Spatiotemporal relationships of dike magmatism in the Kola region, the Fennoscandian Shield

A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74?C1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8?C0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth??s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.     

Comprehensive paleoseismic geological studies in a key site in southwestern Kola Peninsula (Northeast of the Fennoscandian Shield)

This paper considers the results of detailed paleoseismic and geological investigations in a key site in the wall of the Imandra Lake depression (Kola Peninsula Region, Northeast of the Fennoscandian Shield). Study of different groups of paleoseismic deformations developed in the fault zone and the application of new methods and techniques made it possible to identify a large seismotectonic zone characterized by great earthquakes at the end of the Late Glacial and in the Holocene. The investigation data are indicative of the necessity to estimate the seismic potential in the Kola Atomic plant area in a different way.     

A palaeomagnetic analysis of rapakivi intrusions and related dykes in the Fennoscandian Shield

In the Fennoscandian Shield (Baltica) there are seven major rapakivi plutons and fifteen minor ones ranging in age from ca 1.66 to 1.50 Ga. These plutons are distributed in a broad WNW zone and if the most eastern pluton is excluded there is a westward trend of decreasing age of the intrusions. A palaeomagnetic study has been performed on 4 minor plutons (Rödö, Mårdsjö, Norsjö and Mullnäset) and associated dykes in central Sweden. The results were combined with palaeomagnetic data from other rapakivi complexes in Fennoscandia in order to test if a stationary hot spot may be the origin of these anorogenic intrusions. Plotting the pole positions of this study together with poles of other complexes, poles calculated from rapakivi rocks and related dykes in Finland are located at somewhat lower latitudes and more eastern longitudes than poles of corresponding rocks in Sweden, probably reflecting an APW related to the general age differences between the plutons. The palaeolatitudes for the Fennoscandian Shield at the time of the rapakivi intrusions are restricted to a latitudinal range between ca 16° south and 27° north and there is a weak trend of increasing palaeolatitude with decreasing age of the rocks. A trend of gradually changing palaeolatitudinal positions has also been observed for the intrusion of Proterozoic anorthosite-rapakivi plutons in the Ukranian Shield. Such differences in palaeolatitudes is not expected in case of a single stationary hot spot being the source of the rapakivi intrusions, as the rock then should carry a magnetization reflecting the same latitudinal position.     

A 2.44 Ga syn-tectonic mafic dyke swarm in the Kolvitsa Belt,Kola Peninsula,Russia: implications for early Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield

The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with the earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Palaeoproterozoic history of this belt comprises the deposition of the 2.47 Ga Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneisses, the intrusion of the 2.45–2.46 Ga Kolvitsa Massif of gabbro-anorthosite and the subsequent multiple injection of mafic dykes and magmatic brecciation, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive shearing at 2.43–2.42 Ga. The gabbro-anorthosite and dykes contain high-pressure garnet-bearing assemblages that have previously been considered as evidence for metamorphism in a compressional setting of the Kolvitsa Belt at 2.45–2.42 Ga, i.e. coeval with the formation of the Imandra–Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape study area on the White Sea coast presents an unique remnant of a 2.44 Ga mafic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-trending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent shearing during or immediately after solidification, and later dykes intruded into already sheared dykes. Thus, rocks of the Kolvitsa Massif and its dyke complex were successively injected into a large-scale shear zone which was active from ca. 2.46 to 2.42 Ga. Multiple injection of mafic melts, the presence of mutually intruding, composite, sheared mafic dykes, of magmatic breccias with gabbroic groundmass, and of host rocks fragments (showing no evidence of tectonic stacking at the time of brecciation), all indicate an extensional setting. Shearing was also extensional as it occurred simultaneously with the multistage magmatism. The asymmetric morphology of deformed dykes, and asymmetric flexures within weakly deformed lenses show that all these extensional shear zones, apart from a few exceptions, are dextral, were formed in a transtensional setting and are attributed to general W–E to WSW–ENE extension. Structural data available for 2.4–2.5 Ga magmatic rocks elsewhere in the Kola region suggest that the same kinematics operated on a regional scale. The presence of the garnet-bearing assemblages in gabbro-anorthosite and dykes may be explained by crystallisation and shearing of the magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the juxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvitsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, an overall extensional setting rather than a combination of compressional and extensional zones is preferable for Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield at 2.5–2.4 Ga.     

