Eruptive stratigraphy of the Tatara-San Pedro complex, 36 degrees S, southern volcanic zone, Chilean Andes: Reconstruction method and implications for magma evolution at long-lived arc volcanic centers

(2)DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 0100 3-5820, USA (US)-U-3 GEOLOGICAL SURVEY (MS 913), DENVER FEDERAL CENTER, DEN VER, CO 80225, USA The Quaternary Tatara-San Pedro volcanic complex (36 deg reesS, Chilean Andes) comprises eight or more unconformity-bound volcanic s equences, representing variably preserved erosional remnants of volcanic ce nters generated during similar to 930 ky of activity. activity. The interna l eruptive histories of several dominantly mafic to intermediate sequences have been reconstructed on the basis of correlations of whole-rock major an d trace element chemistry of flows between multiple sampled sections, but w ith critical contributions from photogrammetric, geochronologic, and paleom agnetic data. Many groups of flows representing discrete eruptive events de fine internal variation trends that reflect extrusion of heterogeneous or r apidly evolving magma batches from conduit-reservoir systems in which open- system processes typically played a large role. Long-term progressive evolu tion trends are extremely rare and the magma compositions of successive eru ptive events rarely lie on precisely the same differentiation trend, even w here they have evolved from similar parent magmas by similar processes. The se observations al-e not consistent with magma differentiation in large lon g-lived reservoirs, but they may be accommodated by diverse inter-actions b etween newly arrived magma inputs and multiple resident pockets of evolved magma and/or crystal mush residing in conduit-dominated subvolcanic reservo irs. Without constraints provided by the reconstructed stratigraphic relati ons, the framework for petrologic modeling would be far different. A well-e stablished eruptive stratigraphy may provide independent constraints on til e petrologic processes involved in magma evolution-simply on the basis of t he specific order in which diverse, broadly cogenetic magmas have been erup ted. The Tatara-San Pedro complex includes lavas ranging from primitive bas alt to high-SiO2 rhyolite, and although the dominant erupted magma tyke was basaltic andesite (similar to 52-55 wt % SiO2) each sequence is characteri zed by unique proportions of mafic, intermediate, and silicic eruptive prod ucts. Intermediate lava compositions also record different evolution paths, both within and between sequences. No systematic long-term pattern is evid ent from comparisons at the level of sequences. The considerable diversity of mafic and evolved magmas of the Tatara-San Pedro complex bean on interpr etations of regional geochemical trends. The variable role of open-system p rocesses in shaping the compositions of evolved Tatara-San Pedro complex ma gmas, and even some basaltic magmas, leads to the conclusion that addressin g Problems such as arc magma genesis and elemental fluxes through subductio n Zones on the basis of averaged or regressed reconnaissance geochemical da tasets is a tenuous exercise. Such compositional indices are highly instruc tive for identifying broad regional trends and first-order problems, but th ey should be used with extreme caution in attempts to quantify processes an d magma sources, including crustal components, implicated in these trends.