Sierra Buttes intrusive complex
Sierra Buttes intrusive complex
earliest Mississippian and Late to latest Middle Devonian, ~352-385 Ma
In central and northeastern part of complex, chiefly granite, granodiorite, trondhjemite. Unit also includes rhyolite and andesite dikes. Major units are the Bowman Lake batholith, Wolf Creek Stock, and coeval hypabyssal intrusions. Farther south, felsic orthogneiss and local metagabbro are inferred to be a continuation of the unit.
The Bowman Lake batholith is a composite batholith composed chiefly of trondhjemite, granodiorite, biotite granite, and hornblende tonalite, formed from discrete batches of magma injected into a common plutonic chamber (Hanson and others, 1988). The trondhjemitic rocks form inclusion-rich and inclusion-poor facies. The inclusion-rich facies contains abundant ovoid inclusions of biotite-hornblende tonalite and less common hornblende gabbro, less than 4 m diameter within a trondhjemite and trondhjemitic tonalite host containing biotite and hornblende in smaller amounts. Inclusions in the inclusion-poor facies are petrographically identical to those in the inclusion-rich facies but are smaller and present in much lower amounts. The inclusions in many cases exhibit fluidal cuspate margins against the more felsic host rocks and in some cases also show chilled margins, supporting the interpretation that the enclaves represent blobs of gabbroic to tonalitic magma that mingled with cooler, more felsic magma during emplacement of these parts of the batholith. The Bowman Lake batholith has zircon U-Pb ages of ~353-409 Ma (Girty and others, 1984; Hanson and others, 1988; Rotberg, 2008; Powerman and others, 2020). Both Girty and others (1984) and Hanson and others (1988) analyzed multigrain zircon fractions, which can yield unreliable results. The result from Girty and others (1984) is now inferred to reflect inheritance. The results from Hanson and others (1988) suggest a modelled age range of ~364-385 Ma, but they note that the data record multiple mechanisms of discordance. More recent U-Pb zircon SHRIMP analyses by Powerman and others (2020) yielded ages of ~353-371 Ma, with a weighted mean age for the batholith of ~361 Ma. The smaller Wolf Creek stock, in the northernmost part of this unit, is biotite granite. Saleeby and others (1987) obtained an age of ~378 Ma for this pluton, using multigrain zircon analyses. Powerman and others (2020) reported an age of ~352 Ma (Early Mississippian), which is considered a more reliable result. 
The more southerly bodies in this unit include the Mokelumne Peak roof pendant of McKee and Howe (1981) at the Mokelumne River east of Salt Springs Reservoir and scattered small bodies around the Stanislaus River northeast of the Calaveras-Shoo Fly thrust. They are predominantly granitic gneiss, with lesser amounts of syenitic gneiss. The latter rock type has yielded a preliminary whole rock Rb-Sr isochron age of ~363 Ma (Merguerian and Schweickert, 1987) and a zircon U-Pb age of ~275 Ma from a granitic body (Sharp and others, 1982), which clearly falls well outside the time frame of this unit. An additional age from a blastomylonitic granite was originally reported with an anomalously low zircon U-Pb age of ~216 Ma (Saleeby and Sharp <em>in Bedrossian and Saucedo, 1981) but later reported as ~370 Ma (Merguerian and Schweickert, 1987). This U-Pb age is based on multigrain zircon analyses, so should be regarded as preliminary. The Mokelumne Peak roof pendant has yielded biotite and hornblende K-Ar ages of ~87-98 Ma, the same as those from the surrounding Cretaceous plutonic rocks (McKee and Howe, 1981).
The Devonian part of the intrusive complex is intrusively linked to the coeval Sierra Buttes Formation, which consists of arc-derived sedimentary and volcanic rocks of Taylorsville sequence (in unit NSSB) in the Northern Sierra terrane (Hanson and others, 1988). Together, the plutonic and volcanic rocks comprise the Sierra Buttes arc. Younger U-Pb zircon ages of ~275 Ma represent later plutonism, though still prior to the several Permian(?) to Middle Jurassic phases of regional metamorphic overprint (Merguerian and Schweickert, 1987). 
Unit regionally deformed with an older generation of E to NE-appressed folds and parallel schistosity, and a younger generation of NW-trending appressed folds and parallel schistosity. The younger deformation is equivalent to the Middle Jurassic deformation of the Tuttle Lake Formation (unit NSTT) during emplacement of the Emigrant Gap and related plutons (unit Gegg) as described above (Hanson and others, 1996). Plutons crosscut older schistosity in Shoo Fly Complex marine sedimentary rocks (unit NSSF).