HR: 17:30h
AN: V32H-07    [PDF]
TI: Composite Chemostratigraphy of Lavas From the Casitas Shield, Descabezado Grande-Cerro Azul Volcanic Complex, Chilean Andes
AU: * Wulff, A H
EM: andrew.wulff@wku.edu
AF: Western Kentucky University, Dept. of Geography and Geology 1 Big Red Way, Bowling Green, KY 42103 United States
AB: The Descabezado Grande-Cerro Azul (DGCA) volcanic complex is located at approximately 35.5 degrees S in the transitional Southern Volcanic Zone (SVZ) of the Chilean Andes. The complex is characterized by two large latest Pliocene to early Holocene volcanic edifices and many smaller vents rising above a plateau comprising lavas of an earlier shield-building stage, the Casitas Shield. This plateau has been deeply encised, revealing stacks of lava flows exposed along the valley walls. More than 100 lava flows from eight vertical stratigraphic sections have been analyzed for complete major and trace element abundances. These have been compiled in a composite chemical stratigraphy representing much of the history of this stage of volcanism. At least twelve eruptive episodes were identified based on field observations (soil development, erosional features, mingled top and bottom rubble zones, etc.). Lavas flowed south and west from the proposed vent location(s) primarily filling paleovalleys, constrained to the west by topographic highs composed of tilted metasediments and a granodioritic stock. A number of long flows served as the basis in the field for physical correlations between sections, and were used to evaluate within-flow compositional variation. The top flows on the shield have a single date of .34 My. The composite chemical stratigraphy shows changes in lava compositions reflecting both differences in parental magmas and the secular dominance of certain petrogenetic processes over others. In general, lavas become more evolved through episodes 1-5, and are relatively constant in composition during episodes 6-8. More mafic lavas erupted during episode 9, including lavas that are among the most primitive in the SVZ. These lavas are characterized by high Cr (254-267 ppm) and MgO (8-8.3%), and low LILE, P2O5, Al2O3, Na2O, and K2O, and probably represent a different parental magma. Lavas erupted during episodes 10 and 11 are progressively more evolved, becoming similar to lavas erupted more recently from Volcan Cerro Azul. Several cross-cutting basaltic dikes are characterized by high MgO (11.7%), Ni (226 ppm) and Cr (776 ppm) contents, and along with high-Sr lavas (972-1061 ppm) erupted during episode 3, extend the known compositional ranges for mafic lavas in the SVZ. Most of the compositional diversity occurs during periods of apparent low eruption rate with few lava flows, perhaps including some from separate short-lived vents. Periods of higher eruptive activity are characterized by less compositional range and flows often appear in the field with flow tops mingled with the bottom of the next flow. This composite chemical stratigraphy will be compared to one constructed at the nearby Tatara-San Pedro complex.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting