THERMAL HISTORIES OF EPISODICALLY CONSTRUCTED, LARGE VOLUME MAGMA CHAMBERS: IMPLICATIONS FOR PROCESSES ALONG INTERNAL CONTACTS

We have used FD thermal modeling to evaluate crystallization and cooling histories of large, episodically constructed magma conduits by comparing conduits of sheeted, nested cylindrical, disk-shaped, and irregular-shaped magma pulses. For the latter, the maps or cross-sections are digitally rendered into rock types, assigned thermal properties, and emplaced into the model at specified times as new thermal pulses. All of these models indicate that crystallization times are much slower than for smaller magma conduits and that melt may reside in large magma conduits for millions of years. Our modeling also emphasizes that the thermal effects of irregular margins dissipate rapidly and that the overall pattern of isotherms depends on the center of "thermal" mass of each pulse. Our modeling of the Tuolumne Batholith (TB), California indicates that subsequent pulses are centered on the highest isotherms established from heating by previous pulses suggesting that the ascent of younger pulses is thermally (rheologically?) controlled. Episodically constructed chambers result in numerous internal contacts between magma pulses. We thus are also constructing plots which track cooling histories at these internal contacts to aid in establishing the crystallization/cooling/reheating histories along internal contacts, predicted crystal size distributions, cooling ages, and histories of rheological contrasts across contacts. In the TB, we have completed detailed mapping across these internal boundaries the characteristics of which vary tremendously. Some are rather sharp and structurally and petrologically simple whereas others display spectacular complexities the details of which indicate that they are zones of mechanical and chemical disequilibrium. We have established local growth histories along these contacts which typically indicate outward younging of pulses, indicating local reversals from the overall inwards growth established by published geochronology. We also see a range of rheological behaviors in different pulses ranging from brittle cracking and stoping to complex magmatic flow. Finally we have established that multiple magmatic fabrics exist along these contacts and that at least one cuts internal boundaries indicating that some melt remained in both pulses after juxtaposition as predicted by the thermal modeling.