Compositional relations among pyroxenes,amphiboles and other mafic phases in the Oslo Region plutonic rocks: Publication No. 115 in the Norwegian Geotraverse project

Mafic minerals in samples from older and younger intrusions in the Permian Oslo rift were analysed to determine temperatures and oxygen fugacities of crystallization and possible genetic relations between different rock types. Applications of various geothermometers and oxygen barometers to plutonic rocks have been assessed in connection with this work.Pyroxenes in the peralkaline rocks define a continuous trend from augites to pure aegirines. This trend was found to be the result of fractional crystallization of plagioclase (and augite), suggesting that the peralkaline rocks formed from alkaline initial liquids.     

Fluid inclusion studies of the Drammen Granite,Oslo Paleorift,Norway

The Drammen Granite is a subvolcanic complex, intruded during the Permian igneous activity in the Oslo Paleorift. Molybdenite deposits occur within the complex in large discrete quartz veins, accompanied by moderate alteration of the granite. Microchemical and Sr-isotopic studies of fluids trapped in quartz from miarolytic cavities allow characterization of fluids in the granite.The results compare well with compositions interpreted from microthermometry studies. The observed range in K/Na, Ca/Na and the calculated concentrations of metals in the original fluids suggest that different stages of development are represented. Some of the fluids probably represent early magmatic stages. K/Rb results and the average Sr-isotope composition indicate equilibrium between the fluids and the granite magma.     

Petrology of the Skien basaltic rocks,southwestern Oslo Region,Norway

The pile of Permian continental basaltic rocks near Skien has a minimum thickness of 1 500 m, dipping 40^g towards NE, consisting of several thin lava flows. Most flows are porphyric, though both tuffs and aphyric flows occur to a lesser extent. Amygdules are abundant and a characteristic feature. The flows in the lower 2/3 of the basalt pile are mainly classified as melanite-ankaramite; some in the middle part as melanite-nephelinite; and the upper 1/3 of the flows as basanite. The basaltic rocks evolved from an assumed olivine-nephelinitic magma at great depths principally by clinopyroxene crystal fractionation accompanied by later olivine and maybe melilite reactions at moderate pressure in minor shallower magma chambers.     

Olivine clinopyroxenite xenoliths in the Oslo Rift,SE Norway

Olivine clinopyroxenite xenoliths in a basalt flow at Krokskogen in the Oslo rift consist mainly of Al-Ti-rich clinopyroxene and alteration products after olivine (possibly also orthopyroxene). The clinopyroxene contains primary inclusions of Cr-Al-bearing titanomagnetite, pyrite and devitrified glass, and secondary fluid inclusions rich in CO₂. On the basis of petrography, mineral compositions and bulk major and trace element chemistry, it is concluded that the xenoliths represent cumulates with about 5% trapped liquid, formed from a mildly alkaline basaltic magma. Microthermometrical analysis of secondary or pseudosecondary fluid inclusions give a minimum pressure of formation of 5.5 to 6 kbars, that is a depth greater than 16–17 km. The host lava has initial _Nd=+4.16±0.17 and _Sr=–5.50±0.26, which is believed to reflect the isotopic composition of the lithospheric mantle source region under south Norway in early Permian time. The isotopic character of the magma which gave rise to the xenoliths is preserved in clinopyroxenes which have _Nd ^t =+1.9 to +2.6 and _Sr ^t = –1.1 to –1.8. The isotopic differences between the host magma and the xenoliths reflect some degree of crustal contamination of the xenolith's parent magma.The xenoliths of this study represent an important source of information about the large masses of dense cumulates found at depth in the crust under the Oslo rift.     

Columnar-Dendritic Feldspars in the Lardalite Intrusion, Oslo Region, Norway: 1. Implications for Unilateral Solidification of a Stagnant Boundary Layer

