Structural repetition of the Hutchison Group metasediments,Eyre Peninsula,South Australia

Progressive deformation of the Palaeoproterozoic Hutchison Group metasediments, eastern Eyre Peninsula, South Australia, accompanied the development of a ～200 km‐long suture, the Kalinjala Shear Zone. High‐strain structures that developed during the Kimban Orogeny preserved in this supracrustal sequence at Sleaford Bay, southern Eyre Peninsula are: (i) KF₁ sheath folds; and (ii) KD₂ tight folds and reverse shears. Basement Archaean Sleaford Complex gneisses are interpreted to have been thrust over their cover rocks. Interleaving of individual Hutchison Group units by imbricate shears and folds are suggested to have caused an estimated strike‐normal shortening of up to 50%. However, this calculation is of less significance than the >15:1 strike‐parallel elongation of the package during KD₁. Identical structural elements reported by earlier workers are also preserved in the stratigraphic type areas of the Hutchison Group, central Eyre Peninsula. Importantly, the stratigraphic Upper and Lower Middleback Jaspilites, in the Middleback Range, are reinterpreted as one unit structurally repeated by a kilometre‐scale KF₁ sheath fold. Imbricate KD₂ shear zones are inferred to have inverted original disconformities throughout the package. In light of these observations, we suggest that the currently constructed stratigraphic succession of the Hutchison Group should be regionally re‐examined, with a special focus in those areas less affected by the imbricate structures.     

镁铁岩脉侵位机制及伴随变形

南澳的ＥＹＲＥ半岛位于ＧＡＷＬＥＲ克拉通南部，包含了ＧＡＷＬＥＲ克拉通太古界至中元古界结晶基底的主要部分，全区于１４２３Ｍａ克拉通化，此后除了局部的，较小的地壳运动外，一直是稳定的克拉通地块，研究区ＪＵＳＳＩＥＵ半岛为ＦＹＲＥ半岛南部的次级半岛，镁铁岩脉群以及韧性剪切糜棱岩带主要沿海岩分布，区内出露岩石变形复杂，脉岩强烈的布丁化并重结晶，围岩中的转换拉伸构造及转换挤压构造可追踪识别，基性岩浆的侵位是转换拉伸力和岩浆压力联合作用的结果，脉岩群的传播侵位(PROPAGATION)与转换拉伸作用(TRANSTENSION)密切相关。多次的转换拉伸与挤压作用，还导致镁铁岩脉边缘成为高应变带，并形成复杂的变形图案 此外，围岩中伴随的变形以次剪切带(SUBSHEAR ZONE)最为显著，是作动力学分析最好的匹配构造。     

A radiometric estimate of the duration of sedimentation in the Adelaide geosyncline,south Australia

The Adelaide System forms the uppermost Precambrian sequence in South Australia and the Wooltana Volcanics lie near its base. Though affected by Palaeozoic metamorphism, the least‐altered samples give a minimum age of 850 ± 50 m.y., so that the base of the System is about 900 m.y. old or more. The unmetamorphbsed Roopena Volcanics of northeastern Eyre Peninsula are 1,345 ± 30 m.y. old and if correlated with the Wooltana Volcanics the base of the system becomes about 1,400 m.y. old. The data for the Wooltana Volcanics are consistent with this, provided that even the least‐altered total‐rock samples were open systems during the later metamorphism. Ages of basement in the Mount Painter and Olary districts (1,600 m.y.) and data for Willouran shales overlying the Wooltana Volcanics can fit both minimum and maximum estimates for the Volcanics.

Lower Cambrian shales give a range of 530–690 m.y.; though some Palaeozoic isotopic movement occurred, the ages are approximately correct. Shales from the top of the Torrensian Series range from 660–840 m.y. (700 m.y. preferred value). If the base of the system is at 1,400 m.y., this is surprisingly young. It suggests either a hiatus between the Wooltana Volcanics and the Torrensian or that the correlation of the former with the Roopena Volcanics is wrong (and that the base is at about 900 m.y.). Alternatively, the shales may be abnormally updated.

