庐枞盆地侵入岩的时空格架及其对成矿的制约

庐枞中生代火山盆地位于长江中下游断陷带内,地处扬子板块的北缘。庐枞盆地内火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山四组火山岩以及34个侵入岩体。本次工作在详细野外地质调查的基础上,结合作者已有的5个岩体(巴家滩岩体、城山岩体、花山岩体、黄梅尖岩体和枞阳岩体)年代学的研究工作,系统开展了盆地侵入岩体的年代学和时空分布特征研究。通过侵入岩锆石LA-ICP-MSU-Pb定年方法,本次研究确定庐枞盆地内15个主要侵入岩体的成岩时代分别为:黄屯岩体134.4±2.2Ma、岳山岩体132.7±1.5Ma、拔茅山岩体132.7±1.9Ma、尖山岩体132.0±1.3Ma、谢瓦泥岩体131.6±1.1Ma、龙桥岩体131.1±1.5Ma、焦冲岩体129.6±1.3Ma、土地山岩体127.4±2.8Ma、凤凰山岩体128.4±0.9Ma、罗岭岩体126.3±2.0Ma、龙王尖岩体126.5±1.5Ma、小岭岩体126.2±1.8Ma、大缸窑岩体125.9±1.3Ma、巴坛岩体125.1±1.1Ma、毛王庙岩体123.9±1.9Ma。从而得出庐枞盆地内侵入岩的成岩时代界于134～123Ma之间,属于早白垩世,并进一步将庐枞盆地内侵入岩划分成2期,其中早期侵入岩主要为二长岩和闪长岩类,主要分布在盆地北部,与龙门院旋回和砖桥旋回火山活动关系密切,岩体侵位受火山机构和北东向构造联合控制,成岩时代为134～130Ma;晚期侵入岩还可分为两类,第一类主要为正长岩类,分布在盆地南部,主要受盆地内火山机构和北北东向断裂控制,侵入活动与双庙旋回和浮山旋回火山岩浆活动相对应,成岩时代为129～123Ma;第二类主要为A型花岗岩,分布于盆地东南缘,成岩时代为126～123Ma,主要受区域北北东向大断裂控制,而与盆地火山机构无关。庐枞盆地内与岩浆岩有关的铁、铜、金、铅、锌、铀矿床可划分为3个成矿系列,其中砖桥旋回形成的二长岩类与罗河、泥河和龙桥等铁矿床、岳山铅锌矿床及井边和拔茅山铜矿床关系密切,双庙旋回形成的正长岩类与马口等铁矿床关系密切,而晚期的A型花岗岩与3440矿床等金、铀矿化关系密切。长江中下游地区燕山期存在145～136Ma、135～127Ma、126～123Ma等三期成岩(成矿)作用,庐枞盆地内侵入岩对应于该区域第二和第三期岩浆活动的产物,其形成时代明显晚于长江中下游成矿带断隆区内与斑岩型-矽卡岩型铜(铁)、金矿床有关的高钾钙碱性岩体(第一期)。庐枞盆地内侵入岩形成于区域岩石圈伸展的构造环境。     

Experimental study of long wave generation on sloping bottoms

Low-frequency waves generated on steep (1:10) and mild (1:40) slopes by six series of bichromatic wave groups are studied experimentally. The shorelines for both slopes are replaced by horizontal reaches of small depth. This reduces the reflection of long waves near the shoreline significantly, which for the first time makes possible the explicit observation of outgoing breakpoint forced long waves. The breakpoint and released bound long wave mechanisms on the different slopes are compared. Generally, the breakpoint forced long waves dominate the low-frequency wave field on the steep slope, while the released bound long waves are found to be more significant on the mild slope. Two parameters indexing the effectiveness of the breakpoint mechanism are compared and the normalized slope tends to give more realistic results. Shoaling of bound long waves is analyzed and the shallow-water equilibrium limit ~ h^−5/2 exhibits a good prediction of the variation of the bound long waves on both slopes.     

