HR: 1330h
AN: V52A-0420    [PDF]
TI: Insights into off-axis volcanism in the Trans Mexican Volcanic Belt: B-Li-Be systematics of lavas from the Colima Graben and the Michouacan-Guanojuato Volcanic Field
AU: * Fuller, T
EM: tkfuller@mail.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., SCA 528, Tampa, FL 33620 United States
AU: Johnston, R
EM: raymj@tampabay.rr.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., SCA 528, Tampa, FL 33620 United States
AU: Mui, E
EM: lmui@tampabay.rr.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., SCA 528, Tampa, FL 33620 United States
AU: Ryan, J G
EM: ryan@chuma.cas.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., SCA 528, Tampa, FL 33620 United States
AU: Allan, J F
EM: jallan@nsf.gov
AF: Division of Ocean Sciences, National Science Foundation, 4201 Wilson Boulevard, Arlington, VA 22230 United States
AB: In the Trans-Mexican Volcanic Belt (TMVB) major calc-alkaline stratovolcanoes are associated with alkaline and calc-alkaline monogenetic centers in extensional basins. A past B-Be survey of TMVB lavas (Hochstaedter et al. 1996) suggested a distinction between lavas from stratocones and associated monogenetic centers, possibly related to differences in the mantle sources of these rocks. We are examining this distinction further, utilizing Li-Be-B systematics and other trace element variations to constrain the roles of melting, assimilation/ crystallization, and source differences in the genesis of TMVB lava types. Samples analyzed thus far include relatively primitive and evolved alkaline lavas from the Colima graben (minettes and lamprophyres); calc-alkaline andesites from Volcan de Colima; gabbroic xenoliths entrained in Colima lamprophyres; and alkaline and calc-alkaline lavas from small centers in the Michoacan-Guanojuato volcanic field (MGV), including suites (and associated crustal xenoliths) from Jorullo and Paricutin. TMVB alkaline lavas vary from 3-13 ppm Li, 1-5 ppm Be, and 1-12 ppm B. Calc-alkaline Colima andesites range from 5-18 ppm Li, 0.2-1 ppm Be, and 2-8 ppm B. B/Be ratios of Colima alkaline rocks are $<$5 and distinct from calc-alkaline lavas (7-12), due primarily to lower Be. Li/Yb ratios are high and similar in both lava types (4-16). Lavas from Jorullo widely in Li and B (8-20 ppm Li and 1-47 ppm B) suggesting incorporation of Li and B-enriched crustal materials (Jorullo xenolith: 25 ppm Li, 95 ppm B). Gabbroic xenoliths from the Colima graben are much lower in Li and Be ($<$3 ppm and 0.01 ppm). B systematics in TMVB monogenetic centers suggest limited H$_{2}$O-rich inputs from the slab, and point to a significant role for crustal assimilation during magma evolution. Li systematics record the anomalous crystallizing assemblages of TMVB alkaline lavas, and may, along with elevated Ba and Be contents, indicate mantle sources modified by a higher temperature slab component.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting