Late Cretaceous-Paleocene magmatic complexes of central Kamchatka: Geological settings and compositional features

A comparative analysis of the Late Cretaceous-Paleocene volcanism was conducted for four areas of Kamchatka: the Pravyi Tolbachik-Levaya Shchapina-Adrianovka interlfuve (the northern part of the Tumrok Range), the area south of the Ipuin River and Mt. Khrebtovaya (the northern Valaginsky Range), the area of Mt. Savul’ch (the upper reaches of the Kitil’gina River, northern Valaginsky Range), and the Kirganik-Levaya Kolpakova interfluve (the Sredinny Range). New petrochemical, geochemical, and isotopic data on the volcanic rocks from these areas are reported. The examination of this material, together with already published data on volcanic and plutonic rocks of similar composition and age, made it possible to establish the following: (1) the considered basaltoids are ascribed to the subalkali basalt-trachyandesite series with transition toward a meymechite-picrite rock association; (2) the alkali content in the rocks of the Valaginsky-Tumrok-Sredinny ranges increases simultaneously with the increase of the Rb content, while the contents of HFSE and radioactive elements decrease and then again increase. Two trends are identified in the Yb_n-Ce_n diagram: a positive trend spanning most of the volcanic and plutonic rocks and a negative trend defined by the data points of the meymechite-picrite association. The first trend reflects the rock evolution during crystallization differentiation, while the second trend was produced by different degrees of melting of initial protolith. The possible geodynamic reconstructions of this volcanism are discussed as well.     

Distinctive features of tin-ore potential of central Chukotka

During the Early Triassic, the lacustrine facies of the Moscow syneclise was regularly replaced eastward by alluvial-lacustrine and alluvial facies. The Early Triassic accumulation involved, in addition to the Moscow syneclise (its axial zone and northwestern slope), the Latvian saddle and the Polish-Lithuanian syneclise, that is, a sedimentation area significantly larger than the present one. Triassic sedimentation within the Moscow syneclise marked a new transgressive stage of the Hercynian tectogenic cycle and differed markedly from the Late Permian regressive stage of sedimentation. —Authors.     

赣中变质岩带的组成及构造变质变形特征

赣中变质岩带不是简单的一套震旦纪地层 ,而是由结晶基底中元古界中深变质岩系 (斜长角闪岩的Sm Nd全岩等时线年龄为 1113± 4 9Ma)和褶皱基底变质较浅的震旦系组成。两者之间以具热流体参与的混合岩化、韧性剪切带和递进变质三位一体组成的动热变质带接触。结晶基底经历了4期构造变质变形的叠加改造 ,每期构造变形都在变质岩石构造单元内留下各种变形形迹 ,变质作用表现为时间上的递进和空间上的叠加演化系列 ,是一套以众多的不平衡结构和多相共生混存的矿物组合 ,热变质带为一套动热变质塑性变形带 ,空间上依次形成绢云母—绿泥石带、黑云母带、石榴石带、十字石带以及夕线石带     

Surface features associated with transform faults: A comparison between observed examples and an experimental model

In several tectonic provinces where active ridge segments are offset, transform faults are expected but not observed. This paper discusses the evolution of the surface expression of some transform faults with the help of a few geological examples and a simple experimental clay model in which the importance of en-échelon fault systems is assessed. We conclude that the azimuth of observed fault traces may not coincide with the direction of movement, but be oblique to it. Thus we must be cautious when using a fieldobserved fault direction to infer a transform-fault direction for use in plate-tectonics models. This study also suggests the scale at which the assumption of rigid plates fails.     

四川盆地川中地区走滑断裂的分布、类型与成因

近期研究发现四川克拉通盆地内存在大型走滑断裂带,是断控油气藏勘探开发的有利新领域,走滑断裂的分布与成因对油气目标评价具有重要意义。本文在三维结合二维地震资料解释的基础上,分析走滑断裂的分布与成因类型,并探讨走滑断裂的形成机制。结果表明,川中三维地震工区发育北西向走向为主的大型板内走滑断裂系统;走滑断裂呈雁列、斜列的不连续的带状分布,断裂发育成熟度低,具有断裂样式的多样性与分层分布的差异性;四川盆地存在川中克拉通内走滑断层、川东南帚状走滑构造、川东调节走滑带、川东北楔入走滑带、川西北斜向冲断走滑带等5个地区发育5种类型走滑断裂带;走滑断裂形成于震旦系灯影组沉积晚期,受控于原特提斯洋俯冲背景下的斜向伸展作用,基底北西向先存断裂构造复活,形成了调节斜向伸展裂陷槽的川中板内走滑断裂系统。结果揭示克拉通盆地可能发育大型的板内走滑断裂系统,不同于常规的板缘与板内调节走滑断裂系统。     

