大别（超）包产到户铁岩和阜新火山岩铂族元素斌存规律研究

The platinum-group elements (PGE), including Os, Ir, Ru, Rh, Pt and Pd, axe strongly siderophile and chalcophile. On the basis of melting temperature, the PGE may be divided into two groups: the Ir group (IPGE, >2000°C) consisting of Os, Ir and Ru, and the Pd group (PPGE, <20GO°C) consisting of Rh, Pt and Pd. Because of their unique geochemical properties, PGE provide critical information on global-scale differentiation processes, such as core-mantle segregation, late accretionary history, and core-mantle exchange. In addition, they may be used to identify magma source regions and unravel complex petrogenetic processes including partial melting, melt percolation and metasomatism in the mantle, magma mixing and crustal contamination in magma chambers and melt crystallization.Compared with other rocks, (ultra)mafic rocks have lower REE content but higher PGE content, so PGE have advantages in studying the petrogeneses and evolution of them. In this study, we selected (ultra)mafic rocks collected in Dabie Orogen and volcanic rocks from Fuxin Region. Based on the distribution and behaviour of platinum-group elements, combined with other elements, we speculate the magma evolution and source mantle of these (ultra)mafic rocks and volcanic rocks.Many (ultra)mafic rocks are widely distributed in Dabie Region. According to their deformation and metamorphism, we classed them into three types. One is intrusive (ultra)mafic rocks, which are generally undeformed and show no or little sign of metamorphism, such as (ultra)mafic intrusions in Shacun, zhujiapu, Banzhufan, qingshan, Xiaohekou, Jiaoziyan, Renjiawan and Daoshichong. The other one is ultrahigh pressure metamorphic (ultra)mafic rocks, some of them appeared as eelogites, such as complex in Bixiling and adjacent Maowu. Another one is intense deformed and metamorphic, termed as tectonic slice, alpine-type (ultra)mafic rocks. The most representative is Raobazhai and Dahuapin. However, there are many controversies about the formation of those (ultra)mafic rocks. Here, we select typical rocks of the three types. The PGE were determined by inductively coupled plasma mass spectrometry (ICP-MS) ater NiS fire-assay and tellurium co-precipitation.The PGE tracing shows that three components are needed in the source of the cretaceous (uitra)mafic intrusions. They could be old enriched sub-continental lithospheric mantle, lower crust and depleted asthenospheric mantle. The pattern of PGE also shows the primitive magma of these intrusions underwent S saturation. According to palladium, we can conclude that the mantle enrich in PGE. Distribution of PGE in Bixiiing and Maowu (ultra)mafic rocks display they are products of magmas fractional crystallization. The (ultra)mafic rocks in Bixiiing and Maowu are controlled by various magmatic processes and the source mantle is depleted in PGE. Of interest is that the mantle produced UHP (ultra)mafic rocks are PGE-depleted, whereas the mantle of cretaceous (ultra)mafic intrusions are enrich in PGE. This couldindicate that the mantle change from PGE-enriched to PGE-depleted during120-OOMa, which in accord with the time of tectonic system change in the East China. At the same time, (ultra)mafic intrusions in cretaceous took information of deep mantle, which means the processes in deep mantle arose structural movement in the crust The character of PGE in alpine-type (ultra)mafic rocks declared that the rocks had experienced two types of metasomatic processes - hydrous melt derived from slab and silicate melt. In addition, we analyze the platinum-group elements in volcanic rocks on the northern margin of the North China Craton, Fuxin. The volcanic rocks characterized by negative anomalies of platinum. This indicates that platinum alloys, which may host some Pt resided in the mantle. The PGE patterns also show that Jianguo alkali basalts derived from asthenospheric mantle source, but wulahada high-Mg andesites derived from lithospheric mantle.

