Geochemical constraints on the relationship between the Miocene-Pliocene volcanism and tectonics in the Palaoco and Fortunoso volcanic fields, Mendoza Region, Argentina:new insights from 40Ar/39Ar dating, Sr-Nd-Pb isotopes and trace elements

New 40Ar/39Ar analyses constrain the formation of the volcanic succession of Sierra de Palaoco in the presentback-arc of the Andean Southern Volcanic Zone (SVZ), near 36°S, to the Late Miocene and assigns them to theHuincán II Formation. The composition of major and trace elements, Sr, Nd and Pb isotopes of the Palaoco andnearby Río Grande rocks require a strong arc-like component in the mantle that is absent or weak in bothEarly Miocene (Fortunoso Group) and Pleistocene alkaline lavas (Llancanelo Group) erupted in the same area.Weevaluate the relative roles of varyingmantle source compositions and crustal contamination in the generationof geochemically very different lavas from the Palaoco, Fortunoso and Río Grande volcanic fields, north of thePayún Matrú Volcano. The source for the Early Miocene Fortunoso(I) basalts was a OIB-type mantle devoid ofsubduction zone input. This type of OIB-like volcanic activity terminated due to a change from an extensionalto a compressional tectonic regime. Towards the end of the Miocene renewed alkaline volcanism at Fortunoso(II) display a transition to arc-type incompatible element enrichment. Shortly after the calc-alkaline Palaoco volcanismstarted with a very strong geochemical arc-signature including Ba/La ≈ 60 and La/Nb = 2?3. After aquiesence of 1 Ma the major part of the voluminous Late Palaoco basalts were erupted around 7.5 Ma over afew hundred ka. These are less enriched in Ba and Sr and have compositions like many Holocene rocks of theSouthern Volcanic Zone. Isotopically the Fortunoso I and Palaoco rocks are distinct. Regional volcanism of theCharilehue, Huincán I and II mostly has a moderate arc-type enrichment indicating incipient arc developments.However, Palaoco and La Brea at (c. 35°S) showfull geochemical arc-signature, andwe infer that a frontal arcwasestablished. The subsequent development in the Palaoco-Río Grande area encompasses renewed late Pliocenecalc-alkaline low volume volcanic eruptions (Río Grande group) succeeded in the Late Pleistocene by alkalineOIB-type eruptions (Llancanelo group). In the light of the course of volcanism to the east, in the Nevado area,where late Miocene?Pliocene calc-alkaline volcanism was followed by Late Pliocene?Pleistocene alkaline volcanism.We propose a scenario where the Nazca plate developed an eastwards widening flat slab from which theeast dipping slab before the Late Pliocene translated from Palaoco to Nevado and subsequently retreated passingRío Grande in the Late Pliocene. Alkaline back-arc volcanism was active east of the arc-volcanism and expandedwestwards during the Late Pliocene and Pleistocene.Fil: Dyhr, Charlotte T.. Universidad de Copenhagen; DinamarcaFil: Holm, Paul M.. Universidad de Copenhagen; DinamarcaFil: Llambias, Eduardo Jorge. Universidad Nacional de La Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Enrichments of the mantle sources beneath the Southern Volcanic Zone (Andes) by fluids and melts derived from abraded upper continental crust

Mafic basaltic-andesitic volcanic rocks from the Andean Southern Volcanic Zone (SVZ) exhibit a northward increase in crustal components in primitive arc magmas from the Central through the Transitional and Northern SVZ segments. New elemental and Sr-Nd-high-precision Pb isotope data from the Quaternary arc volcanic centres of Maipo (NSVZ) and Infernillo and Laguna del Maule (TSVZ) are argued to reflect mainly their mantle source and its melting. For the C-T-NSVZ, we identify two types of source enrichment: one, represented by Antuco in CSVZ, but also present northward along the arc, was dominated by fluids which enriched a pre-metasomatic South Atlantic depleted MORB mantle type asthenosphere. The second enrichment was by melts having the characteristics of upper continental crust (UCC), distinctly different from Chile trench sediments. We suggest that granitic rocks entered the source mantle by means of subduction erosion in response to the northward increasingly strong coupling of the converging plates. Both types of enrichment had the same Pb isotope composition in the TSVZ with no significant component derived from the subducting oceanic crust. Pb-Sr-Nd isotopes indicate a major crustal compositional change at the southern end of the NSVZ. Modelling suggests addition of around 2 % UCC for Infernillo and 5 % for Maip

Subduction controls on Miocene back-arc lavas from Sierra de Huantraico and La Matancilla, Argentina and new ⁴⁰Ar/³⁹Ar dating from the Mendoza Region, Argentina

Back-arc volcanism in the western Argentinian provinces of Mendoza and Neuquen has been widespread from the Miocene to historic times. We present a detailed investigation of profiles through two of the major Miocene volcanic areas of the region, the neighboring Huantraico and La Matancilla plateaus, including new Ar-40/Ar-39 age results of major and trace elements as well as Nd, Sr and Pb isotopic data. Four million years of eruptions from 24.4 +/- 0.3 Ma (2 sigma) of alkali olivine basalts with OIB-type incompatible trace element enrichments at La Matancilla (similar to 36.50 degrees S) provide evidence for the presence of back-arc mantle devoid of subduction-related components. In contrast, the lower Huantraico lavas (similar to 37.30 degrees S) require an atypical back-arc mantle, almost devoid of arc-like components (e.g. low La/Ta = 15-18 and Ba/La = 12-18), but with a more depleted isotopic signature (e.g. Sr-87/Sr-86, 0.7033-0.7037) than observed elsewhere in the Andean back-arc. The Lower to Upper Series development in the Huantraico sequence represents a gradual change from basaltic to trachyandesitic back-arc lavas with a weak but temporally increasing arc geochemical signature (e.g. La/Ta = 15-21; Ba/La = 12-45), which is accompanied by Sr, Nd and Pb isotopic compositions approaching present day values of the Andes arc. The compositional change is accompanied by a gradually decreasing role for garnet in the mantle source, a decreasing degree of melting, but also simultaneously increasing influence from subducted fluids, probably as the slab geometry changes through time. The volcanism at Huantraico ceased when a flat slab was established around 15 Ma. (C) 2013 Elsevier B.V. All fights reserved