Petrology and thermobarometry of eclogite rocks in the Krasnaya Guba dike field,belomorian mobile belt

A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all, pressure decreased, the SiO₂ activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes, which were restricted to zones of intense deformations, at variable SiO₂ activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels.     

The Belomorian eclogite province: sequence of events and age of the igneous and metamorphic rocks of the Gridino association

Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U–Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granitic leucosome; U–Pb dating of rutile from mafic dikes; ⁴⁰Ar/³⁹Ar dating of amphibole and mica; and Sm–Nd studies of rocks and minerals. The Sm–Nd model ages of felsic (2.9–3.1 Ga) and mafic (3.0–3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircon grains with an age of ca. 3.0 Ga indicate the Mesoarchean age of the metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the crystallization age of magmatic zircon from metagabbro. Superimposed amphibolitic metamorphism and the “final” exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of the metamorphic associations to 300 °C at 1.9–1.7 Ga is shown by U–Pb rutile dating and ⁴⁰Ar/³⁹Ar mica dating.     

关于大别山榴辉岩相变质作用

大别山产出的榴辉岩相岩石包括石榴橄榄岩、榴辉岩、榴云片岩、榴辉片麻岩、榴玉英岩和榴辉大理岩等不同系列,它们均分布于花岗质片麻岩中。矿物共生序列研究表明,榴辉岩相岩石经历了从绿帘角闪岩相、柯石英榴辉岩相、角闪榴辉岩相、绿帘角闪岩相到绿片岩相的演化过程。花岗质片麻岩及变质火山—沉积岩系并未经历超高压变质作用,但却与榴辉岩相岩石经历了同一期绿帘角闪岩相变质事件,证明二者在地壳范围内发生了构造合并     

Baddeleyite: A promising geochronometer for alkaline and basic magmatism

The paper is devoted to the history of the discovery, petrological and mineralogical characterization, and U-Pb dating of baddeleyite ZrO₂, which was separated from various Archean, Proterozoic, and Paleozoic rocks from Fennoscandia. The morphology of this mineral and its U-Pb age values were examined in the Archean carbonatites (2613 ± 18 Ma) of Siilinjarvi, Finland, and gabbronorite dikes (2738 ± 6 Ma) at the Kirovogorskoe deposit. U-Pb isochrons are reported for the baddeleyite-zircon pair obtained from the gabbronorites and anorthosites of the Proterozoic pyroxenite-gabbronorite-anorthosite association. The U-Pb baddeleyite dates for the early gabbronorite phase (2.5 Ga) and for a gabbronorite dike (late phase, 2.4 Ga) suggest that the basic magmatism evolved over a long time period (100 m.y.) in the Proterozoic. U-Pb dates are also reported for baddeleyite from the Paleozoic carbonatites of Kovdor, Sebljarv, and Vuorijarvi.     

Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value T_Hf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).     

Protolith and metamorphic ages of the Haiyangsuo Complex, eastern China: a non-UHP exotic tectonic slab in the Sulu ultrahigh-pressure terrane

Summary The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km². The U–Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ∼1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ∼340–460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ∼1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation.     

黄陵野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成

采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术测定野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成,以探讨黄陵地区TTG片麻岩原岩的形成及变质时间、是否存在比崆岭群更古老的地壳等问题。野马洞辉绿岩脉(1850 Ma)侵入TTG片麻岩,并从TTG片麻岩中捕获了大量捕掳晶锆石。捕掳晶锆石岩浆结晶核部的U-Pb年龄分别为2842 Ma、2900 Ma和2949 Ma,指示TTG花岗岩体为复式岩体,其至少经历了2949 Ma、2900 Ma和2842 Ma三期岩浆作用。捕掳晶锆石变质边部的U-Pb年龄为2557 Ma,指示TTG花岗岩体转变为TTG片麻岩,是"水月寺运动"及其构造热事件共同作用的结果,其变形变质的时间为2557～2511 Ma。捕掳晶锆石的εHf(t)为-9.85～0.89、平均值为-4.07,亏损地幔模式年龄TDM为3.6～3.2 Ga,指示黄陵地区存在比崆岭群(3.2 Ga)更古老的陆壳。     

New U-Pb and Sm-Nd ages and P-T estimates for eclogitization in the Fe-rich gabbro dyke in Gridino area (Belomorian Mobile Belt)

New data from isotope geochronology (U-Pb, Sm-Nd) petrological study provide evidence of the Svecofennian age (～1.9 Ga) for eclogitization in the Fe-gabbro dyke inductile shear zones of Gridinrea. P-T estimates of eclogitization were computed using the THERIAK/DOMINO software. A close timing relationship between dyke magmatism and eclogitization is inferred.     

