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

Abstract

SummaryGradual 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 metaarenites (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 87 Sr =86 Sr (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 AE 13 Ma. This age coincides with the Rb-Sr whole rock emplacement age of the Ordovician granitoids (466 AE 5 Ma).Correspondence: L. Pinarelli, CNR -Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via La Pira 4, 50121 Firenze, Italy e-mail: lapina@igg.cnr.itThe 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 87 Sr =86 Sr (0.7132-0.7147 vs. 0.7031-0.7046) and lower 143 Nd =144 Nd (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.

Basic Dykes Crosscutting the Crystalline Basement of Valsugana (Italy): New Evidence of Early Triassic Volcanism in the Southern Alps

“…In any case, the vestiges of Permo-Triassic volcanic events aligned along a 300-km E-W belt in the South Alpine region indicate the existence of tectonic structures that were effective at least since the Variscan tectonomagmatic phase, well before the Alpine cycle. Beccaluva et al, 2007), the Tertiary calc-alkaline mafic rocks from Adamello (Alagna et al, 2010;Kagami et al, 1991), Triassic shoshonite rocks from the dolomites (Bonadiman et al, 1994), Permian high-K calc-alkaline basalts and gabbros (Macera et al, 1994;Rottura et al, 1998), and Permian andesites from Lugano and Val di Sesia (Sinigoi et al, 2016); since Nd isotopic values are not available for the metamorphic rocks of the studied area, isotopic values of the basement are referred to a neighboring sector of the South Alpine region characterized by similar micashists and paragneisses (Pinarelli et al, 2008).…”

“…Permo-Triassic basic magmas characterized by calc-alkaline/shoshonite affinity are widespread in the South Alpine domain (Figure 8), suggesting that the underlying mantle has been metasomatized by subductionrelated fluids. Some authors (e.g., Dallagiovanna et al, 2009;Giacomini et al, 2006;Spiess et al, 2010;Stampfli & Borel, 2002;von Raumer et al, 2003) relate the Permian-Triassic orogenic volcanism observed in the southern Variscides to Carboniferous subduction events leading to northward consumption of the (Nimis et al, 2012, and references therein) and that of the South Alpine basement (Pinarelli et al, 2008) are reported for comparison.…”

“…Figure6. (a) Rb-Sr pseudoisochron of the studied dykes from Valsugana (black dots) obtained using87 Sr/ 86 Sr recorded on nonleached powder, compared with that of the host metamorphic rocks from the basement collected exactly in the same area(Meli & Sassi, 2004) and (b) 143 Nd/ 144 Nd versus 87 Sr/ 86Sr diagram (reporting data obtained on leached powders) in which the studied dykes from Valsugana are compared with the neighboring magmatic occurrences such as the Calceranica lamprophyre and the Tertiary anorogenic lavas (Na-alkaline basalts and tholeiites) of the Veneto Volcanic Province (VVP;Beccaluva et al, 2007), the Tertiary calc-alkaline mafic rocks from Adamello(Alagna et al, 2010;Kagami et al, 1991), Triassic shoshonite rocks from the dolomites(Bonadiman et al, 1994), Permian high-K calc-alkaline basalts and gabbros(Macera et al, 1994;Rottura et al, 1998), and Permian andesites from Lugano and Val di Sesia(Sinigoi et al, 2016); since Nd isotopic values are not available for the metamorphic rocks of the studied area, isotopic values of the basement are referred to a neighboring sector of the South Alpine region characterized by similar micashists and paragneisses(Pinarelli et al, 2008).…”

“…Lead isotopic composition of the studied basic dykes from Valsugana (black dots). Compositions of other rocks from neighboring magmatic occurrences(Nimis et al, 2012, and references therein) and that of the South Alpine basement(Pinarelli et al, 2008) are reported for comparison.…”

Geological position and origin of augen gneisses from the Polička Unit, eastern Bohemian Massif

“…Augen gneisses were also interpreted as results of metasomatic processes in metasediments. For instance, Pinarelli et al (2008) explained development of K-feldspar bearing augen gneisses by a model of infiltration of residual hydrous magmas into a metasedimentary protolith. Other types of augen gneisses are thought to be formed by segregation of feldspar (and quartz) crystals and aggregates during migmatization (Mehnert 1968).…”