K‐Ar dating of Permian and Tertiary igneous activity in the Southeastern Sydney Basin,New South Wales

The southern part of the Sydney Basin of New South Wales is comprised mainly of Permian and Triassic marine to freshwater clastic sedimentary rocks. Within this sequence there are six latite extrusive units, several medium‐sized monzonite intrusions and a large number of small to medium‐sized basic to intermediate intrusions. Thin basaltic flows were extruded onto the Tertiary topographic surface. All of these rocks are relatively undeformed.

Radiometric (K‐Ar) dating has previously been carried out on Mesozoic and Tertiary intrusions and flows of the southwestern portion of the Sydney Basin. However, relatively few Permian, and no post‐Permian, K‐Ar dates have been published for the southeastern portion of the basin. The present investigation provides nine K‐Ar dates from the latter area.

Four extrusive and intrusive units have been confirmed as Permian in age (238 ± 6; 241 ± 4; 245 ± 6; and 251 ± 5 m.y.). Five post‐Permian (on stratigraphic criteria) intrusions yielded Tertiary ages (26.2 ± 3.0; 47.9 ± 2.5; 49.0 ± 4.0; 49.4 ± 2.0; and 58.8 ± 3.5 m.y.). The Permian ages agree with previously published K‐Ar data from the southeastern Sydney Basin, and the Tertiary ages complement and extend the data from the southwestern portion of the basin. However, no Mesozoic K‐Ar dates were obtained from the southeastern Sydney Basin. The Tertiary intrusions may have been emplaced as a result of rifting between Australia and New Zealand, or between Australia and Antarctica, or both.     

A survey of the age relations of Shetland Rocks

The survey is based on field work by Flinn, on forty-two K-Ar age determinations by Miller, and on previously published work on Shetland. Most of the metamorphic rocks give K-Ar ages of about 420 m.y. It is clear from petrological and stratigraphical evidence that this age is not the age of the metamorphisms and migmatizations responsible for the more obvious features of the rocks, and also that the metamorphic rocks in different areas have had different histories of development. Ages up to 515 m.y. have been found in various areas, and these may be more closely related to the main metamorphisms than the more common 420 m.y. ages. In the Mainland the 420 m.y. age may be related to a late porphyroblast metamorphism: in Unst and Fetlar it seems to be the age of Read's second metamorphism which accompanied the emplacement of nappes and the formation of orogenic sediments. About 400 m.y. ago a series of postorogenic granitic and appinitic complexes were emplaced in the southern part of Shetland. By 380 m.y. ago erosion had reached migmatitic rocks and they were being buried again beneath Old Red sediments and contemporaneous volcanics. Later still, possibly 350 m.y. ago, in Upper Devonian times, granites were emplaced in the west of Shetland cutting the Old Red rocks. Finally the Walls Boundary Fault (Great Glen Fault ?) cut one of these late granites.     

Fission track dating,thermal histories and tectonics of igneous intrusions in East Greenland

Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks.Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y.Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y.Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °.     

Potassium-argon ages from the Ceneri Zone,Southern Swiss Alps

The Ceneri Zone is a unit of the crystalline basement of the Southern Alps. Its northern boundary is the Tonale Line segment of the Periadriatic Line, an important tectonic lineament separating the Oligocene and younger features of the Central Alps from the older metamorphic and structural trends of the Southern Alps. Unmetamorphosed Permian and younger sedimentary units lap onto the Southern Alpine basement from the south.Potassium-argon results from the Ceneri Zone define a Hercynian age pattern typical for the basement of continental Europe. This pattern extends to within at least 100 meters of the Tonale Line. Thus, amphibolite facies metamorphism in this region occurred around 325 m.y. ago. The geochronologic similarity of the Southern Alps to many other European regions must be taken into account in megatectonic theories.In detail, the Hercynian age pattern of the Ceneri Zone is complicated. Some hornblendes have apparent ages between the Hercynian and a Caledonian value (430 m.y.). They probably retained some radiogenic argon during the Hercynian upper amphibolite facies metamorphism. In addition, mica results between 200 and 300 m.y. have a strong geographic correlation. Apparently, the northwestern portion of the Ceneri Zone was reheated or mildly metamorphosed during the Upper Triassic to Lower Jurassic. A relationship between these ages and 170–180 m.y. ages from the neighboring Ivrea-Verbano Zone seems likely. No geologic evidence for any post-Hercynian event has been noted as yet in the Ceneri Zone.     