Geochronology/geochemistry of the Washan dioritic porphyry associated with Kiruna-type iron ores,Middle-Lower Yangtze River Valley,eastern China: implications for petrogenesis/mineralization

The Early Cretaceous Washan dioritic porphyry is spatially and temporally associated with Kiruna-type iron oxide deposits in the Ningwu basin, Middle-Lower Yangtze River Valley (MLYRV). We present new LA-ICP-MS U–Pb dating + zircon Lu–Hf isotopic studies, as well as bulk-rock major + trace element and Sr + Nd isotopic compositions of the porphyry. LA-ICP-MS U–Pb zircon analyses suggest that the pluton formed at 130.8?±?0.9 Ma. Analysed zircon ?_Hf(t) values range from –7.0 to –4.1. The dioritic rocks are significantly enriched in Pb and light rare earth elements, relative to high-field strength elements (Nb + Ti), coupled in the absence of significant Eu anomalies. They exhibit age-corrected ?_Nd(t) (t?=?130 million years) values of??3.5 to??3.9 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70553–0.70653. The ore-bearing dioritic porphyry was derived from a parental basaltic liquid that was produced by partial melting of an enriched spinel-facies lherzolite in the Yangtze lithospheric mantle. This basaltic melt underwent a fractionation of plagioclase and clinopyroxene during ascent towards the surface, which led to the relative enrichment of iron in the residual melt. This type of magma was widespread in the MLYRV area but did not generate widespread Fe mineralization. In the Ningwu area, the dioritic magma was modified by minor assimilation of phosphorus-bearing rocks in the Yangtze upper crust. The special crustal characteristics of the Ningwu basin, i.e. phosphorus-rich strata, were likely a crucial factor controlling the formation of Kiruna-type iron oxide deposits.     

热带低压远距离对西南涡稳定加强的作用

利用地面观测资料、FY-2C卫星红外云图及中尺度数值模式WRF对2010年7月19-20日河北省南部和中东部一次区域性大暴雨过程进行了诊断分析和数值模拟研究。结果表明,海上热带低压对西南涡的稳定和维持有远距离作用。由于热带低压的存在而促使西南低涡的发展,使西南低空急流在河北突然增强,降水得以再度加强;冷空气从高空侵入、下沉,加强了大气斜压性,迫使对流层中层空气的抬升,为西南涡的发展提供了动力条件;暖湿气流主要来自西南涡携带的西南气流及海上低压和西太平洋副热带高压的东南气流输送,为暴雨加强提供了充沛的水汽和热力条件。     

西藏古乡沟泥石流的数值模拟

泥石流运动数值模拟是确定泥石流泛滥范围、进行危险性分区、辅助减灾工程设计与检验的重要方法。以西藏波密古乡沟为实例,从泥石流的二维运动方程出发,使用一种新的数值方法对古乡沟50a一遇泥石流在堆积扇上的运动过程进行了数值模拟,获得了其最大流速、流深、动量和动能的空间分布,并据此确定了受泥石流危害较小的川藏公路穿越堆积扇方案。将泥石流数值模拟结果与游勇等1997年做的模型试验结果进行了比较,具有较好的相似性。     

辽代西拉木伦河流域聚落分布与环境选择

辽代西拉木伦河流域及毗邻地区的聚落是分析论证农业开垦范围与环境扰动的重要依据,作者将这一地区的聚落分为两类,其中之一为散居在各处的基层聚落,规模小,人口少,围绕聚落形成的耕地范围也有限,不是造成环境扰动的主要方面。另一类以府、州城为中心形成的聚落,人口集中,耕地范围大,且大部分州治城邑属契丹历史纪念地,而不是农业垦殖适宜地;个别州治城邑地处沙地中部,不具备农业开垦的条件,由于人口集中,这些农田插花地不但范围大,而且是引起环境问题的关键地区。     

