Plagioclase microfabrics in a ductile shear zone from the Jotun Nappe,Norway

Gabbroic and anorthositic rocks from the Jotun Nappe are transected by small ductile shear zones in which a high-grade paragenesis and a new foliation are formed. Most plagioclase grains show evidence of plastic deformation, and have recrystallized by subgrain rotation and ‘bulge’ processes to fine-grained mylonite. During these processes a plagioclase grain-shape fabric was destroyed, and with increasing strain a pre-existing plagioclase crystal fabric was successively modified, enhanced and finally obliterated. This could be evidence of superplastic flow. Inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that (021) ⊥ 〈a〉 could be an operative slip system, and planes close to (132) could be possible slip planes.     

The trace element geochemistry of some high-grade Bergen-Jotun kindred gneisses from the Jotun Nappe around Gjendebu, central southern Norway

Abstract The high-grade meta-plutonic rocks of this study lie entirely within the Jotun Nappe of the southern Norwegian Caledonides. They are divisible on the basis of metamorphic grade and petrographic character into three units, the Storadalen Complex (SCX), the Svartdalen Gneiss (SG), and the Mjølkedøla Purple Gabbro (MPG). The SCX is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinel-Iherzolite granulite facies grade. The broadly monzonitic SG is weakly tectonized and internally differentiated. Its metamorphic grade does not exceed plagioclase-lherzolite granulite facies grade. The mis-named MPG is also broadly of monzonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the MPG and SG, but the contact between these two units and the SCX is the steeply dipping Tyin-Gjende Fault. The three units represent a comagmatic body of mid-Proterozoic age, metamorphosed during a Sveconorwegian event and finally dismembered and upthrust during the Caledonian Orogeny. The new trace element analyses reported here show that the three rock units have remarkably similar trace element abundances and trends. K-Rb covariation shows increasing K/Rb ratios with increasing K. These patterns were produced by magmatic fractionation processes acting at deep crustal levels, possibly in the presence of a non-aqueous fluid phase. With the exception of K and Sr, close similarities exist between the rocks of this study and present-day calc-alkaline basalts and andesites from island arcs. The high K content is regarded as a primary magmatic feature, but the available data are insufficient to indicate its origin. The Sr contents are abnormally high and are ascribed to metasomatism which occurred during either high-grade metamorphism or post-climactic cooling. There are no systematic geochemical variations with metamorphic grade or degree of deformation.     

Late-orogenic Sveconorwegian massif anorthosite in the Jotun Nappe Complex,SW Norway,and causes of repeated AMCG magmatism along the Baltoscandian margin

The age and tectonometamorphic history of massif anorthosite in the Jotun Nappe Complex, SW Norway, were investigated by zircon and titanite U–Pb ID-TIMS. The anorthosite contains sparse zircons showing complex U–Pb systematics reflecting events dated at 965 ± 4 and 913 ± 2 Ma, and a pronounced Caledonian metamorphic overprint. The oldest age is interpreted as the protolith age of the massif anorthosite. We propose that the Jotun anorthosite is related to 970–960 Ma magmatism in the Western Gneiss Region and coeval, orogen-perpendicular extension. Conversely, a 930 Ma high-grade metamorphic event in the Jotun Nappe Complex and the related Lindås Nappe is likely related to formation of the autochthonous ca. 930 Ma Rogaland anorthosite complex. We suggest that the two late- to post-orogenic AMCG events reflect two instances of lithospheric foundering below the orogen separated by ca. 20–30 my. The 913 ± 2 Ma metamorphic episode appears to date a heating event restricted to the outermost edge of the Western Gneiss Region. Leucosome formation in high-grade gneisses geographically close to the Jotun anorthosite is dated at 892 ± 4 Ma and suggested to reflect CO₂-rich (?) fluid flux along shear zones.     

Discovery of diamond in the Tromsø Nappe,Scandinavian Caledonides (N. Norway)

We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm^?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm^?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp²‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.     

