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By using 40Ar/39Ar incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in the xenocrysts is as much as [approximately]450 times older than the host lava, the age of which is independently known, and is an obstacle to dating the lava by using a whole-rock sample. The xenocrysts are impossible to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plagioclase and other phenocrysts, gives an accurate 40Ar/39Ar age for the lava. The xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from Paleozoic rocks buried under kilometers of Mesozoic and Tertiary arc rocks composing the Cordillera in central Chile. The required magma ascent velocity, on the order of kilometers/day, is extraordinarily high compared to the [10.sup.-4] km/day minimum implied by the 226Ra excess in continental arc lavas. These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.

Geologic mapping, together with 73 new K-Ar and 40Ar/39Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry are used to define the chronostratigraphy of the Tatara-San Pedro complex, an eruptive center at 36 [degrees] S on the volcanic front of the Andean southern volcanic zone. The Tatara-San Pedro complex preserves [approximately equals]55 [km.sup.3] of lavas that erupted from at least three 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 eruption. From 780 ka onward, more than 80% of the preserved volume is basaltic andesite (52%-57% Si[O.sub.2]), but petrographically and geochemically diverse dacitic magmas (63%-69% Si[O.sub.2]) erupted sporadically throughout this younger, dominantly mafic phase of activity. A few basaltic lavas (49%-52% Si[O.sub.2]) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical diversity, and the temporal distribution of mafic and silicic lavas are consistent 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 active between ca. 188-83 ka and 90-19 ka. Repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre-200 ka volcanic record, particularly on 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 oldest lava above a lacuna may be used to estimate the timing of deglaciation. On this basis, the argon ages of basal lavas of multiple sequences indicate minimum upper limits of la-cunae at ca. 830, 790, 610, 400, 330, 230, 110, and 17 ka. These are broadly consistent with global ice-volume peaks predicted by the oxygen isotope-based astronomical time scale and with age brackets on North American glacial advances. Estimated growth rates for the two young volcanoes are 0.2 to 0.3 [km.sup.3]/k.y.; these are three to five times greater than a growth rate estimated from all preserved lavas in the complex (0.06 [km.sup.3]/k.y.). Removal of up to 50%-95% of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy, and it raises the possibility that episodic erosion of mid-latitude frontal arc complexes may be extensive and common., Chronostratigraphic analysis of the Tatara-San Pedro complex located in central Chile indicated the presence of a steep erosional topography due to repeated glacial advances. The region which is located on a transition zone between the southern and northern parts of the Chilean volcanic zone is composed of the Pellado and the Tatara-San Pedro volcanoes which developed in a period of 90-100 k.y. Furthermore, analysis of lava and erosional hiatuses in the area indicated the presence of successive glacial advances that caused multiple episodes of erosion.