Composite Devonian island-arc batholith in the northern Sierra Nevada, California

Abstract

The Bowman Lake batholith intrudes the pre-Upper Devonian Shoo Fly Complex in the northern Sierra Nevada. The eastern margin of the batholith extends to within 1.5 km of the base of a thick sequence of Paleozoic island-arc rocks that rests unconformably on the Shoo Fly Complex. Hypabyssal silicic intrusions associated with the batholith penetrate the Upper Devonian Sierra Buttes Formation, the lowest volcanic unit of the arc sequence, but do not extend to higher stratigraphic levels. The intrusions in the Sierra Buttes Formation possess marginal peperites and show other evidence of their injection into wet, unconsolidated sediments. On the basis of these field relations, the batholith is interpreted to represent a subvolcanic magma chamber emplaced concurrently with deposition of Sierra Buttes arc rocks. The batholith is composite and consists of trondhjemite, granodiorite, biotite granite, and hornblende tonalite, formed from discrete batches of magma injected into a common plutonic chamber. Mingling of tonalitic and trondhjemitic magmas produced abundant rounded inclusions of tonalite dispersed within trondhjemite and led to hybridization of the two magmas by fine-scale intermixing. Zircons from the trondhjemitic, granodioritic, and granitic phases of the batholith and an associated Sierra Buttes rhyolite sill show complex U-Pb isotopic systematics. A multistage history is suggested, involving the incorporation of earliest Paleozoic and/or Proterozoic zircons during magma generation or ascent in Devonian time and one or more stages of disturbance in Mesozoic-Cenozoic(?) time. Consideration of these different discordance mechanisms results in a model U-Pb igneous age for the batholith of 364 to 385 Ma (Middle to Late Devonian); on the basis of the field relations, a Late Devonian age for the batholith is adopted. Zircon characteristics and isotopic systematics suggest that petrogenesis of the granite involved mixing of trondhjemitic and/or granodioritic magmas with partial melt from a separate, large-ion-lithophile-enriched source.