Gravimetric data on structure of the crust in Central and Western Caucasus

Analysis of gravimetric data in the Central and Western Caucasus shows three zones within the main range. These zones coincide with regions of linear, transitional, and domed folding which occur in sequence in the Central Caucasus, Western Caucasus, and western orogenic end of the main range. The Central Caucasus is an area of maximum depth of the M surface. The 'thickness of the granite layer decreases westward and increases eastward. The,position of the deep structures leaves no doubt that the strike of the Caucasus is the dominant direction in this area, though this is most clearly seen in the granite layer. The Caucasus as a folded area arose in relation to the production of the granite layer, the basalt masses being much less active in producing the mountain system of the Caucasus. Maps showing depths of the granite, basalt, Conrad surface, and M surface as well as structural sections and gravity profiles are included. — C. E. Sears.     

Structure and late cenozoic evolution of the upper lithosphere in Southwest Tuscany (Italy)

Geological and geophysical data on southwest Tuscany are reviewed in order to define the structure and evolution of the upper lithosphere from the Miocene to the Quaternary. Petrologic studies reveal the existence, below all of Tuscany, of Hercynian and older polyphased metamorphic rocks and of Hercynian granite, whose top is an important seismic reflecting horizon. The basement is characterized by NE-SW trending structures, in contrast with the main NW-SE “Alpine” structures of the uppermost levels. The heat flow map shows two broad areas with values higher than 80 mW/m², reaching maximum values of 10.5 and 15 H.F.U. in the geothermal areas, which are also characterized by negative Bouguer anomalies. A Landsat study revealed a NE-SW band of subcircular structures passing through Larderello and coinciding with a regional fault system and a steep rise in the Moho. Petrologic, geochemical and radiometric data on the Tuscan igneous rocks show that partial melting took place in the Tuscan crust at different levels and to varying degrees from the Miocene to Quaternary, producing a continuous “Alpine” granitic layer. The known Tuscan intrusive bodies and two batholiths below the Larderello and Mt. Amiata geothermal fields represent culminations of the “Alpine” granite. The rise of the Tuscan magmas was closely correlated to a post-Tortonian tensional tectonics and followed its N-E migration. Tensional tectonics started after the last compressional phase (10–11 Ma B.P.) as a consequence of the anticlockwise rotation of Italy, the opening of the Tyrrhenian Sea and the swelling of the mantle below southwest Tuscany.     

Topography of the crust–mantle boundary beneath the Black Sea Basin

A map of Moho depth for the Black Sea and its immediate surroundings has been inferred from 3-D gravity modelling, and crustal structure has been clarified. Beneath the basin centre, the thickness of the crystalline layer is similar to that of the oceanic crust. In the Western and Eastern Black Sea basins, the Moho shallows to 19 and 22 km, respectively. Below the Tuapse Trough (northeastern margin, adjacent to the Caucasus orogen), the base of the crust is at 28 km, whereas in the Sorokin Trough, it is as deep as 34 km. The base of the crust lies at 29 and 33 km depths respectively below the southern and northern parts of the Mid-Black Sea Ridge. For the Shatsky Ridge (between the Tuapse Trough and the Eastern Black Sea Basin), the Moho plunges from the northwest (33 km) to the southeast (40 km). The Arkhangelsky Ridge (south of the Eastern Black Sea Basin) is characterised by a Moho depth of 32 km. The crust beneath these ridges is of continental type.     

Neotectonic transformation of Cenozoic fold structures in the northwestern Caucasus

The performed morphotectonic regionalization of the northwestern Caucasus shows that the fold structures directly expressed in the topography of the territory and continuing to evolve under the settings of contemporary lateral shortening predominate in the northwestern Caucasus. A map of fold structures expressed in the topography of the northwestern Caucasus is presented. The districts distinguished therein correspond to the largest regional tectonic units, the fold topography of which occurs at various stages of tectonic evolution from primary brachyanticlinal ridges of the Taman and Sochi districts to the complex fold–thrust and inversion fold ridges of the axial zone. Data on active newly formed fold and inversion structures are given. These inherited structures develop under the combined action of selective denudation, beddingplane upthrow faulting, and thrusting.     

