Petrologic and Geochemical Characterization of Cretaceous Plutons in the Pine Forest Range, Northwest Nevada: Regional Implications for Granitoid Magmatism

Document Type

Conference Proceeding

Publication Date

10-14-2026

Abstract

Northwestern Nevada preserves an extensive belt of Cretaceous plutons that span a critical, yet poorly studied ~400 km gap between the Sierra Nevada and Idaho batholiths. Emplacement history, magma sources, and tectonomagmatic evolution of these Nevada plutons are poorly constrained, limiting our understanding of along-strike variations in magmatism between arc segments with contrasting ages, compositions, and lithospheric architectures. This study presents an initial petrologic and geochemical investigation of Cretaceous granitoids emplaced within the Black Rock arc terrane in the Pine Forest Range (PFR) of northwestern Nevada.

The northern PFR contains the zoned Mahogany Mountain plutonic complex (MMPC; ca. 115-104 Ma; U-Pb zircon). The outer MMPC contains the older (ca. 115-110 Ma), more mafic Black Mountain, Cold Spring, and Lone Mountain phases, which display well-developed mylonitic fabrics recording dip-slip syn-plutonic deformation. These intrusions comprise a heterogeneous assemblage of hornblende gabbros, quartz diorites, quartz monozodiorites, and monzonites, with minor leucogranite. The internal Thacker Canyon and Rattlesnake Spring phases are younger (ca. 108-106 Ma) and more homogeneous biotite hornblende granodiorites to quartz monzodiorites. The MMPC core is occupied by the youngest (ca. 105 Ma) Mahogany Mountain phase, a leucocratic two-mica granite with minor garnet and orthoclase megacrysts. The central PFR, in contrast, is underlain by the voluminous Duffer Peak pluton (DPP; ca. 104 Ma), which consists of a uniformly coarse-grained, leucocratic biotite granodiorite to granite with locally abundant orthoclase megacrysts.

Bulk rock geochemistry reveals that PFR intrusions are predominantly magnesian, calc-alkalic granitoids with arc signatures defined by LILE enrichments and HFSE depletions. Within the MMPC, compositions evolve from more mafic and metaluminous granitoids in the outer zones to a felsic, peraluminous granitoid within the core. Although each phase exhibits compositional heterogeneity, the MMPC intrusions consistently contain high Sr (389-856 ppm) and Ba (487-1998 ppm), low Rb/Sr ratios (0.05- 0.28), elevated Sr/Y ratios (~20-160), and steep REE patterns (LaN/YbN = 8-30) with pronounced HREE depletions and minor positive to negative Eu/Eu* anomalies. The DPP shares similar geochemical characteristics to the Mahogany Mountain phase, but lacks the zoning of the MMPC. These geochemical patterns are consistent with a petrogenetic model involving partial melting of deep crustal sources in equilibrium with feldspar-poor, garnet-, clinopyroxene-, and amphibole-rich residues; however, this preliminary interpretation is being evaluated alongside alternative models.

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