塔里木盆地西南部叶城凹陷下白垩统克孜勒苏群扇三角洲沉积特征及模式

以野外露头、岩心观察为基础,结合录井、测井及粒度分析,认为叶城凹陷下白垩统克孜勒苏群发育进积型扇三角洲,划分出扇三角洲平原与扇三角洲前缘水下分流河道、分流间湾与河口坝。扇三角洲平原受重力流作用明显;扇三角洲前缘主要受牵引流作用,在盆地的短轴方向延伸范围较小,而在长轴方向延伸范围较大,与相邻扇三角洲前缘朵叶叠置连片。扇三角洲由山前向断陷湖盆进积,在盆地的短轴方向构成一侧为陡坡、另一侧为宽缓斜坡的单断式断陷盆地充填模式。这种沉积模式不仅控制了扇三角洲各亚相单元的发育特征,而且对微相类型与砂体分布有控制作用。     

Possible persistence of the morganucodontans in the Lower Cretaceous Purbeck Limestone Group (Dorset, England)

‘Triconodonts’ are relatively rare in the new Purbeck Limestone Group mammal fauna, especially in comparison with the finds made in the nineteenth century in the same beds. However, besides some remains of the already known Triconodontidae, a new possible morganucodontan of uncertain familial affinities, Purbeckodon batei gen. et sp. nov., has been discovered, extending considerably the chronological range of the order while constituting another testimony to the vitality and diversity of Mesozoic mammals.     

Age and nature of the late Early Cretaceous Zhaga Formation,northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.     

Petrography and geochemistry of the Late Cretaceous redbeds in the Gan-Hang Belt,southeast China: implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in southeast China were generated in a crustal extension setting during the late Mesozoic. Compared with the adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstone samples from the Late Cretaceous Guifeng Group of the Yongchong Basin in the Gan-Hang Belt, southeast China. Detrital grains are commonly subangular to subrounded, poorly sorted, and are rich in lithic fragments. The variable pre-metasomatic Chemical Index of Alternation (CIA* = 62–85), Chemical Index of Weathering (CIW = 70.90–98.76, avg. 85.62), Plagioclase Index of Alteration (PIA = 60.23–98.35, avg. 79.91), and high Index of Compositional Variability (ICV = 0.67–3.08, avg. 1.40) values collectively suggest an overall intermediate degree of chemical weathering and intense physical erosion of the source rocks, but a relatively decreased degree of chemical weathering during the late stage (Lianhe Formation) of the Guifeng Group is observed. Several chemical ratios (e.g. Al₂O₃/TiO₂, La/Th, Cr/Th, Th/Sc, Zr/Sc) also suggest a dominant felsic source nature, significant first-cycle sediment supply, and low sedimentary recycling. Such features are consistent with active extension tectonic setting. Sandstone framework models and geochemical characteristics suggest the provenance is related to passive margin (PM), active continental margin (ACM), and continental island arc (CIA) tectonic settings. Sediment derivation from the Neoproterozoic metamorphic rocks and Silurian–Devonian granites indicates a PM provenance, whereas sediments derived from the Early Cretaceous volcanic-intrusive complexes suggest an ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben under the back-arc extension regime when southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

Provenance of the Langjiexue Group to the south of the Yarlung-Tsangpo Suture Zone in southeastern Tibet: Insights on the evolution of the Neo-Tethys Ocean in the Late Triassic

ABSTRACT

The Upper Triassic Langjiexue Group, which lies immediately south of the Yarlung-Tsangpo Suture Zone in the Shannan area of southeastern Tibet, represents an important part of the Tethyan Himalayan Sequence (THS). Its provenance and palaeogeography have been the subject of debate. We present new data on petrographic composition, whole-rock geochemistry, and detrital zircon U–Pb geochronology to constrain the provenance of the Langjiexue Group. The dominance of quartz grains and felsic volcanic lithic fragments suggests that the sandstones are litho-quartzose. The trace element geochemical signatures (V–Ni–Th*10, Co/Th–La/Sc, Eu/Eu*–Th/Sc) suggest derivation from felsic igneous sources. The detrital zircon age spectra display three major peaks: a Meso-to-Neoproterozoic peak (1200–900 Ma, 7–18%), a Neoproterozoic-to-Late Cambrian peak (750–500 Ma, 32–65%), and a Late Carboniferous-to-Late Triassic peak (300–200 Ma, 11–33%). The maximum depositional age of early Carnian (236–235 Ma) is obtained by calculating weighted average ages of the youngest zircons (≤250 Ma). The youngest age cluster (300–200 Ma) is incompatible with sources from neighbouring terranes, including the South Qiangtang terrane, Lhasa terrane, THS, and Higher Himalayan Crystalline. Correlations of the Permian–Triassic zircons with those of time-equivalent strata in northwest Australia, west Burma, and the Banda Arc unveil a potential connection to the Tasmanides along the convergent margin of eastern Australia. The New England Orogen (300–230 Ma) could have supplied the Langjiexue Group with magmatic materials via continent-scale drainage systems or a submarine fan complex. This scenario provides a new perspective into the transport of detritus from distal orogens to sedimentary basins thousands of kilometres away.     

