U–Pb dating and geochemistry of granite porphyry dykes in the Xicha gold–(silver) deposit,southern Jilin Province,China,and their metallogenic significance

We report U–Pb dating of zircon,as well as geochemical and Hf isotope data,in order to constrain the formation time,magma source,and tectonic setting of granite porphyry dykes in the Xicha gold–(silver) district in southern Jilin Province,Northeast China.The zircon grains are euhedral–subhedral,display oscillatory growth zoning and have Th/U ratios varying between 0.11 and 0.78,which together imply a magmatic origin.The dating results indicate the porphyry formed in the Early Cretaceous (122±1 Ma)and it contains SiO_2=70.64–72.31 wt%,Al_2O_(3-)=13.99–14.64 wt%,K_2O+Na_2O=6.96–7.81 wt%K_2O/Na_2O=1.24–2.10,and A/CNK=1.11–1.41.Chemically,the porphyry belongs to a high-K calc-alkaline S-type granite.Chondrite-normalized rare earth elements (REE)patterns show LREE enrichment,light rare earth elements(LREE)/heavy rare earth elements (HREE)=9.93–11.97(La/Yb)_N=11.08–15.16,and d Eu=0.69–0.95.On the trace element spider diagram,large ion lithophile elements such as Rb,Ba,K,Th,and U are enriched,whereas the high field strength elements Ti and P are depleted.The e Hf(t) values of zircon from the granite porphyry vary between-17.1 and-13.2,and their Hf two-stage model ages vary from 2.01 to 2.26 Ga,implying that the magma was derived from partial melting of old lower crust.The granite porphyry dykes and many A-type granites in the region formed at the same time,suggesting an extensiona environment.The combination of the occurrence of strong magmatism,large-scale mineralization,and extensiona tectonics throughout much of Eastern China indicate that the Early Cretaceous was a period of significant lithospheric thinning.The southern Jilin Province,therefore,experienced lithospheric thinning during the Early Cretaceous.     

Origin of the Yueguang gold deposit in Xinhua,Hunan Province,South China: insights from fluid inclusion and hydrogen–oxygen stable isotope analysis

The Yueguang gold deposit is located in Fengjia, Xinhua County, Hunan Province, South China. It represents a recently discovered small-scale gold deposit situated in the southwestern region of the Jiangnan Orogenic Belt, west of the Baimashan granitic batholith. In order to discern the characteristics of the ore-forming fluids, the underlying mineralization processes, and establish a foundation for the origin of the Yueguang gold deposit fluid inclusion micro-thermometry, as well as quartz hydro...     

Zircon U–Pb dating and whole-rock elemental geochemistry of the Shazi anatase deposit in Qinglong,Western Guizhou,SW China

The Shazi deposit is a large-scale anatase deposit in Qinglong, Guizhou Province. Zircon grains from this deposit yielded a zircon U–Pb age of *259 Ma, representing the formation age of the deposit's parent rocks.This age is identical to the eruption age of the Emeishan large igneous province, indicating a synchronous magmatic event. The rare-earth-element patterns of laterite samples were similar to those of the weathered basalt sample, and sub-parallel to those of the Emeishan high-Ti basalts,implying a genetic relationship between the laterite and the basalt. The Chemical Index of Alteration values of laterite ranged from 96 to 98, suggesting a high degree of weathering. SiO_2, MgO, and alkaline metal elements decreased with increasing degree of weathering, while Al_2O_3, Fe_2O_3,and TiO_2 increased. We found the highest TiO_2 in laterite and the lowest in pillow basalts, indicating that Ti migrated from basalt to laterite.Our U–Pb dating and whole-rock elemental geochemistry analyses suggest that the Emeishan basalt is the parent rock of the Shazi anatase ore deposit.Based on our analysis, we propose a metallogenic model to explain the ore-forming process, in which the karst terrain formed by the Emeishan mantle plume and the subsequent basaltic magma eruption were the key factors in the formation the Shazi anatase ore deposit.     

