| Lithostratigraphic, ;The Picotani Group and Intrusive Suite are exposed in the Picotani, Cayconi, Antauta and Quenamari volcanic fields and comprise, respectively, ten volcanic formations and twelve individual hypabyssal intrusions or intrusive complexes, incorporating a diverse assemblage of mantle- and crust-derived igneous rocks. These include lamprophyres (minette), and both high-K calc-alkaline and shoshonitic basalts, as well as true S-type, peraluminous, rhyodacitic pyroclastic and hypabyssal units, many of which display incontrovertible evidence of magma commingling and mixing. The compositions of the mafic members suggest derivation from both enriched lithospheric, and depleted asthenospheric mantle sources, whereas the silicic members are inferred to have been generated through dehydration melting of a semi-pelitic upper crustal protolith during incursion of the mantle-derived melts.;The younger Quenamari Group and Intrusive Suite, incorporating three eruptive and at least eight hypabyssal units, constitute a series of entirely silicic, two-mica ;The Late Cenozoic geodynamic evolution of the southern Peruvian Andes was initiated and subsequently controlled by a flat-slab subduction regime, itself a response to the onset of rapid plate convergence at ca. 52 Ma, and culminating, at ca. 38 Ma, in massive crustal ramping along the orogenic foreland to generate the Incaic Zongo-San Gaban tectonothermal zone. The subsequent collapse of the flat slab, and its progressive foundering into the asthenosphere, generated anomalous mantle flow patterns which produced a wide swath of heated and weakened lower crust. The latter provided the focus for ductile shortening of the lower orogenic crust, the initiation of antithetic subduction of Brazilian Shield lithosphere, and hence thrusting and folding along the Sub-Andean belt.;During the 25 Ma-to-Present interval, the calc-alkaline and shoshonitic igneous suites of the Main Arc domain are ascribed to "normal" arc processes, but those of the Inner Arc were generated by two distinct petrogenetic mechanisms. Mafic assemblages of the Picotani Group probably record small degrees of partial melting of asthenospheric and eroded, sub-continental lithospheric mantle, whereas the silicic S-type assemblages are products of the anatexis of semi-pelitic upper crust. Subsequent commingling and mixing of the mantle- and crust-derived melts in high-level magma chambers yielded a great variety of intermediate rocks with "incompatible" mineral assemblages. The later magmatism characterizing the 17-to-6 Ma interval originated with partial melting of delaminated or detached Brazilian Shield mantle lithospere. (Abstract shortened by UMI.)... |
