Barbara den Brok and Wolfgang Hofmeister Institut für Geowissenschaften, Becherweg 21, 55099 Mainz, Germany
At the margin of the Tertiary volcanic field in the Hocheifel area, three remnants of former volcanos crop out (Fig. 1). These remnants are aligned north-south at a distance of about 300 m from each other. They are (i) the Hillscheider Basalt in the north, (ii) the Hillscheider Diatrem in the centre, and (iii) the Eckfelder Maar in the south. These rocks are of Tertiary age (~44-45 Ma), indicated by fossils in the lake sediments of the Eckfelder Maar (Franzen 1993), and Ar-Ar dating of volcanics from the Hillscheider Basalt and from the Eckfelder Maar (Scheibner 1998).
According to Huckenholz and Büchel (1988) the Tertiary volcanic field in the Eifel can be subdivided in a central area with differentiated and a rim with undifferentiated volcanic rocks (Fig. 2). However, in the area of Eckfeld and Hillscheid, i.e. at the rim of the Eifel, undifferentiated volcanic rocks (the Hillscheider Basalt and samples from the drilling cores of the Eckfelder Maar) as well as differentiated volcanic rocks (Hillscheider Diatrem and "Ostschurf'" of the Eckfelder Maar) are reported to occur (Lutz 1993, Negendank 1982). This is not in agreement with Huckenholz and Büchel (1988), according to which differentiated rocks should not occur in this area.
The argument that these rocks are differentiated is based on a macroscopic petrographic analysis of Lutz (1993), as well as on the low geomagnetic anomaly measured over the Eckfelder Maar and the Hillscheider Diatrem by Pirrung (1998), but not (yet) on a detailed geochemical analysis. The reason that such a geochemical analysis has not been carried out before is that these rocks are heavily altered. Only the rocks belonging to the vent of the Hillscheider Basalt are fresh.
Another question in connection with the samples from the Hillscheder Diatrem is their position in a volcanic system. Pirrung (1991) assumed that they were deposited at their crater rim. That the Hillscheider Diatrem existed as an independent volcanic system was concluded from (i) the fact that volcanic deposits occur at a topographically lower level than the Tertiary surface (the pyroclastics must have deposited in a topographical depression in the surface) and from (ii) geomagnetic anomalies near the outcrop of the Hillscheider Diatrem. Both outcrops, i.e. the "Ostschurf" of the Eckfelder Maar and the Hillscheider Diatrem, are believed to belong to the former crater rim (Pirrung 1991).
Volcanic samples from the "Ostschurf" of the Eckfelder Maar and from the Hillscheider Diatrem both consist of yellowish to brown, dull, resinous, wax-like, 'shaled-out' matrix. This matrix is commonly massive and breaks with a conchoidal fracture. It includes some minerals (spinel) and to a different extent mm- to cm-size xenoliths. For further examination by X-ray, RFA and ICP-MS the matrix of every sample was separated ('picked').
X-ray diffraction shows that the matrix of the samples from the Eckfelder Maar consists of montmorillonite (smectite), whereas the matrix of the samples of the Hillscheider Diatrem consists of glass. SEM-studies of the clay minerals from the samples from the "Ostschurf" show spheroidal structures consisting of small clay minerals. From the major elements from the 'picked' clay matrix it could be shown for samples from the "Ostschurf" in comparison with samples from the Hillscheider Basalt that K, Na, Ca, and Mg and to lesser extent Fe are relatively depleted, and that Si, Ti, and Al are relatively enriched. The samples of the Hillscheider Diatrem show, to a smaller extent, a depletion of the major elements and further, a relative enrichment of Fe. Samples from both locations have an LOI (a 'loss on ignition') of 20 % and higher. Carbon and sulfur appeared to be present in very small quantities only. Therefore, the LOI can be assumed to consist almost entirely of water. From the major elements the 'index of alteration' (CIA) could be calculated. The CIA gives a measure of the alteration (Nesbitt and Young 1982). It depends on the mixing of different minerals and their contents of Al, Ca, Na and K. From fresh to altered rocks, the CIA may vary from 30 (for fresh mafic rocks) to 100 (for kaolinite and chlorite). Samples from the "Ostschurf" show CIA-values of about 99, indicating that they were altered completely, and mainly consist of kaolinite. By contrast, samples from the Hillscheider Diatrem show CIA-values of about 81 to 87, indicating that they consist of montmorillonite and kaolinite. Also, normative calculation of clay minerals from the samples of the "Ostschurf" indicates that their matrix mainly consists of kaolinite. Normative mineral calculations of the samples of the Hillscheider Diatrem indicate that they mainly consist of montmorillonites. These calculations are based on a simplification of the normative clay mineral calculations by Pfeffer (1996). From these results it can be concluded, that the samples are altered and depleted in major elements to a different extent in dependence of sample locations. The matrix from the samples of the "Ostschurf" is totally composed of impoverished montmorillonites, and contains some spinels and various amounts of xenoliths. The matrix of the samples from the Hillscheid Diatrem consists of a not completely 'shaled-out' glassy matrix of montmorillonite, plus some spinels and a variable amount of xenoliths.