利用宽频带台阵资料研究华北地区地壳及上地幔间断面结构

Intense tectonic renovation has occurred in the eastern continent of china since Mesozoic, as evidenced by the high heat flow, widespread magma extrusion and volcanic activities, and development of large sedimentary basins. To explain the cause and mechanism for the tectonic process in this period, some researchers have put forward various models, such as mantle plume, subduction of the Pacific slab, Yangtze Block-North China Block collision, etc. Their seismological evidence, however, is still scarce..During the period from 2000 to 2003, large temporary seismic arrays were established in North China by the Institute of the Geology and Geophysics, Chinese Academy of Sciences. Total 129 portable seismic stations were linearly emplaced across the western and eastern boundaries of the Bohai Bay Basin, and accumulated a large amount of high-quality data. Moreover, abundant data were also collected at the capital digital seismic network established in the ninth five-year period of national economic and social development. These provide an unprecedented opportunity for us to study the deep structure and associated geodynamic mechanism of lithospheric processes in North China using seismological techniques.Seismology is a kind of observation-based science. The development of seismic observations greatly promotes the improvement of seismologic theory and methodology. At the beginning of this thesis, I review the history of seismic observation progress, and present some routine processing techniques used in the array seismology. I also introduce two popular seismic imaging methods (receiver function method and seismic tomography).Receiver function method has been widely used to study the crustal and upper mantle structures, and many relevant research results have been published. In this thesis I elaborate the theory of this method, including the basic concept of receiver functions and the methodology for data pre-processing, stacking and migration. I also address some problems often encountered in practical applications of receiver function imaging.By using the teleseismic data collected at the temporary seismic arrays in North China, in particular, the traveltime information of P-to-S conversion and multiple reverberations of the Moho discontinuity, I obtain the distributions of the crustal thickness and the poisson ratio at the northwest boundary area of the Bohai Bay Basin and discuss the geological implications of the results.Through detailed intestigations on the crustal structural feature around the middle part of the Tanlu fault, considerable disparity in poisson ratios is found in the western and eastern sides of the Tanlu fault. Moreover, an obvious Moho offset is coincidently observed at the same surface location. A reasonable density model for the Tanlu fault area is also derived by simulating the observed gravity variations. Both receiver function study and gravity anomaly modeling suggest that the crustal difference between the western and eastern sides of the Tanlu fault is mainly resulted from their different compositions.With common conversion point imaging of receiver functions, I estimate the depths of the upper and lower boundaries of the mantle transition zone, i.e., the 410 and 660 km discontinuities, beneath most part of the North China continent The thickness of the transition zone (TTZ) in the study area is calculated by subtracting the depth of .410 km discontinuity from that of the 660km discontinuity. The resultant TTZ is 10-15 km larger in the east than in the west of the study area. Phase transitions at the 410 km and the 660 km discontinuities are known to have different Clapeyron slopes. Therefore, the TTZ is sensitive to the temperature changes in the transition zone. Previous studies have shown that the TTZ would be smaller in the mantle plume areas and become larger when the remnants of subducted slabs are present The hypothesis of mantle plume cannot give a reasonable interpretation to the observed TTZ beneath North China, Instead, the receiver function imaging results favor a dynamic model that correlates the thermal structure of the mantle transition zone and associated upper mantle dynamics of North China to the Pacific plate subduction process.

