Paper No. 13
Presentation Time: 4:30 PM


MILLER, Jonathan S., Department of Geology, San Jose State University, San Jose, CA 95192-0102 and LACKEY, Jade Star, Geology Department, Pomona College, Claremont, CA 91711,

The defining characteristic of the Late Cretaceous zoned intrusive suites of the Sierra Nevada Batholith (SNB) are large, K-feldspar megacryst-bearing granodiorites/granites (e.g. Cathedral Peak, Mono Creek, Whitney, Topaz Lake) comprising the inner and volumetrically dominant parts of the suites. They essentially represent the terminal stages of the high-flux Cretaceous arc but their petrogenesis remains controversial. They are characterized by a wide variety of enigmatic magmatic structures, some relatively common in other intrusions (e.g. planar and irregular mafic schlieren) and others relatively uncommon (ladder dikes, nests and “dikes” of nearly pure K-feldspar). Zircon ‘antecrysts’ are a hallmark of the well-dated Cathedral Peak granodiorite; the other megacrystic intrusions have not been as thoroughly dated.

Zircon trace element data from the inner megacryst-bearing intrusions are also quite distinctive: low Ti-in-zircon (Tzrc,Ti) model temperatures for most zircons (ca. 650-720°C, regardless of uncertainties in aTiO2), low concentrations of MREEs, high Yb/Gd, low Th/U, high Hf, and high Eu/Eu*. In addition, the Tzrc,Ti temperatures are mostly below the calculated bulk zircon saturation temperatures (Tzrc,sat) for the rocks, which are also relatively low (ca. 720-750°C).

The above indicates that zircon grew at low temperature (H2O-saturated granite solidus), high fO2, and in equilibrium with titanite, which exerts strong control on the REEs, particularly the MREEs. The offset of Tzrc,sat and Tzrc,Ti can be explained by either saturation and retention of ‘inherited’ zircon during anatexis, or zircon ‘accumulation’ (or loss of residual melt) after zircon saturation was attained. Zircon accumulation and mixing of 'antecrysts' (as is required by the age data) is difficult at the wet solidus; 'antecrysts' may instead be slightly older 'inherited' crystals. We suggest that the megacrystic intrusions formed by repeated cycles of granite anatexis and freezing under relatively cold, hydrous, (near H2O-saturation), and zircon-saturated conditions. The megacrystic intrusions that mark the terminal stages of the arc may thus have formed at antectic conditions atypical for much of the the SNB. Regardless of their exact origin, water likely exerted strong control on their petrogenesis.