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311.
Geological mapping, petrography, geochemistry, and isotope studies enable the division of the Pelotas Batholith into six granitic suites: Pinheiro Machado (PMS), Erval (ES), Viamão (VS), Encruzilhada do Sul (ESS), Cordilheira (CS), and Dom Feliciano (DFS). The rocks of the PMS show a large compositional range (granite through granodiorite to tonalite), and the suite is considered pre- to syncollisional. Other suites show restricted compositional variations (granite to granodiorite) and are late to postcollisional. In general, the suites are metaluminous to slightly peraluminous (PMS, ES, and VS) or peraluminous (CS) or have alkaline tendencies (ESS and DFS). The magmatic evolution corresponds to high-K calc-alkaline to alkaline magmatism. The suites are enriched in K, Rb, and REE compared with rocks of typical calc-alkaline series. Initial 87Sr/86Sr ratios vary from 0.705 to 0.716, except in the CS, where they attain values of 0.732–0.740. Sm–Nd TDM model ages vary between 0.98 and 2.0 Ga, with initial εNd values ranging from −0.3 to −10. U–Pb zircon dates of samples from PMS, VS, and ESS suggest an age between 0.63 and 0.59 Ga for magmatism. Rb–Sr dates of samples of alkaline granites from DFS present ages between 0.57 and 0.55 Ga. The main tectonic controls of the magmatism of the Pelotas Batholith are high-dip sinistral shear zones. 相似文献
312.
Astolfo G.M. Araujo Walter A. Neves Luís B. Pil Joo Paulo V. Atui 《Quaternary Research》2005,64(3):298-307
An overview of the archaeological data produced over the last decades for Brazil, coupled with a background of recent paleoenvironmental studies, suggests that during the mid-Holocene vast areas of Central Brazil ceased to be occupied by human groups. Independent data from dated human skeletons, rock-shelter stratigraphy, and chronology of open-air sites converge to support the inference that these areas were depopulated or altogether abandoned. Paleoenvironmental data suggest that dryness events constitute the major cause behind the observed trends. This phenomenon expands the already perceived notion that climatic stresses had a major role in the shaping of human settlement patterns in marginal environments, such as deserts and high-altitude settings. 相似文献
313.
Michael Holz Juliano Küchle Ruy Paulo Philipp Alan Patrick Bischoff Naoki Arima 《Journal of South American Earth Sciences》2006,22(3-4):185
The Lower Permian sedimentary succession of the Paraná Basin in southernmost Brazil has an overall transgressive sedimentation regime, recorded by a clear retrogradation of the facies belt. However, important depositional strike-orientated variations and regional inversions occur in the sedimentation regime along the paleo-shoreline (i.e., along-strike) of the basin. At the regional scale, a huge source area was uplifted by the end of the Artinskian in the north and caused regression; the southern part of the study area increasingly was transgressed by the epicontinental sea (= regional inversion). This important tectonic overprint on the stratigraphic signature of the basin’s infill has a tectonic origin. The variable sedimentation regime along the paleo-shoreline is controlled by the structural framework of the basement, which is formed by several crustal blocks with different responses to tectonic strain induced by terrain accretion on the occidental margin of Gondwana during the Permian. Stratigraphic data indicate that during the Early Permian, there were at least two differential subsidence and uplift events, one by the end of the Sakmarian–Artinskian and another during the Late Artinskian–Kungurian. 相似文献
314.
Major Ion Chemistry in a Freshwater Coastal Lagoon from Southern Brazil (Mangueira Lagoon): Influence of Groundwater Inputs 总被引:1,自引:0,他引:1
Isaac R. Santos Maria I. Machado Luis F. Niencheski William Burnett Idel B. Milani Carlos F. F. Andrade Richard N. Peterson Jeffrey Chanton Paulo Baisch 《Aquatic Geochemistry》2008,14(2):133-146
This paper characterizes major ion distributions and investigates whether groundwater exerts a major control on the chemical
functioning of Mangueira Lagoon, a large (90 km long), shallow (∼4–5 m deep), and fresh coastal lagoon in southern Brazil.
Water volumes equivalent to ∼80% of the total annual input are used in the summer for irrigating nearby rice plantations,
the most important regional economic activity. While Na+ and Cl− are the major ions in local groundwater, Na+ and HCO3− are the most enriched ions in lagoon water. The ion concentrations measured in Mangueira Lagoon were homogeneous, except
for a few samples affected by rainwater and groundwater inputs. A shore-normal transect starting at the pump house of a rice
irrigation canal indicated strong groundwater input at this canal. In spite of the small volume contribution (∼2% of precipitation),
groundwater discharge accounts for 50–70% of major ion inputs into the lagoon, with ∼70% of the groundwater inputs being anthropogenically
derived (e.g., from the rice irrigation canals). This may have serious implications for the management of the coastal water
resources from Mangueira Lagoon and other similar areas as groundwater associated with agricultural systems may be contaminated
by fertilizers and pesticides. The results imply that groundwater should not be neglected in dissolved species’ budgets even
when its volume contribution is small. 相似文献
315.
