Depositional Age, Provenance Characteristics and Tectonic Setting of the Ailaoshan Group in the Southwestern South China Block

The depositional and metamorphic ages and provenances of the Ailaoshan(ALS) Group in the Ailaoshan-Red River(ALS-RR) shear zone, southwestern South China Block(SCB), were investigated to constrain the tectonic history of the southwestern SCB. In this study, we use petrology, geochemical analysis, zircon cathodoluminescence imaging and UPb geochronology to analyse samples of quartzite, garnet-bearing two-mica schist and metapelite. The age spectra of detrital zircon grains from these metasediments show two dominant age peaks at 550–424 Ma and 876–730 Ma and two subordinate peaks at 970–955 Ma and ~2450 Ma. The youngest peak, corresponding to the early Palaeozoic, accounts for more than 20% of the total dates and constrains the deposition of the ALS Group to the Palaeozoic rather than the Palaeoproterozoic as traditionally thought. Moreover, two peaks of metamorphic ages corresponding to the Permo-Triassic and Cenozoic were also identified, and these ages document the tectonothermal events associated with the Indosinian collision between the Indochina Block and the SCB and the Himalayan collision between the Indian and Asian plates. Geochemical data suggest that the provenances of the ALS Group were dominated by continental arc and recycled metasedimentary rocks. The comparison of probability density distribution plots of the detrital zircon U-Pb age data indicates that the Neoproterozoic detritus in the ALS Group was probably derived from the arc-related Neoproterozoic intrusive bodies in the northwestern and southwestern SCB. Furthermore, the early Palaeozoic detritus might have been sourced from eroded early Palaeozoic strata and magmatic plutons in Cathaysia and volcanic rocks in the western Indochina Block.

Maximum depositional age estimation revisited

In a recent review published in this journal,Coutts et al.(2019)compared nine different ways to estimate the maximum depositional age(MDA)of siliclastic rocks by means of detrital geochronology.Their results show that among these methods three are positively and six negatively biased.This paper investigates the cause of these biases and proposes a solution to it.A simple toy example shows that it is theoretically impossible for the reviewed methods to find the correct depositional age in even a best case scenario:the MDA estimates drift to ever smaller values with increasing sample size.The issue can be solved using a maximum likelihood model that was originally developed for fission track thermochronology by Galbraith and Laslett(1993).This approach parameterises the MDA estimation problem with a binary mixture of discrete and continuous distributions.The‘Maximum Likelihood Age’(MLA)algorithm converges to a unique MDA value,unlike the ad hoc methods reviewed by Coutts et al.(2019).It successfully recovers the depositional age for the toy example,and produces sensible results for realistic distributions.This is illustrated with an application to a published dataset of 13 sandstone samples that were analysed by both LA-ICPMS and CA-TIMS U–Pb geochronology.The ad hoc algorithms produce unrealistic MDA estimates that are systematically younger for the LA-ICPMS data than for the CA-TIMS data.The MLA algorithm does not suffer from this negative bias.The MLA method is a purely statistical approach to MDA estimation.Like the ad hoc methods,it does not readily accommodate geological complications such as post-depositional Pb-loss,or analytical issues causing erroneously young outliers.The best approach in such complex cases is to re-analyse the youngest grains using more accurate dating techniques.The results of the MLA method are best visualised on radial plots.Both the model and the plots have applications outside detrital geochronology,for example to determine volcanic eruption ages.