地幔对流理论及其在解释地球物理观测的应用

With the improving of mantle convection theory, the developing of computing method and increasing of the measurement data, we can numerically simulate more clearly about the effects on some geophysical observed phenomenons such as the global heat flow and global lithospheric stress field in the Earth's surface caused by mantle convection, which is the primary mechanism for the transport of heat from the Earth's deep interior to its surface and the underlying force mechanism of dynamics in the Earth.Chapter 1 reviews the historical background and present research state of mantle convection theory.In Chapter 2, the basic conception of thermal convection and the basic theory about mantle flow.The effects on generation and distribution of global lithospheric stres s field induced by mantle flow are the subject of Chapter 3. Mantle convection causes normal stress and tangential stresses at the bottom of the lithosphere, and then the sublithospheric stress field induces the lithospheric deformation as sixrface force and results in the stress field within the lithosphere. The simulation shows that the agreement between predictions and observations is good in most regions. Most of subduction zones and continental collisions are under compressive. While ocean ridges, such as the east Pacific ridge, the Atlantic ridge and the east African rift valley, are under tensile. And most of the hotspots preferentially occur in regions where calculated stress is tensile. The calculated directions of the most compressive principal horizontal stress are largely in accord with that of the observation except for some regions such as the NW-Pacifie subduction zone and Qinghai-Tibet Plateau, in which the directions of the most compressive principal horizontal stress are different. It shows that the mantel flow plays an important role in causing or affecting the large-scale stress field within the lithosphere.The global heat flow simulation based on a kinematic model of mantle convection is given in Chapter 4. Mantle convection velocities are calculated based on the internal loading theory at first, the velocity field is used as the input to solve the thermal problem. Results show that calculated depth derivatives of the near surface temperature are closely correlated to the observed surface heat flow pattern. Higher heat flow values around midocean ridge systems can be reproduced very well. The predicted average temperature as a function of function of depth reveals that there are two thermal boundary layers, one is close to the surface and another is close to the core-mantle boundary, the rest of the mantle is nearly isothermal. Although, in most of the mantle, advection dominates the heat transfer, the conductive heat transfer is still locally important in the boundary layers and plays an important role for the surface heat flow pattern. The existence of surface plates is responsible for the long wavelength surface heat flow pattern.In Chapter 5, the effects on present-day crustal movement in the China Mainland resulted from the mantle convection are introduced. Using a dynamic method, we present a quantitative model for the present-day crustal movement in China. We consider not only the effect of the India-Eurasia collision, the gravitational potential energy difference of the Tibet Plateau, but also the contribution of the shear traction on the bottom of the lithosphere induced by the global mantle convection. The comparison between our results and the velocity field obtained from the GPS observation shows that our model satisfactorily reproduces the general picture of crustal deformation in China. Numerical modeling results reveal that the stress field on the base of the lithosphere induced by the mantle flow is probably a considerable factor that causes the movement and deformation of the lithosphere in continental China with its eflfcet focuing on the Eastern China A numerical research on the small-scale convection with variable viscosity in the upper mantle is introduced in Chapter 6. Based on a two-dimensional model, small-scale convection in the mantle-lithosphere system with variable viscosity is researched by using of finite element method. Variation of viscosity in exponential form with temperature is considered in this paper The results show that if viscosity is strongly temperature-dependent, the upper part of the system does not take a share in the convection and a stagnant lid, which is identified as lithosphere, is formed on the top of system because of low temperature and high viscosity. The calculated surface heat flow, topography and gravity anomaly are associated well with the convection pattern, namely, the regions with high heat flow and uplift correspond to the upwelling flow, and vice versa.In Chapter 7, we give a brief of future research subject: The inversion of lateral density heterogeneity in the mantle by minimizing the viscous dissipation.

Isotope geochronology of Dapingzhang spilite-keratophyre formation in Yunnan Province and its geological significance

On the basis of the geological field investigations and isotope geochronological studies the Sm-Nd isochron age (513 Ma?0 Ma), Rb-Sr isochron age (511 Ma? Ma) and K-Ar age (312-317 Ma) of the Dapingzhang spilite-keratophyre formation in Yunnan Province are presented. From these geochronological data it is evidenced that this suite of volcanic rocks was formed in the Cambrian and the parent magma was derived from a depleted mantle, which was influenced by crustal contamination and/or seawater hydrothermal