Feldspars with exceptionally coarse, columnar-dendritie morphologies,oriented perpendicular to the border, are developed in the contactzones of the lardalite intrusion, Oslo Region, Norway. A marginalpicrite zone with dendritic olivines and feldspars is succeededby a monomineralic zone of columnar feldspars, attaining a remarkablyregular textural pattern which resembles the cellular substructureof metallic substances. The inner portion of the comb-texturedcontact zone shows successive layers of dendritic feldsparsalternating with dendritic Ca-rich pyroxenes and nephelines. Orientational studies suggest that the columnar and dendriticfeldspars are elongated normal to (010) and seem to possessa regular orientation in three dimensions. The perpendicularfabric is the result of heterogeneous nucleation and constrained,competitive growth parallel to heat flow. Some dendritic feldsparspossess characters of faceted growth and originated under moderatesupersaturation, presumably by a screw-dislocation growth mechanism.The relative growth rates along crystallographic directionsof the perpendicular feldspars can be established as b >a > c. The spacing of the columnar feldspars seems to reflect the interdendriticsolute-distribution, and thus the characteristic diffusion distanceof constituent atoms. Applying experimentally determined alkali-diffusioncoefficients, maximum growth rates of certain columnar feldsparscan then be determined as roughly 5?10^–7 cm s¹, assumingsteady-state growth. Compositional variations along growth directionsappear in two major categories. One type, showing regular fluctuationsabout a constant value, forms as a result of minor changes ingrowth conditions under steady-state solidification. The othertype shows cyclic appearance of complementary phases duringgrowth, including dendritic feldspars, Ca-rich pyroxene andnepheline. This type of variation develops in supersaturatedmelts as the result of competitive growth of successive phasesand accompanying solute enrichment in the boundary layer duringnearly isothermal crystallization. Complete transition from perpendicular to porphyritic-laminartextures, with similar orientational characters, suggests thatthese textural types result from solidification under changingconditions of growth. The initial growth is parallel to themaximum heat-flow, while decreasing supersaturation graduallyencourages growth along isotherms, i.e. contact-parallel. The study of perpendicular, comb-textured contact rocks, allowsa qualitative understanding of crystallization mechanisms andchemical segregation during unilateral solidification. Theseresults are relevant to the understanding of magmatic featuressuch as chemical fractionation, modal layering and compositionalzonation in cumulate, equigranular textures. A textural development similar to that of cast metal ingots,implies that the formation of equigranular or columnar-dendritictextures, may reflect the presence or absence, respectively,of fluid-flow near the crystallization and nucleation front,whereas the chemical variations depend solely on diffusionalprocesses in the innermost, stagnant portion of the boundarylayer.     

The Skien lavas, Oslo Rift: petrological disequilibrium and geochemical evolution

The Skien lavas, which form the earliest phase of basaltic magmatism within the Permo-Carboniferous Oslo Rift, contain multiple generations of clinopyroxene which exhibit strong petrological and geochemical disequilibrium. Three principal core compositions have been identified: (1) low-jadeite, high-Mg, Cr-diopside cores (CrMgDi) with strongly depleted trace-element signatures, which are believed to be xenocrystic in origin; (2) Mg-rich, Cr-poor diopside cores (MgDi) with moderately depleted trace-element signatures which probably represent early cognate growth; and (3) more dominant, low-Mg, phenocrystic diopside cores (PhenDi). Several samples contain CrMgDi or MgDi cores which have been subjected to resorption and partial re-equilibration with their host melts, indicative of extensive disequilibrium and magma mixing. These three core types are overgrown by trace-element-enriched Ti-augite, which also forms megacrysts and late-stage lava groundmass. Calculated Ti-augite/melt partition coefficients show clinopyroxene compatibility of the M-HREE, Zr, Hf and Y. The LILE, Sr, and Nb remain incompatible. )Sr₃₀₀ and )Nd₃₀₀ of Ti-augite overgrowths, phenocrystic diopside, and MgDi diopside cores show that intrasample isotopic disequilibrium existed when the host basalts were erupted. All epsilon values lie within the range of data previously published for the Skien lavas. Detailed examination of the chemical, isotopic and textural disequilibrium features seen in these lavas has enabled us to place constraints upon the magmatic evolution of this basalt suite, ranging from xenocryst incorporation to cognate multistage pyroxene growth, as well as identifying clear evidence of magma mixing and possible crustal contamination.     

201、325和706铀矿床蚀变带绿泥石研究

以岩矿鉴定结果和电子探针绿泥石分析数据为依据,将325、706花岗岩型铀矿床蚀变带绿泥石分为假象绿泥石和鳞片状绿泥石。后者由前者转变而成,转变过程中存在着铁的迁出与镁的加入,迁出的铁形成赤铁矿,可能是造成碱性蚀变带呈红色的原因之一。201、325铀矿床蚀变带绿泥石为铁镁绿泥石和蠕绿泥石,706铀矿床蚀变带绿泥石主要属密绿泥石和铁斜绿泥石,少数属铁镁绿泥石。研究发现绿泥石变种由蚀变带原岩的∑FeO与MgO比值大小决定,与铀矿蚀变带是否为酸性和碱性没有必然的对应关系;绿泥石晶胞中镁羟基和铝羟基相对比例大小不同,是导致其吸收位置在2259-2262nm和2348-2359nm的诊断性吸收峰发育程度存在差别的原因。     

