FIELD, GEOCHEMICAL, AND GEOCHRONOLOGIC EVIDENCE FOR RECHARGING IN A MUSHY MAGMA RESERVOIR: CATHEDRAL PEAK GRANODIORITE, TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA

The Cretaceous Tuolumne Intrusive Series (TIS), part of the Sierra Nevada batholith, is a spectacularly exposed, normally zoned composite arc intrusion. Early work on the TIS suggested that chemical variation resulted from in situ igneous processes. However, recent U/Pb geochronology (Coleman et al 2004) constrains TIS emplacement to roughly 10 Ma, leading to the model that incremental filling of a shallow magma chamber created the entirety of the suite. This model suggests that the outer, older units were completely solid when the inner units were emplaced. The Cathedral Peak granodiorite (Kcp), which is the largest of the units comprising the TIS has been previously considered as a relatively homogenous granodiorite, showing modest chemical and isotopic variation. However, if Kcp was constructed over > 10⁶ m.y. by numerous inputs of magma, then field, petrographic and geochemical variability is expected.

Bulk composition varies modestly and relatively smoothly from outer to inner parts of Kcp (69 – 73 wt. % SiO₂) but many trace element concentrations (e.g. Sr, Y, Zr, Nd) and ratios (e.g. Rb/Sr) show appreciable scatter. Chondrite normalized La/Yb ratios generally increase inward with some local perturbations. Field observations such as textural and mineralogic variation, schlieren zones, magmatic faults, metamorphic xenolith concentrations, sparse microgranular mafic enclaves, alkali feldspar accumulations and zones of ladder dikes are evidence of Kcp heterogeneity. Preliminary U/Pb zircon dates within Kcp for samples separated by ~ 4 km along a transect from the porphyritic Half Dome granodiorite (Khdp) contact to the Johnson Granite porphyry (Kjgp) contact have indistinguishable ages and overlap ages from other parts of the Kcp (Coleman et al 2004).

These data and observations may be explained best by a model where significant melt is present during Kcp construction, resulting in a high-crystallinity (mushy?) chamber. Some of the features above are indicative of differential melt flow in mush (Weinberg et al 2001), possibly linked to instability arising from new inputs, thereby causing domainal redistribution of melt and/or disruption of mush.