Application of grouped detrital zircon analyses to determine provenance and closely approximate true depositional age:Early Cretaceous McMurray-Clearwater succession,Canada

In the Lower CretaceousMcMurray-Clearwater succession of the intracontinental Alberta Foreland Basin,Canada,detrital zircon U-Pb geochronology samples(referred to herein as DZ samples)have been used to interpret the strata as representing a paleo-continental-scale drainage system.However,the majority of DZ samples are relatively small(n≈90–100),and syndepositional DZ(i.e.,crystallization age<5 Ma older than depositional age)are rare.This has forced a reliance on dinocysts with long stratigraphic ranges to chronostratigraphically subdivide the McMurray-Clearwater succession rather than employing maximum depositional ages(MDAs)derived from DZ samples.Herein,43 DZ samples(taken from20 subsurface cores)are assigned to 1 of 5 stratigraphic intervals,and in each stratigraphic interval all associated DZ samples are combined to produce a grouped DZ sample.Analysis and comparison of individual and grouped DZ samples are used to(1)assess variability in provenance through time and space,and(2)assess the accuracy of chronostratigraphically subdividing the succession using MDAs.Along the main paleo-drainage axis,a comparison of dissimilarity between DZ samples from the same stratigraphic interval,as well as between stratigraphic intervals,reveals increasing average dissimilarity between individual DZ samples and their respective grouped DZ samplewith increasing spatial separation of samples.These data indicate that in the McMurray Depocenter some sediment is sourced from local tributaries,leading to geographical provenance variability.CalculatedMDAs for all 43 DZ samples and groupedMDAs(gMDAs)for the 5 grouped DZ samples are compared to an ash-derived absolute age and existing biostratigraphy.In theMcMurray Formation,comparison of MDAs to gMDAs shows that in basins with rare syndepositional DZ,the gMDA method improved depositional age estimates by transforming low-confidence MDAs(e.g.,youngest single grains)into high-confidence(multi-grain)gMDAs.In the Clearwater Formationwhere syndepositional DZ are plentiful(i.e.,>5%of the total DZ population),calculating maximumlikelihood ages fromgrouped DZ samples avoids negatively biased(i.e.,too young)MDAs.We suggest grouped DZ samples and the gMDA method be used in systems with multiple DZ samples from a well-defined stratigraphic interval as a means of assessing variability in provenancewithin a depositional system and for improving estimates of depositional ages using DZ.

Zircon U–Pb Ages and Lu–Hf Isotope of the Dongchuan Group in Central Yunnan, China, and their Geological Significance

On the southwestern margin of the Yangtze Block, the Dongchuan Group consists of slightly metamorphosed sedimentary rocks, including silty slate, argillaceous slate, clayey slate, arkose, dolomite, and minor volcanic rocks. To date, it is still a controversy over the depositional age and stratigraphic sequence of the Dongchuan Group. In this study, we analyzed five samples of meta-sedimentary rocks and one sample of meta-tuff from the Yinmin, Luoxue and Etouchang Formations of the Dongchuan Group in the Yuxi region for detrital zircon U-Pb ages and Lu-Hf isotope. The detrital zircon ages of the meta-sediments vary from 3073 to 1703 Ma, mainly clustered at three periods, from 1889 to 1840, 2490 to 2008 and 2878 to 2844 Ma. The youngest age peak of all the samples is ~1859 Ma, with the εHf(t) values of the zircons ranging from-20.3 to +4.3 and more than 90% being negative, indicating that the Paleoproterozoic crustal accretion on the southwestern margin of the Yangtze Block was dominated by reworking of the ancient crustal materials involved in the assembly and breakup of the Columbia supercontinent. Another important age range is between 2490 Ma and 2008 Ma, with εHf(t) values from-14.7 to +8.9 and 70% of them are negative, suggesting that the magmatism in the source area was also dominated by reworking and recycling of the ancient crustal materials, with minor juvenile mantle substances added. The detritus was probably derived from the Paleoproterozoic crystalline basement in the southern Yuxi region. The oldest peak age is ~2847 Ma and the εHf(t) values are from-7.7 to +7.0 with 50% of both positive and negative values, demonstrating a possible ~2.85 Ga ancient continental nucleus on the southwestern margin of the Yangtze Block and substantial growth in juvenile crust materials during this period. Besides, the weighted average age of the zircons from the meta-tuff of the Etouchang Formation is 1677 ± 14 Ma. Combining the previous research data and this study, we can constrain the depositional age of the Dongchuan Group in central Yunnan Province to the period from the late Paleoproterozoic to early Mesoproterozoic, slightly earlier than that of the Dongchuan Group in the Dongchuan area near to the southwestern Sichuan Province. The depositional age of the Dongchuan Group is older than that of the Kunyang Group.

