宁远保安地区基性─超基性火山岩基本特征

本文论述了保安地区火山岩的地质、岩石、副矿物、岩石化学、微量元素、稀土元素、稳定同位素、火山岩相及火山机构等特征。并对其演化规律、形成机制与成矿作用的关系，作了初步探讨。     

商丹断裂构造特征及其演化

商丹断裂带是北秦岭加里东造山带与中秦岭海西造山带的分界断裂,由韧性剪切应变带、逆冲推覆构造带、地堑断陷带及其所夹持的岩块、岩片、构造岩、褶曲等多种地质体组合成复杂的断裂带。其演化历史至少经历晋宁期俯冲、加里东期碰撞、海西—印支期陆内俯冲、燕山期滑脱推覆和喜山期地堑断陷的复杂演化过程而形成今日所见的复杂断裂构造。     

从辽宁东、西部地质对比论郯庐断裂的平移运动

辽宁东、西部地质上诸多差异并非是原地升降运动造成的，而是二个异地岩块经郯庐断裂大规模左旋平移运动迁移到一起的结果。辽东半岛原曾与鲁西－徐淮地区处于相同的古纬度。郯庐断裂北延的主干断裂应是抚顺－敦化断裂，它在辽宁境内是地层区划的重要界线。郯庐断裂在太古宙末期即已出现，曾多次变换其平移方向，最近一次大规模左行平移活动的高峰期是在晚侏罗世晚期－早白垩世，结束于孙家湾组或泉头组堆积之前。     

铜川王石凹黑色高岭石粘土岩的成因分析

黑色高岭石粘土岩，是铜川王石凹煤矿石炭纪煤系地层顶部，五号煤层的上部一层稳定的煤矸石。经偏光显微镜及费氏台测试光性特征、化学试验及分析鉴定表明，这是一种由高岭石组成的，成分很纯、颗粒大、结晶良好的高岭石粘土岩。据微量元素含量分析，此高岭石系淡水沉积物。     

中国北西部金矿主要类型,分布规律和成矿带划分

本文较系统地总结了我国北西部金矿主要类型、矿化特征和金矿时空分布规律,在此基础上划分出4个成矿区、12条成矿带、32条成矿亚带,从而指明了找矿方向,为贯彻“以铀为主,综合找矿,多种经营,搞活地质”的方针,提供了较好的参考资料。     

Alluvial architecture of the Holocene Rhine–Meuse delta (the Netherlands)

Ancient fluvial successions often act as hydrocarbon reservoirs. Sub‐surface data on the alluvial architecture of fluvial successions are often incomplete and modelling is performed to reconstruct the stratigraphy. However, all alluvial architecture models suffer from the scarcity of field data to test and calibrate them. The purposes of this study were to quantify the alluvial architecture of the Holocene Rhine–Meuse delta (the Netherlands) and to determine spatio‐temporal trends in the architecture. Five north–south orientated cross‐sections, perpendicular to the general flow direction, were compiled for the fluvial‐dominated part of the delta. These sections were used to calculate the width/thickness ratios of fluvial sandbodies (SBW/SBT) and the proportions of channel‐belt deposits (CDP), clastic overbank deposits (ODP) and organic material (OP) in the succession. Furthermore, the connectedness ratio (CR) between channel belts was calculated for each cross‐section. Distinct spatial and temporal trends in the alluvial architecture were found. SBW/SBT ratios decrease by a factor of ca 4 in a downstream direction. CDP decreases from ca 0·7 (upstream) to ca 0·3 (downstream). OP increases from less than 0·05 in the upstream part of the delta to more than 0·25 in the downstream delta. ODP is approximately constant (0·4). CR is ca 0·25 upstream, which is approximately two times larger than in the downstream part of the delta. Furthermore, CDP in the downstream Rhine–Meuse delta increases after 3000 cal yr BP. These trends are attributed to variations in available accommodation space, floodplain geometry and channel‐belt size. For instance, channel belts tend to narrow in a downstream direction, which reduces SBW/SBT, CDP and CR. Tectonics cause local deviations in the general architectural trends. In addition, the positive correlation between avulsion frequency and the ratio of local to regional aggradation rate probably influenced alluvial architecture in the Rhine–Meuse delta. The Rhine–Meuse data set can be a great resource when developing more sophisticated models for alluvial architecture simulation, which eventually could lead to better characterizations of hydrocarbon reservoirs. To aid such usage of the Rhine–Meuse data set, constraints for relevant parameters are provided at the end of the paper.     