Discovery of eclogite facies carbonate rocks from the Lindås Nappe, Caledonides, Western Norway

Eclogite facies carbonate rocks have been discovered associated with the granulite–eclogite transitional rocks within Bergen Arc system, Caledonian Orogen of western Norway. The local occurrences of marbles and calc‐silicates are found subparallel to the mafic eclogite facies shear zones on Holsnøy Island. Marbles contain the assemblage calcite (Ca_0.99Sr_0.01CO₃), calcian strontianite (Ca_0.18?0.44Sr_0.53?0.84CO₃), clinopyroxene (Jd_7?32), epidote/allanite (Ps_0?33), titanite, garnet (Alm_52?56Grs_28?33Pyp_11?16), barite (Ba_0.90?0.99Sr_0.01?0.10SO₄), celestine (Sr_0.67?0.98Ba_0.01?0.23Ca_0.01?0.11SO₄), and one apparently homogeneous grain of intermediate composition (Ba_0.49Ca_0.01Sr_0.50SO₄). Adjacent eclogites have clinopyroxene with similar jadeite contents (Jd_14?34) and similar garnet (Alm_51?60Grs_26?36Pyp_8?14) compositions. The marbles have high contents of Sr (9500–11000 p.p.m) and Y (115–130 p.p.m). However, low concentrations of some key trace elements (110–160 p.p.m. Ba and <5 p.p.m. Nb) appear to indicate that the marble is not a metamorphosed carbonatite. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.7051 to 0.7059. Field and petrological relationships suggest that metasomatic reactions and fluids played a significant role in producing and/or modifying the marbles. The breakdown of scapolite in the granulite into carbonates and sulphates during eclogite facies metamorphism may have contributed to the metasomatic formation of the marbles along shear zones. Fluids involved during subduction are an important catalyst for metamorphism and are recognized to have played a critical role in the localized transformation from granulite to eclogite in the Holsnøy Island area. Thermobarometry indicates 640–690 °C and 18–20 kbar for adjacent eclogites and temperatures of 580–650 °C for the calc‐silicates. The marble assemblages are consistent with fluid that is dominantly comprised of H₂O (X_CO2 < 0.03) under high‐pressure conditions. Phase equilibria of the marbles constrain the fO₂ of the fluids and imply oxidizing conditions of the deep crustal fluids. At present the source of the fluids remains unresolved. The results provide additional insights into the variable and evolving nature of fluids related to subduction and high‐pressure metamorphism.     

挪威中部双峰态火山岩系地质,地球化学特征及其成因

挪威中部Ｇｊｅｒｓｖｉｋ地区加里东造山带由一系列地体组成。Ｇｊｅｒｓｖｉｋ地体内的火山岩系具有明显的双峰态特征，主要由深色和浅色拉斑玄武质绿岩以及石英角斑质火山碎屑岩组成。火山活动分为三个阶段，各阶段火山活动特征与裂谷构造演化息息相关。地质和地球化学证据表明，裂谷演化早期形成来源于亏损地幔未分异型拉斑玄武岩，中期形成由玄武岩或辉长岩局部重熔产生的长英质火山岩，晚期则形成来源于更深部的富集地幔分异型拉斑玄武岩。Ｇｊｅｒｓｖｉｋ地体内已发现一系列的与火山活动有关、空间上与长英质火山碎屑岩紧密共生的块状硫化物矿床。     

A reconnaissance study of the distribution of Ba, Nb, Y, and Zr in some Jotun kindred gneisses from Central Jotunheimen, southern Norway

Abstract The Bergen-Jotun kindred rocks of this study, the Storådalen Complex (SCX), Svartdalen Gneiss (SG) and Mjølkedøla Purple Gabbro (MPG), have been shown to be a co-magmatic series with calc-alkaline affinities. The analyses of Ba, Nb, Y, and Zr presented here show no variation in these elements between the three rock units and are consistent with the calc-alkaline character of the rocks. The lithophile elements Ba, K, and Sr are enriched relative to MORB and the high field strength elements Nb, Y, and Zr are depleted relative to MORB, Zr especially so.
The SCX contains rocks with low (>30) differentiation indices which are interpreted as plagioclase + pyroxene ± olivine ± amphibole cumulates. The remainder of the SCX, together with the MPG and SG, is regarded as the congealed liquid in equilibrium with these cumulates. The distribution of trace elements between these two components of the SCX can be adequately modelled using a Rayleigh fractionation process, measured 'liquid'compositions, and calculated bulk distribution coefficients. It is thus concluded that the trace element geochemistry of the rocks of this study is consistent with subduction-related, mantle-derived magmas that fractionate within a continental or mature island arc environment. Subsequent high-grade metamorphism and deformation of Sveconorwegian age have been essentially isochemical.     

