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The Geological Setting of the HPPP

  • Harald G. Dill
Chapter
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Part of the Modern Approaches in Solid Earth Sciences book series (MASE, volume 15)

Abstract

Contact metasomatism and contact metamorphism is closely related with the pegmatites of the HPPP and places the thermal “hot spot” for the early mineralization in the area between Pleystein and Gsteinach. Wollastonite is the diagnostic mineral found in the skarn and calcsilicates next to the Kreuzberg pegmatite. The intimate intertonguing of scheelite with calcsilicate rocks near Pleystein, accompanied by silicates such as grossularite (hessonite), vesuvianite, zoisite, and minerals of the diopside-hedenbergite s.s.s., in context within a well-defined zone of minerals typical of a high-T contact metamorphism lends support to a W mineralization genetically related to the pegmatites. The presence of W-bearing “nigrine” supports this idea. Neither the large granite massifs, e.g., the Flossenbürg Granit Massif at the northern rim of the HPPP, nor the shallow granitic stocks intruded along deep-seated lineamentary fault zones (e.g., Křížový kámen granite, Czech Republic) had the thermal potential to provoke the crystallization of wollastonite within their calcareous country rocks. Consequently, magmatic rocks of dioritic to even gabbroic composition area around Pleystein possess the thermal potential to raise the temperature in the wall rocks of the HPPP so that minerals of the uppermost hornfels facies can develop in the pertinent host rock lithology. The intimate spatial relationship between the pegmatites and a wide variety of ultrabasic, basic magmatic rocks of subcrustal origin and even metacarbonates has to be paid more attention to during the study of pegmatites. The stock-like pegmatites have a characteristic aureole with stockwork-like veins and veinlets in the hanging wall zone. Within this zone the amount of pegmatitic veinlets gradually increases at the expense of aplites when approaching the pegmatite stock. It is accompanied by a strong kaolinization representing a supergene alteration onto a hypogene one. By contrast, the footwall sequence of the pegmatite consists of a layered aplite. The intensity of the kaolinization gradually changes towards the pegmatite reached its maximum in zone underneath and to the east of the Kreuzberg Pegmatite where feldspar was no longer stable under the existing physical-chemical regime and completely eradicated from the aplites, aploids and pegmatoids. The hypogene kaolinization which has neither an equivalent in the remaining pegmatites of the HPPP nor elsewhere in the granitic complexes in NE Bavaria has eaten away the feldspar rim of the Pleystein pegmatite and left behind the “quartz pegmatite ruin”. The timing of the process may be seen in context with the proposed epithermal processes (shallow hydrothermal mineralization) at the end of the Variscan orogeny. Corresponding to their structural setting, the complete suite from pre-kinematic meta-lamprophyres to post-kinematic lamprophyres could be mapped in the environs of the Kreuzberg Pegmatite. They point to a subcrustal source. High Nb, Ta, P and Ti contents underscore the impact of these mafic rocks on the felsic intrusive rocks around Pleystein. From the structural point of view the lamprophyres are part of a dyke swarm, whose individual dykes are arranged NNE-SSW being an integral part of a shear zone or strike-slip fault together with the Kreuzberg Pegmatite and the newly discovered aplite. The pegmatite at Pleystein developed in an open cylindrical fold whose axis gently plunged towards the WSW. The fold axis has been calculated to dip away at angle between 10° and 15° towards the WSW at a distance of approximately 100 m west off the quartz core. All tabular pegmatites and aplites strike NW to NNW. Their stock-like counterparts, such as Hagendorf-South reside on fold structures running almost in the same direction NW-SE (2nd order fold). The pegmatite bodies of the stocks pinch out towards the WSW to SW. The central facies of the pegmatite system composed of the Hagendorf-South, Hagendorf-North and Pleystein stocks plunges towards the E and SE. Mimetic or facsimile crystallization played the most decisive role from the micro- to the larger scales when it came to the emplacement of the stock-like and tabular pegmatites and aplites in the HPPP. Preexisting fold structures created during the Variscan Orogeny behaved in two different ways as conduit and as traps for the mineralizing solutions and melt, all in one.

Keywords

Mineral Assemblage Contact Metamorphism Granitic Pegmatite Variscan Orogeny Footwall Rock 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer International Publishing Switzerland 2015

Authors and Affiliations

  • Harald G. Dill
    • 1
  1. 1.Gottfried-Wilhelm-Leibniz UniversityHannoverGermany

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