PREDICTING PARTIAL-MELT SOURCES AND COMPOSITIONS IN THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE THROUGH BULK-ROCK COMPOSITIONAL MODELLING

Voluminous granites are pervasive in the Ruby Mountains (RM) – East Humboldt Range (EHR), Nevada. The granites are the product of partial-melting of metasedimentary crustal sources and have crystallization ages between 80 and 30 Ma (Romanoski, 2012). In the EHR voluminous biotite monzogranites were attributed to partial melting of Neoproterozoic McCoy Creek Group rocks (Sicard, 2012). A similar source was proposed for the emplacement of Cretaceous sheets, dikes and pods of leucocratic granite, particularly in Lamoille Canyon (RM).

Assemblages for a range of effective bulk compositions were calculated to determine the P-T limits of partial melting. Results were compatible with crustal anatexis perhaps during mylonitization of the mid-crust during Tertiary extension after Nevadaplano collapse, and prograde dehydration during crustal over-thickening in the Sevier hinterland. Melt compositions were calculated and compared to granite compositions.

Modelling of McCoy Creek compositions from the south EHR suggest partial melting occurred at 6.5-7 kb and ~700 ºC producing highly leucocratic compositions. These reactions are exposed as stromatic migmatites, and modeled compositions are comparable to sampled late Cretaceous peraluminous melts. The occurrence of sheet-like leucogranite melts in the south EHR suggest Sevier crustal thickening as a mechanism to produce the small volume, but wide-spread low-temperature granites of late-Cretaceous age.

TWQ-estimated metamorphic conditions of 7.2 kb and 735ºC for samples at Clover Hill near the north EHR are above the modelled partial melting curve (Sicard, 2012). Modeling suggests P-T conditions in the upper amphibolite facies zone, again compatible with Sevier Orogenesis and crustal thickening.

Partial melting of metapelites richer in Fe, Mg and K may have produced a peraluminous melt with biotite, comparable to the biotite monzogranites. In contrast, McCoy Creek compositions with lower Fe, Mg and K abundance may have resulted in final produced the sheets and dikes of leucocratic granite. Compositional variability in the McCoy Creek Group protolith may explain how a single unit could produce the observed diversity of magmatic rocks.

Romanoski, A., 2012, MS thesis, Texas Tech University

Sicard, K., 2012, MS Thesis, University of Wyoming