Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (Pliensbachian–Toarcian)

In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (δ¹³C_bel, δ¹³C_carb) and oxygen (δ¹⁸O_bel, δ¹⁸O_carb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra‐rostral carbon and oxygen‐isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra‐rostral carbon‐isotopes are influenced by vital effects, whereas oxygen‐isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from δ¹⁸O_bel‐isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least ?3‰ (Vienna‐Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between δ¹³C_carb and δ¹³C_bel exists, indicating well‐ventilated bottom‐waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south gradient in salinity stratification and surface‐water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen‐isotope records in epicontinental seaways.     

特提斯喜马拉雅带江孜地区古近纪地层源区分析——对造山带早期地壳加厚的制约

特提斯喜马拉雅北亚带江孜地区上古新统-下始新统甲查拉组记录了喜马拉雅碰撞造山带的早期地壳加厚和沉积历史。本文我们报道了甲查拉组详细的碎屑锆石U-Pb年龄和全岩Sm-Nd同位素数据。甲查拉组由青灰色厚层的岩屑砂岩夹泥岩组成,不整合覆盖在宗卓组之上,碎屑锆石主要的峰值介于350～80 Ma, 900～470 Ma以及1 300～950 Ma,次要的峰值介于2 800～1 500 Ma。全岩⁸⁷Sr/⁸⁶Sr介于0.707 505～0.713 174,¹⁴³Nd/¹⁴⁴Nd介于0.512 206～0.512 355,ε_Nd(0)介于-5.52～-8.43。甲查拉组物源区以再循环的日喀则弧前盆地和上三叠统郎杰学群为主,少量物质来自雅鲁藏布江缝合带。上述研究表明,甲查拉组沉积在周缘前陆盆地的背景下,且特提斯喜马拉雅北亚带在始新世期间经历了明显的地壳加厚。     

特提斯洋对流上地幔Os同位素组成估算的新方法及初步运用

对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。     

Great Victoria Desert: Development and sand provenance

Sands of the Great Victoria Desert, south‐central Australia, can be divided into three main groups on the basis of their physical and chemical characteristics (colour, grainsize parameters, mineralogy of heavy‐mineral suites, quartz oxygen isotopic composition, zircon U–Pb ages). The groups occupy the western, central and eastern Great Victoria Desert respectively, boundaries between them corresponding approximately to changes in the underlying rocks associated with the Yilgarn Craton to Officer Basin to Arckaringa Basin. Several lines of evidence suggest derivation of the sands mainly from local bedrock with very little subsequent aeolian transport. Ultimate protosources for the sands, each in order of importance, are: western Great Victoria Desert—Yilgarn Craton, Albany‐Fraser Orogen, Musgrave Complex; central Great Victoria Desert—Musgrave Complex; eastern Great Victoria Desert—Gawler and Curnamona Blocks, Adelaide Geosyncline, Musgrave Complex. Sediment from the Adelaide Geosyncline includes in addition an ‘exotic’ component from Palaeozoic sedimentary rocks probably derived mainly from Antarctica. Sediment transport of several hundred kilometres from these protosources to the sedimentary basins was dominantly by fluvial, not aeolian, means. Post‐Tertiary aeolian transport or reworking has been minimal, serving only to shape sand eroded from underlying sedimentary rocks or residual products of local basement weathering into the current dunes.     

Obituary

Geological sections are still best drawn at natural scale. Sections with vertical exaggerations are rarely drawn correctly, and even when computed carefully give a false notion of the structural features. Many schematic sections which are not drawn to scale and are supposed to explain a proposed new tectonic interpretation give such a distorted and frequently absurd picture of the structural features that they cannot be regarded as valid.     

Granite ages within the Archaean Pilbara Block,Western Australia

Within the Pilbara Block of Western Australia, a complex of migmatite, gneissic and foliated granite near Marble Bar is intruded by a stock of younger massive granite (the Moolyella Granite) with which swarms of tin‐bearing pegmatites are associated. The age of the older granite has been determined by the Rb‐Sr method as 3,125 ± 366 m.y., and that of the Moolyella Granite as 2,670 ± 95 m.y. Initial Sr⁸⁷/Sr⁸⁶ ratios suggest that the older granite is close to primary crustal material, but that the Moolyella Granite consists of reworked material. It probably formed by partial remelting of the older granite.     

Rb‐Sr geochronology and Sr isotopic composition of Devonian granitoids,eastern Tasmania

Biotite igneous ages and well‐defined isochron ages of plutons from the composite Blue Tier Batholith and the Coles Bay area in northeastern Tasmania range from 395 to 370 Ma. The older limit of this range, for the George River granodiorite, is considerably older than any age previously recorded for NE Tasmania. The ages of the youngest plutons (Mt Paris and Anchor granites), which host cassiterite ores, record pervasive hydrothermal alteration events. The initial ⁸⁷Sr/⁸⁰Sr ratios of the granitoids range from 0.7061 to 0.7136 and suggest different protolith compositions, consistent with mineralogical and geochemical characteristics of each pluton. The S‐type garnetbiotite granites (Ansons Bay and Booby alia granites) have initial ratios greater than 0.7119, indicative of enriched, high Rb/Sr ratio, crustal source‐rocks of Proterozoic age (1700–800 Ma). The S‐type biotite granites (Poimena and Pearson granites) have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7070, 0.7105) but overlap with those of the I‐type granodiorites (George River, Scamander Tier, Pyengana and Coles Bay granodiorites) which are in the range of 0.7061 to 0.7073. The initial ratios of the enriched altered plutons are poorly constrained, and on both hand‐specimen and thin‐section scales, reveal open‐system Sr isotopic patterns.

