MECHANISMS OF MAFIC DISAGGREGATION IN A CRYSTAL-RICH GRANITIC MUSH AT AZTEC WASH PLUTON, NV

Most of Aztec Wash (AWP), a mid-sized Miocene pluton emplaced from 15.8-15.6 Ma in the Northern Colorado Extensional corridor, is characterized by mafic, intermediate, and felsic rocks that are interspersed on very small to large scale. Field relations suggest that this heterogeneous assemblage is the product of repeated injection of basaltic magma into a variably crystal-rich magma chamber and subsequent mechanical and chemical interactions. The most pristine manifestations of the intruding mafic magma are sheets and discrete pillow-like masses of basaltic material with chilled, crenulate margins against granite that indicate quenching of mafic liquid against melt-bearing granite. The petrographic and textural similarity between sheets and pillows suggests that the latter formed from the former by disaggregation while still molten. The goal of this study is to understand how disaggregation—a process that may have facilitated mixing and heat transfer into host granite—worked.

Pillows and sheets are present over a several km paleothickness within AWP, from the deepest exposed levels to near the top. Well-defined sheets are ~0.5 to 3 m thick; thicker sheets have quenched bases but grade upward to intermediate compositions and granite. They commonly display load casts in coarse underlying felsic cumulate host and are penetrated by granitic flame structures; individual sheets may be disrupted by boudinage-like necking. The pillow-like structures are ellipsoidal and vary from ~0.2 to ~5 meters in maximum dimension. Host material beneath sheets and pillows is typically a closely packed, feldspar-rich cumulate, but fine-grained granite zones a few mm thick are commonly present adjacent to the mafic rocks. The pillows appear to be the end result of disaggregation of coherent sheets. Our favored interpretation is that heat introduced by the mafic injections weaken the underlying crystal-rich granitic mush (melt fraction increases), initiating both upward injection of flame-like structures into the sheet and collapse of the dense sheets into the underlying mush.