The Gawler Range Volcanics of Eyre Peninsula have been dated accurately at 1,535 ± 25 m.y. and illitic shale from the penecontemporaneous Corunna Conglomerate gives nearly the same value. These ages indirectly set a maximum for the age of the base of the system, as stratigraphy suggests that they are older. Granites underlying the Gawler Range Volcanics are about 1,600 m.y. old; some may be 1,800 m.y. old.

Final Palaeozoic metamorphism in the northern Flinders Ranges was at 465 m.y. The ages of several post‐orogenic intrusions are given.     

Tectonic evolution of the southern Gawler Craton,South Australia,from electromagnetic sounding

Long-period natural-source electromagnetic data have been recorded using portable three-component magnetometers at 39 sites in 1998 and 2002 across the southern Eyre Peninsula, South Australia that forms part of the Gawler Craton. Site spacing was of order 5 km, but reduced to 1 km or less near known geological boundaries, with a total survey length of approximately 50 km. A profile trending east – west was inverted for a 2D electrical resistivity model to a depth of 20 km across the southern Eyre Peninsula. The main features from the models are: (i) on the eastern side of the Gawler Craton, the Donington Suite granitoids to the east of the Kalinjala Shear Zone are resistive (>1000 Ωm); (ii) the boundary between the Donington Suite granitoids and the Archaean Sleaford Complex, which has much lower resistivity of 10 – 100 Ωm, is almost vertical in the top 10 km and dips slightly westwards; and (iii) two very low resistivity (<1 Ωm) arcuate zones in the top 3 km of Hutchison Group sediments correlate with banded iron-formations, and are probably related to biogenic-origin graphite deposits concentrated in fold hinges. Such features suggest an extensional regime during the time period 2.00 – 1.85 Ga. We suggest that the resistivity boundary between the Donington Suite and the Archaean Sleaford Complex represents a growth fault, typical for rift systems that evolve into a half-graben structure. In the graben basin, low-resistivity shallow-marine Hutchison Group sediments were deposited. Folding of the sediments during the Kimban Orogeny between 1.74 and 1.70 Ga has led to migration of graphite to the fold hinges resulting in linear zones of very low resistivity that correlate with banded iron-formation magnetic anomalies.     

Three Palaeoproterozoic basins—Yerrida,Bryah and Padbury—Capricorn Orogen,Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Basins record periods of sedimentation and magmatism along the northern margin of the Archaean Yilgarn Craton. Each basin is characterised by distinct stratigraphy, igneous activity, structural and metamorphic history and mineral deposit types. The oldest of these basins, the Yerrida Basin (ca 2200 Ma) is floored by rocks of the Archaean Yilgarn Craton. Important features of this basin are the presence of evaporites and continental flood basalts. The ca 2000 Ma Bryah Basin developed on the northern margin of the Yilgarn Craton during backarc sea‐floor spreading and rifting, the result of which was the emplacement of voluminous mafic and ultramafic volcanic rocks. During the waning stages of the Bryah Basin this mafic to ultramafic volcanism gave way to deposition of clastic and chemical sedimentary rocks. At a later stage, the Padbury Basin developed as a retroarc foreland basin on top of the Bryah Basin in a fold‐and‐thrust belt. This resulted from either the collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny) or the ca 2000 Ma westward collision of the southern part of the Gascoyne Complex and the Yilgarn Craton (Glenburgh Orogeny). During the Capricorn Orogeny the Bryah Group was thrust to the southeast, over the Yerrida Group. Important mineral deposits are contained in the Yerrida, Bryah and Padbury Basins. In the Yerrida Basin a large Pb–carbonate deposit (Magellan) and black shale‐hosted gossans containing anomalous abundances of Ba, Cu, Zn and Pd are present. The Pb–carbonate deposit is hosted by the upper units of the Juderina Formation, and the lower unit of the unconformably overlying Earaheedy Group. The Bryah and Padbury Basins contain orogenic gold, copper‐gold volcanogenic massive sulfides, manganese and iron ore. The origin of the gold mineralisation is probably related to tectonothermal activity during the Capricorn Orogeny at ca 1800 Ma.     

