ediacaran Costeiro back-arc basin is represented by three units of the São Fidélis group. The proximal Macuco unit, comprising meta psammitic rocks with marble, yielded a bimodal provenance distribution (852–815 Ma; 643–634 Ma), which reflects only the SP and RN magmatic arcs sources. The pelitic distal Upper and Lower units point out multimodal sources, coming from the SP-RN arc-rocks (ca 950–734 Ma; ca. 693–621 Ma) and Archean to Mesoproterozoic sources (ca. 3.12–3.0 Ga; 2.98–2.49 Ga; 2.29–1.50 Ga; 1.49–1.01 Ga), derived from ancient cratonic blocks located eastern wards, in African side. All units of the OMAS recorded two metamorphic episodes, with the oldest M1 (ca. 600–590 Ma) related to the collision of the OMAS against the Inner Magmatic Arc System (IMAS) and the younger M2 (ca. 545–533 Ma) that resulted from the final docking of the Cabo Frio terrane (Angola block) and finally closed the OMAS back-arc basin. Data obtained reinforces the long-term E-verging subduction that built the OMAS.

Long-lived Nb-Ta mineralization in Mufushan, NE Hunan, South China: Geological, geochemical, and geochronological constraints

The Mufushan Complex (MFSC), located in northeastern Hunan, is a significant producer of Nb-Ta-Li-Be rare metals in South China. The present study examines the genetic relationship, material provenance, fluid evolution, and metallogeny of the co-developed ore-free pegmatite (OFP) and ore-bearing pegmatite (OBP) in granite-related pegmatite-type Nb-Ta rare-metal deposits in MFSC. Three minerals (columbite-tantalite (coltan), zircon, and monazite) were chosen for analysis. The coltan grains display both primary crystallization structures (crystal homogeneity, oscillatory zonings, and primary growth rims) resulting from equilibrium and disequilibrium reactions due to localized changes in the physicochemical conditions and environment, as well as later replacement structures (alteration rims, patches, irregular zonations, and complex zonations) from metasomatic replacement processes related to hydrothermal fluid activity. The coltan yielded two weighted mean 206Pb/238U ages of 138.1 ± 2.1 Ma and 125.3 ± 2.0 Ma corresponding to magmatic and hydrothermal Nb-Ta mineralization ages. For the OFP, zircons also yielded two weighted mean 206Pb/238U ages of 138.4 ± 0.8 Ma and 131.5 ± 0.7 Ma, whereas monazite gave a weighted mean U-Pb age 142.9 ± 1.2 Ma. The ages of 142–138 Ma and 131 Ma represent the early and late stages of OFP crystallization and barren pegmatites in the MFSC, respectively. Zircon Lu-Hf isotopic compositions link rare-metal metallogenesis to the Lengjiaxi Group, which was the source material to the Mufushan composite batholith. Calculated εHf(t) values and TDM2 ages from the OFP (−7.6 to −3.6 and 1676–1418 Ma, respectively) and the OBP (−14.1 to +4.9 and 2976–1548 Ma, respectively) are akin to those of schists and metasandstones of the metasedimentary Lengjiaxi Group. We propose a long-lived (ca. 13-Myr) event involving two metallogenic episodes of Nb-Ta mineralization in the Mufushan region. This study demonstrates the potential of zircon, coltan, and monazite for fingerprinting minerals and classifying the mineralization potential of pegmatite veins.

The Fucino 250–170 ka tephra record: New insights on peri-Tyrrhenian explosive volcanism, central mediterranean tephrochronology, and timing of the MIS 8-6 climate variability

The Fucino Basin, central Italy, with its long and continuous history of Quaternary sediment accumulation, is one of the richest Mediterranean Middle Pleistocene tephra records. Here, we present a new detailed investigation of tephra layers (tephras) of the 250–170 thousand years before present (ka) interval, corresponding to the entire Marine Isotope Stage (MIS) 7 and parts of the MIS 8 and MIS 6. The investigated tephras have been characterised in terms of major, minor and trace elements, Sr–Nd isotopic compositions and 40Ar/39Ar ages. For correlation purposes, glass compositions and several new 40Ar/39Ar ages of selected proximal pyroclastic units spanning the same temporal interval from Vulsini (Latera Volcanic Complex), Sabatini, and Vico volcanic systems, central Italy, were measured. The late MIS 8-early MIS 6 Fucino tephras were backtracked to their corresponding volcanic sources, which include the Vulsini, Vico, Sabatini, Roccamonfina, Ischia and Campi Flegrei volcanic systems. While some of these tephras have been correlated to specific eruption units, other layers are currently not documented or described in near-vent sections, thus highlighting previously unrecognised events generated by these volcanic systems. Furthermore, the new high precision 40Ar/39Ar ages provide improved temporal constraints for Fucino making it one of the most detailed and chronologically best constrained tephra records for central Mediterranean MIS 7 tephrochronology. The Fucino record thus provides new integrative information for reconstructing the explosive history of Italian volcanoes during the investigated time interval. Furthermore, the geochronological constraints provide the basis for future paleoclimatic investigations at local and regional scale.

