Pre-metamorphic melt infiltration in metasediments: geochemical, isotopic (Sr, Nd, and Pb), and field evidence from Serie dei Laghi (Southern Alps, Italy)

Abstract

Gradual transitions from K-feldspar free gneisses to K-feldspar bearing augengneisses are sometimes observed in metamorphic terranes. They have been explained with metasomatic porphyroblastic growth connected with regional metamorphism, or with pre-metamorphic presence of magmatic megacrysts. A transition of this kind can be observed in the Serie dei Laghi (Southern Alps, Italy), where coarse-grained meta-arenites (Cenerigneiss) grade into Ceneri augengneisses with large K-feldspar porphyroclasts, and banded amphibolites of the “Strona Ceneri Border Zone” grade into Hbl augengneisses rich in K-feldspar. The Ceneri augengneisses are chemically indistinguishable from the Cenerigneiss, but have higher 87Sr/86Sr (0.7256–0.7258 vs. 0.7215–0.7233), similar to those of the Ordovician granites that were intruded, before the regional metamorphism, into the protoliths of both Cenerigneiss and amphibolites. The Cenerigneiss contains two types of zircons: (1) highly luminescent, rounded grains or fragments, yielding U–Pb SHRIMP ages from 0.43 to 1.0 Ga; (2) euhedral grains with oscillatory zoning (magmatic), with U–Pb SHRIMP concordant ages of 466 ± 13 Ma. This age coincides with the Rb–Sr whole rock emplacement age of the Ordovician granitoids (466 ± 5 Ma). The Hbl augengneisses form three groups with distinct geochemical patterns, whose distributions on inter-element diagrams trend towards the Ordovician metagranites and meta-aplites. In addition, the Hbl augengneisses have higher 87Sr/86Sr (0.7132–0.7147 vs. 0.7031–0.7046) and lower 143Nd/144Nd (0.51214–0.51219 vs. 0.51273–0.51297) than the amphibolites, suggesting the addition of an isotopically evolved component. The observed chemical and isotope patterns, as well as the vicinity of the augen gneisses to the Ordovician intrusions, lead us to conclude that the Ceneri augengneisses and Hbl augengneisses are the result of infiltration of residual hydrous magmas into the protolith of both the Cenerigneiss and the amphibolites at the time of Ordovician granite emplacement, long before the regional metamorphism in the Serie dei Laghi.