10-8 What might the zoning of shallow plutons tell us about the deeper reservoirs that underlie them? Insights from the Mount Whitney Intrusive Suite, Sierra Nevada, California

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

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Leucogranite compositions indicate the exposed part of the Mount Whitney Intrusive Suite (MWIS) crystallized at a depth of ~4 km (p = 100 MPa). Al-in-hornblende barometry indicates, however, that the magma which formed its oldest member crystallized at ~10 km (250 MPa) and those which formed its two younger members crystallized at ~6 km (150 MPa). Although the oldest pluton consists of several discrete intrusive phases, the younger ones lack prominent internal contacts and are zoned on kilometer scales, suggesting they were fed from integrated reservoirs that developed as the upper crust matured thermally.

The suite’s lobate map pattern and the domical upper contact of its youngest member suggest the MWIS plutons are laccoliths. If these intrusions were emplaced without significant internal mixing, we might expect to find the earliest magmas (drawn from the upper parts of the reservoirs) distributed distally and later magmas (drawn from the deeper parts of the reservoirs) located centrally.  Neglecting the younger plutons’ marginal zones, which gradients in age and composition indicate were modified by local mixing with older magmas, both grade from distal phases that contain small alkali-feldspar megacrysts to central phases that contain large ones. In the youngest pluton the increase in megacryst size is correlated with increases in groundmass crystal size and color index. If the younger members of the MWIS grew as described above, their zoning suggests megacryst sizes and ferromagnesian mineral abundances increased downward in the underlying reservoirs.

The sawtooth zoning of megacrysts and notched size distributions of alkali-feldspar crystals in the younger MWIS plutons indicate the magmas that built them were cycled thermally (Rout et al., 2021) with more cycling, and thus larger megacrysts, deeper in the reservoirs where hotter magmas impinged from below. Chronological studies of megacrysts from similar systems (Chambers et al., 2020; Rout et al., 2021) suggest that magmas in the MWIS reservoirs likely remained above their alkali-feldspar saturation temperatures, at least intermittently, for 10⁵-10⁶ years. This would have afforded time for processes such as hindered settling and compaction to enrich rhyolitic melts upward and make magmas in the upper parts of the reservoirs more felsic.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7048

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