INSIGHTS FROM ZIRCON CHEMISTRY ON THE CRYSTALLIZATION HISTORY OF THE WHITNEY PLUTON, SIERRA NEVADA, CALIFORNIA

The Whitney pluton is the youngest member of the Mount Whitney Intrusive Suite and is inferred to preserve the most thermally mature part of this long-lived (90.6-82.6 Ma) magmatic system. The central part of pluton is exposed over vertical extent of about 3000 m and consists of a compositionally graded granodiorite overlain by relatively uniform granite that, in turn, hosts sheet-like bodies of locally segregated leucogranite at high elevations. Modeling of whole-rock major and trace element compositions indicate the granite can be derived from granodiorite by ≅10% crystallization of an assemblage dominated by plagioclase and hornblende whereas the leucogranite can be derived from the granite by ≅50% crystallization of an assemblage dominated by feldspars and quartz in which titanite has strongly affected REE behavior and produced marked MREE depletion. Zr-in-titanite thermometry (Moore and Sisson, 2008) indicates titanite in the granite crystallized at ≅715-740°C during alkali-feldspar megacryst growth and 710-680°C during subsequent groundmass crystallization assuming a(SiO₂) = 1 and a(TiO₂) = 0.7.

Ti-in-zircon temperatures calculated using these same SiO₂ and TiO₂ activities indicate zircon in the granite crystallized at 680±40°C. Zircon REE distributions do not, however, display the MREE/HREE depletions that would be expected if their growth had followed significant titanite crystallization (e.g. Kneeling Nun Tuff, Szymanowski et al., 2017).

This lack of MREE depletion suggests zircon crystallized from the granite prior to titanite and, thus, that the calculated Ti-in-zircon temperatures may be too low. Crystallization of both minerals from melts with lower a(TiO₂) could resolve the apparent discrepancy but is at odds with estimates of typical a(TiO₂) for felsic magmas. The substitution of REE^{3+, VIII} + P^{5+, IV} for Zr^{4+, VIII} + (Si, Ti)^{4+, IV} could also lower XTi^IV in zircon but appears to be of limited extent in the Whitney zircons.