CONSTRAINTS ON THE ORIGIN OF SUBDUCTION-RELATED IGNIMBRITE FLAREUPS FROM SOURCE VOLUME CALCULATIONS: THE SOUTHERN GREAT BASIN IGNIMBRITE PROVINCE

In the broad spectrum of arc volcanism, the ignimbrite flareup end member contrasts sharply with the normal end member where small and frequent extrusions of intermediate composition lava and minor ignimbrite create stratovolcanoes. The flareup end member is exceptionally well represented by the 36-18 Ma Southern Great Basin ignimbrite province. Caldera-forming supereruptions of silicic magma occurred on a high orogenic plateau underlain by crust as thick as about 70 km. During the flareup, an underlying subducting oceanic plate rolled back from a previous flat configuration. Hundreds of thousands of cubic kilometers of basalt from the mantle invaded this unusually thick crust to drive production of >70,000 km³ of explosively erupted silicic magma. Several investigators have suggested that the basaltic magmas formed by partial melting of the lithospheric mantle that had been hydrated during the flat slab phase.

However, estimates of the mantle source volume of such a large amount of basalt reveal that such a scenario is unlikely. The hydrated zone would necessarily be thin (<10 km). Assuming 5% melting and a hydrated layer 10 km thick, a lithospheric source volume would need to have a diameter of 720 to 960 km to produce enough basaltic magma (250,000 to 430,000 km³) to power the silicic magma system beneath just the Indian Peak-Caliente field. This hypothetical source zone would have stretched underneath the adjacent volcanic fields–Marysvale to the east and Central Nevada to the west–and starved them of their basaltic power supplies.On the other hand, assuming the width of the source volume corresponded to the width of the volcanic field (~180 km), the calculated height of the mantle source column for the Indian Peak-Caliente field is 175 to 300 km; thicker than the entire mantle lithosphere.

Thus, a sublithospheric source for the voluminous mantle-derived magma that powered these complexes is required. We conclude that melting occurred, not just in the lithospheric mantle, but in a growing “wedge” of asthenosphere above the foundering slab. Dehydration of the slab, flux melting, and decompression melting all contributed to the development of the mantle component of these vast by short-lived systems.