Eruptive Stratigraphy of the Tatara-San Pedro Complex, 36°S, Southern Volcanic Zone, Chilean Andes: Reconstruction Method and Implications for Magma Evolution at Long-lived Arc Volcanic Centers

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(1)JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 3. PAGES 555–626. 2001. Eruptive Stratigraphy of the Tatara–San Pedro Complex, 36°S, Southern Volcanic Zone, Chilean Andes: Reconstruction Method and Implications for Magma Evolution at Long-lived Arc Volcanic Centers MICHAEL A. DUNGAN1∗, ANDREW WULFF2† AND REN THOMPSON3 SECTION DES SCIENCES DE LA TERRE, UNIVERSITE´ DE GENE`VE, 13, RUE DES MARAIˆCHERS, 1211 GENE`VE 4,. 1. SWITZERLAND 2. DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003-5820, USA. 3. US GEOLOGICAL SURVEY (MS 913), DENVER FEDERAL CENTER, DENVER, CO 80225, USA. RECEIVED JULY 1, 1999; REVISED TYPESCRIPT ACCEPTED JULY 21, 2000. The Quaternary Tatara–San Pedro volcanic complex (36°S, Chilean Andes) comprises eight or more unconformity-bound volcanic sequences, representing variably preserved erosional remnants of volcanic centers generated during >930 ky of activity. The internal eruptive histories of several dominantly mafic to intermediate sequences have been reconstructed, on the basis of correlations of whole-rock major and trace element chemistry of flows between multiple sampled sections, but with critical contributions from photogrammetric, geochronologic, and paleomagnetic data. Many groups of flows representing discrete eruptive events define internal variation trends that reflect extrusion of heterogeneous or rapidly evolving magma batches from conduit–reservoir systems in which open-system processes typically played a large role. Long-term progressive evolution trends are extremely rare and the magma compositions of successive eruptive events rarely lie on precisely the same differentiation trend, even where they have evolved from similar parent magmas by similar processes. These observations are not consistent with magma differentiation in large long-lived reservoirs, but they may be accommodated by diverse interactions between newly arrived magma inputs and multiple resident pockets of evolved magma and/or crystal mush residing in conduit-dominated. subvolcanic reservoirs. Without constraints provided by the reconstructed stratigraphic relations, the framework for petrologic modeling would be far different. A well-established eruptive stratigraphy may provide independent constraints on the petrologic processes involved in magma evolution—simply on the basis of the specific order in which diverse, broadly cogenetic magmas have been erupted. The Tatara–San Pedro complex includes lavas ranging from primitive basalt to high-SiO2 rhyolite, and although the dominant erupted magma type was basaltic andesite (>52–55 wt % SiO2) each sequence is characterized by unique proportions of mafic, intermediate, and silicic eruptive products. Intermediate lava compositions also record different evolution paths, both within and between sequences. No systematic long-term pattern is evident from comparisons at the level of sequences. The considerable diversity of mafic and evolved magmas of the Tatara–San Pedro complex bears on interpretations of regional geochemical trends. The variable role of open-system processes in shaping the compositions of evolved Tatara–San Pedro complex magmas, and even some basaltic magmas, leads to the conclusion that addressing problems such as arc magma genesis and elemental fluxes through subduction zones on the basis of averaged or regressed reconnaissance geochemical. ∗Corresponding author. Telephone: +41-22-702-6630. Fax: +41-22320-5732. E-mail: michael.dungan@terre.unige.ch †Present address: Department of Geosciences, University of Iowa, Iowa City, IA 52242, USA. Extended dataset can be found at http://www.petrology. oupjournals.org.  Oxford University Press 2001.

(2) JOURNAL OF PETROLOGY. VOLUME 42. datasets is a tenuous exercise. Such compositional indices are highly instructive for identifying broad regional trends and first-order problems, but they should be used with extreme caution in attempts to quantify processes and magma sources, including crustal components, implicated in these trends.. Andean volcanism; Tatara–San Pedro complex; magmatic differentiation; volcanic stratigraphy; petrologic modeling KEY WORDS:. CONTENTS INTRODUCTION PREVIOUS AND CURRENT WORK GEOLOGIC SETTING AND PETROLOGIC CONTEXT METHOD OF STRATIGRAPHIC RECONSTRUCTIONS Physiography and sampling Chemical stratigraphy: whole-rock X-ray fluorescence analyses Analytical data Reconstruction criteria Revised 40Ar/39Ar chronology Petrologic and geochemical overview Compatible elements: closed vs open systems LILE variations and crustal contributions Relative enrichment of Y and HREE Limited isotopic variations Diversity among parent magmas PRE-ESTERO MOLINO SEQUENCES Munõz sequence (>930–825 ka) Quebrada Turbia sequence (QTS: >785–771 ka) ESTERO MOLINO SEQUENCE (EMS) Lower Estero Molino sequence (>620–600 ka) Middle (>579 ka) and upper (>495 ka) Estero Molino sequence Contact relations and lacunae Middle Estero Molino sequence: stratigraphy QTW12.22 and package B Packages C and D Package F QCNE.1 Middle EMS: summary Subdividing the upper EMS: Laguna Verde and Laguna Azul lavas Laguna Verde lavas (upper EMS): stratigraphy Non-correlative flows Packages H–I–J–K. 557 558 558 562 562 562 562 563 567 570 570 571 571 571 572 572 572 574 574 575 578 578 579 581 581 581 581 581 581 583 583 583. NUMBER 3. MARCH 2001. Laguna Verde lavas: summary Laguna Azul lavas (upper EMS): stratigraphy Laguna Azul lavas (upper EMS): lower eruptive episode Packages M and Q Package R and associated lavas Laguna Azul lavas (upper EMS): upper eruptive episode Packages S and U Package W Temporal evolution of the Laguna Azul lavas (upper EMS): implications Mixing vs fractionation-dominated evolution? Divergent open-system evolution paths and multi-component mixing PLACETA SAN PEDRO SEQUENCES Lower Placeta San Pedro sequence (LPSPS) Upper Placeta San Pedro sequence (UPSPS) ´ N TATARA VOLCA Contact relations, lacunae, and preservation Comparisons with previous work Lower Volcań Tatara Mingled Quebrada Turbia Dacite (QTD) Alpha Gamma Epsilon Theta Tatara Dacite and andesitic flows Tatara rhyolite Lower Volcań Tatara: summary Upper Volcań Tatara Package Non-correlative flows The Omega eruptive episode: insights into magmatic processes on the basis of variations in magma composition in the context of a high-resolution stratigraphic reconstruction Petrography Chemical stratigraphy Petrologic evolution Summary remarks ASSESSMENT OF THE RECONSTRUCTION METHOD IMPLICATIONS FOR PETROLOGIC MODELING STUDIES Implications of intra-package compositional variations Implications of intra-sequence compositional variations. 556. 585 586 587 587 587 587 587 587 589 589 589 590 590 591 595 595 596 596 596 597 601 603 603 604 604 604 606 606 607. 610 612 613 613 615 615 617 617 617.

(3) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Problems related to a fragmentary eruptive record 618 Summary remarks 618 MAGMATIC PROCESS RATES 619 SOURCES OF SILICIC MAGMAS 620 IMPLICATIONS FOR INTERPRETATIONS OF ALONGSTRIKE REGIONAL COMPOSITIONAL PATTERNS 621 ELECTRONIC APPENDICES (may be downloaded from the Journal of Petrology Web site at http:// www.petrology.oupjournals.org) Appendix I: Geological map of the Tatara–San Pedro Volcanic Complex Appendix II: Supporting data, graphs, and discussion for the upper Placeta San Pedro sequence Appendix III: Supporting data, graphs and discussion for packages 1–2–3, upper Volcań Tatara. INTRODUCTION Placing the eruptive record of a prehistoric volcanic center into a well-calibrated temporal framework is essential to meaningful modeling of the origin and evolution of its magmas. The first steps in reconstructing the eruptive history of an edifice are to define appropriate stratigraphic units and then to establish their relative ages. Highprecision geochronologic studies, with superposition relations as an internal check on the reliability of direct or indirect dating techniques, may provide sufficient temporal resolution to permit estimates of (1) the ages and durations of eruptive phases, (2) the lengths of time gaps between them, and (3) long-term volumetric eruption rates. In addition, they may eliminate or support correlations among similar but physically separate units (e.g. Bacon, 1983; Hildreth & Lanphere, 1994; Druitt et al., 2000). Whereas these tenets are self-evident, establishing a high-resolution stratigraphic framework for the entire history of any long-lived arc volcanic complex is a difficult and resource-intensive exercise. Apart from uncertainties associated with geochronological measurements, investigations of large volcanic centers are intrinsically limited in terms of attainable stratigraphic control, as a result of incomplete preservation of eruptive products and limited exposures. The fraction of a volcano’s eruptive products that is accessible for sampling depends on natural exposures that penetrate the flanks and summit region of the edifice,. and this fraction will be different for every volcano as a complex function of the non-uniform rates (temporally and spatially) of aggradation (growth and burial) and degradation (collapse, mass wasting, and erosive excavation of valleys), and of the original geometry and distribution of eruptive units. Even where the flanks of a large central-vent volcano may be sampled in deep valleys, on caldera walls, or on sector collapse scarps, individual vertical sections will inevitably be under-representative of the overall history of a volcanic center because of the consequences of edifice geometry. The following factors may play important roles: (1) the often eccentric positions of point-source and fissure vents relative to a volcano’s summit frequently result in correspondingly asymmetrical distributions of both effusive and pyroclastic eruption products; (2) lavas characterized by relatively high viscosities and/or low effusion rates may be restricted to vent-proximal locations (e.g. Rhodes, 1996); (3) lava flows are confined to topographic depressions generated either by volcanic construction or by degradation. The degree to which eruptive products accumulate asymmetrically will be much higher if distal flank vents are important and this may lead to greater overall stratigraphic and petrologic uncertainty. Thus, regardless of whether or not a significant fraction of an edifice has been removed, some or even most of the center’s eruptive products will be absent from any given exposed section. The distributions of recent lavas erupted from active volcanoes, which are commonly highly divergent in flow direction during short periods of time, suggest that typically 101–103 eruptions might be unrepresented between successive flows exposed in a particular vertical section depending on eruption frequency and vent position (e.g. Albare`de et al., 1997, p. 174). Edifice degradation may amplify the tendency for biased local records by selectively removing: (1) stratigraphic units emplaced immediately before eruptive hiatuses, (2) deposits on the pole-facing flank, where glaciation is the main erosion mechanism, and (3) unconsolidated pyroclastic deposits, particularly those of minor volume (Hackett & Houghton, 1989). Vent-proximal units, such as silicic domes, may be susceptible to short preservation times because of a tendency for explosive auto-destruction and the consequences of episodic summit crater formation. In light of these hindrances, how may petrologists devise strategies for the study of prehistoric arc volcanoes that provide a representative record of a volcano’s magmatic evolution in combination with the relative and absolute temporal constraints required for meaningful modeling of this evolution? The Tatara–San Pedro Project (Dungan et al., 1992) was undertaken, in part, to respond to this question. In the course of mapping and characterizing the well-exposed Tatara–San Pedro complex (TSPC: 930 ka to late Holocene; Fig. 1), the scientific team sampled 30 flow-by-flow. 557.

(4) JOURNAL OF PETROLOGY. VOLUME 42. vertical sections in eight deep glacial valleys around the complex (Fig. 2; Table 1). The chemistry and petrography of samples from 23 of these sections have been used as the basis for correlations, in conjunction with supporting paleomagnetic, photogrammetric, and geochronologic data, to determine the relative eruptive orders of flows in sequences of the Tatara–San Pedro complex that are amenable to such reconstructions, and for which we have adequate stratigraphic control. Volcań Tatara (>100–60 ka), the upper Placeta San Pedro sequence (>234 ka), and an older edifice remnant represented by the Estero Molino sequence (>620–495 ka) are discussed in some detail. Three large edifices that are not treated here are the Holocene Volcań San Pedro, Volcań Pellado, and the lavas of Cordoń El Guadal (Feeley & Dungan, 1996; Feeley et al., 1998). The strengths, weaknesses, and benefits of this approach are evaluated with respect to the general problem of reconstructing eruptive histories and refining temporal resolution within lava sequences at Quaternary arc volcanic centers. These results offer encouragement concerning the stratigraphic and chronological control that may be achieved for such edifices, but they also sound a loud cautionary note with respect to the pitfalls of petrologic modeling on the basis of poorly constrained stratigraphic relations.. PREVIOUS AND CURRENT WORK The Tatara–San Pedro complex (TSPC; 36°S, Southern Volcanic Zone, Chilean Andes) is a large arc-front Quaternary volcanic center that was first identified by Gonza´lez & Vergara (1962). Subsequent reconnaissance mapping during the first stage of this study (1984–1986), and regional studies by Munõz & Niemeyer (1984), led to the recognition of multiple edifices of variable age. The application of field mapping, photogrammetry, K–Ar and 40Ar/39Ar dating, and paleomagnetic data to the elucidation of the broad evolution of the TSPC has allowed us to define its overall eruptive and erosional history, to demonstrate that vent positions migrated though time, and to identify multiple unconformitybound volcanic sequences (Singer et al., 1997). Volcanic sequences are the first-order stratigraphic unit referred to throughout this paper. Each sequence is inferred to represent a separate volcanic edifice. Consecutive sequences are usually separated in time by lengthy lacunae, indicating that a continuous record has not been preserved. Sequences older than >150–200 ka are erosional remnants of volcanic constructs that have been greatly reduced in volume, mainly by glaciation. The volcanic stratigraphic nomenclature employed in this paper generally follows that of Singer et al. (1997), but a few modifications are proposed in this paper. A modified. NUMBER 3. MARCH 2001. geologic map of the TSPC, with a stratigraphic column and age data, is included in Electronic Appendix I (available on the Journal of Petrology Web site at http:// www.petrology.oupjournals.org) as Fig. I-1. Petrologic studies up to now provide evidence for a range of mafic parent magmas and multiple differentiation processes (Davidson et al., 1987, 1988; Ferguson et al., 1992; Singer et al., 1995, 1997; Feeley & Dungan, 1996; Feeley et al., 1998). Sampling for paleomagnetic studies has been conducted at a total of 243 sites within the TSPC, of which 30 within the Quebrada Turbia sequence record the Bruhnes–Matuyama magnetic polarity transition (Brown et al., 1994; Singer & Pringle, 1996).. GEOLOGIC SETTING AND PETROLOGIC CONTEXT Previous investigations of individual Andean Southern Volcanic Zone (SVZ) volcanoes (e.g. Calbuco, HickeyVargas et al., 1995; Lo´pez-Escobar et al., 1995; Villarrica– Lanin, Hickey-Vargas et al., 1989; Puyehue–Cordon Caulle, Gerlach et al., 1988; Mocho–Choshuenco, McMillan et al., 1989; Sollipulli, Murphy & Brewer, 1994; Gilbert et al., 1996; Lonquimay, Moreno & Gardeweg, 1989; Antuco, Lo´pez-Escobar et al., 1981; Nevado de Longavı´, Gardeweg, 1981; Laguna del Maule–Puelche, Frey et al., 1984; Hildreth et al., 1999; Calabozos caldera, Hildreth et al., 1984; Grunder, 1987; Grunder & Mahood, 1988; Quizapu, Hildreth & Drake, 1992; Planchon– Peteroa–Azufre, Tormey et al., 1995) combined with regional compositional profiles on the basis of multicenter geochemical traverses along and across the arc (e.g. Lo´pez-Escobar et al., 1977, 1992, 1993; De´ruelle, 1982; Harmon & Hoefs, 1984; Hickey et al., 1984, 1986; Lo´pez-Escobar, 1984; Stern et al., 1984, 1991; Notsu et al., 1986; Munõz & Stern, 1987, 1989; Futa & Stern, 1988; Hildreth & Moorbath, 1988, 1991; Stern, 1988, 1989, 1991a, 1991b; Davidson, 1991; Tormey et al., 1991) provide the petrologic context of this study. This body of literature addresses issues central to an understanding of continental arc volcanism: (1) What are the contributions of asthenospheric and subcontinental lithospheric mantle sources to the petrogenesis of the parental basaltic magmas? (2) Do variations in the age of the subducted plate, thickness of arc-trench sediment, and/ or vigor of subduction erosion play roles in along-arc variability? (3) How do differentiation mechanisms and crustal contributions to the petrogenesis of evolved magmas vary as functions of crustal thickness, density, structure, age, and lithologic character along the arc? The purpose of this paper is not to respond directly to the questions listed above, but to refine the geologic framework of the TSPC so that ultimately the regional. 558.