Mantle-derived gaseous components in ore-forming fluids of the Xiangshan uranium

The Xiangshan U deposit, the largest hydrothermal U deposit in China, is hosted in late Jurassic felsic volcanic rocks although the U mineralization post dates the volcanics by at least 20 Ma. The mineralization coincides with intrusion of local mantle-derived mafic dykes formed during Cretaceous crustal extension in South China. Ore-forming fluids are rich in CO2, and U in the fluid is thought to have been dissolved in the form of UO2 (CO3)22− and UO2 (CO3) 34− complexes. This paper provides He and Ar isotope data of fluid inclusions in pyrites and C isotope data of calcites associated with U mineralization (pitchblende) in the Xiangshan U deposit. He isotopic compositions range between 0.1 and 2.0Ra (where Ra is the 3He/4He ratio of air=1.39×10−6) and correlates with 40Ar/36Ar; although there is potential for significant 3He production via 6Li(n,α)3H(β)3He reactions in a U deposit (due to abundant neutrons), nucleogenic production cannot account for either the 3He concentration in these fluids, nor the correlations between He and Ar isotopic compositions. It is more likely that the high 3He/4He ratios represent trapped mantle-derived gases. A mantle origin for the volatiles of Xiangshan is consistent with the δ13C values of calcites, which vary from −3.5‰ to −7.7‰, overlapping the range of mantle CO2. The He, Ar and CO2 characteristics of the ore-forming fluids responsible for the deposit are consistent with mixing between 3He- and CO2-rich mantle-derived fluids and CO2-poor meteoric fluids. These fluids were likely produced during Cretaceous extension and dyke intrusion which permitted mantle-derived CO2 to migrate upward and remobilize U from the acid volcanic source rocks, resulting in the formation of the U deposit. Subsequent decay of U within the fluid inclusions has reduced the 3He/4He ratio, and variations in U/3He result in the range in 3He/4He observed with U/3He ratios in the range 5–17×103 likely corresponding to U concentrations in the fluids b0.2 ppm.

山东地区中生代岩浆作用与地壳拉张-兼论煌斑岩与金成矿的关系

燕山期（205～65Ma）山东地区地壳活动强烈，构造体系已由古亚洲构造域完全转化为滨太平洋构造域，构造活动导源于太平洋板块对欧亚板块的俯冲。由于太平洋板块对欧亚板块的俯冲（NW向），鲁东地区岩石圈发生了快速拆沉减薄作用．同时鲁东地区也可能会出现地l漫柱的活动；另外，在太平洋板块俯冲作用影响下，炎区庐断裂（山东称沂沐断裂）带发生了大型左行走滑剪切和拉张活动。以上构造因素加上早白至世末一晚白至世期间燕山造山带的垮塌，都可能为山东地区中生代地壳拉张提供了动力条件。山东地区中生代（燕山期）基性脉岩特别发育，这些慢源基性岩脉充填张性裂隙，是大陆地壳拉张的标志；另外，山东地区也存在大量拉张背景下的燕山期火山岩和碱性岩。但关于它们的年代学和系统的地球化学研究还比较薄弱，且对其成因和形成的构造环境，仍存在着争议。本论文主要从同位素年代学、岩石化学、地球化学和Sr-Nd-Pb同位素方面对山东地区燕山期基性脉岩、火山岩和碱性超基性脉岩进行了系统研究。同时，考虑到鲁东地区煌斑岩中金含量普遍较高，且燕山期又是山东金矿的主成矿期，论文中对煌斑岩与金成矿之间的关系也作了一定的研究。通过研究，得出以下主要认识：1、火山岩为一套以钙碱性安山岩为主，含少量拉斑玄武岩和英安岩。成因上为富集地慢部分熔融作用的结果，但在成岩过程中也可能存在单斜辉石、斜长石、橄榄石和Ti-Fe氧化物等矿物的分离结晶作用。碱性超基性脉岩岩性上为单一的橄榄辉石岩，为富集地慢源低度（3.4％）部分熔融作用的产物，岩浆演化过程经历了以橄榄石为主的分馏作用。基性脉岩主要包括辉长岩、辉绿岩（主要分布在鲁西地区）和煌斑岩（以斜闪煌斑岩为主，同时含部分拉辉煌斑岩和角闪煌斑岩）（主要分布在鲁东地区），都为富集岩石圈地慢部分熔融的产物。三类岩石在侵位结晶过程都不存在明显的地壳混染。2、火山岩、碱性超基性脉岩和基性脉岩（除少数外）都形成于大陆板内拉张环境。3、富集地慢源区（EMI）的产生是俯冲并熔融的扬子下地壳物质进入华北岩石圈地慢并与之相互交代作用形成的。4、研究区中生代基性脉岩K-Ar年龄分布范围为72.2±1.70Ma～204.2±5.4Ma，且基本上在90～140Ma之间变化。综合碱性超基性脉岩和已知的青山组的火山岩、基性脉岩年龄数据，认为山东地区中生代地壳拉张至少存在四次：即约80Ma、100Ma、120Ma和 140Ma。但鲁东地区在地壳拉张方面可能存在着与鲁西地区不同的制约因素：即鲁东地区存在拆沉作用和可能存在地慢柱的影响，而鲁西地区可能受到了郊庐断裂的左行走滑剪切和拉张活动的影响。5、胶北地区煌斑岩为钙碱性系列，且金含量普遍较高（平均28ppb），该研究对胶北地区的找矿勘探工作具有一定意义。

