Microbially induced sedimentary structures in Archean sandstones: A new window into early life

Until now, the most valuable information on the early life on the Archean Earth derived from bacterial fossils and stromatolites preserved in precipitated lithologies such as chert or carbonates. Also, shales contain complex biomarker molecules, and specific isotopes constitute an important evidence for biogeneicity.In contrast, because of their low potential of fossil preservation, sandstones have been less investigated. But recent studies revealed a variety of ‘microbially induced sedimentary structures — MISS’ that differ greatly from any other fossils or sedimentary structures. ‘Wrinkle structures’, ‘multidirected ripple marks’, ‘biolaminites’, and other macrostructures indicate the former presence of photoautotrophic microbial mats in shallow-marine to tidal paleoenvironments. The MISS form by the mechanical interaction of microbial mats with physical sediment dynamics that is the erosion and deposition by water agitation. The structures occur not only in Archean tidal flats, but in equivalent settings throughout Earth history until today.MISS are not identified alone by their macroscopic morphologies. In thin-sections, the structures display the carpet-like fabrics of intertwined filaments of the ancient mat-constructing microorganisms. Geochemical analyses of the filaments proof their composition of iron minerals associated with organic carbon.In conclusion, microbial mats colonize sandy tidal settings at least for 3.2 Ga years. Therefore, Archean sandstones constitute an important archive for the exploration of early life.     

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR₂) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.     

硅质碎屑岩中的微生物席相关构造——联接现代与过去的纽带

微生物通过自身生命活动可以在硅质碎屑物上形成微生物席.微生物席具有黏结性、柔韧性、抗剥蚀性等特征,与物理营力相互作用可以形成一系列独特的原生沉积构造,即微生物诱发的沉积构造(MISS).形成和保存MISS的基本条件包括经历较低的变质程度、发育于海退—海侵的转折点、出现在有利的“微生物席沉积相”中.微生物席本身很难保存在岩石序列中,所以识别石化MISS具有一定难度.与现代环境中发育的MISS具有相似的几何形态和包含古代微生物席组构是识别石化MISS的有效途径.微生物生长、稳化、障积、捕获和黏结作用可以形成形态各异的MISS,根据这些作用和特征可以将其划分为5大类17小类.对保存在硅质碎屑岩中的微生物诱发的原生沉积构造和现代潮坪中发育的MISS进行综合对比研究,有利于微生物席沉积学的发展,也为进一步理解古代硅质碎屑岩沉积环境提供重要线索.     

Characteristic and Geological Significance of Microbially Induced Sedimentary Structures(MISS) in Terrestrial P-T Boundary in Xingyang,Western He’nan Province

Microbially Induced Sedimentary Structures (MISS) are primary sedimentary structures that arise syndepositionally from microbial community activity. Especially valuable are MISS for the analysis of early Archean (and extraterrestrial) deposits. However, most reports of MISS have focused on the Precambrian and Phanerozoic mass extinction marine sediments, and only a few and terrestrial MISS have been studied. The MISS presented in this paper, which mainly consists of mat growth feature, mat destruction feature and mat decay feature, are well preserved in terrestrial clastic rocks around the P-T boundary in Xiyang area, western Henan. Large U ridges, oriented sand quartz, mica grains and thin clayey laminae arranged parallel to bedding plane are the main features visible under the microscope. Several lines of evidence indicate that the Xingyang MISS are of biogenic origin. Abundant MISS in Xingyang may indicate the degradation of terrestrial ecosystems and proliferation of microbial mats immediately after the severe Permian-Triassic mass extinction. Study aiming at MISS helps to analyze their space distribution in the globe and to probe into links between microbial proliferation and environmental stresses following the end-Permian mass extinction in terrestrial ecosystems.     

华北燕山期区域变质与内生成矿作用关系雏议

初步讨论了华北地区燕山期区域变质作用与内生成矿作用的关系。指出华北地区众多内生金属矿床产于绿片岩相岩石组合中，其中包括太古宙岩石的退变质、沉积盖层和中生代火山-侵入杂岩的进变质岩石。因此，这些矿床的形成与燕山期区域变质作用及其相应的韧性-韧脆性变形作用密切相关。     

Timing relationships between pegmatite emplacement,metamorphism and deformation during the intra‐plate Alice Springs Orogeny,central Australia

