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 t he Rangeley, Perry Mountain, and Smalls Falls formations of western Ma ine were analyzed for major elements, trace elements, and neodymium is otopes. These formations were derived from highlands created during th e Taconian orogeny, deposited into a trough, and metamorphosed during subsequent orogenic events. Most major and trace element abundances re lative to Al2O3 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 system atic variations in some elements. Notably, plots of SiO2 vs. Al2O3 and K2O vs. Al2O3 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. Dif ferent amounts of these minerals should not influence the shape of the REE patterns of the metapelites, but higher amounts of quartz and fel dspar may dilute the REEs and most elemental abundances of the clay mi nerals and lead to lower elemental abundances. The major difference be tween the samples within the Perry Mountain Formation are different LR EE and MREE abundances relative to Al2O3 which are not correlated to d ifferences in major element or other trace element abundances relative to Al2O3 The samples in the Ferry Mountain with higher, LREE and MREE abundances have, for example, 42.3 +/- 8.3 ppm, and those with low ab undances have 5.6 +/- 3.6 ppm. The samples with the high REE abundance s of the Ferry Mountain Formation are similar in abundances and REE pa tterns to those of the Rangeley and Smalls Falls formations typical of mudstones derived from granitoids. Another difference between the low and high KEE abundance samples are the calculated TDM model ages. The high REE abundance samples of the Ferry Mountain Formation show T-DM, similar to the samples of the Rangeley Formation, with ages of about 1.7-1.8 Ga. The Ferry Mountain samples with low REE abundances, howeve r, give unrealistically old TDMS between 2.5 and 5.3 Ga. These unreali stically old T(DM)s are due to the relatively high Sm/Nd ratios (compa red to crustal values) which are characteristic of samples of the Ferr y Mountain Formation with lower REE abundances. We therefore suggest t hat these samples may be indicators for open system behavior of the ne odymium isotopic system. The timing of this disturbance of the neodymi um isotope system is difficult to determine and cannot be tied to weat hering or a definite postdepositional event. The complexities of the d ata 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 an d turbidite mudstones during diagenesis, but some migration may have c ontinued during metamorphism in order to reconcile the neodymium isoto pic data. (C) 1997 Elsevier Science Ltd.