H-TYPE (HYBRID) GRANITOIDS - A PROPOSED REVISION OF THE GRANITE-TYPE CLASSIFICATION AND NOMENCLATURE

Recent studies on calc-alkaline plutonic rocks reveal field and petrographic relationships with strong implications for the processes involved in their genesis. The presence of magmatic inclusions and magma mingling zones in these rocks supports an origin by magma mixing. Compositional variations and isotopic anomalies are a good test for such petrogenetic models. Most of these calc-alkaline plutonic rocks which characterize both collision-related and active plate margin environments have been classically identified as I (igneous)-type granitoids, following Chappell and White's (1974) classification, and then interpreted as derived from partial melting of older igneous rocks (restite model). If the magma mixing origin of a type of granitoids can be demonstrated from field and petrographic evidence and supported by chemical and isotopic variations, the nomenclature must be changed. In this sense the new category of H (hybrid)-type is tentatively introduced. It includes most of the so-called I-type granitoids and some of the S-type. As deduced from field relationships of magma mingling zones, a mixing model is suggested that can operate on a large scale to explain the petrogenesis of the Hercynian calc-alkaline granodiorites of Iberia. Finally, a modified classification of common plutonic rocks in orogenic environments is proposed.