WHERE’S THE RHYOLITE? EVIDENCE FOR MELT LOSS AND CRYSTAL ACCUMULATION IN THE TUOLUMNE INTRUSIVE COMPLEX

Bulk-rock compositional trends are commonly used to infer magmatic processes. For example, bulk-rock compositions from the concentrically zoned Tuolumne Intrusive Complex (TIC) have been used to infer inward fractionation within the main intrusion [1] and within smaller peripheral lobes [2,3]. However, crystal accumulation and/or melt loss may be difficult to recognize [4] within an evolved magma system. Here we demonstrate that TIC amphiboles (Amp), with magnesiohornblende compositions are primarily in equilibrium with rhyolitic melts and that the bulk-rock compositions from the outer Kuna Crest (KC) and equigranular Half Dome (eHD) units are typically too mafic to be in equilibrium with their Amp.

Amp/bulk-rock ^Fe-MgK_D values range from 0.2 - 0.64. Several analyses are outside the recommended range of 0.28 ± 0.12 for ^Fe-MgK_D equilibria [5]. Most analyses with ^Fe-MgK_D values that lie outside the equilibrium range occur in either KC or eHD samples with bulk-rock SiO₂ contents < 65 wt%; whereas in more silicic samples, ^Fe-MgK_D indicates that bulk-rock compositions reflect melt compositions. A direct link between Amp and melt composition has also been proposed [5,6,7]. These approaches suggest that most TIC Amp were in equilibrium with rhyolitic melts (> 65 SiO₂, < 0.5 MgO, < 1.5 FeO wt%). Finally, several compositional similarities can be drawn between Amp from the TIC and those from the Fish Canyon Tuff, which have been experimentally determined to have crystallized from rhyolitic melts [8]. Therefore, it seems probable that crystal accumulation and/or melt extraction (e.g. eruption) has played an important role in the compositional variability of the outer KC and eHD units whereas the more interior porphyritic Half Dome and Cathedral Peak units have bulk-rock compositions that are permissively similar to melt compositions from which TIC Amp crystallized.

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