A transect across the Andean cordillerâ at latitude 22°S in Chile is investigated using geochemical and geochronological techniques.
The oldest rocks in the region are the Limon Verde schists, which yield a Rb-Sr whole rock age of 251+50 Ma. This is considered to reflect resetting by the intrusion of the adjacent Limon Verde pluton. TNDDM ages range from 913 - 1308 Ma indicating the presence of old lithosphere on the western flanks of the Andes. The Limon Verde pluton gives a whole rock Rb-Sr age of 266+42 Ma (e = iSr = 26.5). Its Sr and Nd isotopes are slightly enriched relative to 'bulk Earth' and derivation from isotopically enriched mantle or equilibration of the whole, more depleted magma with a crustal melt prior to high level fractionation is favoured. Rhyolitic lavas of the Cerro Crespo Formation yield a whole rock Rb-Sr age -of 233+36 Ma (e iSR = 40.2). Their petrogenesis is considered to be similar to that of the Limon Verde pluton.
The La Negra Formation outcrops in the Cordillera de la Costa, and marks the start of the 'Andean Orogeny' in this transect. The lavas yield a whole rock Rb-Sr age of 186+14 Ma (e iSR = -18.8). They are mainly plagioclase phyric, high K basaltic andesites exhibiting tholeiitic fractionation trends. Sr and Nd isotope systematics indicate derivation from a depleted mantle source. More than 80% of Sr, K, Rb, Ba and Th, and less than 60% Ce, Nd, P and Sm are introduced as the subduction zone component. Plots of Hf/LILE and Hf/HFSE against these proportions yield information regarding the elemental concentrations in the mantle prior to the addition of the Subduction zone component. This mantle had e 1Nd ~ +10, and the subduction zone component had eiNd ~ -2. To satisfy trace element criteria, the addition of the subduction zone component must have been a multistage process. The La Negra Formation is considered to have been erupted through fissures in an ensialic back, arc basin.
Jurassic plutons in the Cordillera de la Costa yield ages between 158 - 154 Ma. Parts of the Tocopilla pluton experienced late-stage alkali metasomatism resulting in the formation of "monzonites". Trace element and isotopic differences between the plutons are due to derivation from a heterogeneous, isotopically depleted mantle source.
Jurassic marine and lower Cretaceous continental sedimentation around Cerritos Bayos was succeeded by the lavas of the Indio Muerto Formation. Immobile inter HFSE ratios suggest derivation from a variety of mantle sources: Nd isotope data imply that significant crustal contamination may have occurred. The Augusta Victoria Formation gives an age of 105+19 Ma (e1Sr = -4.3). Two magma types are present, the high K group being derived from a more trace element enriched source than the low K group. The Cerro Negro Formation, which is composed mainly of tuffs with isolated lavas, was derived from an isotopically depleted source within the garnet stability field.
Emplacement of plutons during the Cretaceous occurred at 100 Ma in the Cerros de Montecristo and at Cerritos Bayos, at 79 Ma in the Pampa Negra and at 66 Ma at Cerro Colorado. Whilst Cerro Colorado is highly potassic, the other plutons follow calcalkaline trends. The Cerritos Bayos pluton has been affected by late-stage alkali metasomatism and post consolidation silicification which has disturbed much of its geochemistry. Isotope systematics suggest that the Cerros de Montecristo, Cerritos Bayos and Cerro Colorado plutons have tapped isotopically distinct sources, whereas the Pampa Negra pluton may have experienced complex crustal interaction.
The El Abra pluton yields an age of 39+.2 Ma (e1Sr -2). There is no evidence to indicate that it has suffered significant crustal contamination. Tertiary volcanics between 15.2 and 2.5 Ma all have slightly enriched isotopic characteristics. Their TNdDM ages range from 798 - 1151 Ma, again implying that the Western Cordillera is underlain by old lithosphere. Recent basic monogenetic centres also have enriched isotopic characteristics and subduction-related trace element geochemistry. An alkali basalt from Bolivia, however, is unrelated to subduction and has more depleted isotope systematics; its source is probably below the continental lithosphere. Revised and additional Sr and Nd isotope data for San Pedro and Ecuador are presented.
There has been a general eastwards migration in the locus of magmatism since the Jurassic. Isotope data show that younger, more easterly units tap progressively less depleted sources, and that to the east of the Rio Loa these are enriched relative to 'bulk Earth'. Trace element data indicate that the proportion of the subduction zone component in volcanic rocks decreases from the Jurassic - Recent. Variations in the subduction zone component and progressively increasing degrees of crustal contamination of depleted mantle derived magmas are not thought to be responsible for the observed isotopic and trace element variations. Instead, as the locus of magmatism has migrated eastwards it has tapped more trace element and isotopically enriched, and probably older mantle source regions.
The increase in crustal thickness since the Miocene may be attributable to tectonic underthrusting of the continental margin. The thickening crust promotes the likelihood of crustal anatexis and contamination. This contamination, however, operates on magmas derived from old, enriched mantle sources
