Discovery of Paleoproterozoic rapakivi granite on the northern margin of the Yangtze block and its geological significance

The Huashanguan rapakivi pluton in Zhongxiang,Hubei Province,China,is the first discovered Proterozoic rapakivi pluton in the Yangtze block.Based on field and petrographical observations,a typical rapakivi texture was found in the northern portion of the Huashanguan granitic pluton.Almost all the K-feldspar phenocrysts were round to oval in shape and most had plagioclase coatings known as rapakivi phenocrysts.Alkali feldspars and quartz had two or more generations.Petrochemically,the Huashanguan rapakivi granites were characterized as having high values of Si,K,Fe,Th,U,La,Ga,Ce,Sm and LREE,low values of Ca,Mg,Sr,Nb,Y and HREE,and a negative Eu anomaly.These geochemical characteristics of the Huashanguan granites were concordant with typical rapakivi granites,and had an affinity to A-type granites.LA-ICP-MS U-Pb zircon dating also was conducted.The dating yielded a 207Pb/206Pb weighted mean age of 1851±18 Ma (MSWD =1.2),which represents the age of the pluton emplacement.The age of 803±170 Ma at the lower intercept in the concordia diagram corresponds to the age of a later deformation event which affected the pluton,and suggests that the Huashanguan pluton was influenced by Neoproterozoic thermo-tectonic events after its formation.The discovery of Paleoproterozoic Huashanguan rapakivi granites indicates continental rifting or a post-orogenic extensional event that took place in the Paleoproterozoic in the Yangtze block.These events may be related to the breakup of the Paleoproterozoic Columbia supercontinent.     

碎屑锆石U-Pb定年在准噶尔盆地南缘物源研究中的应用

伊连哈比尔尕山山前断褶带是准噶尔南缘前陆盆地和北天山造山带的重要结合部位,从该区采集了4件中生界中的砂岩样品,用LA-ICP-MS方法对其中的250个锆石颗粒进行了U-Pb年龄测定.根据锆石的矿物学特征、CL图像特点以及碎屑锆石年龄分布特征,结合古水流条件的约束,对其沉积物源和构造环境进行了分析.结果表明,锆石形成年龄的范围在170~2 886 Ma间.最年轻的碎屑锆石年龄是(170±1)Ma,说明这套地层不老于中侏罗世;最古老锆石的年龄峰值为2 200~2 900 Ma和1 000~1 600 Ma,源区应为天山在燕山期造山后已有古老基岩的剥露区;290~310 Ma和400~410 Ma的锆石,物源可能包括两部分:伊山为主和部分来自南部的中天山地区;碎屑锆石中出现170~200 Ma的年龄峰值,表明晚侏罗世北天山及盆地南缘发生了较明显的隆升和剥蚀作用,判断其物源为天山北坡.     

An erosional hiatus in Chinese loess sequences revealed by closely spaced optical dating

1 Introduction Loess deposits in China are the thickest and most extensively distributed in the world. Investigations overthe past around one century have significantly im- proved our understanding of the origin, transportation, deposition and evolution o…     

^14C年代测定样品碳转化率探讨

用液体冷闪烁法测定＾１４Ｃ年供工作的前期样品制作技术的实验探讨，对提高＾１４Ｃ制样技术的操作技能和用液体闪烁法测定＾１４Ｃ年代的精度有着十分重要的意义。通过对＾１４Ｃ样品制作三个步骤的分析，找出了影响＾１４Ｃ样品碳转化率的因素；适当调整以往＾１４Ｃ制样的技术指标，可将样品碳转化率从８０％左右提高到８５５以上，并稳定在８５％－９０％之间；同时，顺利地完成了＾１４Ｃ国际比样品的制作１。     

海南岛西北岸三万年前后浅海相地层的发现及其意义

在海南岛西北部神尖沿岸地区,广泛出露一套富含海相化石的粉细砂—砾石沉积层,组成了高达20米左右的沿岸台地。该地层中所含化石组合主要有海扇、曼氐孔楯海胆、舌骨牡蛎和个体很大的盖虫。根据岩性、沉积相及~(14)C年代数据可知,在距今三万年前后,这里是一个环境比较稳定的热带浅海湾。海水的温度比现在高。我国其它沿海地区,在0—-27米也普遍存在三万年前后的海相沉积层。不同的是,海南岛西北岸三万年前后的海相层,因受后期火山作用而被抬升至高于现代海面20米左右。     

Application of cathodoluminescence to zircon in FEG-ESEM

Cathodoluminescence (CL) image analysis of zircon is a key step of mineral micro?area compositional investigation and U-(Th)-Pb dating. We introduce a system consisting of field emission ESEM and high performance cathodoluminescence. This system has obvious advantages in imaging resolution and CL spectrum. We acquired some zircon CL images and spectra to demonstrate the ability of this system. Our results show that this system has a great potential application to characterize and identify the genesis of zircon.     

