关于华北元古宙富钾、富稀土沉积岩是白云鄂博大型稀土矿床矿源层的讨论

文章对大连、北京元古宙沉积岩及白云鄂博稀土矿围岩的岩石化学成分、独居石形态和稀土元素特征等进行了分析与总结,结果显示三者特征相似.因此认为白云鄂博的稀土矿不是来自地幔岩而是源于大陆增生,华北元古宙富钾、富稀土沉积岩可能是白云鄂博巨型稀土矿的矿源层.文章还提出了一个白云鄂博稀土矿的理想大陆增生成矿模式图.     

Pb-Pb age of Jatulian carbonate rocks: The Tulomozero Formation of southeast Karelia

The U-Pb systematics of 25 carbonate samples collected from the Upper Jatulian Tulomozero Formation in southeastern area of the Baltic shield has been studied. The U-Pb systems of Jatulian carbonates metamorphosed here under conditions of the greenschist facies likely have not been reset completely. Samples have been collected from core sections recovered by drilling 60 to 70 km apart from each other in western and eastern areas of the Onega Lake region. In majority, the rocks sampled characterize a thick upper member of the Tulomozero Formation, and a few samples have been collected in addition from its middle and lower members. The analyzed dolomitic rocks (Mg/Ca=0.60–0.68) have variable Mn (54–1450 ppm) and Sr (31–438 ppm) concentrations. Powdered dolomite samples have been treated preliminary in 1 N solution of ammonium acetate in order to get material for analysis enriched in pre-metamorphic carbonate phases in terms of U-Pb systematics. Five samples have been subjected to stepwise dissolution in 0.5 N HBr to analyze their carbonate phases L-1, L-2, L-3 and siliciclastic fraction for getting a deeper insight into the U-Pb systematics. The U-Pb characteristics of carbonate and siliciclastic fractions suggest deposition of studied carbonate sediments in two different paleobasins. In addition, they show for same samples the partial Pb redistribution between carbonate and siliciclastic components in the course of metamorphism and Pb gain from allogenic metamorphic fluids. The Pb-Pb date of 2090 ± 70 Ma (MSWD = 2.0) is estimated for the least altered dolomite samples from upper member of the Tulomozero Formation, which represent marine sediments of a paleobasin and contain a minimum of siliciclastic material, being the least-contaminated by gained Pb. The date obtained is well consistent with U-Pb and Sm-Nd ages established for the Jatulian volcanogenic rocks in northern and western areas of the Baltic shield.     

Neoproterozoic India within East Gondwana: Constraints from recent geochronologic data from Himalaya

Recent geochronologic data of detrital zircons and neodymium isotopic signatures of the Himalaya, Arabian–Nubian Shield, and Western Australia–East Antarctica (the Pinjarra Orogen/Circum-East Antarctic Orogen) are assessed to estimate the location of Neoproterozoic basement of the Himalaya.

The protolith of the Higher Himalayan Gneisses is considered to have been derived from the Pinjarra Orogen/Circum-East Antarctic Orogen of Western Australia–East Antarctica, and not from the Indian Craton to the south. This conclusion strongly suggests the juxtaposition of the Indian Craton, which forms the basement of the Himalaya, with the Circum-East Antarctic Orogen during the Neoproterozoic when the protolith of the Higher Himalayan Gneisses deposited.     

内蒙古拜仁达坝维拉斯托 超大型银铅锌矿的发现及找矿意义

大兴安岭中南段西坡新近发现的拜仁达坝维拉斯托超大型银铅锌矿,是内蒙古地矿局最新勘查成果,也是自20世纪50年代开展物化探工作以来,应用综合物化探方法找到超大型矿床的典型案例。拜仁达坝矿区银的远景规模大于8000t,铅锌远景规模可达300万t;维拉斯托矿区银的远景规模大于2000t,铅锌铜可达300万t。两矿区可能会发展成为超大型银多金属矿田。为大兴安岭中南段地质矿产勘查和矿产资源评价开创了美好的前景。     

Rb-Sr, K-Ar, H-and O-Isotope systematics of the Middle Riphean shales from the Debengda Formation, the Olenek Uplift, North Sibera

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ¹⁸O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.     

Tectonostratigraphy and base-metal mineralization controls, Aravalli province (western India): New interpretations from geophysical data analysis

Analysis and synthesis of multi-disciplinary geoscience information from geological literature/maps and from digitally-processed aeromagnetic and gravity data pertinent to the Aravalli province were carried out to address some hitherto unresolved questions about the tectonostratigraphy of this Archaean–Proterozoic metallogenic province. Based on the magnetic anomalies, several tectonic domains were identified. These domains, bounded by regional-scale geophysical lineaments, have distinct crustal, lithological, metamorphic, and metallogenic characteristics and correlate broadly with lithostratigraphic belts identified by several earlier workers. New interpretations on the tectonostratigraphy and the base-metal mineralization controls in the Aravalli province are as follows. The Hindoli sequences, in the eastern parts of the province, constitute an independent Palaeo–Proterozoic tectonic domain and do not form part of the Archaean basement complex. The base-metal-bearing metasedimentary enclaves in the central parts of the province also constitute an independent Palaeo–Proterozoic tectonic domain, which is quite distinct from the surrounding (basement complex?) rocks. The base-metal-bearing metavolcano-sedimentary sequences in the western parts of the province constitute an independent Neo–Proterozoic tectonic domain. The base-metal deposits in the province are spatially associated with the regional-scale lineaments and with the mafic metavolcanic rocks deduced from the aeromagnetic data. The regional-scale lineaments, which possibly represent Proterozoic crustal-scale faults, are plausible structural controls on the base-metal mineralization in the province. The mafic metavolcanic rocks are plausible heat-source controls on the SEDEX- and/or VMS-type base-metal mineralizations and are possible metal-source controls on the VMS-type base-metal mineralization in the province.     

