Geology and K-Ar dating of the Tuxtla Volcanic Field,Veracruz, Mexico

The Tuxtla Volcanic Field (TVF) is located on the coast of the Gulf of Mexico in the southern part of the state of Veracruz, Mexico. Volcanism began about 7 my ago, in the Late Miocene, and continued to recent times with historical eruptions in ad 1664 and 1793. The oldest rocks occur as highly eroded remnants of lava flows in the area surrounding the historically active cone of San Martín Tuxtla. Between about 3 and 1 my ago, four large composite volcanoes were built in the eastern part of the area. Rocks from these structures are hydrothermally altered and covered with lateritic soils, and their northern slopes show extensive erosional dissection that has widened preexisting craters to form erosional calderas. The eastern volcanoes are composed of alkali basalts, hawaiites, mugearites, and benmoreites, with less common calc-alkaline basaltic andesites and andesites. In the western part of the area, San Martín Tuxtla Volcano and its over 250 satellite cinder cones and maars produced about 120 km³ of lava over the last 0.8 my. A ridge of flank cinder cones blocked drainage to the north to form Laguna Catemaco. Lavas erupted from San Martín and its flank vents are restricted to compositions between basanite and alkali basalt. The alignment of major volcanoes and flank vents along a N55°W trend suggests an extensional stress field in the crust with a minimum compressional stress orientation of N35° E. In total, about 800 km³ of lava has been erupted in the TVF in the last 7 my. This gives a magma output rate of about 0.1 km³/1000 year, a value smaller than most composite cones, but similar to cinder cone fields that occur in central Mexico. Individual eruptions over the last 5000 years had volumes on the order of 0.1km³, with average recurrence intervals of 600 years. The alkaline compositions of the TVF lavas contrast markedly with the calc-alkaline compositions erupted in the subduction-related Mexican Volcanic Belt to the west, leading previous workers to suggest that the TVF is not related to subduction. Trace-element signatures of TVF lavas indicate, however, that they are probably related to subduction. We suggest that the alkaline character of the TVF lavas is the result of low degrees of melting of a mantle source coupled with a stress regime that allows these small-volume melts to reach the surface in the TVF.     

Dynamics of cracking and swelling clay soils: Displacement of skeletal grains,optimum depth of slickensides,and rate of intra-pedonic turbation

Grumusols (Vertisols), though having a uniformly high clay content with depth, commonly exhibit a coarsening upward in the sand fraction, either of skeletal grains or of nodules. The non-uniform way in which the wetting front advances in swelling clay soils, with preferred advance and swelling around the coarse grains, produces upward directional forces and uplifting of the grains, thus producing the observed textural differentiation in the coarse fraction. From observations of slickenside distribution in the profile, and using published data on the ratio of lateral to vertical stresses and swelling pressure distribution with depth, a concept of optimum depth for slickensides is developed. Generally at 150–200cm, below the depth of cracking, this depth depends in part on climatic conditions. Below the optimal depth increased overburden pressure and smaller moisture variations restrain the extent of swelling and deformation, above it drying produces cracks and obliterates large slickensides, though a strong bimasepic microfabric remains. The extent of intrapedonic turbation was estimated from measurements of the volume of cracks and of surface material falling into the cracks. It indicates a turnover time of several hundred to a few thousand of years. This agrees with the known increase of radiocarbon ages of organic matter (MRT) with depth which in Vertisols (Grumusols) is only slightly slower than in non-turbating Udolls (Chernozems) and Udalfs (Lessivés). The moderate rate of homogenization due to intrapedonic turbation does not prevent the development of a normal organic carbon profile and it is slower than the upswelling of coarse grains by upward directional forces. The development of shear planes and slickenside structures is a rapid process and not dependent on turbation.     

Quaternary glaciations in the Verkhoyansk Mountains, Northeast Siberia

Geomorphological mapping revealed five terminal moraines in the central Verkhoyansk Mountains. The youngest terminal moraine (I) was formed at least 50 ka ago according to new IRSL (infrared optically stimulated luminescence) dates. Older terminal moraines in the western foreland of the mountains are much more extensive in size. Although the smallest of these older moraines, moraine II, has not been dated, moraine III is 80 to 90 ka, moraine IV is 100 to 120 ka, and the outermost moraine V was deposited around 135 ka. This glaciation history is comparable to that of the Barents and Kara ice sheet and partly to that of the Polar Ural Mountains regarding the timing of the glaciations. However, no glaciation occurred during the global last glacial maximum (MIS 2). Based on cirque orientation and different glacier extent on the eastern and western flanks of the Verkhoyansk Mountains, local glaciations are mainly controlled by moisture transport from the west across the Eurasian continent. Thus glaciations in the Verkhoyansk Mountains not only express local climate changes but also are strongly influenced by the extent of the Eurasian ice sheets.     