Late Panafrican evolution of the main Hoggar fault zones: Implications of magnetic fabric study in the In Telloukh pluton (Tin Serririne basin, Algeria)

The Late Panafrican evolution of the Hoggar shield is characterized by emplacement of magmatic intrusions and by occurrence of major shear zones separating different terranes. In Telloukh granite is close to the In Guezzam faults (western border of the Tin Serririne basin). Analysis of its visible and magnetic fabrics suggests an emplacement mode and deformation that are not related to the In Guezzam faults, but most likely to a N–S compression, an event not yet identified. Dioritic dykes crosscutting the granite have a very different magnetic fabric, which is related on the contrary to dextral strike-slip movements along the In Guezzam faults. In both cases, no visible fabric can be correlated with the magnetic fabric, which has been likely acquired during late magmatic stages. This magnetic fabric was not significantly affected by the tectonic events that took place after entire crystallization of the magma. The In Guezzam faults and the major 7°30 and 4°50 shear zones are close to intrusions such as In Telloukh dykes and the Alous En Tides and Tesnou plutons where quite similar magnetic fabrics are observed, all related with dextral strike-slip movements along these structures.     

南岭稀土花岗岩、钨锡花岗岩及其成矿作用的对比

南岭地区的钨锡和稀土矿床都与花岗岩类有直接成因联系,但二者的成矿作用有许多不同之处.钨锡是典型的热液成矿,而稀土则主要形成于风化作用.随着花岗岩类的分异演化,岩石中的W、Sn等元素含量逐渐增加,因此钨锡等矿床主要与高度分异演化的晚阶段小岩体有关;但是稀土的表现与钨锡不同,由于花岗岩类的分异演化导致稀土栽体黑云母及许多副矿物的减少,因此稀土元素含量在晚阶段岩体中反而降低.赣南的五里亭-大吉山岩体、桂东北的花山-姑婆山岩体等提供了很好的范例.因此,南岭地区与风化壳型稀土矿床有关的岩石主要有:印支期准铝质花岗岩,燕山期A型花岗岩,燕山中-晚期黑云母二长花岗岩等.     

Tectonic setting of post-orogenic granites within SW Fennoscandia based on deep seismic and gravity data

Within the Svecononvegian Province of SW Scandinavia granite intrusions are abundant. Prominent ones are the Bohus-Iddefjord and the Flå granites, the so- called Bohus-Flå Granite belt. The age of these granites, consistent at ≅ 900 Ma, coincides with the late extensional stage of the Svecononvegian-Grenvillian orogen. Gravity observations and deep seismic profiling lines are presented that also cover the Skagerrak Sea. The geophysical data suggest that the Bohus granite continues seaward for at least 80 km and its thickness, offshore, is estimated to be 2–4 km. Where the seismic profiling lines intersect the inferred seaward extension of the Bohus granite, a distinct reflection pattern is observed at ≅ 1.6 s. TWT. This coincides with a gravity modelled thickness of ≅ 4 km. The seismic profiles also show a large Moho offset beneath the modelled granite. It is proposed that this offset is related to Svecononvegian crustal underthrusting and that the granite melt could have formed by anatexis of mid-crustal rocks downthrusted to greater depths in the vicinity of the seismically observed Moho offset.     

Ulkan-Dzhugdzhur ore-bearing anorthosite-rapakivi granite-peralkaline granite association,Siberian Craton: Age,tectonic setting,sources, and metallogeny

The paper systematizes and integrates the results of geological, isotopic geochronological, and geochemical studies of the igneous rocks that make up the Ulkan-Dzhugdzhur anorthosite-rapakivi granite-peralkaline granite association and related mineralization. This association is a typical example of anorogenic igneous rocks that formed in the within-plate geodynamic setting most likely under effect of the mantle superplume, which was active in the territory of the Siberian Craton 1.75–1.70 Ga ago. The igneous rock association formed in a discrete regime that reflected the pulsatory evolution of a sublithospheric mantle source. The prerift (1736–1727 Ma) and rift proper (1722–1705 Ma) stages and a number of substages are distinguished. All igneous rocks pertaining to this association have mixed mantle-crustal origin. Basic rocks crystallized from the OIB-type basaltic magma, which underwent crustal contamination at various depths. Felsic rocks are products of mantle and crustal magma mixing. The contribution of mantle component progressively increased in a time-dependent sequence: moderately alkaline subsolvus granite → moderately alkaline and alkaline hypersolvus granites → peralkaline hypersolvus granite. All endogenic deposits in the studied district are related to a single source represented by the mantle plume and its derivatives. The Fe-Ti-apatite deposits hosted in anorthosite formed as a result of intense lower crustal contamination of basaltic magma near the Moho discontinuity and two stages of fractional crystallization at lower and upper crustal depth levels. The rare-metal deposits are genetically related to peralkaline granite. Formation of uranium deposits was most likely caused by Middle Riphean rejuvenation of the region, which also involved rocks of the Ulkan-Dzhugdzhur association.     

