Crystallization and welding variations in a widespread ignimbrite sheet; the Rattlesnake Tuff,eastern Oregon,USA

The 7.05 Ma Rattlesnake Tuff covers ca. 9000 km², but the reconstructed original coverage was between 30000 and 40000 km². Thicknesses are remarkably uniform, ranging between 15 and 30 m for the most complete sections. Only 13% of the area is covered with tuff thicker than 30 m, to a maximum of 70 m. The present day estimated tuff volume is 130 km³ and the reconstructed magma volume of the outflow is 280 km³ DRE (dense rock equivalent). The source area of the tuff is inferred to be in the western Harney Basin, near the center of the tuff distribution, based mainly on a radial exponential decrease in average pumice size, and is consistent with a general radial decrease in welding and degree of post-emplacement crystallization. Rheomorphic tuff is found to a radius of 40–60 km from the inferred source.Four facies of welding and four of post-emplacement crystallization are distinguishable. They are: non-welded, incipiently welded, partially welded and densely welded zones; and vapor phase, pervasively devitrified, spherulite and lithophysae zones. The vapor phase, pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones. At constant thickness (20±3 m), and over a distance of 1–3 km, nonrheomorphic sections can cary between two extremes: (a) entirely vitric sections grading from nonwelded to incipiently welded; and (b) highly zoned sections. Highly zoned sections have a basal non- to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae-dominated zone to a zone of pervasive devitrification, which, in turn, is overlain by a zone of vapor-phase crystallization and is capped by partially welded vitric tuff. A three-dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections.Strong local variations are interpreted to be the result of threshold-governed welding and crystallization controlled by residence time above a critical temperature, which is achieved through differences in thickness and accumulation rate.     

Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models

We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Origin and development of tafoni in Tunnel Spring Tuff,Crystal Peak,Utah, USA

Bedding‐parallel tafoni are well developed over much of the surface of the Tunnel Spring Tuff (Oligocene) exposed in 300‐m‐high Crystal Peak, an inselberg. The Tunnel Spring Tuff is a crudely stratified, non‐welded rhyolite ash‐flow tuff with > 30 per cent porosity. Clasts of Palaeozoic dolomite, limestone and quartzite make up 10 per cent of the tuff. The tafoni are remarkable because of their size (up to 20 m wide but rarely wider than 4 m), shape of the openings (spherical, arch‐like or crescent‐shaped) and abundance (up to 50 per cent of an outcrop face). They are actively forming today. Calcite is abundant (10 to 40 per cent by weight) in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls. Halite and gypsum generally make up less than 0·01 per cent of the efflorescence. The absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni. Calcite, aragonite, halite and gypsum dust from modern salt pans less than 20 km from Crystal Peak are potential sources of salt in the tuff, but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg. Calcite is the only evaporite mineral present in the tafoni in more than trace amounts, and this mineral is readily available within the tuff itself as a result of rock weathering. We propose that meteoric water containing carbonic acid infiltrates the tuff, dissolves carbonate clasts, and migrates to the steep flanks (>20°) of the peak through abundant megapores and micropores. There it evaporates and precipitates calcite. Crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering. The quasi‐uniform spacing of tafoni suggests that a self‐organization process is active in the water flow. Copyright © 2000 John Wiley & Sons, Ltd.     

川滇地块古新统宁蒗组磁组构特征及构造意义

对川滇地块程海断裂附近的宁蒗地区古新统宁蒗组地层进行了详细的磁组构研究,沿战河-西布河布置了22个采点（钻取287块样品）,综合分析表明研究区内存在四种磁组构类型,分别为初始变形磁组构和铅笔状磁组构以及介于上述两者之间的两种过渡型磁组构. 研究区西侧（采点1—9）K1轴优势方向为近NNE-SSW方向,东侧（采点13—22）K1轴优势方向则为近S-N向,K1轴方向的突然变化可能与研究区内的隐伏断层活动有关.另外,磁组构也可以很好判断断层所夹持块体之间的相对旋转运动.将两组K1轴优势方向经过旋转校正之后,发现研究区内中-晚始新世时古应力方向为近E-W向,该应力方向主要与新生代印欧碰撞有关.此外,E-W向的古应力场明显不同于现今的近S-N向的应力场方向,这可能与印欧碰撞后青藏高原从前期的挤压缩短阶段进入到后期的E-W向伸展阶段有关.     

