Phase contrast synchrotron X-ray microtomography of Ediacaran (Doushantuo) metazoan microfossils: Phylogenetic diversity and evolutionary implications

Microfossils from the Ediacaran Weng’an Phosphate Member of the Doushantuo Formation (Guizhou Province, southern China) have received widespread attention. The Doushantuo, which overlies the glacial deposits of the Nantuo Formation, was deposited following the Marinoan glaciation, the last extensive glaciation of Snowball Earth. Radiometric age dating indicates that the Doushantuo is older than 580 my, and hence that these microfossils are older than the Ediacara Biota. However, the diversity represented by these fossils has yet to be fully documented. A recent technological approach that has increasingly been used to image fossils, propagation phase contrast synchrotron X-ray microtomography, has allowed non-destructive study of both exterior and interior features of a variety of Doushantuo microfossils from the gray facies of the Weng’an Phosphate Member, cropping out along the axis of the Mt. Beidou anticline. Studies of Doushantuo embryos demonstrate the existence of a large suite of modern embryonic features, including macromeres and micromeres, cell lineage, polar lobes, compacted epithelia, equal and unequal cleavage, blastulation and gastrulation, and chorionic protection. Because embryos such as those here studied provide only a limited amount of phylogenetic information, and because adult metazoans of the types that produced these embryos have yet to be discovered in Doushantuo-age rocks, these fossilized embryonic forms can at present be assigned only to the various superclades represented amongst living Metazoa. The diversity of the embryos here studied suggests that the metazoan fauna of the Doushantuo may well have included animals of poriferan, cnidarian, and both protostomial (representatives possibly of basal protostome lineages) and deuterostomial affinity. If this interpretation is correct, it would then follow that the last common ancestor of the bilaterian metazoan lineage, as well as the last common ancestor of sponges, cnidarians and bilaterians, pre-dated deposition of the Doushantuo strata.     

Petrology and geochemical characteristics of Precambrian granitic basement complex rocks in the southernmost part of North-Central Nigeria

The Precambrian basement complex in the southernmost part of North-Central Nigeria is underlain by migmatitic banded gneisses, granitic intrusions and dykes of dolerite, rhyolite porphyry and pegmatite. The rocks are generally felsic, containing modal and normative hypersthene, as well as normative corundum. The basement complex has experienced high-grade regional metamorphism as indicated by the presence of hypersthene and plagioclase of andesine composition. Anatectic melting is suggested by the occurrence of ptygmatic folds, folded gneissose foliation, numerous quartzo-feldspathic veins and lenses of dark-colured, micaceous schistose rocks. Geochemically, the rocks have magnesian, calc-alkalic and strongly peraluminous characteristics. Their overall characteristics suggest derivation from progressive (fractional) partial melting of pelitic rocks during high-grade regional metamorphism, possibly associated with intense hydrothermal activities. The magnesian characteristics reflect close affinity to relatively hydrous, oxidizing melts and source regions in settings broadly related to subduction.     

浅论津巴布韦康尼玛拉（Connemara）金矿床地质特征及其找矿意义

康尼玛拉金矿床的形成与太古宙绿岩带、前寒武系变质岩关系密切,矿体、矿化蚀变体主要受NNW向断裂构造控制,属破碎蚀变岩型金矿,围岩蚀变发育,硅化、黄铁矿化、绢云母化与矿化关系密切,该矿区具有良好的找矿前景。     

山东早前寒武纪变质地层形成年代——锆石SHRIMP U-Pb测年的证据

在鲁西地区,确定了泰山岩群主体形成时代为2 750-2 700 Ma,属新太古代早期。锆石SHRIMP U-Pb定年：雁翎关组角闪变粒岩（2 747±7）Ma,柳杭组黑云变粒岩（2 739±16）Ma、（2 703±6）Ma,侵入雁翎关组底部透闪阳起片岩的片麻状石英闪长岩（2 740±6）Ma。济宁（岩）群碎屑锆石年龄（2 700-2 665）Ma,岩浆锆石年龄（2 561±24）Ma,所以济宁（岩）群形成于新太古代晚期,而不是以往认为的古元古代。在鲁东地区,发现了中太古代表壳岩系、花岗质岩石和新太古代早期TTG花岗质岩石;荆山群和粉子山群的形成时代为古元古代晚期。荆山群大理岩被锆石SHRIMP U-Pb年龄（1 868±11）Ma、（1 852±9）Ma的莱州组合西水夼单元辉长岩、闪长岩侵入,并一起遭受变质变形作用改造。     

