Dissolved strontium in the oceans today has a value of 0.709 that is dependent on the relative input from the continents and the ridges.
In the geologic past, changes in the activity of these two sources produced varying Sr ratios over time.
The pre-Variscan basement of Iberia contains two contrasting associations of silicic igneous rocks, one composed of intermediate to felsic peraluminous rocks with ages clustering around 486 Ma, and the other dominated by felsic peralkaline rocks considered to be of similar age.
The most voluminous manifestation of peralkaline magmatism is the Galiñeiro Igneous Complex, mainly consisting of A1-type granitoids locally extremely enriched in HFSE and REE, which underwent medium-grade metamorphism during the Variscan.
Using estimates of measurement precision, the crucial question of whether or not scatter outside of measurement error exists is addressed.
Such scatter would constitute a geologic component, indicating that one or more of the underlying assumptions has been violated and that the age indicated is probably not valid.
Although the Rb-Sr isotope system composition was totally reset at 325 Ma, coincidental with the peak of the Variscan metamorphism, the Nd isotope composition has not been perturbed, and yielded εNd ≈ 0.9–1.25 Ga, thus indicating mantle-derived magmas with a perceptible crustal component.
If, on the other hand, the underplating magmas were more alkaline, they released hydrothermal alkaline fluids that percolated through the lower crust to form such compositionally anomalous rocks as the peralkaline Galiñeiro gneisses.With one exception all the other apparent ages fall short of the estimated age of deposition by as much as 22%.Low ages, the pattern customarily observed, are attributed to postdepositional loss of radiogenic Sr from expandable layers by weathering or during diagenesis.Previous attempts at dating these rocks using zircon concentrates have not succeeded.Using zircon U-Pb ion microprobe we found these granitoids have a crystallization age of 482 ± 2 Ma, and they are therefore coeval with the spatially related peraluminous magmas.