Discussion and Reply Lithostratigraphic revision and correlation of the Oligo‐Miocene Murray Supergroup,western Murray Basin,South Australia

Sequence‐stratigraphic interpretations of the 4200 m‐thick Palaeoproterozoic (1700–1650 Ma) Mt Isa Group and underlying Surprise Creek Formation identify three unconformity‐bounded packages termed the Prize, Gun and Loretta Supersequences. Siliciclastic rocks of the Surprise Creek Formation and Warrina Park Quartzite comprise the Prize Supersequence. Rapid facies changes from proximal, conglomeratic fluvial packages to distal, fine‐grained and deep‐water, rhythmites characterise this supersequence. Conglomeratic intervals in the Mt Isa area reflect syndepositional movement along basin‐margin faults during the period of supersequence initiation. A major unconformity, which extends over a period of about 25 million years, separates the Gun and Prize Supersequences. In the Leichhardt River Fault Trough uplift and incision of Prize sedimentary rocks coincided with emplacement of the Sybella Granite (1671±8 Ma) and Carters Bore Rhyolite (1678±2 Ma) and the removal of an unknown thickness of Prize Supersequence section. Deep‐water, turbiditic rhythmites of the Mt Isa Group dominated the Gun and Loretta Supersequences. Tempestites are present over discrete intervals and represent times of relative shallowing. High accommodation and sedimentation rates at the base of the Gun Supersequence resulted in the deposition of transgressive nearshore facies (uppermost Warrina Park Quartzite) overlain by a thick interval of deep‐water, siltstone‐mudstone rhythmites of the Moondarra Siltstone and Breakaway Shale. With declining rates of siliciclastic sedimentation and shallowing of the succession, calcareous sediments of the Native Bee Siltstone prograded over the deeper water deposits. Two third‐order sequences, Gun 1 and 2, characterise these lower parts of the Gun Supersequence. An increase in accommodation rates near the top of the Native Bee Siltstone in Gun 3 time, resulted in a return to deep‐water sedimentation with deposition of dolomitic rhythmites of the Urquhart Shale and Spear Siltstone. The Pb–Zn–Ag ore‐hosting interval of the Urquhart Shale is interpreted to occur in progradational highstand deposits of the Gun 3 Sequence. In the Leichhardt River Fault Trough the Loretta Supersequence boundary forms a correlative conformity. Coarser grained and thicker bedded sediments of the Kennedy Siltstone comprise lowstand deposits at the base of this cycle. These sediments fine up into the transgressive, deep‐water, siliciclastic facies of the Magazine Shale, which in turn are truncated against the Mt Isa Fault.     

Geochronological constraints on early volcanic evolution of the Pilbara Block,Western Australia

U‐Pb isotopic systems of zircons from the Boobina and Spinaway Porphyries from the Precambrian Pilbara Block of Western Australia indicate ages of 3307± 19 Ma and 2768 ± 16 Ma, respectively. The Boobina Porphyry intrudes upper members of the Archaean greenstones of the Warrawoona Group. The Spinaway Porphyry intrudes basal units of the unconformably overlying volcanics and sediments of the Mt Bruce Supergroup. The age of the Boobina Porphyry, together with previous zircon U‐Pb and whole rock Sm‐Nd age determinations on stratigraphically older units, indicate that early Archaean volcanism in the Pilbara took place between 3560 Ma and 3300 Ma. On the basis of the age determination of the Spinaway Porphyry, and the chronometric definition of 2500 Ma for the Archaean—Proterozoic boundary, by the International Subcommis‐sion on Precambrian Stratigraphy (James H. L. 1978, Precambrian Res. 7, 193–204), the lower units of the Mt Bruce Supergroup should now be assigned to the Archaean.     

The age of the Stuart Dyke Swarm and its bearing on the onset of late Precambrian sedimentation in central Australia

A Rb‐Sr age of 897 ± 9 m.y. is obtained for dolerite from the Stuart Dyke Swarm in the southern part of the Arunta Block, Northern Territory. The dyke swarm presents an older age limit for the unconformably overlying Heavitree Quartzite, basal formation of the Amadeus Basin sequence. This limit is consistent with all isotopic data with the exception of previously determined glauconite ages from the Vaughan Springs Quartzite, a correlative of the Heavitree Quartzite in the Ngalia Basin.     