Deformation events in the Gridino zone,Belomorian Province,Fennoscandian Shield: relationships between mafic dike swarms and eclogite-bearing mélange

Relationships between reference mafic dikes and deformations in the Gridino zone, Belomorian province, Fennoscandian Shield, make it possible to subdivide the deformations into three groups: pre-dike, synmagmatic, and post-dike. The Neoarchaean eclogite-bearing mélange was formed by disintegration of large eclogite slices in the course of ductile flow, which was associated with synkinematic granitoid magmatism and metamorphism varying from the granulite to amphibolite facies. Exotic blocks, including those of eclogites, are distributed in the TTG gneisses as layers and lenses, whose thicknesses range from a few to a few hundred metres and which are conformable with the foliation. Ductile flow brought the rock complexes to the depth level where brittle–ductile deformations were possible. As a result, certain parts of the mélange were deformed in a more rigid setting. A number of mafic dike swarms were emplaced into relatively cold rocks in an extensional environment in the earliest Palaeoproterozoic. The dikes cut across all earlier structures and are thus an important benchmark for distinguishing Neoarchaean and Palaeoproterozoic processes. Post-dike (~1.9 Ga) tectonic activity was associated with local deformations and discrete metamorphic retrogression to amphibolite facies. None of them significantly affected the pre-existing regional structure.     

The Narryer Gneiss Complex of the Yilgarn Block, Western Australia: a segment of Archaean lower crust uplifted during Proterozoic orogeny

The Narryer Gneiss Complex of the Yilgarn Block is a key segment of the Western Australian Precambrian Shield. It is a regional granulite facies terrain comprised of predominantly quartzo-feldspathic gneisses derived from granitic intrusions c. 3.6–3.4 Ga old. Granulite facies metamorphism occurred c. 3.3 Ga ago, and conditions of 750–850°C and 7–10 kbar are estimated for the Mukalo Creek Area (MCA) near Errabiddy in the north. The P–T path of the MCA has been derived from metamorphic assemblages in younger rocks that intruded the gneisses during at least three subsequent events, and this path is supported by reaction coronas in the older gneisses. There is no evidence for uplift immediately following peak metamorphism of the MCA, and a period of isobaric cooling is inferred from the pressures recorded in younger rocks. Pressures and temperatures estimated from metadolerites, which intruded the older gneisses during ‘granite–greenstone’tectonism at about 2.6 Ga and during early Proterozoic thrusting show that the Errabiddy area remained in the lower crust, although it was probably reheated during the younger events. Isothermal uplift to upper crustal levels occurred at c. 1.6 Ga ago, and was followed by further deformation and patchy retrogression of high-grade assemblages. The effects of younger deformation, cooling and reheating can be discerned in the older gneisses, but as there has been no pervasive deformation or rehydration, the minerals and microstructures formed during early Archaean granulite facies metamorphism for the most part are retained. The MCA remained in the lower crust for about 1700 Ma following peak metamorphism and some event unrelated to the original metamorphism was required to exhume it. Uplift occurred during development of the Capricorn Orogen, when some 30–35 km were added to the crust beneath the Errabiddy area. The recognition of early Proterozoic thrusting, plus crustal thickening, suggests that the Capricorn Orogen is a belt of regional compression which resulted from convergence of the Yilgarn and Pilbara Cratons.     

Tectonic evolution of the North Qinling Orogen from subduction to collision and exhumation: Evidence from zircons in metamorphic rocks of the Qinling Group

The North Qinling Block (NQB) is an important segment of the Qinling Orogen in Central China. Here we report the results from SIMS geochronology and oxygen isotopes, as well as LA-MC-ICPMS Hf isotopic analyses on zircon grains from a suite of metamorphic rocks (felsic gneisses, garnet plagioclase amphibolites, and retrograde eclogite dikes) in the Qinling Group of the NQB. The age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483–501 Ma, followed by 454–470 Ma retrograde metamorphism. These results are generally coeval with the periods of 500–480 Ma for peak metamorphism and 460–420 Ma for retrograde metamorphism previously obtained from the HP/UHP metamorphic rocks of the NQB. During the prograde and retrograde metamorphism, widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons. The fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low Th/U ratios, positive ε_Hf(t) (> 5) and extremely δ¹⁸O (10.01–13.91‰) values in metamorphic zircons. In contrast, the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high U and Th contents and Th/U ratios and enriched Hf (ε_Hf(t) = − 5.42 to − 0.18) and oxygen isotope composition (δ¹⁸O around 8‰). Geochronological and geochemical features of the magmatic cores of the clear core-rim textured zircons demonstrate that the protoliths of the gneisses were intermediate-acid volcanic rocks erupted before Neoproterozoic (800 Ma), which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes, with the oldest magmatic zircon formed at 789 Ma. The protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism. Combined with the previous studies, the Qinling Group experienced overall subduction in the Early Paleozoic. The NQB as represented by the Qinling Group was most likely a discrete micro-block in the Neoproterozoic, and underwent deep subduction in the Cambrian (483–501 Ma) and exhumation in Ordovician (454–470 Ma). We propose that the NQB preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading, and micro-continent formation to deep subduction and exhumation.     