Crustal structure of the western part of the Southern Granulite Terrain of Indian Peninsular Shield derived from gravity data

The Southern Granulite Terrain (SGT) is composed of high-grade granulite domain occurring to the south of Dharwar Craton (DC). The structural units of SGT show a marked change in the structural trend from the dominant north–south in DC to east–west trend in SGT and primarily consist of different crustal blocks divided by major shear zones. The Bouguer anomaly map prepared based on nearly 3900 gravity observations shows that the anomalies are predominantly negative and vary between −125 mGal and +22 mGal. The trends of the anomalies follow structural grain of the terrain and exhibit considerable variations within the charnockite bodies. Two-dimensional wavelength filtering as well as Zero Free-air based (ZFb) analysis of the Geoid-Corrected Bouguer Anomaly map of the region is found to be very useful in preparing regional gravity anomaly map and inversion of this map gave rise to crustal thicknesses of 37–44 km in the SGT. Crustal density structure along four regional gravity profiles cutting across major shear zones, lineaments, plateaus and other important geological structures bring out the following structural information. The Bavali Shear Zone extending at least up to 10 km depth is manifested as a plane separating two contrasting upper crustal blocks on both sides and the gravity high north of it reveals the presence of a high density mass at the base of the crust below Coorg. The steepness of the Moyar and Bhavani shears on either side of Nilgiri plateau indicates uplift of the plateau due to block faulting with a high density mass at the crustal base. The Bhavani Shear Zone is manifested as a steep southerly dipping plane extending to deeper levels along which alkaline and granite rocks intruded into the top crustal layer. The gravity high over Palghat gap is due to the upwarping of Moho by 1–2 km with the presence of a high density mass at intermediate crustal levels. The gravity low in Periyar plateau is due to the granite emplacement, mid-crustal interface and the thicker crust. The feeble gravity signature across the Achankovil shear characterized by sharp velocity contrast indicates that the shear is not a superficial structure but a crustal scale zone of deformation reaching up to mid-crustal level.     

Lithospheric Evolution and Geodynamic Process of the Qinghai-Tibet Plateau:An Inspiration from the Yadong-Golmud-Ejin Geoscience Transect

The Tibet Geoscience Transect (Yadong-Golmud-Ejin) has revealed the basic structures, tectonic evolution and geodynamic process of the lithosphere of the Qinghai-Tibet plateau. The evidence of northward thrusting of the Indian plate beneath the Himalayans on the southern margin and to southward compression of the Alxa block on the northern margin has been found. They were the driving forces causing the plateau uplift. The plateau is a continent resulting from amalgamation of eight terranes. These tenanes are separated by sutures or large-scale faults, and different terranes have different lateral inhomogeneities and multi-layered lithospheric structures. At depths of about 20-30 km of the crust in the ulterior of the plateau there commonly exists a low-velocity layer. It is an uncoupled layer of the tectonic stress; above the layer, the upper crustal slices were thrust and overlapped each other and the rocks underwent brittle deformation, thus leading to shortening and thickening of the upper crust Belo     

Late Panafrican evolution of the main Hoggar fault zones: Implications of magnetic fabric study in the In Telloukh pluton (Tin Serririne basin,Algeria)

The Late Panafrican evolution of the Hoggar shield is characterized by emplacement of magmatic intrusions and by occurrence of major shear zones separating different terranes. In Telloukh granite is close to the In Guezzam faults (western border of the Tin Serririne basin). Analysis of its visible and magnetic fabrics suggests an emplacement mode and deformation that are not related to the In Guezzam faults, but most likely to a N–S compression, an event not yet identified. Dioritic dykes crosscutting the granite have a very different magnetic fabric, which is related on the contrary to dextral strike-slip movements along the In Guezzam faults. In both cases, no visible fabric can be correlated with the magnetic fabric, which has been likely acquired during late magmatic stages. This magnetic fabric was not significantly affected by the tectonic events that took place after entire crystallization of the magma. The In Guezzam faults and the major 7°30 and 4°50 shear zones are close to intrusions such as In Telloukh dykes and the Alous En Tides and Tesnou plutons where quite similar magnetic fabrics are observed, all related with dextral strike-slip movements along these structures.     

