The interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust,Fiordland, New Zealand     

《Journal of Structural Geology》2016

Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition.  相似文献   

Atmospheric palaeo-CO2 estimates based on the carbon isotope and stomatal data of Cheirolepidiaceae from the Lower Cretaceous of the Jiuquan Basin,Gansu Province     

《Cretaceous Research》2016

The stable carbon isotope compositions and the stomatal parameters (stomatal density and stomatal index) of four Cheirolepidiaceae species, Brachyphyllum ningxiaensis, Brachyphyllum obtusum, Pseudofrenelopsis dalatzensis and Pseudofrenelopsis gansuensis, were analyzed to recover the late Early Cretaceous atmospheric CO₂ levels. The fossil plants were collected from 5 consecutive sedimentary members of the uppermost Zhonggou Formation. Based on the stomatal data, the estimated palaeo-atmospheric CO₂ concentrations in the Jiuquan Basin during the late Early Cretaceous were 1060–882 ppmv based on the carboniferous standardization and were 641–531 ppmv based on the recent standardization; the pCO₂ values present at first a decreasing and then an increasing trend within the sedimentary time of the five members. The δ¹³Cp values based on the 21 Brachyphyllum specimens showed a large variation, which ranged from −20.98‰ to −25.69‰, with an average of −24.2‰. The values also identified a C3 photosynthetic pathway for the Brachyphyllum specimens. The predicted δ¹³Ca values varied from −2.1‰ to −6.38‰, with an average of −5.03‰. These two proxies were irregular within the different members; therefore, the correlation with the change in atmospheric CO₂ concentrations was not significant. Moreover, a water-stressed environment was proposed based on the δ¹³C values of the present fossil plants, a proposal that was also supported by the previous palaeobotanical, palynological and stratigraphical evidence. In the present study, an inconsistent relationship between the stable carbon isotope and the stomata values was apparent, which most likely indicated that the stomata numbers of the plant were more sensitive to the variation in the concentration of the atmospheric CO₂, whereas the δ¹³C values were sensitive to the moisture conditions.  相似文献   

四川龙门山地区早中泥盆世孢子化石研究   总被引：1，自引：0，他引：1  

黄敏  朱怀诚  刘锋 《地质论评》2016,62(6):1533-1548

本文系统研究了四川龙门山地区下泥盆统桂溪组、木耳厂组、观音庙组、关山坡组以及中泥盆统金宝石组的孢子化石,共鉴定孢子41属81种,建立了三个孢子组合带,由下至上分别是:Retusotriletes rotundus—Calamospora panucea(RP)孢子组合、Synorisporites downtoensis—Verrucosisporites scurrus(DS)孢子组合、Densosporites inaequus—Grandispora inculta(Ⅱ)孢子组合。通过与研究较为详细的西欧、北美地区的经典资料,包括老红砂岩大陆、北欧、加拿大等地区,以及国内相关资料的对比,结合特征种的地层分布、重要属种的含量、孢子大小等组合特征,确定RP组合时代为Lochkovian,DS组合时代为Pragian至早Emsian,Ⅱ组合时代为Eifelian中晚期—Givetian早期。这也是首次在金宝石组中发现孢子化石并对其进行研究。另外,通过对孢子直径的统计分析从另一个角度探讨了古植物的演化历程。  相似文献   

