晋北后所金矿床地质特征及矿床成因

后所金矿是产在五台群变质岩中的金矿床，有含金黄铁矿石英脉型和蚀变岩型两种矿化类型本文论述了后所金矿的地质特征，矿物组成，微量元素，稳定同位素地球化学和矿物馀本学特征，探讨了金矿的物质来源和金矿成因，指出了金矿的演化和最后定位于燕山中晚期。     

关于金矿成矿物质来源的初步探讨

近年来“矿源层”这一概念已被广大地质学家所接受，他们认为矿床中的金是从围岩中萃取的，本文认为矿床中的金来源于岩浆残余热液。在地壳深部，当丰度值高的基性岩或其他岩石，在深熔作用下产生花岗质岩浆时，同时带出许多金，当岩浆侵入到上部地壳冷凝结晶时，其中的大部金进入残余热液，最后富集成矿。     

青海德令哈巴音郭勒河断裂带的新活动特征

在青海德令哈巴音郭勒河北侧山前冲洪积扇上新发现了一条长约60km的逆断裂带,属于本区NNW-NWW向的柴达木盆地北缘活动断裂系内的一条次级挤压构造。断裂在地貌上表现为明显的挤压逆冲断层陡坎,晚更新世晚期以来的平均垂直滑动速率为0.41±0.27mm/a。探槽剖面确定了三次古地震事件,其年代分别为距今约32.7±1.45ka、15.54±1.32ka和3.2±0.33ka。     

赣南中侏罗世流纹岩地球化学及成因研究：上地壳成因的的微量元素和Pb-Nd-Sr同位素地球化学制约

江西南部龙南-寻邬地区余田群菖蒲组的流纹岩形成于中侏罗世(164．8．1Ma～174．9Ma)，为双峰式火山岩组合的酸性端元，属弱过铝质(ACNK平均为1．04)钙碱性系列(σ平均为1．90)。稀土元素含量高(平均为278．9μg/g)，轻稀土富集(IMH平均为7．94)，Eu亏损中等(δEu平均为0．41)。微量元素以富集Rb，Ba，Th，U，Zr，Y，Pb，而亏损Sr，Nb为特征。Pb，Nd，Sr同位素组成为：(^206Pb/^204Pb)t=17．89～18．58，(^207Pb/^204Pb)t=15．58～15．70，(^208Pb/^204Pb)t=37．94～38．82；εNd(t)=-7．44～-11．9；ISr=0．71126～0．71228。对赣南流纹岩微量元素采用蛛网图和模糊聚类分析方法进行对比研究的结果及Pb-Nd-Sr同位素相关特征为其上地壳成因提供了地球化学佐证。赣南流纹岩是由进入上地壳内高位岩浆房的石英拉斑玄武岩浆的底侵作用直接使上地壳部分熔融形成的产物。     

U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.     

晋北区中小地震活动与晋冀蒙交界区6级地震的关系

分析了晋北区中小地震(3.0≤ML≤5.0)的活动特征。统计发现,在1989 年山西大同-阳高6.1、1996年内蒙包头6.4和1998年河北张北6.2 级中强地震前1~2 年内晋北地区的中小地震活动年频次有明显的增强-衰减-发震这一特征;震前主震近场区的地震条带活动对主震位置有预示意义,而较远场的地震活动仅反映主震前应力水平的增强。并有震前活动向未来强震区靠近的特征。     

First estimate for the age of a mesoproterozoic palaeomagnetic pole from the Volodarsk-Volynsky Massif,the Ukrainian Shield