The Orosirian-Statherian banded iron formation-bearing sequences of the southern border of the Espinhaço Range,Southeast Brazil

The Serra da Serpentina and the Serra de São José groups are two distinct banded iron formation-bearing metasedimentary sequences along the eastern border of the southern Espinhaço Range that were deposited on the boundary between the Orosirian and Statherian periods.The Serra da Serpentina Group (SSG) has an Orosirian maximum depositional age (youngest detrital zircon grain age = 1990 ± 16 Ma) and consists of fine clastic metasediments at the base and chemical sediments, including banded iron formations (BIFs), on the top, corresponding to the Meloso and Serra do Sapo formations, respectively, and correlating with the pre-Espinhaço Costa Sena Group. The SSG represents sedimentary deposition on an epicontinental-epeiric, slow downwarping sag basin with little tectonic activity.The younger Serra de São José Group (SJG) is separated from the older SSG by an erosional unconformity and was deposited in a tectonically active continental rift-basin in the early stages of the opening of the Espinhaço Trough. The Serra do São José sediments stretch along the north-south axis of the rift and comprise a complete cycle of transgressive sedimentary deposits, which were subdivided, from base to top, into the Lapão, Itapanhoacanga, Jacém and Canjica formations. The Itapanhoacanga Formation has a maximum depositional age of 1666 ± 32 Ma (Statherian), which coincides with the maximum depositional age (i.e., 1683 ± 11 Ma) of the São João da Chapada Formation, one of the Espinhaço Supergroup's basal units. The Serra de São José Rift and the Espinhaço Rift likely represent the same system, with basal units that are facies variations of the same sequence.The supracrustal rocks have undergone two stages of deformation during the west-verging Brasiliano orogeny that affected the eastern margin of the São Francisco Craton and generated a regional-scale, foreland N–S trending fold-thrust belt, which partially involved the crystalline basement. Thrust faults have segmented the terrain into a large number of tectonic blocks, where the stratigraphic sequence was nevertheless well preserved.     

西昆仑塔什库尔干陆块赞坎铁矿赋矿地层形成时代及其地质意义

布伦阔勒岩群既是西昆仑塔什库尔干陆块的主体组成部分,也是该区域铁矿床的赋矿地层,在其中已经发现了赞坎、老并、莫喀尔和叶里克等大型磁铁矿床。文章通过对塔什库尔干陆块内赞坎铁矿区布伦阔勒岩群及侵入地层的岩浆岩进行精细的岩相学和SHRIMP年代学研究,确定赋矿地层的形成时代,进而探讨该矿床的形成时代。研究表明布伦阔勒岩群斜长角闪片岩中岩浆结晶锆石的的形成年龄为(1845.0±11.0)Ma,限定了地层的形成时代可能晚于(1845.0±11.0)Ma。再根据次斜长花岗斑岩侵入体的形成年龄(544.5±4.7)Ma,限定地层的形成时代要早于(544.5±4.7)Ma。因此推测赞坎铁矿区布伦阔勒岩群的形成年龄介于1845.0~544.5 Ma,为元古宙。而且在英安岩中的部分锆石核部还发现有3048.0~3054.0 Ma和2032.0 Ma的年龄信息,说明地层中还保留有更古老的基底物质(古元古代甚至中太古代基底岩石),也进一步说明塔县—瓦恰隆起带所出露的布伦阔勒群岩是最古老的地层之一,为塔什库尔干古陆块的存在提供了依据。由于赞坎磁铁矿床的形成主要与沉积成矿作用密切相关,其主要矿体是与布伦阔勒岩群底部含铁岩系同生的,因此铁矿床的形成时代应与布伦阔勒岩群含铁岩系的形成时代一致,为元古宙全球性前寒武纪铁矿成矿事件的产物。     