东北地区花岗岩地质图(1∶1500000)编图及其说明

在收集前人资料和最新区调及科研成果文献的基础上,梳理厘定公开发表文献的花岗岩岩体同位素年龄数据1093个,硅酸盐数据772套,微量元素数据627套,稀土元素数据653套.依据这些数据及最新调查成果编制东北地区花岗岩地质图(1∶1 500 000).该图突出了构造-花岗岩区划作用.此次研究将前中生代构造-花岗岩类划分为2个构造花岗岩域、2个构造花岗岩省和7个构造花岗岩区(带),将中新生代构造-花岗岩类划分为2个构造花岗岩域、3个构造花岗岩省和7个构造花岗岩区(带),使得花岗岩类的时空演化特征更加明显.     

黑龙江伊春地区早中生代花岗岩成矿作用 及成矿能力差异性探讨

伊春地区晚三叠世-早侏罗世花岗岩分布广泛,岩石主要类型有似斑状二长花岗岩-二长花岗斑岩、正长-碱长-碱性花岗岩等,在该地区已发现的与早中生代花岗岩侵入密切相关的铁多金属及金等大小矿床的成因类型主要为斑岩型、矽卡岩型、矽卡岩-热液型、岩浆热液型、浅成低温热液型等,成岩、成矿时代为晚三叠世-早侏罗世(U-Pb锆石LA-ICPMS年龄为191～225Ma之间),成矿时代略晚于成岩时代.依据早中生代花岗岩岩石类型、时代、构造-岩浆演化、花岗岩成因类型及其陆陆碰撞不同阶段构造背景下的成矿作用特征,将矿床成矿系列划分为:碰撞-碰撞后构造转换型似斑状二长花岗岩-二长花岗斑岩Mo-Pb-Zn-Fe-W-Au (Ag)成矿亚系列和碰撞后崩塌型正长-碱长-碱性花岗岩Fe-Mo-Pb-Zn-NbTa成矿亚系列,其成矿元素、矿化强度、类型等成矿特征上的差异,可能与源区物源、壳幔混合程度、混合比例和侵位深度,以及与成矿构造背景不同等综合因素有关.     

Tungsten Mineralization and Mica REE Geochemistry,Huangsha,Jiangxi

The Huangsha Ag-rich tungsten deposit is genetically related to a buried granite which shows apparent vertical zoning in alteration.Greisen-type W(Mo) ores coccur at the top or the intrusive and sulfide-wolframite-quartz veins developed at the major stage of mineralization are present in low-grade metamorphic rocks in the outer-contacts.The veins exhibit a reversed zonation in the vertical section with silver concentrated in the lower part in association with sulfides.Micas,characterized by high Si and low Al.are extensively developed both in the granite and in the veins.They have similar cell parameters.belonging to 2M1 type,but those in the veins are understanding of the petrogenesis, evolution and mineralization of the granite.     

裂谷盆地下的壳-幔结构及其成因机制讨论——以由近垂直反射地震剖面揭示的松辽盆地下的壳-幔结构为例

详细研究了七段、约130km长的近垂直反射地震剖面所揭示的松辽盆地下的壳幔结构。松辽盆地之下壳幔之间为相对厚的Moho过渡带。可分为薄Moho和透镜状Moho以及过渡型Moho,与之对应的壳幔结构也有差异。松辽盆地的发育阶段明显地可分为断陷期、拗陷期以及夭折期,不同的壳幔结构可能与松辽盆地不同的发育期存在密切联系。此外,通过对比,认为裂谷盆地区具有类似的特殊壳幔结构。在裂谷区,壳幔之间强烈相互作用,不同的地壳组成和裂谷形成的不同阶段是形成不同壳幔结构的主要原因,而且它们之间具有成因上的联系,并与裂谷的形成阶段相一致。     

The evolution of the layered lower crust and the Moho through geological time in Western Europe: contribution of deep seismic reflection profiles

Deep seismic reflection images from a set of profiles shot in Western Europe have been reviewed and compared, and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the Moho. The disappearance of Variscan mountain roots is related to the set-up of a new Moho at a typical 30-km depth and the creation of seismic layering in the lower crust. Deep seismic profiles suggest that these processes resulted, at least in part, from magmatic intrusion, partial crustal melting and metamorphism of deep crustal rocks into eclogite. On the other hand, the layered lower crust is greatly attenuated beneath Cretaceous basins and Tertiary rifts in relation to prominent Moho upwellings. The unusual amplitude of the Moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the Tertiary rifts suggest that the Moho and part of the layering are comparatively young features related to interactions between crust and mantle. Beneath Triassic-Jurassic basins, the layered lower crust was not affected by the subsidence of the basement, with the whole crustal thinning being entirely concentrated in the upper crust. This indicates that the layered lower crust and the Moho were formed or restored during or after the main rifting phase. Seismic data reveal constraints on the processes that affect the crust-mantle transition and seem to restore the Moho to its typical depth after any mechanical deformation of the lithosphere.     

