The Smartville intrusive complex, Sierra Nevada, California: The core of a rifted volcanic arc

Abstract

The Smartville complex is a Jurassic volcanic and plutonic arc in the northwestern Sierra Nevada that was deformed during the Late Jurassic Nevadan “orogeny.” We interpret the Smartville intrusive complex to have formed during the incipient rifting of an active volcanic arc. This interpretation is supported by the close relationship between volcanism and plutonism and by the close association between sheeted dikes and the younger plutonic rocks. Further support is found in the similarities that exist between the Smartville complex and modern arcs that developed on oceanic crust, either at a continental margin or in the ocean basins. Smartville volcanic rocks consist of a lower unit of tholeiitic submarine flows and pillowed flows that grades upward into an upper unit of calc-alkaline pyroclastic and volcaniclastic deposits. Intrusive rocks include older units of metamorphosed gabbro and massive diabase, which are intruded by a unit of 100% mafic and felsic, sheeted and unsheeted dikes. Biotite-hornblende tonalite and granophyric hornblende tonalite plutons are coeval with the dike complex, and both rock types occur locally as dikes within the dike unit. Continuously and reversely zoned gabbro-diorite plutons are also coeval with the dike complex. Granodiorite plutons are the youngest intrusive unit and may be related to the Sierra Nevada batholith. Relative age relations suggest that the Smartville complex formed in a single volcanic-plutonic system of pre-Nevadan age. The dike complex intrudes and is intruded by both the tonalite and zoned gabbro-diorite plutons. Both the upper and lower volcanic units are intruded by all of the plutonic and hypabyssal units and were deformed prior to the intrusion of the sheeted dike complex. Clasts of plutonic and hypabyssal rocks resembling the younger intrusives, however, occur locally in some of the youngest volcaniclastic rocks, suggesting that shallow plutonism and volcanism could be broadly coeval. Early Nevadan thrust faults juxtapose volcanic and older plutonic rocks of the Smartville complex and Mesozoic(?) chert-argillite broken formation to the east. The younger plutons and the dike complex in the Smartville are deformed by steep, late Nevadan faults and do not intrude chert-argillite broken formation. The youngest granodiorite plutons in the area truncate Nevadan faults, intrude the chert-argillite formation, and appear to be unrelated to the Smartville complex. The Smartville volcanic arc underwent pre-Nevadan intra-arc extension. The sheeted dike complex is the primary manifestation of the rifting event. The elongate shapes of the younger plutons, which are coeval with the dike complex, reflect extensional control on their emplacement. Volcanic rocks in the western and southern Smartville complex were deformed prior to the intrusion of the dike complex.