Magnetic fabrics and compositional evidence for the construction of the Caleu pluton by multiple injections, Coastal Range of central Chile

Abstract

The Caleu pluton, which is located in the Coastal Range of central Chile, consists of three N-S elongated lithologic zones which define an across-pluton compositional variation characterized by a westward increase in SiO2 content, the Gabbro-Diorite (GDZ), Tonalite (TZ) and Granodiorite (GZ) Zones. A total of 547 cores were drilled at 68 sites within the different zones to carry out magnetic fabric and paleomagnetic studies. The highest values of magnetic susceptibility and remanent magnetization are found in the GDZ (0.04-0.1 SI), whereas the lowest values are observed in the GZ (0.01-0.02 SI). This is consistent with the significant decrease in magnetite content from the GDZ to the GZ. The dips of the magnetic foliation planes are steep within GDZ and TZ, and shallower dips are observed in GZ. Magnetic foliations along some borders of the pluton are parallel to the corresponding contacts. The parallelism observed between the magnetic foliations and the contacts between zones suggests that these zones have recorded the same mechanism of emplacement at the magmatic state. The peculiar foliation pattern observed in the central parts of TZ and GZ is attributed to a lobe-like structure that was acquired during the late stages of emplacement and inflation.

The distinct compositional characteristics of these three zones were not acquired in situ. Rather they were emplaced successively from mafic to felsic from a subjacent stratified sill-like magma reservoir, which was filled after tilting of the host volcanic succession. The rapid upward transport of these melts of different compositions up to a shallow depth is attested by both the equilibration pressure of late magmatic homblende (∼ 200 MPa) and the pressure of early magmatic epidote crystallization (∼ 500-600 MPa). We conclude that space was created by lateral flow propagation, in which the intrusion of later pulses pushed earlier ones away from the feeder system, followed by roof lifting and possibly floor depression. © 2005 Elsevier B.V All rights reserved.