Abstract
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 f lat 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 mineralphase 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 f lattening. Particularly, the mantle-derived magmas from Lomas del Campanario Formation (Western Precordillera) are enriched by subduction related f luids 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 ref lect astenospheric-derived melts that interacted through a heterogeneous and thickened crust toward the surface.
Highlights
A Miocene to Pliocene pyroclastic sequence with related lava f lows and subvolcanic rocks is exposed along the Iglesia Valley Basin and surrounding areas, between Western Precordillera and Cordillera Frontal, in San Juan Province
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
The geodynamic evolution of the Paleogene to Neogene main volcanic arc has been studied in the last years on the basis of chemical, petrographic and radiometric ages relationships of the involved volcanic units (Kay et al, 1991, 1999; Kay and Mpodozis, 2002; Bissig et al, 2001, 2003; Litvak et al, 2007; Litvak and Poma, 2005, 2010, 2014; Jones et al, 2016); magmatic activity developed at the same latitudes but in a back arc position is less known
Summary
A Miocene to Pliocene pyroclastic sequence with related lava f lows and subvolcanic rocks is exposed along the Iglesia Valley Basin and surrounding areas, between Western Precordillera and Cordillera Frontal, in San Juan Province. The geodynamic evolution of the Paleogene to Neogene main volcanic arc has been studied in the last years on the basis of chemical, petrographic and radiometric ages relationships of the involved volcanic units (Kay et al, 1991, 1999; Kay and Mpodozis, 2002; Bissig et al, 2001, 2003; Litvak et al, 2007; Litvak and Poma, 2005, 2010, 2014; Jones et al, 2016); magmatic activity developed at the same latitudes but in a back arc position is less known These deposits include a sedimentary and pyroclastic rock sequence known as Lomas del Campanario Formation (Wetten, 1975, 2005; Contreras et al, 1990) and lava f lows related to shallow intrusives, named Miocene Intrusives and Tertiary Intrusives (Cardó and Díaz, 1999; Cardó et al, 2007; Llambías et al, 1990). We use in this work the denomination Lomas del Campanario Formation to refer to all the lava f lows, subvolcanic intrusives and pyroclastic facies that constitute the eastern expression of the arc-related volcanic sequences during EarlyMid Miocene to early Pliocene times (18-5 Ma)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