Coronas from the Thessaloniki gabbros,North Greece

Chemical composition of the phases occurring as corona structures in the Thessaloniki gabbros are subjected to algebraic method of treatment following Korzhinskii [1]. The analysis reveals that the mineralogical reactions responsible for the growth of the corona structures were possibly allochemical in nature in regard to the non-volatile components. The chemical similarity of the pyroxenes occurring within and beyond the corona structures in the gabbros appears compatible with such a model.     

Genesis of garnet-bearing rocks at the Berezitovoe deposit,Upper Amur Region,Russia

The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.     

Deposition and diagenesis of debris flows in Upper Ordovician limestones, Hadeland, Norway

The Upper Ordovician rocks of Hadeland, Norway, form a sequence of thin bedded nodular limestones (wackestones) and shales, hosting five distinctive sedimentary breccia complexes. These breccias contain blocks of varying sizes and shapes in a wackestone and grainstone matrix. Blocks differ in lithology, and in their included biotas and cement sequences. The thin bedded limestones are interpreted as turbidites, deposited against a background of hemipelagic calcareous shales. The breccias occupy channels cut into this sequence. The lithologies and biotas of blocks in the breccias record deposition in differing sedimentary environments, whereas their cements are the results of contrasting diagenetic histories. Blocks were eroded from a diverse and mature carbonate platform, close to sea level, which probably lay 5–10 km east of Hadeland. The breccias are interpreted as debris flow deposits, transported as channellized flows. Following channel cutting events, perhaps triggered by sea level change, channels were characterized by deposition rather than erosion. Wackestones and grainstones associated with the breccias also reflect resedimentation, their less diverse biota suggesting local derivation on the slope. The reworking of calcarenaceous muds locally produced clean washed calcarenites (now grainstones). A fall in sea level resulted in emergence of the upper slope and erosion of the debris flow complex to form caverns and fissures. As sea level rose again crinoidal calcarenites, now grainstones, were deposited within these cavities. Cement sequences in blocks record early marine and burial conditions on the shelf, and also precipitation of new marine cements following downslope transport. Those cements in lithologies formed in situ document later shallowing, culminating in emergence. The localized dissolution of cements in both blocks and associated grainstones reflects the infiltration of ‘aggressive’meteoric waters through permeable channel deposits. A subsequent rise in sea level is recorded in the generation of an additional marine cement with final burial reflected in the deposition of blocky calcite. The debris flow deposits therefore maintained their distinctive character from deposition through diagenesis.     

江西南城地区中—新生代推滑覆构造研究

通过对江西南城地区侏罗系林山群与下伏前泥盆纪变质地层、白垩纪地层与前白垩纪地层接触关系调查和谢家构造窗、竺由新村构造窗、上乌石构造窗、望天石飞来峰的发现及钻孔验证,初步厘定了区内燕山期滑覆构造和喜马拉雅期推覆构造。燕山期滑覆构造发生在中侏罗世末,主滑面沿华力西—印支构造层(D—T2)、燕山构造层(T3—K1)与下伏地层间角度不整合面扩展而成,泥盆纪—早白垩世地层为滑覆体,滑覆在前泥盆纪变质地层之上,动力学来源为太平洋板块由南东向北西对欧亚板块俯冲。喜马拉雅期推覆构造规模大、动力学机制复杂(顺层和切层推覆),彻底打破了华南地区原有的地层沉积层序,形成了现存的横向分带、纵向分层的多层叠瓦、堆垛推覆构造格局。前白垩纪地质体为外来推覆体,由南东向北西推覆堆垛在白垩纪地层之上,白垩纪地层呈"构造窗"分布于前白垩纪地质体之下,动力学来源可能为印度板块与欧亚板块碰撞形成。     

四川盆地上三叠统砂体大面积分布的成因

四川盆地上三叠统须家河组二、四、六段以砂质沉积为主,砂体厚度大,分布面积广泛。弄清砂体大面积分布的成因,对预测优质储层分布具有重要的实践意义。通过系统的层序地层划分、物源分析、砂体展布研究和沉积相编图,并结合水槽实验和现代地质调研,认为晚三叠世四川盆地古地形平坦,发育多个物源。以冲积扇、河流、三角洲和湖泊4类沉积体系为...     

Anorthosites and Related Megacrystic Units in the Evolution of Archean Crust

Archean anorthositic complexes occur in essentially all Archeancratons and contain large equidimensional plagioclase crystals(up to 30 cm. diam.) with highly calcic compositions (An₈₀ toAn₉₀) but are not readily amenable to determination of theirparent melt compositions. However, insight into petrogenesisof the complexes is provided by megacrysts of plagioclase thatare identical to those in the complexes and occur in many Archeanflows, sills, and dikes whose matrices display REE and fractionationpatterns that indicate tholeiitic trends and are compatiblewith prior subtraction of plagioclase during earlier evolutionof the melts. Included blocks of anorthosite and megacrystswith very thin rims that approach the more sodic compositionof lathy plagioclase in the matrices indicate an earlier stageof cryst formation under different conditions of crystallizationthan the matrices. The megacrystic units occur both in greenstonebelts that have oceanic affinities and stable cratonic dikeswarms that have continental affinities. Both major and traceelement contents of the matrices of the megacrystic units differbetween greenstone and cratonic dike environments; the dikesbeing higher in Si0₂, TiO₂ FeO, Na₂, K₂O, and light REEs butlower in Al₂O₃ and CaO. The matrices of both environments followseparate but parallel tholeiitic fractionation with high Fe-enrichmenttrends similar to Skaergaard liquids suggesting relatively lowvolatiles and fo₂. Experimental data and projections in CMAFspace suggest a multistage petrogenesis involving a relativelyhigh-pressure fractionation of olivine and/or orthopyroxenefrom a primitive mafic melt followed by ascension of the fractionated,less-dense melt, probably in several pulses, to a low-pressurechamber, probably at 1 to 2 kb. The depressurization accompaniedby cooling could easily place the melt composition in the plagioclasefield and significantly below the liquidus resulting in severalcrystallization cycles of plagioclase in the low pressure chamber.The melts would crystallize as anorthositic complexes and periodicallyexpel pulses that would form the observed megacrystic flows,sills, and dikes.     

禹县煤田上石盒子组紫斑岩的成因

禹县煤田晚二叠世上石盒子组为一套含煤的陆源碎屑岩沉积,包括六、七、八三个含煤段,共厚250m左右（图1）,与上覆石千峰组和下伏下石盒子组呈连续沉积。近年的研究表明,本区上石盒子组是一套河流注入到淡化海湾的浅水三角洲沉积。以往,人们根据上石盒子组中含有多层紫色斑块岩的现象,认为晚二叠世的古气候是属于半干燥到千燥类型的。     

Coronas in olivine metagabbros from the Proterozoic Chotanagpur terrain at Mathurapur, Bihar, India

The Proterozoic Mathurapur olivine metagabbros possess several types of coronas due to subsolidus reactions between igneous mineral pairs. viz. olivine-plagioclase, ilmenite-plagioclase and pyroxenes-plagioclase. Microprobe analyses of coexisting primary and coronitic minerals from different corona domains, indicate attainment of equilibrium. Mineral chemical data of primary (reactants) and coronitic (products) minerals from the present metagabbros and also from similar studies in the literature are critically examined to evaluate the chemistry of corona reactions by mixing calculations assuming boundary migration. The mass-balanced corona equations are consistent with the allochemical system and do not show volume imbalance. However, SiO₂ and Al₂O₃ in olivine-plagioclase coronas remain immobile, which indicates that the corona reactions therein proceeded with minimum structural rearrangements of (Si.Al)---O bonds.     

