Ulrich Kramm

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.

Volume characteristics and element transfer of fenite aureoles: a case study from the Iivaara alkaline complex, Finland

Abstract Fenitization in granodioritic rocks has been investigated taking the Iivaara alkaline complex (Finland) as a case study. Five main fenitization reactions responsible for the transition from a granodioritic precursor mineral assemblage to a syenitic or even cancrinite–syenitic mineral assemblage of the fenites are set up. An assessment of element transfer and volume change associated with each of these reactions indicates a general volume loss of about 20% due to fenitization. High activities of Na, Ca, Mg and Fe as well as a low H2O/CO2 ratio in the fenitizing fluid phase are indicated. Si, which is the main component released from the precursor rock, was redistributed within the aureole and formed pyroxene dominated mineral assemblages in a reaction with the fenitizing fluid. These vein-type pyroxenites can be found cross-cutting all fenite types. Compositional data of aegirine augite in the fenites reveal disequilibria and feldspar compositions point to minimum temperatures for the production of fenites with syenitic compositions of 660°C and also indicate fast cooling. A Sr isotope characterization of the fenites allows assessment of minimum fluid–rock weight-ratios in the fenite aureole of 0.023–2.3. The element transfer scheme developed in this study is applicable to other localities since many aureoles formed in granitic to granodioritic host rocks have petrographic characteristics similar to the Iivaara fenites.

In situ U-Pb (Shrimp) dating of zircons from granosyenite of the Troitsk pluton, Kvarkush-Kamennogorsk Anticlinorium, central Urals

The Troitsk granosyenite pluton is located among the Neoproterozoic sedimentary sequences of the Kvarkush‐Kamennogorsk Anticlinorium on the western slope of the Urals. The sedimentary sequences comprise the Kedrovka and Basegi groups of the Upper Riphean and the Serebryanka and Sylvitsa groups of the Vendian [1, 2]. In the mid-1980s, the Troitsk pluton was used as a reference intrusive body for timing the lower boundary(620 ± 15 Ma) of the Upper Vendian [3]. However, later on, the validity of the Rb‐Sr wholerock isochron age of carbonated granosyenites was questioned [4]. In addition to the Troitsk pluton, a number of other igneous complexes (Shpalorezov, Dvoretsk, Blagodatny, Kus’ya, Zhuravlik, and others) are localized in the Vendian sedimentary rocks of the Kvarkush‐Kamennogorsk Anticlinorium. Their petrology, geochemistry, and age are discussed in many publications. The results of geological mapping under the supervision of Suslov [5] made an important contribution to recognition of the geological position of some igneous complexes. The most reliable correlation of the Late Riphean and Vendian igneous complexes in the Kvarkush‐Kamennogorsk Anticlinorium was given in [6‐8]. Karpukhina et al. [9] made an attempt to reconstruct geodynamic settings of the Dvoretsk, Kus’ya, and Blagodatny complexes and estimated their Sm‐Nd and Rb‐Sr ages. The modern data on geochemistry, geodynamics, and geochronology of the Neoproterozoic igneous complexes on the western slope of the central Urals are summarized in [10].

Oldest (3.5 Ga) Zircons of the Urals: U-Pb (SHRIMP-II) and T DM Constraints

The Archean domains of the Earth composed of granite‐greenstone and granulite‐gneiss associations are now sufficiently well studied. Nevertheless, many aspects related to the early history of the planet are still far from being solved [1, 2, and others]. Such a situation is also characteristic of the Uralian foldbelt. The formation of the crust in this region spanned a long period comprising the Archean‐Early Proterozoic, Late Proterozoic, Paleozoic, and post-Paleozoic stages [3]. In recent years, considerable progress has been achieved in reconstruction of the major features of the last three stages in the foldbelt evolution mainly due to the active introduction of advanced methods of isotope geology based on sophisticated analytical equipment into geological studies. At the same time, significant difficulties remain in the reconstruction of the preRiphean stage for the following reasons: first, the reliably defined Archean metamorphic domain in the Urals is extremely limited and represented, in fact, only by the Taratash polymetamorphic complex of the Central Uralian Uplift with the single Neoarchean date [4] obtained only recently by the classical U‐Pb zircon method; second, the dating of Neoarchean rocks is hampered by several methodological and analytical problems, the correct solution of which is essential for validity of the results obtained. The Taratash polymetamorphic complex is located among volcanosedimentary rocks of the Lower Riphean Ai and Satka formations at the junction of the Bashkir and Uraltau anticlinoriums located between the South and Middle Urals (Fig. 1). It occupies an area of 400 km 2 and is composed of lithologically variable gneisses, two-pyroxene crystalline schists, quartz-bearing diorites and gabbrodiorites, and quartz‐feldspar rocks. The geological structure of the Taratash Complex, the petrology of its rocks, and metamorphism evolution are described in detail in [5]. The history of the study of the Taratash Complex by methods of isotopic geology (K‐Ar, α -Pb, 207 Pb/ 206 Pb thermal isochron, and classical U‐Pb) is already more than 35 years old [4‐7]. These studies revealed several stages in the evolution of the rock complex corresponding to granulite metamorphism, high-temperature amphibolite-facies diaphthoresis, amphibolite-facies metamorphism, and greenschist diaphthoresis. Nevertheless, many aspects of its formation remain unclear. Moreover, they are conflicting to a significant extent in light of recent data [8].