See more from this Division: Topical Sessions
See more from this Session: Evolution of Simple Granite Systems (Haplogranites) and Rhyolites: A 50th Anniversary Perspective of the Tuttle and Bowen Studies
Abstract:
Aplite dikes from Mesozoic and Tertiary plutonic rocks in the western US have consistent HSR compositions, but their trace-element patterns vary widely and correlate closely with the presence or absence of primary titanite in the host rock. Aplites in titanite-free hosts are characterized by Eu depletion (yielding seagull-shaped REE patterns), whereas aplites in titanite-bearing hosts are depleted in Y and all MREEs, yielding deep U-shaped REE patterns. Similarly, residual HSLs in the titanite-bearing Half Dome Granodiorite of the Tuolumne Intrusive Suite have distinctly U-shaped REE patterns that are indistinguishable from the REE pattern of glass from the titanite-bearing Fish Canyon Tuff. U-shaped REE patterns are essentially unknown among erupted HSRs. Because HSRs all have seagull REE patterns and aplite geochemistry closely correlates with host-rock mineralogy, we conclude that HSLs in titanite-bearing granodiorite bodies do not travel far from their hosts. HSRs thus appear to have a different origin from HSLs developed in long-lived magma bodies.
High-silica granites with negative Eu anomalies like those in HSRs are sparse, but known. However, plutonic rocks with positive Eu anomalies that could be the solid residues of fractionated HSR liquids are equally rare. These observations suggest that the plutons commonly represent unerupted equivalents of volcanic rocks rather than residua from extraction of erupted magma.
See more from this Division: Topical Sessions
See more from this Session: Evolution of Simple Granite Systems (Haplogranites) and Rhyolites: A 50th Anniversary Perspective of the Tuttle and Bowen Studies