青藏高原祖尔肯乌拉山地区火山岩Ar-Ar年代学初步研究

对祖尔肯乌拉山地区 4个火山岩样品进行了Ar -Ar年龄测定 ,其坪年龄分别为 (40 91± 1 18)Ma、(41 0 7± 0 80 )Ma、(42 0 0± 1 31)Ma、(39 0 0± 2 0 6 )Ma。它们代表了这些火山岩的形成时代 ,表明本区在古近纪始新世中期发生过大规模的火山活动     

腾冲热海火山地热区近期水热爆炸的阶段性演化特征

水热爆炸是活动性地热田的典型显示。腾冲热海火山地热区历史上曾发生过强烈的水热爆炸活动，但前期一度沉寂。1993年以来水热爆炸活动再度活跃。在过去十年间，研究区内发生过较大的爆炸喷发事件20余次，且规模越来越大。本文根据对爆炸形成泉点的选出气体化学和氦同位素组成的测试结果，研究了区内近期水热爆炸活动的演化特征。逸出气体化学和氦同位素组成特征指示，区内近期引发水热爆炸活动的气体源区有从浅层、中层向深层发展的趋势；作者认为，区内未来可能发生更大规模的爆炸活动，其危险性应引起高度关注。     

Sequence of faulting and deformation during the 2000 eruptions of the Usu Volcano, Hokkaido, Japan—interpretation and image analyses of aerial photographs

Significant faulting and deformation of the ground surface has been rarely known during volcanic eruptions. Usu Volcano, Hokkaido, Japan, is a unique example of deformation due to felsic magma intrusion. Usu Volcano has a history of such types of eruptions as phreatic, pumice eruption (Plinian type), pyroclastic flowing and lava doming since 1663. On March 31, 2000, phreatomagmatic to phreatic eruptions took place after 23 years of dormancy in the western piedmont, followed by explosions on the western flank of Usu Volcano. They were associated with significant deformation including faulting and uplift. The eruptions and deformation were continuing up to the end of May 2000.We identified the faulting using total nine sets of aerial photographs taken from before the eruption (March 31, 2000) to more than 1 year (April 27, 2001) after the end of the activity, and traced deformation processes through image processing using aerial photographs. We found that some of the new faults and the associated phreatic eruptions were related to old faults formed during the 1977–1981 eruptive episode.The image processing has revealed that the surface deformation is coincident with the area of faulting forming small grabens and the phreatic explosion vents. However, the faulting and main explosive eruptions did not take place in the highest uplift area, but along the margin. This suggests that the faulting and explosive activities were affected by small feeder channels diverging from the main magma body which caused the highest uplift.     

(40)~Ar/(39)~Ar chronology and geochemistry of high-K volcanic rocks in the Mangkang basin, Tibet

Our two newly obtained high-quality 40Ar/39Ar ages suggest that the high-K volcanic rocks of the Lawuxiang Formation in the Mangkang basin, Tibet were formed at 33.5±0.2 Ma. The tracing of elemental and Pb-Sr-Nd isotopic geochemistry indicates that they were derived from an EM2 enriched mantle in continental subduction caused by transpression. Their evidently negative anomalies in HFSEs such as Nb and Ta make clear that there is an input of continental material into the mantle source. The high-K rocks at 33.5±0.2 Ma in the Mangkang basin may temporally, spatially and compositionally compare with the early one of two-pulse high-K rocks in eastern Tibet distinguished by Wang J. H. et al., implying that they were formed in the same tectonic setting.     

Caldera formation by magma withdrawal from a reservoir beneath a volcanic edifice

