Helmut Schleicher

Petrology and geochronology of eclogites from the Variscan Schwarzwald (F.R.G.)

The Moldanubian basement of the Schwarzwald contains basic to ultrabasic rocks of both crustal and mantle origin which display high-pressure mineral assemblages or relics of such. In order to constrain the P-T-t evolution of the crustal high-pressure rocks, petrological and geochronological studies have been carried out on three eclogite samples. Geothermobarometric estimations indicate minimum metamorphic pressures of 1.6 GPa and equilibration temperatures of 670 750°C. Reaction textures document various metamorphic stages during exhumation of the high-pressure rocks. The age of high-pressure metamorphism is constrained by Sm-Nd isochrons of 332±13 Ma, 334±11 Ma, and 337±6 Ma defined by garnet, whole rock and clinopyroxene. For one sample, large garnets show prominent growth zoning in terms of major elements, Sm, Nd, and inclusions, dividing the grains into two growth stages. Sm-Nd isotope analyses on these garnets indicate that the time span between the two growth stages is too small to be resolved, reflecting a rather rapid metamorphic evolution. This result is further constrained by a Rb-Sr isochron age of 325±6 Ma on retrograde biotite and whole rock on the same sample. For one of the studied eclogites, formation of the magmatic precursor rocks is possibly approximated by the Ordovician U-Pb upper intercept age of a discordia from zircons.

Isotope studies on alkaline volcanics and carbonatites from the Kaiserstuhl, Federal Republic of Germany

Sr, Nd and Pb isotopic data on alkaline volcanic rocks and carbonatites from the Kaiserstuhl Miocene alkaline volcano in the southern part of the Rhinegraben rift valley are presented. Isotopically, three distinct groups of volcanic rocks can be distinguished which are correlated to different magmatic series: 1. (1) Unfractionated primary mantle melts (olivine nephelinites, olivine melilitites) and nepheline basanites: 87Sr86Sr=0.7032-0.7040; ϵNd=4.18-5.03; 206Pb204Pb=18.95-18.98). Geochemically, these rocks correspond to a Na-dominated magmatic series. 2. (2) Fractionated alkaline rocks (tephrites, phonolites: 87Sr86Sr=0.7039-0.7051; ϵNd=2.07-3.96; 206Pb204Pb=19.10−19.42), corresponding to a K-dominated magmatic series. 3. (3) A group with carbonatites, bergalites, hauynophyres (87Sr86Sr=0.7036-0.7040; ϵNd=2.81-4.04; 206Pb204Pb=19.24-19.66) These results are discussed in terms of derivation from different mantle sources and interaction of asthenospheric melts with subcontinental lithospheric mantle during their ascent. Contamination with crustal material is important in the tephritic magma series, also in some phonolites. The isotopic data point to a metasomatic overprinting of the upper mantle below the Kaiserstuhl. This metasomatism is thought to be directly connected with the updomed mantle structure in this region. For the carbonatitic rocks a genetic linkage to olivine nephelinites is most likely. This implies an asthenospheric origin of the carbonatitic melts.

Phanerozoic Time-Scale

The field of geochronology involves more than just age determination. It also includes the chronostratigraphy of the Earth, in a wider sense, the eosmochronology of the universe. On the basis of the development of the biological world found in them, chronostratigraphic sequences reflect the history of the Earth in a qualitative way. Their significance lies in the possibility widely scattered natural records, often no longer in their original formation, and assign them to a previously reconstructed time sequence. In chronostratigraphic studies, attributes that can be assigned to a specific period of time, e.g., a certain isotopic composition (Chap. 6), are required. In biostratigraphic studies, key fossils that permit global correlations are of importance for correlating geological sequences whose biofacies and lithofacies differ completely from place to place.

Pb isotopic systematics of alkaline volcanic rocks and carbonatites from the Kaiserstuhl, Upper Rhine rift valley, F.R.G.

