Premium
The rapakivi texture of feldspars in a plutonic mixing environment: A dissolution‐recrystallization process?
Author(s) -
Bussy François
Publication year - 1990
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350250314
Subject(s) - plagioclase , mafic , igneous differentiation , porphyritic , recrystallization (geology) , feldspar , geology , geochemistry , alkali feldspar , crystallization , mantle (geology) , mineralogy , petrology , chemical engineering , paleontology , quartz , engineering
A new hypothesis is formulated to explain the development of rapakivi texture in and around the mafic enclaves of porphyritic granitoids, i.e. in environments involving magma mixing and mingling. The formation of a plagioclase mantle around alkali feldspar megacrysts is attributed to the localized presence of a melt resulting from the reaction of these megacrysts, with host hybrid magma with which they are in disequilibrium. This feldspathic melt adheres to the resorbed crystals and is virtually immiscible with the surrounding magma. Its composition is modified in terms of the relative proportions of K 2 O, Na 2 O, and CaO through selective diffusion of these elements, thus allowing the specific crystallization of andesine. With decreasing temperature, the K‐feldspar, again stable, crystallizes along with the plagioclase, leading to mixed mantle structures.