可可西里北缘中新世火山活动带的基本特征

可可西里北缘火山活动带为可可西里中新世火山活动区数条火山活动带之一,其特征十分典型。本文以火山地质、同位素、岩石化学以及稀土元素地球化学为基础对其岩浆来源、演化、火山活动规律进行了研究。该火山带粗安岩-粗面岩-亚碱性流纹岩组合与世界高原溢流玄武岩类似,同时反映大陆伸展的构造环境。     

Miocene tectonics of the Maramures area (Northern Romania): implications for the Mid-Hungarian fault zone

The interplay between the emplacement of crustal blocks (e.g. “ALCAPA”, “Tisza”, “Dacia”) and subduction retreat is a key issue for understanding the Miocene tectonic history of the Carpathians. Coeval thrusting and basin formation is linked by transfer zones, such as the Mid-Hungarian fault zone, which seperates ALCAPA from Tisza-Dacia. The presented study provides new kinematic data from this transfer zone. Early Burdigalian (20.5 to ∼18.5 Ma) SE-directed thrusting of the easternmost tip of ALCAPA (Pienides), over Tisza-Dacia is linked to movements along the Mid-Hungarian fault zone and the Periadriatic line, accommodating the lateral extrusion of ALCAPA. Minor Late Burdigalian (∼18.5 to 16 Ma) NE-SW extension is interpreted as related to back-arc extension. Post Burdigalian (post-16 Ma) NE–SW shortening and NW–SE extension correlate with “soft collision” of Tisza-Dacia with the European foreland coupled with southward migration of active subduction. During this stage the Bogdan-Voda and Dragos-Voda faults were kinematically linked to the Mid-Hungarian fault zone. Sinistral transpression (16 to 12 Ma) at the Bogdan-Voda fault was followed by sinistral transtension (12–10 Ma) along the coupled Bogdan-Dragos-Voda fault system. During the transtensional stage left-lateral offset was reduced eastwards by SW trending normal faults, the fault system finally terminating in an extensional horse-tail splay.     

Statistics of sediment mass in the South China Sea: Method and results

Sediment mass volumes for periods since the Oligocene (E₃, N₁ ¹, N₁ ², N₁ ³, N₂, and Q) in the South China Sea (SCS) were calculated on the basis of geological and geophysical data including ODP borehole data. Above the pre-Oligocene base, the estimated sediment volume is 7.01 × 10⁶ km³ and the mass is 1.44 × 10¹⁶ t for the entire SCS, with an average sedimentation rate of 6.22 cm/ka and accumulation rate of 12.8 g/(cm²·ka) since the Oligocene. Most of the sediment was deposited on the continent shelf and slope, while only 5% of the total mass accumulated in the Central Basin. Sediment basins on shelf and slope occupy 34% of the entire SCS area bounded by the 2 000 m thickness contour, but receive more than 82% of the total sediment mass, indicating that sediment basins played a central role of sedimentary process in the SCS. The highest sedimentation rate and the largest mass volume occurred in the Oligocene SCS, which is quite different from global sedimentation rate patterns. Therefore, we conclude that deposition in marginal seas was primarily controlled by local tectonics. Translated from Advances in Earth Science, 2006, 21(5): 465–473 [译自: 地球科学进展]     

Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on Ar/Ar step heating data

New ⁴⁰Ar/³⁹Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhön and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhön ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously.The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhön area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.     

Distribution and geochronology of Oregon Plateau (U.S.A.) flood basalt volcanism: The Steens Basalt revisited

The timing and petrogenesis of mid-Miocene flood basalt volcanism in the northwest United States has been extensively addressed, yet the chemical characteristics and temporal details of the Steens Basalt, exposed on the Oregon Plateau, are poorly defined. Steens Basalt volcanism has generally been accepted to have occurred at ∼ 16.6 Ma, coeval and/or just prior to the onset of Columbia River Basalt Group volcanism to the north. New major and trace element analyses and nine ⁴⁰Ar/³⁹Ar ages ranging from 15.51 ± 0.28 to 16.58 ± 0.18 Ma were obtained on Oregon Plateau flood basalt lava flows from stratigraphic sections in close proximity to Steens Mountain. Additionally, new ⁴⁰Ar/³⁹Ar ages were obtained on the uppermost and thirty-first lava flow down from the top of the ∼ 1 km section of Steens Basalt exposed at Steens Mountain and yield eruption ages of 16.59 ± 0.10 and 16.55 ±0.10 Ma, respectively. Field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of Steens Mountain.     

