Age, origin and geodynamic significance of a polymetamorphic felsic intrusion in the Otztal Crystalline Basement, Tirol, Austria

A multi-method approach was applied to derive the age and origin of an orthogneiss body located in the central Kaunertal, western Otztal Crystalline Basement (OCB). The Tieftal orthogneiss body is an internally differentiated, polymetamorphosed epizonal intrusion, embedded in amphibolites. It comprises leucocratic hedenbergite-hornblende-, hornblende- and biotite-hornblende-gneisses, but also some melanocratic rock types. The leucocratic Tieftal gneisses are granitic, have a near eutectic melt composition and share some features of A-type granites, such as high Na2O + K2O (8.07 to 8.58 wt%), Zr (379 to 554 ppm) and Y (58 to 79 ppm) contents. The REE-patterns are rather Bat ((La/Yb)(N) = 2.4 to 3.7), with distinct negative Eu anomalies. Single zircon evaporation dating of two samples and Sm-Nd dating of relict magmatic titanite resulted in ages of 487 +/- 7, 484 +/- 3 and 487 +/- 5 Ma, respectively. The weighted mean of 485 +/- 3 Ma is interpreted as the primary crystallization age of the Tieftal orthogneiss body. Rb-Sr whole rock dating results in a well defined regression line, corresponding to an age of 411 +/- 9 Ma. This age clearly documents at least a partial resetting of the whole rock Rb-Sr system, which is most probably due to subsequent metamorphic overprint. The leucocratic Tieftal gneisses are isotopically rather primitive with an epsilon Nd-CHUR(485 Ma) value of + 1.7 and a calculated magmatic initial Sr-87/Sr-86 ratio of 0.7047. These data suggest a major mantle contribution. Most probably, they originated through fractionation of the magmatic precursors of the accompanying tholeiitic metabasites. The more primitive isotopic composition of OCB metabasites and some late Archean/early Proterozoic and Cambrian inheritance in Tieftal gneiss zircons suggest some involvement of old crustal rocks, too. The amount of crustal contamination can be calculated to be in the range of 10 to 40%. The Tieftal gneisses and the accompanying metabasites are interpreted as remnants of igneous rocks related to an early Ordovician rifting and incipient formation of new oceanic crust, an event which can be traced throughout the central and western European Variscan and Alpine terranes.