Thermodynamic regime of Svecofennian (1.9 Ga) metamorphism of the Umba nappe of the Lapland collisional orogen

Thermodynamic regime of culmination phase of high-grade metamorphism of the Umba nappe (Lapland allochthon) was studied, and peak metamorphic monazite was dated. Based on calculation of end member reactions, the metamorphic assemblages of aluminous gneisses from the upper and lower parts of the nappe are close to equilibrium. The metamorphic conditions of the rocks are estimated to be about 800°C and 7 kbar in its upper part and 9 kbar in its lower part. The formation of orthopyroxene-sillimanite aggregates points to increasing pressure and temperature at the prograde stage of PT path, whereas cordierite rims mark the onset of decompression and cooling. The pressure difference of 2–2.5 kbar identified by thermobarometric methods between aluminous gneisses from the upper and lower parts of the Umba nappe corresponds to a depth gradient about 7.5 km, which agrees with approximate thickness of the Umba nappe. The study of the eclogitelike rocks developed after the rocks of the Paleoproterozoic Kolvitsa gabbronorite massif made it possible to trace the P-T evolution of metamorphsim: the temperature peak of granulite stage (11 kbar, 860°C) was followed by pressure increase (up to 14 kbar and more), and then decompressional cooling due to the exhumation of the Por’ya Guba nappe together with the Kolvitsa Massif. The U-Pb monazite age of 1904 ± 3 Ma obtained for aluminous gneisses from the upper part of the Umba nappe corresponds within error to the timing of metamorphic events determined from metamorphic zircon in the anorthosites of the Kolvitsa Massif (1907 ± 2 Ma) and zircon from aluminous gneisses in the melange zone (1906 ± 3 Ma). These isotopic data confirm the conclusion of simultaneous high-pressure granulite metamorphism in the upper and lower portions of the Umba nappe.     

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8?C2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ??1.9 Ga) episode of eclogite metamorphism of Archean rocks.     

大别山榴辉岩一片麻岩杂岩的成因

大别山榴辉岩由辉长岩、大陆拉斑玄武岩和少量泥灰质经高压变质作用形成。大别地块可划分出四个形成条件不同的榴辉岩区,它们代表一种构造－岩石组合体。片麻岩杂岩中各种高压变质岩类的发现证明它们与榴辉岩一起经历了原地高压变质过程。二者变质作用Ｐ－Ｔ参数的差异归因于抬升过程中退变质反应速度的不同。不同地区榴辉岩退变质组合及Ｐ－Ｔ条件与围岩的一致性表明,大别杂岩现今所展示的“递增”变质带是由榴辉岩相退变质作用形成的。高压榴辉岩－片麻岩杂岩的产生是印支期扬子与华北两个大陆板块碰撞的结果。     

Paleoproterozoic metamorphism in the northern Fennoscandian Shield: age constraints revealed by monazite

ABSTRACT

We have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling.

U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76?1.81 Ga with only a few older exceptions in samples showing a spread of ²⁰⁷Pb/²⁰⁶Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum.

In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86?1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of ²⁰⁷Pb/²⁰⁶Pb ages from 1.89?1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80?1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of ²⁰⁷Pb/²⁰⁶Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86?1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes.     

Mesoarchean mafic dykes of the Belomorian eclogite province (Gridino Village Area,Russia)

Archean processes of eclogitization in the Gridino metamorphic association (the Belomorian eclogite province) developed in mafic dykes, boudins, and acidic rocks of the Archean continental crusts. To determine the U-Pb age of the intrusion of the latest dykes, the geochronological samples were taken from the dyke of ferriferious metagabbro that cross-cuts the dyke of eclogitzed and granulitized olivine gabbronorite. The igneous zircons were dated by the SHRIMP II technique. The zircons showed a concordia age of 2846 ± 7 Ma, which is considered as the time of intrusion of a mafic melt. The younger low-thorium zircon rims of 2.78–2.81 Ga age around the igneous cores are typical formations that appeared under metamorphic conditions in equilibrium with a migmatite melt, and may characterize the time of formation of the granite leucosome under metamorphism, probably of eclogite facies.     

