武汉东湖是如何形成的?

东湖位于湖北省武汉市中心城区,是中国乃至亚洲最大的城中湖之一.她是首批国家重点风景名胜区和国家5A级旅游景区.武汉市正在打造东湖城市生态绿心.东湖对于武汉在资源、环境、生态、人文各方面均具有重要意义.关于东湖的成因长期众说纷纭.研究首次从地质、地貌、沉积等方面对东湖的成因进行了分析,并对东湖与长江的关系进行了讨论.（1）东湖位于中-晚更新世形成的岗地区.湖汊发育,岸线蜿蜒,岬湾交错,是东湖最大的特点.（2）东湖的湖相沉积厚度各处不一,总体呈现南薄北厚、边缘薄中间厚的特点.下伏主要地层为晚更新世下蜀黄土,在靠近基岩残丘的南部边缘局部为晚更新世坡积层,两者之间为明显的侵蚀接触关系.（3）东湖的湖盆形成于距今2万年左右的末次冰期盛期.东海海平面的大幅度下降,长江河床深切.发育于长江南岸珞珈山、南望山、喻家山一带的地表径流,在汇入长江时因江水水位较低而发生侵蚀,形成多条冲沟组合而成的侵蚀洼地.之后,随着冰后期的全球变暖,长江水位快速上升,两岸天然堤发育、壮大,使侵蚀洼地的出口被淤塞,逐渐积水成湖,即东湖为沟谷壅塞湖.（4）根据湖泊地质地貌特征,东湖与沙湖是两个不同成因且相对独立的湖泊;长江并未经东湖流过.但东湖的形成与长江有关,乃是全球气候变化驱动下海、江、湖相互作用的产物.（5）东湖之美,美于自然.保护其自然特质,顺应其自然规律,是东湖保护与利用必须坚持的原则.将湖域、湖岸、岸上作为一个整体,将水域和流域作为一个系统,按照山水林田湖草生命共同体的科学理念,对东湖进行整体性和系统性的保护与治理是十分必要的.      

藏东西藏岩的电气石

西藏岩是以发现地命名的一类新火成岩,相当于电英质火山岩。作为西藏岩最重要 造岩矿物之一的电气石,以其独特的产状、结构和构造等,显著有别于深成岩、交代岩和富电气石岩的电气石。所研究电气石产出的宏观、微观地质特征并借鉴富硼添加剂花岗岩体系 的岩石学实验资料表明,赋存在西藏岩内的电气石,大多数是由一类富硼和水的酸性岩浆- 热液过渡流体（但其总性质偏岩浆,即佤山岩浆）,因近地表和内压大于外压,引发在一个较大而有限空间内隐爆、爆破,骤然冷凝结晶,并同石英及其晶屑一起胶结爆破角砾;佤山岩浆溢出期间,电气石与石英可各自聚集结晶并形成韵律状“层纹”。     

扬子地块东段若干橄揽岩包体的温度－压力计算

采自江苏省六合地区的１１个尖晶石相橄榄岩包体，用电子探针和质子探针精确测定其中橄榄石和辉石的主量和微量成分，采用新近校准的适用于尖晶石相包体的橄榄石一单斜辉石地质压力计和二辉石地质温度计，计算了包体中共生矿物对的平衡温度和压力。其中ＢＭ８５计算的温度比ＢＫＮ９０的约低５０℃左右。而经Ｂｒｅｙ和Ｋｏｈｌｅｒ修改的ＢＭ８５温度计得到的结果与ＢＫＮ９０的几乎相同。假定压力为１．５ＧＰａ，用ＢＫＮ９０计算，１１个包体样品的温度范围为７２２℃～１１９３℃，它大体上反映了扬子地块东段大陆岩石圈地使尖晶石相部分的温度状态。橄榄石一单斜辉石地质压力计用于本样品组计算，仅部分样品获得合理的结果。由于该压力计自身的误差较大，尚不能精确确定尖晶石相橄榄岩的压力。     

