Fluid dynamic modelling of crystallising calc-alkalic magma bodies has predicted that differentiated liquids will ascend as boundary layers and that accumulation of these buoyant liquids near chamber roofs will result in compositionally stratified magma chambers. This paper reports physical features in La Gloria Pluton that can be interpreted as trapped ascending differentiated liquids. Leucogranitic layers decimetres thick, which are locally stratified, are trapped beneath overhanging wall contacts. The same felsic magmas were also preserved where they were injected into the wall rocks as dykes and as large sill complexes. These rocks do not represent differentiated magmas produced by crystallisation along the exposed walls because the felsic layers occur at the wall rock contact, not inboard of it. Rather, we speculate that evolved felsic liquids are generated by crystallisation all across the deep levels of chambers and that initial melt segregation occurs by flowage of melt into tension fractures. Melt bodies so formed may be large enough to have significant ascent velocities as diapirs and/or dykes. The other way in which the leucogranite occurrence is at variance with the convective fractionation model is that the ascending liquids did not feed a highly differentiated cap to the chamber, as the composition at the roof, although the most felsic in this vertically and concentrically zoned pluton, is considerably more mafic than the trapped leucogranitic liquids. This suggests that these evolved liquids were usually mixed back into the main body of the chamber. Backmixing may be general in continental-margin calc-alkalic magmatic systems, which, in contrast to those in intracontinental settings, rarely produce volcanic rocks more silicic than rhyodacite. That the highly differentiated liquids are preserved at all at La Gloria is a result of the unusual stepped nature of the contact and the entirely passive mode of emplacement of the pluton, which, in contrast to ballooning in place, does not result in wall zones being “scoured”.

An integrated structural, stratigraphic, geochronological, and geochemical investigation of Cenozoic volcanic and sedimentary rocks within a highly extended part of the eastern Great Basin sheds light on the interplay between magmatism and extensional tectonism. Tertiary rocks in east-central Nevada and west-central Utah can be divided into three broad groups: (1) 40 to 35 Ma, locally derived sequences of andesite and rhyolite lava flows and ash-flow tuffs; (2) the voluminous 35 Ma Kalamazoo volcanic rocks, including the compositionally zoned (rhyolite to dacite) Kalamazoo Tuff, crystal-rich hornblende dacite lavas, and the K-rich dacite tuff of North Creek and associated lavas; and (3) 35 to 20(?) Ma, predominantly sedimentary sequences. Crosscutting relations between faults and subvolcanic intrusions, decreasing tilts upward within the Tertiary sections, and sedimentologic evidence for rapid unroofing of deep structural levels demonstrate that rapid, large-magnitude extension in this region began at least 36 Ma during some of the earliest eruptions, was ongoing at 35 Ma during the culminating eruptions of Kalamazoo volcanic rocks, and continued after volcanism had largely ceased. These synextensional volcanic rocks constitute a high-K calc-alkaline andesite to rhyolite series, and closely resemble suites from the central Andes rather than the bimodal or alkalic suites commonly associated with continental rifts. Trace-element systematics and reconnaissance Sr and Nd isotopic data suggest that the suite formed by extensive contamination of mantle-derived basalt by crustal partial melts in the deep crust, followed by relatively minor wall-rock assimilation during fractionation from andesite to rhyolite, presumably at shallower levels. Modeling of the isotopic data suggests that the most voluminous rock type, hornblende dacite, consists of 30 to 50 percent mantle material. Thus, intrusions associated with Cenozoic volcanic rocks represent a significant addition of new mantle-derived material to the continental crust. A comparison of the eastern Great Basin with other highly extended parts of the Basin and Range province reveals striking similarities in eruptive and extensional histories, despite important regional variations in absolute timing. These similarities are best explained by an active rifting model that invokes a flux of basaltic magma into the crust, hybridization and mixing of these magmas with crustal melts to produce intermediate magmas that differentiate in shallower magma reservoirs, and magmatically induced thermal weakening of the crust culminating in brittle failure of the upper crust and ductile flow at depth. This model helps explain (1) the close spatial and temporal association between the onset of large-magnitude extension and voluminous volcanism throughout the province; (2) the general decrease in extensional strain rates through time; (3) the typical progression of magma compositions from early, intermediate to silicic rocks to late, relatively primitive basaltic or bimodal suites; and (4) the uniform crustal thickness and the reflective mafic lower crust of the Basin and Range province.