FIELD EVIDENCE FOR THE ASSEMBLY OF THE HALF DOME PLUTON BY AMALGAMATION OF SMALL INTRUSIONS

Detailed mapping (1:10 000) of petrography and magnetic susceptibility of the Half Dome Granodiorite of the Sierra Nevada batholith, California, reveals abundant previously unrecognized internal contacts formed by discrete pulses of magma. These results are consistent with the formation of a superficially homogeneous Half Dome pluton through amalgamation of many discrete batches of magma. The age variation across the Half Dome (>3 m.y.) exceeds age differences across both the inner (~ 1 m.y.) and outer (<1 m.y.) contacts with adjacent map units, demonstrating that internal contacts distinguished by subtle changes in petrography can be more significant than contacts distinguished by obvious changes in petrography.

We distinguish two types and scales of internal contacts in the Half Dome Granodiorite. At the km-scale, there are gradational changes in color index (~20 to <2) that correlate with changes in bulk magnetic susceptibility (~0.03 to < 0.005 SI). The western edges of these bodies are marked by mappable abrupt increases in both color index and magnetic susceptibility back to values of ~20 and 0.03, respectively. Rocks on the mafic side of such contacts typically contain abundant schlieren and swarms of mafic enclaves. These contacts are continuous along strike and parallel previously mapped contacts between plutons within the Tuolumne Intrusive Suite. At the meter to decameter-scale, internal contacts are variably marked by chilled margins, truncated enclaves or schlieren, and igneous layering. Unlike the km-scale contacts, few of these are traceable along strike for more than a few tens of meters before becoming cryptic, and have no consistent orientation.

We propose that the mapped internal contacts delineate discrete additions of intrusive rock that amalgamated to form the Half Dome pluton. The variations in km-scale map units suggest repeating variations in the chemistry of the magma building the pluton, or repeating processes driving magma evolution at the level of emplacement. Field data are consistent with geochronologic data that require multiple pulses of magma, and are inconsistent with emplacement of the Half Dome as a single batch of magma.