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The 160-Ma Grayback pluton provides ample evidence for the role of mantle-derived magmas in the crystallization history of the magma chamber. Mafic magmas tapped several distinct mantle sources, and evidence for magma mixing is manifest in (1) widespread mafic enclave swarms in which basaltic magma mingled with basaltic to andesitic magma, (2) late-stage granodioritic dikes containing pillow-shaped enclaves of basalt and hybrid andesite, and (3) late-stage tonalitic dikes including microdioritic enclaves contained within a dark-colored hybrid matrix. These magma-mingling zones reflect large-scale magma mixing processes in the Grayback system. Much of the compositional variation among Grayback main-stage samples was apparently produced by magma mixing between mafic and intermediate magmas similar to those exposed in some of the mafic enclave swarms. Mixing between basaltic and granitic end-members at deeper levels in the Grayback magma chamber produced hybrid an-desite. Enclaves of basalt and hybrid andesite are observed in late-stage granodioritic dikes that cut the mafic enclave swarms. Mingling between partially crystalline basaltic and tonalitic mushes, resulting in a mega- and microscopically complex hybrid, is observed in the late-stage tonalitic dikes. Few Grayback main-stage samples were apparently produced in this manner.

The Grayback pluton is one of several similar-aged plutons in the Wooley Creek suite, in which mafic magma played a prominent role in petrogenesis. The Grayback system is unusual in that mafic magmas from several obviously different sources were present throughout its crystallization history. This circumstance probably resulted from its position in an extensional, back-arc setting.

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