The metamorphic evolution of migmatites from the Ötztal Complex (Tyrol, Austria) and constraints on the timing of the pre-Variscan high-T event in the Eastern Alps

Abstract

Within the Ötztal complex (Öc), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the Öc, the Winnebach migmatite in the central part and the Verpeil-and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central Öc. the P-t data range from 670-750 °c and < 2.8 kbar for the pre-Variscan event, 550-650 °c and 4-7 kbar for the Variscan event and 430-490 °c and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-silurian metamorphic event in the Öc.

The Grand St Bernard‐Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U‐Pb Dating

“…The occurrence of such bimodal magmatism in a relatively short time interval (470–450 Ma) is supportive of magma emplacement within a transtentional to extensional setting along the northern Gondwana margin, which is also in line with the high‐T metamorphic event at 450–420 Ma reported in the Eastern Austroalpine and Helvetic domains (Rode et al, ; Schulz et al, ; Thöny et al, ; von Raumer et al, ). These bimodal magmatic features are common to all of the pre‐Variscan basement units of the Central and Western Alps, which may suggest that all of these units were possibly part of the same thinned‐continental‐crust domain in northern Gondwana during Middle to Late Ordovician times.…”

Pre-Mesozoic Alpine basements—Their place in the European Paleozoic framework

“…In the Alpine domain, the partial anatectic melting dated by Schaltegger (1993) and Thöny et al (2008) most likely resulted from a thermal event (450-430 Ma) triggered by crustal extension, confi rmed recently for the external domain by new monazite ages (Schulz and von Raumer, 2011). Located along the south Chinese (Gondwana) margin, these areas provide evidence of the transform-type eastern Rheic margin.…”

“…Located along the south Chinese (Gondwana) margin, these areas provide evidence of the transform-type eastern Rheic margin. The emplacement of 450 Ma gabbros along the Gondwana margin, specifi cally in the external domain (Paquette et al, 1989;Rubatto et al, 2001); the general thermal event in the Austroalpine domain (Schulz et al, 2008;Thöny et al, 2008;Rode et al, 2012); and the extrusion of acidic volcanics in the Noric terrane (Frisch and Neubauer, 1989) are, again, the signature of an extending crust in the Alpine domain. The observation of alkalic metabasites from the Paleozoic Austroalpine Graz (Loeschke and Heinisch, 1993) and Early Silurian, 430 Ma mid-ocean ridge and within-plate basalts from the Austroalpine basement to the south of the Tauern Window (Schulz et al, 2004) are equally attributed to the Late Ordovician-Silurian crustal extension.…”

Monazite Microstructures and Their Interpretation in Petrochronology

“…Monazite has proven its great potential to record the chronology of polymetamorphism and also of successive events along a single metamorphic circle in many case studies (e.g., Finger et al, 2002;Foster et al, 2004;Finger and Krenn, 2007;Simmat and Raith, 2008;Thöny et al, 2008;Krenn et al, 2009Krenn et al, , 2012Schulz and von Raumer, 2011;Wawrzenitz et al, 2012;Imayama and Suzuki, 2013;Mottrama et al, 2014;Williams et al, 2017). Finger et al (2016) have claimed that the monazite satellite microstructure gives evidence of a distinct second crystallization event along increasing temperatures, subsequent to a retrogression with the formation of the corona microstructure.…”