Geochemistry and Zircon U–Pb age of the Yao'an pseudoleucite porphyry,Yunnan Province,China

The Yao'an Pb–Ag deposit, located in the Chuxiong Basin, western Yangtze Block, is an important component of the Jinshajiang–Ailaoshan alkaline porphyry–related polymetallic intrusive belt. This complex suite of rock bodies includes a vein of pseudoleucite porphyry within deposits of syenite porphyry and trachyte.The pseudoleucite is characterized by a variable greyish,greyish-white, and greyish-green porphyritic texture. Phenocrysts are mainly pseudoleucite with small amounts of alkali feldspar and biotite. In an intense event, leucite phenocrysts altered to orthoclase, kaolinite, and quartz.Both the pseudoleucite porphyry and the syenite porphyry samples were typical alkali-rich, K-rich, al-rich rocks with high LaN/YbNratios; enriched in light rare earth elements and large-ion lithophile elements, and depleted in high field strength elements; and with strongly negative Ta, Nb, and Ti(TNT) anomalies and slightly negative Eu anomalies—all characteristics of subduction-zone mantle-derived rock.We obtained a LA-ICP-MS zircon U–Pb age of 34.1 ± 0.3 Ma(MSWD = 2.4), which is younger than the established age of the Indian and Eurasian Plate collision.The magma derived from a Type-II enriched mantle formed in a post-collisional plate tectonic setting. The geochemical characteristics of the Yao'an pseudoleucite porphyry are powerful evidence that the porphyry'sdevelopment was closely linked to the Jinshajiang–Ailaoshan fault and to the Indian-Eurasian collision.     

Re-Os isotope dating of molybdenites in the Huang-shaping Pb-Zn-W-Mo polymetallic deposit, Hunan Province, South China and its geological significance

The large-scale Huangshaping Pb-Zn-W-Mo polymetallic deposit is located in the central Nanling min- eralization zone, South China. Six molybdenite samples from the Huangshaping deposit were selected for Re-Os isotope measurement in order to define the mineralization age of the deposit. It yields a Re-Os isochron age of 154.8±1.9 Ma (2σ ), which is in accordance with the Re-Os model ages of 150.9― 156.9 Ma. This age is about 7 Ma younger than their host granite porphyry, which was dated as 161.6±1.1 Ma by zircon U-Pb method using LA-ICPMS. All these ages demonstrate that the Huang- shaping granite and related Pb-Zn-W-Mo deposit occurred in the middle Yanshanian period, when many other granitoid and related ore deposits emplaced and formed, e.g. the Qitianling granite and Furong tin deposit, the Qianlishan granite and giant Shizhuyuan W-Sn-Mo-Bi deposit and Jinchuantang Sn-Bi deposit in the nearby area. They constitute the main part of the magmatic-metallogenic belt of southern Hunan, and represent the large-scale metallogeny in middle Yanshanian in the area. The lower rhenium content in molybdenite of Huangshaping deposit suggests that the ore-forming material was mainly of crust origin.     

Genesis and metallogenic characteristic of Dongnan Cu–Mo deposit associated granitoids: LA-ICP-MS zircon U–Pb dating and isotope constraint from Zijinshan ore field in southeastern China

The Dongnan Cu–Mo deposit, located in the southeast of the Zijinshan ore field(the largest porphyry–epithermal system in Southeast China), represents the complex magmatic and metallogenesis events in the region. The petrogenesis and metallogenesis of granitoids from the deposit are not determined, especially the interactions between ore-bearing(granodiorite porphyry) and barren samples(granodiorite and diorite). In the paper, the whole rock geochemical features shared a similar affinity to the m...     

