Elemental distributions and neodymium isotopic compositions of Silurian metasediments, western Maine, USA: Redistribution of the rare earth elements
Pelitic schists from the lower garnet to lower staurolite zones from the Rangeley, Perry Mountain, and Smalls Falls formations of western Maine were analyzed for major elements, trace elements, and neodymium isotopes. These formations were derived from highlands created during the Taconian orogeny, deposited into a trough, and metamorphosed during subsequent orogenic events. Most major and trace element abundances relative to Al 2O 3 were statistically identical between zones of the same formation, as well as between formations. Although the average major element composition of these formations are the same, there are systematic variations in some elements. Notably, plots of SiO 2 vs. Al 2O 3 and K 2O vs. Al 2O 3 suggest that most of the variation could be produced by mixing of a fairly constant ratio of clay minerals and feldspar with varied amounts of quartz due to sorting in the sedimentary system. Different amounts of these minerals should not influence the shape of the REE patterns of the metapelites, but higher amounts of quartz and feldspar may dilute the REEs and most elemental abundances of the clay minerals and lead to lower elemental abundances. The major difference between the samples within the Perry Mountain Formation are different LREE and MREE abundances relative to Al 2O 3 which are not correlated to differences in major element or other trace element abundances relative to Al 2O 3. The samples in the Perry Mountain with higher LREE and MREE abundances have, for example, 42.3 ± 8.3 ppm, and those with low abundances have 5.6 ± 3.6 ppm. The samples with the high REE abundances of the Perry Mountain Formation are similar in abundances and REE patterns to those of the Rangeley and Smalls Falls formations typical of mudstones derived from granitoids. Another difference between the low and high REE abundance samples are the calculated T DM model ages. The high REE abundance samples of the Perry Mountain Formation show T DM similar to the samples of the Rangeley Formation, with ages of about 1.7-1.8 Ga. The Perry Mountain samples with low REE abundances, however, give unrealistically old T DMS between 2.5 and 5.3 Ga. These unrealistically old T DMS are due to the relatively high Sm/Nd ratios (compared to crustal values) which are characteristic of samples of the Perry Mountain Formation with lower REE abundances. We therefore suggest that these samples may be indicators for open system behavior of the neodymium isotopic system. The timing of this disturbance of the neodymium isotope system is difficult to determine and cannot be tied to weathering or a definite postdepositional event. The complexities of the data suggest more than one resetting event. The most likely event that could have produced much of the movement of the LREEs and MREEs could have been due to small scale migration between anoxic hemipelagites and turbidite mudstones during diagenesis, but some migration may have continued during metamorphism in order to reconcile the neodymium isotopic data.