ABSTRACT
Magmatic structures in the Aztec Wash pluton preserve a record of repeated injections of mafic magma into a felsic magma chamber and provide information about subsequent tilting. The pluton is divided into a southern (lower) heterogeneous zone (HZ), which contains the abundant magmatic structures, and a northern (upper) homogeneous granite (HG), which is virtually structureless. HG is entirely felsic (most-72 wt.% SiO2), whereas the HZ is extremely diverse in composition (44–76 wt% SiO2).
Sequences of meter-scale mafic sheets with intervening felsic material mark repeated injection of mafic magma into the HZ. The sheets spread laterally and ponded, trapping more felsic magma or crystal mush beneath them. Most of the sheets are fine-grained with delicately crenulated, quenched margins against felsic rock, but thick sheets apparently cooled slowly enough to crystallize coarse cumulates. Gravitationally unstable felsic melt in the trapped felsic layers intruded the overlying mafic layers to form vertical sheets and pipes. The lateral spreading of the mafic sheets was interrupted near the margins of the HZ, where magma injected and disaggregated the wall rock to form abundant xenolithic blocks. After the HZ was semisolid, composite vertical sheets with mafic pillows in a felsic matrix intruded, apparently as a coarse slurry, in response to fresh injections of mafic magma into the base of the chamber. A gradational contact with the overlying HG suggests that HG was still at least partly molten when late HZ magmas intruded, but preservation of significant topography on the HZ-HG surface and of trains of mafic material above the contact indicate that it had appreciable strength.
The structures in the HZ demonstrate the complex, protracted way in which magma chambers can fill. This provides a mechanism for maintaining a chamber in a partially molten state for a much longer interval than if it had been emplaced all at once, or had contained only mafic or felsic magma. Furthermore, because many of the structures are inferred to have been either horizontal or vertical when they formed, they provide a means of estimating crustal tilting and therefore of reconstructing the original geometry of the pluton. The tilt estimates also clarify important extensional structures in this area that were difficult to define because of the absence of stratified rocks.