Caratterizzazione geo-petrografica, petrologica e geochimica di rocce anatettiche del nord Sardegna

The High Grade Metamorphic Complex (HGMC) of Variscan basement of north Sardinia is characterized by the widespread of migmatites. This study is focused on two localities of NE Sardinia (Porto Ottiolu and Punta Sirenella) where ortho- and para-derivates migmatites outcrop. A geological and structural survey was carried out, leading to the realization of a geological schematic map of Punta Sirenella area. Several samples of different rocks were collected for petrographic, micro-structural minero-chemical and geochemical analyses. In the Porto Ottiolu area three main deformation phases have been identified; D1, characterized by tight folds with sub-horizontal axes, rarely preserved in paragneisses; D2, that produce a pervasive foliation oriented N100° 45°SW marked by biotite and sillimanite blastesis and locally transposed by shear zone oriented N170°; D3, late deformation phase caused symmetric folds with sub-horizontal axes with no axial plane schistosity. Leucosomes form pods and layers along S2 schistosity but also leucosomes along shear zones have been observed. In the Punta Sirenella area, three main deformation phases have been identified; D1, is manifested by the transposition of centimeter-sized leucosomes and is rarely observed in paragneisses were produce open folds with sub-vertical axes; D2, NW-SE oriented on whose XY plane three mineralogical lineation (quartz+plagioclase, fibrolite+quarz and muscovite) lie; D3, a ductile-brittle deformation phase that produce a mylonitc S3 foliation that locally become the most evident schistosity in the field oriented N140° steeply dipping toward NE. In both areas, leucosomes of sedimentary-derived migmatites are generally trondhjemitic pointing out for a H2O fluxed melting reaction, but also granitic leucosomes have been found, produced by muscovite dehydration melting. Leucosomes of migmatitic orthogneiss instead, have granitic compositions. Migmatization started early, during the compressional and crustal thickening (sin-D1, pre-D2) and was still active during exhumation stage. For each studied outcrop of migmatite pseudosections for the average mesosome composition have been calculated; these pseudosections have been used to model the P-T conditions of anatexis on the basis of the melt volume (%) of melt, Si/Al and Na/K molar ratios, modal content of garnet and Si content in metamorphic white mica. Further pseudosections have been calculated for the average composition of leucosomes in order to define the P-T conditions of the end of the crystallization through intersection of solidus curve and isopleths of Si content in white mica and/or XMg ratio in biotite. Thermodynamic modeling on ortho- and sedimentary-derived migmatites of Punta Sirenella yield P-T conditions of 1.1-1.3 GPa - 670-740°C for migmatitic event and 0.75-0.90 GPa - 660-730°C for the end of crystallization. These conditions are fit well with previous studies on adjacent rocks. Modeling of Porto Ottiolu ortho- and sedimentary-derived migmatites yield P-T conditions of 0.85-1.05 GPa - 690-730°C for migmatitic event and 0.35-0.55 GPa - 630-690°C strongly affected by re-equilibration during exhumation, expecially for crystallization conditions. Geochemical analyses of samples belonging to Porto Ottiolu and Punta Sirenella orthogneisses show a strong link with those of other orthogneisses outcropping in NE Sardinia (for instance, Lode-Mamone and Golfo Aranci) that are considered the intrusive counterparts of middle-Ordovician metavolcanics rocks outcropping in the Nappe Zone. Thus, the studied ortogneiss bodies, even lacking radiometric data, can be considered as belonging to the same magmatic cycle.