Deciphering polymetamorphic episodes in high-grade metamorphic orogens: Constraints from PbSL, Sm/Nd and Lu/Hf garnet dating of low- to high-grade metasedimentary rocks from the Kaoko belt (Namibia)

Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation.     

Relationship between footwall composition,crustal contamination,and fluid–rock interaction in the Platreef,Bushveld Complex,South Africa

In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave. −7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef has generally higher plagioclase and pyroxene δ ¹⁸O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic evidence for sub-solidus interaction with fluids, and the Δ _{plagioclase–pyroxene} values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ _{plagioclase–pyroxene} in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO₂-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale. The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO₂ produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation.     

Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros

The regionally extensive, coarse-grained Bakhtiyari Formation represents the youngest synorogenic fill in the Zagros foreland basin of Iran. The Bakhtiyari is present throughout the Zagros fold-thrust belt and consists of conglomerate with subordinate sandstone and marl. The formation is up to 3000 m thick and was deposited in foredeep and wedge-top depocenters flanked by fold-thrust structures. Although the Bakhtiyari concordantly overlies Miocene deposits in foreland regions, an angular unconformity above tilted Paleozoic to Miocene rocks is expressed in the hinterland (High Zagros).

The Bakhtiyari Formation has been widely considered to be a regional sheet of Pliocene–Pleistocene conglomerate deposited during and after major late Miocene–Pliocene shortening. It is further believed that rapid fold growth and Bakhtiyari deposition commenced simultaneously across the fold-thrust belt, with limited migration from hinterland (NE) to foreland (SW). Thus, the Bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the Zagros. However, new structural and stratigraphic data show that the most-proximal Bakhtiyari exposures, in the High Zagros south of Shahr-kord, were deposited during the early Miocene and probably Oligocene. In this locality, a coarse-grained Bakhtiyari succession several hundred meters thick contains gray marl, limestone, and sandstone with diagnostic marine pelecypod, gastropod, coral, and coralline algae fossils. Foraminiferal and palynological species indicate deposition during early Miocene time. However, the lower Miocene marine interval lies in angular unconformity above ~ 150 m of Bakhtiyari conglomerate that, in turn, unconformably caps an Oligocene marine sequence. These relationships attest to syndepositional deformation and suggest that the oldest Bakhtiyari conglomerate could be Oligocene in age.

The new age information constrains the timing of initial foreland-basin development and proximal Bakhtiyari deposition in the Zagros hinterland. These findings reveal that structural evolution of the High Zagros was underway by early Miocene and probably Oligocene time, earlier than commonly envisioned. The age of the Bakhtiyari Formation in the High Zagros contrasts significantly with the Pliocene–Quaternary Bakhtiyari deposits near the modern deformation front, suggesting a long-term (> 20 Myr) advance of deformation toward the foreland.     

Integrated approach for identification of potential groundwater zones in Seethanagaram Mandal of Vizianagaram District,Andhra Pradesh,India

Identifying a good site for groundwater exploration in hard rock terrain is a challenging task. In hard rocks, groundwater occurs in secondary porosity developed due to weathering, fracturing, faulting, etc., which is highly variable within short distance and contributing to near-surface inhomogeneity. In such situations topographic, hydrogeological and geomorphological features provide useful clues for the selection of suitable sites. Initially, based on satellite imagery, topographical, geomorphological and hydrogeological features, an area of about 149 km² was demarcated as a promising zone for groundwater exploration in the hard rock tract of Seethanagaram Mandal, Vizianagaram District, Andhra Pradesh, India. A total of 50 Vertical Electrical Soundings (VES) were carried out using Wenner electrode configuration. An interactive interpretation of the VES data sharpened the information inferred from geomorphological and hydrogeological reconnaissance. Ten sites were recommended for drilling. Drilling with Down-The-Hole Hammer (DTH) was carried out at the recommended sites down to 50 to 70 m depths. The interpreted VES results matched well with the drilled bore well lithologs. The yields of bore wells vary from 900 to 9000 liters per hour (lph).