Normal faults associated with volcanic activity arc

Volcanic centers (volcanoes, fumaroles or solfatara fields), epicenters of strong shallow earthquakes (with focal depths up to 20 km) and epicenters of intermediate depth strong earthquakes (with focal depths between 120 and 160 km) in the southern Aegean volcanic arc can be grouped into five, well defined, linear clusters trending about N60°E. This lineation of shallow earthquakes and volcanic activity is attributed to five corresponding normal faults which are named after the five corresponding volcanic centers (Sousaki, Methana, Milos, Santorini and Nisyros). This is supported by a similar trend of the geomorphological features (grabens and islands) and of geophysical features (Bouguer anomalies), as well as by other seismological data (fault plane solutions and the origins of tsunamis) and geological information on the Santorini caldera. The greater volcanic activity in the eastern volcanic centers (Santorini and Nysiros) compared to the western volcanic centers (Sousaki, Methana and Milos) is attributed to the higher rate of extensional crustal deformation. In the eastern part of the volcanic arc it is 26 mm/yr: in the west it is 2 mm/yr. The delineation of the epicenters of the intermediate depth earthquakes along the same five lines indicates the existence of five corresponding rupture zones in the lower (leading) part of the descending lithospheric slab (at depths of 120–180 km). These deep zones are probably the sources of hot material which is ascending vertically upwards and intrudes into the crust along its fracture zones. The orientation of these zones explains the focusing of the macro-seismic results of these deep shocks at narrow regions of the sedimentary arc (Peloponnesus, Crete, etc).     

Active faults in the Zagros and central Iran

Active tectonic movements in the northwestern Zagros include right lateral slip at the rate of about 10 mm/a along the Main Recent Fault, which inherits the position of the Main Thrust, now inactive, and active thrusting and accompanying folding distributed between several zones southwest of the Main Recent Fault. In the southeastern Zagros (the Fars Province), there are several right lateral faults that extend N–S obliquely to the overall trend of the Zagros fault-and-fold belt. These may be either branches of the Main Recent Fault, or faults accommodating relative broadening of the outer Zagros in its southeastern segment. The Main Thrust in the southeastern Zagros also remains inactive.

The Ipak, North Tehran, and Mosha fault zones and several minor structures in the eastern Alborz form the E–W-trending active fault system with combined reverse and left lateral slip. On the Ipak and Mosha zones, lateral movements with the late Quaternary mean rate exceeding 1 mm/a dominate over vertical fault movements. Together with right lateral faults stretching northeast of Zagros, the faults of the Alborz may accommodate east-directed motion of the Iranian microplate.     

Formation of gold mineralization in ultramafic alkalic magmatic complexes

Study of mineral inclusions within alluvial gold particles of the Guli Complex (East Siberia) and findings of lode gold in rocks of the same intrusion have demonstrated that gold mineralization occurs in interstitions of both early high–magnesium rocks (dunite) and later alkalic and carbonatite rocks. In dunite the native gold occurs in association with Fe–Ni sulfides (monosulfide solid solution, pentlandite, and heazlewoodite). Formation of the gold-bearing alloys took place under a low oxygen potential over a broad range of temperatures: from those close to 600°C down to below 400°C.     

K-Ar dating of ring complexes and fault systems in Northern Kordofan,Sudan: evidence for independent magmatic and tectonic activity

From an area of brittle shearing and ring complexes in Northern Kordofan, Sudan 28 K-Ar ages were calculated to determine the geochronology of tectonic and magmatic events. The age results for deformational episodes are 560, 340 and 255 Ma, whereas alkaline magma was intruded at 280, 220 and 165 Ma. The incompatibility of age marks leads to the conclusion that magmatic and tectonic activity in this area have to be regarded as independent events. This holds true even when all dated intrusions from the entire region are compared with post-Pan-African continental tectonic features.     

Magnetic susceptibility mapping of felsic magmatic lithounits in the central part of Bundelkhand Massif,central India

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600–2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG), grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10⁻³ SI with an average of 6.35×10⁻³ SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10⁻³ SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10⁻³ SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10⁻³ SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10⁻³ SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31–10.22×10⁻³ SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10⁻³ SI) and pink (av. MS=6.72×10⁻³ SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.     

Mafic magmatic enclaves and mafic rocks associated with some granitoids of the central Sierra Nevada batholith, California: nature, origin, and relations with the hosts

The calc-alkaline granitoids of the central Sierra Nevada batholith are associated with abundant mafic rocks. These include both country-rock xenoliths and mafic magmatic enclaves (MME) that commonly have fine-grained and, less commonly, cumulate textures. Scarce composite enclaves consist of either xenoliths enclosed in MME, or of MME enclosed in other MME with different grain size and texture. Enclaves are often enclosed in mafic aggregates and form meter-size polygenic swarms, mostly in the margins of normally zoned plutons. Enclaves may locally divert schlieren layering. Mafic dikes, which also occur in swarms, are undisturbed, composite, or largely hybridized. In central Sierra Nevada, with the exception of xenoliths that completely differ from the other rocks, host granitoids, mafic aggregates, MME, and some composite dikes exhibit a bulk compositional diversity and, at the same time, important mineralogical and geochemical (including isotopic) similarities. MME and host granitoids display distinct major and trace element compositions. However, strong correlations between MME–host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in each pluton. Identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution. Mafic dikes have broadly the same major and trace element compositions as the MME although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids. The composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred, respectively, at depth, during ascent and emplacement, and after emplacement. The occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas. The sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes. Unusual MME that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth, whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement. MME and host granitoids are considered cogenetic, because both are hybrid rocks that were produced by the mixing of the same two components in different proportions. The felsic component was produced by partial melting of preexisting crustal materials, whereas the dominant mafic component was probably derived from the upper mantle. However, in the lack of a clear mantle signature, the origin of the mafic component remains questionable.     

