Journal of Petrology Volume 42 Number 2 Pages 251-276 2001
© Oxford University Press 2001
Crystal Accumulation and Shearing in a Megacrystic Quartz Monzonite: Bodocó Pluton, Northeastern Brazil
DEPARTMENT OF GEOSCIENCES, TEXAS TECH UNIVERSITY, LUBBOCK, TX 79414, USA
The Bodocó pluton, typical of numerous felsic intrusions in northeastern Brazil that are characterized by blocky megacrysts of K-feldspar, consists mainly of porphyritic coarse-grained quartz monzonite (SiO2 58–70 wt %) and is reversely zoned from a granitic margin to a quartz monzodioritic core. There is little variation in mineral composition throughout the pluton, despite a range of variation in mineral proportions. Isotopic characteristics also are homogeneous, with 
18Oquartz between +9·3 and +9·8
and initial 87Sr/86Sr within limits of 
0·7056–0·7063. Petrogenetic modelling indicates that in situ crystal accumulation processes, accompanied by the upward migration of a crystal-poor felsic melt, can account for many of the observed chemical and isotopic features, petrographic textures, and spatial relationships of rock types. Localized shearing associated with regional ductile deformation produced extensive kilometre-wide bands of strongly foliated megacrystic quartz monzonite intruded by mafic dykes. Shear-related magma mingling and/or mixing were localized post-emplacement differentiation processes, particularly at the upper level of the intrusion and in quartz monzonite border units along the southeast margin.
KEY WORDS: accumulation; Brazil; megacryst; petrogenesis; shearing