LATE CRETACEOUS TO PALEOGENE GRANITIC MAGMATISM, LAMOILLE CANYON AREA, RUBY MOUNTAINS METAMORPHIC CORE COMPLEX, NE NEVADA

In Lamoille Canyon, four generations of Cretaceous to Paleogene granitic magmatism are exposed in the infrastructure of the Ruby Mountains core complex. The oldest (ca. 92–90 Ma) consists of scant, equigranular, biotite monzogranite gneiss. The second generation (ca. 85–69 Ma; most 75.6–69.1 Ma) was the most voluminous, constituting a batholith-scale intrusive complex. This second unit is typically coarse-grained/pegmatitic biotite ± muscovite monzo- and syenogranite gneiss, although grain size is widely variable. These Cretaceous gneisses are related to crustal anatexis following tectonic thickening, with garnet residual in the source (Lee et al., 2003, J. Petrology).

Paleogene magmatism began at ca. 60 Ma, with emplacement of minor monzogranite petrographically similar to second-generation rocks. The major Paleogene pulse of granitic magma spanned 39.9–32.1 Ma and consists of two monzogranite types, Snow Lake Peak (SLP) and Overlook, based on field, zircon, and geochemical features.

SLP intrusions form dike swarms and km-scale sheet-like bodies. U-Pb (zircon) ages range from 39.9–33.1 Ma, typically displaying minor Proterozoic inheritance. SLP monzogranites are distinct in terms of their high Zr and Nb, relatively low Sr, and the lowest eHf (zircon) and eNd (whole rock) of any granites in the area. Overlook-type intrusions (36.0–32.4 Ma) occur as leucosome-like bodies in Late Cretaceous granitic gneiss and as thin, isolated dikes unrelated to SLP intrusions. Overlook types display complex zircon inheritance, including Late Cretaceous grains, and have eHf (zircon) and eNd (whole rock) identical to the Late Cretaceous granitic gneiss. The Paleogene granites attest to trans-crustal magmatism, driven by heat from basaltic magmas, with partial melting of old (Neoarchean–Paleoproterozoic) lower crust (SLP granites) and lower to mid-crustal Late Cretaceous granitic gneiss (Overlook granites). Addition of mafic magma in the lower crust, transfer of granitic magmas into the middle crust, and consequent partial melting in the middle crust predisposed the crust for plastic extensional flow, a potential contributor to extensional collapse of the Nevadaplano.