Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significance

In the Neoproterozoic, a large-scale magmatic activ- ity took place in the northern margin of the South China Block, with ages in a range of 700―800 Ma[1―4]). A systematic zircon U-Pb dating for bimodal metaigneous rocks in the Dabie-Sulu orogen yields ages of 758 ± 15 Ma[5], typifying rift magmatism along the northern margin of the South China Block during the middle Neoproterozoic. In addition, there is a widespread oc- currence of volcanic tuff interlayers around 635 Ma with the se…     

淮南地区新元古代刘老碑组叠层石消失原因初探

安徽淮南地区新元古代刘老碑组主要由页岩和中薄层泥质、粉砂质灰岩组成。在该组上部中薄层砂质灰岩中产有该地区新元古代最古老的叠层石Baicalia和?Linella。大量的臼齿碳酸盐脉体穿插在该组叠层石的基本层中的现象较为独特,该组上部发育的滑塌构造和臼齿碳酸盐岩脉体的特征表明该沉积时期发生过强烈的地震活动;此外,在叠层石礁体周围还有大量的风暴沉积产出。这些现象表明,该组叠层石发育于风暴活动和地震活动交织的环境之中,地震活动是导致该组叠层石消失的主要原因。     

Neoproterozoic magmatic activity and global change

Neoproterozoic is a very important time in the history of the Earth, during which occurred supercontinent breakup, low-latitude glaciation, and biotic diversification.These concern a series of interdisciplinary studies involving ancient plate motion, climate change and life evolution, resulting in many forefront topics of general interest in the earth sciences. These include exact ages bracketing the Cryogenian System and glaciations, initial age and lasted duration of supercontinent breakup, dynamic reconstruction of China continents in supercontinental configurations, the nature of rift magmatism and extent of hydrothermal alteration, paleoclimatic implication of water-rock interaction and Iow-^18O magmatism, and relationship between supercontinental evolution and global change. A number of outstanding advances in the above aspects have being made by Chinese scientists, leaving many important issues to be resolved: (1)did the Cryogenian start at either 800 to 820 Ma or 760 to 780 Ma? (2) was South China in the supercontinental configuration located in either southeast to Australia or north to India? (3) are Paleoproterozoic to Archean ages of crustal rocks a valid parameter in distinguishing North China from South China? Available observations suggest that Neoproterozoic mantle superwelling occurred as conspicuous magmatism in South China but as cryptical magmatism in North China. Mid-Neoproterozoic mantle superplume event and its derived rift-magmatism would not only result in the supercontinental demise, but also play a very important role in the generation and evolution of the snowball Earth event by initiating the global glaciation, causing the local deglaciation and terminating the snowball Earth event.     

Geochemistry of Sinian tillites from Hunan Province, South China--A test of the Snowball Earth hypothesis

A glaciomarine succession of Neoproterozoic age (between 748～584 Ma) is widespread on the Yangtze Platform, South China. The deposition took place during an intermediate to low paleolatitude position of the Yangtze Platform. The Snowball Earth hypothesis offers an explanation for the occurrence of low-latitude tillites in general. It is based on records obtained of deposits underlying and overlying the tillites. In contrast, we focused on the tillites themselves by using geochemistry to obtain detailed information about the conditions during the glaciation. Of particular interest are environmental conditions, which are closely related to the climate development. Additionally, stable isotope geochemistry was used for a paleoclimate interpretation. The geochemical results of the Sinian glacial succession on the Yangtze Platform are compared to the predictions made by the Snowball Earth hypothesis.     

Oxygen isotope geochemistry of phosphorite and dolomite and palaeo-ocean temperature estimation: A case study from the Neoproterozoic Doushantuo Formation, Guizhou Province,South China

