23-7 Basin and Range Extension in the Lassen Volcano Backarc: Hybrid Lava Compositions in the Poison Lake Volcanic Chain, California

Session: Distributed volcanic fields of the southwestern U.S. (Posters)

Poster Booth No.: 14

Presenting Author:

John Chadwick

Authors:

Chadwick, John¹, Jones, Ella², Strobl, Claire³, Stinebrook, Jake⁴, Floss, Jen⁵, Clynne, Michael A.⁶, Ruth, Dawn Catherine Sweeney⁷, Bowden, Shelby⁸

(1) Dept. of Geology, College of Charleston, Charleston, SC, USA, (2) Dept. of Geology, College of Charleston, Charleston, SC, USA, (3) Dept. of Geology, College of Charleston, Charleston, SC, USA, (4) Dept. of Geology, College of Charleston, Charleston, SC, USA, (5) Dept. of Geology, College of Charleston, Charleston, SC, USA, (6) Volcano Science Center, US Geological Survey, CalVO MS 910, Menlo Park, CA, USA, (7) United States Geological Survey, Moffett Field, CA, USA, (8) Savannah River National Laboratory, Aiken, SC, USA,

Abstract:

The Lassen Volcanic Center in California is the southernmost young expression of Cascades arc volcanism and is surrounded by distributed small mafic volcanoes.  Lassen is also located in a region of Basin and Range continental extension, a process that also induces melting and magmas with different compositional properties from arcs, leading to a potential for hybrid magmas to develop.  The Poison Lake Chain of small monogenetic shields and flows lies about 30 km east of Lassen in the backarc region and these have chemical arc affinity, but they extend 20 km along a NE-SW trend reflecting Basin and Range normal fault orientations.  The Poison Lake Chain produced nine mapped eruptive units with distinct petrographic characteristics, erupted between about 100-110 ka (Muffler et al., 2011).  In this study we collected 39 rock samples from eight Poison Lake eruptive units, along with samples of regional basalts, to measure their major and trace element and Sr-Nd-Pb-Hf isotopic compositions to better understand their petrogenesis and to look for evidence of hybridization.  The major and trace element results supplement previous work (Muffler et al., 2011) and show the units occupy variable but partly overlapping ranges on major and trace element variation diagrams.  The Poison Lake lavas have SiO₂ from ~50-54% and range from mostly sub-alkaline basalts and basaltic andesites to minor higher alkali trachybasalts and basaltic trachyandesites. Trace element concentrations and ratios for the different eruptive units are correlated with their total alkalis and FeO*/MgO ratios.  Our preliminary results show Sr, Nd, and Hf isotope ranges are weakly or not correlated with major and trace elements and generally overlap regional basalt values, but Pb isotopes correlate with some major and trace element variability.  For example, ²⁰⁶Pb/²⁰⁴Pb is inversely correlated with Ba/Nb ratios and other large ion lithophile element to high-field strength element (LILE/HFSE) ratios, which may indicate the varying influence of the subduction mixing component. Electron microprobe data for plagioclase, olivine, and pyroxene phenocrysts also show variability in crystal cargo compositions among the flow groups (e.g. Fo in olivines varies by up to ~30%).  Preliminary Magma Chamber Simulator (MCS) software modelling reveals possible variable mixing between Basin and Range and subduction magmas, and potential variations in melting depth and melt fractions among the Poison Lake Chain lavas.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

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