内蒙古色尔腾山──大青山地区推覆构造

内蒙古包尔腾山──大青山地区推覆构造，随着1∶5万区域地质调查工作的开展，在原来发现的几个孤立的推覆构造的基础上，相继识别出一些新的推覆构造，从而使我们有可能纵观本区推覆构造的全貌。它们呈近东西向展布，主要发生在东西向侏罗纪成煤盆地的南北两侧，前中生代地层及侵入体做为外来系统逆冲推覆在中生代盆地之上，并发育大小不等的飞来峰、构造窗。它们形成于中侏罗世末和早白垩世末，是在南北向水平挤压的统一的构造应力场中发生发展的。推覆构造研究，对认识本区地质构造的演化和地壳的层圈式结构将是十分有益的，对指导找矿也有一定的意义。     

叙利亚Oudeh油田上白垩统Shiranish组碳酸盐岩储层特征及成因模式

叙利亚东北Oudeh油田上白垩统Shiranish油藏储层为一套由孔隙性颗粒灰岩构成的碳酸盐岩缓坡沉积,其构造背景总体上为北西高、东南低,并被近东西和近南北走向的两组断层分割成若干断块,其中近南北走向的几条大断层为同生断层,是油田内的主控断层.同生断裂作用形成的古地形高差对沉积和成岩作用有一定的控制作用,从而控制了优质...     

Nappe displacement in the Scandinavian Caledonides

Large areas of the Scandinavian Caledonides are eroded to the level of the basement/cover contact. Relationships between the Precambrian crystalline basement (largely Svecofennian-Dalslandian, 1800-1000 m.y.) and cover sequences are exposed both in transverse profiles through the mountain belt and along the belt in the various windows. These relationships provide an unique opportunity for studying the basement configuration, character of basement involvement and general nappe geometry. Major allochthonous units of the central part of the Scandinavian Caledonides — the Offerdal, Särv and Seve-Köli Nappe Complex — have been shown to wedge out westwards, having been displaced eastwards from environments along and west of the present Norwegian coast. Recent investigations have shown that these units (the Offerdal, Särv and Seve) reappear in western Norway as major pinch-and-swell structures, the lenses reaching thicknesses in the order of 2 km and with long axes of several tens of kilometres. Within the western parts of the Swedish Caledonides the thicker parts of the lenses approximately coincide with the axes of the late synforms which fold basement and cover together. Further west, in Norway, the tensing appears to be unrelated to the geometry of these major folds.This evidence increases estimates of nappe displacement distance (now thought to be in the order of at least 1000 km). At the same time it emphasizes that translation may account for only about half of this amount, the rest being achieved by stretching of the nappes. Apparently, a nappe sequence built up in the west which subsequently collapsed, leading to continued displacement eastwards on to the Baltoscandian Platform. Whereas basement shortening is of the order of several tens to perhaps hundreds of kilometres in the western part of the central Scandes, it is of lesser importance from central Trøndelag eastwards, a distance of about two hundred kilometres, to the Caledonian Front.Biostratigraphic evidence from the late-orogenic intramontane basins, taken in relation to the youngest units involved in the nappes, requires nappe translation into western Norway to have occurred after the Llandoverian (Köli Supergroup) and prior to the (Ludlovian?) Downtonian (Hitra Formation) deposition in the intramontane basins. The nappes contain sequences derived from a variety of probable oceanic and continental margin environments, and this translation may greatly exceed the minimum estimate of five hundred kilometres. Further displacement eastwards occurred during uplift of seaboard Norway and accompanied sedimentation both in the intra- and extramontane basins. The latter were not finally influenced by the décollement tectonics until after the Early Devonian.This evidence suggests that a compressive regime dominated the early phases of orogenesis during basement shortening, build up of the nappe pile and translation of these denser units on to the western margin of the Baltoscandian Platform. This compression subsequently gave way to a gravity regime, collapse and stretching of the nappes dominating the late phases of displacement on to the Baltoscandian Platform.