弧后盆地玄武岩(BABB)数据挖掘:与MORB及IAB的对比

弧后盆地玄武岩(BABB)全球分布有限,与板块俯冲有关,位于岛弧外侧,规模小,寿命短,现代BABB主要分布于西太平洋。通常认为,BABB地球化学成分变化较大,包括正常洋中脊玄武岩(N-MORB)、富集洋中脊玄武岩(E-MORB)、岛弧玄武岩(IAB)及少量洋岛玄武岩(OIB)组分。在玄武岩构造判别图中,BABB大多在洋中脊玄武岩(MORB)范围内,说明BABB类似MORB。新的全球MORB数据研究表明,MORB包含了从MORB到OIB及IAB的组分,而BABB相对于MORB更富集Cs,Rb,U,Ba,Th和Pb等不相容元素,有明显的Nb-Ta负异常,表明BABB兼具MORB和IAB的地球化学特征,是俯冲流体及沉积物参与其岩浆作用过程所致,大多是在湿的条件下部分熔融形成的。弧后盆地可分为初始弧后盆地和成熟弧后盆地,前者玄武岩具有明显的岛弧玄武岩的地球化学特征,而后者玄武岩更接近MORB的特征。     

Diatom response to recent climatic change in a high arctic lake (Char Lake, Cornwallis Island, Nunavut)

From 1968 to 1972, Char Lake (Resolute Bay, Cornwallis Island, Canadian High Arctic) was the site of a large-scale limnological study conducted under the auspices of the International Biological Programme. However, since that time, very little research has been done on the lake. We sampled Char Lake from 1992 to 2000 for a suite of physical, chemical, and biological variables. In general, there were no major differences between our water quality data and those collected 30 years earlier, showing that Char Lake is still oligotrophic, slightly alkaline, and dilute. However, a diatom-based paleolimnological analysis revealed that a subtle, yet distinct species assemblage shift has occurred beginning around 1987. The timing of this species shift does not correspond to the deposition of atmospherically transported persistent organic pollutants into the lake (beginning in the early 1950s) or to minor disturbances within its catchment (early 1970s). Instead, these subtle diatom changes are consistent with recent climatic changes during 1988–1997 (as documented by local meteorological measurements), and are likely related to reduced summer ice cover and a longer growing season.     

芝罘岛黄土状黄土的沉积特征与物源分析

芝罘岛黄土状黄土记录了丰富的古地理信息,利用粒度分析、显微镜碎屑矿物鉴定、黏土矿物X射线衍射峰能谱分析,对芝罘岛2个剖面13个黄土样品进行分析,获得芝罘岛黄土状黄土的典型特征:芝罘岛黄土状黄土以粉砂为主,其他组分砾、砂、粉砂俱全,且在粒度组成上占有一定的比例;在碎屑矿物组成上,芝罘岛黄土状黄土具有含量较大的当地基岩才有的典型矿物;两个黄土剖面特征存在一定的差异性;这些都表明芝罘岛黄土状黄土在物质来源上的多源性和近源性。黏土矿物在垂向上的变化,反映了芝罘岛黄土堆积时的冷暖旋回,为多源的物质堆积提供了条件。     

A comparison of classification systems for the conservation of sparsely wooded plains on Melville Island,Northern Australia

A floristic quadrat study of sparsely wooded plains on western Melville Island was conducted and used to define and describe floristic and structural vegetation types and identify their associated environmental correlates. The results were compared with an existing land‐unit survey of the area in order to identify the relative merits of these schemes for conservation planning. All schemes identify a range of broad structural types including Acacia shrubland, Grevillea pteridifolia low woodland, Lophostemon lactifluus low woodland, and Banksia dentata low woodland. However, some of the structural and land‐unit types are composed of two plant communities with distinctive environmental relations. Despite being structurally distinctive, some community types are floristically indistinguishable from surrounding forest dominated by Eucalyptus miniata and E. tetrodonta. The number of species captured by randomly selecting sites from each classificatory type revealed little difference between schemes. All the classificatory approaches were less effective at species capture than a simple iterative procedure. Some of the plant communities seem to be unique and require preservation initiatives in the face of current forestry operations and planned horticultural development.     