2012年第58卷第5期目录

目录     

Crustal contaminants in the Permian Oslo Rift, South Norway: constraints from Precambrian geochemistry

During the late Paleozoic Oslo rifting event, the SW part of the Baltic Shield was penetrated by mantle-derived magmas from a depleted lithospheric or sublithospheric source. Along the way to their final emplacement, these magmas may have interacted with a heterogeneous continental crust, consisting of a mosaic of continental terranes, each with its unique composition and internal crustal history. Information on radiogenic isotope ratios and trace element distributions in the Precambrian terranes surrounding the rift can be used to define characteristic crustal components. These components may be used as endmembers in petrogenetic modelling of the Oslo Rift magmatic system. Based on available data, six endmember components can be identified, and (semi) quantitatively characterized in terms of Sr, Nd and Pb isotopes and selected trace elements. Data on the distribution of rock-types along the rift flanks allow estimates to be made of the relative importance of the components in different parts of the rift. Combining these data with petrological information may allow a realistic understanding of crust–magma interaction in the Oslo Rift magmatic system.     

The 33rd International Geological Congress, Oslo 2008

The 33rd International Geological Congress is being organised jointly by the Nordic countries and to be held in Oslo, Norway, August 6-14th, 2008. This ＂Geoscience World Congress 2008＂ will run up to 40 parallel science sessions, poster sessions, an extensive exhibition, short-courses, workshops, and business meet- ings; about 50 exciting preand post-Congress excursions are planned. The excursions include all the Nordic countries, as well as Greenland, Svalbard, the Faeroes, Russia and Ukraine. All major geoscientific themes are being covered by the Congress which has been divided into two parts, Sunday lOth August being without Symposia and dedicated to workshops, short courses, business meetings and excursions. Through a series of ＂Themes of the Day＂, seven themes with major societal impact will be covered in plenary sessions with invited lecturers, including a key-note ＂StatoiIHydro lecture＂ given each day during the lunch interval. The venue is set up to offer a compact Congress with easy access to all sessions and other events. An extensive social and cultural programme is also being arranged.     

Did hot,high heat-producing granites determine the location of the Oslo Rift?

The Late Carboniferous–Early Permian Oslo Rift formed in apparently cold, stable lithosphere of the Fennoscandian Shield in a tensional stress regime widely documented in Northwest Europe at that time. The Rift formed obliquely to older, crustal structures that display only limited Permian reactivation, and, although numerical modelling suggests that the present-day lithospheric structure would serve to focus tensional stresses in the Oslo region, the assumption that no lithospheric evolution has occurred since the Palaeozoic is by no means obvious. Here, I show that, up to 5 km thick, regional-scale Late- to Post-Sveconorwegian granites in the vicinity of the Oslo Rift, with heat-production rates averaging ca. 5 μW/m³, nearly three times higher than the surrounding Sveconorwegian gneisses, would have increased the temperature in the lower crust and lithospheric mantle by up to 100 °C, resulting in significant thermal weakening of the lithosphere in this area. Given a tensional stress regime, weakening by these high heat-producing element granites would have made the Oslo area a favoured site for passive rifting and may have been a first-order parameter locating rifting to this part of the Fennoscandian Shield. The thermo-rheological effects of such granites must be considered along with other factors in future models of initial rift mechanisms in the Oslo Rift, and probably in other rifts elsewhere.     

Architecture and early evolution of the Oslo Rift

A revised assessment of architecture and pre-rift fabric connections of the Oslo Rift has been undertaken and linked to a new appraisal of observations and data related to the initial phase of the rift evolution. In addition to half-graben segmentation, accommodation zones and transfer faults are readily identified in the linking sectors between the two main grabens and between graben segments. Axial flexures are proposed between facing half-grabens. The accommodation zones were generally sites of volcanism during rifting. Pre-rift tectonic structures played an influential role in the rift location and development. The deviant N-S axis of the Vestfold graben segment is viewed as related to pre-rift structural control through faults and shear zones. This area was probably a site of Proterozoic/Palaeozoic crustal and lithospheric attenuation.

Field evidence suggests that the rift started as a crustal sag with no apparent surface faulting in a flat and low-lying land at a time about 305–310 Ma. Volcanism, sub-surface sill intrusion and faulting started about simultaneously some time after the initial sag (300–305 Ma). Faulting and basaltic volcanism were initially localized to transfer faults along accommodation zones and a NNW-SSE transtensional zone along the eastern margin of the incipient Vestfold graben segment. This transtensional zone was probably created by right-lateral simple shear tracing pre-rift structures in response to a regional stress field with the tensional axis normal and the maximum compressional axis parallel to the NNE-SSW-trending rift axis.