东喜马拉雅构造结东、西边界断裂对比及其构造演化过程

为了查明东喜马拉雅构造结东、西边界断裂的关系,及其印度与欧亚板块碰撞以来东喜马拉雅构造结的构造演化过程.在综合野外填图、构造观察、代表性岩石的锆石LA-ICP-MS U-Pb测年分析及前人研究的基础上,对东构造结两条边界断裂的几何学、运动学特征进行对比,讨论了两条断裂的多期次、多阶段的变形特征,还探讨了在东构造结地区自印度板块-欧亚板块碰撞以来的演化历史.结果显示,东构造结两条边界断裂几何学和运动学非常相似,构造变形具有明显的同时代、同期次特点,共同经历从碰撞、持续俯冲-折返、直到后期垮塌-隆升等一系列重要的地质事件.      

Geomorphic evidence for active tectonic deformation in the coastal part of Eastern Black Sea,Eastern Pontides,Turkey

The Eastern Pontides (EP), which is the under transpressional deformation zone, is an active mountain belt that has been rising rapidly since the Cenozoic era because of the Arabian-Eurasian convergence. Morphometric studies have been performed to investigate the tectonic activity of this region and better understand the characteristics of the faults geomorphologically; the faults control the mountain fronts in the drainage basin of the EP. The results show the Hypsometric Curve (HC)-Hypsometric Integral (0.37-HI-0.67), Basin-Shaped Analysis (1.2-B_s-7), Valley-Floor-Width to Height-Ratio (0.4-V_f-1.2) and Asymmetry Factor (35-AF-81) applied to 46 drainage basins together with 9 tectonically controlled geomorphic indices (1.2-S_mf-1.5) and a Stream Length Gradient (30-S_L-120) indicate that the EP is tectonically active, and when the areas are evaluated according to S_mf and V_f analyses, the tectonic level is relatively high. According to our conceptual model for the uplifting of the EP, with respect to field studies and morphometric analysis, (i) the EP is the active deformation zone and has a “push-up” geometry in conjunction with the North Anatolian Fault; (ii) the EP is progressively uplifting at a rate of more than 0.5 mm/yr in along with the thrust faults of the Black Sea Fault (BSF) and Borjomi-Kazbegi Fault (BKF).     

Exhumation of the Tauern window,Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.     

塔东地区寒武—奥陶纪岩相古地理分析

通过钻井资料以及露头资料的详细分析,在塔东地区寒武系—奥陶系识别出盆地相、斜坡相、台地相。根据盆地相沉积特征把塔东地区盆地相分为碳酸盐岩盆地、混积盆地、页岩-硅岩盆地三种类型。碳酸盐岩盆地相岩石类型为碳酸盐岩,混积盆地岩石类型为碳酸盐岩与页岩及硅岩互层,页岩-硅岩盆地岩石类型为泥页岩以及硅质岩。台地区发育台地边缘灰泥丘相、台缘滩相、开阔台地相等。灰泥丘主要为藻凝块岩,滩相主要为颗粒灰岩。综合塔里木盆地寒武纪—奥陶纪沉积特征提出了塔东地区碳酸盐岩沉积综合模式,并指出由台地至盆地方向,盆地由碳酸盐岩盆地向混积盆地及页岩-硅岩盆地过渡。在单井分析、露头资料分析、地震剖面分析的基础上,分析了塔东地区寒武纪—奥陶纪古地理展布。     

Osumilite-bearing Granulites in the Eastern Grenville Province, Eastern Labrador, Canada: Mineral Parageneses and Metamorphic Conditions

