Pegmatites and Their Country Rocks in the Central European Variscides

Abstract

Aplites and pegmatites can be correlated with their country rocks using the temporal, the compositional/chemical and physical relationships. The emplacement of pegmatitic rocks began during the early Paleozoic (470–440 Ma) which are encountered as metapegmatites in nappe units in the Bohemian Massif, proper, and in the Ötztal Massif within the Alpine Mts. Range. At the end of the MP-HT metamorphism around 370 Ma pegmatoids came into existence in the allochthonous units. From the core to the margin near the collision zone the granites and the pegmatites get younger. The belong to the Variscan heat event, but a close-up view of some of the pegmatites bear witness of an older cooling age of the muscovite from the HPPP pegmatite than the whole-rock age of the nearby Flossenbürg granite. The pegmatite show different ages as the major-rock-forming silicates and rare-element minerals, e.g. columbite are considered. Considering the element assemblage of the pegmatites and aplites in the Hagendorf-Pleystein-Pegmatite Province reveals that the element has not been derived from one source only. Intracrustal sources may be claimed for Li, F, U, Sn, B, P, As and Mn. Niobium, beryllium and bismuth (?) are of subcrustal derivation. In addition, there are subcrustal - intermediate repositories as it is the case with the Zn, REE and Y. Apart from the granitic intrusive rocks and metamorphic rocks, another group of subcrustal magmatic rocks, (meta)-lamprophyres, has not been drawn the attention to, which they deserve. If the fractionation and zonation in a pegmatite field or province is investigated and an attempt is made to compare these individual pegmatites and aplites with a nearby granite some critical points have to be considered.

• 1. the direction of fractionation in and emplacement of the granite

• 2. the polyphase zonation within the pegmatite field or province.

The Flossenbürg granite shows a E-W direction of emplacement and an opposite trend of fractionation. The HPPP, which is situated to the south of the afore-mentioned granite, shows an emplacement from the SW towards the NE and a polyphase zonation as a function of the mineral association. With this in mind the granites and pegmatites are supposed to be part of the Variscan thermal event but unrelated with regard to the structural and fractionational processes. They are sisters but not parents and children. Geophysical surveys lend support to this structural-compositional plan. The Flossenbürg Granite dips away from the ZEV metamorphics towards the E, similar to its northern counterpart, the Tirschenreuth Granite. South of the Oberpfalz granites, an ENE to NE trending deep structure, called Luhe Line exists. It is marked by ultrabasic to basic rocks of mantle affiliation. At the point of intersection, where it is crossed by another deep structure, this strong linear magnetic anomaly is perforated by a couple of negative rounded to oval-shaped magnetic anomalies like a “Swiss Cheese”. The zone of intersection coincides with the cluster pegmatites and aplites of the HPPP.A low-resistivity zone known from the foreland dips down under the basement. The low resistivity layer at shallow depth was interpreted as an accumulation of graphitic matter or highly saline waters. Graphitic material accounts for the deep low-resistivity zone. Seismic methods were extensively used during DEKORP. They show a series of southward dipping reflectors which formed the guide-lines for the differentiation within the pegmatite-aplite fields located between the core zone and collision front and mark the nape stacking at the western edge of the Bohemian Massif. The intersection of these nappe piles and deep-seated lineamentary fault zone are of ore control for the HPPP.