A Model of Hydrothermal Dolomite Reservoir Facies in Precambrian Dolomite,Central Sichuan Basin,SW China and its Geochemical Characteristics

Hydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show that the hydrothermal dolomite reservoirs of Dengying Formation consist of four main types of pores in the reservoir facies. These include: 1) hydrothermal dissolution vug(or pore), 2) intercrystalline pore, 3) residual inter-breccia vug(or pore), and 4) enlarged dissolved-fracture. There are three different fabrics dolomite in hydrothermal dolomite reservoirs, namely, saddle dolomite, fine-medium dolomite and micritic dolomite. Micritic dolomite is the original lithology of host rock. Saddle dolomite with curved or irregular crystal faces was directly crystallized from hydrothermal fluids(average temperature 192°C). Fine-medium dolomites are the products of recrystallization of micritic dolomite, resulting in abnormal geochemical characteristics, such as slight depletion of δ¹⁸O, significant enrichment of Mn-Fe and ⁸⁷Sr/⁸⁶Sr, and positive Eu anomaly. A model for the distribution of various hydrothermal dolomite reservoir facies is proposed here, which incorporates three fundamental geological controls: 1) extensional tectonics and tectono-hydrothermal events(i.e., the Xingkai Taphrogenesis of Late Sinian-Early Cambrian, and Emei Taphrogenesis of Late Permian), 2) hydrothermal fluid storage in clastic rocks with large thickness(e.g., Nanhua System of Chengjiang Formation and part of Doushantuo Formation), and 3) confining bed for hydrothermal fluids(such as, the shale in Qiongzhusi Formation). The supply of hydrothermal fluid is critical. Large basement-rooted faults and associated grid-like fracture system may function as the channels for upward migration of hydrothermal fluid flow. The intersection of the above-mentioned faults(including the conversion fault), especially transtensional sags above negative flower structures on wrench faults can serve as a key target for future hydrocarbon exploration.     

Provenance and tectonic setting transition as recorded in the Neoproterozoic strata,western Jiangnan Orogen:Implications for South China within Rodinia

The Neoproterozoic Tonian strata(ca.870-725 Ma)in the western Jiangnan Orogen archive the records of sedimentary provenance and tectonic setting which can be used to understand the geological evolution of the South China Continent.These strata are separated into the basement and cover sequences by a regional angular unconformity.The basement sequence can be subdivided into the lower and the upper parts by the widespread interbedded ca.840 Ma basalt with pillow structure.In the present work,234 concordant detrital zircon analyses are obtained from three Tonian sandstone samples in the Fanjingshan district,Guizhou Province.Combined with previous results,a total of 1736 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata in the western Jiangnan Orogen are used to decipher the integrated sedimentary and tectonic histories.The zircons from the lowermost part of the basement sequence(the Yujiagou Formation)show oval morphology and display two Paleoproterozoic age peaks at 2325 Ma and 1845 Ma which are similar with the detrital zircon age peaks from the Late Paleoproterozoic to Early Mesoproterozoic Dongchuan/Dahongshan/Hekou groups,suggesting a passive margin basin in which the sediments were mainly sourced from the southwestern Yangtze Block.However,the zircon age population of the lower part of the basement sequence(the Xiaojiahe,Huixiangping formations and their equivalents)indicates the sedimentary derivation from bidirectional sources(the ca.870 Ma arc materials in the south and the old detritus from the southwestern Yangtze Block)which is consistent with a back arc setting for the deposition of the sediments.Zircons from the upper part of the basement sequence(the Duyantang Formation and its equivalent)show euhedral and subangular morphology and display a unimodal age peak at ca.835 Ma.This sequence was possibly deposited in a convergent setting and the detritus were came from the locally distributed syn-collisional igneous rocks.The lower part of the cover sequence(the Xinzhai and Wuye formations and their equivalents)shows a distinct zircon age peak at 815—809 Ma and two subordinate peaks at 2485 Ma and 2018 Ma,suggesting that the basin had gradually transformed into a continental rift basin and received the detritus from the ca.815 Ma post-collisional magmatic rocks as well as from different Paleoproterozoic source rocks in the northern Yangtze Block.We propose a tectonic evolution model that envisages eruption of ca.840 Ma basalt in a back arc basin that existed during ca.870-835 Ma,an angular unconformity was formed during amalgamation of the Yangtze Block and the Cathaysia Block at ca.835-820 Ma and the rifting of the South China Continent was initiated at ca.800 Ma.Our study concludes that the South China Continent was formed on the periphery of the Rodinia supercontinent.     