In the Harts Range (central Australia), the upper amphibolite facies to lower granulite facies, c. 480–460 Ma Harts Range Metamorphic Complex (HRMC), and the upper amphibolite facies, c. 340–320 Ma Entia Gneiss Complex are cut by numerous, generally peraluminous pegmatites and their deformed equivalents. The pegmatites have previously been interpreted as locally derived partial melts. However, SHRIMP U–Pb monazite and zircon dating of 29 pegmatites or their deformed equivalents, predominantly from the HRMC, reveal that they were emplaced episodically throughout almost the entire duration of the polyphase, c. 450–300 Ma intra‐plate Alice Springs Orogeny. Episodes of pegmatite intrusion correlate with the age of major Alice Springs‐age structures and with deposition of syn‐orogenic sedimentary rocks in the adjacent Centralian Superbasin. Similar Alice Springs ages have not been obtained from anatectic country rocks in the HRMC, suggesting that the pegmatites were not locally derived. Instead, they are interpreted as highly fractionated granites, and imply that much larger parental Alice Springs‐age granites exist at depth. The mechanism to allow repeated felsic magmatism in an intraplate setting, where all exposed rock types had a previous high‐temperature history, is enigmatic. However, we suggest that episodic underthrusting and dehydration of unmetamorphosed Centralian Superbasin sedimentary rocks allowed crustal fertility to maintained over a c. 140 Ma interval during the intra‐plate Alice Springs Orogeny.     

Lithochemical typification of Early Precambrian ferruginous-siliceous formations in the East European Craton

The first comparative paleolithochemical characteristics of Early Precambrian ferruginous-siliceous formations of the East European Craton confined to four stratigraphic levels—Lower Archean, Upper Archean (Lopian), Lower Karelian, and Upper Karelian—are presented. Using the MINLITH method and software package for lithochemical calculations, the possible primary composition of metasedimentary rocks is reconstructed and paleogeographic settings of sedimentation are suggested. It is shown that different age formations represented initially lithogenetic groups with different compositions and quantitative relationships between the major types of sedimentary rocks with gradual transitions and genetic affinity. They accumulated in paleotectonic and facies settings that were specific for each stage of iron ore sedimentation, resulting in the development of four genetic (Bug, Algoma, Okolovo, and Lake Superior) types of ferruginous-siliceous formations.     

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures.Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.     

Comparative study of two relatives,MISS and Stromatolites: example from the Proterozoic Kunihar Formation,Simla Group,Lesser Himalaya

Comparison of microbially induced sedimentary structures (MISS) and stromatolitic bearing horizons from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, has been scrutinised to understand the formative processes and controls on MISS and stromatolites in the context of sedimentary facies and response to sea level fluctuations. MISS structures recorded are wrinkle structures, Kinneyia ripples, load casts, domal structures, sand chips, palimpsest and patchy ripples with limited desiccation cracks. Stromatolitic morphotypes recorded are solitary, branching, wavy and domal forms of stromatolites associated with ooids, peloids and fenestral laminae. MISS structures flourished within tidal flats to shallow intertidal while stromatolites mushroomed in environments ranging from tidal to deep subtidal. MISS structures were favoured by resistant substratum, low energy conditions, consistent water supply and low terrigenous input. Stromatolites boomed when the volume of carbonate accumulation exceeded siliciclastic deposition. Fluctuating environmental conditions and sediment budget controlled morphology of stromatolites. Owing to limited siliciclastic input during deposition of dolomudstones (characterizes transgressive systems tract), microbial growth was enhanced. Calcareous shales were deposited over dolomudstones which marks the maximum flooding surface (MFS) indicating the culmination of transgression. Deposition of storm-dominated sandstone-siltstone (FA1), wave-rippled sandstones (FA2), tide-dominated sandstones (FA3), heteroliths (FA4), wackestone-packestone (FA6), boundstone (FA7) and ooid-peloid grainstone (FA8) on top of the MFS reflects initiation of highstand systems tract (HST) which is mainly characterized by stromatolitic horizons, alternation of carbonates and siliciclastics with flourishing microbial activity. Eventually, increased sedimentation in upper part of Kunihar Formation marks late stage of regression.     

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur.Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.     

鄂尔多斯盆地延长组长7沉积期物源分析及母岩类型研究

通过对鄂尔多斯盆地延长组长7砂岩的类型、轻重矿物特征、重矿物聚类分析及泥岩的地化特征研究,并结合盆地周边源区露头剖面的岩性组合资料,讨论了长7沉积期的物源方向,恢复各源区母岩性质。经研究表明:鄂尔多斯盆地长7沉积期存在五个方向物源,东北、西南为主要的物源方向,西北、南部作为次要物源方向;其中东北源区母岩以中基性岩浆岩、高级变质岩为主,西南源区母岩以白云岩、浅变质岩为主,西北源区母岩以沉积岩为主,浅变质岩次之,南部源区母岩以含灰岩、碎屑岩为主,浅变质岩次之;另外经讨论证实西部存在局限的物源供给区,母岩主要为白云岩。     

Variations in Neoarchean microbialite morphologies: clues to controls on microbialite morphologies through time

