NEW U/PB AGE DATA FROM THE HUMBUG INTRUSIVE COMPLEX OF THE SODA RAVINE BLOCK, NORTHWESTERN SIERRA NEVADA: POTENTIAL TECTONIC IMPLICATIONS

The Soda Ravine block (SRB) is an enigmatic, fault-bound sliver of poorly dated mélange containing McCloud Zone A carbonate blocks and Ladinian or Carnian bioclastic limestone within sheared argillite. Work by others (1980’s) suggested the SRB contains remnants of an east-facing section including strata equivalent to the Baird Fm, McCloud and Hosselkus limestones (olistoliths), Pit Fm, and Brock Shale of the Redding subterrane (Klamath Mtns). The SRB is juxtaposed on the east against Norian through Oxfordian marine strata of the Butt Valley block as well as lower Paleozoic metaclastic rock of the Shoo Fly complex of the Northern Sierra terrane. To the west, the SRB is faulted against the Paleozoic Feather River peridotite.

The SRB mélange is intruded by dikes and one stock of undated felsic or silicic igneous rock. The Humbug intrusive complex (HIC) forms the largest of these exposures. The 3.2 by 1.5 km hypabyssal HIC is holocrystalline at its base and porphyritic (with plagioclase +/- cpx and hornblende) at its top. In contrast to the surrounding mélange, igneous rock of the HIC lacks cleavage suggesting the complex postdates mélange formation. Zircon sampling from the upper HIC supports that interpretation. Twenty-one grains yielded U/Pb ages spanning the late Aalenian through late Pliensbachian with a weighted mean average of 178.0 +/- 1.1 Ma (2σ).

The presence of the mid-Toarcian HIC within the SRB mélange has several implications. First, the lack of similar volcanic rock in the adjacent Butt Valley block suggests the fault separating these blocks must have regional significance. This fault may extend southward to the I-80 corridor, separating smaller fault-bound slivers of Permo-Triassic rock from the Northern Sierra terrane. Displacement on this fault must be Late Jurassic (or younger) based on the youngest strata in the Butt Valley block (late Oxfordian clastics). Secondly, the presence of dikes identical to the HIC within the Feather River peridotite to the west suggests the SRB and peridotite were structurally juxtaposed prior to the Middle Jurassic. Lastly, while some components of the SRB mélange may resemble Permo-Triassic units of the Redding subterrane, mid-Toarcian volcanic rock similar to the HIC has not been identified within that subterrane.