The Neoproterozoic Doushantuo Formation at Weng'an in Guizhou Province of South China consists of phosphorites and dolomites.Three types of samples are recongnized for the phosphate-bearing rocks.In Type I samples, interstitial phosphates occur within the dolomite rocks, whereas in Type III samples, interstitial dolomites occur within the phosphorites; both of them may have diagenetic origin.The Type II samples are interbedded phosphorites and dolomites.Oxygen isotopic compositions of phosphate and coeval carbonate were analyzed.Phosphates of Type I and II samples show similar low δ18Ophos (SMOW) values of 10.9‰～12.1‰ and 10.7‰～12.8‰, respectively, whereas Type III phosphates show higher δ18Ophos (SMOW) values of 13.2‰～15.0‰.Dolomites from Type I and II samples have δ18Ocarb (PDB) and δ13Ccarb (PDB) values of -6.2‰～-3.4‰ and -2.2‰～-0.7‰, respectively, whereas interstitial dolomites from Type III samples show lower δ18Ocarb (PDB) and δ13Ccarb (PDB) values of -12.9‰～-8.6‰ and -4.4‰～-2.6‰.Petrology and oxygen isotope data suggest that diagenetic alteration may have partially affected the studied rocks, but the dolomites from Type I and phosphates from Type III samples represent the most pristine isotopic compositions of primary carbonates and phosphates precipitated from seawater.Calculations of palaeocean temperatures from the most 18O enriched dolomite and phosphate indicate a temperature range of 32.2 ℃ to 34.0 ℃ for the seawater of the Doushantuo Formation period.Ce anomaly also suggests more oxic conditions and possibly shallower water depth in the upper strata of the Doushantuo Formation at Weng'an, Guizhou Province.     

郯庐断裂带鲁南段新元古界石旺庄组的震积岩序列

在郯庐断裂带鲁南段新元古界石旺庄组碳酸盐岩地层中,鉴别出一套震积岩,其主要类型为液化方解石脉、同沉积褶曲、震碎角砾岩、海啸岩等。经过分析,这些现象是地震作用形成的震积岩—啸积岩序列,该序列为地震原地相和海啸异地相震积层组成的双层结构,两层之间被震积不整合面所分隔,因而为鲁西新元古界的地层对比增添了新的材料。     

伊春花岗质片麻岩锆石 U-Pb 年代学特征

通过1∶5万朱拉比拉河等图幅区域地质调查工作,在伊春地区发现了新元古代花岗质片麻岩。应用锆石 U-Pb 测龄方法对花岗质片麻岩进行了年代学研究,23个点的206 Pb／238 U表面谐和年龄均在850 Ma 左右,加权平均为850．2±2．1 Ma。该年龄代表花岗质片麻岩体的侵位时间为新元古代;而较新的499 Ma 年龄可能是后期构造事件改造的新生锆石,反映后期构造热事件的年龄;1518 Ma 的年龄应是本次岩浆作用过程中捕获的早期锆石的年龄,该锆石年龄可能代表本区花岗岩的继承性锆石年龄。伊春花岗质片麻岩所在的地区存在前寒武纪的古老微陆块。     

皖北新元古代刘老碑组滑塌沉积及其地质意义

安徽北部淮南、凤阳地区刘老碑组为一套页岩、钙质泥岩和泥灰岩沉积,厚度685～857m,中上部发育Chuaria化石群。刘老碑组中段发育一套滑塌沉积。滑塌沉积1～3层,单层厚30～200cm,由粉砂质泥岩、钙质泥岩、泥质粉晶灰岩及含石英粉砂粉晶灰岩组成。滑塌沉积中褶皱发育,有平卧不对称褶皱、旋转滑塌等。滑塌褶皱轴面产状表明,存在一个走向近EW,向SE154°±倾斜的大陆边缘斜坡。并且推测发育Chuaria化石群的刘老碑组中段页岩可能形成于水体较深的大陆边缘环境。     

河北隆化县龙王庙两个成矿系统的成矿探讨

河北隆化县龙王庙矿区存在两个相互独立的成矿系统。综合区域成矿规律和矿化特征分析认为:铅锌银多金属矿化是与岩浆期后热液有关的热液脉状矿床,其成矿时代为白垩世,铅锌已达中型,伴生银金,且具有寻找斑岩型矿床的前景;钒钛磁铁矿化是岩浆结晶分异型,矿化时代为新元古代,磁法圈定的磁异常区,有望找到隐伏的钒钛磁铁矿体。     

扬子陆块西缘康定杂岩的形成时代和构造环境

攀西麻粒岩是扬子陆块西缘康定杂岩的重要代表.通过对麻粒岩Rb-Sr、Sm-Nd、40Ar/39Ar和铬石U-Pb测年结果的比较,结合麻粒岩的变质演化历史和岩石地球化学特征分析,认为角闪岩相退变质作用对麻粒岩的Rb-Sr、Sm-Nd和40Ar/39Ar同位素计时体系产生了重要影响.多个岩体锆石U-Pb测年结果的一致性,表明康定岩浆-变质杂岩形成于新元古代(860～750 Ma).岩石的地球化学性质还指示了康定杂岩形成于扬子陆块西缘新元古代活动大陆边缘的岛弧环境.