PETROGENESIS OF TERTIARY INTRUSIONS IN LAMOILLE CANYON AND IMPLICATIONS FOR FORMATION OF RUBY MOUNTAINS CORE COMPLEX, NEVADA

Granitic magmatism in Lamoille Canyon preserves a record of crustal evolution spanning Late Cretaceous to Oligocene times. The Neoproterozoic–Paleozoic metasedimentary rocks are intruded by various Mesozoic and Tertiary granitic rocks; two-mica granitic gneisses (~80 Ma), gneiss of Thorpe Creek (36 – 39 Ma), biotite monozogranite suite (~29 Ma) and gneiss of Seitz Canyon of unknown age.

The gneiss of Thorpe Creek is geochemically similar to Late Cretaceous two-mica gneisses that were interpreted as deep-seated crustal melts in the thickened crust. Depleted HREE and Y in samples of Thorpe Creek gneiss suggest that melting occurred in a thick crust where garnet was stable in residue. The Oligocene intrusions are distinct geochemically from the older gneissic units and are characterized by; (1) high Rb/Sr ratio, (2) lack of HREE depletion, (3) enriched HFS elements. The observed geochemical characteristics are best explained by interaction of mantle-derived basaltic magma with crust at a depth in which garnet was not stable.

Combined with previous thermobarometric studies, the observed temporal variations in geochemistry suggest moderate decompression from Late Cretaceous to Oligocene. Intrusion of ~29 Ma monzogranite suite is coincident with the beginning of accelerated exhumation and mylonitization. Large-scale basaltic magma-crust interaction during Oligocene time produced “hot” hybrid silicic melts with various mixed isotope signatures of mantle and crust. Intrusion of the hybrid melts in the middle crust was an efficient way to provide heat and material to higher crustal levels and to trigger rapid deformation. The study suggests that magmatism played major role in metamorphic core complex formation rather than being a passive response during extension