Paleoseismic Events Reflected in Late Pleistocene and Holocene Deposits of the Terraces of Imandra Lake (Kola Region,Fennoscandian Shield)

Doklady Earth Sciences - Soft-sediment deformation structures have been identified in glaciolacustrine deposits of the terrace complexes of Imandra Lake (Kola Region, northeastern part of the...     

Fine-grained mafic bodies as preserved portions of magma replenishing layered intrusions: the Nadezhda gabbronorite body, Lukkulaisvaara intrusion, Fennoscandian Shield, Russia

Summary The 100 m thick and 700 m long Nadezhda body in the Lukkulaisvaara layered intrusion exhibits concentric zonation with an inward progression from a 0.5 to 1.0 m thick marginal layer of medium- to coarse-grained norites and gabbronorites that abruptly give way to fine-grained oikocrystic gabbronorites composing the rest of the body. The concentric zonation is additionally emphasized by well-developed alignment of plagioclase laths and orthopyroxene oikocrysts parallel to the outer contacts of the body, pegmatitic gabbronorite segregations in the centre of the body and slight inward decrease in whole-rock Mg# and Cr and increase in incompatible elements. The body has distinctly higher whole-rock Mg# and lower concentrations of all incompatible components than its host rocks. It is enveloped by highly altered marginal anorthosites belonging to host norites and gabbronorites. We interpret the Nadezhda body as a portion of high Mg# (∼75%) and incompatible element-poor (∼20 ppm, Zr; ∼10 ppm, total REE; ∼0.20 wt%, TiO₂) magma that replenished the evolving chamber and became trapped within the cumulate pile. Recrystallization of adjacent rocks by volatiles exsolved from the magma upon emplacement resulted in formation of marginal anorthosites. Upon cooling the magma started to crystallize medium- to coarse-grained norites along its margins, but subsequent decompression and loss of volatiles led to rapid crystallization of magma into fine-grained oikocrystic gabbronorites. Solidification of the remaining residual liquid gave rise to pegmatitic gabbronorite segregations. Supplementary material to this paper is available in electronic form at Tables 3–6 available as electronic supplementary material Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200 Apatity, Russia; Present address: Department of Geosciences, University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland     

Archaean Kuru-Vaara eclogites in the northern Belomorian Province,Fennoscandian Shield: crustal architecture,timing, and tectonic implications

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.     

Early Proterozoic regolith at Pasvik, NE Norway: palaeoenvironmental implications for the Baltic Shield

Regolith occurs at a number of localities along the northern margin of the early Proterozoic Pechenga Greenstone Belt marking a profound stratigraphic unconformity at the base of the 10 km thick Petsamo Supergroup. The age of regolith formation can be bracketed between 2.453 ± 42 Ga (Sm-Nd) and 2.330 ± 36 Ga (Rb-Sr). Regolith is developed on a varied substrate of Archaean rocks including early Proterozoic layered gabbros (2.45 Ga). The best example of the regolith occurs in the Pasvik valley at Brattli, Norway. Analogues are widespread on the Baltic Shield at the base of early Proterozoic 'greenstone belts' showing a low degree of chemical weathering and carbonatisation. Fluviatile deposits overlying the Brattli regolith preserve unweathered pyrite-magnetite boulders and terrigenous pyrite grains. These features are considered to relate an arid or semi-arid palaeoenvironment, possibly with some deficiency of oxygen in the atmosphere-hydrosphere system. A systematic and well-preserved regolith or palaeosaprolite at the base of the Petsamo Supergroup is proof that the northern boundary of the Pechenga Greenstone Belt is a fundamental first-order unconformity. This places constraints on geotectonic modelling of this early Proterozoic rift-basin.     

Mineralogy and geochemistry of Devonian ultramafic minor intrusions of the southern Kola Peninsula, Russia: implications for the petrogenesis of kimberlites and melilitites

