The Merensky Cyclic Unit and its impact on footwall cumulates below Normal and Regional Pothole reef types in the Western Bushveld Complex

The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2 reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall.     

Temporal Geochemical Evolution of Neogene Magmatism in the Baguio Gold–Copper Mining District (Northern Luzon, Philippines)

Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene–Pliocene–Quaternary calc‐alkaline and adakitic intrusions. Systematic sampling, K‐Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2–18.7 Ma), a Middle–Late Miocene phase (15.3–8 Ma) and finally a Pliocene–Quaternary event (3–1 Ma). The first phase emplaced evolved calc‐alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward‐dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced first petrogenetically related and relatively unradiogenic low‐K calc‐alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to reflect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversified rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area.     

Notes on non-marine bivalve Trigonioides (Trigonioides?) from the mid-Cretaceous Goshoura Group, Kyushu, Japan

Trigonioides goshourensis n. sp. and Trigonioides amakusensis Kikuchi and Tashiro occur in the late Albian Eboshi Formation of the Goshoura Group in Kyushu, Japan. These Albian species are characterized by three radial pseudocardinal teeth on the thick and wide hinge plate, and are probably ancestors of Cenomanian species of Trigonioides (Kumamotoa) with four radial pseudocardinal teeth. This chronological relation may be important for the correlation of non-marine Cretaceous strata in East Asia. In addition, the habitat of T. amakusensis is interpreted as estuarine tidal flats under brackish water conditions, although Trigonioides is generally a freshwater bivalve genus.     

The Western Bohemia uppermost crust shear wave velocities from Love wave dispersion

We present a detailed study of Western Bohemia Love waves generated by blasts with an intention to estimate the uppermost crust structure for a more detailed layer distribution than previous studies have used. The use of short-period (4 s and shorter) Love waves represents a new approach in the studied region. Properties of multiple filtering as a tool of frequency–time analysis are discussed. A new method of selecting the dispersion ridges is introduced. Tests of filtering are provided by analyzing signals with analytically known dispersion. The isometric algorithm for the inversion problem is applied, the problem of non-uniqueness is discussed, and tests of reliability of the inversion are presented. During the inversion, the forward problem is solved by use of the matrix method. Six records of blasts from the Western Bohemia region are analyzed to separate the fundamental modes of Love wavegroups, and shear wave velocity distributions down to a depth of 3.0 km are inferred. Modal summation is used to compute synthetic velocigrams, which are compared to measured ones. The lateral heterogeneity of the region is discussed and the presented models are compared to those of previous studies and to the geological setting of the region.     

Spatial Analysis of Airborne Geophysical Data Applied to Geological Mapping and Mineral Prospecting in the Serra Leste Region, Carajás Mineral Province, Brazil

The Serra Leste region, located in the northeast portion of Carajás Mineral Province (PA), hosts important chrome-PGE, Au–Pd–Pt, Cu–Mo–Au, iron, and manganese deposits and occurrences related to different geological settings. We present a regional prospective model with focus on PGE mineralizations based on airborne geophysics (magnetometry, gamma-ray spectrometry and time domain electromagnetometry) and geological data. Geological and geophysics data analysis indicate a plausible correspondence between areas with meta-mafic–ultramafic rocks and low values in gamma spectrometry, mainly for the thorium channel. The meta-ultramafic units of the complexes, the main host rock of the PGE mineralization, are well defined by analytic signal from the anomalous magnetic field and by the apparent conductance anomalies derived from the GEOTEM system. The combination of the derivative products with updated geological data of the Serra Leste region and PGE-Luanga deposit enable to establish a regional prospective model for PGE and the identification of new targets within the studied area.     

Tomography and velocity structure of the crust and uppermost mantle in southeastern Europe obtained from surface wave analysis

A set of two hundred shear-wave velocity models of the crust and uppermost mantle in southeast Europe is determined by application of a sequence of methods for surface-waves analysis. Group velocities for about 350 paths have been obtained after analysis of more than 600 broadband waveform records. Two-dimensional surface-wave tomography is applied to the group-velocity measurements at selected periods and after regionalisation, two sets of local dispersion curves (for Rayleigh and Love waves) are constructed in the period range 8–40 s. The shear-wave velocity models are derived by applying non-linear iterative inversion of local dispersion curves for grid cells predetermined by the resolving power of data. The period range of observations limits the velocity models to depths of 70 km in accordance to the penetration of the surface waves with a maximum period of 40 s. Maps of the Moho boundary depth, velocity distribution above and below Moho boundary, as well as velocity distribution at different depths are constructed. Well-known geomorphologic units (e.g. the Pannonian basin, southeastern Carpathians, Dinarides, Hellenides, Rodophean massif, Aegean Sea, western Turkey) are delineated in the obtained models. Specific patterns in the velocity models characterise the southeast Carpathians and adjacent areas, coast of Albania, Adriatic coast of southern Italy and the southern coast of the Black Sea. The models obtained in this study for the western Black Sea basin shows the presence of layers with shear-wave velocities of 3.5 km/s–3.7 km/s in the crust and thus do not support the hypothesis of existence of oceanic structure in this region.     

浙江省蒙山地区平水组成岩环境及其构造变形分析

蒙山地区平水组细碧岩角班岩系经岩石化学、微量元素及稀土元素特征和构造变形特征研究表明,形成于岛弧环境,岩浆物质来源为幔源型,来源深度约为19Km±,是扬子古板块活动陆缘增生阶段的产物;依据其构造变形特征,探讨多期变形构造叠加型式和变质作用,并确定其构造变化序列,同时显示出与双溪坞群不同的成岩构造背景.     

Determination of Distances to Galaxies of the NGC 1023 Group. The Hubble Constant

On the basis of photographs from the 6 m Large Azimuthal Telescope and the Hubble Space Telescope, VRI photometry of stars in 11 galaxies in the NGC 1023 group has been carried out. The distances to these galaxies were determined by the method of brightest stars. The distances to NGC 925 and NGC 1023 were determined from the position of the top of the red giant branch (the TRGB method). From the calculated average and root-mean-square distances to the NGC 1023 group (10.3 ± 2.2 Mpc and 9.7 ± 0.5 Mpc) the Hubble constant in this direction was determined: H _0R = 75 ± 8 km·sec^-1·Mpc^-1 and H _0M = 81 ± 5 km ·sec^-1·Mpc^-1.     

鲁西地区中元古代四堡期辉绿岩基本特征——以蒙阴野店及费县高家围子地区为例

蒙阴野店—费县高家围子一带辉绿岩广泛发育 ,多呈NNW向及近SN向岩墙分布 ,常成群产出。该辉绿岩是幔源基性岩浆沿区域性张裂隙 (刚性陆块伸展作用的结果 )上侵形成的 ,它标志着鲁西乃至整个华北地区早前寒武纪基底第三次克拉通化的完成 ,陆块从此进入稳定发展阶段。     

几种弱向异性介质中qSV波速度和偏振异常

研究了在具有生趣对称轴的横向各向同性介质（ＴＩＶ介质）中ｑＳＶ波群速度及偏振矢量,给出了相应的精确和近似公式,进一步讨论了用较简单的近似公式来代替复杂精确公式的可靠性;最后展示了地球内部几种常见各向异性岩石矿物中地震ｓＳＶ波速度各向异性因子、偏振信息及其与传播方向之间的偏差。