Paper No. 2-9
Presentation Time: 11:15 AM
CRETACEOUS BATHOLITHS OF THE PINE FOREST RANGE, NORTHWEST NEVADA: PETROLOGY, GEOCHRONOLOGY, AND REGIONAL RELATIONS
Batholith-scale mid-Cretaceous granitoid plutons are a hallmark feature of NW Nevada geology (e.g., Smith et al., 1971; Van Buer and Miller, 2010), occupying major areas of outcrop from the Sahwave, Nightingale, Selenite and Granite Ranges east and north of Pyramid Lake to the Pine Forest Range (PFR) and Bilk Creek Mountains at the Oregon border. These plutons cluster near the MSNI dextral strike-slip boundary (Wyld and Wright, 2001, 2005), along which allochthonous terranes to the west were displaced northward ca. 400-500 km in the early Cretaceous. Van Buer and Miller (2010) describe plutons from the southern part of this belt, which are mostly granodioritic and range in age from 110-88 Ma. We here describe batholith-scale Cretaceous plutons of the PFR at the northern end of this belt. The Duffer Peak pluton (DPP), which underlies an area of >190 km2 in the central PFR, over a vertical distance of 3500 m. This pluton is uniformly coarse-grained, leucocratic biotite granodiorite, with k-spar megacrysts. High resolution U-Pb zircon geochronology (same for all ages presented) indicate intrusion at 104 Ma. The northern PFR is underlain by a prominently zoned Cretaceous intrusion (here named the Mahogany Mountain plutonic complex or MMPC). Older, and more mafic external phases occur only along the NW side of the MMPC, close to the MSNI fault. These consist of a complex assemblage of hornblende gabbro, quartz diorite, quartz monzodiorite, monzonite, and locally abundant leucogranite, with ages from 115-110 Ma. Mylonitic fabrics are common within these rocks and reflect re-activation of the MSNI fault during younger syn-plutonic contraction. The interior of the MMPC consists of an outer rim of granodiorite to quartz monzodiorite, dated at 108-106 Ma, and a core of very coarse grained two-mica granite with k-spar megacrysts, dated at 105 Ma. Total area underlain by exposed MMPC is ≥300 km2, and probably larger as similar intrusive rocks are found in the Bilk Creek Mountains. Vertical exposure is ≥2200 m. The PFR Cretaceous plutons form an important link between those of the Sierra Nevada, Sahwave area, and western Idaho, although there are some variations in ages. We infer that the crustal-scale MSNI boundary formed an important pathway for mid-Cretaceous intrusive influx, at least in Nevada.