Una clasificación normativa de las rocas plutónicas y volcánicas mediante el diagrama 2Q-(or+ab)-4an-2F

Abstract

Modal analyses provide a precise classification of the plutonic rocks, as demonstrated by the great acceptance of the QAPF diagram. However, the existence of isomorphic series in feldspars causes some indeterminations and discontinuities in the fields of classification. Consequently, distinct rocks overlap at the same point, as is the case of the gabbro and the diorite. In volcanic rocks, the accuracy of the determination decreases strongly and its classificationcan become a simple approximation, or even become impossible in the hyaline rocks. Another logical drawback that derives from the different resolution is that some classification fields are different in plutonic and volcanic rocks, so there is no nomenclature equivalence for the same compositions. The normative diagram 2Q-(or+ab)-4an-2F proposed in this work allows to discriminate clearly between rocks with plagioclases of different compositions as well as to compare plutonic and volcanic rocks using the same parameters, independently of whether they are phaneritic or aphanitic rocks. As a result, two diagrams are obtained whose classification fields correspond exactly to the plutonic and volcanic rocks. Since the number of fields obtained is greater than those of the QAPF diagram, several new rock names are proposed, some of which may have an unlikely existence in nature