陕西潼关地区太华群变质岩原岩性质的恢复

潼关地区太华群地层是一套以角闪岩相为主(局部达麻粒岩相)的中深变质岩系。本区太华群变质岩的主要岩石类型:斜长片麻岩类的原岩为中酸性火山岩(二长安山质、英安质凝灰岩);浅(变)粒岩类的原岩为中酸性火山岩(英安岩,流纹岩)一中酸性火山碎屑岩(二长安山质、英安质凝灰岩),并以后者为主,还有少量粘土质沉积岩或长石质杂砂岩;斜长角闪岩的原岩为基性火山岩—拉斑玄武岩。     

Methodology for groundwater recharge assessment in carbonate aquifers: application to pilot sites in southern Spain

Aquifer recharge can be determined by conventional methods such as hydrodynamic or hydrologic balance calculations, or numerical, hydrochemical or isotopic models. Such methods are usually developed with respect to detrital aquifers and are then used on carbonate aquifers without taking into consideration their hydrogeological particularities. Moreover, such methods are not always easy to apply, sometimes requiring data that are not available. Neither do they enable determination of the spatial distribution of the recharge. For eight regions in southern Spain, the APLIS method has been used to estimate the mean annual recharge in carbonate aquifers, expressed as a percentage of precipitation, based on the variables altitude, slope, lithology, infiltration landform, and soil type. The aquifers are representative of a broad range of climatic and geologic conditions. Maps of the above variables have been drawn for each aquifer, using a geographic information system; thus they can be superimposed to obtain the mean value and spatial distribution of the recharge. The recharge values for the eight aquifers are similar to those previously calculated by conventional methods and confirmed by discharge values, which corroborates the validity of the method.     

Fate of individual sewage disposal system wastewater within regolith in mountainous terrain

In order to improve understanding of the fate of septic tank or individual sewage disposal system (ISDS) effluent in regolith overlying fractured-rock aquifers, effluent from an ISDS in such a setting was tracked via geophysical, hydrological, and geochemical methods. Under typical precipitation conditions, the effluent entered the fractured bedrock within 5 m of the boundary of the constructed infiltration area. During a period of unusually high spring recharge, the plume migrated between 50 and 100 m within the regolith before infiltrating the fractured bedrock. The chemical signature of the effluent is similar to that required to account for the decline in water quality, suggesting a causative relationship (as estimated from mass-balance models of the surface-water chemistry near the mouth of the basin). The elevated salt content of the effluent during periods of high natural recharge to the infiltration area correlates with elevated salt concentrations in surface and groundwater at the basin scale, suggesting that some of the effluent salt load may be stored in the unsaturated zone during dry periods and flushed during periods of elevated natural recharge.     

Fuzzy logic modeling of the resistivity parameter and topography features for aquifer assessment in hydrogeological investigation of a crystalline basement complex

A linguistic fuzzy logic system (LFLS)-based expert system model has been developed for the assessment of aquifers for the location of productive water boreholes in a crystalline basement complex. The model design employed a multiple input/single output (MISO) approach with geoelectrical parameters and topographic features as input variables and control crisp value as the output. The application of the method to the data acquired in Khondalitic terrain, a basement complex in Vizianagaram District, south India, shows that potential groundwater resource zones that have control output values in the range 0.3295–0.3484 have a yield greater than 6,000 liters per hour (LPH). The range 0.3174–0.3226 gives a yield less than 4,000 LPH. The validation of the control crisp value using data acquired from Oban Massif, a basement complex in southeastern Nigeria, indicates a yield less than 3,000 LPH for control output values in the range 0.2938–0.3065. This validation corroborates the ability of control output values to predict a yield, thereby vindicating the applicability of linguistic fuzzy logic system in siting productive water boreholes in a basement complex.     

花岗岩源岩问题——关于花岗岩研究的思考之四

花岗岩源岩是花岗岩研究中最令人关注的问题之一,许多花岗岩的地球化学性质和分类实际上反映的是花岗岩的源岩问题.花岗岩幔源、壳源和壳幔混合源的说法被证明是不合适的,花岗岩不可能是幔源的,花岗岩都是壳源的,既然没有了幔源,也就无所谓壳幔混合源,因此,壳源本身也失去了意义.流行的花岗岩源岩组分混合计算的方法缺少理论依据,可变的因素太多,计算的结果可能没有多少实际意义.作者指出,对花岗岩来说第1位重要的是源区特征,它决定了花岗岩的基本面貌;其次是部分熔融程度、压力、温度和挥发分加入的情况;岩浆混合的意义可能是第3位的;而结晶分离作用可能是没有地位的.文中按照Nd-Sr同位素比值将花岗岩大致分为3个源区:即B、C和BC源区.B源主要由洋壳组成,C源主要由陆壳组成,BC源是二者的过渡.BC源区的组成很复杂,可能包括由交代地幔部分熔融形成的中基性岩浆岩,也可能是基性岩混染了陆壳物质的产物.B源与BC源可能与地幔亏损程度有关,B源来自强烈亏损的地幔,一部分BC源可能源于大陆下的富集地幔.文中还概略地讨论了中国各地花岗岩复杂的源区情况,指出中国花岗岩具区域性分布的特点说明花岗岩主要受源岩的制约.     

