Geology and chemistry of Variscan-type pegmatite systems (SE Germany) – With special reference to structural and chemical pattern recognition of felsic mobile components in the crust

Abstract

The geology of pegmatite systems encompasses lithology, shape, and structure while the chemistry of major and trace elements is indicative of the ore composition; both are the “pillars” of the CMS classification scheme (Chemical composition-Mineral assemblage-Structural geology) for barren and rare-metal pegmatites, including their granitic affiliates. The term Variscan-type has been coined to describe a style of formation linked to the ensialic orogens and a timebound mineralization sandwiched between the Caledonides and the Alpides. The primary formation covers the time from the Neoproterozoic through the Permian and ends with a hydrothermal phase waning eventually in the supergene alteration and is subdivided into three stages: (1) from diatectic to metatectic gneisses, (2) from metapegmatites, metamorphic pegmatoids to thrusting, (3) from the crust to the mantle and from barren to rare metal pegmatites. This evolution is characterized by a retrograde metamorphism from HP/MP to LP regimes. The tabular and stock-like pegmatitic, aplitic and granitic rocks in autochthonous and allochthonous units are grouped into 8 types (A–H) based on the above qualifiers of the CMS scheme. On a large scale, felsic mobilizates are accumulated by mimetic (facsimile) crystallization in anticlines with the most effective traps encountered where the directions of great circle plunges cut each other at almost right angle (stereonet analysis). The term “mobilizates” is used to describe felsic mobile components in the crust which migrated to a different extent from the site of their formation. On a small scale, where southward-dipping planar architectural elements are cut across by deep-seated lineaments the “temperature depression” of the retrograde system occurs and rare-metal pegmatites are located. This subhorizontal plane is correlated with a gently dipping Moho and vertical lineamentary fault zones with bulges of the Moho (chemical contour map analysis). Spider diagrams whose element contents are normalized to a reference paragneiss are categorized into 4 chemical patterns: (1) circular patterns (= metamorphic mobilizates, magmatic mobilizates), (2) necking-down patterns (= different degrees of fractionation), (3) lens-shaped patterns (= wall rock alteration), (4) stellate pattern (= different degrees of fractionation and mixing of fluids). The marker assemblages among the major elements are: Si-Fe-P: metamorphic to magmatic (sub)crustal mobilizates, K-Na-Al: metamorphic mobilizates, Ti-Mg: restites of metamorphic and magmatic mobilizates, Ca: remnant in the exocontact of pegmatitic systems, Mn: marker of depth-pressure The marker assemblages among the minor elements are: As-Bi: HT hydrothermal-metamorphic fluids, Cu-Ni-Mo: hydrothermal-deep-seated +(ultra)basic sources, U-Zn: hydrothermal-deep-seated sources, Pb: LT hydrothermal, Nb-Ba-Rb: pegmatitic fractionation-Ba (early)-Rb (late), Zr: restites of metamorphic mobilization + fractionation, REE: metamorphic mobilizates. The marker to discriminate hypogene and supergene kaolinization are: (1) hypogene (Ca- Mg out, Zn-Cu-Bi-Rb-Nb in), (2) supergene (Zr-Ti in).