The Upper Devonian orbital cyclostratigraphy and numerical dating conodont zones from Guangxi,South China

The hierarchically organized laminae, bundles, bundlesets and superbundlesets which correspond to a sub-Milankovitch, obliquity or precession, eccentricity and long eccentricity cyclothems, respectively, have been distinguished from the Upper Devonian Fras-nian-Famennian (F-F) transitional carbonate successions deposited in the carbonate-basin and slope facies of Guangxi, South China. The durations of cyclothems are 8000-10000a, 16667a or 33333a, 100000a and 400000a, respectively. The ratio of eccentricity to precession, eccentricity to obliquity, and long eccentricity to eccentricity is 1 : 6, 1 : 3 and 1 : 4 in the Devonian, respectively. Orbital cyclostratigraphical studies show that the durations of the conodont falsio-valis Zone, transitans Zone, punctate Zone, Lower hassi Zone, Upper hassi Zone, jamieae Zone, Lower rhenana Zone, Upper rhenana Zone, linguiformis Zone, Lower triangularis Zone, Middle triangularis Zone and Upper triangularis Zone are 0.4, 0.4, 0.4, 0.3, 0.4, 0.2, 0.8, 0.6, 0.8, 0.3, 0     

Zircon SHRIMP U–Pb geochronology,geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province,China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed. Herein, the middle-acid volcanic rock, dacite,exposed to the Zhesang gold district, southeast Yunnan Province, has been analyzed. The results show that the dacite belongs to a calc-alkaline series, SiO₂ contents range from 62.79 to 76.66 wt%. Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic(247.8 ± 1.7 Ma, MSWD = 1.2). All samples exhibit enri...     

Rare earth element(REE) geochemistry of different colored fluorites from the Baoshan Cu–Pb–Zn deposit,Southern Hunan,South China

The Baoshan Cu–Pb–Zn deposit, located in the central part of the Qin–Hang belt in South China, is closely related to the granodiorite-porphyry. However, the characteristics and the source of the ore-forming fluid are still ubiquitous. According to the crosscutting relationships between veinlets and their mineral assemblages, three stages of hydrothermal mineralization in this deposit were previously distinguished. In this contribution, two different colored fluorites from the major sulfide miner...     

Zinc,copper, and strontium isotopic variability in the Baiyangping Cu–Pb–Zn–Ag polymetallic ore field,Lanping Basin,Southwest China

The Baiyangping Cu-Ag polymetallic ore district is located in the northern part of the Lanping-Simao foreland fold belt,between the Jinshajiang-Ailaoshan and Lancangjiang faults,and the deposit can be divided into eastern and western ore zones.Based upon microscope observation of ore minerals and analysis of zinc,copper,and strontium isotope composition,we conclude that:(1)the zinc isotopic compositions of sphalerite from the eastern and western ore belt of the Baiyangping polymetallic ore deposits are enriched in both the heavy(-0.09‰ to+0.15‰) and light(-0.19‰ to-0.01‰)zinc isotopes.Rayleigh fractionation is likely the additional factor controlling the observed temporal and spatial variations in zinc isotopes in the two studied ore zones.The zinc isotopic composition in the Baiyangping polymetallic Pb-Zn deposits may have the same fractionation as that of magmatic-hydrothermal,VHMS,SEDEX,and MVT deposits,as demonstrated by geological and other geochemical evidence;(2) the range of δ~(65)Cu in massive tetrahedrite is from-0.06‰ to+0.12 ‰ that relates to the early stages of ore-formation,which are higher than that of venial chalcopyrite(from-0.72‰ to-0.07‰)formed at a late ore-forming stage in the western ore belt.Different ore-forming stages and alteration or leaching processes are likely the main factors controlling the observed variations in copper isotopes in the western ore zone;(3) the ~(87)Sr/~(86)Sr value of hydrothermal calcite in eastern(0.7080-0.7093) and western(0.7085-0.7113) ore belt suggested that mineralization of early calcite,with~(87)Sr/~(86)Sr values much higher than in ancient Late Triassic seawater,may be related to recrystallization from a radiogenic Sr-rich or silicifying fluid,either from the strata that the ore-forming fluid flows through or from other fluids.     