(5) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Fig. 1. Location and general tectonic setting of the Tatara–San Pedro complex in the Southern Volcanic Zone, Chilean Andes. This modified version of fig. 1 from Hildreth & Moorbath (1988) illustrates the arc segmentation scheme discussed in the text: TMS, Tupungato–Maipo segment 33–34·2°S; PTS, Palomo–Tatara segment 34·7–36°S; LOS, Longavı´–Osorno segment 36·2–41·5°S (SB, segment boundary). Symbols used to distinguish volcanoes of the three segments (Μ, Ε, Χ) are repeated in Fig. 30. The oldest age of exposed basement units is indicated as Paleozoic (Pz) or Mesozoic (Mz).. trends may be assessed in light of a process-oriented evaluation of the contributions of diverse mantle and crustal sources at a single long-lived center. In this context, the setting of the TSPC is briefly reviewed. A modified version of fig. 1 of Hildreth & Moorbath (1988) is reproduced as Fig. 1 of this paper to illustrate the arc segmentation scheme of Wood & Nelson (1988). The narrow northern chain (33–34·2°S; Tupungato–Maipo segment) is located entirely on exceptionally thick and relatively old crust along the Andean crest. To the south (34·7–36°S; Palomo–Tatara segment), the arc broadens and a N20°E chain of frontal arc centers lies to the. west of the continental divide. The southernmost chain generally lies near the topographic front of the Cordillera, which becomes increasingly less well defined to the south, and it extends from Nevado de Longavı´ to Osorno and Calbuco (not shown in Fig. 1), or beyond (36·2–42°S; Longavı´–Osorno segment). The TSPC is the southernmost volcanic center of an arc segment that is intermediate in terms of geography, Bouguer gravity (hence apparent crustal thickness; see Hildreth & Moorbath, 1988), and some geochemical characteristics between the highly contrasting Tupungato–Maipo and Longavı´–Osorno segments.. 559.

(6) JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 3. MARCH 2001. Fig. 2. Locations of sampled sections (e.g. ΧΧΧ ESPE1) superimposed on a highly generalized geologic map of the TSPC. Geography-based acronyms designating sampled sections are explained in Table 1, along with additional information. The apparently short lengths of many traverses reflect sampling on canyon walls. Abbreviations of geographic features employed on this map: (1) drainages (clockwise from the northwest): ESPN, Estero San Pedro del Norte; QT, Quebrada Turbia; CM, Cajoń de Munõz; CH, Cajoń de Huelmul; RC, Rio Colorado; EP, Estero Pellado; ESP, Estero San Pedro; EM, Estero Molino; QC, Quebrada Castillo; (2) ridges and plateaux (clockwise from the northwest): ˜ , Cordoń Los N ˜ irales. [Refer to figs CLY, Cordoń Las Yeguas; PSP, Placeta San Pedro; CLL, Cordoń Los Lunes; CT, Cordoń Tatara; CLN 2–4 in Singer et al. (1997), and Figs 3–6 and 9 in this paper for additional illustrations bearing on stratigraphic relations.] A revised version of the geologic map of the TSPC [modified from Singer et al. (1997)] is included in Electronic Appendix I (Fig. I-1).. 560.

(7) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Table 1: Sequences and stratigraphic sections: abbreviations, geography, and characterization Section∗. Location and description [auxiliary data]. Sequences†. QTW10.1–8. Quebrada Turbia—West wall (medial) [pmag, 40Ar/39Ar]. MS (Sin Nombre), QTS, middle and upper EMS,. QTW11.1–38. Quebrada Turbia—West wall (northern–distal) [pmag, 40Ar/39Ar]. QTW12.0–44. Quebrada Turbia—West wall (southern–proximal). lower, middle, and upper EMS. [pmag, 40Ar/39Ar]. L-TAT. UPSPS QTS, LPSPS, UPSPS. QTW14.1–9. Quebrada Turbia—West wall (medial–proximal). middle EMS. CLL.1–9 (plus). Crest of Cordoń Los Lunes. UPSPS (plus LPSPS). QTE.1–8. Quebrada Turbia—East wall (basal flows–distal). MS. EML.0–23. Estero Molino—Lowest flows (upper NE wall) [pmag, 40Ar/39Ar]. lower, middle, upper EMS. QCSE.1–14. Upper Quebrada Castillo—Southeastern head wall [ 40Ar/39Ar]. upper EMS, UPSPS, L-TAT. QCNE.1–17. Upper Quebrada Castillo—Northeastern head wall [ 40Ar/39Ar]. middle and upper EMS, LPSPS, UPSPS. LV.1–21. North rim of Quebrada Castillo, beginning near Laguna Verde,. middle and upper EMS,. then eastward to rim of Quebrada Turbia [pmag, 40Ar/39Ar]. LPSPS, UPSPS. QC98.1–4. Floor of Quebrada Castillo sampled in 1998. middle EMS. ESPN.1–12. Estero San Pedro del Norte (upper east wall) [ 40Ar/39Ar]. lower and upper EMS, UPSPS,. EMU1–3. Drainage on far NW flank, well removed from Estero San Pedro. L-TAT. Estero Molino—Upper south wall [pmag, 40Ar/39Ar]. L-TAT and U-TAT. Three linked sections: EMU1.1–23—EMU2.1–5—EMU3.1–30 ESPW3.1–23. Estero San Pedro—West wall (northern-proximal). L-TAT. ESPW2.1–23. Estero San Pedro—West wall (medial). L-TAT. ESPW4.1–26. Estero San Pedro—West wall (southern-distal). L-TAT and U-TAT. TR.1–9. Unnamed western extension of Cordoń Tatara (’Tatara Ridge’). U-TAT. ESPE1.1–39. Estero San Pedro—East wall (distal) [pmag, 40Ar/39Ar]. L-TAT and U-TAT. ESPE2.1–10. Estero San Pedro—East wall (still more distal). L-TAT. BP.1–5. Unnamed valley just north of Estero Molino (’Bottomless Pit’). L-TAT and U-TAT. UEP7.1–14. Upper Estero Pellado—western head wall. U-TAT. EPW5.1–20. Estero Pellado (lower)—West wall. U-TAT. EPW6.1–13. Estero Pellado (lower)—West wall [pmag, 40Ar/39Ar]. U-TAT. ∗Sampled sections and flow numbering: individual samples are designated by section (geographic acronyms listed above) and consecutive numbers following a period (e.g. EML.1). Where specific reference is made to a group of consecutive samples, the first and last samples of this group are hyphenated (e.g. EML.1–4). †Sequence abbreviations (used in Table 2): MS, Munõz; QTS, Quebrada Turbia; EMS, Estero Molino; LPSPS, Lower Placeta San Pedro; UPSPS, Upper Placeta San Pedro; L-TAT, Lower Volcań Tatara; U-TAT, upper Volcań Tatara. Other abbreviations: ArrQT, samples collected in upper headwaters of Quebrada Turbia; QTD, samples collected from isolated dacitic lava on western rim of upper Quebrada Turbia near section QTW12; FBD, dacitic lava above Estero Molino; pmag, paleomagnetic data. Sections BP, EMU4, and ESPE3 (Fig. 2) are traverses in which sample numbers do not record stratigraphic order. All other sections were sampled from base to top (numbers increase up-section) on a 100% flow-by-flow basis.. Although this part of the arc could be considered petrologically transitional (Tormey et al., 1991), Wood & Nelson (1988) argued that the volcanoes of each segment correspond to separate petrologic populations, and this point of view is reinforced by new data from the TSPC. One example of the importance of segmentation of the SVZ lies in the observation that many evolved TSPC magmas more closely resemble those of volcanoes to the. north, which were constructed on thicker crust, than those to the south, which lie on crust that does not appear to be substantially thinner than the crust at 36°S. Pre-Pliocene rocks exposed beneath the frontal arc between 37 and 35°S (referred to as basement) are late Mesozoic–Tertiary sedimentary and volcanic rocks and Tertiary granitoids. The TSPC overlies a deformed Tertiary sequence of dominantly volcanic and volcaniclastic. 561.

