Diabase variation and genesis

Boyd, W. W.

doi:10.17741/bgsf/44.1.004

Cited by 4 publications

(4 citation statements)

References 16 publications

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“…These values are similar to those from the least hybrid MMEs (i.e., samples 91053A, 91053B, and 91053E) with Si0 2 from 50.80 to 51.80 wt%, total alkalies from 4.62 to 5.34 wt%, and Mg numbers from 35.84 to 37.54. The Mg numbers are lower than those of the diabase dykes of the Suomenniemi (26 to 49) and Häme (38 to 61) swarms (Laitakari 1969, Boyd 1972, Rämö 1990. In the pillow-like MMEs, the Mg numbers vary from 28.5 to 34.3 possibly reflecting a decrease in the Mg/Fe due to hybridization with the felsic magma.…”

Section: Small Mmes and Pillow-like Mmesmentioning

confidence: 86%

“…Related Subjotnian tholeiitic diabases (Laitakari 1969, Boyd 1972, Ehlers & Ehlers 1978, Laitakari & Leino 1989, Lindberg et ai. 1991, Rämö 1991 form dykes and dyke swarms running in a NE-SW or NW-SE direction (see Fig.…”

Section: Rapakivi Granites Of Finlandmentioning

confidence: 99%

“…It has been shown that the Subjotnian diabase dykes in southeastern Fennoscandia are products of extensive fractionation in intra-crustal magma chambers (Boyd 1972, Rämö 1991 and this probably is true of the mafic end-member of the Jaala-ters magma evolution prior to hybridization. When this mafic magma was intruded into a felsic magma chamber in the crust a stratified magma chamber was formed.…”

Section: Mafic End-membermentioning

confidence: 99%

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Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland

Salonsaari¹

1995

Bull Geol Soc Finland

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SALONSAARI, PEKKA T. 1995. Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland. Bulletin of the Geological of Finland 67, Pan lb, 104 pages, 56 figures, 7 tables, and 2 appendices. The 1630 Ma Jaala-Iitti complex is an example of bimodal rapakivi granite magmatism in which the interaction of granite and diabase magmas have led locally to hybridization. The dyke-like complex is situated at the northwestern margin of the Wiborg rapakivi batholith in southeastern Finland, cutting both the Proterozoic Svecofennian metamorphic crust and the Wiborg batholith. The complex consists mainly of non-hybridized compositionally homogeneous granites, i.e., hornblende granite and hornblende-quartz-feldspar porphyry which represent the felsic end-member (ca. 68 wt% Si0 2) of the complex. The mafic end-member (ca. 51 wt% Si0 2) is present as globules of disaggregated Fe-rich tholeiitic magma forming magmatic mafic enclaves (MMEs) and composite MMEs. Commonly MMEs and large (up to 2 metres in diameter) pillow-like MMEs show magma mixing and mingling characteristics. Hybrid rocks in the complex are monzogranitic and are characterized by (a) quartz grains and quartz aggregates (partially melted xenoliths) mantled by amphibole rims (ocellar texture) with occasional augite, (b) alkali feldspar megacrysts mantled by a micrographic plagioclase-quartz intergrowth, and (c) alkali feldspar megacrysts mantled by a plagioclase shell with occasional amphibole inclusions. These textures are also found in hybrid MMEs and especially in pillow-like MMEs. Alkali feldspar and quartz megacrysts in hybrid rocks are derived from partially crystallized felsic magma and from disaggregated rapakivi granite and granitoid xenoliths whereas in the MMEs the xenocrysts are solely derived from partially crystallized felsic magma. Disaggregation of the mafic magma to form MMEs and equilibration with the host have produced micro-enclaves and recrystallized borders of MMEs. Disaggregation of the mafic magma has also produced needle-like apatite crystals incorporated from the mafic magma into the hybrid magma; acicular apatite is typical in MMEs and pillow-like MMEs and is indicative of rapid crystallization. The mass-fraction of mafic magma (X m) in the hybrid rock is up to 0.3. The X m in hybrid MMEs and pillow-like MMEs varies from 0.4 to 0.9. The temperature difference between the two magmas probably caused convection that spread the disaggregated mafic magma throughout the postulated layered magma chamber. The injected mafic magma simultaneously mingled and mixed with the felsic magma which was superheated, assisting the chemical exchange. The interaction of hybrid MME magma with the hybrid magmas was possibly made more effective by diffusion of fluorine from the felsic magma into the mafic magma. The composition of minerals indicates intensive interaction of the mafic magma and the felsic host, with the temperature of equilibrium being in the range of 900° t...