Osumilite-bearing metasedimentary gneiss occurs in the contactaureole of the Sand Hill Big Pond gabbronorite complex of Labradorianage in eastern Grenville Province, eastern Labrador, Canada.The osumilite (Os) occurs in association with cordierite (Cd),orthopyroxene (Opx), sillimanite (Sil), sapphirine (Sa), spinel(Sp), K-feldspar (Kf), plagioclase (PI), phlogopite (Ph), hematite(Hm),magnetite (Mt), corundum (Co), and quartz (Q) in zones adjacentto the gabbronorite intrusion. The osumilite-in isogard is delineatedat a distance of 1-3km from the intrusive contact. The osumiliteis characterized by enhanced Mg/(Mg + Fe) ratios (0.89-0.92)and relatively high K₂O contents (4. 13-4.78 wt. % ) The compositionalvariation of the osumilite is best understood by the substitutions(Mg+Fe+Mn)+St=2A1 and (K+Na)+(Mg +Fe+Mn)=Al+vacancy. Symplectiticintergrowth of Opx-Cd-Kf-Q, which partly of completely replacedosumilite, is ubiquitously present. The stable osumilite-bearingassemblages (all with Hm, Mt, and P1) deduced from petrographicfeatures and from the phase relations in the KMAS system areOs-Sa-Cd, Os-Cd-Sil-Q, Os-Cd-Kf-Q, Os-Cd-Opx-Q, and Os-Opx-Kf-Q.The saphirine-bearing assemblages are restricted to silica-deficient(quartz-absent) zones of the gnesis, which include Sa-Os-Cd,Sa-Sil-Cd, Sa-Opx-Sp, Sa-Sp-Cd, and Sa-Sp-Co. Orthopyroxenecoronas mantling phologite reflect the breakdown reaction Ph+ Q= Opx+Kf+vapor under extremely low water activity in thevapour. Petrogenetic grids in MAS, KMAS, and KMAS-H₂O-CO₂appropriateto the mineral assemblages in the metasedimentary gneisses suggestthat the changes of the mineral assemblages in the area studiedreflect dehydration reactions Ph+Sil+Q=Cd+Kf+V and Ph+Q=Opx+Kf+V, and osumilite and sapphirine-forming reactions Opx+Cd+Kf+Q=Os,Cd+Kf+Q=Os+Sil+, Opx+Sil+Kf=Os+Sa, and Opx+Sil =Sa+ Cd. Relativelyhigh Mg/(Mg+Fe²⁺) (0. 64-0.88) in the whole-rock compositionand high oxygen fugacity (near hematite-magnetite buffer), togetherwith exceptionally high temperature ( 1000?C) and extremelylow water activity (0.2) at peak meta-morphic conditions mightstabilize the osumilite- and sapphirine-bearing assemblagesat middle or lower crustal levels. Relatively low water activityis probably caused by the relatively dry precursor, which hadbeen metamorphosed at upper amphibolite facies metamorphic conditionsbefore gabbronorite emplacement.     

Lake Van, Eastern Anatolia, Hydrochemistry and History

Saline, 450-m-deep Lake Van (Eastern Anatolia, Turkey) is, with 576 km³, the third largest closed lake on Earth and its largest soda lake. In 1989 and 1990, we investigated the hydrochemistry of the lake’s water column and of the tributary rivers. We also cored the Postglacial sediment column at various water depths. The sediment is varved throughout, allowing precise dating back to ca. 15 ka BP. Furthermore, lake terrace sediments provided a 606-year-long floating chronology of the Glacial high-stand of the lake dating to 21 cal. ka BP. The sediments were investigated for their general mineralogical composition, important geochemical parameters, and pore water chemistry as well. These data allow reconstructing the history of the lake level that has seen several regressions and transgressions since the high-stand at the end of the Last Glacial Maximum. Today, the lake is very alkaline, highly supersaturated with Ca-carbonate and has a salt content of about 22 g kg^?1. In summer, the warmer epilimnion is diluted with river water and forms a stable surface layer. Depth of winter mixing differs from year to year but during time of investigation the lake was oxygenated down to its bottom. In general, the lake is characterized by an Na–CO₃–Cl–(SO₄)-chemistry that evolved from the continuous loss of calcium as carbonate and magnesium in the form of Mg-silica-rich mineral phases. The Mg cycle is closely related to that of silica which in turn is governed by the production and dissolution of diatoms as the dominant phytoplankton species in Lake Van. In addition to Ca and Mg, a mass balance approach based on the recent lake chemistry and river influx suggests a fractional loss of potassium, sodium, sulfur, and carbon in comparison to chloride in the compositional history of Lake Van. Within the last 3 ka, minor lake level changes seem to control the frequency of deep water renewal, the depth of stratification, and the redox state of the hypolimnion. Former major regressions are marked by Mg-carbonate occurrences in the otherwise Ca-carbonate dominated sediment record. Pore water data suggest that, subsequent to the major regression culminating at 10.7 ka BP, a brine layer formed in the deep basin that existed for about 7 ka. Final overturn of the lake, triggered by the last major regression starting at about 3.5 ka BP, may partly account for the relative depletion in sulfur and carbon due to rapid loss of accumulated gases. An even stronger desiccation phase is proposed for the time span between about 20 and 15 ka BP following the LGM, during which major salts could have been lost by precipitation of Na-carbonates and Na-sulfates.     