First Report of Zircon U-Pb Ages from Lubei Cu-Ni Sulfide Deposit in East Tianshan of Central Asian Orogenic Belt, NW China

Objective The East Tianshan mafic-ultramafic rocks belt mainly produced in the eastern Jueluotage belt is an important part of the Central Asia Orogenic Belt （CAOB）. The well- known deposits including Huangshan, Huangshandong, Tulaergen, Hulu, Xiangshan were have been consecutively discovered in this belt （Duan Xingxing et al., 2016）. The new discovery of the Lubei Cu-Ni sulfide deposit in recent years, which locates in the west of Jueluotage belt, has great significance to the westward extension of the East Tianshan Cu-Ni metallogenic belt. To determine whether the mineralization age of the Lubei Cu-Ni sulfide deposit is consistent with other typical deposits, this study conducted zircon U-Pb geochronology on the diorite from the Lubei Cu-Ni sulfide deposit in order to provide new information for further exploring direction of Cu-Ni prospecting in East Tianshan.

New Zircon U-Pb Age of Granodiorite from the Shayikenbulake Be Deposit in Altay, Xinjiang and its Significance

Objective Intrusive rocks are widely distributed in Altay, Xinjiang, and appear in every structural belt. The rocks are mainly granite （Song Peng et al., 2017）, which formed from 523 Ma to 202 Ma and can be divided into five periods： 479-421 Ma, 410-370 Ma, 368-313 Ma, 300- 252 Ma and 247-202 Ma. However, intrusive rocks earlier than the Ordovician are rarely found. The small- scale low-grade metamorphic granite intruded in the Kanasi Group is the oldest intrusion reported up to now, which is the only one formed in the Early Cambrian （5234-19 Ma, Liu Yuan et al., 2013）. Few Cambrian intrusions limit the study of early magmatic-tectonic evolution in Altay. Medium-free-grained granodiorite is exposed widely in the Shayikenbulake deposit of Central Altay, and occurs as batholith. It formed in the Early Cambrian indicated by U-Pb dating, and is an ideal intrusion for studying Cambrian magmatic-tectonic events in Altay.

New Zircon U-Pb Age of the Ore-bearing Porphyry from the Kuga Copper Deposit in the Eastern Bangongco–Nujiang Matallogenic Belt, Tibet

Objective As the third most important copper polymetallic metallogenic belt in Tibet, the Bangongco-Nujiang metallogenic belt （BNMB） has attracted much attention among geoscientists all over the world （Lin Bin et al., 2017a）. There are two ore clusters in the westem of BNMB, the Duolong giant porphyry-epitherrnal Cu （Au, Ag） ore cluster and the Ga＇erqiong-Galalelarge porphyry- skarn Cu （Au） ore cluster （Lin Bin et al., 2017a; 2017b）. Now, the latest exploration advances show that the Kuga project is the first economic porphyry-skam copper deposit in the eastern of BNMB, with over 0.4 Mt melt copper （333＋334） @ 0.9%. However, the Kuga deposit is poorly studied about its diagenetic age. In this study, we present a zircon U-Pb LA-ICP-MS dating of ore-bearing biotite granite, in order to identify the time of the ore- related magmatism and reveal the relationship with the westem of BNMB.