朝鲜平南盆地古元古界-下古生界沉积岩碎屑锆石年龄谱对比及意义

中朝古陆(华北古陆)平南盆地面积~25000km~2,位于朝鲜半岛中部,发育从中元古界到下古生界地层,但经历了低级变质作用(绿片岩相及以下)。变质基底岩石中有一套角闪岩相-麻粒岩相的变质的古元古界地层。本文根据盆地不同时代沉积岩碎屑锆石/变质锆石U-Pb LA-ICP MS年龄数据讨论沉积源区的变化,并对区域演化进行制约。甑山群/杂岩为盆地基底岩系,变质砂岩样品中碎屑锆石出现ca.2500~2100Ma的年龄峰值。另外,36.5亿年的碎屑锆石是朝鲜迄今发现的最古老碎屑锆石;夕线榴片麻岩样品记录了~1850Ma(1859±9Ma)的变质年龄;推测甑山群沉积于ca.2100~1900Ma,变质于1850 Ma。黄海群局限分布于朝鲜半岛中部,碎屑锆石年龄谱显示~1850 Ma的峰值,可见~1250 Ma的年龄,推测对应物源为古元古代基底岩浆岩和变质岩系;结合其上覆直岘群的沉积时代,推测地层沉积于ca.1250~1000Ma。直岘群是平南盆地分布最广的地层之一,底部长峰组样品显示明显的~1850Ma的峰值,而其上第二个和第三个组则显示明显的ca.1400~1600Ma和ca.1000~1200 Ma年龄峰值,~1850 Ma年龄很少;推测直岘群开始沉积时,物源主体是盆地基底岩系,但之后出现大量中元古代物质;推测其沉积时代为ca.1000~900Ma。黄州群有~1850Ma和~2500Ma的峰值,另外,还有较少的ca.1000~1200Ma及1400~1600 Ma年龄,表明沉积物源主体仍是基底岩系,可能有中新元古代沉积岩(黄州群-直岘群)的再沉积。这些沉积岩碎屑锆石年龄峰值与辽东和山东半岛沉积地层相似,并且中新元古代地层中均有大量1000~1200Ma及1400~1600Ma的物质,推测可能来自华北古陆之外,如圣弗朗西斯科克拉通。     

Geology of southeast Bohol,central Philippines: Accretion and sedimentation in a marginal basin

Recent field mapping has refined our understanding of the stratigraphy and geology of southeastern Bohol, which is composed of a Cretaceous basement complex subdivided into three distinct formations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by the Cansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex. The entire basement complex is overlain unconformably by a ～2000 m thick sequence of Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identified lithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpreted as an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene Carmen Formation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphic complex, suggesting that the Alicia Schist was not formed in response to emplacement of the Southeast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and the presence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearc setting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a suprasubduction zone environment related to a southeast‐facing arc (using present‐day geographical references). The accretion of this ophiolite complex was followed by a period of erosion and then later by extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone and Maribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document intermittent Tertiary volcanism in southeastern Bohol.     

U?Pb and 40Ar?39Ar geochronology of Palaeozoic units in the northern Apennines: determining protolith age and alpine evolution using the Calamita Schist and Ortano Porphyroid

In the northern Apennines, the Palaeozoic basement involved in the Late Oligocene–Middle Miocene nappe stack contains metamorphic units for which hypothetical ages have been assigned on the basis of lithological correlations with the Palaeozoic formations of the Variscan chain in Sardinia. This uncertainty concerning the age poses limitations to reconstructing the Palaeozoic stratigraphy, defining the Alpine and pre‐Alpine histories and correlations with other domains of the Variscan chain. We present the U Pb age of detrital zircon and the ⁴⁰Ar ³⁹Ar age of metamorphic muscovite for the Calamita Schist and Ortano Porphyroid, two metamorphic units of undetermined Palaeozoic age cropping out in the eastern Elba Island. The radioisotopic data allows us to: (i) define the Early Carboniferous and Middle Ordovician ages for the Calamita Schist and Ortano Porphyroid, respectively, as well as their derivation (flysch deposit and magmatic rocks); (ii) pose some constraints concerning their alpine tectonic and metamorphic histories. These new data generate a more precise reconstruction of the Palaeozoic sequence in the northern Apennines, and they document that the Palaeozoic basement involved in the alpine deformation underwent internal stacking with an inversion of the original sequence. Copyright © 2010 John Wiley & Sons, Ltd.     

