LITHOSPHERIC RAINDROPS BENEATH THE PERUVIAN ALTIPLANO: EVIDENCE FROM BACK ARC POTASSIC MAFIC VOLCANIC ROCKS

Small-volume, young (Pliocene to Quaternary), back arc shoshonitic volcanic rocks of southeastern Peru permit evaluation of models for the removal of crustal and mantle lithosphere beneath the northwestern Altiplano. Trace element patterns and Sr and Nd isotopic values indicate that these mafic potassic rocks were derived from an enriched magma source with a “subduction signature.” Clinopyroxene-liquid thermobarometry suggests equilibration over a range of depths from deep (~ 35 km) to near-surface (<5 km) levels at temperatures of 1000 to 1150 ˚C. These results, taken with preliminary observations from lower crustal xenoliths, suggest that these magmas were most likely generated through partial melting of an amphibolitic to eclogitic deep crustal root and not from underlying metasomatized mantle lithosphere. The persistence of a dense keel of lower crust beneath the Peruvian Altiplano argues strongly against wholesale removal of a large body of lithospheric material. Further, shoshonitic magmatism in the back arc is volumetrically minor, contrary to the copious amounts of depleted magma expected from upwelling asthenosphere during a large-scale delamination event. Alternatively, we suggest that small-volume potassic mafic magmas resulted from dehydration partial melting in small (km-scale) downgoing drips, resembling raindrops of foundering lower crustal material. The apparent spatial restriction of hypothetical small delaminating domains to a zone of active transtension (the Cusco-Vilcanota fault system) may reflect an increase in gravitational potential energy associated with drip detachment.