Diques lamprofidicos de olivino-hornblenda de la quebrada los Sapos, El Teniente, Chile central (34°S): implicancias para la evolucion temporal de la geoquimica del manto subarco Andino.

Abstract

AbSTRACT. Mafic Mg-olivine (Fo88)+hornblende lamprophyre dikes, with Ni ~190 ppm and Cr ~390 ppm, cut late Miocene lavas in the Quebrada los Sapos a few kilometers west of the El Teniente Cu-Mo mine. These dikes have petrochemical affinities with other less primitive, Pliocene (2.9-3.9 Ma), olivine-free lamprophyres previously described from both within and in the vicinity of El Teniente. The mafic mantle-derived lamprophyre dikes from Quebrada los Sapos have La/Yb ratios of 10-13, higher than the ratios of 4-9 for older Late Miocene El Teniente Mafic Complex olivine basalts, suggesting a temporal decrease in the percent of partial mantle melting, consistent with the observed decrease in the volume of igneous rocks through time at this latitude, as well as the ultimate cessation of magmatism and >40 km eastward arc migration by the Late Pliocene. Less primitive olivine-free lamprophyres have higher La and lower Yb, resulting in higher La/Yb ratios of 15-44, due to crystal-liquid fractionation involving hornblende, but not plagioclase, the crystallization of which is suppressed by the high H 2 O contents of the lamprophyres. The lamprophyre dikes, as well as younger ( 1. Pb, and lower Є Nd for mantle-derived mafic magmas, and by implication their mantle source region. Significantly Sr, Nd and Pb isotopic compositions are independent of SiO 2 content for rocks in each age group, which precludes contamination by isotopically heterogeneous Paleozoic and Mesozoic continental crust during evolution of intermediate and silicic rocks from mantle-derived mafic magmas. The Oligocene to Pliocene isotopic evolution of the mantle source of the mafic magmas may be explained by an increase from 1% to 6% in the extent of mantle source region contamination by subducted components, including continental crust tectonically eroded off the continental margin. We attribute this to both decreasing subduction angle and increasing rates of subduction erosion associated with the southward migration of the locus of subduction of the Juan Fernández Ridge. The lamprophyres also imply increased hydration of the mantle below this portion of the arc by the Pliocene, which may have played an important role in producing oxidized volatile-rich magmas and mineralization at El Teniente. sapOs, el TenienTe, CenTral Chile (34°s)... RESumEn. Diques lamprofídicos de olivino-hornblenda de la quebrada los Sapos, El Teniente, Chile central (34°S): implicancias para la evolución temporal de la geoquímica del manto subarco Andino. Diques de lampró-fidos máficos, con fenocristales de olivino magnésicos (Fo88)+hornblendas y contenidos de Ni ~190 ppm y Cr 390 ppm, cortan lavas miocénicas superior en la quebrada los Sapos a escasos kilómetros al oeste del depósito de Cu-Mo El Teniente. Estos diques tienen afinidades petroquímicas con otros diques lamprofídicos del Plioceno (2,9-3,9Ma), menos primitivos y sin fenocristales de olivinos, los cuales han sido descritos tanto dentro como alrededor de El Teniente. Estos diques de lamprófidos máficos en la quebrada lo...

Payenia volcanic province, southern Mendoza, Argentina: OIB mantle upwelling in a backarc environment

“…Another possible explanation for the apparently large amount of lower crustal melt in some Payenia basalts could be that the assimilated component of lower crust was derived from the arc or fore-arc, either by delamination or subduction erosion. Subduction erosion has been shown to be an important process in the northern Southern Volcanic Zone (NSVZ) due to the strong compressional tectonics caused by the Pampean flat slab (Stern, 1991(Stern, , 2011Stern and Skewes, 1995;Kay et al, 2005;Stern et al, 2007Stern et al, , 2011. Likewise, the Miocene to Pleistocene shallow subduction zone in the Payenia region may have enhanced the subduction erosion and caused a source region contamination with lower crustal rocks beneath the province.…”