滇南新元古代的岩浆作用：来自瑶山群深变质岩SHRIMP锆石 U-Pb年代学证据

滇南瑶山—哀牢山地区出露一套呈NW—SE向狭长条带状展布的深变质岩系——下元古界瑶山群和哀牢山群,并认为存在前寒武纪的结晶基底。本文选取瑶山群眼球状花岗质糜棱岩和金平阿得博的片麻状花岗岩作为研究对象,进行SHRIMP锆石U-Pb定年。结合阴极发光图像,获得岩浆锆石的结晶年龄为828±6.2Ma(N=11,MSWD=1.8)和748～801Ma。同时,作者在墨江-元江剖面的哀牢山群黑云母花岗闪长质片麻岩中获得SHRIMP锆石核部U-Pb年龄为769～893Ma。研究揭示滇南的瑶山和哀牢山地区存在新元古代岩浆作用。瑶山群中古老残余锆石核部的984Ma年龄和哀牢山群中古老残余锆石核部年龄1104～1353Ma及2655Ma,表明该区存在中元古代或太古代的结晶基底,研究区很可能位于南华裂谷与康滇裂谷的交汇部位,新元古代的岩浆活动可能与Rodinia超大陆聚合——裂解过程有关。这为构筑Rodinia超大陆聚散过程和揭示华南大地构造演化史提供年代学方面证据。     

图像分析技术在碎屑岩粒度分析中的应用

在详细介绍一种获得粒度资料的新技术——图像分析技术的基础上,选取青藏高原乌丽地区乌丽群含石榴子石的纯石英砂岩作为研究对象,对其进行了详细粒度分析研究.结果表明,该技术不仅原理可靠,实现容易,应用方便,且检测速度快,测量精度高,结果稳定性好.该技术主要通过图像采集,图像处理、目标测量、数据处理和结果解释等一系列必要的操作...     

浙江金衢盆地白垩系衢江群沉积相类型及沉积环境演化

本文在野外露头观察和前人研究成果整理的基础上,通过对金衢盆地白垩系衢江群的岩性组合、沉积构造、地层序列及沉积中心、岩相变化等的研究,将衢江群划分出山麓洪积相、河流相、湖泊相、河湖相4种沉积相类型.认为盆地发育大致经历了3个阶段:形成于中戴期早期,于金华期处于全盛时期,萎缩于衢县期.衢江群岩性总体上反映了盆地的发生、扩展至萎缩各个时期的变化.     

Chemical indexes of Paleoproterozoic sedimentary rocks from Liaohe Group,North China Craton:Implications for paleoclimate and provenance

Paleoproterozoic supracrustal materials have been widely identified in North China Craton, such as the Liaohe-, Hutuo-and Lyuliang groups. The Liaohe Group in the eastern part of the North China Craton is dominated by deformed and metamorphosed sedimentary and volcanic successions. Compared with that of the coeval volcanic rocks, geochemistry of the sedimentary rocks from the Liaohe Group has rarely been studied in detail, which can possibly provide information on the paleoclimate and provenance. The authors analyzed the whole-rock and detrital zircon geochemistry of sedimentary rocks from the Liaohe Group against a uniform process and proposed their different ways of paleoweathering of the lower and upper formations. That is to say, although the lower and upper formations within the Liaohe Group might be derived from the similar source composition in a tectonically active setting, the paleoclimate that they experienced was not exactly the same. The predominant derivations are the Paleoproterozoic granitoids and basalts within the Liaohe Group, with minor input of Archean continental crust. This study highlights the contributions of the Paleoproterozoic mafic sources, which has been generally overlooked in the previous researches.     

浙江横山组的时代和区域地层对比

浙江横山组被认为与寿昌组上段呈整合接触而归入建德群,由于生物化石稀少,又无年龄资料,因而地层时代和层位认识分歧较大。对寿昌盆地横山组与寿昌组之间的接触关系作了重新研究,证实是假整合而不是整合。在兰溪墩头盆地、淳安甘坞盆地以及常山朱村盆地,横山组均假整合或不整合覆于老地层上,在墩头盆地横山组下部发现大量永康馆头组中常见的...