Timing of Holocene lake highstands around Dawa Co in inner Tibetan Plateau: Comparison of quartz and feldspar luminescence dating with radiocarbon age

Lakes over the inner Tibetan Plateau (TP) are very sensitive to the regional environmental transformations and climate changes. Well-preserved lake sediments around these lakes provide critical geomorphological and sedimentary evidence that can be used to infer the past hydroclimate changes. In this study, a lacustrine section from a sandy shoreline (∼74 m above the modern lake) situated to the northwest of modern Dawa Co in the inner TP was investigated using both luminescence and radiocarbon dating methods. Our results demonstrated: (1) the quartz optically simulated luminescence (OSL) dating yielded much younger ages (∼4 ka) than that of the post-infrared IRSL (pIRIR) dating of the K-feldspar fraction; (2) fading test showed g-values ranging between 1.34 and 4.46%/decade for quartz OSL signals, which is considered to be responsible for the underestimation of the corresponding ages; (3) the AMS ¹⁴C age of the charcoal sample from the section is in line with the K-feldspar pIRIR₂₂₅ ages, confirming the reliability of the pIRIR₂₂₅ dates and the underestimation of the quartz OSL ages. The anomalous fading of quartz OSL signals and the consequent age underestimation have been reported in several other lakes on the TP, we presented here for the first time firm evidence of the phenomenon with the help of a robust independent control of AMS ¹⁴C age of the charcoal. Based on the pIRIR₂₂₅ and AMS ¹⁴C ages, we conclude that Dawa Co underwent a prominent highstand during the early Holocene (∼9–7 ka), which was probably controlled by the large amounts of glacial meltwater input and the increased monsoonal precipitation.     

Application and evaluation of multiple-centres ESR dating of Pliocene-Quaternary fluvial sediments: A case study from the Zhoulao core from the Jianghan Basin,middle Yangtze River basin,China

Multiple-centres electron spin resonance (MC-ESR) dating of quartz grains has been commonly applied to fluvial and lacustrine deposits and can provide a precise chronological framework for depositional histories. However, the reliability of this method for quartz grains obtained from sediments of boreholes, which are usually deposited continuously and record information regarding basin evolution and climate change, has not yet been assessed. In this study, we have initially applied the MC-ESR dating method to borehole sediments from the Zhoulao core (ZLC), located in the depocenter of the Jianghan Basin, middle Yangtze River, China. Dating of quartz grains from the ZLC using MC-ESR yields estimated ages that are generally consistent with the established paleomagnetic chronological framework. For Middle Pleistocene samples (i.e., 0.7–0.3 Ma), the Ti–Li centre provides more accurate ages than those of Al centre, which are overestimated. For Early Pleistocene samples (i.e., 2.3–0.8 Ma), both the Al centre and Ti–Li centre give highly consistent estimate ages, indicating that this is a favorable dating range for MC-ESR. Overall, the Al centre shows promise for dating Pliocene samples, whereas the Ti–Li centre is more suitable for Middle-Early Pleistocene (2.3–0.3 Ma) sediments. In addition, the deposition rate of depth ＜170 m in the ZLC is greater than those of depth ＞170 m sediments; however, the specific tectonic, climatic, or geomorphic mechanism for this change in sedimentation rate is still unclear.     

ESR dating of optically bleached quartz grains from Plio-Pleistocene to Holocene coastal dune deposits (Wilderness-Knysna area,South Africa): a comparison with luminescence