广西隆安下石炭统都结微生物岩微相特征与沉积环境研究     

苗卓伟  张永利  巩恩普  关长庆  阮氏梅香  黄文韬  王邓 《地质论评》2016,62(5):1134-1148

广西隆安县都结乡发育的微生物岩产于下石炭统都安组中—上部地层,分布广、厚度大,是中国南方早石炭世报道较少的碳酸盐岩建造类型,具有较高的研究价值。该微生物岩以颗粒灰岩和粘结岩为主,生物含量丰富,包括微生物、有孔虫、棘皮类、钙球、腕足等。通过岩相分析,可识别出以下8种微相类型:MF1含亮晶胶结物的泥晶包壳生屑灰岩、MF2含腕足似球粒灰岩、MF3无纹层似球粒颗粒灰岩、MF4集合颗粒灰岩、MF5细密纹层状粘结岩、MF6内碎屑鲕粒灰岩、MF7鲕粒灰岩、MF8白云质鲕粒灰岩。伴随海平面升降变化,都结微生物岩沉积环境先后经历了FZ1台地边缘浅滩相、FZ2开阔台地相与FZ3局限台地相这三种沉积相带之间往复变化的沉积相模式。结合前人基础地质资料,该沉积相模式表明该微生物岩发育于低纬度孤立碳酸盐岩台地浅水环境,并经历了初期、主体时期和消亡期3个发育阶段,海平面虽伴有小幅波动,但整体呈下降趋势。微生物岩发育于晚古生代冰期与谢尔普霍夫生物灭绝事件前夕,该时期南盘江盆地微生物岩的富集与后生动物礁的缺失相耦合,海洋生态系统中生物组成发生显著变化。在此基础上,本文从微生物岩的角度初步探讨了冈瓦纳大陆冰川作用对北半球低纬度地区的影响。  相似文献   

下扬子PTB界线深水相区粘土岩的火山成因研究及其对LPME的指示意义     

廖志伟  胡文瑄  王小林  曹剑  姚素平  万野 《地质学报》2016,90(4):785-800

全球,尤其是特提斯域二叠—三叠纪之交(PTB)剖面中普遍发育火山成因的粘土岩,对理解晚二叠世末生物大灭绝(LPME)的触发机制及相关基础地质问题具有重要意义。本文报道了在华南下扬子区新发现两条深水PTB剖面(皖南牛山和蔡村)的粘土岩工作,包括岩石学、矿物学和全岩地球化学等,填补了区域研究空白。研究结果发现,粘土岩主要由伊利石等粘土矿物,以及石英、岩浆锆石、长石等斑晶矿物组成。在地球化学上具有高K_2O、低Na_2O、相对富集大离子亲石元素(LILE)、相对亏损高场强元素(HFSE)等特征。据此,认为这些粘土岩为火山成因的钾质斑脱岩,其原岩可能为中酸性流纹英安岩,具有弧岩浆作用的源区背景。对比华南其他地区已发现的PTB界线粘土岩,发现它们成因类似,可能来源于古特提斯洋周缘陆陆碰撞,抑或是泛大洋俯冲潘吉亚大陆东缘(包括华南板块)所导致的长英质火山岩浆喷发,且具有多期多源性特点。PTB时期全球活跃的火山岩浆活动(包括镁铁质火山作用和华南地区火山灰所指征的长英质火山作用)可能是导致LPME的主要原因。  相似文献   

Saurischian track assemblages from the Lower Cretaceous Shenhuangshan Formation in the Yuanma Basin,Southern China     

《Cretaceous Research》2016

Cretaceous dinosaur tracks from Hunan Province are historically significant as the basis for three ichnotaxa: Xiangxipus chenxiensis, Xiangxipus youngi, and Hunanpus jiuquwanensis all representing theropodan tracks, described from a single site in 1982. Although the type locality has since been destroyed, the type specimen and replicas remained available for restudy in 2006, when paratype Hunanpus tracks and sauropod footprints were described from a second, nearby locality. Material from both localities is here re-described in detail. It is proposed that while Xiangxipus chenxiensis can be regarded as a distinct ichnospecies, probably representing an ornithomimid trackmaker, Xiangxipus youngi cannot be accommodated in the same ichnogenus. We consider it similar to the ichnogenus Wupus from the Lower Cretaceous of Sichuan Province, and therefore of possible avian affinity. We also find no justification for regarding Hunanpus jiuquwanensis as generically distinct from the widespread ichnogenus Grallator, and therefore rename it Grallator jiuquwanensis comb. nov. The Hunan track record is generally consistent with the sparse record of theropod skeletal remains in the province, but adds evidence of sauropods that was previously lacking.  相似文献   