Runoff generation in zero-order precambrian shield catchments: The stormflow response of a heterogeneous landscape

Hydrometric and isotopic (oxygen-18) observations were used to delineate the runoff processes operating in several headwater catchments on the Precambrian Shield of Canada. The catchments comprise patches of conifer forest situated on thin soils among areas of lichen-covered granitic bedrock. Horton overland flow occurred from the lichen-bedrock areas in all precipitation events that exceeded 4–6 mm. Runoff from the forest stands occurred mainly as subsurface stormflow, but in some instances saturation overland flow was observed. The occurrence of saturation overland flow was controlled by the topography of the bedrock beneath the forest soils. The area contributing runoff and the pathway by which water was conveyed to the catchment outflow switched from the open lichen-bedrock areas producing overland flow on the rising limb of the storm hydrograph to the forest stands contributing subsurface stormflow on the recession limb of the hydrograph. The areal extent and position of the landscape units in the basin were important to the rate and magnitude of stormflow production. Runoff was generated from the catchments only during and immediately after snowmelt and/or rainfall events. The catchments were dry and/or frozen for about 70% of the year.     

粤西云开前寒武纪基底麻粒岩、紫苏花岗岩放射性元素分布特征与岩石成因讨论

粤西云开前寒武纪基底的麻粒岩、紫苏花岗闪长岩和紫苏闪长岩的放射性元素分布具有明显的规律，Ｋ／Ｒｂ比值分别为１１７～２０２，９４～１２９，７８～８９，Ｔｈ／Ｕ比值分别为１５～３０，１１～２０，２５～７８，Ｕ的含量范围分别为０．５×１０－６～０．９９×１０－６，１．２５×１０－６～１．６９×１０－６，１．６５×１０－６～５．３７×１０－６．这种分布规律反映了在低ＸＨ２Ｏ、高ＣＯ２流体参与下麻粒岩的熔融、紫苏花岗质岩浆形成、难熔铁镁相和副矿物相富集的过程     

浙闽前寒武纪基底地壳的形成和增长时代

对浙闽地区变质岩Ｓｍ－Ｎｄ及锆石Ｕ－Ｐｂ年龄资料研究后认为，浙闽地区存在前寒武纪地壳基底，还可能存在晚太古代古陆核，基底地壳具有幕式增长的特点，经历了２４００Ｍａ（早元古代）１８００Ｍａ（中元古代）和１４００Ｍａ（中元古代）三个主要的地壳增长时期。     

Sr、C同位素对苏皖北部上前寒武系时代的界定

对苏皖北部上前寒武系碳酸盐岩地层系统采集灰岩样品 ,采用弱酸处理方法测定清液部分的 Sr同位素组成 ,对全岩样品测定 C同位素组成。测定结果与 Shields(1999)发表的新元古代古海水 Sr、C同位素组成随时间的演化曲线对比表明 ,苏皖北部上前寒武系是跨越北方青白口系与南方震旦系之间的一段连续地层 ,其时代约从70 0 Ma到 85 0 Ma,该研究进一步印证了淮南生物群属于先伊迪卡拉期的学术观点。