Bulk inclusion dating: a geochronological tool to date low-grade metamorphism
Authors: Hollinetz Marianne Sophie , Schneider David A. , McFarlane Christopher R. M. , Huet Benjamin , Grasemann Bernhard 
Published: March 2020

&lt;p&gt;The petrologic evolution of low-grade metamorphic rocks is essential for a coherent understanding of subduction- and exhumation-related processes during collisional orogeny. Retrieving useful P-T-t-d data from low-grade metamorphic units however is challenging as these rocks commonly lack suitable target minerals for geothermobarometry and/or geochronology. Herein we introduce a new geochronological method termed &amp;#8216;bulk inclusion dating&amp;#8217; and present an example of a rock sampled at the base of the Stauffen-H&amp;#246;llengebirge Nappe (Austroalpine Unit, Eastern Alps, Austria) that witnessed an Eo-Alpine tectono-metamorphic event in the Late Cretaceous.&lt;/p&gt;&lt;p&gt;The investigated schist contains mm-scale chloritoid porphyroblasts in a foliated matrix consisting of chlorite, muscovite and quartz. Accessory minerals include ilmenite, hematite, rutile, zoned epidote with REE-rich cores, euhedral apatite and zircon. Thermodynamic modeling in the MnCNKFMASHTO system predicts the stability of the equilibrium assemblage in a narrow P-T field between 450&amp;#8211;490&amp;#176;C and 5&amp;#8211;7 kbar. Ilmenite, rutile and hematite inclusions in chloritoid cores indicate porphyroblast growth within this field, which is consistent with the observed chemical zoning of the chloritoid. The interpreted peak P-T conditions agree with the observation of garnet in a sample from the same outcrop and independent peak temperature constraints around 490&amp;#176;C derived from Raman spectroscopy of carbonaceous material.&lt;/p&gt;&lt;p&gt;Detailed petrographic investigations using high-resolution SEM imaging combined with EDX analysis revealed abundant minute (100 nm &amp;#8211; 3 &amp;#181;m), idiomorphic zircons both included in chloritoid porphyroblasts and in the matrix. In the chloritoid rim, zircon comprises &gt;95% of the inclusionary phases. &amp;#160;Based on grain size distribution, we interpret zircon growth during prograde metamorphism via dissolution-precipitation mechanism and progressive coarsening due to Ostwald ripening. In situ laser ablation ICP-MS analysis of the bulk zircon population included in the chloritoid rim using a 120 &amp;#181;m spot size yields a U-Pb age of 116.7 &amp;#177; 6.4 Ma (MSWD: 1.5; n: 79). Combined with the results of thermodynamic forward modeling, we link the age to the late prograde part of the P-T evolution. The latest synorogenic sediments on top of the Stauffen-H&amp;#246;llengebirge Nappe were deposited at ca. 120 Ma, giving a consistent upper bound the late prograde age. An apatite U-Pb age from the same sample yields 429.3 &amp;#177; 14.6 Ma (MSWD: 1.2; n: 60). Considering the protolith is an altered tuff and the apatite is likely magmatic, a Devonian protolith age is inferred. That the apatite age was not reset during Eo-Alpine metamorphism is in agreement with the inferred metamorphic conditions. We emphasize that the strength of the bulk inclusion dating approach lies in the improved link of P-T and age data and its relative ease of application compared to other geochronological methods.&lt;/p&gt;