Ordovician system

Rocks in the Brungle‐Darbalara area of the Tumut Trough form two distinct domains: basement (mainly Bullawyarra Schist), of Cambrian‐Ordovician age, and an Ordovician ‐ Early Silurian sedimentary and volcanic cover sequence. These two domains are separated by a sharp discontinuity that marks an abrupt change in rock type, structure, metamorphic grade and deformation style. Cover sequences have undergone only one major penetrative deformation during the Late Silurian, involving sub‐greenschist facies metamorphism and upright folding. In contrast, the basement also underwent at least two older deformations at greenschist facies and contains distinct high‐strain zones subconcordant with the basement‐cover contact. The high‐strain zones, characterized by a ubiquitous south‐southeast trending mineral lineation, record a discontinuous history of ductile followed by brittle behaviour, consistent with an extensional origin.

The structural and metamorphic discontinuity separating basement from Silurian cover is characterized by widespread cataclasis and alteration and is interpreted as a major detachment fault associated with lithospheric extension and the development of the Tumut Trough in the Early Silurian. During the main period of movement on the detachment, which took place prior to intrusion of the Blacks Flat Diorite into the Bullawyarra Schist, mafic and serpentinized ultramafic rocks either were tectonically emplaced or intruded into the high strain zones. This preceded and accompanied extensional faulting of the cover and deposition of Silurian trough sediments and volcanics which unconformably overlie and onlap older units.

The development of the Tumut Trough, in the Brungle‐Darbalara area, bears many similarities with that of Cordilleran metamorphic core complexes. Such a model is consistent with environments suggested for the trough by previous workers. The south‐southeast extension direction parallels the trough‐bounding faults and implies an overall strike‐slip tectonic setting.     

Potassium‐argon dates from the Arltunga Nappe complex,northern territory

Twenty‐four mineral separates from the Arunta Complex, four from the metamorphosed Heavitree Quartzite (White Range Quartzite), and one whole rock sample of metamorphosed Bitter Springs Formation, all from the western part of the White Range Nappe of the Arltunga Nappe Complex, and two samples from the autochthonous basement west of the nappe have been dated by the K‐Ar method. The samples from the basement rocks form two groups. Those in the southern or frontal part of the nappe are of Middle Proterozoic (Carpentarian) age (1660–1368 m.y.), determined on hornblende, biotite, and muscovite. In the northern or rear part of the nappe, all but one of the muscovite samples and two biotites are of Middle Silurian to Early Carboniferous age (431–345 m.y.); the remainder of the biotite dates range from 1775 to 548 m.y. (including the two samples from the autochthon), and two hornblendes gave dates of 1639 and 2132 m.y. respectively. All the muscovite samples from the Heavitree Quartzite, and the whole rock sample from the Bitter Springs Formation gave Early to Middle Carboniferous dates (358–322 m.y.). The findings support the identification of the White Range Quartzite as the metamorphosed part of the Heavitree Quartzite, which in turn supports the interpretation of the structure of the area as a large, basement‐cored fold nappe. In addition, they date the time of the Alice Springs Orogeny as pre‐Late Carboniferous, which agrees with fossil evidence from elsewhere in the area. The Alice Springs Orogeny was accompanied by widespread greenschist facies meta‐morphism that progressively metamorphosed the Heavitree Quartzite and Bitter Springs Formation, and retrogressively metamorphosed the Arunta Complex. However, the basement rocks in the southern part of the nappe escaped this metamorphism and retain a Middle Proterozoic age, thus dating the time of the Arunta Orogeny in this region as Carpentarian or older.     