Plasma acceleration above Martian magnetic anomalies

Grande, Manuel, et al., 'Plasma acceleration above Martian magnetic anomalies', Science (2006) 311(5763) pp.980-983 RAE2008

Particulate and dissolved aluminum and titanium in the upper water column of the Atlantic Ocean

This study investigates the oceanic behavior of the lithogenic trace elements Al and Ti in the upper 200 m of the Atlantic Ocean. The distribution of both metals in the dissolved and particulate phases was assessed along an E-W transect in the eastern tropical North Atlantic (December 2009) and along a meridional Atlantic transect (April-May 2010). The surface water concentrations of particulate and dissolved Al and Ti reflected the previously observed pattern of atmospheric inputs into the Atlantic Ocean. Subsurface minima at stations with pronounced fluorescence maxima were observed, suggesting a link between biological productivity and the removal of both dissolved and particulate Al and Ti. This may include uptake mechanisms, adsorption and aggregation processes on biogenic particle surfaces and the formation of large, fast sinking biogenic particles, e.g., fecal pellets. Residence times in the upper water column (100 m) of the tropical and subtropical North Atlantic were estimated to range in the order of days to weeks in the particulate phases (Al: 3-22 days, Ti: 4-37 days) and were 0.9-3.8 years for Al and 10-31 years for Ti in the dissolved phases. Longer residence times in both phases in the South Atlantic are consistent with lower biological productivity and decreased removal rates. In the upper water column, Al was predominantly present in the dissolved form, whereas Ti mostly occurred in the particulate form. Largest deviations in the partition coefficients between the particulate and dissolved phases were found in the surface waters, together with excess dissolved Al over Ti compared to the crustal source. This likely reflects elevated dissolution of Al compared to Ti from atmospheric mineral particles.

Low colloidal associations of aluminium and titanium in surface waters of the tropical Atlantic

The distribution of dissolved, soluble and colloidal fractions of Al and Ti was assessed by ultrafiltration studies in the upper water column of the eastern tropical North Atlantic. The dissolved fractions of both metals were found to be dominated by the soluble phase smaller than 10 kDa. The colloidal associations were very low (0.2–3.4%) for Al and not detectable for Ti. These findings are in some contrast to previous estimations for Ti and to the predominant occurrence of both metals as hydrolyzed species in seawater. However, low tendencies to form inorganic colloids can be expected, as in seawater dissolved Al and dissolved Ti are present within their inorganic solubility levels. In addition, association with functional organic groups in the colloidal phase is unlikely for both metals. Vertical distributions of the dissolved fractions showed surface maxima with up to 43 nM of Al and 157 pM of Ti, reflecting their predominant supply from atmospheric sources to the open ocean. In the surface waters, excess dissolved Al over dissolved Ti was present compared to the crustal source, indicating higher solubility and thus elevated inputs of dissolved Al from atmospheric mineral particles. At most stations, subsurface minima of Al and Ti were observed and can be ascribed to scavenging processes and/or biological uptake. The dissolved Al concentrations decreased by 80–90% from the surface maximum to the subsurface minimum. Estimated residence times in the upper 100 m of the water column ranged between 1.6 and 4 years for dissolved Al and between 14 and 17 years for dissolved Ti. The short residence times are in some contrast to the low colloidal associations of Al and Ti and the assumed role of colloids as intermediates in scavenging processes. This suggests that either the removal of both metals occurs predominantly via direct transfer of the hydrolyzed species into the particulate fraction or that the colloidal phase is rapidly turned over in the upper water column.

Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Rapid Petrogenesis of Dacites at Fonualei Volcano

Fonualei is unusual amongst subaerial volcanoes in the Tonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km(3) and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100-1000 degrees C, 2-4 wt % H2O and oxygen fugacities 1-2 log units above the quartz-fayalite-magnetite buffer. Major and trace element data, along with Sr-Nd-Pb and U-Th-Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2-6 km) depths beneath the volcano. U-Th-Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust.

Rapid magmatic processes accompany arc-continent collision: the Western Bismarck arc, Papua New Guinea

New U-Th-Ra, major and trace element, and Sr-Nd-Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc-continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc-continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated Pb-208/Pb-204. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. U-238 and Ra-226 excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.