Seismic Anisotropy Observations in the Mexicali Valley,Baja California,México

A study of seismic anisotropy was performed using data from earthquakes of the Mexicali Valley. The investigated region encompasses the Cerro Prieto Geothermal Field (CPGF), one of the most important fields in the world. The results showed that at most of our stations the average polarization directions of the fast S waves range from N14°W to N17°E. A N-S polarization direction was obtained for the whole area by averaging the polarization directions from all stations used. In terms of the EDA hypothesis, this average trend agrees with the postulated state of stress for southern California, and with fault plane solutions for events of the Mexicali Valley. Notorious deviations from the N-S global trend were found southeast of the CPGF, with polarization trends between N25°E and N67°E, and in the geothermal field, with polarization directions between N7°W and N14°W. The polarization results for these zones indicated stress conditions that are different from the more regional stress pattern. The delay times that were measured between the fast and slow shear waves reached values of up to 0.6 sec, with a mean value of 0.35 sec. Consistent with our polarization results, the larger delay times (0.2–0.6 sec) were found in the CPGF. Smaller or null values were observed at the periphery of the study area. No temporal trends in the delay times were apparent, as shown by data from the two stations that recorded the larger number of events. Overall, we conclude that the splitting effects of this study result from a shallow anisotropy volume. The splitting results are thus interpreted as caused by the preferred orientation of vertical fluid-filled microcracks aligned in a direction that is parallel to the regional stress field. The stronger splitting effects that were observed in the area of the CPGF were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone, at depths of 1 to 4–5 km from the surface. We thus believe that such marked splitting effects have a direct relation with the reservoir of the CPGF.We are grateful to Miguel Navarro, Tito Valdez, and Manuel Luna for their contribution in the operation of RANM and for processing and cataloguing the strong-motion data. Ignacio Méndez and Francisco Farfán helped us with data from the RESNOM system. The study benefited from funding provided by CICESE and from grants awarded by CONACYT to Luis Munguía (Grants F195T and PCCNCNA-031339).     

Early Permian Palaeoenvironment in Damodar Valley Coalfields, India: an Overview

In the Damodar Valley Basin, coalfields containing coal bearing Barakar Formation are Raniganj, Jharia, Bokaro, Ramgarh, and Karanpura. The Barakar Formation is composed of conglomerate, sandstone, siltstone, shale, fireclay, and coal. The lower part of Barakar Formation represents a braided channel deposit, and also in few places glacio-fluvial deposit which changes to meandering channel system with the formation of some ox-bow lake, and cut-off channel in the middle part. In a few places deltaic/brackish water condition possibly existed along with this meandering channel system. In the upper part of Barakar Formation, marine signatures are more prominant. Marine signatures/influences have been reported from Barakar Formation of Ramgarh, South Karanpura, and West Bokaro coalfields on the basis of trace fossil assemblage, sedimentation character, and trace element content. Although, definite marine signatures have not been observed from Jharia, and Raniganj coalfields, high concentration of boron, vanadium, and chromium, and presence of skolithos, and thalassinoides burrows possibly suggest a brackish water condition. The upper part clearly suggest that the sediments were deposited in a geographic setting very close to the sea or at the edge of the sea possibly in a peritidal setting where storm activity played a vital role during sedimentation which in turn suggests the presence of a broad shallow sea (epeiric/epicontinental sea) that develop during times of high sea level. The sea water possibly entered from the northeastern side as vast seaways or as embayment through the Damodar Valley which acted as a channel.     

A tectono-genetic model for porphyry–skarn–stratabound Cu–Au–Mo–Fe and magnetite–apatite deposits along the Middle–Lower Yangtze River Valley, Eastern China

The Middle–Lower Yangtze River Valley metallogenic belt (YRB), situated along the northern margin of the Yangtze craton, is characterized by porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits in the areas of uplift and magnetite–apatite deposits in Cretaceous fault basins. Following detailed field investigations and a review of published data, we recognize two episodes of magmatism and mineralization in the YRB: 1) 156–137 Ma high-K calc-alkaline granitoids associated with 148–135 Ma porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits and 2) 135–123 Ma shoshonitic series, associated with 134.9–122.9 Ma magnetite–apatite deposits. A-type granitoids and associated alkaline volcanic have a small age range from 126.5 to 124.8 Ma and are temporally, spatially and genetically associated with the second episode. The geodynamic history of the YRB did not experience the Paleozoic to Mesozoic lithospheric thickening that took place in the North China craton. This process is inferred to be linked to partial melting of the delaminated lower crust at high pressures, resulting in the development of C-type adakitic rocks. The petrochemical and Sr/Nd isotopic data show that both the shoshonitic series and A-type granitoids are quite different from adakites, with only some of the K-calc-alkaline granitoids having adakitic signatures. Previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono-thermal evolution from 150 to 100 Ma.All data obtained for the Middle–Lower Yangtze River region consistently show that the Tan–Lu regional strike-slip fault zone, initiated at 233 ± 6 to 225 ± 6 Ma from the collision between the North China and Yangtze cratons and was reactivated at ca. 160 Ma. The Tan–Lu fault was caused by the oblique subduction of the Izanagi plate, which along the YRB the low-angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks. The high-K calc-alkaline granitoids magmas were derived from melting of the subducted slab, with some input of crustal material. These magmas were emplaced at the intersections between NE- and EW-trending faults and formed porphyry–skarn–stratabound Cu–Au–Mo–Fe deposits between 156 and 137 Ma. After 135 Ma the subducted plate changed its direction of motion to northeast, now running parallel to the Eurasian continental margin, and leading to large-scale continental extension. The shoshonitic series and subsequent A-type granitoids magmatism and the development of magnetite–apatite ores in the YRB, took place in both fault basins and NE-trending rifts between 135 and 124 Ma.     