The role of lower continental crust and lithospheric mantle in the genesis of Plio–Pleistocene volcanic rocks from Sardinia (Italy)

The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high ⁸⁷Sr/⁸⁶Sr (0.7043–0.7051), low ¹⁴³Nd/¹⁴⁴Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (²⁰⁶Pb/²⁰⁴Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (⁸⁷Sr/⁸⁶Sr=0.7031–0.7040; ¹⁴³Nd/¹⁴⁴Nd=0.5127–0.5129; ²⁰⁶Pb/²⁰⁴Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb<9, Ce/Pb=24–28 and Nb/U=46–54 for the RPV). The sources of the UPV could have been stabilised in the Precambrian after low amounts of lower crustal input (about 3%), or later, during the Hercynian Orogeny, after input of Precambrian lower crust in the source region, whereas the sources of the RPV could be related to processes that occurred in the late Palaeozoic–early Mesozoic, possibly via recycling of proto-Tethys oceanic lithosphere by subduction.     

Magnetic fabric and strain in Miocene volcanics in the southern margin of the Bükk Mountains, Hungary

The magnetic anisotropy of two Miocene volcanic horizons south of the Bükk Mountains, northeast Hungary, was investigated (28 geographically distributed sites, over 400 samples). The horizons contain rocks from ignimbrites to porous tuffs. Microscopic and rock magnetic investigations showed that the main carrier of anisotropy is multidomain maghemite with varying composition. The complex magnetic fabric was separated into deformational and flow fabric in the upper horizon. This was carried out by monitoring the distribution of minimum susceptibility directions supposed to be elongated towards intermediates as a response to compressional strain. Jelínek's tensor statistics were used in the computations. Filtering out the effects of volcanic flow and compaction, north-south striking horizontal compressional strain was determined for the upper horizon in the geographic system. Owing to the lack of elongated distribution of minima, the characteristic intermediate susceptibility directions were taken as markers of strain directions in the lower horizon (WNW-ESE and NNE-SSW in the geographic system). The strain directions showed correlation with compressional stress orientations observed by microtectonics, thus a homogeneous stress-strain field was supposed within the area investigated. The tectonic component of the strain can be determined by taking the respective paleomagnetic declination rotations as the rotational components of the deformation.     

Vent area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (Campi Flegrei, Italy): new insights from directional fabric through image analysis

In order to provide new information about the source area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (NYT), a prominent pyroclastic deposit of the Campi Flegrei Volcanic District (southern Italy), statistics on directional fabric, by means of computer-assisted image analysis on 32 rock samples, were compiled. Seventeen samples were collected along vertical direction on two selected exposures and fifteen were taken from outcrops widely distributed all around the Campi Flegrei Volcanic District. Fabric measurements within the investigated successions reveal a vertically homogeneous direction of the mean particle iso-orientation, with considerable variability in the strength of particle iso-orientation even at cm-scale. The existence of particle iso-orientation can be related to continuous sedimentation from a concentrated bedload region beneath suspension currents, producing massive or inversely graded beds by traction carpet sedimentation. The considerable vertical variability in the strength of iso-orientation is the result of very unstable flow regimes, up to the extreme condition of discrete depositional events, with a variable combination of traction carpet and/or direct suspension sedimentation. The vertical homogeneity in the mean orientation values, found in the investigated sections, may derive from the sequential deposition of laminae to thin beds, whose relatively flat upper surfaces were unable to significantly deflect the depositional system of the following currents. According to the observed homogeneous mean particle orientation values along the investigated vertical profiles, samples collected through areal distribution are considered representative of the local paleo-flow directions of the whole deposit. The mean directions of the samples collected areally show two different coherent patterns which point to the existence of two different source areas. The first, which includes all samples from the northern outcrops, appears to converge in a narrow area about 2 km NE of the town of Pozzuoli, largely in coincidence with the inferred area on the basis of the pumice fall distribution. The second, which includes samples from Capo Miseno and Posillipo areas, points to the central part of the Pozzuoli Bay, about 4 km offshore the town of Pozzuoli.     

