DOI: http://dx.doi.org/10.5027/andgeo%25x

Southern Central Andes Neogene Back Arc Magmatism Over the Pampean Flat Slab: Implications on Crustal and Slab Melts Contribution to Magma Generation in Precordillera, Western Argentina

Stella Poma

Abstract


A Miocene to Pliocene (13 to 4.6 Ma) mostly pyroclastic sequence is exposed along the Iglesia Valley, to the east of the coetaneous main volcanic arc. At 29º30’SL to 30º00’SL this is a transitional region between Frontal Cordillera and Precordillera, over the flat slab segment of the Southern Central Andes. Radiometric ages, geochemical data, petrography and field relationships are evaluated to establish differences and similarities between the main arc and back arc magmatism during Miocene times. Analyzed rocks have a magmatic arc signature similar to the former main volcanic arc front 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 as pressures increase with retention of HREEs in residual hornblende, and particularly garnet at Sm/Yb>4. Rocks evidence an increase in the Sm/Yb with time, and a relatively minor LREE enrichment. Both magmatic belts, the volcanic activity in Frontal Cordillera and the volcanic-volcaniclastic expression in the back arc, 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 crustal thickening as a result of tectonic compressive regime consequence of the shallow subduction. Particularly, the mantle derived magmas in Lomas del Campanario Unit are enriched by subduction related fluids and melts but also by crustal components. Presented data allow identifying 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, western area of Cordillera Frontal. The very amplitude of the orogenic front is in itself another indication of the slab gradual subhorizontalization. It is interpreted that the cause of the geochemical differences between the backarc position rocks and the main arc lay in the heterogeneous composition of the overlain crust involved in both locations. Moreover, some volcanic rocks identified with adakitic geochemical affinity, probably reflect partial melts of a mantle hydrated and chemically modified by slab melts and also by interaction with melts of the heterogeneous and gradually thickened crust.

How to cite this article Poma, S., 2017, Southern Central Andes Neogene Back Arc Magmatism Over the Pampean Flat Slab: Implications on Crustal and Slab Melts Contribution to Magma Generation in Precordillera, Western Argentina: Andean Geology, v. 44, no. 3, , doi:http://dx.doi.org/10.5027/andgeo%x