Characteristics of Breccias and C-O-Sr-S Isotope Geochemistry of the Duocaima Pb-Zn Deposit in Tuotuohe, Qinghai Province: Implications for the Ore-forming Process

The Duocaima carbonate-hosted Pb-Zn deposit is a newly found large deposit in the southern area of Qinghai Province.In this paper, the characteristics, genesis, significance to Pb-Zn mineralization of the widely developed breccias, and the ore-forming process have been carefully studied based on geological documentation of drilling holes, microscopic observations of petrography and microstructure and some stable isotope measurements.Based on the compositions of the clast and matrix, the breccias can be classified into three types: limestone clasts cemented by marl; limestone clasts with fine-grained calcareous materials; and limestone clasts cemented by hydrothermal calcite.The mineralization in the first type of breccia is weak, whereas it is strong in the latter two types of breccias.According to the locations of occurrence and structural characteristics of the breccias along with the relationship between the breccias and mineralization, part of the limestone clasts that are cemented by marl and outcrop in the contact zone between the Wudaoliang Formation(Nw) and the underlying Jiushidaoban Formation(Pj) are attributed to synsedimentary fault-genetic breccia, whereas the last of the limestone clasts that are cemented by marl and developed in the Jiushidaoban Formation(Pj) are attributed to the breccia generated by karst cave collapse; the limestone clasts with fine-grained calcareous materials and the limestone clasts cemented by hydrothermal calcite are attributed to breccia formed by hydrothermal dissolution.The breccia formed by karst collapse had consistently evolved for a long period of time, while the breccias with other origins were formed around the period of mineralization(i.e., about or slightly later than 20–16 Ma).The breccia generated by karst cave collapse and hydrothermal dissolution are somewhat related; the formation of the breccia from karst cave collapse provided open space for the later mineralization and reaction between hydrothermal fluids and host rocks, and the subsequent strong dissolution by hydrothermal fluids transformed some of the breccia formed earlier by karst cave collapse.Meanwhile, carbonate host rocks with breccias and brecciaed mineralization can be a potential sign of Mississippi Valley Type(MVT) deposits and important indicators for regional mineral exploration.The δ13CV-PDB, δ18OVSMOW, and 87Sr/86 Sr values of hydrothermal calcite in the Duocaima deposit range from 4.3‰ to 7.1‰, 14.9‰ to 20.1‰, and 0.707494 to 0.708185, respectively; the δ13CV-PDB, δ18OV-SMOW, and 87Sr/86 Sr values of the host limestones of the Jiushidaoban Formation range from 3.6‰ to 5.3‰, 18.0‰ to 20.5‰, and 0.707372 to 0.707945, respectively.The δ13CV-PDB and 87Sr/86 Sr values of hydrothermal calcite and limestone are similar, indicating single sources of C and Sr in this deposit, with the likely source being the limestone of the Jiushidaoban Formation.The minor scattering of the δ18OV-SMOW values suggests that different O isotope fluids underwent the isotope exchange reaction.The C-O-Sr isotope characteristics indicate that the host limestones experienced a dissolution and precipitation process during mineralization, which is beneficial to improving the porosity of host rocks and promoting the precipitation of metal sulfides.The δ34SV-CDT value of the breccia-type mineralization sulfides ranges from-30.4‰ to-0.3‰; that is, the δ34SV-CDT value is negative with considerable variation, illustrating that during the breccia-type mineralization process, the bacteriogenic reduction of sulfates provided the vast majority of sulfur, whereas the thermochemical reduction of sulfates was relatively unimportant.The brecciation that occurred as a result of karst cave collapse was mainly generated by the dissolution of groundwater; however, the brecciation related to hydrothermal dissolution and mineralization processes were caused by mixing of different fluids.