EXPLORING VOLCANIC-PLUTONIC CONNECTIONS IN THE BONANZA CALDERA, COLORADO

Arc magmatic systems are often studied through investigations of either volcanic or plutonic exposures, which often lead to inconsistent interpretations as volcanic and plutonic rocks represent magmatic products of different depths and durations. Investigating a tilted caldera where both rocks types are exposed and of the same age is thus an advantage as hypotheses about potential links can be directly tested and the system examined as a whole.

The Bonanza Caldera in the Southern Rocky Mountain Volcanic Field, Colorado, is a ~20 km large caldera that was tilted by 40-50° during extensional faulting in the Rio Grande Rift system and deeply eroded resulting in exceptional exposures of volcanic and plutonic rocks of the same magma plumbing system. Volcanics consist of pre-caldera rhyolite lavas (33.83±0.08 Ma; all ages are ⁴⁰Ar/³⁹Ar ages by W. McIntosh), the >1000 km³ Bonanza Tuff (33.19±0.04 Ma), and post-caldera rhyolite to andesite lavas (33.38 ±0.13 to 32.78±0.04 Ma). The plutonic intrusions (33.28±0.06 Ma) are the same age within uncertainty as the Bonanza Tuff. The Bonanza Tuff alternates from dacite to rhyolite (64-73% SiO₂) and from crystal-rich to crystal poor several times. The plutonic rocks vary from diorites to high silica granites (53-76% SiO₂) and texturally from fine to coarse grained. All Bonanza magmas erupted through significantly older basement rocks allowing us to identify crustal assimilation.

Preliminary interpretations of whole rock oxide and trace element data suggest that the volcanic and plutonic rocks have similar geochemical compositions and are equivalent and not complementary in composition. Sr, Nd and Pb isotope analyses also yielded a consistent overlap between both rock types suggesting the magmas were derived from an isotopically similar source. Some isotopic variation in Sr_i 0.70512-0.71170, εNd 6.6-9.1, Pb^206/204=17.44-17.87, Pb^207/204=15.47-15.51 and Pb^{208/204 =}37.25-37.61 implies that all magmas jointly underwent host rock assimilation as well. A Pb isochron calculation produced a 1.42 Ga source age for the assimilant. Details regarding the magma processes that produced lava and ignimbrite eruptions in the same magma plumbing and caused compositional oscillations in the ignimbrites are still under examination and require mineral scale geochemistry studies.