海南岛白垩纪古地磁结果及其构造地质意义

对海南岛地区白垩系鹿母湾组和报万组碎屑岩219个独立定向岩芯样品(29个采点)的岩石磁学和古地磁学研究表明,白垩纪的碎屑岩以赤铁矿为主要载磁矿物。逐步热退磁分析表明,绝大多数样品可分离出特征剩磁分量。综合前人的结果,获早白垩世特征剩磁方向D=6.5°,I=42.7°,κ=73.4,α95=8.2°;晚白垩世特征剩磁方向D=6.7°,I=44.7°,κ=125.5,α95=5.4°。其早白垩世古纬度24.8°(+6.2°/-5.8°),晚白垩世古纬度26.3°(+4.6°/-4.0°);均位于现在地理位置以北约5°～6°。与华南板块东南缘白垩纪的古地磁数据对比表明,晚白垩世海南地块仍是华南板块的一部分。海南岛白沙断裂东西两侧早白垩世古地磁数据的差异,表明存在一个北东向的构造走滑带,白沙断裂可能是华南沿海北东向构造带的南延部分。海南岛白垩纪古地磁结果也表明,相对印支地块,海南岛在早白垩世时发生了25°左右局部顺时针旋转。推测此局部旋转很可能与晚侏罗世—晚白垩世早期,印度洋开始第一次海底扩张,印度板块向北运动有关。     

Groundwater in a coral island

 Freshwater aquifers in small coral islands usually occur as thin lenses which are significantly influenced by stresses such as groundwater pumping, sea tides, etc. Kavaratti, a coral island, is one of the 36 such islands in the Arabian Sea off the western coast of India. Detailed hydrogeological investigations were carried out to determine the quantity of freshwater that could be pumped in addition to the present usage. Salinity of groundwater must remain within permissible limits and a 2-D solute mass transport model with a vertical section of the island was constructed by the computer code SUTRA. The model was calibrated by obtaining a match of computed and observed values of the water table, tidal efficiency, and salinity of groundwater at the water table. The model analysis showed that the salinity of groundwater continues to increase unless groundwater pumping is kept below a certain rate. Groundwater potential can be augmented by reducing the subsurface outflow to the sea and by raising the water table by a subsurface dam. Received: 8 September 1997 · Accepted: 27 April 1998     

Structural geology of the Mesozoic Miers Bluff Formation and crosscutting Paleogene dikes (Livingston Island, South Shetland Islands, Antarctica) – Insights into the geodynamic history of the northern Antarctic Peninsula

The Antarctic Peninsula has been part of a magmatic arc since at least Jurassic times. The South Shetland Islands archipelago forms part of this arc, but it was separated from the Peninsula following the Pliocene opening of the Bransfield Strait. Dikes are widespread throughout the archipelago and are particularly accessible on the Hurd Peninsula of Livingston Island. The host rocks for the dikes are represented by the Miers Bluff Formation, which forms the overturned limb of a large-scale fold oriented 63/23 NW. The orientation of minor structures indicates a fold axis oriented NNE–SSW (24/0). Structural analysis of the dikes and their host rocks shows that the tectonic regime was similar to other parts of the archipelago and that only minor changes of the stress field occurred during dike emplacement.Based on crosscutting field relationships and geochemical data, six early Paleocene to late Eocene intrusive events can be distinguished on Hurd Peninsula. In contrast to calc-alkaline dikes from other parts of the South Shetland Islands, the majority of the Hurd Peninsula dikes are of tholeiitic affinity. Nd and Pb isotope data indicate a significant crustal component, particularly during initial magmatic activity.Plagioclase ⁴⁰Ar/³⁹Ar and whole rock K–Ar ages show that dike emplacement peaked during the Lutetian (48.3 ± 1.5, 47.4 ± 2.1, 44.5 ± 1.8 and 43.3 ± 1.7 Ma) on Hurd Peninsula and also further northeast on King George Island. Dike intrusion continued on Livingston Island at least until the Priabonian (37.2 ± 0.9 Ma). The type of magma sources (mantle, slab, crust and sediment) did not change, though their relative magmatic contributions varied with time.During Cretaceous and Early Paleogene times, the Antarctic Peninsula including the South Shetland Islands was situated southwest of Patagonia; final separation from South America occurred not before the Eocene. Thus, the geological evolution of Livingston Island is related as much to the development of Patagonia as of Antarctica, and needs to be considered within the history of southernmost South America.     