Five Plio-Pleistocene to Holocene aeolian quartz samples from the coastal dune deposits of the Wilderness-Knysna area (South Africa) previously dated by OSL were selected for ESR dating. Samples were processed following the Multiple Centre approach and using the Multiple Aliquot Additive dose method. Aluminium (Al) and Titanium (Ti) signals were systematically measured in all samples.Our study shows that ESR results obtained for Middle Pleistocene to Holocene samples may be strongly impacted by (i) the presence of a significant high frequency noise in the ESR spectra acquired for the Ti signals and (ii) the choice of the fitting function employed. In particular, if not taken into account, very noisy spectra can lead to a significant overestimation of the true ESR intensity measured for the Ti–H signal. These sources of uncertainty are however not sufficient to remove the ESR age overestimations. Consequently, our results indicate that the Al and Ti ESR signals of these quartz samples have not been fully reset during their aeolian transport.While this work contributes to improve our understanding of the ESR method applied to quartz grains, and especially of the potential and limitations of the Ti signals, it also provides additional baseline data to illustrate the existing variability among quartz samples of different origins or sedimentary context. Our results are consistent with previous studies by confirming that the Ti–H signal shows the best potential for the evaluation of low dose values (<100 Gy for these samples), whereas it becomes inappropriate for the higher dose range, and the Ti–Li–H (option D sensu Duval and Guilarte, 2015) should be used instead.Beyond the methodological outcome, this ESR dating study also provides a useful addition to the existing chronology of the aeolian deposits in this region. In particular, new (and possibly) finite numerical age results were obtained for the two oldest samples, constraining the aeolianite landward barrier dune and the coversand formations to the MIS 10-8 and Pliocene, respectively.     

The build-up of the laboratory-generated dose-response curve and underestimation of equivalent dose for quartz OSL in the high dose region: A critical modelling study

The application of a regeneration procedure for optically stimulated luminescence (OSL) dating requires that the dose-response curve (DRC) of a natural sample is the same as that of a laboratory-generated one. However, the build-up of the laboratory-generated DRCs of quartz has been widely reported in the literature, i.e., the laboratory-generated DRCs are significantly higher than the natural counterparts in the high dose region (above 150 or 200 Gy). This results in severe underestimation of equivalent dose (D_e) for quartz OSL in the high dose region during the application of a single-aliquot regenerative-dose (SAR) protocol. However, the potential mechanism governing the build-up of the laboratory-generated DRC is still unclear. In this study, we performed a comprehensive investigation of the natural and laboratory-generated OSL signals and DRCs using a kinetic model for quartz. We compared the differences in charge concentrations between natural and laboratory-irradiated aliquots following irradiation and monitored the competition for holes and electrons during preheat and stimulation, for the natural, regenerative, and test dose cycles. In the course of the modelling, we could see the build-up of laboratory-generated DRCs, the underestimation of D_e, and a double exponential saturation characteristic of the DRCs. We demonstrated a discrepancy in competition for electrons in the deep electron trap and recombination centres during stimulation between the natural, regenerative, and test dose cycles. The simulation results are directly relevant to quartz OSL D_e determination using the SAR protocol and reveal the mechanisms responsible for the experimentally observed different behaviours between natural and laboratory-generated DRCs.     

Downstream changes in quartz OSL sensitivity in modern river sand reflects sediment source variability: Case studies from Rocky Mountain and Andean rivers

OSL (optically stimulated luminescence) sensitivity of quartz has been shown to either (1) record downstream sediment transport related to increased number and duration of light exposure cycles or (2) be a function of source geology, presenting a potential tool to track sediment provenance. To test these competing models this study leverages a suite of modern river samples from an extensional basin system in the Rocky Mountains of western USA (Bear River) and a retroarc foreland basin system in the southern Central Andes of Argentina (Río San Juan) to understand the relationships among quartz luminescence sensitivity, sediment transport distance, and catchment lithologies. We compare our results with petrographic analysis of the river sand composition, and characterization of the lithology and morphometrics of the river catchments. Samples taken along the Bear River and its major tributaries in the Rocky Mountains show a progressive downstream decrease in quartz OSL sensitivity that records variation in sediment provenance and steady contribution of lower OSL sensitivity quartz recycled from Palaeozoic passive margin stratigraphy. Andean river sand samples from the Rio San Juan network exhibit pervasive low sensitivity quartz derived from Andean arc volcanic rocks and recycled Neogene foreland basin strata. These modern river sand data do not show increased sensitivity with transport distance and instead indicates that in these landscapes, quartz OSL sensitivity is an intrinsic property of the source rock. Our study shows that river sands primarily composed of older, recycled low-strained quartz derived from quartzite lithologies exhibit the highest quartz sensitivity values. Moderate quartz OSL sensitivity values are observed in rivers with young igneous quartz derived directly from the volcanic and intrusive rocks. Conversely, microcrystalline quartz in chert lithic grains or polycrystalline quartz found in composite metamorphic lithic grains record the lowest quartz OSL sensitivity values. Determining the controlling factors of quartz sensitivity in river sand provides a current baseline for resolving paleogeographic and paleodrainage histories in the sedimentary record and provides further understanding of how sediments are eroded, transported, and deposited in fluvial systems with diverse tectonic settings and geologic source rocks.     