西秦岭温泉岩体的磁组构特征及其侵位机制意义

结合岩石磁学、磁化率各向异性度与区域构造分析了西秦岭温泉岩体的侵位机制及意义.温泉岩体的样品的平均磁化率k_m值总体很大,岩石磁学表明对于磁化率较低的样品,顺磁性矿物（如黑云母等）对磁化率的贡献较大,而少量铁磁性矿物（如磁铁矿等）可能作为剩磁载体.对于磁化率较高的样品,其主要载磁矿物为磁铁矿;花岗岩样品的校正磁化率各向异性度P_J总体小于1.2,显示了岩体为流动磁组构的特征,磁化率椭球体形态参数T总体大于0,扁率E总体大于1,以压扁椭球体为主;岩体的磁面理同磁线理相比更为发育,样品的磁面理普遍表现出围绕岩体边界分布的特点,且倾角较陡;而在岩体内部磁线理与磁面理分布相对散乱,定向性差,这一特征说明温泉岩体的磁组构主要由侵位时的侧向挤压作用形成的;虽然岩体的磁组构特征总体显示了N-NEE和SW向的挤压作用,但岩体侵位时由商丹缝合带闭合所产生的垂直于缝合带方向的挤压作用已相对较弱.本文认为,温泉岩体侵位时是一种弱挤压环境,或者是一种相对稳定的环境甚至可能是一种相对引张的背景,这与温泉岩体形成于后碰撞环境,秦岭造山带已演化至后碰撞拆沉作用发生的伸展阶段所反映的区域构造背景是一致的.     

龙门山南段山前天全-乐山剖面磁组构研究及其对新生代构造变形的指示意义

四川盆地西南部西侧为龙门山冲断带,南面紧挨川西南褶皱带,其新生代构造变形特征对于认识青藏高原东南缘的变形机制具有一定的指示意义.磁组构是一种灵敏的应变指示计,在变形微弱的沉积岩地区尤为适用.本文在雅安-乐山剖面选取12个采样点进行磁组构分析,结合已有的天全-雅安飞仙关剖面的27个采样点数据,综合讨论川西南地区的构造变形特征.所有采样点的磁组构测试结果显示出3种弱变形的磁组构类型:沉积磁组构、初始变形磁组构和铅笔状磁组构.雅安-乐山剖面采样点的磁线理绝大部分为北东-南西走向,和龙门山南段的整体延伸方向一致,表明四川盆地西南缘新生代构造变形主要受控于龙门山的构造作用.飞仙关剖面的磁组构测试结果显示44%的采样点表现出磁线理和地层走向斜交的特征,由初始变形磁组构演变而来,并且所有异常磁组构仅局限在断层上盘,本文认为这是雅安地区新生代期间局部逆时针旋转引起变形叠加的结果.     

长江中游砂山沉积物磁组构特征及其指示的古风场

本文通过对新发现的江西九江新港砂山剖面沉积物磁化率各向异性测量和磁化率椭球体主轴的统计分析,揭示了该风成砂剖面磁组构参数在不同层位的变化特征,获得了古风向特征及其演变规律.(1)整个时期内该区的主导风向为NW-SE和NNW-SSE向,但在不同时期风向和风力强度又有所不同,变化最为剧烈的时期是砂2层,风向发生了根本性的改变,由原来的NW-SE为主,变为NE-SW向为主,且该阶段沉积环境相对比较稳定;(2)砂2、砂5、砂6层具有较大P、F、L和较小的q值,说明在其形成时气候最为寒冷,冬季风的风力最强、风速比较稳定,这与野外观察到的在这几层中大型板状斜层理发育,砂层粒度较粗,黏土含量较少的结果一致;(3)晚更新世末期长江中游的风成沙丘广泛发育,表明该时段是长江中下游地区气候最干冷、风力作用最强的时期,有着与北方沙漠—黄土区相类似的气候环境;(4)研究表明,AMS主轴的等面积赤平投影法和玫瑰花图,可以用来分析古风向的变化规律,是一种简易有效的方法.     