Minor magmatic intrusions of kimberlite, melilitite and cpx-melilitite occur in the southern part of the Kola Peninsula, Russia, on the Terskii Coast and near the town of Kandalaksha. They yield K-Ar ages of 382 ± 14 Ma and 365 ± 16 Ma, similar to the magmatic rocks from the Kola Alkaline Province. The Terskii Coast kimberlites have mineralogical and geochemical affinities with group 1 kimberlites, whereas the Kandalaksha monticellite kimberlite more closely resembles calcite kimberlites. The lower Al₂O₃ content in the Kola kimberlites indicates a strongly depleted harzburgitic source, while higher Al₂O₃ in the melilitites suggests a lherzolitic source. The Terskii Coast kimberlites are anomalously potassic and significantly enriched in P and Ba compared to other group 1 kimberlites. In contrast, the melilitites are sodic and are anomalously depleted in P compared to worldwide melilitites. Trace element patterns of the Kola kimberlites and melilitites indicate the presence of K- and P-rich phases in the mantle source. To account for the K-troughs shown by both magma types, a K-rich phase such as phlogopite is thought to be residual in their sources; however, the anomalous K-enrichment in the Terskii Coast kimberlites may indicate that an additional metasomatic K-rich phase (e.g. K-richterite and/or a complex K-Ba-phosphate) existed in the kimberlite source. The P-depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual in the melilititic source. However, anomalous P-enrichment in the kimberlites cannot be explained by complete melting of the same phase because the kimberlites are a smaller degree melt; thus, it is most likely that another metasomatic phosphate mineral existed in the source of the kimberlites. The Kola kimberlites and melilitites are all strongly LREE-enriched but the kimberlites have a steeper REE pattern and are significantly more depleted in HREE, indicating a higher proportion of garnet in their source. Higher Nb/Y ratios and lower SiO₂ values in the kimberlites indicate that they were a smaller degree partial melt than the melilitites. The presence of diamonds in the Terskii Coast kimberlites indicates a relatively deep origin, while the melilitites originated from shallower depth. The non-diamondiferous Kandalaksha monticellite kimberlite has lower abundances of all incompatible trace elements, suggesting a higher degree of partial melting and/or a less enriched and shallower source than the Terskii Coast kimberlites. The ⁸⁷Sr/⁸⁶Sr_i, ¹⁴³Nd/¹⁴⁴Nd_i and Pb isotope compositions confirm that the Terskii Coast kimberlites have close affinities with group 1 kimberlites and were derived from an asthenospheric mantle source, while the Kandalaksha monticellite kimberlite and Terskii Coast melilitites were derived from lithospheric mantle. Impact of a Devonian asthenospheric mantle plume on the base of the Archaean-Proterozoic lithosphere of the Kola Peninsula caused widespread emplacement of kimberlites, melilitites, ultramafic lamprophyres and other more fractionated alkaline magmas. The nature of the mantle affected by metasomatism associated with the plume and, in particular, the depth of melting and the stability of the metasomatic phases, gave rise to the observed differences between kimberlites and the related melilitites and other magmas. Received: 3 March 1997 / Accepted: 7 October 1997     

Rheological structure and dynamical response of the DSS profile BALTIC in the SE Fennoscandian Shield

Numerical modelling was applied to study the present-day state of stress and deformation under different tectonic loading conditions at the seismic BALTIC–SKJ profile in south-eastern Finland and in Estonia. The finite element method was used to solve the numerical problem. The two-dimensional model was constructed using the results from both seismic and thermal studies along the profile. The model is 700 km long and 200 km deep, and is roughly divided into an inhomogeneous, laterally layered crust and a homogeneous mantle lithosphere. Both the linear elastic and non-linear elasto-plastic rheologies were used. Elasto-plasticity was achieved by calculating a rheological strength as a function of depth along the profile. Different tectonic load cases were analysed with displacement, force and pressure type boundary conditions. Also, the effect of different strain rates was investigated. The results suggest that even with relatively low compressive stress levels the lower crust deforms in a plastic manner for a wet crustal rheology. When applying a dry crustal rheology, plastic yielding is attained only with much higher stress fields.     

Integrated 3D density modelling and segmentation of the Dead Sea Transform

A 3D interpretation of the newly compiled Bouguer anomaly in the area of the “Dead Sea Rift” is presented. A high-resolution 3D model constrained with the seismic results reveals the crustal thickness and density distribution beneath the Arava/Araba Valley (AV), the region between the Dead Sea and the Gulf of Aqaba/Elat. The Bouguer anomalies along the axial portion of the AV, as deduced from the modelling results, are mainly caused by deep-seated sedimentary basins (D > 10 km). An inferred zone of intrusion coincides with the maximum gravity anomaly on the eastern flank of the AV. The intrusion is displaced at different sectors along the NNW–SSE direction. The zone of maximum crustal thinning (depth 30 km) is attained in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal thickness of the region (38–42 km). Linked to the left lateral movement of approx. 105 km at the boundary between the African and Arabian plate, and constrained with recent seismic data, a small asymmetric topography of the Moho beneath the Dead Sea Transform (DST) was modelled. The thickness and density of the crust suggest that the AV is underlain by continental crust. The deep basins, the relatively large intrusion and the asymmetric topography of the Moho lead to the conclusion that a small-scale asthenospheric upwelling could be responsible for the thinning of the crust and subsequent creation of the Dead Sea basin during the left lateral movement. A clear segmentation along the strike of the DST was obtained by curvature analysis: the northern part in the neighbourhood of the Dead Sea is characterised by high curvature of the residual gravity field. Flexural rigidity calculations result in very low values of effective elastic lithospheric thickness (t _e < 5 km). This points to decoupling of crust in the Dead Sea area. In the central, AV the curvature is less pronounced and t _e increases to approximately 10 km. Curvature is high again in the southernmost part near the Aqaba region. Solutions of Euler deconvolution were visualised together with modelled density bodies and fit very well into the density model structures. An erratum to this article can be found at     