Structural evolution of the Llanos foothills, Eastern Cordillera, Colombia

The Llanos foothills are located in the frontal thrust zone of the Eastern Cordillera in Colombia in a complex environment that BP has been exploring actively since 1988. This exploration has resulted in the discovery of several fields with a variety of hydrocarbon fluids (gas condensate and volatile oil) in very tight quartz-arenites. The structural style and complexity of this fold-and-thrust belt changes along the trend from single frontal structures to an imbricate of up to five thrust sheets in a triangle zone. In highly complex environments, the seismic image quality is poor, and interpretation becomes very challenging. The structural models of the area have evolved as more data have been acquired. The initial structural model required inversion of the basin at the end of the Andean orogeny. The structural style changed to an in-sequence imbricate thrust stack with very long, trailing back limbs that return to regional elevation and finalize in a tighter structures with short back limbs. The concept of early deformation and multiple phases has been introduced. Three main phases have been distinguished: (1) an early event during the deposition of the Lower Carbonera (39–29 Ma), with incipient structures formed to create syntectonic deposition; (2) a phase of steady subsidence that increased notably at the end of the period (29–7 Ma); and (3) the latest phase (7–0 Ma), when most deformation and uplifting occurred. The migration of hydrocarbons happened simultaneously with the deformation, and its final distribution, amount, and variation in composition is related to the structural evolution of the area.     

Morphotectonics of the Baikal rift valley,Eastern Siberia,USSR

The Baikal rift valley, the central segment of the Baikal rift zone located in southern East Siberia, consists of two large depressions separated by an interdepressional uplift. The thickness of the Neogene-Quaternary sediments filling in the depression amounts to 5 km (Logatchev and Florensov 1978). The interdepressional uplift consists of subsiding residual steps and active tilted horsts.The NW slope of the Baikal rift is controlled by a system of faults diverging to the N. This system comprises tectonic scarps (faceted ridge spurs), an inclined piedmont surface and a summit slope. The facets indicate the position of the main dip slip faults behind which longitudinal strike slip faults are distributed. Between the branching faults, the so-called intermediate steps are situated. Their subsidence and destruction result in expansion of the rift valley. Transformation of normal faults into listric faults is manifested in the tectonic topography in the areas of the residual and intermediate steps. The large dimensions of the Baikal rift valley are evidently due to its being confined to the faults striking NE-SW.     

