Insights From Megacryst-Included Zircon Dates on the Spatial Extent of Magma Mixing in the Tuolumne Intrusive Suite, California, USA

Abstract

The spatial extent of mixing between separately emplaced batches of magma is a key component in understanding the incremental assembly of plutons. Potassium feldspar megacrysts (>3 cm length) in granodioritic rocks are hypothesized to record magma mixing and transport over hundred kyr timescales. CA-ID-TIMS U-Pb dates from zircon inclusions within 11 new megacryst samples and their surrounding matrix from the Tuolumne Intrusive Suite are presented as a means of evaluating the extent of mixing across a less than two km wide gradational contact. Megacrysts from within the gradational contact yielded zircon dates consistent with mixing or transport from the older porphyritic Half Dome Granodiorite, but the zircon included in megacrysts from the younger Cathedral Peak Granodiorite show no evidence of interaction with the porphyritic Half Dome. These results, along with the trace element geochemistry of the zircon, suggest that the porphyritic Half Dome and Cathedral Peak magmas where we sampled were not comagmatic, which constrains the width of a mixing front to the kilometer scale gradational contact between these units. From within this gradational contact, we do find evidence of mixing consistent with prior studies. Differences between the core- and rim-included zircon date spectra also suggest that protracted growth is recorded in some K-feldspar megacrysts but may not be a requirement for forming them.