中国休眠火山区岩浆来源气体地球化学特征

中国休眠火山区岩浆来源气体的最主要成分是CO2,含量占80％～99％;其它气体组分还有CH4、He、H2、N2、Ar、O2、H2S、SO2、CO等,其中CH4和He是CO2以外最值得重视的直接来自地幔岩浆体的气体组分。He的同位素组成（^3He／^4He比值）是休眠火山区最可靠的幔源物质的指示性参数;岩浆来源的CO2和CH4也具有特征的区别于浅源气体的稳定同位素组成（δ^13C值）。长白山火山区近期CO2和CH4的碳同位素分馏监测结果显示,2002年汪清7．2级深震虽然可能引发了深部岩浆的局部扰动,但是它可能并没有产生特别大量的、持续不断的上地幔岩浆物质流的上升迁移。这意味着,长白山天池火山区近期可能没有火山喷发活动的现实危险。     

长白山火山区高精度Rayleigh面波相速度结构与岩浆系统

近年来,有关长白山火山是否存在潜在喷发危险的讨论引起了国内外地学研究者的广泛关注,但人们目前对其壳幔岩浆系统的了解却十分不足,已有的研究对长白山火山壳内岩浆房存在的深度位置、形态规模及其部分熔融程度的认识仍存在较大分歧。本研究通过汇集长白山火山及邻区（包括朝鲜境内）多个密集流动地震台阵和区域固定地震台网的观测资料,采用背景噪声成像方法获得了长白山火山区高精度的Rayleigh面波相速度模型。成像结果表明：长白山火山中-下地壳深度位置存在显著低波速异常,可能指示了岩浆房的存在。上地壳局部低速区可能反映了壳内深部岩浆向上运移的通道或者是区域小尺度的岩浆聚集体。长白山火山上地幔顶部的低速可能揭示了软流圈热物质上涌,其减压熔融为壳内岩浆房提供了幔源岩浆补给。

     

Distinctive petrological, geochemical, and geodynamic features of subduction-related magmatism

Geological-petrological and geochemical data on subduction-related magmatism (including the volumes and compositions of the corresponding magmatic series) are compared to the results of experiments and numerical simulation. The subduction zone is subdivided into five depth sectors and volcanic zones I, II, and III: 1 is the accretionary wedge that controls the geodynamic stability of subduction; 2 is the sector of dehydration and fluid filtration; 3 is the zone of eclogitization and initial partial melting in the slab above which boninite volcanic zone I is formed during early stages; 4 is the main zone of melting of the sedimentary-basite layer and the development of volcanic zone II with the predominance of andesites; and 5 is the zone of higher degree melting, above which volcanic zone III (basaltic andesite and alkali basalt) is formed. The criterion of volcanism intensity, which was obtained within the scope of the melting model, is proportional to the subduction velocity and the thickness of the melting zone, and the distance between the groups of volcanics along the subduction zone is 75–100 km, at a thickness of the melting zone of 15–20 km. The calculated isotherm of 600°C, which controls the stability of serpentine and chlorite, is not identified at depth above 150 km, and this is confirmed by the composition and P-T conditions of the high-pressure rocks (containing diamond and coesite), which were brought from depths of 150–200 km in subduction zones. Seismic sections constructed with regard for the amplitude characteristics of seismic waves show two melting zones (“wet” melting at a depth of 100–200 km and “dry” melting at a depth of 150–200 km) and a complicated thermal structure of the suprasthenospheric wedge, which can include slant magma conduits. The mineralogical and geochemical features of arc magmatic series are formed at a decisive role of an H₂O-CO₂ fluid and an elevated oxidation potential. The predominant buffer minerals are as follows: garnet in the slab melting zone; magnetite, Ca-pyroxene, and amphibole in intermediate magmatic chambers; and amphibole, protoenstatite-bronzite (in place of olivine), and Cr-spinel (in place of magnetite) for boninite series generated in a “hot” asthenospheric wedge at interaction with fluids or water-rich melts. Actively disputable problems are the interactions scale of melts and fluids generated in a subduction zone with a “hot” mantle wedge, the possibility of transporting water-rich minerals deep into the mantle (to depths greater than 150 km), and the evolution of the scale at which young continental crust is generated by subduction melts.