Palaeomagnetic data are presented from the southern Volodarsk-Volynsky Massif (VVM) of the Korosten Pluton, the Ukrainian Shield. Laboratory experiments (AF and thermal demagnetization, IRM acquisition, thermal separation), field tests (consistency and secular variation methods) and optical observations indicate that single domain and nearly single domain magnetite is the dominant carrier of a primary TRM in the anorthosites. Palaeomagnetic poles from the three sampling sites (Golovino and Turchinka quarries) are indistinguishable at the 95% confidence level and have been combined to yield a mean pole at P_lat = 30 °N, P_lon = 178 °E, a₉₅ = 3.4 °.In the large slow cooling Korosten Pluton the U-Pb zircon/baddeleyite (U_zb) technique gives an age for the anorthosites, which are not equivalent to the time of magnetic blocking. Based on integrated analysis of geochronologic information and blocking-temperature data for magnetic minerals proposed by Briden et al. (1993), a first attempt has been undertaken to estimate the palaeomagnetic pole age from the Mesoproterozoic anorthosites. The Korosten Pluton has cooled from 850 °C (the closure temperature of U-Pb systematics in zircon/baddeleyite) to 350 °C (the closure temperature of K-Ar systematics in biotite) during 150 Ma after the emplacement of the anorthosites. Assuming a uniform cooling of the intrusion yields a rate of 3.3 °C/Ma. The cooling rate for the granites is 3.1 °C/Ma. The mafic and acid rocks have an average rate of 3.2 °C/Ma. Using the cooling gradient for the VVM (3.2 °C/Ma) and the mean natural blocking temperature of magnetite (520 °C) can be determined a remanence age. The estimate for TRM acquisition is 1656 ± 10.0 Ma.The magnetic pole for the VVM is in good agreement with the mean pole from the Baltic quartz porphyry dykes with an age of 1630 – 1648 Ma. The VVM pole is best dated and requires a revision of the latest paleogeographic reconstructions for the Fennoscandian and Ukrainian Shields at 1770 and 1650 Ma. (Pesonen et al., 2003).     

The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism

The internal architecture of the immense volumes of eruptive products in Continental Flood Basalt Provinces (CFBPs) provides vital clues, through the constraint of a chrono-stratigraphic framework, to the origins of major intraplate melting events. This work presents close examination of the internal facies architecture and structure, duration of volcanism, epeirogenetic uplift associated with CFBPs, and the potential environmental impacts of three intensely studied CFBPs (the Parana-Etendeka, Deccan Traps and North Atlantic Igneous Province). Such a combination of key volcanological, stratigraphic and chronologic observations can reveal how a CFBP is constructed spatially and temporally to provide crucial geological constraints regarding their development.

Using this approach, a typical model can be generated, on the basis of the three selected CFBPs, that describes three main phases of flood basalt volcanism. These phases are recognized in Phanerozoic CFBPs globally. At the inception of CFBP volcanism, relatively low-volume transitional-alkaline eruptions are forcibly erupted into exposed cratonic basement lithologies, sediments, and in some cases, water. Distribution of initial volcanism is strongly controlled by the arrangement of pre-existing topography, the presence of water bodies and local sedimentary systems, but is primarily controlled by existing lithospheric and crustal weaknesses and concurrent regional stress patterns. The main phase of volcanism is typically characterised by a culmination of repeated episodes of large volume tholeiitic flows that predominantly generate large tabular flows and flow fields from a number of spatially restricted eruption sites and fissures. These tabular flows build a thick lava flow stratigraphy in a relatively short period of time (c. 1–5 Ma). With the overall duration of flood volcanism lasting 5–10 Ma (the main phase accounting for less than half the overall eruptive time in each specific case). This main phase or ‘acme’ of volcanism accounts for much of the CFBP eruptive volume, indicating that eruption rates are extremely variable over the whole duration of the CFBP. During the waning phase of flood volcanism, the volume of eruptions rapidly decrease and more widely distributed localised centres of eruption begin to develop. These late-stage eruptions are commonly associated with increasing silica content and highly explosive eruptive products. Posteruptive modification is characterised by continued episodes of regional uplift, associated erosion, and often the persistence of a lower-volume mantle melting anomaly in the offshore parts of those CFBPs at volcanic rifted margins.     

福建省水氡观测资料变化规律与质量分析

本文对福建省水氡观测资料变化规律进行分析,并试图利用相对均方差、拟合噪声水平和相对年变幅等三项指标,对福建省8个水氡点的观测资料质量进行初步探索。