Basalts of the Eastern Deccan Volcanic Province,India

The Mandla lobe in the eastern part of the Deccan volcanic province represents an isolated lava pile having a thickness of ∼900 m. The large thickness of this lava pile and its spatial detachment from the western Deccan outcrop points to a plausible second source. The stratigraphic configuration of the central and eastern Deccan lava sequences and their possible stratigraphic correlation are primarily based on geology and chemical signatures of the lava flows. Based on variations in the incompatible element ratios, the lava sequences of Chindwara, Jabalpur-Seoni and Jabalpur-Piparia sections were classified into four informal formations showing similarity with the southwestern formations. Major and trace element abundances in fifteen lava flows of Jabalpur area are similar to that of the southwestern Deccan lava flows. It has been found that the Ambenali Fm. and a few Khandala and Bushe Fm. flows are present in the northeastern Deccan. The regional mapping and detailed petrographic studies coupled with the lateral tracing have enabled the recognition of thirty-seven physically distinct lava flows and is justified by their major-elemental chemistry. The ‘intraflow variations’ studied in some of the flows is very low for most of the major oxides. These thirty-seven lava flows are grouped into eight chemical types. The order of superposition in this sequence reflects that the older flows occur in the west of the outlier at the Seoni-Jabalpur-Sahapura sector whereas, the younger flows are confined to the Dindori-Amarkantak sector in the east. The spatial disposition of the lava flows suggests that the structural complexity in the lava flow sequence in the Mandla lobe lies between Jabalpur and Dindori. The juxtaposition of distinct groups of lava flows are observed near Deori (flows 1 to 4 abeted aginst flows 5 to 14) and Dindori areas. At Dindori and towards its south the distinct lava packages (flows 15 to 27 and flows 28 to 37) are juxtaposed along the course of Narmada river. The possible explanation for this could be the presence of four post-Deccan faults at Nagapahar, Kundam, Deori and Dindori areas. The vertical shift of chemically distinct lava packages at different sectors in the outlier contravenes the idea of small regional dip and favours the presence of four NE-SW trending post-Deccan faults. Major geochemical breaks, when traced out from section to section, exhibit shifting in heights by approximately 150 m near Nagapahar and 300 m near Deori and Dindori areas. The field, petrographic and major-oxide data sets considered in conjuction with the magnetic chron reversal heights, support the inference that four faults trending NE-SW are present in the Mandla lobe.A commonality in the mineralo-chemical attributes of the infra (Lametas)-/inter-trappean as well as weathered Deccan basalt further favours their derivation from Deccan basalt, implying the availability of Deccan basalt during the Maastrichtian Lameta sedimentation. This observation does not match with the models suggesting an extremely short duration of Deccan volcanism (<0.5 Ma) at the KTB, but is congruent with the models advocating a more prolonged Deccan volcanism.     

The Albera zoned pegmatite field,Eastern Pyrenees,France

Summary Four types of pegmatites comprise the zoned pegmatite field in the eastern sector of the Albera Massif. Type I is represented by barren pegmatites with graphic textures; type II comprises transitional varieties with Li-Fe-Mn phosphates, Be (chrysoberyl) and scarce Nb-Ta and U oxide minerals; type III consists of pegmatites with significant zones of replacement containing Li-Fe-Mn phosphates, beryl and more abundant Nb-Ta oxide minerals; and type IV, muscovite-quartz-albite pegmatites are highly mineralized with Be, Nb-Ta and HREE. REE mineralization is strongly related to abundance of graphite in the late pegmatite units and in the host-rock. The individual pegmatite types are distributed within four subparallel zones concentric around anatectic muscovite-biotite leucogranites, with type I within the granites or close to the contact, and type IV pegmatites in the outermost areas. The zoning from type I to type IV could relate to fractionation processes which generated the pegmatites and is characterized by an enrichment of Mn, Ta, Na, Li, P, Be and REE. According to the pegmatite distribution and their fractionation trends, we propose an origin by differentiation of a granitic melt.

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Résumé On a établi quatre types de pegmatites dans le champ pegmatitique zoné du secteur est du Massif des Albères (Pyrénées Orientales, France). Celles de type I sont des pegmatites non minéralisées avec des textures graphiques, celles de type II sont des variétés intermediaires avec des phosphates à Li-Fe-Mn, Be (chrysobéryl) et des rares oxides à Nb-Ta et U; celles de type Ill sont des pegmatites avec des zones de réplacement bien dévéloppées et qui contiennent des phosphates à Li-Fe-Mn, du béryl et des oxides à Nb-Ta plus abondants; celles de type IV sont des pegmatites bien minéralisées à Be, Nb-Ta et des T.R. La minéralisation à T.R. est liée à des phénomènes de graphitisation répandus dans les unités tardives de la pegmatite et dans l'encaissant. La distribution de chaque type de pegmatite correspond à quatre zones à peu près parallèles et concentriques autour des granites anatectiques à muscovite-biotite, avec le type I dans les granites ou prochain au contact, et les pegmatites à type IV dans la bande plus externe. La zonation serait due à des processus de fractionnement qui auraient généré les pegmatites et qui sont caracterisés par un enrichissement en Mn, Ta, Na, Li, P, Be et T.R. dès les pegmatites de type I vers celles de type IV. On propose un origine par différentiation des granites en vue de la distribution des pegmatites.