皖东地区燕山期高镁中酸性侵入岩地球化学特征及动力学意义

皖东地区中酸性侵入岩为一套石英闪长岩-石英二长闪长岩-花岗闪长岩-二长花岗岩组合,主要属于高钾钙碱性岩系列,具有高Al2O3、Sr、Sr/Y、(La/Yb)N、富集LREE和LILE、亏损HREE和Y、Yb、Eu弱负异常—正异常、岩石高(87Sr/86Sr)i、低εNd(t)等特征。与中国东部多数埃达克质岩石相比,岩石具有较高的Mg#值和相容元素Cr、Ni含量。研究表明:(1)皖东地区中酸性侵入岩可能是拆沉的下地壳基性物质经部分熔融作用及岩浆在上升过程中与地幔橄榄岩相互反应的产物,石榴石作为残留相存在。(2)燕山早期皖东地区存在受特提斯构造域挤压构造控制的地壳加厚过程,早白垩世受太平洋构造域影响的郯庐断裂带大规模左行平移活动是触发下地壳拆沉的主要因素。皖东地区中酸性侵入岩是两大构造体制域发生转换的岩浆岩石记录。     

大陆岩石圈对扩张机制的响应——辽蒙地质走廊中—新生代火山活动的时空分布

选择以辽西为中心,近东西向延伸800km的辽蒙地质走廊为研究区,通过年代学研究,确认130Ma以来的中、新生代火山活动对称分布的时空格局具有“中间老、两侧新”的特点,而且随着时间的推移,软流圈来源的岩浆向东西两侧侧向流动,岩浆来源不断加深。在此基础上提出“软流圈底辟体上涌和水平侧向流动”的模式。     

西秦岭月照 琵琶寺地区火山岩地球化学特征及地质意义

月照琵琶寺地区的变质火山岩出露于甘肃武都县境内,夹在泥盆系和前震旦碧口群地层之间。按地球化学分类,火山岩可划分为双峰式和碱性两个系列。双峰式火山岩系由低K富Na拉斑玄武岩和酸性英安岩、流纹岩组成。该玄武岩具有类似MORB的微量元素特征,平坦的稀土配分模式,但富Th贫Nb显示其受到陆壳混染的影响。该套火山岩总体特征指示其为大陆裂谷向成熟洋盆转化阶段的产物,从而推测本区火山岩可能为勉略古洋盆西延的分支产物。     

Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China

The Late Middle Permian ( 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma.The oxide-bearing intrusions have higher Al₂O₃, FeO (as total iron) and total alkalis (Na₂O + K₂O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of Nd(t) values. All the intrusions, however, have Nd(t) values ranging from − 3.9 to + 4.6, and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe₂O₃ (2.5 to 13 wt.%) with initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.     

Mineralogical,petrological, and geochemical studies of the Limahe mafic–ultramatic intrusion and associated Ni–Cu sulfide ores,SW China

The Limahe Ni–Cu sulfide deposit is hosted by a small mafic–ultramafic intrusion (800 × 200 × 300 m) that is temporally associated with the voluminous Permian flood basalts in SW China. The objective of this study is to better understand the origin of the deposit in the context of regional magmatism which is important for the ongoing mineral exploration in the region. The Limahe intrusion is a multiphase intrusion with an ultramafic unit at the base and a mafic unit at the top. The two rock units have intrusive contacts and exhibit similar mantle-normalized trace element patterns and Sr–Nd isotopic compositions but significantly different cumulus mineralogy and major element compositions. The similarities suggest that they are related to a common parental liquid, whereas the differences point to magma differentiation by olivine crystallization at depth. Sulfide mineralization is restricted to the ultramafic unit. The abundances of sulfides in the ultramafic unit generally increase towards the basal contacts with sedimentary footwall. The δ ³⁴S values of sulfide minerals from the Limahe deposit are elevated, ranging from +2.4 to +5.4‰. These values suggest the involvement of external S with elevated δ ³⁴S values. The mantle-normalized platinum-group element (PGE) patterns of bulk sulfide ores are similar to those of picrites associated with flood basalts in the region. The abundances of PGE in the sulfide ores, however, are significantly lower than that of sulfide liquid expected to segregate from undepleted picrite magma. Cr-spinel and olivine are present in the Limahe ultramafic rocks as well as in the picrites. Mantle-normalized trace element patterns of the Limahe intrusion generally resemble those of the picrites. However, negative Nb–Ta anomalies, common features of contamination with the lower or middle crust, are present in the intrusion but absent in the picrites. Sr–Nd isotopes suggest that the Limahe intrusion experienced higher degrees of contamination with the upper crust than did the picrites. The results of this study permit us to suggest that the parental magma of the Limahe intrusion was derived from picritic magma by olivine fractionation and contamination in a staging chamber at mid-crustal levels. Depletion of PGE in the sulfide ores in the Limahe intrusion is likely due to previous sulfide segregation of the parental magmas in the staging chamber. Sulfide mineralization in the Limahe intrusion is related to second-stage sulfide segregation after the fractionated magmas acquired external S from pyrite-bearing country rocks during magma ascent to the Limahe chamber. The abrupt change in mineralogical and chemical compositions between the ultramafic unit and the overlying unit suggests that at least two separate pulses of magma were involved in the development of the Limahe intrusion. We propose that the Limahe intrusion was once a wider part of a dynamic conduit that fed magma to the overlying subvolcanic dykes/sills or lavas. The ultramafic unit formed by the first, relatively more primitive magma, and the mafic unit formed by the second, relatively more fractionated magma. Immiscible sulfide droplets that segregated from the first magma settled down with olivine crystals to form the sulfide-bearing, olivine-rich rocks in the base of the intrusion. The overlying residual liquids were then pushed out of the chamber by the second magma. Critical factors for the formation of an economic Ni–Cu sulfide deposit in such a small intrusion include the dynamic petrologic processes involved and the availability of external sulfur. The Limahe deposit reminds us that small, multiphase, mafic–ultramafic intrusions in the region should not be overlooked for the potential of economic Ni–Cu sulfide deposits.