PETROLOGY AND GEOCHEMISTRY OF THE MELROSE STOCK, EAST CENTRAL NEVADA

The Melrose stock in the Dolly Varden Mountains in East Central Nevada is one of many Mesozoic intrusions in the Basin and Range. This 12.5 square mile large stock consists of syenodiorites, syenites, monzonites, quartz monzonites, and granodiorites sharply intruding Mississippian to Permian units. Phenocrysts of Plagioclase (An₃₈ – An₂₄) with oscillatory zoning and albitic rims, hornblende, biotite, ± diopside ± orthoclase are common, + accessory magnetite, apatite, titanite, ilmenite, and allanite. Mineral compositions suggest that the intrusion was emplaced at ~700°C and 1.8 – 2.3 kbar.

All rocks are metaluminous to slightly peraluminous defining a calcalkalic trend in which the monzonites and syenites are shoshonitic. Rare earth element patterns indicate that all rock types are comagmatic. Harker plots show curvilinear trends with kinks consistent with plagioclase fractionation. Major element modeling and petrographic evidence suggest three stages of fractionation/ mixing: stage 1 marked by the fractionation of 12% diopside, 11% plagioclase; stage 2 fractionation of 15- 20% plagioclase, 20-28% hornblende, ± orthoclase ± biotite, accompanied by mixing through convection; stage 3 the fractionation of 0.25 % biotite, 6% hornblende, 9% plagioclase and 2% orthoclase.

Mineralogic, petrographic, and major and trace element data show that all rocks are I-type granitoids, indicating a significant mantle contribution. Spider diagrams show troughs for Ti, P, and Nb, indicating magma genesis in a subduction zone setting. Discrimination diagrams classify all rocks as late orogenic within plate granitoids. Magma was therefore generated from mantle metasomatized by subduction, and emplaced in the continental crust late in the orogenic cycle as a monzonitic magma that differentiated to quartz monzonites and granodiorites.