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With 5 Figures     

A geological study of lake van,Eastern Turkey

Lake Van is a lake with a volume of 607 km³ and a maximum depth of 451 m in a tectonically active zone in eastern Anatolia. It may be divided into three physiographic provinces: lacustrine shelf, sublacustrine slope and lake basin. The shelf is largely made up of submerged fluvial valleys and alluvial plains, as evidenced by the local occurrence of karst-like topography, buried, leveed river valleys and sediment wedges interpretable as relict deltas. The shelf and basinal deposits are characterized by an alternating succession of well-stratified and chaotically reflecting layers consisting of turbidites and slump deposits respectively. This thick sequence of unconsolidated to consolidated sediments is underlain by a Paleozoic metamorphic or Upper Cretaceous limestone basement.Holocene lake level fluctuations are highly correlatable with solar activity. Heat budget estimates imply a high geothermal gradient in the Van region. Because of winter convection, chemical composition of the lake water is homogeneous throughout the lake. Though they bring in waters of varying compositions, the rivers discharge a larger quantity of bicarbonate than alkaline earth metals. This soda-chemistry may be attributed to postvolcanic CO₂-activity.The fine laminations of the sediments of Lake Van are interpreted as varves: a white carbonate layer is deposited in winter and a dark layer during the summer. Using these varves for age dating, the sedimentation rate during the Holocene may be determined. This varies between 40 and 90 cm/1000 years.The lake level was at its highest at 72 m above present at the height of the last ice age about 18,000 yr B. P. A dramatic drop to over 300 m below present occurred about 9500 yr B. P., with an equally dramatic rise around 6500 yr B. P. These sudden variations are attributed to the morphometric pattern of the lake, and they left distinctive imprints on the sedimentary, geochemical and pollen record.

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Zusammenfassung Der Van-See hat ein Volumen von 607 km³ und eine maximale Wassertiefe von 451 m. Er liegt in einer tektonisch aktiven Zone und ist von Vulkanen umgeben. Drei physiographische Provinzen lassen sich ausscheiden: Schelf, Seeabhang und Seebecken. Der Schelf besteht weitgehend aus ehemaligen Flußtälern und alluvialen Ebenen mit ausgeprägter karstartiger Topographie, Deltaschüttungen und mäandrierenden Flußtälern. Schelf- und Beckensedimente setzen sich zusammen aus unverfestigten wechselnden Abfolgen von gut geschichteten und chaotischen Lagen, die ihrerseits von paläozoischen Metamorphiten und Oberkretazischen Kalken unterlagert sind.Rezente Seespiegelschwankungen sind mit Sonnenfleckenzyklen korrelierbar. Wärmekapazitätsberechnungen für das gesamte Wasservolumen sind nur durch die Annahme eines hohen geothermischen Wärmeflusses in der Van-Region zu deuten. Bedingt durch Winterkonvektion wird die Wassersäule gut durchmischt und chemisch einheitlich. Der hohe Sodagehalt des Sees ist aus postvulkanischen CO₂-Emissionen abzuleiten.Sedimentkerne umfassen das gesamte Holozän, und eine ausgeprägte Warvenschichtung erlaubt eine genaue Datierung der Profile. Die Sedimentationsrate schwankt zwischen 40 und 90 cm pro 1000 Jahre.Der Seespiegel hatte seinen höchsten Stand mit 72 m über dem heutigen vor ca. 18 000 Jahren, also dem klimatischen Minimum der Weichseleiszeit. An der Wende Pleistozän-Holozän erfolgte eine schnelle Absenkung des Spiegels um ca. 300 m ( 9500 Jahre v. d. Gegenwart), um 3000 Jahre später, während des Atlantikums, in wenigen Jahrhunderten um 200 m wieder anzusteigen. Diese plötzlichen Ereignisse sind Ausdruck des morphometrischen Musters des Sees in Verbindung mit klimatischen Veränderungen im Gefolge deletzten Eiszeit. Die Sedimentfazies spiegelt diese Geschehnisse durch charakteristische sedimentologische, paläontologische oder geochemische Indikatoren wider.

