Understanding the evolution of compositional zoning in San Juan Volcanic Field ignimbrites using iron isotope ratios

Geology;igneous petrology;San Juan Volcanic Field;Colorado;caldera;ignimbrite;rhyolite;dacite;aplite, granodiorite;stable isotopes;iron;strontium;compositional zoning;trace elements;stratigraphy

Finlayson, Valerie A. ; Lundstrom ; Craig C.

关键词: Geology;    igneous petrology;    San Juan Volcanic Field;    Colorado;    caldera;    ignimbrite;    rhyolite;    dacite;    aplite, granodiorite;    stable isotopes;    iron;    strontium;    compositional zoning;    trace elements;    stratigraphy;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/26165/Finlayson_Valerie.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The San Juan Volcanic Field (SJVF) of southwestern Colorado is an erosional remnant of a large Tertiary aged calc-alkaline ignimbrite flare up. Previous work has led to detailed characterization of the volcanic stratigraphy and temporal evolution of many large calderas. Ignimbrites include both compositionally zoned rhyolitic to dacitic eruptions as well as monotonous intermediates. However, the petrogenetic origin of the volcanic field and the mechanisms of magma compositional evolution within magma chambers prior to eruptions remain largely unknown.Here I present new iron (Fe) isotope ratio data along with 87Sr/86Sr ratios and major and trace elements for 37 samples from the SJVF with the goal of improved understanding of the origin of zoned ignimbrites. I sampled in detail the zoned 33.2 Ma Bonanza Tuff as well as two intrusive units from the Bonanza Caldera. I also sampled in detail the 27.55 Ma Carpenter Ridge Tuff and collected representative samples from other major eruptions: the 28.02 Ma Fish Canyon tuff, the 26.9 Ma Nelson Mountain tuff, and the 26.9 Ma Rat Creek tuff (all ages given are from Lipman and McIntosh, 2008). Detailed X-ray mapping and compositional analysis were performed; compositions range from andesite to rhyolite, and granodiorite to aplite. Fe was purified from dissolved whole rock samples and analyzed for isotopic ratios using a high resolution MC-ICPMS.Results for δ56Fe are consistent with previous work, which has shown δ56Fe values of igneous rocks hover around the mean mafic earth value of 0.09‰ and increase in the most evolved high silica compositions. δ56Fe in the Carpenter Ridge ignimbrites shows a systematic increase in δ56Fe over much of the compositional range (68% to 78% SiO2) but importantly, increases to higher δ56Fe (up to 0.26‰) at the highest silica contents (74% and higher). Similarly, Bonanza intrusive rocks also follow a trend of increasing δ56Fe with silica content. Granodiorite samples exhibit slightly increased values (