CONSTRUCTION OF LITHOLOGY -DEPTH PROFILES

Voll (1983) presented a crustal profile for the Eifel which is based solelyon the abundances of rock types observed within his xenolith collection. There is, however, no convincing evidence that the sampling process by uprising magmas is representative with respect to actual abundances of crustal rock types within a crustal segment. As an alternative it is assumed here that aB major crustal rock types have actually been sampled but that the relative abundances of individual rock types have no significance.

The comparison of P-wave velocities of xenoliths with velocity-depth profiles along the EGT suggests the model crustal structures as presented in Figure 4-17. P-wave velocities for xenoliths fram the Eifel, North Hessian Depression and Urach/Hegau are reported in Mengel et al. (1991). Vp data for the Heldburg Gangschar xenoliths have also been calculated from their modal mineralogy according to the procedure described in Section 4.3.1. The pressures and temperatures for wh ich the P-wave velocities are calculated are those expected in the middle or lower crust and in the upper mantle.

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Figure 4-17. Xenolith-based crustal profilesfor the Hessian Depression, Elfei, Heldburg Gangschar and Urach-Hegau localities. The Vp-depth relations are from Aichroth and Prodehl (1990), exceptfor the Elfel which isfrom Mechie et al. (1983). The relative positions ofrock types are arranged according to calculated P-wave velocities of xenoliths.

North Hessian Depression

Beneath an 8-10 km thick Phanerozoic cover which includes about 2 km of Mesozoic sediments, there is a low velocity layer between about 9 and 11 km depth with a small high velocity layer immediately beneath. The P-wave velocities of leuco-granitic gneisses and low-grade meta-sediments match the lower velocities whereas the high velocity layer may be explained by sillimanite bearing meta-sediments.

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P-wave velocities measured in the middle crust between 13 and 20 km depth are conformable with felsic rock types found as xenoliths. The NHD granitic gneisses have somewhat 10wer velocities than the NHD tonalitic granulites and it appears likely that the latter originate from depths of 20-25 km. The increase in _Vp between depths of 25 and 32 km of about 6.6-7.3 kms-^I suggests that the mafic granulites constitute the lowermost section of the NHD crust.

The fact that the seismic Moho is not a sharp discontinuity implies that the mafic granulites may grade into ultramafic rocks below 30 km depth. Compressional wave velocities calculated for websterites and spinel peridotites also fall in this range of high velocity values (Mengel et al. 1991). The crust-mantle transition zone is believed to consist of a complex mafic/ultramafic layer.

Elfel

The crustal profile for the Eifel has much in common with that of the NHD, with the exception that calcsilicate rocks, meta-quartzites, and mica-schists occur in the middle part of the Eifel crust. The depth of origin of meta-igneous gneis ses is not well constrained because the observed increase in Vp with depth is quite continuous and a further subdivision is beyond the resolution of the approach employed.

Like the NHD, observed P-wave velocities increase from values near 6.7 kms-^I around 28 km to >8 kms-^I below 33 km. The only metamorphic rock types that match such values are mafic pyroxene and gamet granulites. It has been suggested by Stosch (1987) that these rocks grade into spinel peridotite upper mantle, thereby also forming a comp1ex mafic/ultramafic crust-mantle transition zone.

Heldburg Gangschar

Compared with the Rheno-Hercynian crustal sections of the NHD and the Eifel, the distribution of P-wave velocities in the central part of the Saxo-Thuringian is more simple:

between 8 and 19 km depth, observed Vp values range between 5.9 and 6.2 kms-^1• The lower crust (20-27 km) is separated from the middle crust and from the upper mantle by sharp discontinuities. Within the xenolith suite from the Heldburg Gangschar, there is currently no information about the upper and middle crust. P-wave velocities calculated for the pyroxene granulites range from 6.8-7.5 kms-1with an average value of7.2 kms-^I which is slightly too high for the Heldburg Gangschar lower crustal segment between 20 and 27 km depth. It is therefore suggested that the lowermost part of the Saxo-Thuringian lower crust is composed of mafic granulites which are compositionally similar to those of the Rheno-Hercynian lower crust as exemplified by the Eifel and NHD mafic xeno1iths. P-wave ve10cities calculated for the websterite xenoliths from Heldburg Gangschar vary between 7.8 and 8.3 kms-^I and it seems likely that these rocks are present in a thin crust-mantle transition zone between 27 and 29 km depth and in the uppermost Saxo-Thuringian mantle.

Urach/Hegau

In southem Germany near shotpoint CE (Figure 3-1), the crustal P-wave velocity distribution is characterized by a steady increase from values of 6 kms-¹ at about 7 km depth to 6.8 kms-^I near 28 km. The Urach/Hegau xenolith suite is dominated by medium to high-grade felsic meta-sedimentary and meta-igneous rocks and it is quite problematic to construct a fine scaled Iithology profile. However, Sachs (1988) and GIahn et al. (1992) suggested that

A CONTINENT REVEALED

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gamet-sillimanite bearing sampies with P-wave veloeities of7.0 to 7.6 kms-¹ originate from the lowermost part of the erust; the few meta-mafie xenoliths are expeeted to originate from the same zone. The other felsie xenoliths eannot be assigned to erustal depths more speeifieally. The distribution ofroek types shown in the Uraeh/Hegau eolumn ofFigure 4-17 is drawn tentatively by assuming that the slightly faster high-grade roeks originated from greater depth than the medium grade eordierite gneisses. Moldanubian type tonalitie and granitie gneisses probably oeeur in the uppermost part of the seetion.

Summarizing the main aspeets of the crustal sections presented it is important to note that each xenolith loeality comprises a distinct lithology suite. The differences between the Rheno-Hercynian xenolith profiles (NHD and Eifel) and the Moldanubian Urach/Hegau zone seem to reflect eontrasting tectonie styles as weH as substantial eompositional differ-ences, probably derived by pre-Variscan crustal evolution in independent terranes whieh were welded together during Variscan collision teetonism (see Chapter 6.3).