Coupling restites and mobilizates – Geological and litho-chemical investigations of paired belts of calcsilicate fels and quartzite (SE German Basement) – Quo vadis David London’s pegmatology?

Abstract

In some areas of the Variscan orogen felsic mobilizates (pegmatitic and aplitic rocks) are closely associated with stratiform and stockwork-like bodies enriched in Ca minerals (e.g. wollastonite, diopside-hedenbergite s.s.s., grossular-spessartine s.s.s., siderite..) and bodies aligned to them similar in structure but abundant in quartz, plagioclase and mica. Geological mapping and lithochemical studies are the tools to decipher the nature of these crystalline rocks which are common to the Hagendorf-Pleystein Pegmatite Province, SE Germany, and present in many ensialic orogens elsewhere. Geological and chemical data suggest paired belts of a restite-mobilization system. The Ca and Si metasomatites are different from calcareous metasediments and quartzites elsewhere in the SE German basement devoid of mobilizates (parent rocks: limestones and cherts). Mobilization conducive to this paired belt of metasomatites involved silica mobilized from a deep level of the crust as a result of metamorphic-metasomatic alteration of Precambrian to Early Paleozoic metagreywackes during retrograde metamorphism from HP to LP metamorphism around 680–600 °C. The arrangement of mobilizates and restites in the field has been denominated as metamorphic differentiation sensu lato. The zone of silica mobilization is transitional into a zone of pegmatoids and aploids that overlaps with another one characterized by rocks derived from Ca metasomatism the footwall facies of which developed in the range 750–400 °C while in the hanging wall metamorphic rocks of rare-element pegmatites 570–430 °C occurred. The intensity of Ca metasomatism diminishes from the footwall to the hanging wall rocks and reflects a subcrustal impact. These investigations call attention among exploration geologists and petrologists to an alternative origin of “metasilica” and “metacarbonate” rocks being encountered in a zoned arrangement with felsic mobilizates (pegmatitic and aplitic rocks). The current study also raises the question “Quo vadis” pegmatology? It is an amendment to the mainstream geoscientific handling of pegmatitic rocks as “..texturally complex igneous rocks” genetically linked to granitic plutons (see review of London (2018) in Ore Geology Reviews). Taking a holistic approach can give us a reality check and prevent pegmatology from converting into a one-way street (granites-only) that eventually ends up in a dead-end street. The field evidence is the litmus test for all our models created in the laboratory and on the PC. There is no ore geology without field geology.