胶东邓格庄金矿成矿流体、成矿物质来源与矿床成因

邓格庄金矿是胶东牟平-乳山成矿带第二大石英脉型金矿床,其空间产出受断裂构造、荆山群变质地层和岩浆活动联合制约。对不同类型蚀变岩和不同阶段金脉体流体包裹体研究表明:包裹体可划分为液相包裹体(Ⅰ)、气相包裹体(Ⅱ)、含液体CO_2包裹体(Ⅲ)和含子矿物包裹体(Ⅳ)四类。从热液蚀变期到主成矿期,包裹体的种类增多,数量增多,主成矿期可见Ⅲ和Ⅳ型包裹体。激光拉曼探针分析结果显示成矿流体的气相成分类型包括CO_2-CH_4-H_2O、CO_2-H_2O、CO_2-CO_2和CO_2-CH_4四种,以CO_2为主,H_2O次之,主成矿期出现了少量的CH_4,成矿流体总体属CO_2-H_2O-NaCl体系。成矿流体完全均一温度变化范围为177～361℃,峰值240～280℃;盐度为1.7%～16.3%NaCleqv,密度变化范围为0.65～0.97g/cm~3;表明该矿床属于中低温、中低盐度、中低密度热液脉型矿床,成矿流体为酸性、弱酸性,且富含CO_2、CH_4等还原性质的热液体系。从热液蚀变期到成矿期各个阶段成矿温度、盐度、密度总体显示降低趋势。邓格庄金矿石英的δD值为-87.6‰～-80.7‰,δ18O_(H_2 O)值为5.87‰～7.49‰;δ13C_(V-PDB)值为-3.6‰～0.7‰,δ18O_(V-SMOW)值为1.3‰～9.1‰;δ34S值的变化范围在8.4‰～10.8‰之间;表明成矿流体来源于深部流体,以岩浆水为主,少量的大气降水参与了成矿过程。流体包裹体及C-H-O-S同位素研究,并结合地质特征,表明邓格庄金矿是与白垩系岩浆岩有关的,受断裂构造控制,并以大面积钾长石化为特征标志的中温岩浆热液型矿床,充填作用和混合作用可能是金矿成矿物质大规模沉淀的机制。     

山东邹平王家庄铜(钼)矿床蚀变围岩中含云母流体包裹体的成因及其意义

王家庄铜(钼)矿床位于鲁西邹平火山岩盆地。矿床围岩,即王家庄复合岩体,主要由闪长岩、二长岩和石英二长(闪长)岩等组成。从岩体周边向内到矿体,依次出现钾蚀变带、钾硅化蚀变带和强钾硅化蚀变带。矿床主要有两种类型矿石:深部蚀变石英二长(闪长)岩中的早期浸染状矿石和浅部角砾岩化石英二长(闪长)岩中的块状硫化物石英脉矿石。矿石矿物主要有黄铁矿、黄铜矿、砷黝铜矿和辉钼矿等。流体包裹体研究表明,除了证实有前人确定的富液相包裹体、富气相包裹体和含石盐的高盐度包裹体外,还在钾硅化蚀变-矿化围岩中发现含云母的流体包裹体。它们多随机或成群分布于石英晶核和生长环带中。岩相学、拉曼光谱和电子探针分析表明,含云母的流体包裹体主要由水液相+气相+白云母(黑云母)组成,其相比例随石英晶体不同或是相对稳定,或是有很大变化。显微测温表明,含云母的流体包裹体中水溶液的冰点和均一温度有很大变化,而主要集中在-5～10℃和310～360℃。热液金刚石压腔(HDAC)加热实验显示,含云母流体包裹体的均一温度可高达773～790℃,由此估算的均一压力高达275～380MPa,将大大超过地质推断的合理范围。对含云母流体包裹体的成因机制进行了讨论。认为它们不属于熔融包裹体范畴,而是钾硅化蚀变阶段捕获的原生流体包裹体,是从中酸性岩浆结晶作用晚期分离出来的富K-Al-Si组分、挥发分、盐类和成矿金属元素的含水流体中非均匀捕获的产物。伴随裂隙开放和压力温度迅速降低,导致大量白云母(少量黑云母和硅酸盐等)从富K-Al-Si组分的含水流体中结晶出来,沉淀在正在结晶的石英核部或生长表面坑穴,随后与石英周围的流体介质一起被捕获,成为含云母流体包裹体。含云母流体包裹体属于岩浆作用晚期自交代作用产物,它们与含石盐包裹体的共存反映了这两种矿物在岩浆-热液中达到饱和并沉淀出来,对于流体流动和成矿作用具有重要意义,并有助于我们对王家庄铜(钼)矿床成矿流体的性质和岩浆-热液演化过程有深入了解。     

