'DEFROSTING' MICROSTRUCTURES IN THE JACKASS LAKES PLUTON AND THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CALIFORNIA

Magma ‘defrosting’ microstructures are well known in extrusive igneous rocks, however, there is limited work that identifies these microstructures to this re-heating and partial re-melting process in plutonic systems after they have undergone a fair amount of solidification. In the Sierra Nevada batholith, we investigated whether these microstructures occurred in both the ca. 175 km2 Jackass Lakes pluton (JLP) and the ca. 1,100 km2 Tuolumne Intrusive Complex (TIC). Further, we examined where in the pluton the ‘defrosting’ was most prominent.

Petrographic analysis of thin sections from both plutons indicate that primarily low-T mineral phases were involved in the defrosting process. These minerals include quartz, muscovite, albite, and K-feldspar. Cathodoluminescence imaging was completed to better detect and highlight defrosting microstructures.

In the JLP, we observe an abundance of defrosting microstructures, primarily within the older units. These older units, as determined by crosscutting relationships in the field, exhibit a lot more defrosting microstructures than the younger intruding phases, where defrosting microstructures were minor to non-existent. Recurring defrosting microstructures observed include 1) “islands” or fragments of resorbed minerals such as quartz or plagioclase occupied by another mineral, 2) deeply embayed biotite, 3) apatite inclusion accumulations incorporated into surrounding minerals, 4) blocky-sieve texture with plagioclase and K-feldspar, 5) lobate grain margins, 6) resorbed titanite, and resorbed hornblende. In the TIC, defrosting microstructures occur largely in the oldest Kuna Crest unit, including 1) resorbed hornblende, 2) lobate grain margins, 3) apatite inclusions, and 4) resorbed biotite. The younger, interior TIC units show little to no signs of defrosting microstructures.

Our data indicates that the occurrence of defrosting microstructures in plutons may depend on its size and is more likely to be found in smaller plutons, given smaller plutons are more likely to cool significantly more between recharge events in contrast to larger plutons.