青岛灵山岛晚中生代重力流沉积特征及环境分析

青岛灵山岛晚中生代发育一套重力流沉积,国内学者对其成因机制存在争议。笔者通过对造船厂、灯塔、千层崖、羊礁洞、老虎嘴、背来石等剖面的重点观察发现:灵山岛晚中生代地层出露以厚度不等的细粒砂泥岩与火山熔岩、火山碎屑岩为主,夹沉火山碎屑岩和侵入体,地层从老到新组合为一个水体变浅的水退序列。地层中发育不同类型、不同尺度、不同期次的软沉积变形构造,如软沉积变形褶皱、布丁构造、负载构造和火焰构造、软双重构造等。灵山岛晚中生代重力流沉积碎屑岩主要是块体搬运沉积(砂质滑塌和砂质碎屑流)和浊流沉积,向上过渡为陆相三角洲前缘沉积,其成因机制是物源供应充足的湖盆三角洲前缘在构造坡度或沉积坡度控制下,受地震和火山活动影响发生的块体搬运沉积和浊流沉积。初步研究认为沉积时的古斜坡方向为南东—北西方向。     

Timing of the Northern Prince Gustav Ice Stream retreat and the deglaciation of northern James Ross Island,Antarctic Peninsula during the last glacial–interglacial transition

The Northern Prince Gustav Ice Stream located in Prince Gustav Channel, drained the northeastern portion of the Antarctic Peninsula Ice Sheet during the last glacial maximum. Here we present a chronology of its retreat based on in situ produced cosmogenic ¹⁰Be from erratic boulders at Cape Lachman, northern James Ross Island. Schmidt hammer testing was adopted to assess the weathering state of erratic boulders in order to better interpret excess cosmogenic ¹⁰Be from cumulative periods of pre-exposure or earlier release from the glacier. The weighted mean exposure age of five boulders based on Schmidt hammer data is 12.9 ± 1.2 ka representing the beginning of the deglaciation of lower-lying areas (< 60 m a.s.l.) of the northern James Ross Island, when Northern Prince Gustav Ice Stream split from the remaining James Ross Island ice cover. This age represents the minimum age of the transition from grounded ice stream to floating ice shelf in the middle continental shelf areas of the northern Prince Gustav Channel. The remaining ice cover located at higher elevations of northern James Ross Island retreated during the early Holocene due to gradual decay of terrestrial ice and increase of equilibrium line altitude. Schmidt hammer R-values are inversely correlated with ¹⁰Be exposure ages and could be used as a proxy for exposure history of individual granite boulders in this region and favour the hypothesis of earlier release of boulders with excessive ¹⁰Be concentrations from glacier directly at this site. These data provide evidences for an earlier deglaciation of northern James Ross Island when compared with other recently presented cosmogenic nuclide based deglaciation chronologies, but this timing coincides with rapid increase of atmospheric temperature in this marginal part of Antarctica.     

海南岛西部燕山期逆冲推覆构造特征

海南岛燕山期逆冲推覆构造有NE向有军营-红岭构造带、戈枕构造带。两构造带都倾向NW,具有由NW往SE向逆冲推覆的运动学特征。在生成时间上,戈枕构造带略早,而军营-红岭逆冲构造带略晚,属于后从式逆冲推覆构造体系。其生成与印支块体和亚洲板块的相互作用有关。该逆冲推覆构造的厘定,为深入研究海南岛燕山阶段构造环境转化及其控矿作用具有重要的地质意义。