滇西南澜沧县老厂浅水相“老厂组”时代之订正

对澜沧县老厂水库东侧公路剖面开展系统的生物地层学研究,证实西盟至竹塘公路１３～１４ｋｍ 区段属Ｅｏｓｔａｆｆｅｌｌａ 带,１２．５～１３ｋｍ 区段属Ｐｓｅｕｄｏｓｔａｆｆｅｌｌａ 带。所谓的浅水相“老厂组”和冬瓜林火山岩的时代应修正为早石炭世德坞阶至晚石炭世滑石板阶,宜归入平掌组的范畴。确认老厂矿区存在有两套不同时代、不同构造背景的火山岩     

内蒙古查干敖包地区上石炭—下二叠统宝力高庙组特征及时代

提要：笔者在查干敖包地区开展1∶5万区域地质调查工作时,发现本区出露的宝力高庙组层序齐全、植物化石丰富、划分标志明显,通过路线调查、剖面测量对其进行了重新划分,即据岩性组合、生物面貌等特征划分为4个岩性段,一段为河湖相灰色、灰绿色碎屑岩夹少量中性火山岩,在粉砂岩中采到了大量安格拉植物化石;二段为中偏碱性-酸性砖红色、紫色火山岩夹砂（砾）岩,其中粗面岩获得锆石U-Pb同位素年龄为(297.0±1.2) Ma;三段为灰色、灰紫色安山质火山岩夹碎屑;四段以灰白色流纹质火山岩为特征,夹砂岩、泥灰岩等,在砂岩中采到了大量安格拉植物群化石。据植物化石鉴定结果、同位素年龄资料将其形成时代确定为晚石炭—早二叠世。     

豫西宜阳上二叠统孙家沟组孢粉组合及其地质意义*

豫西宜阳上二叠统孙家沟组为一套陆相碎屑岩,在该组中部土门段砂泥互层中发现大量的植物化石碎片和孢粉化石。文中依据孙家沟组沉积特征及孢粉组合区域对比,厘定了土门段的地质时代,并依据孢粉化石的亲缘植物关系和信息函数,结合微量元素Sr/Cu值指标,定性和半定量分析了宜阳地区的古气候特征。结果显示: 土门段共发现52属孢粉化石,以裸子植物花粉占优势,与华北地区晚二叠世孢粉组合特征有很大的相似性,推测其地质时代相当于晚二叠世长兴期。孢粉化石属种的植物亲缘关系与欧美镁灰岩统植物成分类似,表明整体上为较炎热的半干旱古气候。该成果可为华北地区晚二叠世晚期的陆相沉积环境、古植物背景以及古气候演化研究提供参考依据。     

K-Ar and Rb-Sr Dating of blue amphiboles,micas, and associated minerals from the Western Alps

The results of 63 new radiometric K-Ar and Rb-Sr measurements on metamorphic minerals from the internal units of the Western Alps show Hercynian, Permian, as well as three Alpine age groups. The first of the Alpine ages cover the period between 78 and 100 m.y. and refer to high pressure parageneses. The second group comprises K-Ar 39 to 50 m.y. ages; these values are affected by some inherited argon, as indicated by Rb-Sr measurements which point to 35–36±4–5 m.y., i.e. similar to the culmination of the Lepontine crystallization. The final group includes 15 to 30 m.y. ages. It is not yet clear which geologic processes have led to this isotope re-equilibration. Large amounts of inherited argon have been found in Alpine metamorphic minerals of the basement rocks.     

Evidence of multiple intrusion,possible resetting of U-Pb ages,and new crystallization of zircons in the post-tectonic intrusions (‘Rapakivi granites’) and gneisses from South Greenland