Structure,Matter Composition,and Deep Structure of the Oceanic Slope of the Central Kuril Islands: New Evidence

The results of reinterpretation of the geophysical data obtained during the study of the central sector of the Kuril Island Arc (2005–2010) are reported. The new boundaries of the shallow bedding of the basement and its block uplifts of varied composition are defined within the previously discovered zone of tectonic stretching and destruction of the Vityaz submarine ridge and interarc trough. Interblock depressions filled with sedimentary and volcanogenic-sedimentary rocks are distinguished. Areas represented by volcanoplutonic complexes including basic and sialic series (up to granite) are distinguished within the unbroken basement of the Vityaz Ridge. Intrusions and volcanic edifices composed of basic rocks are registered on the whole area studied. The relationship between the formation of the destruction zone and the geodynamic processes in the mantle is illustrated by the map of the Moho relief and structural-density model of the Earth’s crust.     

湖南船岭脚锡矿区矿化特征及成因探讨

船岭脚锡矿为南岭锡多金属成矿带的一大型锡矿床。矿化类型较多,以构造蚀变带-矽卡岩复合型、接触带矽卡岩型锡矿为主,分布于中侏罗世姑婆山岩体接触带,且具分带性,自岩体内接触带往外依次为构造蚀变带-矽卡岩复合型、接触带矽卡岩型。矿化与成矿岩体及构造复合等因素密切相关,成矿物质主要来源于岩浆,为一与花岗岩有关的中高温矽卡岩型矿床。     

The role of dextral transpressional faulting in the evolution of an early Carboniferous mafic–felsic plutonic and volcanic complex: Cobequid Highlands, Nova Scotia, Canada

The Wentworth plutonic complex, consisting of gabbro and granite, was emplaced in the earliest Carboniferous in the Cobequid shear zone of the northern Appalachians. The plutonic complex is coeval with a 5-km-thick pile of volcanic rocks. Early alkalic A-type granite correlates with thick felsic pyroclastics and minor basalt, which are overlain by 1.5-km-thick basalts that correlate with a large gabbro pluton that is intruded, in turn, by late granites. The basalt and gabbro are Fe-rich tholeiites. The geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma. The Wentworth plutonic complex lies on the north side of the dextral Rockland Brook fault, near the western tip of wedge-shaped basement block of the Avalon terrane. Field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike–slip faults. Dykes parallel to the mylonitic foliation in the Rockland Brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction. The plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone-bounded blocks, while the sinking of those blocks finally provided the space for mafic magma emplacement. Dyking was active over at least a 10-Ma time period. The overall location of plutonism in the Cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the Magdalen basin and the E–W transpressional contact of the Avalon and Meguma terranes. Magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge-shaped basement blocks, whose movement controlled the localisation of the voluminous magmatic activity.     

江西龙南稀土花岗岩的锆石U-Pb年龄、内生矿化特征及成因讨论

江西省龙南地区离子吸附型稀土成矿花岗岩出露广泛,然而由于缺乏精确的同位素年代学依据,致使对各岩体的侵位时代、岩石成因等方面的认识存在分歧。本文对足洞、牛坑及半坑花岗岩的风化壳(或基岩)样品进行了LA-MC-ICPMS锆石U-Pb定年,获得206Pb/238U加权平均年龄分别为:(168.2±1.2)Ma、(168.3±1.7)Ma和(209.75±0.86)Ma,表明足洞和牛坑花岗岩体的侵位时代一致,均形成于燕山期,晚于寨背—关西岩体(~195 Ma),更晚于印支期侵位的半坑岩体。足洞—牛坑岩体的稀土配分类型为重稀土型,岩石学、矿物学方面具有相似性,可能为同源岩浆同期分离结晶的产物;寨背—关西岩体和半坑岩体的稀土配分类型均为轻稀土型,岩石学、矿物学方面具相似性,可能为同源岩浆不同期次形成的产物。而足洞—牛坑岩体与寨背—关西岩体具有不同的稀土矿物组合、稀土配分模式和微量元素特征(寨背—关西花岗岩风化壳的Zr/Hf比值(20~60)大于足洞—牛坑岩体(20),且Zr/Hf比值与Nb/Ta比值正相关),可能来自不同的岩浆源区。