The Neoarchean Carawine Formation, Hamersley Group, Western Australia is a carbonate ramp that preserves diverse microbial structures which are characteristic of specific depositional environments. These distinctive structures are distributed in five shallow subtidal and two deeper‐water facies in the Oakover area of the Carawine Formation. The shallow subtidal facies are composed of biohermal and bedded stromatolites, centimetre‐scale ridge‐shaped microbialites and wavy‐laminated microbialites. The deeper‐water facies are composed of fenestrate microbialites, planar laminated dolostone and dolostone with rolled‐up microbial laminae. Microbialites in the Carawine Formation lie within a continuum of Archean to Proterozoic microbial facies. Some shallow‐water microbial facies in the Carawine Formation are similar to Proterozoic facies, such as large bioherms internally composed of a variety of stromatolite morphologies. In contrast, fenestrate microbialites grew in quiet subtidal environments and are common in Archean rocks but have not been documented in similar Proterozoic environments. The similarity of shallow‐water facies across the Archean–Proterozoic transition, before and after the oxidation of the atmosphere and surface oceans, indicates that stromatolite growth in shallow subtidal environments was not strongly affected by the chemical changes associated with oxidation of the oceans or by biological responses to those chemical changes. Rather, stromatolite morphology was controlled mostly by the physical environment and the corresponding biological responses to that environment. In contrast, the absence of fenestrate microbialites from Proterozoic deep subtidal environments suggests that the morphology of deep subtidal microbial structures was influenced by chemical or biological changes that occurred in association with oxidation of the surface oceans.     

胜利油区早古生代沉积相

早古生代，胜利油区作为华北地台的一部分，接受了一套以海相碳酸盐岩为主的沉积，其沉积相与其他地区既有相似性，又有特殊性。文中首先按沉积岩类型，将沉积相划分为碎屑沉积相系和碳酸盐岩沉积相系。根据岩石的沉积构造、指相矿物、生物化石等，将下古生界沉积相进一步划分为４个相９种亚相，并对各种相的特征进行了简要说明。在单井相分析的基础上，利用单因素和三端元作图法编制了基础图件，结合相序的变化规律和区域地质背景，对区内早古生代不同时期的沉积相展布特征进行了探讨：馒头期—徐庄期，基本为潮上到潮间带；张夏期—崮山期，发育有潮下浅滩和开阔海两种主要沉积相；长山期—早奥陶世，以潮间—潮下带为主；中奥陶世，以浅水的潮上、潮间带与较深水的潮下带间互出现。研究早古生代沉积相，对于评价碳酸盐岩储层和烃源岩有重要意义。     

Precambrian metamorphic conditions and crustal evolution,northeastern Alberta,Canada

A complex of Precambrian polymetamorphic gneisses and granitoids of the Churchill structural province, northeastern Alberta, Canada has been examined structurally, petrographically, chemically and geochronologically. An Archean basement gneiss complex is indicated by Rb-Sr dating of pegmatites which cut both gneisses and granitoids (2470 ± 26 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7030 ± 0.0008). A high-pressure granulite facies (M₁) mineral assemblage and older structures (D₁) are assigned to the Archean. A moderate-pressure granulite facies (M_2.1), a low-pressure amphibolite facies (M_2.2), a greenschist facies (M_2.3), and younger structures (D₂) are of Aphebian age. Formation of granitoids by anatexis of the pre-existing Archean basement complex during M_2.1 is indicated by their Aphebian ages (ca. 1900 Ma) and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7100 ± 0.0018). The path of retrograde metamorphism is linked with relatively slow rates of uplift and cooling. Late Aphebian sediments attained low-grade greenschist facies metamorphism only and are younger than the other metamorphic rocks. The tectonic evolution of this Precambrian mobile belt during the Aphebian contrasts with the stable Archean cratonic block in the Slave province to the north.     

滇中地区昆阳群物源及构造环境

昆阳群的形成时代、沉积环境、源岩性质等一直存在较大争议,为了查明滇中地区昆阳群的物源及其形成的构造环境,文章在分析昆阳群沉积组合和沉积相的基础上,对昆阳群3件变质砂岩样品中的碎屑锆石进行LA-ICPMS锆石U-Pb年龄测定,对昆阳群20件极低级变质碎屑岩进行地球化学分析。从昆阳群黄草岭组、黑山头组和美党组中分别获得了最年轻的谐和年龄为984.0 Ma、945.0 Ma和954.0 Ma;碎屑锆石年龄峰谱显示,在1.0 Ga、1.35Ga、1.73 Ga和2.44 Ga出现了统计峰值,其年龄主要集中在1.73 Ga和1.35 Ga。表明昆阳群源区主要经历了1.0 Ga、1.35 Ga、1.73 Ga和2.44 Ga的构造热事件,资料显示扬子地块西南缘出露的大红山群形成时代为1.7 Ga,格林威尔期的构造热事件时期为1.0~1.3 Ga。此外,地球化学分析结果表明昆阳群源岩主要是形成于大陆岛弧—活动大陆边缘的石英质旋回沉积、长英质岩石和少量镁铁质岩石。在中元古代晚期—新元古代早期(0.95~1.0 Ga),Rodinia超大陆形成阶段,在扬子地块西南缘的弧后前陆盆地中形成昆阳群的沉积组合,物源主要来自扬子地块西南缘的大红山群和格林威尔期岛弧的岩石。     