U-Pb data for zircons from post-tectonic monzonite, syenite and norite, and country rock gneiss and migmatite from the Kap Farvel-Prins Christian Sund area of South Greenland indicate two distinct intrusive episodes at ～1740 m.y. and ～1755 m.y. The norites have the same age as the older granitic rocks. Similar intrusions further north along the southeast coast have the same age (～1755 m.y.) and geochemical character as those of the Kap Farvel-Prins Christian Sund area. Our U-Pb ages are about 100 m.y. older than previously determined K-Ar ages.The youngest peak of regional Ketilidian metamorphism in this area is placed at about 1800 m.y. based on a linear array of seven magnetic and size fractions from a syntectonic granitic part of a migmatite. This age is distinctly different from the post-tectonic intrusions and, along with other data, precludes the possibility of in situ formation of the intrusions by remelting of the country rocks.In one hypersthene migmatite sample collected near a post-tectonic intrusion, clear overgrowths and separate clear grains with low uranium concentrations were identified. The clear grains of zircon have the same age as the intrusion, indicative of new zircon growth during the granulite facies recrystallization of the gneiss. In contrast, rounded red zircons from an early subconcordant granitic sheet have clear uranium-rich overgrowths which probably formed during regional metamorphism. If the deep-red zircons observed in one of the post-tectonic intrusions were derived from the surrounding metamorphic rocks, they have had their U-Pb systems reset to the time of intrusion.In accord with observations (Krogh, 1971) on non-magnetic zircon fractions from volcanics, the Greenland zircons contain very low amounts of common lead ranging from <0.01 ppm in the least magnetic fractions in the post-tectonic intrusions, to 6.5 ppm in the most magnetic fractions of the gneisses. These two rock groups can be differentiated on the basis of their common lead content.     

黑龙江省东部马家街群碎屑锆石年代学及其大地构造意义

马家街群分布在黑龙江省东部佳木斯地块桦南隆起的西南缘,主要由一套经历了接触变质作用的富铝、富碳沉积碎屑岩所组成。区域上,这套接触变质岩系具有变质矿物分带特征,由西向东依次出现十字石、红柱石、石榴石和黑云母。红柱石碳质板岩和石榴云母石英片岩2件样品获得的LA-ICP-MS U-Pb碎屑锆石年龄谱均显示有272~310Ma、479~533Ma和>800Ma三组年龄。根据两件样品显示的最小年龄均未小于272Ma,而且二者的最小年龄组（272~310Ma）具有类似的峰值年龄,分别为276Ma和279Ma,这限定了马家街群主体岩石沉积年龄的下限应在中二叠世之前。侵入马家街群的花岗岩的锆石年龄为259Ma,说明其接触变质作用时代为晚二叠世早期,限定了马家街群形成时代的上限。479~533Ma年龄组中,2件样品的峰值年龄分别为499Ma和522Ma,这是佳木斯地块麻山群中最为重要的高级变质和花岗质岩浆作用年龄。>800Ma的年龄组具有多个峰值年龄,说明源区（佳木斯地块）具有前寒武纪-早前寒武纪地壳。上述证据表明,马家街群是晚二叠世早期形成的一套接触变质岩系,而非前寒武纪区域变质岩系。鉴于479~533Ma的麻山群在佳木斯地块中普遍存在,说明以麻山群为代表的早古生代变质结晶岩系既是马家街群沉积的基底,也是重要的物源区;而276~279Ma的早二叠世火山岩在佳木斯地块东缘分布广泛,表明其对马家街群的沉积也具有一定的贡献。     

Paleozoic migmatites affected by high-grade tertiary metamorphism in the central Alps (Valle Bodengo,Italy)

The Lepontine Gneiss Complex of southern Switzerland and northern Italy is characterized by high-grade metamorphism and intensive deformation of Alpine age with migmatites prevalent in the area with the highest metamorphic grade. Petrological and structural observations are generally inconclusive but indicate in some places an Alpine age for the migmatite formation. To determine the time of migmatite formation a geochronologic study was undertaken in one of the best exposed areas, the Valle Bodengo, Italy. Rb-Sr whole-rock errorchrons of intrusive migmatite phases and of two rather homogeneous granitoid gneiss bodies yield apparent ages between 280 and 350 m.y. They suggest a Hercynian or older igneous history for these rocks. The U-Pb ages of the euhedral zircons are highly discordant, but they do point to the presence of zircon components more than 450 m.y. old. The concordia-intercept ages are incompatible with the Rb-Sr data and the low initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.706. These low initial ratios suggest that either the bulk of the granitoid material is not much older than Hercynian, or older crustal material was isotopically homogenized on a regional scale with rocks that had low Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios (e.g. the lower crust or upper mantle) during a Hercynian metamorphism. Rb-Sr small-scale whole-rock isochrons and tie lines of adjacent, lithologically different rock phases give Alpine ages, the best isochron yielding 22 m.y. This coincides with concordant U-Pb ages of monazites of 23 to 24 m.y. Rb-Sr mineral isoohrons (muscovite, biotite, feldspars, apatite) give ages of 18–21 m.y. Our interpretation is that this age pattern resulted due to rapid cooling after the climax of the last phase of the Alpine metamorphism and we conclude that high-grade metamorphic conditions existed during the upper Oligocene or early Miocene. Other investigators have suggested that the Alpine metamorphism had a climax 35–40 m.y. ago and that the younger mineral ages are a result of simple continuous cooling due to uplift. Based on this study and other recent geochronological studies in the Lepotine Gneiss Complex we suggest that there had to be a thermal maximum at about 20–25 m.y. The example of Valle Bodengo demonstrates that the areal coincidence of the zone of highest-grade metamorphism with the occurrence of migmatites does not necessarily mean that metamorphism and migmatite formation were coeval and related to each other.     