(8) JOURNAL OF PETROLOGY. VOLUME 42. rocks that are intruded by two granitic plutons (Huemul and Cerro Risco Bayo), both of which have been dated at >6·5 Ma by 40Ar/39Ar (Nelson et al., 1999). Apart from a suite of partially melted granitic xenoliths present in a pyroclastic deposit on Volcań Pellado, the vast majority of xenoliths in TSPC lavas are mafic plutonic rocks (troctolite, gabbro, and norite; Costa, 2000).. METHOD OF STRATIGRAPHIC RECONSTRUCTIONS Physiography and sampling Along the northwest margin of Placeta San Pedro, a large plateau on the northwest flank of the TSPC (Figs 2–5), lavas of the pre-Volcań Tatara sequences, which flowed mainly from east to west, banked against a north– south basement ridge (Cordoń Las Yeguas; CLY in Fig. 2). The confinement of these units within a perched basin contributed to partial preservation during multiple episodes of glacial valley incision. None the less, the overlying north flank of Volcań Tatara has been largely stripped away by ice flowing to the west and north (Figs 2 and 5), exposing underlying units. Pre-Volcań Tatara sequences are absent on the south flank of the complex, with the exception of Cordoń El Guadal lavas. The southwest flank of Volcań Tatara thins to the west against ˜ irales (CLN ˜ in Fig. 2), the southward Cordoń Los N continuation of Cordoń Las Yeguas (CLY in Fig. 2). Important consequences of these paleotopographic accidents are that thick sections of pre-Volcań Tatara sequences on the northwest flank are dissected by Quebrada Turbia (QT) and other drainages incised into Placeta San Pedro (PSP), and that much of the history of Volcań Tatara is exposed in Estero San Pedro (ESP), Estero Molino (EM) and Quebrada Turbia. Many of the early volcanic sequences of the TSPC (>200 ka), plus the northern flank of Volcań Tatara (>100–60 ka), crop out on and around the margins of Placeta San Pedro. The Munõz, Quebrada Turbia, Estero Molino, and lower and upper Placeta San Pedro sequences have been sampled in 12 flow-by-flow sections located in four valleys (see Fig. 2 and Table 1 for section locations and acronyms employed in the text), comprising 250 analyzed samples. Quebrada Turbia (south to north; QTW12, QTW14, QTW10, QTW11), which heads just below the eroded vent region of Volcań Tatara, marks the eastern limit of Placeta San Pedro. From south to north, the east to west drainages of Estero Molino (EML) and Quebrada Castillo (QCSE, QCNE, LV, QC98), and the south to north canyon of Estero San Pedro del Norte (ESPN) are incised into its western margin. Additional samples from these early sequences (CLL, QTE, and assorted localities) were collected from Cordoń Los Lunes,. NUMBER 3. MARCH 2001. the ridge separating Quebrada Turbia and Cajoń Munõz (CM). The northwest flank of Volcań Tatara overlies the Estero Molino and Placeta San Pedro sequences on southern Placeta San Pedro (Figs 3–5), but is in contact with basement between Estero Molino and Rio de la Puente (Fig. 6). The west flank of Volcań Tatara is divided into northern and southern domains by the pyroclastic and intrusive near-vent facies exposed on eastern Cordoń Tatara, in the divide between Estero San Pedro and Quebrada Turbia (VF; Fig. 3), across which stratigraphic units cannot be readily traced physically; the north slope of Cordoń Tatara is covered almost entirely by talus (Figs 3–4) and the south face is an inaccessible cliff (Fig. 6). Approximately 280 Volcań Tatara samples have been analyzed from 13 sections, including: (1) to the NNW in the upper walls of Quebrada Turbia (QTW12), Estero Molino (EMU1–3), in an adjacent valley (BP), and in the largely moraine- and snowcovered region between Estero Molino and Quebrada Turbia (EMU4); (2) to the SSW in six sections within Estero San Pedro (ESPW2–ESPW3–ESPW4, ESPE1– ESPE2, and TR); (3) on the east wall in upper (UEP7) and lower (EPW5, EPW6) Estero Pellado. Proximal and distal sections in Estero San Pedro (ESPW3 and ESPW4) correspond to sections 1 and 2 of Ferguson et al. (1992, fig. 4). Lava samples were collected almost exclusively on a flow-by-flow basis from continuous sections exposed on steep canyon walls. The vast majority of samples are from very fresh non-vesicular flow interiors lacking any macroscopic or microscopic evidence of alteration such as secondary phases in vesicles or veins, groundmass oxidation, or even iddingsitized olivine. Variably but mildly altered samples that constitute exceptions to these rules were collected from the lower 10 lavas of the QTW12 and ESPE1 sections, where groundwater flow has been concentrated at the interface between permeable lavas and relatively impermeable basement. Although neither the internal stratigraphic relations nor the petrology–geochemistry of Cordoń El Guadal (Feeley & Dungan, 1996; Singer et al., 1997; Feeley et al., 1998), Volcań Pellado (immediately preceded Volcań Tatara), and Volcań San Pedro (Holocene) are discussed in detail, their compositions are shown in Figs 7 and 8 for comparative purposes, as these edifices are volumetrically and petrologically significant.. Chemical stratigraphy: whole-rock X-ray fluorescence analyses Analytical data. 562. The geochemical data presented here were obtained at the University of Massachusetts following the methods.

(9) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Fig. 3. Oblique aerial view (roughly due south) looking into upper Quebrada Turbia. Annotations (red) illustrate contact relations among multiple volcanic sequences and basement outcrops (TVS). Important points: (1) the generally lozenge-shaped cross-sections of the older sequences exposed within Quebrada Turbia reflect the fact that most of these units accumulated by the emplacement of lavas flowing from west to east (left to right) along the axes of east–west-trending paleovalleys (i.e. with axes approximately at right angles to present-day Quebrada Turbia), and (2) the thin remnant veneer of lower Volcań Tatara lavas plastered against the upper wall of Quebrada Turbia reflects the re-excavation of a paleovalley with essentially the same orientation (south–north) and position as present-day Quebrada Turbia. Stratigraphic units and acronyms (black): Munõz [Los Lunes Rhyolite (LLR) and Sin Nombre lavas], Quebrada Turbia (QTS), Estero Molino (lower–middle–upper EMS), lower Placeta San Pedro (LPSPS), and upper Placeta San Pedro (UPSPS), lower Volcań Tatara (LTAT) including Tatara Dacite (Tat Dac) and the vent facies of Volcań Tatara (VF), upper Volcań Tatara (UTAT), and Volcań San Pedro. Labeled geographic features (white) include Cordoń los Lunes (CLL) to the east of Quebrada Turbia, Placeta San Pedro (PSP) to the west, the drainage of Quebrada Castillo (QC), and Laguna Azul (LA). Two common points also visible in Figs 4 and 6 are marked on the crest of Cordoń Tatara (Μ, yellow). The interbedded clastic units (sed) separating various units of the Estero Molino sequence on the west wall of Quebrada Turbia are hachured for emphasis: these bench-forming units appear anomalously thick from this perspective (see Fig. 9). Surficial deposits are ignored for the purpose of simplifying the geologic relations. The summit elevation of San Pedro is 3621 m. The ‘intersection’ of Quebrada Turbia with the bottom of the image is at >1850 m and it lies >5 km NNW of the San Pedro summit.. of Rhodes (1988). Information concerning the precision of major and trace element analyses in this laboratory can be found in table 2 of Rhodes (1996). Major elements plus Nb, Zr, Sr, Rb, Ba, Y, Ni, Cr, V, Ce, and Pb concentrations are reported in Table 2 for selected samples and for average chemical compositions of flow packages [point (1) below]. Total iron is reported as Fe2O3∗. Samples that have been analyzed for Cs, Sc, Cr, Co, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, and Th by instrumental neutron activation analysis (INAA) at MIT (F. A. Frey & M. A. Dungan, unpublished data, 2000) are marked by an asterisk in Table 2. Specific references to analyses in Table 2 throughout the text refer to the index number at the head of each column. Whole-rock analyses, including INAA data, are provided in spreadsheet format in Electronic Appendix I. The entire TSPC dataset is arranged by sampled section in TSPC.xls, whereas reconstructed sequences are presented in stratigraphic order in QTS.xls, EMS.xls, UPSPS.xls,. LTAT.xls, and UTAT.xls. The TSPC comprises mainly lavas ranging from medium-K basalt and basaltic andesite to high-K dacite and rhyolite. The following ranges of wt % SiO2 are used to define rock names: basalt <52, basaltic andesite 52–56, andesite 56–63, dacite 63–70, rhyolite >70, high-SiO2 rhyolite 72–76.. Reconstruction criteria. 563. Internal reconstructions of multiple volcanic sequences of the TSPC have been rendered feasible by the presence of widespread chemically distinctive packages of lavas [point (1) below] that are correlative between multiple sampled sections on the basis of chemistry and petrography. Although some packages were identified as physically separate but correlative units in the field, the approach of tying flow-by-flow sampling traverses to images of canyon walls and establishing correlations among them is more efficient and potentially more certain than field-mapping of such units, which are commonly.

