The CMS classification scheme (Chemical composition – Mineral assemblage – Structural geology) – linking geology to mineralogy of pegmatitic and aplitic rocks
Dill, Harald G.
published: Aug 1, 2016
ArtNo. ESP154019303001, Price: 29.00 €
Pegmatitic rocks take on a special role. On one hand they closely resemble one of the most abundant rocks in the earth's crust, the granitoids; on the other hand they strongly contrast with these plutonic rocks by spreading the boundary from metamorphic to intrusive rocks. They are outstanding as to the texture and grain size and they have in parts a chemical and mineralogical composition which is nearly unrivaled, as it is the number of classification schemes of pegmatites put forward during the last decades. The present CMS classification scheme (Chemical composition – M ineral assemblage – Structural geology) is the first of its kind that goes beyond the mineralogical disciplines and makes geology the centerpiece of the classification. This descriptive CMS classification is a binary system, addressing the ore body and the ore composition. The ore body is subdivided into the type of pegmatites (pseudopegmatite, metapegmatite, pegmatoid, pegmatite, and plutonic pegmatite), the shape and the structure of pegmatites, while the ore composition is characterized by the chemical and mineralogical qualifiers which allow an open access for all element symbols and mineral names, respectively. The CMS scheme can be run as long and short version, which both are suitable for being used in a digital database and can also be linked via alpha-numerical codes to the Chessboard classification scheme of mineral deposits which is at a superior level of classification. The descriptive terms in the CMS directly translate into genetic statements. One of fundamental changes concerns the role of granites. Fractional crystallization of a granitic magma is only one process out of five accountable for the emplacements of pegmatites and aplites; the others are metasomatic processes, metamorphic differentiation, prograde metamorphism and partial melting/anatexis, and retrograde metamorphism. The parental granite only has a say in the plutonic pegmatites. Pegmatites, in general, are a mixture of chemical components derived from crustal and subcrustal sources. Elements and minerals in pegmatites and aplites reflect the source area and the geodynamic setting. Three geodynamic settings, Alpine-type, Variscan-type and Rift-type are prone to pegmatitic rocks. The most preferential area for pegmatites is the ensialic orogen with a thickened crust. Ensimatic crustal sections are only favorite areas for pegmatites during their incipient or embryonic stages of rifting.