柴达木盆地中新世叠层石成因与古环境研究

柴达木盆地西部干柴沟一带发育了大量湖相叠层石，主要存在于新近纪中新世地层中。柴西地区的叠层石形态各异，成因各具特点，而且部分叠层石与现代叠层石完全不同而是更类似于前寒武时期的对应物，根据国内外叠层石研究的最新成果和成因分类标准，柴西地区主要发育了骨架、凝集和细粒叠层石。对应于不同的分类标准其成因也各不相同，细粒叠层石的形成主要是通过微生物自身的钙化作用和生物膜上过饱和碳酸盐等物质以及微生物的引发的沉淀作用；凝集型叠层石的形成主要与原核生物和真核藻类共同发挥粘附作用有关；骨架型叠层石的形成主要是真核藻类等的钙化与原地碳酸盐的沉淀综合作用的结果。柴达木盆地频繁变化的湖相水体条件和周围的构造运动是叠层石形态多样，成因各异的主要外部和环境因素。对各类叠层石的深入细致研究将对沉积学和环境学的研究提供宝贯的现代对应物。     

Early Miocene lamproite from the Colorado Plateau tectonic province, Southeastern Utah, USA

Newly discovered olivine phlogopite lamproite dikes intrude Jurassic siliciclastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 22 Ma both from ⁴⁰Ar/³⁹Ar step-heating of phlogopite and from isochron modeling of laser-fused sanidine. This age is similar to those of mica-rich minettes and melanephelinites of the Wasatch Plateau about 125 km northwest and within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fracture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes are highly LREE-enriched, and display lamproite-specific REE ratios and phlogopite and sanidine compositions. Incompatible element and radiogenic isotope (Nd–Sr–Pb) ratios suggest that lithospheric source material modified by ancient subduction processes, together with younger asthenospheric source components, produced the melt. Timing of the intrusion coincides with the transition from Early–Middle Cenozoic, calc-alkaline plutonism to the dominantly mafic, Basin and Range type volcanism of the Late Cenozoic. While the lamproite occurrence indicates thermal input from the mantle, model non-uniqueness for both magma source depths and geophysical structure prevents quantitative comparison of Early Miocene with present-day lithospheric thickness.     

Geotechnical evaluation of Miocene gypsum from Sivas (Turkey)

Gypsum is widely distributed in Sivas (Turkey), occurring chiefly in the formations of Miocene age. The gypsum is generally of the alabastrine type with a little of the porphiroblast type and contains traces of clay minerals, anhydrite and calcite. It is a moderately strong rock in terms of its unconfined compressive strength which is related to the crystal size. Strength increases as the crystal size decreases. The strength of gypsum was reduced by approximately 55%, when the samples were kept in the water for 20 days. Most samples exhibit plastic–elastic–plastic deformation. The gypsum has a high modulus ratio, and is a moderately deformable rock. High lateral deformations and low vertical deformations are observed under the vertical stress conditions in accordance with the relationship between modulus ratio and strain.     

Pb, Nd, and Sr isotopic constraints on the origin of Miocene basaltic rocks from northeast Hokkaido, Japan: Implications for opening of the Kurile back-arc basin

Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given ⁸⁷Sr/⁸⁶Sr, Hokkaido BABB have slightly lower ¹⁴³Nd/¹⁴⁴Nd and slightly less radiogenic ²⁰⁶Pb/²⁰⁴Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).