(10) JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 3. MARCH 2001. Fig. 4. Oblique aerial view (roughly E 20°S) of the west flank of Volcań San Pedro and underlying units. Foreground: Laguna Azul is a moraine-dammed lake perched on the drainage divide between Quebrada Castillo and Estero Molino. The major unconformity should be noted in Estero Molino at the base of lower Volcań Tatara (LTAT), which completely cuts out the underlying sequences (UPSPS, upper Placeta San Pedro; LPSPS, lower Placeta San Pedro; EMS, Estero Molino). In contrast, the basal contact of lower Volcań Tatara is quasi-conformable with these earlier sequences just to the ENE. The upper Volcań Tatara lavas (UTAT, EMU3.15–30) on the upper slopes of the north wall of Estero Molino are a remnant valley-wall veneer that originally filled a paleovalley cut into the Tatara Dacite and other lower Volcań Tatara units. The elongate unit forming the ridge crest on the divide between Laguna Azul and Estero Molino (CF, causeway flow) is inferred to be valleyfilling unit near the base of lower Volcań Tatara (package 0) on the basis of field relations and geochemistry (no geochronological confirmation). The original elevation of the beheaded summit region of Volcań Tatara was probably higher than the current summit elevation of Volcań San Pedro. Also labeled are two common points on the crest of Cordoń Tatara that are similarly marked in Figs 3 and 6 (Μ, yellow). Surficial deposits are ignored for the purpose of simplifying the geologic relations. Laguna Azul (2423 m) lies >5·5 km to the NW of San Pedro summit (3621 m).. only subtly different in hand specimen, along canyon walls and from valley to valley. Sample traverse selection is a key element in the success of this approach. Our experience favors acquisition and photogrammetric analysis of canyon-wall images before sampling, to maximize the utility of each traverse. For example, subtle or even marked unconformities between sequences and packages that are evident from a distance are rarely so obvious when one is working on a steep rock face. We have found that representing stratigraphic subdivisions at the level of packages on a 1:25 000 geologic map would be virtually impossible without first compiling the integrated information from stereo images of canyon walls. The reconstruction technique has been formulated on the basis of the following reasoning, criteria, and limitations: (1) All sampled sections include groups of consecutive lava flows with closely similar [see points (3) and (4)] or virtually identical whole-rock chemistry and petrography that are unquestionably distinct from underlying and. overlying flows. Where such groups of cogenetic flows may be correlated among multiple sections on the basis of their chemistry, or where a single distinctive flow unit is demonstrably laterally extensive, these lavas have been assembled as informal stratigraphic units herein referred to as flow packages (generally <1 km3), which are taken to represent discrete eruptive events. By analogy with historic observations of stratocone activity, eruptive events are inferred to represent continuous or intermittent effusion of magma from a reservoir–conduit system over periods ranging from weeks to decades. In cases where two adjacent groups of flows have similar chemical signatures, but they are distinguished by a small, abrupt shift in composition, separate packages also have been defined and their mutual affinities are noted. Where consecutive packages ± non-correlative flows [point (2)] are apparently closely related petrologically (e.g. to a particular parent magma type), the ensemble is tentatively referred to as the record of an eruptive episode, implying successive eruptive events during a relatively restricted. 564.

(11) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Fig. 5. Oblique aerial view (roughly N 50°E) of the Estero Molino and Quebrada Castillo drainages (LA, Laguna Azul; LV, Laguna Verde) illustrating stratigraphic subdivisions within the Estero Molino sequence (LEMS, lower Estero Molino; MEMS, middle Estero Molino; UEMS, upper Estero Molino). Also shown: LPSPS, lower Placeta San Pedro sequence; UPSPS, upper Placeta San Pedro sequence; LTAT, lower Volcań Tatara (CF, causeway flow, see Fig. 4). Two roughly east–west-trending paleovalleys with ages of >620 ka, to which the present-day orientations and locations of Estero Molino and Quebrada Castillo closely correspond, trapped relatively thick accumulations of lower and middle Estero Molino lavas, respectively; that is, the basement high that constitutes the present-day drainage divide between Quebrada Castillo and Estero Molino was established as a topographic feature before emplacement of the Estero Molino sequence. The extensive erosional removal of the northern flank of Volcań Tatara (LTAT) can be appreciated by noting that a thin remnant of lower Tatara flows (Fig. 2) overlies the upper Placeta San Pedro sequence (UPSPS) just off the left edge of this image, >5 km to the north of the present limit of preservation of lower Tatara flows between Estero Molino and upper Quebrada Turbia. Surficial deposits are ignored for the purpose of simplifying the geologic relations. The distance between the Laguna Verde (2072 m) and Laguna Azul (2423 m) is >2·6 km. Cordon Los Lunes can be seen beyond Quebrada Turbia.. but undefined period; such inferences are subject to further evaluation. (2) Conversely, some magma compositions were sampled at only one locality. As all sampled sections are incomplete in the sense that a significant proportion of the eruptive products of the relevant center(s) are not present, the stratigraphic positions of these unique flows usually are imperfectly defined with respect to certain other flows or flow packages in other sections, which are, none the less, constrained to be close in time. These noncorrelative flows (NC in Table 2) are discussed, as they are equally important with respect to magmatic evolution, but for the purpose of establishing a consistent and meaningful nomenclature they have been accorded package status only in special circumstances. There is potential for circular reasoning in generating stratigraphic reconstructions on the basis of chemistry, in the sense that there is a logical but subjective tendency to ‘resolve’ uncertainties by placing apparently cogenetic flows in. stratigraphic proximity where a lack of constraints permits multiple interpretations. Despite the high density of sampling in this study, and abundant supporting geochronologic and paleomagnetic data, such dilemmas do arise [point (5)], and they are noted accordingly. (3) Natural variability within the same lava flow or package [e.g. arising from zoned or heterogeneous magma reservoirs, eruption associated with incomplete mixing of two or more magmas, heterogeneous distribution of phenocrysts or fragments of cognate cumulate at the centimeter scale, or dispersed fragments of crustal xenoliths—point (4)] frequently renders reported lava compositions from single packages non-identical (i.e. outside analytical uncertainties). Decisions about intra-section flow groupings, and their correlative status among sampled sections, have been made on the basis of an empirical inspection of chemical data and thin sections, rather than on statistical criteria, because small variations do not negate fundamental petrogenetic relations among. 565.