Single zircon LA-ICPMS U-Pb dating of Weishan granite (Hunan, South China) and its petrogenetic significance

It is well-known that South China experienced suc- there are also the Indosinian granitoids in Jiangxi, cessively two important tectonic movements during Guangdong, Guangxi, Hainan and so on[4], most of Mesozoic time, i.e. Indosinian and Yanshanian move- them, in the mass, are fairly concentrated in Hunan ment[1], which yield widely-distributed granitoids and Province, in which the Indosinian granites outcrop abundant mineral resources[2]. Therefore, the geologic over an area of ca. 5000 k…     

U–Pb dating of calcite–aragonite layers in speleothems from hominin sites in South Africa by MC-ICP-MS

Speleothems are found in association with hominin fossil-bearing cave deposits in South Africa and can be used to provide valuable chronological constraints. Such material is generally too old for U–Th dating and, although U–Pb geochronology presents a suitable alternative, bulk U concentrations are typically too low (<0.05 μg/g) to provide useful ages. For this reason, we used a simple non-invasive beta-scanner imaging screening step to identify U-rich (≥1 μg/g) domains that could be analyzed with MC-ICP-MS techniques to provide U–Pb ages. We demonstrate the technique using samples from Sterkfontein cave that exhibit infrequent <1 cm-thick layers with U concentrations ≥1 μg/g. Relict aragonite needles are found exclusively in these U-rich layers. We analyzed material from the same flowstone suite as Walker et al. (2006) and obtained a U–Pb age of ～2.3 Ma that agrees well with their estimate of 2.24 ± 0.09 Ma. We also obtained similar U–Pb (0.164 ± 0.026 to 0.200 ± 0.052 Ma) and U–Th (0.148 ± 0.003 Ma) ages for another sample exhibiting U-rich layers. We recognize that open-system behaviour during the partial transformation of aragonite to calcite is a potential problem and argue, on the basis of geochemistry and age consistencies, that recrystallization took place rapidly after speleothem formation and did not significantly affect the U–Pb ages.     

Zircon provenance of Quaternary cover beds using U–Pb dating: Regional differences in the Southwestern USA

U–Pb dating of detrital zircons (DZ) in Quaternary slope deposits (cover beds) and examination of the surface structure of single zircon grains may enable reconstruction of the provenance of their aeolian components and help to reconstruct sedimentary palaeo-transportation cascades. We distinguish several layers of slope deposits (cover beds) using a palaeosol-supported sequence-stratigraphic approach. In a pilot study, we demonstrate the usability of using end-member-modelled analyses of grain sizes to identify the aeolian matter, multi-dimensional scaling and density functions (probability density functions (PDF) and Kernel density estimations (KDE)) of the U–Pb data, and the grain morphology of DZ, and show that the age and appearance variation of DZ in the Great Basin differ remarkably from those of the Colorado Plateau. All samples contain aeolian matter. Density functions of the DZ ages show similarities within each of the two regions. The Great Basin samples are dominated by late Cretaceous and Paleogene zircons, which are assumed to derive from volcanism. In contrast, the Colorado Plateau samples are almost free of these contributions. Presumably, the difference is due to palaeolakes, which were frequent in the Great Basin only, whose sediments contain a similar spectrum of DZ ages. Zircon morphology indicates that most grains have a history of aeolian transportation, whereas the grains assumed to be of volcanic origin often do not show similar transportation marks; this indicates that their transportation was dominated by the eruptive process but fewer steps of a transportation cascade. Multidimensional scaling of age data defines clusters of samples with similar DZ ages and – compared with published ages from possible sources – allows first interpretations of provenance. Quaternary aeolian sediments have not been studied in, and the applied mix of methods has not been applied to, the southwestern USA before. We demonstrate their feasibility for this study area.     

Geochemistry,zircon U–Pb geochronology,and Hf isotopes of S-type granite in the Baoshan block,constraints on the age and evolution of the Proto-Tethys

Geochemistry, zircon U–Pb geochronology, and Hf isotope data for the Early Paleozoic granites in the Baoshan Block reveal the Early Paleozoic tectonic evolution of the Proto-Tethys. The samples are high-K, calcalkaline, strongly peraluminous rocks with A/CNK values of 1.37–1.46, are enriched in SiO₂, K₂O, and Rb, and are depleted in Nb, P, Ti, Eu, and heavy rare earth elements,which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indica...     

