Mafic enclaves in the Wilson Ridge Pluton, northwestern Arizona: Implications for the generation of a calc‐alkaline intermediate pluton in an extensional environment

Abstract

The Wilson Ridge pluton is an epizonal calc‐alkaline pluton that formed about 13.5 Ma during a period of mid‐Miocene extension. Faulting and erosional dissection provide a cross section of the pluton. The apex of the pluton, in the Boulder Wash area, Nevada, is composed of hypabyssal quartz monzonite and dacite. The base of the pluton is 20 km to the south where quartz monzodiorite, monzodiorite, and diorite are in low‐angle intrusive contact with Precambrian basement. The pluton was separated from cogenetic volcanic rocks in the River Mountains by movement along the Saddle Island detachment fault at about 13.4 Ma. The River Mountains now lie 20 km to the west of the pluton. The Wilson Ridge pluton is composed of the Teakettle Pass suite consisting of foliated monzodiorite and quartz monzodiorite and unfoliated quartz monzonite and the older Horsethief Canyon diorite. Rocks of the pluton contain 2–4 modal percent sphene. Intermediate rocks of the Teakettle Pass suite contain abundant basaltic and diorite enclaves. Basaltic enclaves are lensoidal and pillow‐like and commonly have crenulate and fine‐grained margins. Enclaves are chemically similar to mafic dikes of the Wilson Ridge pluton and to cogenetic alkali basalt flows in the River Mountains. They probably represent blobs of mafic liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton. Basaltic enclaves commonly occur as inclusion‐rich zones that represent synplutonic mafic dikes that were injected into a quartz monzonite host. Mafic magma was entrained and mechanically broken down by magmatic flow shear. A continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin, tabular mafic selvages and schlieren and ultimately to the mafic component in foliated quartz monzodiorite and monzodiorite. Diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths. Field evidence and major and trace element models suggest that the intermediate rocks of the pluton were produced by the commingling of a large volume of mafic magma with a smaller volume of felsic magma (a mafic‐felsic ratio of about 70∶30) as well as fractional crystallization. Similar open system processes may be responsible for the production of calc‐alkaline intermediate rocks in other parts of the Great Basin.