(12) JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 3. MARCH 2001. Fig. 6. Oblique aerial view (roughly N 30°W) of the upper southwest flank of Volcań Tatara and the upper Estero San Pedro drainage. The two prominent triangular cliff faces at the head of the valley underlie Cordoń Tatara; common points similarly marked in Figs 3 and 4 should be noted (Μ, yellow). Annotations in red illustrate the contact relations between upper and lower Volcań Tatara and Volcań San Pedro, as well as the presence of basement. It should be noted that the older units present on the northwest flank are absent, presumably as a result of erosion; hence, lower Tatara lavas (LTAT) are directly in contact with Tertiary basement (TVS). The large ridge in the right foreground (divide between Estero San Pedro and Quebrada Honda) comprises upper and lower Tatara lavas overlain by glacial till. No basement is exposed between the floor of Estero San Pedro and west wall of Estero Pellado, except beneath the most distal flanks, indicating that the southern flank of Volcań ˜ irales (CLN ˜ ) and Cordoń El Guadal (Fig. 2). Surficial deposits are ignored for the Tatara accumulated in a broad basin between Cordoń Los N purpose of simplifying the geologic relations. The eastern yellow triangle on Cordoń Tatara (3224 m, Fig. 2) is located >5 km due north of the intersection of Estero San Pedro and the lower left corner of the image (>1500 m).. multiple lavas generated during the same eruptive event when the data are viewed from the perspective of covariations among multiple elements and elemental ratios complemented by petrographic observations. (4) Diverse trends with respect to eruptive order are recognized within flow packages comprising basaltic andesitic to andesitic lavas. In some cases, consecutive flows display no significant chemical variations, or no systematic variations as a function of stratigraphic position. More commonly, they are marked by progressive changes such that the early and late flows are significantly different; flow-to-flow changes are typically on the order of analytical uncertainties, without major internal discontinuities in either major or trace elements. Up-section trends toward more evolved and less evolved magmas are observed, and reversals of these internal trends are occasionally present. In all cases where internally variable packages have been identified, intra-package chemical variations are of a lesser magnitude than those that distinguish such a package from underlying and overlying packages or non-correlative flows.. (5) Eruptive episodes are rarely characterized by progressive trends toward greater degrees of evolution from a single parent magma, implying that the petrologic differences between many adjacent flow packages reflect discrete parent magma batches and unique evolution paths. If this inference is correct, duplication of the major and trace element characteristics of one magma batch by another that differs significantly in time should be rare, because multiple variables are involved. There are, none the less, examples of compositionally similar lavas that are unequivocally separated by intervening units; these can normally be distinguished because the concentrations of one or a few elements (and elemental ratios) are different between them. There is, in addition, one example of lavas from two different volcanic sequences that are nearly indistinguishable for major and trace elements. These occurrences may be explained as fortuitous repetitions of parent magma compositions and evolutionary histories, but they are impossible to rationalize as the persistence of a mafic magma body that remained unchanged during >105 years. This observation. 566.

(13) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. Fig. 7. Summary diagrams comparing the samples of different sequences as a function of mg-number [100 × Mg/(Mg + Fet)] vs SiO2 wt % (a–c), and K/Rb vs SiO2 wt % (d–f ). Colors for pre-Volcań Tatara units are keyed to colors used in Fig. 9. The limiting differentiation trends are reference curves corresponding to basalt–rhyolite mixing (10% increments) and to an approximate closed-system fractional crystallization path (FC) inferred from Volcań Puyehue (Gerlach et al., 1988). The parent magma is a relatively primitive (Fo85) basalt (QTW14.9; Table 2c, 53) from the middle Estero Molino sequence, and the rhyolitic end-member is the most evolved composition in the Los Lunes rhyolite of the Munõz sequence (QTPum; Table 2a, 8). Divergence among possible fractional crystallization paths will depend on parent magma composition and conditions of differentiation (P, f O2, H2O), and differentiation trends reflecting AFC will describe trajectories intermediate between the FC and mixing trends. Data points from the Munõz sequence (mafic Sin Nombre lavas, Munõz dacite, and Los Lunes rhyolite: Μ, black) are shown individually in (a) and (d). The inset in (d) illustrates the relatively low K/Rb of the basement granitoids (inverted red triangles in pink shaded field) compared with the volcanic trend.. emphasizes the care required in utilizing chemical criteria in the reconstruction of eruptive histories in the absence of independent constraints derived from physical stratigraphy, petrography, paleomagnetic data, and/or geochronology; that is, chemical similarity, or even virtual identity, is not a guarantee of stratigraphic equivalence.. Revised 40Ar/39Ar chronology A major emphasis of this paper is applying the reconstruction method outlined above to the Estero Molino sequence and to Volcań Tatara: both are thick, widespread and complex. New 40Ar/39Ar ages have been acquired from these sequences to clarify stratigraphic. 567.

(14) JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 3. MARCH 2001. Fig. 8. Summary diagrams comparing the samples of different sequences [Rb/Y–SiO2 wt % (a–c) and Y–Ba/Y (d–f )], with limiting mixing and fractional crystallization trends (as defined by Volcań Puyehue lavas). Data points from the Munõz sequence (mafic Sin Nombre lavas, Munõz dacite, and Los Lunes rhyolite: Μ, black) are shown individually in (a) and (d). Inset in (a) illustrates the generally elevated Rb/Y in basement granitoids (inverted red triangles in pink shaded field) relative to the TSPC lavas. Inset in (d) illustrates the pronounced Y enrichments in increasingly evolved Puyehue lavas compared with the arrays defined for different sequences of the TSPC (d–f ). The inset in (f ) emphasizes the variable Y enrichment trends in TSPC lavas (Y–SiO2).. relations where uncertainties or conflicts existed as a result of K–Ar determinations characterized by (1) low radiogenic argon yields (<5%) and consequently low precision, (2) discordant groundmass–whole-rock ages (groundmass younger), and/or (3) out-of-sequence ages. It is probable that argon loss and xenocrystic argon contamination both affected the K–Ar measurements. Argon loss from glass-bearing groundmass, leading to disturbed Ar-release spectra and anomalously young ages, was encountered during dating of some groundmass separates. These results will be presented in detail in another paper, but the revised conclusions concerning the ages of the dated sequences are summarized here (Fig. 9).. The Estero Molino sequence is subdivided on the basis of chemical and 40Ar/39Ar age constraints into lower (>620–600 ± 20 ka), middle (579·3 ± 10·5 ka), and upper (493·7 ± 10·8 ka) lavas. Thus, the upper age limit of the Estero Molino sequence has been revised from <320 ka to >495 ka, and the age of the intra-Estero Molino sequence lacuna (middle–upper EMS) is now constrained to be much shorter than previously estimated. K–Ar ages previously determined for Volcań Tatara lavas range from 90 ± 19 ka to 19 ± 13 ka (Singer et al., 1997). New 40Ar/39Ar age determinations for much of lower and upper Tatara yield concordant plateau and inverse isochron ages in the range of 100–60 ka with variable uncertainties (typically ±10–20 ky on inverse. 568.

(15) Fig. 9. Photogrammetric projection of the west wall of Quebrada Turbia [modified from fig. 4 of Singer et al. (1997)]. Original contacts between volcanic sequences are essentially unchanged, but the Estero Molino sequence (EMS) and lower Volca´ n Tatara lavas have been annotated to reflect newly recognized stratigraphic subdivisions (lower EMS, middle EMS, upper EMS) and flow packages within the upper EMS and lower Volca´ n Tatara. It should be noted that the Sin Nombre lavas of the Mun˜ oz sequence (MSSN), the Quebrada Turbia sequence (QTS), and the lower Estero Molino sequence fill successively incised paleodepressions (N→S) with axes that trend at high angles to the present-day Quebrada Turbia drainage. The basement high at the north end of this image (TVS, Tertiary volcanic and sedimentary rocks) projects to the west toward Cordo´ n Las Yeguas (Fig. 2), forming a topographic divide between paleo-Quebrada Castillo and paleo-Estero San Pedro del Norte (Figs 2 and 5). Erosion surfaces at the base of the lower Placeta San Pedro sequence (LPSPS) and upper Placeta San Pedro sequence (UPSPS) are characterized by similar geometries; i.e. glacial ice that incised these valleys flowed to the WNW from the summit regions of volcanic centers that were located to the east of Quebrada Turbia (destroyed by erosion before the construction of Volca´ n Tatara). To highlight the long durations of lacunae separating unconformity-bound sequences relative to the shorter durations of most sequences the basal contact of the UPSPS is marked by a thick line and the durations of the time gaps between the UPSPS and underlying units are shown in two boxes (UPSPS–UEMS, UPSPS–QTS). Contact relations are nearly conformable at these localities, even though they are discordant elsewhere and different intervening units have been cut out by erosion. The unrepresented time interval between the UPSPS and the QTS (>540 ky) is >60% of the entire duration of the Tatara–San Pedro complex (>930 ky). The Quebrada Turbia Dacite (QTD) is an isolated lava of lower Volca´ n Tatara (no relation to Quebrada Turbia sequence). The thin unit marked ‘MSLLR’ is a zone of reworked blocks of Los Lunes Rhyolite overlying a thick valley-filling unit (QTW10.1) of the Sin Nombre lavas (both Mun˜ oz sequence). The basal Mun˜ oz Dacite and Los Lunes Rhyolite flows are exposed as thick units on Cordo´ n Los Lunes (i.e. east wall of Quebrada Turbia) but are not present on the west wall. The LPSPS flow exposed in the upper west wall of Quebrada Turbia (near QTD) continues into Quebrada Castillo (Fig. 5; Table 2f, 101–102).. DUNGAN et al. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. 569.

