Decompression melting of peridotitic mantle is unlikely to produce the large volumes of magma erupted during the early stages of extension in continental extensional provinces such as the Great Basin of western North America. First, a substantial amount of extension is required before asthenospheric mantle ascends to depths shallow enough to begin melting, even if it is unusually hot. The lithospheric mantle is too cold to undergo melting, even after large amounts of extension. Second, decompression melting of peridotite produces progressively greater amounts of magma during extension; in the Great Basin silicic magmatism was voluminous during the early stages of extension but diminished in volume as extension progressed. Comparison of empirical melting relations with modeled pressure and temperature conditions in the lithosphere during extension indicates that large volumes of magma may be produced by partial melting of melt metasomatized subcontinental mantle (SCM) during the earliest stages of extension.

Melt-metasomatized SCM is expected to contain components less mafic than peridotite, emplaced during older magmatic episodes. Mafic components within the SCM will begin to melt during the earliest stages of extension if the base of the lithosphere is ~1300 ¿C or warmer and at depths greater than 75 km. Interaction of these melts with crustal rocks, fractionation processes, and crustal anatexis driven by the heat contained in the ascending mantle melts can produce the silicic to intermediate compositions observed in Great Basin Middle Tertiary magmas. A suite of models comparing different parameters show that the potential for the lower portions of the SCM to produce melt during the early stages of extension depends most strongly on the pre-extension thickness of the lithosphere. If the lithosphere is thicker than about 150 km, the base of the SCM will lie well within the subsolidus field for mafic rocks and a substantial amount of extension is required before it begins to melt. At pressures less than about 2.0 GPa, ascent paths in the SCM are nearly parallel to the basalt solidus, and so little melting occurs if the lithosphere is thinner than about 60 km. Optimal conditions for producing large volumes of melt during the early stages of extension require an initial lithosphere ~100--150 km thick. Under these conditions the lower 25 km of the SCM may undergo partial melting during extension.

Predicted magma flux rates match the space-time pattern of Middle Tertiary silicic magmatism in the Great Basin; the model predicts a high rate of melt production at the onset of extension which peaks within the first 10--20 m.y. and then decreases rapidly with continuing extension. Melt production within the SCM is consistent with trace element and isotope characteristics of mid-Miocene and younger basalts in the Great Basin, which suggest a transition at 5 Ma from old, geochemically evolved magma sources within the lithosphere to later asthenospheric sources. The later asthenospheric melts are compatible with decompression melting of peridotitic mantle after ≥50% extension. ¿ American Geophysical Union 1995