Metamorphism of the Permo-Triassic Cape Fold Belt and its basement, South Africa

Summary  The Permo-Triassic Cape Fold Belt around the southern tip of Africa consists of a thick sequence of Palaezoic siliciclastic sedimentary and pre-Cape basement rocks believed to be of Pan-African age. Both the basement rocks and the supracrustal rocks of the Cape Supergroup display only low metamorphic grades. Application of chlorite, chlorite-chloritoid Fe-Mg exchange, and calcite-graphite carbon isotope geothermometry to rocks from the unconformable contact between pre-Cape basement and the Cape Supergroup made it possible to distinguish pre-Cape and syn-Cape metamorphic overprints. During Pan-African metamorphism temperatures of up to middle greenschist facies conditions (around 400 °C) were reached, whereas lowermost greenschist facies conditions (around 300 °C) were not exceeded during the 220–290 Ma Cape orogeny. In the past, most if not all of the pre-Cape basement rocks, which form the Pan-African Saldania Belt, were considered to be of Neoproterozoic age. A hiatus of about 100 °C observed between two adjacent limestone horizons that previously had been grouped together into a single formation at the bottom of the allegedly Neoproterozoic Kango Group indicates that almost all of this group is syn- to post-orogenic with respect to the Pan-African orogeny. A revision of the stratigraphy of the Kango Group is therefore suggested. Only its lowermost member is truly Pan-African and probably related to about 620–740 Ma post-Sturtian cap carbonates in other Pan-African belts of southern Africa. The remainder of the Kango Group reflects the successive development of two stages of orogen-related intra-continental basins: The older stage led to a typical syn-orogenic foreland basin related to tectonic loading in the Gariep and Damara orogenic belts further north(west) between 570 and 540 Ma; the younger is believed to have formed either a further foreland basin or an intra-orogen pull-apart basin caused by later tectonic loading in the Ross orogenic belt and its continuation into the southern Saldania Belt between 510 and 480 Ma. Received May 7, 2000;/revised version accepted January 15, 2001     

Late Paleozoic to Jurassic chronostratigraphy of coastal southern Peru: Temporal evolution of sedimentation along an active margin

We present an integrated geochronological and sedimentological study that significantly revises the basin and magmatic history associated with lithospheric thinning in southern coastal Peru (15–18°S) since the onset of subduction at ∼530 Ma. Until now, estimating the age of the sedimentary and volcanic rocks has heavily relied on paleontologic determinations. Our new geochronological data, combined with numerous field observations, provide the first robust constraints on their chronostratigraphy, which is discussed in the light of biostratigraphical attributions. A detailed review of the existing local units simplifies the current stratigraphic nomenclature and clarifies its absolute chronology using zircon U–Pb ages. We observe that the Late Paleozoic to Jurassic stratigraphy of coastal southern Peru consists of two first-order units, namely (1) the Yamayo Group, a sedimentary succession of variable (0–2 km) thickness, with apparently no nearby volcanic lateral equivalent, and (2) the overlying Yura Group, consisting of a lower, 1–6 km-thick volcanic and volcaniclastic unit, the Chocolate Formation, and an upper, 1–2 km-thick sedimentary succession that are in markedly diachronous contact across the coeval arc and back-arc. We date the local base of the Chocolate Formation, and thus of the Yura Group, to 216 Ma, and show that the underlying Yamayo Group spans a >110 Myr-long time interval, from at least the Late Visean to the Late Triassic, and is apparently devoid of significant internal discontinuities. The age of the top of the Chocolate Formation, i.e. of the volcanic arc pile, varies from ∼194 Ma to less than ∼135 Ma across the study area. We suggest that this simplified and updated stratigraphic framework can be reliably used as a reference for future studies.     

安徽洪镇地区董岭群碎屑锆石U-Pb年代学及其地质意义

安徽洪镇地区的董岭群为一套以二长片麻岩为主的中深变质岩系,长期以来被认为是长江中下游地区古元古代的变质结晶基底,因作为洪镇变质核杂岩的"变质核"而受到广泛关注。为了能够更好地揭示华南前寒武纪的大地构造演化和洪镇"变质核杂岩"的成因,本文对洪镇地区董岭群主体的二长片麻岩进行了碎屑锆石LA-ICP-MS U-Pb定年,获得了760~830 Ma、1930~2060 Ma和2360~2530 Ma 3个主要年龄区间。这一年龄谱与扬子克拉通前寒武纪的碎屑锆石年龄分布特征类似,暗示董岭群的物质源于其附近克拉通物质的剥蚀及搬运;其中最年轻的锆石年龄为768±16 Ma,这一年龄限定了董岭群地层时代下限,结合其上覆盖层为震旦系(底界小于635 Ma),将董岭群的地层时代限定于新元古代,这一结论否定了其为古元古代古老变质结晶基底的说法。综合近年来的研究,扬子板块西缘及东南缘目前并未发现太古宙-古元古代的结晶基底。另外,由于董岭群地层时代的重新厘定,得知这一地区并不存在大规模地层缺失和大规模剥离断层,因此前人提出的洪镇"变质核杂岩"的构造模型值得商榷。     

