Abstract

In this paper we interpret new and previously published U-Pb zircon data in light of structures mapped within ∼360 km2 of the southwestern Sierra Nevada batholith. Traces of intrusive contacts and igneous foliations reveal the presence of two ring dike complexes: the eastern ring complex and the western ring(?) complex. These subvolcanic complexes formed during overlapping periods: the eastern ring complex between 123 and 117 Ma (n = 5) and the western ring complex between 120 and 115 Ma (n = 5). Each complex may have been emplaced during a minimum of two events, each 2 to 3 m.y. long and separated by 3 to 4 m.y. In the western ring complex, the presence of a 120 Ma xenolith of silicified porphyry enclosed by unaltered tonalite implies that the ring complexes intruded the shallow crust and stoped the overlying volcanic-hypabyssal constructs. Rare mafic mylonites suggest that collapse of a mafic-ultramafic mass may have assisted western ring complex emplacement.

Near-synchronous emplacement of the two ring complexes is consistent with textures and structures indicative of intense magma mingling and synmagmatic deformation preserved in the north-northeast–trending Stone Corral shear zone. The ≥ 13-km-long Stone Corral shear zone separates the impinging magmatic centers and was active ca. 116 ± 2 Ma. Blocks of layered plagioclase-olivine-orthopyroxene cumulates, 0.2–5.5 km long, also record variable synmagmatic deformation ca. 123 ± 3 Ma, thus indicating that the cumulates crystallized in a shallow, dynamic environment immediately preceding or during the earliest stages of ring dike emplacement.

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