THE ASSEMBLY RATE OF HALF DOME'S BEDROCK

The rates and mechanisms by which large magma bodies are assembled is critical to understanding construction of continental crust. Geochronologic data demonstrate that the Tuolumne Intrusive Suite of Yosemite National Park accumulated over at least 10 Ma. Coleman et al. (Geosphere, 2012) suggested that the assembly of the suite occurred as downward-stacking, domed laccoliths, leading to a pattern of ages in which the oldest rocks crop out at the margins with the youngest in the center. A corollary to this model is that individual map units should have the same age pattern. We are dating a transect through the western exposures of the Half Dome Granodiorite, from its outer, western contact with older tonalite to its inner, eastern contact with the Cathedral Peak Granodiorite, using CA-TIMS U-Pb geochronology. Our goal is to evaluate whether there is predictable age variation in the pluton from west to east and to estimate the rate of emplacement of the magmas. This transect crosses the intrusive cycles referred to above and includes Half Dome.

Previous U-Pb dates from the Half Dome Granodiorite span ~ 4 Ma across a 15 km transect of the western lobe, a lateral accretion rate of ~4 mm/a; if the units have been tilted 45° to the west, this translates to a vertical accretion rate of ~2.5 mm/a. These rates predict an age difference of ~750 ka across Half Dome proper, easily resolvable with CA-TIMS. The ~1 km spacing of cycles predicts an age difference of ~250 ka between cycle tops and bottoms, also resolvable. Half Dome encompasses a cycle, showing a progression of diking from layered granodiorite dikes on the northeast to a dense plexus of aplite dikes on the southwest (Glazner et al., this meeting).

CA-TIMS dating should allow us to refine the assembly rate of the Half Dome Granodiorite. Preliminary redating of one sample from Coleman et al. (2004) that showed a spread of ages along concordia gives a tight cluster of zircons at 92.10 +/- 0.12 Ma. This age better fits the age variation expected in the pluton, with older ages occurring in the western exposures. Further dating is in progress.