Ediacaran in Uruguay: palaeoclimatic and palaeobiological implications

The Ediacaran to lowermost Cambrian successions of south‐eastern Uruguay preserve an unusual and significant record of deposits generated during the Gondwana assembly (ca 590 to 535 Ma). This study presents a review of data obtained through extensive field‐based mapping coupled with detailed sedimentology and stratigraphy of key formations. The geological units within the study area consist of the Maldonado Group (Playa Hermosa, Las Ventanas and San Carlos formations), the Arroyo del Soldado Group (Yerbal, Polanco Limestones, Barriga Negra and Cerro Espuelitas formations) and the Arroyo de la Pedrera Group (Piedras de Afilar and Cerro Victoria formations). The Maldonado Group is characterized by a glacially influenced volcanogenic‐sedimentary sequence with ice‐rafted debris and dropstones in the Playa Hermosa and Las Ventanas formations. The Arroyo del Soldado Group is a mixed siliciclastic‐carbonate succession, mainly represented by an intercalation of basal pink dolostones, banded siltstones, rhythmites of dolostone‐limestone, iron formations, cherts and conglomerates. Carbonates in the Polanco Limestones Formation are characterized by a negative δ¹³C excursion up to ?3·26‰ PeeDeeBelemnite. The Arroyo de la Pedrera Group consists of quartz arenites and stromatolitic/oolitic dolostones. Preliminary data indicate that the Precambrian–Cambrian could be contained within or at the base of this group. The entire succession is almost 6000 m thick, and contains a rich fossil assemblage composed of organic‐walled microfossils and small shelly fauna, including the index fossil Cloudina riemkeae. The stratigraphic and chemostratigraphic features are suggestive of a Gaskier age (ca 580 Ma) for the basal glacial‐related units. In this scenario, the results show the importance of lithostratigraphic, biostratigraphic and chemostratigraphic data of these Ediacaran units in the global correlation of terminal Proterozoic sedimentary rocks.     

华北南缘中元古界高山河群碎屑锆石U-Pb年代学及其地质意义*

为了探讨华北板块南缘中元古代沉积地层的时代归属和物质来源、区域古地理格局和大地构造特征,对豫西灵宝福地地区的高山河群进行碎屑锆石U-Pb年代学和锆石微量元素特征研究。获得的高山河群年龄最小(年轻)的单颗碎屑锆石 ²⁰⁷Pb/²⁰⁶Pb 年龄值为1685±39 Ma,从而限制了高山河群最早沉积年龄不早于1700 Ma。结合上覆的龙家园组年代学标定(1594±12 Ma),将高山河群的形成年代限定为1700—1600 Ma,即中元古代长城纪的中晚期,属国际地质年表的“固结纪”。高山河群中碎屑锆石 ²⁰⁷Pb/²⁰⁶Pb 年龄范围为1685—2751 Ma,呈现1850 Ma、2150 Ma、2300 Ma和2500 Ma共4个年龄峰值,对应于华北克拉通古元古代重要的地质事件,并且高山河群以1850 Ma和2500 Ma峰值年龄段的地质体为主要的物源区。根据高山河群与云梦山组碎屑锆石年龄频率对比,推测在豫西地区西侧存在以往报道较少的年龄为2500 Ma的地质体。根据熊耳群火山岩及其对应锆石的地球化学特征和熊耳期盆地动力学性质,并结合高山河群沉积相特征和沉积盆地构造属性,认为熊耳群形成于与“岛弧”共生的拉张性质的弧后盆地地区,而其上覆的高山河群为弧后盆地靠近大陆一侧的具有被动大陆边缘性质的滨浅海沉积。