Abstract Lead isotopic data of alkaline volcanic rocks and carbonatites from the Kaiserstuhl are presented in combination with Sr isotopic ratios. As a whole the initial lead ratios cover the range typical for other Tertiary to Recent alkaline rock provinces in Europe: 206 Pb 204 Pb = 18.9–19.7 , 207 Pb 204 Pb = 15.61–15.70 and 208 Pb 204 Pb = 38.7–39.7 . Especially in the 208 Pb 204 Pb vs. 206 Pb 204 Pb diagram the data reveal a linear array which is interpreted in terms of mixing of two mantle components. In detail the data comprise three distinct groups which are correlated with different magmatic rock types: unfractionated primary mantle melts (olivine nephelinites, olivine melilitites), fractionated alkaline rocks (tephrites, phonolites), and a group with carbonatites, bergalites and hauynophyres. These results are interpreted to reflect either derivation from different mantle sources, or the data trace a mixing line between two mantle reservoirs, one of them being more enriched. Contamination with crustal lead is unlikely: it may have affected the tephritic magma series, but not the carbonatitic rocks. The uppermost part of the updomed mantle beneath the Kaiserstuhl, which is known by petrographic evidence to be partially metasomatized, probably is the site of the enriched reservoir. A “plume”-like structure within the deeper mantle is suggested as the initial source of this mechanism.

Rb-Sr systematics of Permian volcanites in the Schwarzwald (SW-Germany): Part II: Age of eruption and the mechanism of Rb-Sr whole rock age distortions

AbstractFrom the quartz porphyry occurrences of Haigerach and Geisberg (Central Schwarzwald), which were dated by minerals and whole-rock samples, the whole-rock samples generally yielded lower ages than the minerals. This result agrees with other studies on apparently rejuvinated whole-rock ages of acid volcanic rocks (compare McKerrow et al. 1980). Petrographic and geochemical observations show that two processes disturbed the Rb-Sr relations:1)fast alterations immediately after extrusion, such as sericitization during autohydrothermal bleaching in the pipe area of the volcanoes, gain of Rb, addition of common Sr.2)loss of radiogenic 87Sr due to long-lasting exchange processes with groundwaters. The combined result of the two processes is a kind of isochron rotation simulating younger whole-rock ages and higher initial Sr isotope ratios. Our results on the two quartz porphyries indicate that whole-rock dating of such acid volcanics is not succesful.

Time Scales and Ages

The question as to whether physical and chemical dating methods yield absolute or relative ages is the subject of continuing debate. Fuel for this discussion has been supplied especially by those methods that, for example, utilize cosmogenic isotopes (Sect. 6.2), and have time scales which deviate systematically from the solar calendar (Fig. 2.1).

Treatment and Interpretation of the Raw Data

The evaluation, treatment, and interpretation of the results of an age determination require long experience and an extensive knowledge of the literature. The laboratory analysis for physical and chemical age determinations is often depreciated as routine work, but careful laboratory work requires great skill and the guidance of an experienced scientist.

Chronostratigraphic Methods Using Global Time Markers

Several widely applied chronostratigraphic dating methods are based on globally distributed strata that contain physical or chemical anomalies, climatic rhythms or seasonal cycles resulting from global events (Walliser 1983/1984). To yield time marks, these globally present anomalies or stratigraphic records must be very accurately dated by suitable methods. Paleomagnetic remanence in magmatic rocks and sediments (Sect. 7.1) recording changes in the Earth’s magnetic field is an example of time markers. Variations in the stable oxygen isotope ratio in planktonic foraminiferal tests from pelagic sediments caused by variations in climate (Sect. 7.2) are another example of a natural stratigraphic record.

Physical Dating Methods

Most physical dating methods are based on processes that are strictly a function of time or that contain a prominent time-dependent component. This is especially true for the radiometric methods. Even with these methods, however, derivations from the siderial time-scale sometimes occur as a result of processes not taken into consideration by the assumptions of the model (Fig. 2.1). Such processes can be geochemical or geophysical, e.g., diagenetic mobilization of parent or daughter nuclides in a mineral or rock system, the occurrence or annealing of radiation damage, isotope fractionation, or long-term fluctuations in the production of cosmogenic radionuclides.

Radiometric Dating Methods

Except for Quaternary times, the major dating methods for geology and petrology are based on parent/daughter isotope ratios. The most important of these methods for dating igneous and metamorphic rocks and their minerals are the K/Ar (Sect. 6.1.1), Rb/Sr (Sect. 6.1.3), U/Th/Pb (Sect. 6.1.9), and Sm/Nd (Sect. 6.1.6) methods. For example, only with these methods can time relationships be determined for the Precambrian. This is also true for younger basement rocks, for instance, for deciphering the development of an orogen, especially where there are no associated sediments that can be dated by other means.