First results of zoned garnet Lu-Hf geochronology in micaschists from the Eoalpine high-pressure-belt, Eastern Alps
Authors: Fassmer Kathrin , Tropper Peter , Pomella Hannah , Hauzenberger Christoph , Münker Carsten , Fügenschuh Bernhard 
Published: March 2022

&lt;p&gt;In collisional orogens continental crust is subducted to (ultra-)high-pressure (HP/UHP) conditions as constrained by petrologic, tectonic and geophysical observations. Despite a wealth of studies on the subduction and exhumation of UHP rocks, the duration of prograde metamorphism during subduction is still not well constrained. With dating different parts of single garnet crystals separately with Lu-Hf and Sm-Nd geochronology, we will add tight time constraints to the PT-path and constrain the duration of garnet growth.&lt;/p&gt;&lt;p&gt;We apply Lu-Hf and Sm-Nd geochronology on metamorphic rock samples to date the duration of garnet growth, which represents a major part of prograde metamorphism from the greenschist-facies onward. Micaschist samples from the Schneeberg and Radenthein Units in the Eoalpine high-pressure belt (Eastern Alps) are used for dating as they contain cm- to dm-sized garnet blasts, which experienced only one subduction-exhumation cycle. We present first Lu-Hf ages of three different zones of two garnet grains from two different samples. For these analyzes garnet grains were cut through their geometric core, which we also assume to be the chemical center of the crystals. These garnet grains were then analyzed with EPMA, &amp;#181;-XRF and LA-ICPMS in order to determine their chemical zoning of major and trace elements. Afterwards the profiles and maps of the chemical compositions were used to divide each garnet into three zones. These zones were cut with a Well microsaw with a diamond coated wire, crushed and then carefully handpicked to get inclusions-poor garnet separates for Lu and Hf isotopic analyses. The garnet separates together with a whole rock aliquot of each sample were then dissolved and elements separated with a column chemistry in order to measure Lu and Hf isotope ratios. The different garnet separates each together with the whole rock aliquot will then give separate ages for the rims, intermediate parts and cores of the garnets. We will also show first results of pseudosection modeling combined conventional thermobarmetry.&lt;/p&gt;&lt;p&gt;We already have preliminary results for Zr-in-rutile thermometry of rutile inclusions in garnets and matrix rutiles for samples from both locations: the obtained temperatures lay in a range of c. 500-600 &amp;#176;C and are in accordance with previously published date (e.g. Kaindl &amp; Abart 2002, Konzett &amp; Hoinkes 1996). &amp;#160;We measured Zr content with an EPMA and used the calibrations of Tomkins et al. (2007) and Kohn (2020).&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;Kaindl, R. &amp; Abart, R. (2002). Reequilibration of fluid inclusions in garnet and kyanite from metapelites of the Radenthein Complex, Austroalpine Basement, Austria. Schweiz Mineral Petrogr(82), 467&amp;#8211;486.&lt;/p&gt;&lt;p&gt;Kohn, M.J. (2020). A refined zirconium-in-rutile thermometer. American Mineralogist(105), 963-971.&lt;/p&gt;&lt;p&gt;Konzett, J., &amp; Hoinkes, G. (1996). Paragonite-hornblende assemblages and their petrological significance: An example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy. J Metamorph Geol(14), 85&amp;#8211;101.&lt;/p&gt;&lt;p&gt;Tomkins, H.S., Powell, R. &amp; Ellis, D.J. (2007). The pressure dependence of the zirconium-in-rutile thermometer. Journal of Metamorphic Geology(25), 703-713.&lt;/p&gt;