柴北缘骆驼泉剖面新生代地层磁组构特征及其构造意义

青藏高原东北部新生代构造演化对理解高原隆升和变形模式具有重要意义, 而目前对于该地区挤压应力方向转变过程仍存在很大争议.本文对柴北缘逆冲带北西部骆驼泉剖面新生代地层开展系统磁组构(本文特指磁化率各项异性)研究, 以揭示该地区挤压应力方向的转变特征.系统岩石磁学结果表明, 骆驼泉剖面新生代样品中主要磁性矿物是顺磁性组分和赤铁矿以及少量磁铁矿.通过对磁组构特征分析及其与古水流方向对比表明, 骆驼泉剖面新生代地层磁组构主要为初始变形磁组构, 可用于指示沉积成岩时期的挤压应力方向.磁组构结果揭示, 骆驼泉地区挤压应力方向在上干柴沟组下部沉积时期为NNE-SSW向, 而上干柴沟组上部和油砂山组沉积时期转变为NE-SW向.结合柴北缘逆冲带已有磁组构结果指出, 该地区早期N-S向或NNE-SSW向挤压应力可能与印度—欧亚板块早新生代以来近N-S向碰撞挤压过程有关, 指示印度—欧亚碰撞的挤压应力自下干柴沟组下部沉积时期就已传播至高原东北部地区; 而后期NE-SW向挤压应力方向与该地区现今GPS揭示的上地壳运动方向一致, 可能与该时段高原东北部巨型走滑断裂构造体系(尤其是阿尔金断裂)有关.此外, 柴北缘逆冲带新生代挤压应力方向转变在其北西部起始于上干柴沟组下部沉积时期, 而南东部起始于上油砂山组下部沉积时期, 与地震反射剖面揭示的断裂活动等地质证据共同揭示柴北缘逆冲带新生代的构造活动自靠近阿尔金断裂的北西部向南东部传播和扩展.综合分析青藏高原东北部地区挤压应力方向转变和其他地质证据发现, 挤压应力方向转变显示出自柴北缘逆冲带北西部向东、西和南向扩展特征, 与阿尔金断裂在上干柴沟组下部-上油砂山组下部沉积时期剪切应力集中于断裂本身, 而上油砂山组下部沉积以来开始散布于高原东北部内部地区的两阶段走滑活动相关.

     

Sediment characteristics and macrofauna distribution along a human-modified inlet in the Gulf of Oristano (Sardinia, Italy)

We studied the spatial variability and within-year temporal changes in hydrological features, grain size composition and chemical characteristics of sediments, as well as macrofaunal assemblages, along a heavily modified inlet in the Gulf of Oristano (western Sardinia, Italy). The inlet connects the Cabras lagoon to the gulf through a series of convoluted creeks and man-made structures, including a dam and fish barriers built in the last three decades. Sediments were muddy and mainly composed of the "non-sortable" fraction (i.e., <8 microm particle size) in all four areas investigated: Lagoon, Creeks, Channel and Seaward. Along the inlet, however, the ratio between the <8 microm and the 8-64 microm fractions was highest in Creeks and Channel, between the fish barriers and the dam, suggesting impaired hydrodynamics. Consistently, steep gradients in water salinity, temperature and dissolved oxygen concentrations were found in proximity to the fish barriers. The whole inlet was characterized by a major organic enrichment of sediments, with up to an annual mean of 33.6% of organic matter and 11.7% of total organic carbon in Seaward due to the presence of seagrass leaf litter. Acid-volatile sulphide and chromium-reduced sulphur concentrations were highest throughout the year in Seaward and Lagoon, respectively, with a peak in summer. Consistently, the whole inlet supported low structured macrofaunal assemblages dominated by few opportunist species, with a relatively lower diversity in Lagoon throughout the year and the highest abundances in Seaward in summer. We infer that the presence of artificial structures along the inlet, such as fish barriers and the dam, impair the lagoon-gulf hydrodynamics, sediment exchange and animal recruitment and colonization. We suggest that the removal of these structures would favour water renewal in the Cabras lagoon, but would also increase the outflow of organic C-bonding fine particles into the gulf with serious consequences for Posidonia oceanica and Cymodocea nodosa seagrass meadows. We conclude that all possible consequences of such initiatives should be carefully considered before any action is taken.     

Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei,Italy)

The Campi Flegrei (Campanian Region, Italy) experienced two cataclysmic caldera-forming eruptions which produced the Campanian Ignimbrite (39 ka, CI) and the Neapolitan Yellow Tuff (15 ka, NYT). We studied the minor eruptions before both these large events to understand magma chamber evolution leading towards such catastrophic eruptions. Major, trace element, and Sr and Nd isotope compositions of pre-Campanian Ignimbrite and pre-Neapolitan Yellow Tuff products define distinct geochemical groups, which are here interpreted as distinct magma batches. These batches do not show any transitional trend towards the CI and NYT eruptions. The CI and NYT systems are decoupled geochemically and isotopically. At least one of the pre-CI and one of the pre-NYT erupted magma batches qualifies as mixing endmembers for the large CI and NYT eruptions, and thus, must have been stored in reservoirs for some time to remain available for the CI and NYT eruptions. The least evolved, isotopically distinct magma compositions that are typical of the last phases of the NYT and CI eruptions did not occur before caldera-forming events. Based on the new data, we propose the following scenario: Multiple magma chambers with distinct compositions existed below the Campi Flegrei before the CI and NYT eruptions and remained generally separated for some time unless new magma was recharged. In each case, one of the residing magma reservoirs was recharged by a new large-volume magma input of intermediate composition from a deeper differentiating magma reservoir. This may have triggered the coalescence of the previously separated reservoirs into one large chamber which fed the cataclysmic caldera-forming eruption. Large magma chambers in the Campi Flegrei may therefore be ephemeral features, interrupted by periods of evolution in individual, separated magma reservoirs.     