U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre,Southern Calabria (Italy)

An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses.Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism.Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages.Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The ²⁰⁷Pb/²⁰⁶Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite.Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y.The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism.In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism.On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section.A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.     

Geochronology of West Indonesia and its implication on plate tectonics

The results of radiometric dating of granitic rocks around Kotanopan near the west coast of Central Sumatra indicate an average age of 45 million years.Granites from the Lassi Mass in the Padang Highlands, Central Sumatra, and the Lampong Mass, South Sumatra, possess radiometric ages of ca. 112 and ca. 88 m.y., respectively. Granites and other rocks from the offshore areas north of Java indicate an average age of 100 m.y.Late Cretaceous granitic rocks are present in the islands of the Sunda Shelf namely Anambas (ca. 86 m.y.), Tembelan (ca. 85 m.y.) and Natuna (ca. 75 m.y.).Late Paleozoic granites possessing ages of ca. 276–298 m.y. are encountered in the basement rocks near Djambi, South Sumatra.The outcome of this radiometric age dating proves to be significant for it permits a fresh analysis of the geological evolution of Indonesia based on the plate-tectonics concept.The Tertiary volcano-plutonic arc exposed along the west coast of Sumatra can be traced to the south coast of Java. The corresponding subduction zone can be found in the islands west of Sumatra and the submarine ridge south of Java.The Late Cretaceous plutonic belt of Sumatra does not continue to Java but passes north of it, running however parallel to the subduction zone of Java. These two zones merge in the Meratus Mountains of Southeast Kalimantan.Sumatra was already a volcano-plutonic arc during Permian time, suggesting that since this Period the margins of at least four lithospheric plates have remained near the side of the active Sumatran arc.The presence of Permian volcanic and granitic rocks in the Malay Peninsula and West Kalimantan, and the results of the radiometric age determination of granitic rocks from the islands situated in the Sunda Shelf area, point to the existence of other Permian and Cretaceous volcano-plutonic arcs east and north of the arcs previously described in Sumatra and Java. Thus a double volcano-plutonic arc with opposing Benioff zones must have existed during Permian and Cretaceous time in this area.The Schwaner Mountains of West Kalimantan are considered to be the place where volcano-plutonic arcs of different ages have merged together. The correlative subduction zones have to be sought in the so-called Danau Formation of West Kalimantan and the northern part of the Kuching zone, the Sibu zone of Serawak situated north of the Schwaner Mountains.The evolution and complex geology of the western part of Indonesia can only be understood by the supposition of the existence of megaplates and sub-plates generated from spreading centers situated in the Indian Ocean and presumably in the area of the South China Sea, respectively.     

论卡以头组的时代

中国科学院南京地质古生物研究所的有关人员 (1980 )曾做过大量地层古生物工作 ,证实卡以头砂页岩层或卡以头组的时代为三叠纪最早期 ,是飞仙关组底部地层的相变。“论卡以头组”一文认为该组下部的时代为二叠纪 ,上部则属三叠纪。对该文提供的地层古生物资料进行分析后 ,本文作者认为该论点缺乏证据 ,不能成立 ;将陆相的威宁哲觉剖面代替标准地点宣威羊场的过渡相剖面作为卡以头组的标准剖面 ,也是不合适的     

Response of U‐Pb Zircon and Rb‐Sr total‐rock and mineral systems to low‐grade regional metamorphism in proterozoic igneous rocks,mount Isa,Australia

A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 ⁺²⁷ _‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in ⁸⁷Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).     