U–Pb dating of speleothems from Spannagel Cave,Austrian Alps: A high resolution comparison with U–series ages

U–Pb dating is increasingly used to date speleothems that are too old for precise U–Th disequilibrium dating; however there is little data that can independently validate its application to such material. This study presents U–Pb ages for speleothems from the Spannagel Cave in the Austrian Alps including a detailed comparison with U–Th ages from an unusually U–rich sample that yields precise ages by both methods. Sample SPA4 is a flowstone with three growth phases separated by distinct hiatuses. For the youngest growth phase the U–Pb and U–Th ages are 267 ± 1 ka and 267 ± 5 ka respectively; the middle growth phase is 291 ± 1 versus 295 ± 11 ka while for the oldest growth phase a single sub-sample, assuming the same initial Pb composition as for the younger phases, yields an age of 340 ± 2 ka compared to 353 ± 9 ka by U–Th. Correlation of these ages with the marine isotope stages confirms that these speleothems grew during glacial stages as suggested by previous work on the same sample. Sample SPA 15 has U–Th isotopic compositions indistinguishable from secular equilibrium; the U–Pb data on the main growth phase of this sample give an age of 551 ± 10 ka, whereas a single analysis from the oldest phase suggests it may be on the order of 40 ka older. This detailed comparison of U–Pb and U–Th ages provides important support for the potential validity of the U–Pb method in older samples beyond the range of U–Th.     

Zircon U–Pb age and geochemical constraints on the origin and tectonic implication of the Tuotuohe Cenozoic alkaline magmatism in Qinghai–Tibet Plateau

A large number of Eocene-Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau.In this study,we present zircon U-Pb ages,Hf isotope data,and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block.Zircon U-Pb ages were measured via LA-ICP-MS to be39.6,37.6 and 32.0 Ma.The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents.The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O,but deficient in MgO.All of the samples belong to the alkaline rocks.These rocks display enrichment in REE,LREE,and LILE,depletion in HFSE,and no obvious Eu anomalies.Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region:(1)the removed mafic lower crust dropped into the asthenosphere,forming the mix magma(Nariniya trachyte);(2)the upwelling asthenosphere triggered the crustal melting(Nariniya and Zamaqu orthophyre).The Eocene-Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic-magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination.This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region(*92°E)from Banda Co(*82°E).     

A study on the Dushiling tungsten-copper deposit in the Miao’ershan-Yuechengling area,Northern Guangxi,China: Implications for variations in the mineralization of multi-aged composite granite plutons

The polymetallic Dushiling W-Cu deposit is a large, altered, skarn-type deposit, located in the northeastern part of the Miao'ershan-Yuechengling pluton, China. Two types of granite have been identified in the deposit: a medium-grained porphyritic biotite granite, and a medium- to fine-grained biotite granite. Both are spatially and temporally related to ore bodies, suggesting they may be the source of mineralization in the deposit. A medium- to fine-grained porphyritic biotite granite is exposed at the surface in the region of mineralization. U-Pb dating of zircons yielded magmatic ages of 423 Ma for the medium-grained porphyritic biotite granite and 421 Ma for the medium- to fine-grained porphyritic biotite granite, while a younger age(217 Ma) obtained for surface samples indicates later diagenesis. Thus, magmatism occurred during the Caledonian and Indosinian, respectively. The petrological and geochemical characteristics of the two Caledonian granites show that both are calc-alkaline and peraluminous.They are moderately enriched in Cs, Rb, U, and REE, and strongly depleted in Sr,Ba,P,and Ti; they show similar REE behavior,including negative Eu anomalies. These geochemical similarities suggest that the two granites were derived from the same source,although they were emplaced during different stages of the evolution of the magma. Furthermore, the granites are associated with mineralization, suggesting they were the source of mineralization in the Dushiling W-Cu deposit. Sm-Nd ages of scheelite from the Dushiling W-Cu deposit indicate that metallogenesis occurred at 417±35 Ma, while the two types of titanite, intergrown with scheelite, yield U-Pb ages of 423–425 Ma(in altered granite sample) and 218 Ma(in skarn sample). These ages place the main mineralization event in the late Caledonian, and later magmatic-hydrothermal activity occurred in the Indosinian. The ages obtained for the Dushiling W-Cu deposit in the western Nanling Range, northern Yuechengling, together with the occurrence and ages of the Niutangjie W deposit in southern Yuechengling, provide insight into the process of ore concentration during the Caledonian and Indosinian.     