Benthic foraminifera as bio-indicators of trace element pollution in the heavily contaminated Santa Gilla lagoon (Cagliari, Italy)

In order to assess the response of benthic foraminifera to trace element pollution, a study of benthic foraminiferal assemblages was carried out into sediment samples collected from the Santa Gilla lagoon (Sardinia, Italy). The lagoon has been contaminated by industrial waste, mainly trace elements, as well as by agricultural and domestic effluent. The analysis of surficial sediment shows enrichment in trace elements, including Cr, Cu, Hg, Ni, Pb and Zn. Biotic and abiotic data, analyzed with multivariate techniques of statistical analysis, reveal a distinct separation of both the highly polluted and less polluted sampling sites. The innermost part of the lagoon, comprising the industrial complex at Macchiareddu, is exposed to a high load of trace elements which are probably enhanced by their accumulation in the finer sediment fraction. This area reveals lower diversity and higher percentages of abnormalities when compared to the outermost part of the lagoon.     

秦祁接合带造山缝合带磁组构特征及其构造意义

结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及Kmin与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸.     

青藏高原东缘新生代早期古应力环境:来自四川盆地西南缘磁组构的记录

四川盆地西南缘紧邻龙门山褶皱冲断带(青藏高原东边界)南段.该地区的新生代早期红层沉积记录了青藏高原东缘的隆升历史及构造演变.本研究选取四川盆地西南缘芦山地区古新统一下渐新统名山组-芦山组地层剖面为研究对象,利用磁组构方法,结合前人对研究区古地磁及构造变形的研究,恢复了该地区新生代早期的古应力方向.本研究获取了548块样...     

A facies interpretation of the eruption and emplacement mechanisms of the upper part of the Neapolitan Yellow Tuff,Campi Flegrei,southern Italy

This study focuses on the upper part, Member B, of the Neapolitan Yellow Tuff (NYT). Detailed measurements of stratigraphic sections within the unlithified pozzolana facies show that Member B is composed of at least six distinct depositional units which each record a complex fluctuation between different styles of deposition from pyroclastic density flows. Six lithofacies have been identified: (1) massive valleyponded facies, the product of non-turbulent flows; (2) inverse-graded facies formed by flows that were turbulent for the majority of transport but were deposited through a non-tubulent basal regime; (3) regressive sand-wave facies, the product of high-concentration, turbulent flows; (4) stratified facies, the product of deposition from turbulent, low-particle-concentration, flows; (5) particle aggregate and (6) vesicular ash lithofacies, both of which are considered to have formed by deposition from turbulent, low-concentration flows. Although the whole eruption may have been phreatomagmatic, facies 1–4 are interpreted to be the product of dry eruptive activity, whereas facies 5 and 6 are considered to be of wet phreatomagmatic eruptive phases. Small-scale horizontal variations between facies include inverse-graded lithofacies that pass laterally into regressive sand-wave structures and stratified deposits. This indicates rapid transition from non-turbulent to turbulent deposition within the same flow. Thin vesicular ash and particle aggregate layers pass laterally into massive valley-ponded vesicular lithofacies, suggesting contemporaneous wet pyroclastic surges and cohesive mud flows. Three common vertical facies relations were recognised. (1) Massive valley-ponded and inverse-graded facies are overlain by stratified facies, suggesting decreasing particle concentration with time during passage of a flow. (2) Repeated vertical gradation from massive up into stratified facies and back into massive beds, is indicative of flow fluctuating between non-turbulent and turbulent depositional conditions. (3) Vertical alternation between particle aggregates and vesicular facies is interpreted as the product of many flow pulses, each of which involved deposition of a single particle aggregate and vesicular ash layer. It is possible that the different facies record stages in a continuum of flow processes. The deposits formed are dependent on the presence, thickness and behaviour of a high-concentration, non-turbulent boundary layer at the base of the flow. The end members of this process are (a) flows that transported and deposited material from a non-turbulent flow regime and (b) flows that transported and deposited material from a turbulent flow regime.     

