16-49 Crystals in Conversation: Zircon Records of Magmatic Mixing in the Bitterroot lobe of the Idaho Batholith

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

The Bitterroot Lobe of the Idaho batholith contains many mafic dikes and sills that intrude the granites of its Cretaceous-Paleocene main phase.  Some of these features have sharp contacts with the host granite, but others show evidence of local magma mingling and hybridization, so it remains uncertain to what extent these features are co-magmatic with the Cretaceous-Paleocene granites or younger features related to the Eocene Challis magmatic event which also affects this region.  Here, we present new LA-ICP-MS U-Pb ages for zircons separated from these rocks. 

Zircons were recovered from samples of four different mafic features at two different localities, and these zircons show major differences in size and shape.  Each of the four samples contained a few relatively large grains with distinct morphologies that yielded U-Pb ages mostly between 60 and 70 Ma, although one Proterozoic grain was also observed.  However, two of the samples yielded an additional population of smaller grains with Eocene ages between 48 and 50 Ma.  We conclude that these small Eocene grains are native to the mafic magma that formed those dikes, whereas the larger grains are xenocrysts that have been incorporated into the mafic magma from the hosting Cretaceous-Paleocene granite and Proterozoic country rocks of the batholith.  The two dikes containing the small grains are therefore interpreted to be Eocene Challis features.  The crystallization ages of the two dikes containing only large xenocrystic zircons remain uncertain with either Cretaceous-Paleocene or Eocene being possible.  

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