Fine-grained felsic enclaves in the homogeneous granite zone of
the Aztec Wash Pluton, Nevada
Danny Lazzareschi
Faculty Advisor: Rick Hazlett
The Aztec Wash pluton has been tilted to expose a cross section consisting of
relatively homogeneous granite and a heterogeneous zone containing granitic,
basaltic, and intermediate rocks. The heterogeneous zone is interpreted to reflect
repeated injection of and interaction between mafic and felsic magmas, with mafic
and hybrid products accumulating near the bottom and interior of the chamber and
felsic magma at the margin and roof. Fine-grained felsic enclaves are widely
distributed within the granite zone. They are mostly ovoid and range in diameter
from centimeter scale to several meters, commonly 5 to 20 cm. Enclaves are
present from the basal and lateral contacts with the heterogeneous zone up to the
roof of the chamber, and are seen in isolation, clusters and trains. Very fine-grained
to aphanitic textures contrast strongly with those of the host granite and
demonstrate that the enclaves are essentially quenched melts. Large feldspars in
enclaves are similar to those seen in the host granite, but they commonly have
reaction rims suggesting that they may include xenocrysts as well as phenocrysts.
Compositions cluster between 68 and 72% SiO2 in a host granite typically 73-74%
SiO2, demonstrating that they were not derived directly from the granite as
fractionated liquids. Initial Sr isotope ratios of enclaves (0.7089-0.7116) fall mostly
between those of the host granite (0.7102-0.7113) and heterogeneous zone rocks
(0.708-0.710).
Compositional and textural characteristics and distribution demonstrate that the
enclaves were formed by a process distinct from the evolution of the main granitic
magma and then transported to their present locations. Their compositions suggest
that they are well-mixed hybrids rich in a granitic end member, probably formed in
or at the margin of the heterogeneous zone. Field relations, including angular,
sheet-like enclaves and disaggregated tips of dikes resembling enclaves, indicate
that at least some formed from the dissociation of felsic dikes within the granite.
Other possible sources for the enclaves are quenched margin materials (similar
porphyries are locally exposed at the roof), and hybrid material carried off the
heterogeneous zone by gravity or convection (the most felsic hybrids at upper
surfaces of the heterogeneous zone are fine-grained and have similar
compositions). Field and thin section evidence (mafic inclusions, rapakivi texture
and skeletal amphiboles) suggest a hybrid origin for many enclaves, indicating that
an early solidification front is an unlikely source.
This project was funded by NSF grant #EAR-0107094 to C. Miller, Vanderbilt
University.