GPS Solutions - In addition to traditional tide gauges, the ground-based global navigation satellite system reflectometry (GNSS-R) that utilizes signal-to-noise ratio data from a single GNSS... 相似文献
316.
317.
Predicted changes in climate will lead to seawater intrusion in the Querença-Silves (QS) coastal aquifer (south Portugal) during the coming century if the current water-resource-management strategy is maintained. As for much of the Mediterranean, average rainfall is predicted to decrease along with increasing seasonal and inter-annual variability and there is a need to understand how these changes will affect the sustainable use of groundwater resources. A density-coupled flow and transport model of the QS was used to simulate an ensemble of climate, water-use and adaptation scenarios from 2010 to 2099 taking into account intra- and inter-annual variability in recharge and groundwater use. By considering several climate models, bias correction and recharge calculation methods, a degree of uncertainty was included. Changes in rainfall regimes will have an immediate effect on groundwater discharge; however, the effect on saltwater intrusion is attenuated by the freshwater–saltwater interfaces’ comparatively slow rate of movement. Comparing the effects of adaptation measures demonstrates that the extent of intrusion in the QS is controlled by the long-term water budget, as the effectiveness of both demand and supply oriented measures is proportional to the change in water budget, and that to maintain the current position, average groundwater discharge should be in the order of 50 × 106 m3 yr?1. 相似文献
318.
胶东苏鲁地体范家埠金矿成矿作用与矿床成因浅析:兼与胶北地体金矿对比 总被引:2,自引:0,他引:2
范家埠金矿是胶东半岛苏鲁地体内规模最大的石英脉型金矿。含金石英脉中锆石阴极发光和LA-ICP-MS微量元素及U-Pb同位素分析结果表明,含金石英脉中的锆石均为捕获围岩(新元古代威海片麻状花岗岩)的岩浆锆石,其中15个颗粒在U-Pb图解上位于谐和线上,其206Pb/238U加权平均年龄为(726±14)Ma(MSWD=4.6);另外7颗锆石位于不一致线上,其上交点年龄为(758±25)Ma,下交点年龄为(123±11)Ma(MSWD=0.45)。这些锆石的下交点年龄与金矿脉旁侧斜闪煌斑岩脉的角闪石40Ar/39Ar年龄分别为(118.8±1.6)Ma和(117.5±1.5)Ma,在误差范围内一致,表明范家埠金矿的成矿时代为120Ma左右。由此认为,苏鲁地体和胶北地体的金矿成矿作用均发生于早白垩世晚期120Ma前后,与目前多数人认为的华北克拉通岩石圈减薄的峰期时间一致,表明苏鲁地体的金矿成矿受岩石圈减薄的构造背景控制。范家埠金矿床成矿流体具中高温、中高盐度15.5%~23.2%(NaCleq)、低δ18OH2O(-3.84‰~-4.05‰)和低δDH2O(-82.5‰~-80.8‰)值等特点,载金矿物黄铁矿富32S(δ34S=-5.5‰~-9.1‰),与含油气盆地热卤水的组成相近,暗示范家埠金矿的成矿流体可能来源于中生代胶莱盆地。这种盆地流体沿断裂——五莲—米山深大断裂及其次级构造迁移,同时萃取基底岩石中的金形成含矿热液并在还原的环境中成矿。相反,胶北地体大量金矿床成矿流体具有低盐度、高δ18OH2O和δDH2O,δ34S(‰)为正值等特点。胶北和苏鲁地体早白垩世金成矿强度和成矿作用的差异可能反映了两者在地壳基底、流体来源和成矿环境等方面的不同。 相似文献
319.
氧化锰矿是我国仅次于沉积型碳酸锰矿的重要锰矿床类型,主要分布在广西、湖南、广东和福建等南方省区,但由于定年手段的限制,迄今对我国氧化锰矿形成时代的了解还极其有限.作者近几年利用激光阶段加热40Ar/39Ar同位素分析技术对华南地区典型氧化锰矿进行了系统和精确的年龄测定,获得了一大批可靠的成矿年龄,年龄值介于23.64 ± 0.29 Ma和0.48 ± 0.17 Ma (2σ),表明华南氧化锰矿的大规模成矿至少在渐新世末-中新世初就已开始并一直持续到更新世末期.基于多数氧化锰矿的上部矿石已被剥蚀或开采殆尽这一事实,可将华南氧化锰矿的成矿时间上溯到渐新世早期甚至始新世末期.研究发现,氧化锰矿的保存程度和年龄分布明显受新构造运动的影响;如果剔除后期剥蚀程度和采样系统性的影响,可以认为华南氧化锰矿的形成在区域上具有同时性.本次研究还说明,次生氧化锰矿的成矿是一个极其漫长的过程,形成一个具有工业意义的氧化锰矿可能需要上千万年的时间;这明显有别于多数内生热液矿床的成矿.本项研究为我国氧化锰矿以及其他类型的风化矿床(如红土型金矿和红土型镍矿)的年代学研究提供了新的思路和途径. 相似文献