Geological Survey of Finland Bulletin 289 - arkisto.gsf.fi

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A F Geological Survey of Finland, Bulletin 289 25 M Fig. 23. AFM-diagram showing the composition of the iron-rich Aland diabases compared with the ferrodolerites of W Greenland (Windley 1970) and normal basalt composition according to Kuno (1968). Also included in the diagram are the differentiation trend of the Skaergaard intrusion and the differentiation area for some Tasmanian diabase sills. 1 ) the eas t Aland diabases, 2) normal basalt composition, 3) ferrodolerites on W Greenland. AFM diagram (Fig. 23). The diagram shows that the diabases of the Aland archipelago are very rich in iron and that they differ considerably in their composition from the diabase analyses published previously. The ferrodolerites in the west of Greenland described by Windley (1970) are very similar in chemical composition. A proportion of the high iron content is present in the form of magnetite and pyrite (up to 10 per cent opaques) in the diabases. In the Skaergaard intrusion there are parts rich in iron, the composition of which is also rich in hornblende. This suggests strong fractionation of a basic magma at a low stage oE oxidisation (Wager & Brown 1967). The AFM diagram gives the results oE the analyses not only of the Aland diabases but also oE Windley's (1970) Eerrodolerites in the west of Greenland and the differentiation trends in the Skaergaard massif and so me diabase sills in Tasmania. It should be mentioned that, according to analyses published earlier (Frosterus 1893, Wahl 1906, and Springert 1951), the large pyroxene diabases of Föglö and Korsö fall within the same area on the AFM diagram and chemically they closely resemble the trapp diabases. DISCUSSION Thestrongly iron-rich composltlon suggests that the dykes of the diabase set were formed by basaltic magma that was already greatly diffenrentiated. The chemie al variations within the set are small and it is hardly possible to prove any differentiation differences between different parts of the set. 4 127702109V
26 Geological Surveiy of Finland, Bulletin 289 Yoder's and Tilley's (1962) experimental results show that natural olivinetholeiitic or aluminium-rich basalts crystallise as amphibolitic rocks at high water pressure. Amphibolitic types of rock begin to crystallise when the water pressure exceeds 1500 bars. However, there are no experimental results that are direct1y applicable to basaltic magmas very rich in iron. In an iron-rich magma the pyroxene, for example, must erystallise at a lower temperature: this may lead to the hornblende erystallising in a primary form at a lower water pressure (cf. Yoder and Tilley 1962, p. 449, Hg. 27 ). It is quite dear that apart at least of the plagiodase crystallises at an early stage; in the fine-grained chilIed margins in particular where a dear flow structure has been formed from earlier crystallised plagiodase this is evident. In the middle of the diabase dykes hornblende has often replaced the pyroxene entirely. Some of the hornblende may naturally be primary. This seems to be the case particularly in the fine-grained chilled margins. Here there are no traces of uralitisation and the hornblende is fairly weH crystallised in small prisms that sometimes lie in the form of radial aggregates. In contrast to the middle of the dykes, residues or pseudomorphs of pyroxene have not been encountered in the chilled margins. Table 1 shows the differences in mineral eomposition between the chilled margins and the centres of two diabase dykes 1-2 m in breadth. It can be seen that the proportion of hornblende in the marginal zone is larger and the amount of plagoiodase lower than in the middle of the dykes. What has been said above suggests that thc magma was very rieh in water at the intrusion stage and that hornblende, at least in the chilled margins, crystallised in its primary form. The rapidly cooled chilled margins form a skin of "quenched rock" around the central part of the diabase dyke. The centre then crystallised more slowly and possibly under increasing water pre sure. It is possible that this might explain the more worn and resorbed feldspars in the middle of the dykes. Crystallisation from the water-rieh magma was slower [here and some pyroxene has been formed. The pyroxene was later transformed into hornblende. In certain cases the chilled margin may have melted again as a result of which local melting of the surrounding rock may have ensued (Figs. 9 and 10 ). In general, however, it seems that the hot magma did not melt the bedrock. The origin of the water in the basalt magma is obscure but it seems dear that it was present at the time of the intrusion of the diaba e dykes. It is evident that there must have been great variations in water pressure in the diabase dykes. The pressure within a dyke must have varied as it pushed its way through the earth's crust, perhaps faster than the minerals had time to achieve equilibrium with the varying PT conditions. The water pressure is not just a function oE the water content but it also depends on the pressure of the surrounding rock. When the magma comes into contact with a new system of fissures and rushes into them there may occur temporary reductions in the gas press ure in the magma. In a dyke E of Lappo (no. 289 ) there are a number of armoured relicts indicative of
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Page 3 and 4: Ehlers, C. and Ehlers, M. 1977: She
Page 6 and 7: INTRODUCTION The diabase dykes in t
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Page 32: Rancken, R. (1953) Über eine arch
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