I conducted fieldwork at Ossipee caldera in 2001-2003 and at Lake City caldera in 2002. The magmatic textures observed in the intrusive and extrusive rocks exposed at these calderas show striking similarities. I hypothesize that the textures at both calderas are the result of magma mixing within the magma chamber driven by caldera collapse.
At Ossipee, I compared the mixing and mingling textures in the ash flow sheets with those in the post collapse intrusions. Magmatic textures in the post collapse magmas showed that two distinctly different types of magma interaction occurred, (1) magma mixing during caldera collapse disrupting a compositionally zoned chamber, and, (2) magma mingling after caldera collapse caused by replenishment into the magma chamber. Similarly, the preliminary work done at Lake City shows that the post collapse magmas are significantly more mingled and mixed than those of the early erupted ash flows. I propose an additional field season at Lake City caldera to constrain the timing and degree of mixing. I will map the spatial distribution of changes in mineralogy and textures in the post-collapse magmatic intrusions and compare it with earlier erupted ash flow sheets as done at Ossipee. In particular, I will investigate the composition, distribution, and shape of magmatic inclusions to reveal details of magma mingling. I will also measure crystal size distributions in order to establish whether two crystal populations have mixed (Higgins, 1996). The textures identified at Lake City and Ossipee, may be common at many calderas and I propose that caldera collapse drives thorough magma mixing during collapse and encourages chamber replenishment driving further magma mingling. This contrasts the traditional view that magma mixing drives caldera collapse.
This work will identify the most important physiochemical processes involved in caldera collapse and post-collapse magmatism. Understanding such processes is an important component to any caldera cycle model. Post-collapse magmatism (often referred to as resurgence) has been the subject of debate since the work of Smith and Bailey (1968). A variety of different processes, styles and mechanisms have been described theoretically and at different calderas (Marsh, 1984; McConnell et al., 1995; Hon, 1987). My overall goal is to link the physical processes associated with collapse with the mechanisms that drive resurgence. Also, the occurrence and timing of processes such as magma mixing, renewed intrusion, and magmatic degassing are crucial to the understanding of post-caldera collapse hazards of currently active calderas. In certain cases, such an understanding can have a predictive value in terms of when and where a future eruption might occur.
Higgins, M. D., 1996, Magma dynamics beneath Kameni
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Hon, K. A., Geologic and petrologic evolution of the Lake City Caldera, San Juan Mountains, Colorado. Ph.D. Thesis, University of Colorado, 1987.
Smith, R.L., and R.A. Bailey, Resurgent cauldrons, Geol.
Soc. Am. Mem., 116, 83-104, 1968.