VOLCANISM AND EROSION DURING THE PAST 930 KY AT THE TATARA-SAN-PEDRO COMPLEX, CHILEAN ANDES

Geologic mapping, together with 73 new K-Ar and Ar-40/Ar-39 age determ inations of 45 samples from 17 different volcanic units, plus paleomag netic orientations, geochemical compositions, and terrestrial photogra mmetry are used to define the chronostratigraphy of the Tatara-San Ped ro complex, an eruptive center at 36 degrees S on the volcanic front o f the Andean southern volcanic zone. The Tatara-San Pedro complex pres erves approximate to 55 km(3) of lavas that erupted from at least thre e central vent regions. Remnant, unconformity-bound sequences of lavas are separated by lacunae that include significant periods of erosion. Quaternary volcanism commenced ca. 930 ka with eruption of voluminous dacitic magma, followed 100 k.y. later by the only major rhyolitic er uption. From 780 ka onward, more than 80% of the preserved volume is b asaltic andesite (52%-57% SiO2), but petrographically and geochemicall y diverse dacitic magmas (63%-69% SiO2) erupted sporadically throughou t this younger, dominantly mafic phase of activity. A few basaltic lav as (49%-52% SiO2) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical dive rsity, and the temporal distribution of mafic and silicic lavas are co nsistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the Tatara-San Pedro complex over the past million years. Nearly two-thirds of the preserved volume of the Tatara -San Pedro complex comprises the two youngest volcanoes, which were ac tive between ca. 188-83 ka and 90-19 ka. Repeated advances of mountain glaciers punctuated growth of the complex with major erosional episod es that removed much of the pre-200 ka volcanic record, particularly o n the south flank of the complex. Dating the inception of a glaciation on the basis of preserved material is difficult, but the age of the o ldest lava above a lacuna may be used to estimate the timing of deglac iation. On this basis, the argon ages of basal lavas of multiple seque nces indicate minimum upper limits of lacunae at ca. 830, 790, 610, 40 0, 330, 230, 110, and 17 ka. These are broadly consistent with global ice-volume peaks predicted by the oxygen isotope-based astronomical ti me scale and with age brackets on North American glacial advances. Est imated growth rates for the two young volcanoes are 0.2 to 0.3 km(3)/k .y.; these are three to five times greater than a growth rate estimate d from all preserved lavas in the complex (0.06 km(3)/k.y.). Removal o f up to 50%-95% of the material erupted between 930 and 200 ka by repe ated glacial advances largely explains this discrepancy, and it raises the possibility that episodic erosion of midlatitude frontal are comp lexes may be extensive and common.