(16) JOURNAL OF PETROLOGY. VOLUME 42. isochron ages), although a few samples from basal lower Tatara units have yielded more complicated argon-release spectra that tentatively suggest ages as old as 100–130 ka. Thus, Volca´ n Tatara is older than was inferred from K–Ar measurements.. Petrologic and geochemical overview Detailed petrologic studies with the objectives of identifying and quantifying processes operative during different stages in the complex’s evolution (e.g. Singer et al., 1995; Feeley & Dungan, 1996; Feeley et al., 1998), are currently in progress on the basis of the chemical data presented here combined with additional trace element and isotopic analyses plus petrographic and mineral chemical data. Delving deeply into petrologic interpretations of comprehensively characterized samples is not the main goal of this paper, but preliminary inferences derived from graphical treatments of wholerock chemistry, which require additional testing, are discussed to (1) provide context for the temporal trends revealed by stratigraphic reconstructions and (2) illustrate the implications of these temporal trends for modeling investigations. These preliminary results are offered largely as working hypotheses with the goal of demonstrating the impact of high-density, stratigraphically controlled sampling on the identification of (1) multiple components implicated in the diversity of broadly cogenetic magmas, and (2) the processes that were most probably involved in the immediately pre-eruptive evolution of these magmas. On the basis of previous work it has been suggested that the diverse lavas of the TSPC manifest evidence for a range of basaltic parent magma compositions and for multiple differentiation mechanisms. Eruptive products include the following: (1) heterogeneous mingled andesitic lavas generated by mafic–silicic magma interaction, such as those of Cordo´ n El Guadal (Feeley & Dungan, 1996), the lower Placeta San Pedro sequence (probably correlative with lavas of Cordo´ n El Guadal), and a large late Holocene eruption of Volca´ n San Pedro (Singer et al., 1995); (2) basaltic andesitic and andesitic lavas with phenocryst assemblages approaching equilibrium that were derived primarily by fractional crystallization from mafic parent magmas (Ferguson et al., 1992); (3) blended hybrids characterized by disequilibrium phenocryst assemblages; (4) intermediate magma produced when mafic parent magmas became contaminated by interaction with the crust concurrently with fractional crystallization (Davidson et al., 1987, 1988). The following section (Figs 7 and 8) is a general comparison of lava compositions at the level of sequences, which is designed to introduce the following observations: (1) each sequence is distinct from all others; (2) there. NUMBER 3. MARCH 2001. is no discernible temporal progression defined by lava compositions or differentiation mechanisms at the level of sequences; (3) most of the mafic and intermediate composition lavas of the TSPC display evidence for some type of open-system evolution; (4) the TSPC is substantially different, in terms of differentiation mechanisms, from most volcanoes of the Longavı´–Osorno segment of the SVZ. For comparative purposes, data from the well-studied basalt to rhyolite suite of Volca´ n Puyehue–Cordo´ n Caulle (Gerlach et al., 1988) are shown in several plots, as are fractional crystallization trends derived from these data. Features such as nearly constant ratios of some incompatible elements from basalt to highly differentiated rhyolite and extremely high FeO∗/ MgO in dacites and rhyolites led Gerlach et al. (1988) to conclude that nearly closed-system fractional crystallization was responsible for the spectrum from basalt to rhyolite, although some andesitic magmas formed by mixing of mafic and silicic end-members. Although we do not argue that closed- vs open-system differentiation is the only distinction between TSPC magmas and those at volcanoes to the south, we suggest that the Puyehue– Cordo´ n Caulle lavas represent a limiting case with respect to the SVZ that serve as a useful reference.. Compatible elements: closed vs open systems The compatible elements Mg, Ni, and Cr are higher in intermediate-composition hybrid magmas (i.e. at a given SiO2; Fig. 7a–c) than in magmas generated by closedsystem fractional crystallization under similar P and f O2 conditions. In detail, the graphical trajectories of mafic to silicic differentiation trends defined by these elements may vary in accord with the following: (1) the fractionating mineralogy, which is a function of several factors (e.g. Grove & Kinzler, 1986); (2) the compositions of magmatic end-members in the case of magma mixing; (3) endmember compositions plus the ratio Ma/Mc (DePaolo, 1981) in the case of assimilation–fractional crystallization (AFC). The trends defined by different sequences of the TSPC (Fig. 7a–c) are compared with a reference basalt–rhyolite mixing curve, and with an inferred fractionation-dominated trend that is based empirically on data from Volca´ n Puyehue–Cordo´ n Caulle (Gerlach et al., 1988). Hybrid andesites from Puyehue fall well to the right of the FC curve. Mg-numbers of TSPC mafic andesitic magmas with 55–56·5 wt % SiO2 vary between >61 and >40 (>5·3– 2·6 wt % MgO) and Cr varies sympathetically by more than an order of magnitude (>120–5 ppm): such divergent trends require multiple differentiation mechanisms. Low-mg-number basaltic andesitic to andesitic magmas are generally characterized by a closer approach to textural equilibrium (plag + pyx ± oliv), and are provisionally interpreted as the products of fractionationdominated differentiation. High-mg-number andesitic. 570.

