THE UPPER CRUSTAL EMPLACEMENT OF THE BALBUENA PLUTON BY MULTIPLE MATERIAL TRANSFER PROCESSES

The ascent of melt from source to surface typically involves a wide variety of material transfer processes. Processes accommodating the ascent and emplacement of magma into the brittle upper crust are typically dominated by stoping, roof uplift, and diking. Processes such as ductile flow and large-scale rotation of panels of host rock are commonly subordinate. The material transfer processes associated with the upper crustal (<7 km depth) emplacement of the Balbuena pluton, however, included, at various times, ductile strain, large-scale rotations and stoping of the surrounding host rock. The Balbuena pluton, located ~80 km south southeast of Ensenada, Mexico, intruded the late Early Cretaceous Alisitos Formation after most of the contraction associated with the accretion of the Alisitos arc to the continental margin had ceased. The pluton is comprised of two distinct magmas (tonalite and quartz diorite) separated into three zones. The outer and middle zones are crescent shaped about the inner zone, which is nearly circular. Field and petrographic observations demonstrate that the middle (quartz diorite) zone is the oldest, but was still locally molten when the melts of the inner and outer zones intruded. U/Pb zircon crystallization ages for the different magmas (107.7 +/- 3.4 and 108.6 +/- 4.0 Ma) are within error of one another, which supports this conclusion. While the time between the intrusion of the magmas must have been relatively short, the material transfer processes operating in the structural aureole of the Balbuena pluton were very distinct. Structures in the pluton aureole include high ductile strains that increase towards the pluton margin and folds that rotated clockwise and counter-clockwise by as much as 75° away from regional NW-SE trends. Stoping was an important material transfer process operating during the emplacement of the magma of the inner and outer zone as evidenced by stepped margins partially removing the ductily strained and rotated aureole rocks formed during emplacement of the earlier phase. Additionally, contacts with the inner, outer and middle phase commonly include abundant stoped blocks of the early phase including some that have shattered so completely as to form a slurry-mix of the younger magma choked with particles of the older.