Grain boundary dissolution porosity in quartzofeldspathic ultramylonites: Implications for permeability enhancement and weakening of mid-crustal shear zones

Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼10⁹ cm⁻². A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 10⁸ cm⁻². Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal conditions.     

Development of quartz c-axis crossed/single girdles under simple-pure shear deformation: Results of visco-plastic self-consistent modeling

Quartz c-axis fabrics are widely used to determine the shear plane in ductile shear zones, based upon an assumption that the shear plane is perpendicular to both the central segment of quartz c-axis crossed girdle and single girdle. In this paper the development of quartz c-axis fabric under simple-pure shear deformation is simulated using the visco-plastic self-consistent (VPSC) model so as to re-examine this assumption. In the case of no or weak dynamic recrystallization, the simulated crossed girdles have a central segment perpendicular or nearly perpendicular to the maximum principal finite strain direction (X) and the XY finite strain plane, and at a variable angle relative to the imposed kinematic framework that is dependent on the modeled flow vorticity and finite strain. These crossed girdles have a symmetrical skeleton with respect to the finite strain axes, regardless of the bulk strain and the kinematic vorticity, and rotate in a way similar to the shear sense with increasing bulk strain ratio. The larger the vorticity number the more asymmetrical their legs tend to be. In the case of strong dynamic recrystallization and large bulk strain, under simple shear the crossed girdle switches into single girdles, sub-perpendicular to the shear plane, by losing the weak legs. The numerical results in our models do not confirm the above-mentioned assumption.     

内蒙古额仁陶勒盖银矿区火成岩岩石地球化学特征及锆石SHRIMPU-Pb同位素定年

通过对额仁陶勒盖银矿区的塔木兰沟组中酸性火山岩详细的岩石地球化学特征研究表明,该套火山岩具高硅(SiO₂>57%)、高铝(Al₂O₃>16%)、富碱尤其富钾(K₂O>3%)特征,属HKCA系列;稀土含量中等(187×10^-6~240×10^-6),(La/Yb)_N在18.6~27.9,Yb<1.90×10^-6,Y<18.0×10^-6,δEu=0.85~0.96,轻重稀土分馏强烈,明显亏损重稀土,无或略具负铕异常;微量元素相对富集Rb、Ba、Th、U、K等大离子亲石元素(LILE),亏损高场强元素Nb、Ta、Ti、HREE,与活动陆缘火成岩的微量元素特征一致.塔木兰沟组安山质火山岩特征表明,安山质岩浆来源较深(>50km),起源于加厚地壳底部,源区残留为石榴石、单斜辉石及角闪石,岩浆在部分熔融及后期演化过程中不曾与斜长石平衡过,可能是蒙古-鄂霍茨克洋闭合后进入碰撞后松弛阶段,钙碱性或高钾钙碱性的中基性变火成岩的部分熔融形成.对与额仁陶勒盖银矿关系密切的石英斑岩(脉)测年,获得SHRIMP U-Pb年龄138.6±2.3Ma,为石英斑岩的成岩年龄,进一步限定该银矿床形成于早白垩世早期.对塔木兰沟组安山岩与石英斑岩成岩年龄、矿床成矿特征等方面进行探讨了,认为该银矿床为岩浆热液成因.