Olivine-hornblende-lamprophyre dikes from Quebrada los Sapos, El Teniente, Central Chile (34°S): implications for the temporal geochemical evolution of the Andean subarc mantle

mafic mg-olivine (fo88)4-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 petro-chemical 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 ultímate cessation of magmatism and >40 km eastward are 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 h2o contents of the lamprophyres. the lamprophyre dikes, as well as younger (1.8-2.3 ma) olivine-bearing basaltic-andesite lava flows in the valley of the cachapoal river, have 87sr/86sr=0.7041 to 0.7049, =+1.2 to -1.1 and 206pb/204pb=18.60 to 18.68, while middle to late miocene (6.5-13.9 ma) el teniente volcanic and plutonic complex igneous rocks have lower 87sr/86sr=0.7039 to 0.7041 and 206pb/204pb=18.56 to 18.59, and higher g =+1.9 to +3.8, and older oligocene to early miocene (>15 ma) abanico or coya-machalí formation volcanic and plutonic rocks in the region have even lower 87sr/86sr=0.7033 to 0.7039 and 206pb/204pb=18.45 to 18.57, and higher gnd=+3.8 to +6.2. the data indicate a significant progressive temporal evolution, between the oligocene and the pliocene, to higher 87sr/86sr and 206pb/204pb, and lower for mantle-derived mafic magmas, and by implication their mantle source regio