Isochron ages for the arenite‐lutite and lutite sedimentary associations of the Mathinna Beds, which are intruded by the granitoids, reflect an approach to Sr isotopic equilibrium during regional metamorphism. The metamorphic age (401 ± 7 Ma) of the early Pragian arenite‐lutite association indicates a relatively small time interval between deposition, regional metamorphism and granitoid intrusion. The isotopic age for the lutite sedimentary association (423 ± 22 Ma) is tentatively correlated with a Benambran‐age burial metamorphic event that has not previously been recorded in Tasmania.     

Palaeozoic Victoria,Australia: Igneous rocks,ages and their interpretation

A reconnaissance traverse across Victoria yields 160 K‐Ar dates on igneous rocks from 94 localities. These are supplemented by Rb‐Sr dating in critical cases, and major‐element analyses (some new) on a proportion of the samples. All dates quoted in text and tables, new and previously‐published, have been revised in terms of the latest decay‐constant conventions.

The dates range from Early Ordovician (480 Ma) in the west to Late Devonian (360 Ma) among the high‐level intrusives of Central Victoria. The relatively complex age pattern, and the petrochemical character of the rocks, are compared with the published chronology of neighbouring States, and are interpreted in terms of a long‐duration regime of westwards compression, which began in the deformations of the Adelaide System of South Australia, and continued until the Mid‐Devonian Tabberabberan Movement.     

Isotope systematics of secondary minerals from the Prospect Intrusion,New South Wales

The differentiated Mesozoic alkali dolerite Prospect Intrusion contains a wide range of secondary minerals, including carbonates (primarily calcite), laumontite, prehnite and heulandite, whose stability relationships imply a formation temperature of <200°C. The δ¹⁸O data for carbonates define a higher temperature (160 – 195°C) suite, and a lower temperature (51 – 73°C) suite. The δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotope systematics for these carbonates suggest derivation of the higher temperature group from magmatic fluids, whereas the other group had a major meteoric component that probably originated from porewater in the country rock. Source fluids for prehnite were meteoric rather than magmatic in origin based on their δD and δ¹⁸O ratios. Early in the intrusion's emplacement, CO₂-rich hydrothermal fluids formed a carbonate rind sealing the upper part of the hydrothermal system and produced the higher temperature carbonates (calcite) and laumontite. Later, cooler fluids with a meteoric component infiltrated vesicles and fractures, depositing the lower temperature carbonates (calcite, aragonite), heulandite and prehnite.     

Diatoms and ostracods as mid-Holocene palaeoenvironmental indicators,North Stromatolite Lake,Coorong National Park,South Australia ∗

The Holocene carbonate sequence of perennial North Stromatolite Lake, located adjacent to the Coorong Lagoon near Salt Creek, South Australia, includes a prominent sapropelic unit (7 – 12% total organic carbon), in places more than 2 m thick, that was sampled for the purpose of radiocarbon dating and documenting its diatom and ostracod biostratigraphy. The recovered ostracods were also subjected to carbon and oxygen isotopic analysis. The bulk organic matter at the base of the sapropel yielded an uncalibrated ¹⁴C age of 6080 ± 60 y BP. Diatoms, where preserved, are almost exclusively benthic. Stratigraphic variation of the proportions of key indicator species in diatom assemblages records a marked oscillation between oligosaline and eusaline conditions in the hypolimnion during deposition of the sapropel. Ostracod carbon isotope data indicate that the lake at this time was eutrophic, thereby enriching the dissolved inorganic carbon of the hypolimnion in ¹³C. However, the observed secular variation in δ¹³C implies a mid-sapropel drop in productivity, caused by a freshening of the lake. Ostracod δ¹⁸O values display an overall increase through the sapropel consistent with the rising salinity of the hypolimnion. The existence of a flourishing benthic ostracod community, together with the valve ornamentation of Osticythere baragwanathi, indicates that the bottom waters were well oxygenated. Thus, anoxia was not a prerequisite for sapropel accumulation. The biostratigraphy and chemostratigraphy of the sapropel concur in suggesting a lack of climatic uniformity during its deposition, a period of ～1200 years. This study therefore highlights the potential of diatoms and ostracods in shallow perennial alkaline lakes along the Coorong coastal plain as proxies for short-term (10² – 10³ years) Holocene palaeoenvironmental change in southeastern Australia.