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Description
The Sierra City mélange forms the structurally highest unit within the post-Cambrian and pre-Upper Devonian Shoo Fly Complex. Chemical analyses of inclusions within the mélange indicate a period of magmatism that predates the well documented Late Devonian period of island arc magmatism known to have affected the northern Sierra terrane. Rounded basaltic inclusions within the mélange have chemistries that are like modern day intraplate ocean island alkali basalt, whereas feldspathic sandstone inclusions have modal mineralogies and chemistries that are consistent with derivation from a dissected andesitic magmatic arc. In contrast, the Upper Devonian Bowman Lake batholith and an associated sill intrude the Shoo Fly Complex and exhibit chemical characteristics suggestive of a mixed andesitic to dacitic/rhyodacitic parentage, whereas submarine tuffs within the Upper Devonian Sierra Buttes Formation have chemistries indicative of andesitic volcanism. Data derived during this study are consistent with the fringing island arc model popularized in the 1960's by B. C. Burchfiel and G. A. Davis. In the fringing island arc model, early Paleozoic subduction occurred in association with a west-facing island arc system. During the Late Devonian, arc polarity reversal occurred, and westward dipping subduction beneath the eastern edge of the fringing arc commenced. Eastward migration of the trench hingeline culminated with the emplacement of the Roberts Mountain allochthon onto the outer continental shelf of western North America during the Antler orogeny. With this tectonic framework as background I interpret the alkali basalt inclusions as the remnants of a seamount or ocean island that was subducted during the period of west-facing arc magmatism. In contrast, feldspathic sandstone inclusions are interpreted to have been derived from uplifted and dissected segments of the fringing west-facing island arc, whereas Late Devonian components of island arc magmatism developed during the switch to westward dipping subduction. Thus, the northern Sierra terrane contains the record of both pre- and Late Devonian magmatic activity.
