200 M.Y. RECORD OF MAGMATISM, DEFORMATION, AND ACCRETION IN THE YREKA-TRINITY COMPOSITE TERRANE: 580 MA-380 MA

Yreka (Yt), Trinity (Tt), and YT composite terrane (YTt) history begins with the Neoproterozoic Antelope Mountain Quartzite (AMQ). Derived from a non-Laurentian continent and containing Ediacaran cyclomedusoids (580-550 Ma), AMQ was deposited on a shallow marine continental shelf after the breakup of Rodinia. At the same time, plagiogranites and gabbros of the Neoproterozoic Trinity ophiolite (NTo) crystallized in a nearby ocean basin (579-556 Ma) at a paleolatitude of 7°N or 7°S.

During Cambrian-Ordovician, two blocks within the Tt were ductilely deformed: the Trinity peridotite (Tp), and the NTo whose plagiogranite, gabbro, and harzburgite were ductilely stretched and dismembered. Collision of these two oceanic blocks along ductile shear zones ended with uplift and partial melting of Tp, resulting in intrusion of post-tectonic Early to early Middle Ordovician plutons (480-470 Ma) into the juxtaposed NTo-Th blocks. In the Yt, Middle Ordovician Skookum Gulch mélange (SGm) protolith formed (~467 Ma), undergoing Late Ordovician blueschist metamorphism (~447 Ma), possibly associated with metamorphism and deformation of Schulmeyer Gulch Complex mélange. This Ordovician “Callahan event” is the earliest confirmed collision/accretion within the Yt and Tt.

In the Yt, Silurian trench turbidites were deposited (Moffett-Duzel-Sissel assemblage: detrital zircons ~440-410 Ma), and the Siluro-Devonian Gazelle trench-slope basin (Gtb: detrital zircons ~430-400) formed over active accretionary wedge of the Lower Devonian Gregg Ranch Complex (GRC). Related Tt magmatism produced the Siluro-Devonian Trinity SSZ ophiolite (435-412 Ma). These Siluro-Devonian events record an active convergent margin that terminated with Middle Devonian collision (407-392 Ma) of Forest Mountain terrane (FMt) oceanic lithosphere. FMt accretion stacked thrust sheets of the Yt over the Tt to form the YTt. The Central Metamorphic terrane accreted outboard of the FMt-YTt in Late Devonian (380 Ma).

Lower Devonian mafic volcanics are interbedded with basal Gtb and overlie eroded Tt; feeder dikes and a related stock (400 Ma) cut GRC, implying near-trench volcanism and passage of a RTT or RTF triple junction. Post-tectonic Middle Devonian (~398-385) pillow lavas were erupted through YTt (paleolatitude ~31ºN or 31ºS) and correlate with the Middle Devonian basal Redding terrane.