Normalisation of the major elements, the trace elements, and the RE-elements of the 'picked' matrix of the samples from the Hillscheider Basalt shows that REE, together with Nb, Zr, Ti, Y, Sc (high field strength) are enriched. The REE-enrichment was found earlier by Cullers (1987) in the clay fraction. The samples are completely depeleted in the major elements as well as in in Sr, and, for the samples from the "Ostschurf", also in Rb. The samples from the Hillscheider Diatrem, are enriched in Ba, even though this is a mobile element. REE-diagrams normalised to the Hillscheider Basalt show a trend to enrichment, whereas Ce, Eu and Lu show negative anomalies.From Eu it cannot be excluded that it is a signature of the original rock and Lu seems to be a phenomon typical by research done on clay-sized factions (Cullers 1987).The samples from the Hillscheider Diatrem show a parallel pattern of the REE, with sligth HREE-enrichment (or LRRE-depletion). The samples of the "Ostschurf" show a less parallel pattern of the REE and slight HREE- depletion (or LREE-enrichment).
The relative content of the high field strength elements of the matrix from the samples of the Hillscheider Diatrem and from the "Ostschurf" of the Eckfelder Maar make a classification possible (Winchester and Floyd 1977). They are classified as alkalibasalts, as basanites or as nephelinites. There is no indication for any differentiation. Together with the SEM-study, showing the presence of spheroidal structures consisting of clay minerals, (i) the colour and the wax-like nature of the samples, (ii) the high water content, (iii) the K-depletion, and (iv) the relative enrichment in Fe, suggests that the samples consist of palagonites, an alteration product of sideromelan or glassy basalts (Furnes 1979, Furnes 1980). Palagonites are commonly formed by weathering, sometimes enforced by a high flux of warm water immediately after a volcanic eruption.
The depletion of the mobile elements together with the (relative) enrichment of only the high field strength, the absence of sulfur in the samples and the negative Ce-anomaly of the REE are indicative of the alteration process: (i) There is no indication for secondary processes such as hydrothermal alteration, and (ii) Ce-anomalies are found after weathering in a water saturated surrounding (Braun et al. 1990).The Ba-enrichment seems to be caused by weathering of feldspar. Ba could be hold in zeolites (harmotome), correlating with high Ba-contents of zeolites shown by electron microprobe analysis. Zeolites are typically formed during the first stages of palagonitisation (Fisher and Schmincke 1984). Also LREE-depletion of the samples from the Hillscheider Diatrem could be correlated by water saturated alteration of basaltic glass (Bienvenu et al. 1990). Therefore the alteration could be explained by weathering or palagonitisation only. They do not necessarily include later hydrothermal alteration processes.
The differences in the alteration state of the studied samples from the Hillscheider Diatrem suggest that they are not the products of a phreatomagmatic explosion deposited immediately after explosion at the crater rim. Furthermore, drilling in the assumed centre of the Hillscheider Daitrem in the Summer of 1999 showed that solid bed rocks occur after a depth of only 20 m (Lutz, pers. comm. 2000). These solid bed rocks are overlain by sediments that contain abundant blocks from the Hillscheider Basalt. They could be responsible for the measured geomagnetic anomaly and the samples from the Hillscheider Diatrem could belong to the surrounding breccia of the Hillscheid Basalt. Basalt and breccia from the Hillscheider Basalt could build up epiclastic talus deposits. Therefore, there remain indications for only two former volcanically active centres: the 'Eckfelder Maar' and the 'Hillscheider Basalt', and not for the third so called 'Hillscheider Diatrem' volcanic centre.
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