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Résumé Le Lac de Van a un volume de 607 km³ et une profondeur maximale de 451 m. Il est situé au milieu de volcans dans une zone tectoniquement active en Anatolie orientale. Trois zones physiographiques peuvent être distinguées: le shelf, le versant et le bassin du lac. Le shelf consiste pour la plus grande partie en anciennes vallées fluviales et en plaines alluviales avec topographie karstique bien marquée, en vallées fluviales enterrées et remblayées et des coins sédimentaires, qui peuvent être interprétés comme des débris deltaïques. Les dépôts sédimentaires du shelf et du bassin se caractérisent par des alternances de couches, les unes bien stratifiées, les autres chaotiques, faites de turbidites et de dépôts formés à la suite de glissements, au-dessous desquelles se trouvent des métamorphites paléozoïques et des calcaires formés au cours du Crétacé supérieur.Des oscillations récentes du niveau du lac peuvent être mises en relation avec des cycles de taches solaires. Des calculs de la capacité thermique supposent un gradient géothermique élevé dans la région de Van. A cause de la convection hivernale, la composition chimique de l'eau du lac est homogène dans l'ensemble du lac. Bien que les fleuves apportent des eaux de compositions variables, ils transportent une quantité plus grande de bicarbonate que les métaux alcalins. Ce chimisme sodique peut être attribué à une activité post-volcanique de l'anhydride carbonique.Les fines laminations des sédiments du Lac de Van sont interprétées comme varves: une couche carbonatique blanche est déposée en hiver, et une couche sombre en été. En utilisant ces varves comme moyen de datation on peut déterminer le taux de sédimentation prédominant au cours de l'Holocène. Celui-ci varie entre 40 et 90 cm par millénaire.Le plus haut niveau du lac s'est trouvé à 72 m au-dessus du niveau actuel, à l'apogée de la dernière glaciation qui eut lieu, il y a environ 18 000 ans. Un abaissement rapide à 300 m au-dessous du niveau actuel survint il y a 9500 ans, suivi d'une remontée aussi rapide il y a 6500 ans. Ces oscillations brusques sont attribuées à la condition morphométrique du lac en relation avec les changements climatiques consécutifs à la dernière glaciation. Elles ont laissé des empreintes caractéristiques sur les faciès sédimentaire, géochimique et pollinique.

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Metamorphic rocks of the Araya Peninsula,Eastern Venezuela

Two grades of metamorphism, both subfacies of the greenschist facies of regional metamorphism, were mapped on the Araya Peninsula: 1. the quartzalbite-epidote-almandine subfacies, consisting mainly of a sequence of garnet and kyanite quartz-mica schists, interlayered with quartzites; and 2. the quartzalbite-muscovite-chlorite subfacies, which consists of chloritic phyllites, quartzmica schists and phyllites, metaconglomerates, calcareous quartz-mica schists, limestones and marbles, and calcareous epidote schists of volcanic origin. The two subfacies are separated by faults. The probable age of these rocks ranges from Triassic(?)-Jurassic to Lower and Middle Cretaceous. Serpentinites intrude rocks of the lower metamorphic grade, and are interpreted as tectonically emplaced. The foliation of these rocks is highly folded by mesoscopic folds, whose axes trend east-northeast and which are overturned to the south-southeast. Lineations parallel to the fold axes and thrusting to the south-southeast are common. These structures reflect a macroscopic structure of antiforms and synforms, all affected by a dominant north-northwest to south-southeast tectonic transport. A system of high-angle or vertical longitudinal faults crosses the peninsula from west-southwest to east-northeast. Evidence of recent strike-slip movement was found, although vertical movement has also been important along these faults. Two tectonic styles exist in the metamorphic rocks. The older one is characterized by compression and thrusting in a south-southeast direction. The younger one is represented by longitudinal faults oriented in an east-northeast direction, with vertical and strike-slip movement. They probably reflect the eastward movement of the Caribbean region with respect to South America.     

