Resumen
A Miocene to Pliocene (13 to 4.6 Ma) mostly pyroclastic sequence is exposed along the Iglesia Valley, to
the east of the former main volcanic arc. This area is a transitional region between Cordillera Frontal and Precordillera,
over the flat slab segment of the Southern Central Andes, at 29º30? S to 30º00? S. New radiometric ages, geochemical
data, petrography and field relationships are evaluated to establish differences and similarities between Miocene arcrelated
sequences across the main arc and its expansion towards a back arc position, in western Precordillera. Analyzed
rocks have a magmatic arc signature partially like the former main volcanic arc to the west. The Iglesia Valley rocks
are LREE-enriched (La/Sm: 3.7-6.5) with respect to HREE (Sm/Yb: 2.2-6.0) and define patterns with a pronounced
slope. Sm/Yb ratios generally increase with time, as pressures increase, with retention of HREE in residual mineralogy,
particularly garnet at Sm/Yb>4. Volcanic activity in Cordillera Frontal and the volcanic-volcaniclastic expression in
Precordillera show a continuous increase in the La/Yb ratio with decreasing age. Variations in the residual mineral
phase equilibrating with magmas would be related to the progressive increase in crustal thickness due to the tectonic
compressive regime resulting from shallow subduction since Middle Miocene. The data presented suggest that the arc
magmatic activity during the Miocene was expanded notably to the East in relation to the location of the main arc at
Valle del Cura, in Cordillera Frontal. The extensive amplitude of the volcanic arc activity is indicative of the slab gradual
flattening. Particularly, the mantle-derived magmas from Lomas del Campanario Formation (Western Precordillera) are
enriched by subduction related fluids but also by crustal components. It is interpreted that the cause of the geochemical
differences between the back arc position rocks and the main arc lay in the heterogeneous composition of the underlying
continental crust involved in both locations. Presence of volcanic rocks with adakitic geochemical affinity probably
reflect astenospheric-derived melts that interacted through a heterogeneous and thickened crust toward the surface.