(17) DUNGAN et al.. ERUPTIVE STRATIGRAPHY OF THE TATARA–SAN PEDRO COMPLEX. magmas are texturally variable. Many have disequilibrium textures, multiple populations of plagioclase phenocrysts, and olivine plus two pyroxenes, whereas others contain olivine phenocrysts without pyroxene. These distinctions may reflect mixing among diverse endmembers, evolution by AFC, or polybaric evolution involving multiple processes, which can only be addressed by geochemical modeling integrated with petrographic and mineral chemistry constraints.. LILE variations and crustal contributions Rare primitive, uncontaminated basalts of the Southern Volcanic Zone and other arcs have high K/Rb (>1000– 750; Fig. 7d–f ). The high K/Rb, Sr/Rb, Ba/Rb, and Ba/Th of Rb- and Th-poor basaltic magmas can be lowered by 25–50% relative to primitive values by only 2–10% contamination involving highly Rb- and Thrich crustal components. Consequently, suites of mafic magmas that are not necessarily heavily contaminated in volumetric terms may be characterized by dramatic decreases in these ratios vs SiO2 or Rb. In contrast, these ratios are not changed drastically by closed-system fractional crystallization of an anhydrous mineral assemblage, particularly in the mafic range (e.g. Davidson et al., 1987, 1988). K/Rb in rhyolitic and dacitic magmas of the TSPC is >280–250 (lower than in comparable magmas from Puyehue; Fig. 7e), and some basement granitoids have still lower ratios, and in part, very high Rb contents (insets in Figs 7d and 8a). Thus, assimilation and mixing components with the potential for lowering K/Rb and other such ratios in mafic magmas were available. As the vast majority of mafic to intermediate TSPC magmas have K/Rb <450 (many <350), opensystem behavior during some stage of their evolution is required. As SVZ basaltic magmas rarely have K/Rb in the primitive range, modification of primitive magmas during ascent by interaction with the crust appears to have been nearly ubiquitous (Hildreth & Moorbath, 1988), consistent with thermal–chemical–mechanical modeling (e.g. Huppert & Sparks, 1985; Reiners et al., 1995; Edwards & Russell, 1998) and diverse observations from various tectonic settings (e.g. McBirney et al., 1987; Philpotts & Asher, 1993; Kerr et al., 1995; Luhr et al., 1995; Davidson, 1996).. Relative enrichment of Y and HREE Among the important distinctions between the northern and southern SVZ is the behavior of Y and the heavy rare earth elements (HREE) during progressive evolution from basalt to rhyolite. Some high-Y dacitic and rhyolitic lavas of the southern SVZ (Longavı´–Osorno segment, Fig. 1) that are characterized by unusually high FeO∗/ MgO, iron-rich anhydrous ferromagnesian silicate mineralogy, and strongly decreasing Sr concentrations with. increasing differentiation (e.g. Volca´ n Puyehue–Cordon Caulle; Gerlach et al., 1988) also have nearly the same La/Yb, Th/Yb, Ba/Y (Fig. 8c, inset), and Rb/Y (Fig. 8a–c) as associated mafic lavas. These observations are consistent with production of these intermediate to silicic magmas largely by closed-system fractional crystallization from mafic parent magmas in the upper crust as proposed by Gerlach et al. (1988). In contrast, Hildreth & Moorbath (1988), and Tormey et al. (1991) postulated suppression of Y–HREE enrichments in evolved rocks of the northern SVZ relative to closed-system fractionation trends (particularly the Tupungato–Maipo segment, Fig. 1) as a consequence of higher pressure differentiation in open systems characterized by incorporation of crustal components generated by partial melting wherein garnet remained in the source. Hildreth & Moorbath (1988) interpreted a correlation between the suppression of Y–HREE enrichments (high Ce/Yb) and inferred crustal thickness as an indication that assimilation occurs at increasingly greater depths in increasingly thicker crust (i.e. primarily near the base of the crust). As silicic magmas from 36°S northward commonly contain hornblende and those south of 36°S do not, differentiation of wet parent magmas (i.e. high cpx/plag and hornblende/plag) may have contributed to increases of light REE (LREE) over Y–HREE at the TSPC and at other centers in the northern SVZ, but this mechanism cannot explain the much higher concentrations of LILE in intermediate to silicic magmas of the northern SVZ. In contrast to the strong Y-enrichment trends observed at Puyehue (inset, Fig. 8d), evolved magmas of the TSPC display multiple, divergent trends ranging from no Y enrichment with increasing SiO2 (i.e. the Los Lunes Rhyolite, Mun˜ oz sequence, has lower Y and Yb than most basalts) to enrichment trends that are less pronounced than the most strongly enriched magmas of the Longavı´–Osorno segment of the southern SVZ (inset, Fig. 8f ). The tendency for increasing Ba/Y and Rb/ Y with increasing SiO2 in some sequences is broadly interpreted as a signature of open-system behavior (i.e. involvement of high-Ba/Y and high-Rb/Y crustal endmembers) at the TSPC. Low-mg-number andesitic magmas (mainly Volca´ n Tatara) display the greatest Y–HREE enrichments in combination with low Rb/Y and Ba/Y, thereby approaching the Puyehue trend. This diversity is impossible to interpret in terms of a single differentiation mechanism, or as a function of one crustal variable. A range of differentiation trends linked to multiple depths, assimilated crustal components, and processes is implied.. Limited isotopic variations The tendency of arc magmas of the southern SVZ to display minor variations in isotopic ratios from basalt to. 571.

(18) JOURNAL OF PETROLOGY. VOLUME 42. rhyolite, caused in large part by low crust–magma isotopic contrast, is well established (Hickey et al., 1986; Davidson et al., 1987, 1988; Gerlach et al., 1988; Tormey et al., 1995). This has led to the tradition, which is followed here, of developing arguments that depend on trace element ratios as the basis for discussions of open-system vs closed-system magma evolution. None the less, an incomplete isotopic survey ( 87Sr/86Sr) of mafic to silicic magmas of the TSPC ( J. P. Davidson & M. A. Dungan, unpublished data, 2000) has thus far defined a range of low ratios for primitive magmas (0·70379–0·70398) and generally higher ratios in mafic to silicic magmas that may have crustal contributions. In particular, the voluminous Mun˜ oz dacite (0·704168) and Los Lunes rhyolite (0·704249) of the early Mun˜ oz sequence, and rhyolite ESPE3.3 of Volca´ n Tatara (0·704825), have higher isotopic ratios than any basaltic values. As the Mun˜ oz sequence silicic magmas and ESPE.3 have extreme trace element signatures that render unlikely a derivation by closed-system fractional crystallization from basaltic magmas, there is a strong prima facie case that at least some silicic magmas were generated by partial melting of the crust (possibly including late Tertiary, or even Quaternary, granitoids). As these magma compositions serve rather well as contaminants for many intermediate composition magmas, it is likely that such crustally derived melts were incorporated into hybrids. The mechanisms and locations of melting and assimilation, and the specific compositions of assimilated or mixed components at various stages of the evolution of diverse TSPC magmas, are the topics of subsequent work.. Diversity among parent magmas Some of the most primitive and least contaminated basaltic magmas thus far described from a Quaternary Andean volcano have been recognized at the TSPC. Although this paper does not treat questions related to mantle source heterogeneity or basalt generation, we note that basaltic magma compositions at the TSPC display variable trace and major element signatures and we refer to temporal changes in parent magma character as a source of magmatic diversity. Although elemental co-variations are not perfectly systematic, basaltic magmas with low TiO2 (<0·9 wt %) and P2O5 (<0·15 wt %) are generally characterized by: (1) low LREE and La/Yb; (2) low Nb and Zr, high Ba/Nb (>100), and low Zr/Y (>3·5–5) and Nb/Y (>0·10–0·15); plus (3) high Al2O3 (>18 wt %) and Sr (>650 ppm) in combination with high Sr/Y (>45) and Sr/Rb (>70), and low Ba/Sr (>0·3). In addition, there is a range of basaltic magmas with higher TiO2 (>0·9–1·1 wt %) and P2O5 (>0·2–0·3 wt %) that have higher Nb/Y and Zr/Y, but lower Ba/Nb and K/P. The most robust ratio with respect to parent magma discrimination is Nb/Y, which varies from. NUMBER 3. MARCH 2001. >0·10 to >0·45 among basaltic magmas, and which displays little variation during closed-system evolution and only minor variations among lavas that appear, on the basis of criteria noted above, to be related by opensystem processes. In subsequent treatments of individual sequences, emphasis is placed on comparisons of lavas within sequences on the basis of the major and trace element variation diagrams that most clearly illustrate, in each specific case, distinctions among parent magmas and associated differentiation trends. Data from some sequences are also presented as a function of relative eruptive order as documentation of the general observation that progressive evolution from a single parental magma batch is generally not the cause of compositional changes from one eruptive event to the next, even where quasi-linear arrays on certain variation diagrams suggest a high degree of commonality among closely related, variably evolved magmas.. PRE-ESTERO MOLINO SEQUENCES Mun˜ oz sequence (>930–825 ka) The Mun˜ oz sequence (Figs 7 and 8) is a compositionally bi-modal suite that is dominated volumetrically by two major silicic units, the basal Mun˜ oz Dacite (plag + hbl + biot + ox; Table 2a, index numbers 1 and 2) and the younger Los Lunes Rhyolite (plag + biot + ox + zirc; Table 2a, 8 and 9). These units are separated in time by >100 ky, and by oxygen isotope stage 22 (>860 ka), and both comprise multiple flow units with slightly variable compositions. Intercalated between these large silicic units are the volumetrically minor mafic to intermediate Sin Nombre lavas and laharic breccias (reversed magnetic polarity; figs 3–4 of Singer et al., 1997). Mafic basaltic andesitic lavas exposed between the Mun˜ oz Dacite and Los Lunes Rhyolite on the east wall of Quebrada Turbia (QTE) are diverse (Table 2a, 3–6), including (1) a relatively MgO-rich basaltic andesitic lava (>52·3 wt % SiO2; mg-number 59), and (2) three basaltic andesitic lavas (>52·2–54·5 wt % SiO2) with variable trace element ratios and much lower mg-numbers (>46–42). Relatively mafic compositions with such a strong fractional crystallization imprint are unusual elsewhere in the early preserved remnants of the TSPC. Comparably low mg-numbers at the same SiO2 do not occur again until lower Volca´ n Tatara (e.g. packages and ; Table 2h and i, 155–163). In contrast, a single basaltic andesitic lava flow on the west wall of Quebrada Turbia with reversed magnetic polarity (QTW10.1; >55·4 wt % SiO2; Table 2a, 7), underlying a horizon defined by reworked blocks of Los Lunes Rhyolite, possesses an extreme. 572.