Cambrian-Ordovician evolution of Eastern Alps: New evidences constrain from magmatic rocks in the Schladming Complex (Austroalpine unit)
Authors: Huang Qianwen , Liu Yongjiang , Genser Johann , Neubauer Franz , Yuan Sihua , Yu Shengyao , Bernroider Manfred , Guan Qingbin , Wei Jin , Chang Ruihong 
Published: March 2021

&lt;p&gt;The pre-Mesozoic basements in the Eastern Alps overprinted by the Variscan and alpine metamorphism (Neubauer and Frisch, 1993), which still remained the pre-Variscan tectonic evolution evidences. Many of these basements left away from their lithospheric roots due to large-scale tectonic activities (von Raumer et al., 2001), whereas their origin and tectonic history can be recorded by detailed geochemistry and geochronology. Here we present a study on the Schladming Complex, one part of Silvretta-Seckau nappe system in Austroalpine Unit, that located in the northern part of Alps to discuss their ages, origin, and tectonic relationship with the Proto-Tethys Ocean.&lt;/p&gt;&lt;p&gt;The Schladming Complex basement mainly comprises biotite-plagioclase gneiss, hornblende-gneiss, mica-schists, together with some amphibolites, orthogneisses, paragneisses, metagabbro and migmatites, which covered by sequence of metasedimentary (Slapansky and Frank, 1987). It underwent the medium- to high-grade metamorphism during the Variscan event and is overprinted by the greenschist facies metamorphism during the Alpine orogeny (Slapansky and Frank, 1987).&lt;/p&gt;&lt;p&gt;Granodioritic gneisses (539~538 Ma) and fine-grained amphibolite (531&amp;#177;2 Ma) in the basement represent a bimodal magmatism. Geochemical data show that the granodioritic gneisses belong to A&lt;sub&gt;2&lt;/sub&gt;-type granite and originated from the lower crust, while the fine-grained amphibolites have an E-MORB affinity and the magma origined from the lithospheric mantle and contaminated by the arc-related materials. The data implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian.&lt;/p&gt;&lt;p&gt;A medium-grained amphibolite gives an age of 495&amp;#177;5 Ma, exhibits ocean island basalt-like geochemical features and zircons positive &amp;#949;Hf(t) values (+5.3~+10.9) indicating that the medium-grained amphibolite derived from a depleted mantle source. The monzonite granitic gneiss and plagioclase gneiss yields ages of 464&amp;#177;4 Ma for and 487&amp;#177;3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive &amp;#949;Hf(t) values of +5.9~+7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite.&lt;/p&gt;&lt;p&gt;In summary, our data presents record of the Cambrian to Ordovician magmatism in the Schladming Complex, which provided new evidence of tectonic evolution history between Proto-Tethys and Gondwana. According to the data, we proposed that a series of rift developed in the northern margin of Gondwana during 540-530 Ma, the rifts continually expanded into a back-arc ocean in ~490 Ma and was closed around 460 Ma with S-type granitic magma intruded.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Neubauer, F., Frisch, W. 1993. The Austroalpine metamorphic basement east of the Tauern window.&amp;#160; In: Raumer, J. von &amp; Neubauer, F. (eds.): Pre-Mesozoic Geology in the Alps. Berlin (Springer), pp. 515&amp;#8211;536.&lt;/p&gt;&lt;p&gt;von Raumer, J., Stampfli, G., Borel, G., Bussy, F., 2001. Organization of pre-Variscan basement areas at the north-Gondwanan margin. International Journal of Earth Sciences 91, 35-52.&lt;/p&gt;&lt;p&gt;Slapansky, P., Frank, W. 1987. Structural evolution and geochronology of the northern margin of the Austroalpine in the northwestern Schladming crystalline (NE Tadst&amp;#228;dter Tauern). In: Fl&amp;#252;gel, H. W. &amp; Faupl, P. (eds.), Geodynamics of the Eastern Alps, pp. 244-262.&lt;/p&gt;