THERMAL MODELING OF A COMPOSITE FELSIC INTRUSION: IMPLICATIONS FOR THE ORIGIN OF COMPOSITIONAL ZONING WITHIN MEMBERS OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Mount Whitney Intrusive Suite (MWIS) is a large (80 km by 40 km) composite granitic intrusion whose three major members were emplaced between about 87.5 and 83.5 Ma based on published U/Pb zircon ages. The two younger plutons are partially nested within the older ones, and locally exhibit transverse variations in whole rock and mineral chemistries that are consistent with both km-scale crystal fractionation and local melt segregation. These zoning patterns are particularly well developed in the parts of the younger plutons that were emplaced entirely within the older ones, suggesting that rising magmas were able to accumulate and differentiate within the younger bodies where relatively high host rock temperatures slowed cooling and inhibited rapid crystallization.

We have used a finite-element technique to model conductive cooling of the MWIS and explore the hypothesis that a large nested intrusive suite, growing at a geologically reasonable rate in the upper crust, could sustain a km-scale magmatic reservoir over a period sufficient for crystallization differentiation to occur (perhaps 10⁴ to 10⁵ years). The elongate outcrop patterns and right-lateral offsets of the plutons in the suite suggest that its emplacement was, at least in part, fault controlled. We have therefore adopted the emplacement model of Yoshinobu et al. (J Struct Geol, 20:1205-1218) in which an axial dike feeds the intrusion and crust is translated outward to accommodate its growth. Each pluton is assumed to be 15 km thick, consistent with the magnitude of the negative gravity anomaly over central part of the suite, and to extend from a depth of 5 to 20 km. Growth rates were constrained by the need to emplace plutons with a combined width of 40 km, and cool their upper parts to about 700°C, over a period of approximately 4 Ma. The post-magmatic thermal history of the central pluton is also constrained by published U/Pb zircon and K/Ar biotite ages that indicate it cooled from 700 to 250°C between 83.5 and 81.7 Ma.

The initial results of our modeling agree with previous studies and indicate that a substantial part of oldest member will be comprised of sheeted dikes. Depending on the lengths of the pauses between the emplacements of the younger members, however, large parts of these inner plutons are predicted to remain well above their solidi for periods sufficient for differentiation to occur.