Timing of Tectonic and Magmatic Events in the Philippine Sea Plate Since 50 Ma From High‐Resolution Magnetostratigraphy of IODP Site U1438

The Philippine Sea Plate in the West Pacific is a unique natural laboratory to study subduction dynamics and the evolution of upper plate magmatism following subduction initiation. To investigate these processes, International Ocean Discovery Program Expedition 351 recovered at Site U1438, located in a rear-arc position, a complete sedimentary sequence from recent age to the early Eocene and the top of the underlying volcanic basement. The recovered cores offered the opportunity to study for the first time and in unprecedented detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu-Palau volcanic arc following the inception of the Izu-Bonin-Mariana subduction. Here, we report a magnetostratigraphy for Site U1438 based on ∼60,000 remanence directions isolated from 1,063 archive half core sections and 429 discrete specimens. We identified 142 magnetic reversals and correlated 115 of them with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, our magnetostratigraphy allowed construction of a high-resolution age model for Site U1438 and the determination of changes in sedimentation rates. We show that following subduction initiation at 52–50 Ma and the emplacement of basalts in the rear-arc at 48.7 Ma, a diffuse volcanism in the rear-arc (48.4–45.6 Ma) preceded the true emplacement of the Kyushu-Palau arc at 40.2 Ma, which then grew through four compositionally distinct eruptive phases until 28.8 Ma. Subsequent rollback of the Pacific slab triggered rifting of the arc (28.8–24.3 Ma) and ultimately back-arc spreading in the Shikoku and Parece Vela basins.

Mafic to felsic dyke swarms in coastal Guangdong, China: geochemical and geochronological constraints on the latest Mesozoic geodynamics on the southern margin of the South China Block

AbstractThe coastal region of Guangdong Province, China, is characterized by well-developed dyke swarms. The dykes with widths of decimetres to metres and lengths of tens of metres occur along straight and planar fractures cutting granites or volcanic rocks with Jurassic to Early Cretaceous ages. They show steep attitudes with strikes varying from NNW to NNE and from NE to SE, consistent with a stress regime transition from E–W to N–S extension. Major-element analysis on representative dyke samples reveals a composition range from basaltic to andesitic with a few dacitic outliers. Trace elements of most samples show notable Nb–Ta negative anomalies and Pb positive anomalies on the primitive mantle normalized spidergrams, characteristic of arc-related rocks. Among these, several dacite samples show notable fractionated medium rare earth elements and heavy rare earth elements and high Sr/Y (47–74) and La/Yb (15–21) ratios indicative of adakitic affinity. A few samples lack anomalies in Nb and Ta and have primitive trace-element ratios (e.g. Nb/La and Sr/Nd) or initial ϵNd values resembling ocean island basalt-like rocks. Rare earth element distribution patterns of all samples are right inclined and generally absent of Eu anomalies, which together with other trace-element indexes indicate an origin from heterogeneous mantle sources with depths below the stability field of plagioclase. Radiogenic isotopes, especially those of Nd and Sr (or Nd–Pb), define an array extended from the depleted to enriched mantle (EMII) provenance. LA-ICP-MS dating of zircon obtains ages between 110 and 70 Ma indicating emplacement of these dykes in latest Early to Late Cretaceous time, a time just before initiation of the South China Sea basin. In the context of regional geodynamics, it is proposed that these dykes were formed in a tectonic setting transiting from a Pacific-related back-arc to a passive continental margin pertaining to the development of the South China Sea basin.

Paleomagnetic, tectonic and geochronological constraints for Permian-Triassic oroclinal bending of the Mongolian collage.

Paleomagnetic, tectonic and geochronological constraints for Permian-Triassic oroclinal bending of the Mongolian collage.