John Muir Intrusive Suite
John Muir Intrusive Suite
Late Cretaceous, ~76-106 Ma
Occurs along the eastern crest of the central Sierra Nevada and in scattered plutons extending west near the San Joaquin River. One of the Sierra Crest intrusive suites (e.g., Lackey and others, 2008; Hildebrand and Whalen, 2021). Chiefly granodiorite, granite, and alaskite. Major plutons: Lamarck Granodiorite, Lake Edison Granodiorite, granite of Rock Creek Lake, Round Valley Peak Granodiorite, Mono Creek Granite, Evolution Basin Alaskite, granodiorite of Lone Pine Creek, and Alabama Hills Granite. Unit also includes small intrusions west of the Mount Givens Granodiorite, including the leucogranites of Big Sandy Bluff, Lion Point, and Bald Mountain, and granodiorites of Red Lake, Eagle Peak, and Big Creek, as well as to the southeast the Dragon Pluton, Bullfrog Pluton, Independence Pluton, and granodiorite of Lone Pine Creek. Bateman (1992) states that the optimum ages for this unit range from 88-90 Ma, but zircon (and one titanite) U-Pb ages reported from this unit range from ~76-106 Ma (Stern and others, 1981; Chen and Moore, 1982; Saleeby and others, 1990; Tobisch and others, 1995; Lackey and others, 2008; Davis, 2010; Frazer and others, 2014, 2022). In addition, this unit has yielded ages as follows: whole rock Rb-Sr ~75-87 Ma, biotite K-Ar and Ar-Ar ~75-104 Ma, hornblende K-Ar and Ar-Ar ~77-98 Ma (excluding two ages of ~150-151 Ma that are clearly too old and probably represent wall rock inliers or contamination), K-feldspar Ar-Ar ~76-78 Ma, apatite fission track ~54-93 Ma, sphene fission track ~71-100 Ma, epidote fission track ~84 Ma, and allanite fission track ~86 Ma (Kistler and others, 1965; Naesser and Dodge, 1969; Evernden and Kistler, 1970; Tobisch and others, 1995; Davis, 2010). The unit as mapped here also includes very small slivers of Jurassic plutonic rock (zircon U-Pb ages of 149-181 Ma, Chen and Moore, 1982; Davis, 2010; Attia and others, 2022), which are too small to show at map scale.