A stacked Late Quaternary fluvio-periglacial sequence from the Axe valley, southern England with implications for landscape evolution and Palaeolithic archaeology

The current model of mid-latitude late Quaternary terrace sequences, is that they are uplift-driven but climatically controlled terrace staircases, relating to both regional-scale crustal and tectonic factors, and palaeohydrological variations forced by quasi-cyclic climatic conditions in the 100 K world (post Mid Pleistocene Transition). This model appears to hold for the majority of the river valleys draining into the English Channel which exhibit 8–15 terrace levels over approximately 60–100 m of altitudinal elevation. However, one valley, the Axe, has only one major morphological terrace and has long-been regarded as anomalous. This paper uses both conventional and novel stratigraphical methods (digital granulometry and terrestrial laser scanning) to show that this terrace is a stacked sedimentary sequence of 20–30 m thickness with a quasi-continuous (i.e. with hiatuses) pulsed, record of fluvial and periglacial sedimentation over at least the last 300–400 K yrs as determined principally by OSL dating of the upper two thirds of the sequence. Since uplift has been regional, there is no evidence of anomalous neotectonics, and climatic history must be comparable to the adjacent catchments (both of which have staircase sequences) a catchment-specific mechanism is required. The Axe is the only valley in North West Europe incised entirely into the near-horizontally bedded chert (crypto-crystalline quartz) and sand-rich Lower Cretaceous rocks creating a buried valley. Mapping of the valley slopes has identified many large landslide scars associated with past and present springs. It is proposed that these are thaw-slump scars and represent large hill-slope failures caused by Vauclausian water pressures and hydraulic fracturing of the chert during rapid permafrost melting. A simple 1D model of this thermokarstic process is used to explore this mechanism, and it is proposed that the resultant anomalously high input of chert and sand into the valley during terminations caused pulsed aggradation until the last termination. It is also proposed that interglacial and interstadial incision may have been prevented by the over-sized and interlocking nature of the sub-angular chert clasts until the Lateglacial when confinement of the river overcame this immobility threshold. One result of this hydrogeologically mediated valley evolution was to provide a sequence of proximal Palaeolithic archaeology over two MIS cycles. This study demonstrates that uplift tectonics and climate alone do not fully determine Quaternary valley evolution and that lithological and hydrogeological conditions are a fundamental cause of variation in terrestrial Quaternary records and landform evolution.

Geoquímica dos granitóides de Aguiar da Beira, norte de Portugal

A área de Aguiar da Beira está integrada nos terrenos autóctones da Zona Centro-ibérica e é constituída essencialmente por rochas granitóides variscas instaladas durante e após a terceira fase de deformação (D3). As relações de campo mostram que estes granitóides intruíram formações metassedimentares de idade proterozóica superior-câmbrica e as sequências do Ordovícico e do Carbónico do sinclinal Porto-Sátão, cujo extremo SE aflora na área de estudo. Com base na cartografia publicada e nos dados de campo colhidos no âmbito deste trabalho, foi possível individualizar oito intrusões distintas: o granodiorito -granito biotítico de Sernancelhe, o granito gnaissoso de duas micas, o granito moscovítico-biotítico de Vila Nova de Paiva, o granodiorito-granito biotítico-moscovítico de Lagares e os granitos de Touro (biotítico-moscovítico), Aguiar da Beira (moscovítico-biotítico), Pera Velha / Vila da Ponte (biotítico-moscovítico) e Rei Mouro (moscovítico-biotítico). A presença de encraves microgranulares em cinco dos granitóides estudados sugere que os processos de mistura de magmas desempenharam um papel importante na sua petrogénese. As datações U-Pb obtidas em zircões e monazites durante o presente estudo permitiram subdividir os granitóides de Aguiar da Beira em três grupos, de acordo com as suas relações com a terceira fase de deformação (D3): granitóides sin-tectónicos (Sernancelhe e granito gnaissoso; 322-317 Ma), tardi-tectónicos (Vila Nova de Paiva, Lagares e Touro; 308-306 Ma), e tardi- a pós-tectónicos (Aguiar da Beira, Pera Velha / Vila da Ponte e Rei Mouro; 303297 Ma). As assinaturas geoquímicas de elementos maiores e traço dos granitóides estudados, em conjunto com os dados isotópicos Sr-Nd e δ18 (rocha total e zircão) apontam para uma contribuição significativa de protólitos crustais na génese destes magmas. Á excepção do granito gnaissoso, todos os granitóides possuem um carácter transicional entre os granitos do tipo I e do tipo S, o que apoiado pelos dados de geoquímica de rocha total e isotópica, e pela presença de encraves microgranulares de composição mais máfica presentes em muitos deles, indicia uma forte intervenção de processos de hibridização de líquidos de proveniência distinta (crustais e mantélicos), em diferentes proporções, na sua origem. Pelo contrário, as características geoquímicas e isotópicas do granito gnaissoso revelam claras afinidades com os granitos do tipo S, e sugerem que tenha derivado da anatexia de fontes exclusivamente supracrustais. No entanto, parte da variabilidade geoquímica e isotópica observada em todos os granitóides estudados só poderá ser explicada pela actuação de processos de cristalização fraccionada, especialmente intensos no caso do granito gnaissoso e dos granitos tardi- a PÓS-D3.