2.5 Ga gabbro-anorthosites in the Belomorian Province,Fennoscandian Shield: Petrology and tectonic setting

The Vorochistoozersky, Nizhnepopovsky, and Severo-Pezhostrovsky gabbro-anorthosite massifs have been studied in the central part of the Belomorian Province, Fennoscandian Shield. The similarity of geological setting and rock composition of these massifs suggests their affiliation to a single complex. The age of the gabbro-anorthosites was determined by U-Pb (SHRIMP II) zircon dating of gabbro-pegmatites from the Vorochistoozersky massif at 2505 ± 8 Ma. The studied massifs were overprinted by the high-pressure amphibolite facies metamorphism. Relicts of magmatic layering and primary magmatic assemblages preserved in the largest bodies. The massifs consist mainly of leucocratic gabbros but also contain rocks of the layered series varying in composition from olivinite to anorthosite. The presence of troctolites in the layered series indicates the stability of the olivine–plagioclase liquidus assemblage and, respectively, shallow depths of melt crystallization. Despite the composition differences between gabbro-anorthosites of the Belomorian and peridotite–gabbronorite intrusions Kola provinces, these simultaneously formed massifs presumably mark a single great igneous event. It also includes the gabbronorite dikes in the Vodlozero terrane of the Karelian province, the Mistassini swarm in the Superior province, and the Kaminak swarm in the Hearne Craton, Canadian Shield. The large igneous province of age ~2500 Ma reflects the oldest stage of within-plate magmatism after a consolidation of the Neoarchean crust of the Kenorland Supercontinent (Superia supercraton).     

大别山典型燕山期侵入体中矿物成分特征及其地质意义

分析了大别造山带核部典型燕山期侵入体中的斜长石、黑云母和角闪石的化学成分和成因特点,并从矿物平衡角度讨论了典型侵入体岩石成因。结果表明：（１）石鼓尖侵入岩中,斜长石成分较稳定,共生角闪石和黑云母为岩浆平衡结晶的矿物;（２）天堂寨和九资河侵入岩中,部分斜长石和黑云母可能为深蚀残留矿物,角闪石属于火成岩区的新生矿物,可能为斜长石和黑云母发生融反应所形成;（３）三个侵入体的岩石均为岩浆成因。     

内蒙古长山壕金矿区花岗岩同位素年代学研究及地质意义

长山壕金矿床是近年来在内蒙古中西段找到的一特大型金矿床。尽管金矿化在中元古界白云鄂博群变质沉积岩内呈层状、似层状和透镜状产出,但是与各类花岗岩类侵入岩具有密切空间分布关系。本次研究采用LA-ICP-MS方法分别对有关花岗岩类侵入岩进行了系统年代学研究,花岗斑岩和二长花岗斑岩样品的锆石U-Pb年龄值分别为290.9±2.8Ma (MSWD=1.4)和287.5±1.9Ma (MSWD=2.4);2件黑云母花岗岩样品的年龄值分别为267.9±1.2Ma (MSWD=0.95)和274.0±2.3Ma (MSWD=1.4)。根据上述同位素年龄数据,同时结合金矿区野外地质调查和岩(矿)相学证据,可以认为,长山壕金矿区花岗斑岩和二长花岗斑岩及相关金矿床的形成时间为早二叠世早期,并且遭受到早二叠世晚期-中二叠世早期构造-岩浆活动的叠加改造,矿区切穿含金矿脉黑云母花岗岩体的存在就是很好的例证。强烈的中酸性岩浆作用为金矿床的形成提供了动力、热力和物质来源,初步研究结果表明,长山壕金矿床是海西期构造-岩浆作用及相关流体活动的产物,属于与侵入岩有关的中温热液脉型金矿床。     