New fossil species of the Venicoridae (Heteroptera: Pentatomomorpha) from the Lower Cretaceous of Northeast China     

《Cretaceous Research》2016

Specimens of the Venicoridae (Heteroptera) occur in the Lower Cretaceous Yixian Formation in northeastern China. We describe a new genus and two new species Halonatusivena shii gen. et sp. nov. and Halonatusivena nervosus sp. nov. from this formation. The veins of the membrane are distinctive; the edges of veins are not simple but resemble an irregular stain, with a reduced vasiform structure previously unknown in the Heteroptera. Remarkably, the antennae of one of the specimens of Halonatusivena shii exhibit morphological abnormality. An identification key for the known species of the Venicoridae is provided.  相似文献   

Two new glaphyrids (Coleoptera,Scarabaeoidea) from the Jehol Biota,China     

《Cretaceous Research》2016

Two new species, Cretohypna puncta sp. nov. and Cretohypna robusta sp. nov., of Glaphyridae are described and illustrated. These fossils were collected from the Jehol Biota, Lower Cretaceous Yixian Formation of Liutiaogou Village in Ningcheng, Inner Mongolia, China. Up to date, 18 fossil species in 6 genera of Glaphyridae have been reported, among which ten species are from China. Respective keys to the Mesozoic genera of Glaphyridae and to species of Cretohypna Yan, Nikolajev & Ren, 2012 are provided. This is the first report of spiracles in fossil Glaphyridae leading to a hypothesis that the character of the 7th abdominal spiracles present in pleural membrane is a plesiomorphy for Glaphyridae.  相似文献   

Pes shape variation in an ornithopod dinosaur trackway (Lower Cretaceous,NW Spain): New evidence of an antalgic gait in the fossil track record     

《Cretaceous Research》2016

Trackways can provide unique insight to animals locomotion through quantitative analysis of variation in track morphology. Long trackways additionally permit the study of trackmaker foot anatomy, providing more insight on limb kinematics. In this paper we have restudied the extensive tracksite at Barranco de La Canal-1 (Lower Cretaceous, La Rioja, NW Spain) focussing on a 25-m-long dinosaur (ornithopod) trackway that was noted by an earlier study (Casanovas et al., 1995; Pérez-Lorente, 2003) to display an irregular pace pattern. This asymmetric gait has been quantified and photogrammetric models undertaken for each track, thus revealing distinct differences between the right and the left tracks, particularly in the relative position of the lateral digits II–IV with respect to the central digit III. Given that the substrate at this site is homogenous, the consistent repetition of the collected morphological data suggests that differences recorded between the right and the left tracks can be linked to the foot anatomy, but more interestingly, to an injury or pathology on left digit II. We suggest that the abnormal condition registered in digit II impression of the left pes can be linked to the statistically significant limping behaviour of the trackmaker. Furthermore, the abnormal condition registered did not affect the dinosaur's speed.  相似文献   

A new gigantic lacewing species (Insecta: Neuroptera) from the Lower Cretaceous of Brazil confirms the occurrence of Kalligrammatidae in the Americas     

《Cretaceous Research》2016

Makarkinia kerneri sp. nov. is described from the Lower Cretaceous Crato Formation of Brazil, based on an incomplete but very well-preserved hind wing (Neuroptera: Kalligrammatidae). The previously presumed attribution of the genus is confirmed, based on the diagnostic characters of its wing venation (e.g., dense crossvenation; the anteriorly directed branches of MP) and the presence of a distinct eye-spot on the wing. Makarkinia is the only American representative and youngest known genus of Kalligrammatidae. With an estimated wing length of 100–160 mm, it has the largest wings amongst all fossil and extant Neuroptera.  相似文献