Characterization of wrench tectonics from dating of syn- to post-magmatism in the north-western French Massif Central

This work establishes the relative timing of pluton emplacement and regional deformation from new dating and structural data. (1) Monazite and (2) zircon dating show Tournaisian ages for the Guéret granites [Aulon granite 352 ± 5 Ma (1), 351 ± 5 Ma (2) and Villatange tonalite 353 ± 6 Ma (1)] and Viseo-Namurian ages for the north Millevaches granites [Chavanat granite 336 ± 4 Ma (1), Goutelle granite 336 ± 3 Ma (1), Royère granite 323 ± 2 Ma (1) and 328 ± 6 Ma (2), Courcelles granite 318 ± 3 Ma (1)]. The Guéret and Millevaches granites are separated by the N110 Arrènes–la Courtine Shear Zone (ACSZ), composed from West to East by the Arrènes Fault (AF), the North Millevaches Shear Zone (NMSZ) and the la Courtine Shear Zone (CSZ), respectively. Tournaisian Guéret granites experienced a non-coaxial dextral shearing (NMSZ) recorded by the Villatange granite while the Aulon granite (Guéret granite) cuts across this dextral shear zone which thus stopped shearing during Tournaisian time. Visean to Namurian Millevaches granites experienced a coaxial deformation. Therefore, low displacements along the NMSZ and the CSZ occurred at the emplacement time of Chavanat and Pontarion-Royère granites (336–323 Ma). The structural analyses of Goutelle granite emphasizes a deformation related to the dextral Creuse Fault System (CFS) oriented N150–N160. From 360 to 300 Ma, the Z strain axis is always horizontal inferring a wrench setting for these granite emplacements. During this tectonic evolution, the Argentat zone acted as a minor normal fault and is related with a local Middle Visean (340–335 Ma) syn-orogenic extension on the western border of the Millevaches massif.     

准噶尔卡拉麦里奥塔乌克尔希花岗岩体LA-ICP MS锆石U-Pb年龄与地球化学特征

新疆东准噶尔卡拉麦里构造带分布多种类型花岗岩,主要包括花岗闪长岩、二长花岗岩和钾长花岗岩.对处于卡拉麦里主断裂带西侧的奥塔乌克尔希花岗闪长岩体进行年代学和地球化学特征研究,岩体SiO2含量中等,相对富含钠,A/CNK在0.93~0.99,具准铝质中等钙碱性岩系特征.其轻稀土元素富集,具微弱铕负异常,明显富集Rb,Sr,Th等大离子亲石元素和Zr,Hf等高场强元素,强烈亏损元素Nb,Ta,Ti,为I型花岗岩.锆石LA-ICP MS测年为(329.1±1.3) Ma和(305.7±1.5) Ma,分属于早石炭世维宪期和晚石炭世卡西莫夫期,岩体定位于晚石炭世晚期.主微量及稀土元素综合分析表明该岩体具复杂成因,早石炭世晚期卡拉麦里地区洋壳消减,造山作用形成新生陆壳物质,晚石炭世后碰撞构造阶段地幔底侵岩浆引起地壳物质的部分熔融形成了奥塔乌克尔希岩体.综合研究表明,东准噶尔洋盆闭合发生在早石炭世维宪期,晚石炭世晚期准噶尔地区东部已进入新沉积盆地构造发育时期.     

四川大梁子铅锌矿床单颗粒闪锌矿铷-锶测年及地质意义

大梁子铅锌矿床位于扬子地台西南缘、甘洛-小江深大断裂带西缘,是中国川滇黔地区重要的大型铅锌矿床之一,其金属储量为 180 万吨(Pb Zn),平均品位为11.45%.铅锌矿体产于灯影组顶部,严格受近南北向断层控制.本文首次采用超低本底单颗粒闪锌矿 Rb-Sr 同位素测年方法,测得大梁子铅锌矿床成矿年龄为(366.3±7.7)Ma,代表了矿床的主成矿阶段年龄.通过地球动力背景探讨,认为该矿床的形成可能与晚加里东期扬子地台西缘构造活动有关.锶同位素初始比值测试结果表明大梁子铅锌矿床成矿物质来源可能主要源自围岩碳酸盐岩或基底地层.     