New Zircon U-Pb Age of Devonian Granites in the Niukutou Lead-Zinc Deposit, Qinghai Province and its Significance for Prospecting Blind Orebodies

Object The Eastern Kunlun Orogen(EKO), An important part of the Tethyan orogenic belt in the northern margin of the Qinghai–Tibet Plateau(Li et al., 2014; Ren Haidong et al., 2016), is a key area for geological research and mineral exploration(Li Bile et al., 2015). The Qimantag Mountain is located in middle segment of the EKO, which has experienced the Early Paleozoic and Late Paleozoic–Early

Dating Magmatic Hornblende and Biotite and Hydrothermal Sericite by Laser Probe Technique:Constraints on Genesis of Wangershan Gold Deposit,Eastern Shandong Province,China

The Wangershan gold deposit and spatially related Shangzhuang granite, eastern Shandong Province, have been precisely dated by 40 Ar/ 39 Ar laser incremental heating technique. Magmatic hornblende and biotite, collected from the Shangzhuang granites, yielded well-defined and reproducible plateau ages at 128.1-127.5 and 124.4-124.1 Ma (2 σ ), measuring the cooling ages of the intrusion at ca. 500 ℃ and 300-350 ℃, respectively. Hydrothermal sericite extracted from auriferous vein gave high-quality plateau ages between (120.6±0.3) Ma and (120.0±0.4) Ma (2 σ ). Given the similarity of the closure temperature for argon diffusion (300-350 ℃) in the sericite mineral to the homogenization temperature of primary fluid inclusions in the quartz from gold ores, and the intergrowth of sericite with native gold, present 40 Ar/ 39 Ar sericite ages can be reliably interpreted in terms of the mineralization age for the Wangershan deposit. 40 Ar/ 39 Ar hornblende and biotite ages permit an estimate for the cooling rate of the Shangzhuang granite at about 50 ℃/Ma. There are abundant intermediate-mafic dikes in most gold camps of eastern Shandong, whose ages of formation have been previously constrained mainly at 121-119 Ma. The temporal association between the Shangzhuang granite, the Wangershan gold deposit, and the widespread dikes confirms that intrusive activity, gold mineralization, and dike emplacement in this region were broadly coeval, reflecting significant continental lithosphere thinning and resulting crustal extension of Early Cretaceous in eastern China.

K-Ar isotope evidence for age of the first and only independent tellurium deposit

The Dashuigou tellurium deposit found in 1991 in Shimian County, Sichuan Province of China, is the first and only independent tellurium deposit in the world. The deposit is located on the western margin of the Yangtze Platform. The tellurium ore bodies taking vein and lentiform shapes filled the N-S fractures in the pyrrhotite veins in hornblende schists at the northern limb of the Middle and Lower Triassic metamorphic arch. The main wallrock alterations are biotitization, sericitization, greisenization, dolomitization,

Luminescence Dating and Bracketing Time of the Youngest Toba Tuff Deposits in the Quaternary Sediments of Purna Alluvial Basin,Central India

Light gray to yellowish brown youngest Toba tuff(YTT)ash,preserved in the Quaternary sediments of India,have been used as a tool to know depositional and bracketing time period of the same in the sediments through applications of luminescence dating to interpret age connotation of various geological events.Pre-and post-tephra sediments of YTT ash beds from Hudki and Sukali areas of Purna alluvial basin,Central India have been optically dated to interpret the depositional age and bracketing time of the tephra within sedimentary sequences.These sediments are yellowish brown,thinly bedded,laminated,silty clay and host laterally extend discontinued beds of 0.15–0.20 m thickness.The ash is light gray,fine grained and powdery in nature,massive to structureless and lacks any sign of retransportation and redeposition,hence,considered as primary in nature.Total four sediment samples,one each from pre-and post-tephra lithounits of both the localities have been dated through optically stimulated luminescence technique(OSL)equipped with single aliquot regenerative(SAR)procedure on the feldspar grains.It shows pre-and post-tephra ages of 57±5 and 70±4 ka respectively for Hudki and 66±5 and 67±4 ka for Sukali localities.These depositional ages suggest that the ash was bracketed within sediments thousands years after YTT eruption.It is correlatable with the previously reported depositional and absolute ages for the YTT ash from the localities of Ghoghara and Khuntheli,Son Valley;Tejpur,Madhumati River Basin and Jwalapuram,Jurreru Valley.These ages also provide an idea about the period of existence of biological communities represented by the remains of faunal and vegetational activities before and during settlement of the YTT ash in the basin area.