Low b-value prior to the Indo-Myanmar subduction zone earthquakes and precursory swarm before the May 1995 M 6.3 earthquake

Some 455 events (m_b ⩾ 4.5) in the Indo-Myanmar subduction zone are compiled using the ISC/EHB/NEIC catalogues (1964–2011) for a systematic study of seismic precursors, b-value and swarm activity. Temporal variation of b-value is studied using the maximum likelihood method beside CUSUM algorithm. The b-values vary from 0.95 to 1.4 for the deeper (depth ⩾60 km) earthquakes, and from 0.85 to 1.3 for the shallower (depth <60 km) earthquakes. A sudden drop in the b-value, from 1.4 to 0.9, prior to the occurrence of larger earthquake(s) at the deeper depth is observed. It is also noted that the CUSUM gradient reversed before the occurrence of larger earthquakes. We further examined the seismicity pattern for the period 1988–1995 within a radius of 150 km around the epicentre (latitude: 24.96°N; longitude: 95.30°E) of a deeper event M 6.3 of May 6, 1995 in this subduction zone. A precursory swarm during January 1989 to July 1992 and quiescence during August 1992 to April 1995 are identified before this large earthquake. These observations are encouraging to monitor seismic precursors for the deeper events in this subduction zone.     

No proof from carbon isotopes in the Francevillian (Gabon) and Onega (Fennoscandian shield) basins of a global oxidation event at 1980–2090 Ma following the Great Oxidation Event (GOE)

Highly depleted C isotope composition of organic matters from the Onega (Fennoscandian shield) and Francevillian (Gabon) basins are differently interpreted. Kump et al. (2011) suggested the occurrence of a massive and global oxidation event during the period of 1980–2090 Ma, which follows the Great Oxidation Event (2450–2320 Ma) (Bekker et al., 2004). Inversely, Gauthier-Lafaye and Weber (2003) invoke the possible action of methanotrophic microorganisms to explain the δ¹³C values as low as –46‰ measured in the Franceville basin. Here we present the isotope data available in the Franceville basin in order to discuss these two interpretations. The lack of any δ¹³C correlation between organic matter and carbonate in the Franceville basin does not allow the consideration of a massive and global oxidation event.     

http://www.sciencedirect.com/science/article/pii/S1674987112000898

In more than 4 Ga of geological evolution, the Earth has twice gone through extreme climatic perturbations, when extensive glaciations occurred, together with alternating warm periods which were accompanied by atmospheric oxygenation. The younger of these two episodes of climatic oscillation preceded the Cambrian “explosion” of metazoan life forms, but similar extreme climatic conditions existed between about 2.4 and 2.2 Ga. Over long time periods, changing solar luminosity and mantle temperatures have played important roles in regulating Earth's climate but both periods of climatic upheaval are associated with supercontinents. Enhanced weathering on the orogenically and thermally buoyed supercontinents would have stripped CO₂ from the atmosphere, initiating a cooling trend that resulted in continental glaciation. Ice cover prevented weathering so that CO₂ built up once more, causing collapse of the ice sheets and ushering in a warm climatic episode. This negative feedback loop provides a plausible explanation for multiple glaciations of the Early and Late Proterozoic, and their intimate association with sedimentary rocks formed in warm climates. Between each glacial cycle nutrients were flushed into world oceans, stimulating photosynthetic activity and causing oxygenation of the atmosphere. Accommodation for many ancient glacial deposits was provided by rifting but escape from the climatic cycle was predicated on break-up of the supercontinent, when flooded continental margins had a moderating influence on weathering. The geochemistry of Neoproterozoic cap carbonates carries a strong hydrothermal signal, suggesting that they precipitated from deep sea waters, overturned and spilled onto continental shelves at the termination of glaciations. Paleoproterozoic (Huronian) carbonates of the Espanola Formation were probably formed as a result of ponding and evaporation in a hydrothermally influenced, restricted rift setting. Why did metazoan evolution not take off after the Great Oxidation Event of the Paleoproterozoic? The answer may lie in the huge scar left by the ～2023 Ma Vredefort impact in South Africa, and in the worldwide organic carbon-rich deposits of the Shunga Event, attesting to the near-extirpation of life and possible radical alteration of the course of Earth history.     

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO₂, high MgO, Mg#, K₂O, Ba, and low Na₂O/K₂O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO₂ content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)_N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.