Archaean and palaeoproterozoic migmatizations in the Belomorian Province,Fennoscandian Shield: Petrology,geochronology, and geodynamic settings

Multiple repetitions of migmatization processes are an important indication of the polychronous evolution of Precambrian Mobile Belts: this is certainly true for the Belomorian Belt. In the Belomorian Province of the Fennoscandian Shield, newly obtained data demonstrate the effect of two stages of melting of the Earth’s crust under conditions of higher pressure up to 8–14 kbar. The early stage of the migmatization and genetically related leucogranite formation took place in the Neoarchaean (2710 ± 15 and 2706 ± 14 Ma, U–Pb zircon ages), while the younger one happened in the Palaeoproterozoic (1944 ± 12 and 1882 ± 9 Ma, U–Pb zircon ages of leucosomes). The early stage of crust melting is related to collision in the Belomorian Neoarchaean orogen, while the later stage occurred during formation of the Lapland–Kola orogen.     

Deep 3D structure of the Sydney Basin using gravity modelling

A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north–south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.     

Svecofennian Orogen of the Fennoscandian Shield: Compositional and isotopic zoning and its tectonic interpretation

Endogenic events in the form of intrusive activity and regional metamorphism developed asynchronously in various parts in the Svecofennian Orogen of Fennoscandia. The Early and Late Svecofennian stages of regional high-temperature metamorphism and related plutonism are distinguished from isotopic evidence. The composition, structural features, and asynchronous peaks of endogenic activity within the orogen indicate that at least two zones (inner and outer) should be distinguished in the Svecofennides. The lateral heterogeneity of the orogen in present-day coordinates is traced southward from the margin of the Archean craton. The conjugation zone of the Svecofennian Orogen and the Archean Karelian Craton is characterized by transition from negative to positive εNd (1.9 Ga) values as evidence for a decreased contribution of Archean crustal material to the source of Proterozoic granitoids from the north toward the Proterozoic domain in the south. With allowance for lateral compositional and isotopic heterogeneity of the Svecofennian Orogen and asynchronous culmination of endogenic events in different parts of this orogen, a new scheme of tectonic regionalization has been proposed.     

廊固凹陷的三维精细地质结构

本文通过三维地震资料建立了廊固凹陷的三维精细地质结构.通过重建三维地质结构,可以更为直观的显示其真实地质结构形态.对廊固凹陷三维地质结构的分析表明,认为廊固凹陷新生代以来经历多期构造活动,形成了上、中、下3个构造层:下构造层为孔店组—沙四段,为裂陷初始期沉积;中构造层为沙三段—东营组,为强烈裂陷期沉积;上构造层为明化镇...     

黑龙江三道湾子金矿床火山岩-次火山岩年代学、地球化学和地质意义

三道湾子金矿床位于小兴安岭北麓,是典型的以碲金化合物为主的矿床,为了限定其成矿时代和厘定其成矿地质背景,笔者对同成矿地质体火山岩和成矿期后的玄武粗安玢岩的单颗粒锆石U-Pb年龄和元素地球化学进行测定,获得同成矿地质体粗面英安斑岩及玄武粗安玢岩的~(206)Pb/~(238)U年龄加权平均值分别为(124.2±0.9)Ma(MSWD=0.5,n=18)和(118.9±2.3)Ma(MSWD=0.5,n=6);赋矿围岩为玄武粗安岩与同成矿地质体粗面英安斑岩,Si O2含量为52.75%~62.30%,K_2O和Na_2O含量分别为2.02%~2.85%和3.96%~4.24%。里特曼指数(δ)在0.85~3.67之间,属于钙碱性到高钾钙碱性岩系特征;成矿期后脉岩玄武粗安玢岩,SiO_2含量为51.53%~52.32%,K_2O含量为2.30%~2.36%,Na_2O含量为3.78%~4.14%,里特曼指数(δ)在4.05~4.86之间;微量和稀土元素均显示富集大离子亲石元素(Cs、K、Rb、Ba、LREE等),相对亏损高场强元素(Nb、Ta、Zr、Hf等)和HREE,特别是强烈亏损Th、U和Nb、Ta的特征,与岛弧钙碱性玄武岩相似。研究揭示了该矿床形成于早白垩世(124~119 Ma)或晚中生代造山后伸展构造环境,成矿热动源可能是交代大洋性质的岩石圈地幔。