Lithological and Si–O–S isotope geochemistry: constraints on the origin and genetic environment of the selenium (Se)-rich siliceous rocks in Enshi,Hubei Province,China

Se-rich black rock series of the Middle and Late Permian system is widely distributed in Enshi Prefecture with an exposed area of 850 km~2,among which the unique Yutangba black rock series independent selenium deposit with industrial mining significance in the world is produced.However,the source and metallogenic mechanisms of Se are still controversial.In general,terrestrial weathering and submarine hydrothermal processes are the main source end members of Si and Se,and the related siliceous rocks record the deposition process of Si and Se from different sources.The study of lithofacies and paleogeography shows that western Hubei belongs to the near eastwest turn of the Yangzi platform in the Middle and Late Permian and becomes an inter-platform basin with nearly north-south direction.Therefore,the comparative study of the Yutangba deposit and the selenium-rich black rock series in the northern Shadi with high selenium content is expected to reveal the provenance evolution of the two sections in space,and further restrict the Se mineralization mechanism in the Enshi basin.From the element geochemistry study,the black rock series in two study areas may have formed in a transitional position of either the continental margin or continental slope,in the process of sedimentary,more terrigenous clastic materials entered.They are rich in lithophile elements V and Cr.δU 1.0,U/Th and V/(V+Ni) ratio indicate that the Se-rich strata of black rock series in the Enshi areas occurred in an anoxic reducing environment and formed in an environment between the ocean basin and the continental margin.From Si-O isotope geochemistry,the original Si source of the study area is thought to relate to a volcanic eruption,which leads to the enrichment of Si in the seawater.The determined values of S isotope in the black rock series of the two study areas both show the characteristics related to organic reduction/biogenic.     

Adakitic rocks associated with the Shilu copper–molybdenum deposit in the Yangchun Basin,South China,and their tectonic implications

South China is famous for the extensive magmatism and polymetallic mineralization that took place there in the Mesozoic. Shilu is a large porphyry–skarn Cu–Mo deposit in the Yangchun Basin, South China. The lithology of the Shilu intrusion is granodiorite and quartz diorite, both of which are high-K calc-alkaline series, with high Sr([400 ppm) content along with low Y and Yb contents. Most of the samples have characteristics of adakite except for a few samples that have slightly higher Y and Yb contents, which may be plausibly explained by crustal contamination. Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U–Pb dating revealed ages between 106.6 ± 1.3 and 103.9 ± 0.5 Ma, with multiple magmatic pulses. Molybdenite Re–Os isochron age of 102.2 ± 2.9 Ma(MSWD = 9.4) was determined, which is identical to the youngest zircon U–Pb age(103.9 ± 0.5 Ma) within error.The Shilu intrusion has high oxygen fugacity as indicated by high zircon Ce~(4+)/Ce~(3+) and Eu_N/Eu_N* ratios. Considering the geochemical characteristics(high Sr, and low Y and Yb contents), high oxygen fugacity, and copper mineralization of the Shilu intrusion, it was most likely formed by partial melting of a subducted young oceanic slab. Whole-rock Sr–Nd isotope-, zircon Hf isotope-, and whole-rock trace element analyses show that Shilu adakitic magmas may have interacted with type II enriched mantle and/or crustal materials during ascent. South China was affected by the Pacific tectonic regime to the east and the Neo-Tethys tectonic regime to the south in the Cretaceous. Based on the Pacific Plate drifting and rotation history, it is hard to explain how the Pacific Plate would have subducted and melted, forming adakitic rocks in the Shilu region. Considering the tectonic history of Southeast Asia and the South China Sea, the Neo-Tethys trench should have been much closer to the South China Block in the Cretaceous, and thus have had a greater impact on the South China Block. Based on the subduction direction, time of subduction,and distance between the Neo-Tethys subduction zone and the Shilu deposit, subduction of the Neo-Tethys ridge is the best mechanism for explaining the Shilu adakitic rocks and Cu–Mo mineralization.     

Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex,South Australia,and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern.