Investigating crustal contamination: a case study from the Bolivian Altiplano, Central Andes

Claire McLeod*, Jon Davidson, Geoff Nowell 1 1 NCIET, Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK. * Corresponding Author: c.l.mcleod@dur.ac.uk During ascent towards the earth’s surface primary basaltic magma sourced from the mantle has the potential to interact with overlying crustal rocks. This leads to overprinting of source characteristics. Quantifying the mechanisms and timescales of crustal assimilation by differentiating magmas is key to improving our understanding of mantle source geochemistry. It is widely accepted that eruptives at continental arcs e.g. the Andes are the products of fractional crystallisation combined with intra crustal assimilation (e.g. Davidson et al., 2005). However, direct evidence of this interaction between mantle magmas and crustal rocks and the nature of potential crustal contaminants is rarely preserved. This is largely due to homogenisation and recrystallisation of mantle melts in shallow upper crustal magma chambers. On the Bolivian Altiplano to the east of the active Andean volcanic arc, monogenetic volcanic centres have erupted plagioclase-hornblende phyric lavas which are hosts to a range of crustal xenoliths. Entrained xenoliths contain evidence for partial melting in the form of quenched anatectic melts (now glass). The compositions of these quenched crustal melts hold key information regarding how ascending magmas interact with surrounding crustal wall rocks and how in turn, the crust melts. Analyses of glasses for their major element (EMP), trace element (LA-ICPMS) and Sr isotopic composition (microdrill-TIMS) aim to improve our knowledge of mass transfer during crust-magma interaction and highlight the limitations of current models of crustal contamination which employ bulk rock compositions in calculations e.g. (EC)AFC modelling, DePoalo, 1981; Spera & Bohrson, 2001. References Davidson, J.P., Hora, J.M., Garrison, J.M. and Dungan, M.A., (2005). Crustal Forensics in Arc Magmas: Journal of Volcanology and Geothermal Research, 140, pp. 157-170. DePaolo, D.J., (1981). Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters, 53, pp. 189-202. Spera, F. J., and Bohrson, W, A., (2001). Energy-Constrained Open-System Magmatic Processes I: General Model and Energy-Constrained Assimilation and Fractional Crystallization (EC-AFC) Formulation. Journal of Petrology 42, pp. 999-1018.