Caldera formation has been explained by magma withdrawal from a crustal reservoir, but little is known about the conditions that lead to the critical reservoir pressure for collapse. During an eruption, the reservoir pressure is constrained to lie within a finite range: it cannot exceed the threshold value for eruption, and cannot decrease below another threshold value such that feeder dykes get shut by the confining pressure, which stops the eruption. For caldera collapse to occur, the critical reservoir pressure for roof failure must therefore be within this operating range. We use an analytical elastic model to evaluate the changes of reservoir pressure that are required for failure of roof rocks above the reservoir with and without a volcanic edifice at Earth's surface. With no edifice at Earth's surface, faulting in the roof region can only occur in the initial phase of reservoir inflation and affects a very small part of the focal area. Such conditions do not allow caldera collapse. With a volcanic edifice, large tensile stresses develop in the roof region, whose magnitude increase as the reservoir deflates during an eruption. The edifice size must exceed a threshold value for failure of the roof region before the end of eruption. The largest tensile stresses are reached at Earth's surface, indicating that faulting starts there. Failure affects an area whose horizontal dimensions depend on edifice and chamber dimensions. For small and deep reservoirs, failure conditions cannot be achieved even if the edifice is very large. Quantitative predictions are consistent with observations on a number of volcanoes.     

Did Paleogene North Atlantic rift-related eruptions drive early Eocene climate cooling?

The delivery of volcanogenic sulphur into the upper atmosphere by explosive eruptions is known to cause significant temporary climate cooling. Therefore, phreatomagmatic and phreatoplinian eruptions occurring during the final rifting stages of active flood basalt provinces provide a potent mechanism for triggering climate change.

During the early Eocene, the northeast Atlantic margin was subjected to repeated ashfall for 0.5 m.y. This was the result of extensive phreatomagmatic activity along 3000 km of the opening northeast Atlantic rift. These widespread, predominantly basaltic ashes are now preserved in marine sediments of the Balder Formation and its equivalents, and occur over an area extending from the Faroe Islands to Denmark and southern England. These ash-bearing sediments also contain pollen and spore floras derived from low diversity forests that grew in cooler, drier climates than were experienced either before or after these highly explosive eruptions. In addition, coeval plant macrofossil evidence from the Bighorn Basin, Wyoming, USA, also shows a comparable pattern of vegetation change. The coincidence of the ashes and cooler climate pollen and spore floras in northwest Europe identifies volcanism as the primary cause of climate cooling. Estimates show that whilst relatively few phreatomagmatic eruptive centres along the 3000 km opening rift system could readily generate 0.5–1 °C cooling, on an annual basis, only persistent or repeated volcanic phases would have been able to achieve the long-term cooling effect observed in the floral record. We propose that the cumulative effect of repeated Balder Formation eruptions initiated a biodiversity crisis in the northeast Atlantic margin forests. Only the decline of this persistent volcanic activity, and the subsequent climatic warming at the start of the Eocene Thermal Maximum allowed the growth of subtropical forests to develop across the region.     

PETROLOGY 1.IGNEOUS PETROLOGY

正20140707Bai Yunshan(Wuhan Center,China Geological Survey,Wuhan 430205,China);Duan Qifa Geological Characteristics and Tectonic Significance of Volcanic Rocks from Kenan Formation of the Upper Triassic in Zhahe Area,Southern Qinghai Province(Geology and Mineral Resources of South China,ISSN1007-3701,CN42-1417/P,29(2),     

X射线荧光光谱-电子探针在中酸性火山岩鉴定中的应用

中酸性火山岩多具斑状结构,基质可见微晶状结构、隐晶状结构、玻璃质结构等,由于基质矿物颗粒多细小,常用的偏光显微镜受放大倍数的限制,很难准确鉴定矿物种属及含量,这类岩石仅依靠偏光显微镜分类命名会存在误差。本文采用X射线荧光光谱(XRF)、电子探针(EMPA)和偏光显微镜下观察相结合的方法,对中酸性火山岩进行鉴定。结果表明:对于基质呈隐晶质、显微晶质的中酸性火山岩,基质特征相似,偏光显微镜下无法确定长石、石英的含量,因此无法对岩石准确命名;再通过XRF进行主量元素分析,并对分析结果进行标准矿物QAPF双三角图解分类、TAS图解分类及李氏火山岩定量分类,对比结果显示三种分类命名方法存在差异;通过电子探针对矿物进行校验显示,QAPF及李氏火山岩定量分类图解与显微镜下鉴定相符,TAS图解与其他分析结果存在一定偏差。因此,对于中酸性火山岩准确命名,应采用多种分析方法相结合的方式,避免测试单一引起的误差。