Relações petrogénicas entre o complexo gnáissico migmático e o granito de Banabuiú (Nordeste do Brasil)

A região de Banabuiú situa-se no Domínio Ceará Central, na porção setentrional da Província Borborema, um dos cinturões orogénicos formados durante o evento Brasiliano/Pan-Africano no final do Neoproterozóico. As duas unidades litológicas principais presentes na área são: o complexo gnáissicomigmatítico (metapelitos e metagrauvaques) e granitóides brasilianos. Para além das formações paraderivadas, no substrato da região também foi identificado um conjunto de rochas ortoderivadas, até então não individualizado na cartografia existente. Tanto a sequência paraderivada, como os materiais ortoderivados, foram intensamente afectados por metamorfismo regional da fácies granulítica durante a Orogenia Brasiliana, que atingiu as condições de fusão parcial, gerando migmatitos com um amplo espectro de morfologias. Estes migmatitos apresentam estruturas dominantemente estromáticas, embora localmente se tenham identificado também corpos irregulares de diatexitos de tipo“schlieren”, “schollen” e “maciço (s.s)”, indicando que o processo de migmatização culminou com a produção de maiores quantidades de fundido. Em termos tectónicos, o basamento da região regista os efeitos de três fases de deformação, embora as estruturas concordantes à D3 sejam dominantes e obliterem, em muitos casos, as anisotropias anteriores. A maior parte dos fundidos anatécticos parece ter sido produzida durante tectónica transcorrente D3. No entanto, as condições metamórficas para o início da fusão parcial parece ter sido atingidas antes, durante a D2, já que também existem leucossomas, embora em proporções reduzidas, associados com as estruturas desta fase. A grande quantidade de volumes de leucossomas / veios leucocráticos encontrados na região, está relacionada com a actuação da zona de cisalhamento de Orós e parece corresponder a fundidos anatécticos gerados em níveis mais profundos que foram injectados nas sequências orto- e para-derivadas, devido a notória escassez de leucossomas “in situ” nestas rochas. A presença de fluidos aquosos injectados no complexo migmatítico de Banabuiú terá proporcionado a re-hidratação e retrogradação das rochas hospedeiras, evidenciada, essencialmente, pela presença de moscovite tardia, amplamente distribuída nos metassedimentos e ortognaisses, sobretudo nas zonas próximas aos leucossomas e veios leucocráticos. Dados isotópicos apontam que as rochas da região de Banabuiú apresentam valores fortemente negativos de εNdt e positivos de εSrt sugerindo um significativo envolvimento de materiais supracrustais do grupo Acopiara na formação do complexo migmatítico e na petrogénese do maciço granítico de Banabuiú e o marcado fraccionamento isotópico Sm-Nd observado nalguns dos leucossomas analisados indica que os líquidos anatécticos que lhes deram origem resultaram de processos de fusão em desequilíbrio, em condições anidras, e foram rapidamente extraídos da área-fonte, comprovando o carácter alóctone dos veios leucocráticos intercalados nos ortognaisses e paragnaisses de Banabuiú.