长江中下游金牛盆地花岗斑岩和流纹斑岩的锆石U-Pb年龄、Hf同位素组成及其地质意义

金牛盆地位于鄂东南矿集区,是长江中下游成矿带重要的火山岩盆地之一.相对宁芜和庐枞盆地,金牛盆地找矿一直没有取得重大突破,对其中次火山岩的年代学、成因和成矿属性的研究也很少.本次选择盆地中心吴佰浩地区深部钻孔中的花岗斑岩和流纹斑岩,开展锆石U-Pb年龄和Hf同位素的研究工作,获得了流纹斑岩锆石206Pb/238U的加权平均年龄为128±1Ma (n=14,MSWD=2.5),锆石Hf同位素(176Hf/177Hf),和εHf(t)分别为0.28247～0.28262和-2.5～ -7.7;花岗斑岩锆石206 Pb/238U的加权平均年龄为129±1Ma (n=12,MSWD=1.3),锆石Hf同位素(176Hf/177Hf),和εHf(t)分别为0.28239～0.28259和-3.6～-10.7.本文确定金牛盆地中流纹斑岩和花岗斑岩的形成时代为129～128Ma,与金牛盆地火山岩的形成时代(130～125Ma)相当,暗示金牛盆地火山岩和次火山岩均形成于早白垩世,流纹斑岩和花岗斑岩是壳幔混合的产物.金牛盆地火山岩的形成时代与宁芜、、庐枞盆地相当,次火山岩的形成时代也与宁芜、庐枞盆地玢岩铁矿的成矿岩体相当.提出金牛盆地除应关注玢岩铁矿外,更应该关注与次火山岩有关的热液金矿.     

渭河陇西段第七级阶地年代的确定及其构造意义

昆黄运动是发生在中更新世时期青藏高原及其邻区一次重要的构造抬升事件,河流阶地及地层记录能够较好地反映这次构造事件。渭河陇西段第七级阶地沉积了104.5m厚的黄土,通过对其上覆黄土剖面的古地磁、粒度研究表明,此级阶地形成年代为距今870ka,阶地拔河高度说明自中更新世以来地面至少抬升了205m,其抬升速率约为0.2m/ka。这次构造事件在时间上与昆黄运动相一致,是对青藏高原强烈抬升的响应。     

沥青Re-Os同位素年龄地质意义解读

Re、Os亲有机质的特性使得Re-Os同位素在富有机质地质样品中得到了广泛应用;沥青由大量重组分有机质组成,对Re、Os具有较强的富集能力,并且沥青样品中Re-Os同位素封闭性较好,因此沥青Re-Os同位素已经成为当今国际上研究的一个热点。然而,沥青Re-Os同位素年龄究竟是代表烃源岩的沉积时代、油气成藏时代、还是油气藏破坏后油气运移时代,至今还没有明确的定论。本文对新疆乌尔禾沥青矿、四川矿山梁沥青矿样品进行了Re-Os同位素分析,结果显示,乌尔禾沥青矿Re-Os同位素年龄为269±9Ma,老于周围白垩系砂岩地层沉积时代,Os同位素初始比值¹⁸⁷Os/¹⁸⁸Os（Os_i）为0.367±0.028,小于二叠纪海水的Os_i同位素比值,表明得到的乌尔禾沥青Re-Os同位素年龄反映的是乌尔禾油气藏的成藏时代。矿山梁沥青矿Re-Os同位素年龄为537.3±5.2Ma,老于围岩（寒武系、奥陶系、志留系地层）沉积时代,Os同位素初始比值为0.716±0.028,与早寒武世海水的Os_i同位素比值一致,表明得到的矿山梁沥青Re-Os同位素年龄代表烃源岩的沉积时代。结合前人发表的沥青Re-Os同位素数据,沥青Re-Os同位素等时线年龄可能反映烃源岩的沉积时代,也可能代表油气藏的形成时代,这主要取决于油气成藏过程中,具有不同Re-Os同位素特征的原油是否能够混合均匀。沥青Re-Os同位素年龄能否反映油气藏遭受破坏而发生运移的时代,主要取决于沥青的热演化程度以及油气藏破坏过程中原油的混合程度。沥青Re-Os同位素研究不仅能够为油气成藏时代的厘定提供了一种有力的工具,而且对于油气烃源岩的判别以及油气成因研究都具有十分重要的意义。

     

Thermoluminescence Dating of Alluvial Sequences in Coastal Valleys of Southern New South Wales: Problems and potential

Because of its potential for establishing chronologies far beyond the range of C¹⁴ thermoluminescence (TL) dating has made a significant contribution to the study of the Quaternary history of many Australian landscapes. But, as the reliability of the technique requires the removal by sunlight of any residual TL from quartz grains during transport, inadequate bleaching may yield ages for depositional events that are too old. Inadequate bleaching often can be detected by the shape of curves showing the ratio of natural TL vs laboratory-induced TL with increasing temperature. We use this technique here, together with C 14 dating and pedogenic evidence, to assess the reliability of TL determinations for alluvium in the valleys of the Clyde River and Termeil Creek. The Pleistocene TL ages from these valleys seem reliable, but Holocene dates do not. However, we demonstrate that, even where inadequate bleaching is demonstrable, TL analysis can still yield important insight to depositional processes.