Ordovician meta-pegmatite garnet (N-W Otztal basement, Tyrol, Eastern Alps): preservation of magmatic garnet chemistry and Sm-Nd age during mylonitization

摘要

Meta-pegmatites and meta-pegmatite-hosted garnet from three localities in the westernorthern Otztal basement, Tyrol (Eastern Alps), have been analysed for major and. selected trace element chemistry and for Sm-Nd isotope systematics. The garnets have frequently retained a euhedral shape, even though they are now set in a strongly foliated matrix consisting of dynamically recrystallized quartz and feldspar. They are characterized by high Fe and Mn contents (up to 29 mol% spessartine) and a simple major element zoning with decreasing MnO and increasing FeO from core towards the rim, suggesting a single-phase crystallization and primary, magmatic chemistry. Chondrite-normalized REE patterns display a strong enrichment of HREE relative to LREE, and pronounced negative Eu anomalies. Based on ID-TIMS analysis, neodymium concentration in all but one of the optically clean garnet separates varies between 0.12 and 5.5 ppm, at concomitant high Sm/Nd ratios (1.2-4.9). LA-ICP-MS analysis revealed very strong within-grain variation both for Nd concentration (< 0.1-6.3 ppm) and Sm/Nd ratio (< 1-27.4). Garnet-whole rock or garnet-feldspar Sm-Nd isochron ages range between 445 +/- 3 and 473 +/- 3 Ma, with a distinct concentration of the age data in the 455-462 Ma time interval. These ages indicate pegmatite formation in different areas of the Otztal basement during Middle to Late Ordovician time, related to the widespread generation of crustal melts towards the end of the long-lived Early Palaeozoic magmatic event. The presence of granites and associated pegmatites is a distinctive feature mainly of the north-western parts of the Austroalpine basement in the Eastern Alps and typical for the so-called "Caledonian" era. (C) 2004 Elsevier B.V. All rights reserved.