Emplacement and metamorphism of Archaean mafic volcanics at Kambalda,Western Australia—geochemical and isotopic constraints

Extensive exploration and mining at Kambalda, Western Australia has revealed a conformable mafic-ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units. Geochemical and Rb-Sr, U-Pb isotopic analyses have been carried out on two 70–140 m sections of drill core from the metabasalts and U-Pb isotopic analyses have been made on sulphide ore samples. Greenschist metamorphism of the metabasalts is dated at 2610 ± 30 m.y. and was approximately isochemical except for addition of H₂O and CO₂ and a differential mobilization of K and Rb within the units. The addition of H₂O and CO₂ is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism. The later 2600 m.y. event is correlated with a high thermal regime also detected at Kalgoorlie 50 km to the north. A Pb-Pb isochron age of 2720 ± 105 m.y. may represent the metamorphic event or the time of emplacement of the metabasalts. New Sm-Nd data (Chauvelet al.; claqué--Longet al.) suggest the mafic volcanics are 3200 m.y. old. If true large scale homogenization of Pb isotopes is required. Alternatively the mafic sequence may be only slightly older than 2800 m.y. and the Sm-Nd data does not have time significance.     

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.     

银额盆地西部蒙额地1井二叠纪叶肢介的发现及其意义

近年来,银额盆地油气勘探不断取得新发现与突破,但对钻井地层划分及主要油气产层时代存在巨大争议,或认为产层为白垩系,或认为产层属二叠系。2017年实施的蒙额地1井钻井及取心工作,在930.5~958.0m井段的灰色、灰绿色粉砂质泥岩、泥岩层和1085.0~1095.5m井段的深灰色泥岩层中发现叶肢介化石。第一层段叶肢介为加氏圆通古斯卡叶肢介(比较种)Cyclotunguzites cf.gazimuri Novojilov,第二层段叶肢介为额济纳旗半圆李氏叶肢介(新种)Hemicycloleaia ejinaqiensis Niu(sp.nov.)和内蒙古点列叶肢介(新种)Polygrapta neimengguensis Niu(sp.nov.),均为晚二叠世代表性叶肢介。在此基础上,通过钻井地层对比,明确了银额盆地主要油气发现井的产层为二叠系。叶肢介具有重要的生物古地理指示意义,通过二叠纪叶肢介的区域分布与对比,结合银额盆地石炭纪—二叠纪地层层序分析,进一步限定了古亚洲洋的闭合时限为前石炭纪。     

The age of orogenesis in the Nambucca Slate Belt: A K‐Ar study of low‐grade regional metamorphic rocks

K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess ⁴⁰Ar.

Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region.     

Age and degree of metamorphism and time of nappe emplacement along the southern margin of the Damara Orogen/Namibia (SW-Africa)

Along the southern margin of the Damara orogen age and degree of metamorphism were determined by means of K/Ar dating and illite crystallinity. The investigations include the following units:

1. The southwestern-most part of the east-west striking branch of the Damara orogen.
2. The nappes of the Naukluft Mountains.
3. The Nama-Group from north of the Naukluft Mountains to the Fish River in the south (including the western part of the Dwyka-Formation).

The metamorphism of the Naukluft nappes as well as the underlying Nama beds corresponds to the higher part of the anchi-zone and lower epi-zone. Between the Naukluft nappes and the folded Nama rocks adjoining the southeastern front of the nappes there is an obvious step from higher down to lower metamorphism. Further to the southeast the metamorphism in the Nama beds decreases continuously down to diagenesis. K/Ar age determinations were carried out on the three units mentioned above and also on the basement underlying the Nama sequence. Muscovites of this basement gave an age of about 1160 m. y. Determinations on white micas of the southern Damara belt, the Naukluft Mountains and the northern Nama basin define two isochrons with ages of 495 and 530 m. y The age of 530 m. y. represents the peak of metamorphism and the age of syncrystalline deformation. The age of 495 m. y. can be interpreted as a cooling age of the higher metamorphic rocks or as a dating of rejuvenation caused by a second post-crystalline deformation in parts of the Naukluft and Damara rocks. This age of 495 m. y. was also found in the mylonite of the main thrust plane of the Naukluft nappes and it represents the time of emplacement of the Naukluft nappes.