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Section: Small Mmes and Pillow-like Mmesmentioning

confidence: 86%

Section: Rapakivi Granites Of Finlandmentioning

confidence: 99%

Section: Mafic End-membermentioning

confidence: 99%

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Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland

Salonsaari¹

1995

Bull Geol Soc Finland

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“…Nd isotope data ( ε N d ∼ −1.2 to +1.6) indicate that the diabase dykes were derived from a light‐rare‐earth‐element‐depleted mantle source and contaminated during crustal transit (Rämö 1990). Low Mg # (26–50) suggests substantial low‐pressure fractionation in crustal magma chambers before final emplacement at upper crustal levels (Boyd 1972; Rämö 1990). This interpretation is supported by seismic reflection and refraction experiments that have revealed a ±10 km thick lower crustal high‐velocity ( V p > 7 km s − 1 ) body which may be interpreted as a gabbroic underplate or fossil rift pillow (Luosto et al .…”

Section: Two Real Case Studiesmentioning

confidence: 99%

Models of crustal anatexis in volcanic rifts: applications to southern Finland and the Oslo Graben, southeast Norway

1998

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Crustal anatexis in volcanic rifts is discussed in terms of a 1-D numerical model incorporating lithospheric extension, emplacement of hot mafic magma at crustal levels, various crustal compositions and pre-rift conditions. Only the volume of melt generated by anatexis is considered; crustal melt segregation, transport and emplacement at higher levels are not included. We find that under a broad range of conditions, influx of mafic magma to the crust is required for substantial anatexis to take place, and once this stops, silicic magmatism dies out rapidly. In many rifts the observed change in type of magmatism from predominantly mafic to silicic during rifting is probably more due to a diminishing potential for mafic magmas to ascend though the crust than to an actual halt in mafic melt generation at depth. This has important implications where melt production rates are used to analyse the formation of volcanic rifts and margins. When influx of mafic magmas takes place over a finite period of time, they may be focused in the depth region where the potential for anatexis is largest, and in some cases more anatexis can occur than by instantaneous emplacement of the entire mafic body. Our results support the idea that the 1.6 Gyr old rapakivi granites in southern Finland were formed by anatexis of an intermediate to felsic Svecofennian crust. Provided a mantle thermal anomaly of +200-250°C existed beneath Finland, the seismically observed~10 km thick magmatic underplated body could have produced the exposed silicic rocks which are on average 2-3 km thick. In the Oslo Graben in southeast Norway, where the thickness of magmatic underplating is also about 10 km, the amount of rock produced by anatexis is poorly constrained, but appears to be between 1 and 2 km. Provided the pre-rift crustal thickness in the graben was 35-40 km or more, and heat was transported to crustal levels more efficiently than is feasible by conventional stretching models, the model predicts melt volumes in agreement with these estimates. We propose that secondary convection within the narrow graben could have contributed to the high heat transport efficiency. Finally, a conspicuous feature of the southern Finland magmatic region as well as the Oslo Graben is the large discrepancy between stretching factors derived from analysing dyke intrusions and faulting (Finland: b~1.001; Oslo Graben: b<1.05) and by using variations in crustal thickness (Finland: b~2; Oslo Graben: b≥1.7-2). A possible explanation for this difference may be that when large volumes of mafic rocks cause crustal anatexis, the partially molten crust becomes very weak and acts as a barrier for stress propagation, effectively isolating the upper crust from further deformations. This could add to the thermal weakening of the ductile lower crust due to the influx of hot mafic magmas. This hypothesis could be tested by investigating a large number of magmatic rifts, combined with development of new rheological models of the lithosphere including the effects of underplating and anatex...

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The Jaala-Iitti rapakivi complex. An example of bimodal magmatism and hybridization in the Wiborg rapakivi batholith, southeastern Finland

Salonsaari

Haapala

1994

Mineralogy and Petrology

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Diabase variation and genesis

Cited by 4 publications

References 16 publications

Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland

Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland

Models of crustal anatexis in volcanic rifts: applications to southern Finland and the Oslo Graben, southeast Norway

The Jaala-Iitti rapakivi complex. An example of bimodal magmatism and hybridization in the Wiborg rapakivi batholith, southeastern Finland

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