Laramide exhumation and heating in southeastern Arizona: Low-temperature thermal history and implications for zircon fission-track systematics.

摘要

Fission-track (FT) ages of zircon from Mesozoic sandstones were analyzed to address provenance and post-depositional thermal history of Laramide synorogenic strata. Upper Cretaceous samples show a complex provenance, with significant recycling from underlying rocks. FT peak ages from 14 Jura-Cretaceous sandstones include populations of 570--165, 140--82, and 68--42 Ma. Older single-grain ages (1000--600 Ma) indicate long-term sub-annealing temperatures (180°C, up to 260°C, depending on single-grain response to heating) for portions of the source area. Older FT ages present in all sampled units indicate little burial of the Jura-Cretaceous rocks prior to recycling into the Upper Cretaceous; record little variation in the thermal signature of recycled crustal detritus; and show that the older Jurassic and Cretaceous rocks likely provided an important source for the Fort Crittenden Formation. The uniformity of FT ages necessitates that source rocks all record essentially the same thermal signatures. Accordingly, the relative abundance of rock units in the source terrane was rather uniform, and there was no preferential exhumation of one source area over another.; Some zircons show evidence for significant post-depositional thermal annealing; these strata reached temperatures sufficient to anneal fission tracks in some grains (c. 180°C--260°C). Thicknesses of overlying units was likely less than 2 km; as such, the FT age structure is not purely the result of burial. Regionally, samples with Paleocene-Eocene reset peak ages coincide with proximity to magmatic bodies (75--40 Ma). Besides conductive heating, a hydrothermal system may have locally modified these rocks, causing preferential resetting of certain zircons. This setting provides an opportunity to investigate controls on single-grain thermal response. Geochemical and crystallinity data indicate a correlation between old crystallization ages, high U+Th concentrations, elevated alpha-damage, and young reset FT ages. Resetting occurred in grains with higher radiation damage, and less damaged grains have a higher temperature of track retention and therefore retain a primary detrital signature. Zircon color is related in part to increasing radiation damage, and can be used for identifying different thermal events in both the pre- and post-depositional history of individual zircons. In this study, honey and colorless grains essentially behave as one population in all data sets. Based on these data, this study establishes criteria for identifying reset grains in detrital populations, and suggests revisions in the current methodology.