Further support for a Cretaceous age for the feathered-dinosaur beds of Liaoning,China:New 40Ar÷39Ar dating of the Yixian and Tuchengzi Formations

We report new ⁴⁰Ar÷³⁹Ar dating results obtained from total fusion and incremental-heating analyses of sanidine and biotite from three tuffs found interbedded within the fossil-bearing deposits of Liaoning, northeast China. The first is a new sample of the Bed 6 Sihetun tuff from the Yixian Formation, previously dated by our team as middle Early Cretaceous, and recently considered by Lo et al., partially reset due to metamorphism from a nearby basaltic sill. The second is the Yixian Bed 9 tuff from Hengdaozi considered by Lo et al. to be unaffected by metamorphism and whose age, based on total fusion ⁴⁰Ar÷³⁹Ar dating of biotite, argues for a Jurassic age for the Yixian Formation. The third tuff is a previously undated tuff from the upper part of the underlying Tuchengzi Formation. Single crystal total fusion ⁴⁰Ar÷³⁹Ar analyses of the Sihetun sanidine showed homogeneous radiogenic Ar, Ca÷K ratios, excellent reproducibility and gave a mean age of 125.0±0.18 (1SD)±0.04 (SE) Ma. Single sanidine crystal total fusion ⁴⁰Ar÷³⁹Ar analyses of the Hengdaozi tuff gave a mean age of 125.0±0.19 (1SD)±0.04 (SE) Ma, which is indistinguishable from the Sihetun tuff. The Tuchengzi Formation tuff gave a mean age of 139.4±0.19 (1SD)±0.05 (SE) Ma. Detailed laser incremental-heating analyses of biotite from Sihetun, Hengdaozi, and Tuchengzi tuffs show disturbed Ar release patterns and evidence of trapped argon components. We conclude from these analyses that the total fusion dates on biotite by Lo et al. are erroneously old and isotopic dating of both biotite and sanidine from tuffs of the Yixian Formation point to a middle Early Cretaceous age. The upper part of the Tuchengzi Formation can be referred to the Early Cretaceous.     

用DTA法研究氟硅酸钠分解锆英砂的条件

本文用DTA法研究了氟硅酸钠分解锆英砂的条件。结果表明:锆英砂分解反应在DTA曲线上有一个明显的吸热峰,分解温度为563～588℃,添加剂对分解温度影响不明显,原料配比对反应有一定的影响,粒度对反应的差热峰影响较大。     

Geochronology and geochemistry of the Fangcheng Neoproterozoic alkali-syenites in East Qinling orogen and its geodynamic implications

Finding of Neoproterozoic syenites at Fangcheng in the northern Qinling region of the East Qinling orogen provides an important constraint on timing of tectonic transformation to extensional regime. The alkaline pluton consists mainly of nepheline syenite, aegirine syenite, and alkali-feldspar syenite. The syenites are of intermediate （SiO2 = 54%-62%）, rich in alkali （K2O＋Na2O = 12%-15%）, aluminum （Al2O3= 16.81%-23.26%） and large ion lithophile elements （LILE）, without any obvious Nb, Ta, Zr, and Hf anomalies. The Fangcheng syenites are geochemically characterized by relative enrichment of LREE, minor differentiation of HREE, significant negative Eu anomalies （Eu = 0.13-0.23）, less negative εNd（t） values of -1.37 to-3.90, young Nd model ages of 1364 to 1569 Ma, and high zircon saturation temperatures of 915 to 1044℃. The syenitic magmas probably originated from small proportion melting of upper mantle in an extensional regime of intraplate-anorogenic tectonic setting, and have been slightly contaminated by crustal materials during ascending and/or emplacement. LA-ICP-MS zircon U-Pb dating yields ages of 844.3±1.6 Ma （MSWD=0.86）, suggesting that the Fangcheng alkaline syenites formed in the early Neoproterozoic. They are the oldest Neoproterozoic alkaline rocks ever recognized in the Qinling orogen as well as in South China. This implies that the tectonic regime of the Qinling region would have transformed from post-collisional stretch to intraplate-anorogenic extension no later than 844 Ma.     

Constraints on the upper boundary age of the Tiaojishan Formation volcanic rocks in West Liaoning-North Hebei by LA-ICP-MS dating

In West Liaoning-North Hebei, the Tiaojishan Formation volcanic rocks are developed and many isotopic ages with a very wide age range were obtained in them as well. These different ages have direct influence on confirming and interpreting the related geological issues in West Liaoning-North Hebei and have restricted the study of geological problems related. In the field, the Tiaojishan Formation and the overlying Tuchengzi Formation are in parallel unconformable contact. By systematical sampling and dating of LA-ICP-MS in the Tiaojishan Formation （especially in the upper part） in Luanping, Chengde basins of Northern Hebei Province and in Lingyuan, Jinlingsi-Yangshan basins of Western Liaoning Province, combined with the dating data obtained by previous researchers, results indicate that the upper boundary age of the Tiaojishan Formation is from 156 to 153 Ma, and the lower boundary age of the Tiaojishan Formation is about 165 Ma, the Tiaojishan Formation was formed from the late of middle Jurassic to late Jurassic Epoch.