Guidelines to classification and nomenclature of Arabian felsic plutonic rocks
Well-defined procedures for classifying the felsic plutonic rocks of the Arabian Shield on the basis of petrographic, chemical and lithostratigraphic criteria and mineral-resource potential have been adopted and developed in the Saudi Arabian Deputy Ministry for Mineral Resources over the past decade. A number of problems with conventional classification schemes have been identified and resolved; others, notably those arising from difficulties in identifying precise mineral compositions, continue to present difficulties. The petrographic nomenclature used is essentially that recommended by the International Union of Geological Sciences. Problems that have arisen include the definition of: (1) rocks with sodic, zoned or perthitic feldspar, (2) trondhjemites, and (3) alkali granites. Chemical classification has been largely based on relative molar amounts of alumina, lime and alkalis, and the use of conventional variation diagrams, but pilot studies utilizing univariate and multivariate statistical techniques have been made. The classification used in Saudi Arabia for stratigraphic purposes is a hierarchy of formation-rank units, suites and super-suites as defined in the Saudi Arabian stratigraphic code. For genetic and petrological studies, a grouping as ‘associations’ of similar and genetically related lithologies is commonly used. In order to indicate mineral-resource potential, the felsic plutons are classed as common, precursor, specialized or mineralized, in order of increasing exploration significance.