内蒙古达茂旗北部奥陶纪埃达克岩类的识别及其意义

内蒙古达茂旗北部奥陶纪侵入岩分布于槽台边界断裂(乌兰布拉格—哥舍深大断裂)以北,岩石类型主要有闪长岩、石英闪长岩、英云闪长岩和花岗闪长岩。该套侵入岩具高w(Sr)、低w(Y)和w(Yb)的特征,具高的w(Sr)/w(Y)和w(La)/w(Yb)比值;轻稀土富集,重稀土亏损,具正铕异常或微弱的负铕异常;微量元素蛛网图中具明显的Sr峰和Nb、Ti低谷,与典型的埃达克岩(adakite)特征一致。通过与典型adakite、太古代TTD(G)及岛弧和大陆边缘弧ADR的对比,表明本区奥陶纪侵入岩以具轻、重稀土强烈分馏型而与岛弧和大陆边缘弧ADR区别明显,而与adakite和TTD(G)相似,同时又以具明显的Sr正异常而区别于太古代的TTD(G)。地球物理、重力及航磁资料显示槽台边界断裂两侧有完全不同的地壳结构。北侧可能是在晚古生代早期拼贴到华北陆块的一个小微地块,拼贴过程中二者之间的加里东洋壳向微地块之下俯冲消减,俯冲板片在榴辉岩相或角闪岩相—榴辉岩相过渡相发生部分熔融形成adakite岩浆,熔融残留物主要为石榴石。     

贺兰山——阿拉善地区下、中前寒武系的划分对比及其变质、成矿作用特征

据近期成果,贺兰山—阿拉善地区出露的巨厚变质杂岩可划分为中太古界贺兰山群和叠布斯格群(其全岩Rb—Sr等时年龄为3108.3和3218.8Ma),上太古界阿拉善群和下元古界的赵池沟群、阿拉坦敖包群;它们具不同的变质矿物共生组合,太古界变质岩属低压高温变质的麻粒岩相;下元古界为低—低中压区域动力(热流)变质的绿片岩相岩石。太古界有较强的混合岩化、花岗岩化作用,并蕴藏有铁、石墨、矽线石、刚玉等多种矿产。     

北京地区太古宙岩石同位素年龄及其意义

北京地区太古亩大漕．沙厂表壳岩系的麻粒岩相区域变质作用年龄,苇子峪TTG(A)－MMe杂岩中表壳岩系的高角门岩相区域变质作用年龄,以及两个单元中的早期TTG质岩成岩年龄均为2650Ma左右,推测表壳岩成岩时代为中太古代晚期;四合堂表壳岩系的低角闪岩相区域变质作用年龄和该单元内太古宙TTG质岩的成岩时代约为2539Ma至2580．7Ma,推测表壳岩系成岩时代为新太古代早期（2650～2800Ma）;阳坡地TTG-M-Me杂岩中的太古宙晚期ITG质岩成岩时代为2522~2563Ma,推测其中表壳岩系的角闪岩相区域变质作用及早期TTG质岩的成岩时代稍大于2563Ma;马圈子TTG-M-Me杂岩中TTG质岩成岩时代为新太古代晚期（2500～2650Ma）。结论为本区太古宙表壳岩系峰期区域变质作用和太古宙TTG质岩形成时代均发生于新太古代。     

Mass occurrence of benthic coccoid cyanobacteria and their role in the production of Neoarchean carbonates of South Africa

The sparse Archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post-mortem silicification. Hitherto as an exception, sedimentary carbonate rocks from the Neoarchean Nauga Formation of South Africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria (Chroococcales and Pleurocapsales). Their remains, visible under the scanning electron microscope (SEM) after etching of polished rock samples, comprise capsular envelopes, mucilage sheaths, and groups of cells mineralized by calcium carbonate with an admixture of Al–K–Mg–Fe silicates. The capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the Neoarchean microbiota described and their modern analogues. The new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the Neoarchean, and offer a solution to the problem of the origin of Archean carbonate platforms, stromatolites and microbial reefs.