U-PB GEOCHRONOLOGIC EVIDENCE FOR INCREMENTAL FILLING OF THE TUOLUMNE INTRUSIVE SUITE MAGMA CHAMBER

New U-Pb zircon ages for rocks of the Tuolumne Intrusive Suite (TIS) combined with published Ar thermochronology and thermal modeling suggest that zoned intrusive complexes may result from incremental filling of a magma chamber over significantly longer periods than previously assumed.

An age of 93.1 +/- 0.1 Ma for the tonalite of Glen Aulin pins the onset of magmatism in the TIS at least that old. The outermost Half Dome Granodiorite yielded an age of 92.6 +/- 0.2 Ma, a sample from the top of Half Dome (proper) yielded an age of 91.1 +/- 0.1 Ma, and a sample from the bottom of Half Dome yielded 90.7 +/- 0.2 Ma. Taken with our published data (88.1 +/- 0.2 Ma for the Cathedral Peak Granodiorite), these data indicate that the TIS was emplaced over a span of about 5 Ma, and that the Half Dome pluton was active for at least 2 Ma, far longer than one-pulse thermal models allow. Because contacts between units in the TIS are at least partly gradational, these data require episodic growth of the TIS over a long time span.

Additional evidence for incremental construction of the TIS comes from considering existing Ar thermochronologic data (R. Fleck & R. Kistler) with the new U-Pb dates. Whereas many plutons show rapid cooling from U-Pb zircon closure through hornblende Ar closure, followed by slow cooling through biotite closure, units in the TIS show a markedly different cooling history. TIS units typically required 4-6 Ma to cool through hornblende closure and less than 2 Ma to cool from hornblende through biotite closure. Simple 2-dimensional thermal modeling shows that these “inverted” cooling curves can be reproduced though continuous filling of a magma chamber over a 3 Ma period, and suggest that investigation of the cooling history may be a straightforward way to recognize incrementally filled chambers.

These observations raise several new concerns for understanding magma chamber dynamics. If pluton emplacement can be a continuous process, by what mechanisms are homogeneous magmas produced over millions of years? Or does this suggest that deeper crustal exposures may reveal large homogeneous chambers that periodically send small batches of magma to shallower levels? Similarly, what is the mechanism by which changes in composition arise during incremental filling? Does this suggest that there are large zoned chambers at depth?