ARE THE LATE CRETACEOUS K-FELDSPAR MEGACRYSTIC INTRUSIONS OF THE SIERRA NEVADA BATHOLITH COLD, WET GRANITOIDS?

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 (T_zrc,Ti) model temperatures for most zircons (ca. 650-720°C, regardless of uncertainties in a_TiO2), low concentrations of MREEs, high Yb/Gd, low Th/U, high Hf, and high Eu/Eu*. In addition, the T_zrc,Ti temperatures are mostly below the calculated bulk zircon saturation temperatures (T_zrc,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 T_zrc,sat and T_zrc,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.