Tourmalinites from the Eastern Sierras Pampeanas, Argentina

In the eastern Sierras Pampeanas, Central Argentina, tourmalinites and coticules are found in close association with stratabound scheelite deposits in metamorphic terranes. In Sierra Grande (Agua de Ramón and Ambul districts) and Sierra de Altautina, tourmalinites are associated with stratabound scheelite deposits related to orthoamphibolites. In the Pampa del Tamboreo area, tourmalinites are located in biotite schists stratigraphically related to acid to intermediate metavolcanic rocks and scheelite-bearing quartzites.The mineral chemistry and boron isotopic compositions of tourmalinite-hosted and vein-hosted tourmalines are investigated. Overall, the tourmalines belong to the dravite-schorl series and are generally aluminous; Fe/(Fe+Mg) ranges from 0.33 to 0.85, Al/(Al+Fe+Mg) from 0.66 to 0.76 and the amount of X-site vacancy (0.12–0.48) indicates significant foitite components. Their boron isotopic compositions (δ¹¹B) are from −24.0‰ to−15.0‰.Similar mineral chemistries and boron isotopic values for tourmaline in tourmalinites related to stratabound scheelite mineralisation and in tungsten-bearing quartz veins suggest a common source for the boron and probably the tungsten. The field, chemical and isotopic relationships are consistent with tungsten and boron in quartz-vein deposits being remobilised from stratabound scheelite and tourmalinite, dominantly by liquid-state transfer associated with regional shear zones. Tungsten and boron in the original sedimentary sequence (now meta-exhalites) are ascribed to volcanogenic exhalations.     

A-type volcanics in Central Eastern Sinai, Egypt

Alkaline rhyolitic and minor trachytic volcanics were erupted 580–530 Ma ago. They occur with their A-type intrusive equivalents in Sinai, southern Negev and southwestern Jordan. At Taba-Nuweiba district, these volcanics outcrop in three areas, namely, Wadi El-Mahash, Wadi Khileifiya and Gebel El-Homra. Mineralogically, they comprise alkali feldspars, iron-rich biotite and arfvedsonite together with rare ferro-eckermannite. Geochemically, the older rhyolitic volcanics are highly evolved, enriched in HFSE including REE and depleted in Ca, Mg, Sr and Eu. The rhyolitic rocks of Wadi El-Mahash and Gebel El-Homra are enriched in K₂O content (5.3–10.1 wt.%) and depleted in Na₂O content (0.08–2.97 wt.%), while the rhyolites of Wadi Khileifiya have normal contents of alkalis. Their REE patterns are uniform, parallel to subparallel, fractionated [(La/Yb)_n = 5.4] and show prominent negative Eu-anomalies. They are classified as alkali rhyolites with minor comendites. The younger volcanics are classified as trachyandesite and quartz trachyte (56.6–62.9 wt.% SiO₂). Both older and younger volcanics represent two separate magmatic suites. The overall mineralogical and chemical characteristics of these volcanics are consistent with within plate tectonic setting. It is suggested that partial melting of crustal rocks yielded the source magma. Lithospheric extension and crustal rupture occurred prior to the eruption of these volcanics. The rather thin continental crust (35 km) as well as the continental upheaval and extensive erosion that preceded their emplacement favoured pressure release and increasing mantle contribution. The volatiles of the upper mantle were important agents for heat transfer, and sufficient for the anatexis of the crustal rocks. A petrogenetic hypothesis is proposed for the genesis of the recorded potassic and ultrapotassic rhyolitic rocks through the action of dissolved volatiles and their accumulation in the uppermost part of the magma chamber.