北阿尔金地区古元古代ca.2.0Ga岩浆-变质事件

中国有三个主要的克拉通,分别是华北、华南和塔里木,它们在显生宙经造山过程聚集到一起。塔里木克拉通位于中国的西北部,面积超过60万平方千米,其北侧为中亚造山带,南侧为西昆仑造山带和阿尔金造山带。塔里木克拉通的前寒武纪岩石主要出露在其南北两侧边缘,包括库鲁克塔格、敦煌、阿尔金、铁克里克和阿克苏地块,它们记录了塔里木克拉通早期的构造演化。北阿尔金地块的阿克塔什塔格地区位于塔里木克拉通的东南边缘。该地区最老的岩石被称为米兰群或阿克塔什塔格杂岩。主要岩石包括太古宙的TTG岩石和表壳岩,以及古元古代的片麻状花岗岩,另有少量变质基性岩呈包体状出露在强变形的长英质侵入体中。本文对该区闪长质片麻岩开展了锆石SHRIMP U-Pb定年,同时还对变质基性岩进行了锆石LA-ICP-MS U-Pb测年和地球化学分析,目的是要约束北阿尔金地区古元古代的岩浆-变质事件。闪长质片麻岩的结晶年龄为2.04～2.03Ga,它们形成于岛弧环境。地球化学分析表明,变质基性岩的原岩是拉斑玄武岩。它们有类似于E-MORB的平坦的稀土配分模式,Nb、Ta、Zr、Hf不亏损,说明它们形成于大洋板内环境。在变质基性岩中还识别出两期变质锆石,其中2.05～2.01Ga的早期锆石代表了麻粒岩相的变质作用,而1.98～1.96Ga的晚期锆石可能与角闪岩相的退变质作用有关。无论该区岩浆作用还是变质作用都与约2.0Ga发生的俯冲增生造山事件有关。     

吉林省大荒沟—板石沟早前寒武纪基底的岩石单元划分及其地质演化

根据１５万区域地质调查，将区内的早前寒武纪基底划分为变质上壳岩、中粗粒黑云长英片麻岩、变黑云母钾长花岗岩和变质基性岩四个岩石单元。通过对上述岩石单元岩石类型、地球化学特征、变质变形作用及同位素年代学研究，对其形成时序进行了讨论，由此确定了本区早前寒武纪基底的地质演化轮廓，即在中晚太古时期，本区经历了由玄武安山岩和英安岩双峰式火山建造为主体的上壳岩系的形成阶段，并于２．６Ｇａ遭受角闪岩相变质；随即伴有大规模的ＴＴＧ深成岩浆活动，晚太古末经历绿帘角闪岩相的区域变质作用；至早元古初期，深熔成因的钾质花岗岩侵位，区内已存的早期变质岩石受到该期钾质岩浆的交代改造，并在其成岩之后遭受绿片岩相的区域变质。     

Petrology and structure of the Palaeoproterozoic (1.9 Ga) Rytky nickel sulphide deposit,Central Finland: a comparison with the Kotalahti nickel deposit

The Palaeoproterozoic (1.9 Ga) Rytky and Kotalahti mafic-ultramafic intrusions are located in the contact zone between the Archaean craton and Proterozoic supracrustal rocks. During the second deformation event (D₂) the surrounding country rocks were subjected to intensive metamorphism and deformation associated with the Svecofennian orogeny; the Archaean/Proterozoic boundary controlled both D₂ thrusting and magma ascent. Emplacement of the Rytky and Kotalahti intrusions took place at the culmination of D₂, as shown by the gneiss inclusions with S₂ schistosity within the intrusions. Overthrusting continued after emplacement, with detached fragments of the bodies incorporated into the Archaean gneisses. During the third deformation event (D₃) the originally subhorizontal intrusions were rotated into a subvertical position, so that they now have their stratigraphic top towards the west. The Rytky intrusion is composed mainly of medium- and coarse-grained lherzolite, websterite and gabbronorite. The nickel deposit with pentlandite as the main nickel mineral is associated with the lherzolite and websterite. The coarse-grained lherzolite, websterite and melagabbro represent the first rocks to form, and they contain the nickel sulphide mineralisation. Country rock contamination, as indicated by high TiO₂, P₂O₅, Rb, Zr and light rare earth element contents (LREE), is most pronounced in the marginal part of the intrusion, which was the first to form. The variation in olivine composition (Fo 78.6-84.77 mole %; Ni 630–2386 ppm) and the metal ratio of the sulphide (Ni/Co 19.3 – 50.3) along with the internal stratigraphy of the intrusion indicate an in-situ process of sulphide ore formation.Editorial handling: P. LightfootAn erratum to this article can be found at     

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.