Microstructure and magnetic fabric in the Shuanghe pluton: A synkinematic granite in Eastern Dabie Mountains,China

Strain analyses for the Shuanghe pluton show that the main strain planes suffered distinct deformation. The main strain value (XZ) is up to 1.59-2.18, and the value of Flinn index (K) ranges from 0.11 to 0.82. Anisotropy of magnetic susceptibility (AMS) measurements reveal that the orientations of the magnetic foliation and lineation gently dip SE, consistent with the macroscopic foliation of the pluton. The value of anisotropy degree (P) ranges from 1.109 to 1.639, and the shape parameter (7) from 0.079 to 0.534. These studies prove that the pluton was deformed under strong compression. Quartz c-axis textures, defined by monoclinic or triclinic asymmetry, usually developed the high maxima paralleling the b-axis, which is defined by the developed in the high-ultrahigh pressure rocks (UHP) which were captured in the pluton or country rocks. It is concluded that the Shuanghe pluton emplaced under regional compression slightly after the formation of UHP, and it is characterized by synkinematic granitic deformation.     

Distribution and ecological relevance of fine sediments in organic-enriched lagoons: the case study of the Cabras lagoon (Sardinia, Italy)

In organic-enriched sedimentary systems, like many Mediterranean coastal lagoons, a detailed analysis of sediment grain size composition and partitioning within the muds is crucial to investigate sedimentological trends related to both hydrodynamic energy and basin morphology. In these systems, sediment dynamics are particularly important because the partitioning and transport of fine sediments can strongly influence the redistribution and accumulation of large amounts of organic matter, and consequently the distribution of benthic assemblages and the trophic status and functioning of a lagoon. Nevertheless, studies on benthic-sediment relationships have been based mainly on a rather coarse analysis of sediment grain size features. In muddy systems, however, this approach may impede a proper evaluation of the relationships and effects of the distribution of fine sediment and organic matter on the biotic benthic components. Here we show that the distribution of sedimentary organic matter (OM) and total organic carbon (TOC) in the Cabras lagoon (Sardinia, Italy) can be explained (i.e., predicted) as a function of a nonlinear increase in the amount of the cohesive fraction of sediments (< or = 8 microm grain size particles) and that this fraction strongly influences the structure, composition and distribution of macrobenthic assemblages. Even in such a homogeneously muddy system, characterized by "naturally" occurring impoverished communities, impaired benthic assemblages were found at < or = 8 microm, OM, TOC contents of about 77%, 11% and 3.5%, respectively. A review of studies conducted in Mediterranean coastal lagoons highlighted a lack of direct integrated analysis of sediment features and the biotic components. We suggest that, especially in organic-enriched coastal lagoons, monitoring programs should primarily investigate and consider the cohesive fraction of sediments in order to allow a better assessment of benthic-sediment relationships and ecological quality of the system.     

Nature, distribution and origin of polycyclic aromatic hydrocarbons (PAHs) in the sediments of Olbia harbor (Northern Sardinia, Italy)

The nature, origin and distribution of US EPA polycyclic aromatic hydrocarbons (PAHs) in the sediments of Olbia harbor (North Sardinia, Italy) were investigated by gas-chromatography/mass spectrometry (GCMS). PAH concentrations in the sediments (SigmaPAHs) ranged from 0.16 to 0.77 microg g(-1), indicating a homogeneously low level of pollution. A rather exceptional prevalence of low molecular weight PAHs was substantiated: nearly 80% of SigmaPAHs include naphthalene (15.19%) and phenanthrene (64.47%). Carcinogenic compounds were present in very low (BaP, BkF, BaA and DBA) or negligible (BbF and Inp) concentrations. As indicated by the Phen/Ant molar ratio, the main source of PAHs is petrogenic, probably due to oil spills from shipping. The low/high molecular weight ratio (ranging between 1.2 and 26) distinguishes the sediments of the tourist harbor from those of the commercial/industrial harbor. Moreover, a slight but meaningful pyrolytic contribution to pollution was found in the tourist harbor, which has the most polluted sediments in the whole harbor. Finally, good linear correlations were found between a selected PAH (Phen and Naph) and selected sums of PAHs (i.e. the total amount of the other 15 US EPA and the sum of low molecular weight PAHs).