Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria).

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of ∼0.9-1.1 GPa and ∼780-820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching ∼0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at ∼340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of ∼335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Orbitally paced phosphogenesis in Mediterranean shallow marine carbonates during the middle Miocene Monterey event.

During the Oligo-Miocene, major phases of phosphogenesis occurred in the Earth's oceans. However, most phosphate deposits represent condensed or allochthonous hemipelagic deposits, formed by complex physical and chemical enrichment processes, limiting their applicability for the study regarding the temporal pacing of Miocene phosphogenesis. The Oligo-Miocene Decontra section located on the Maiella Platform (central Apennines, Italy) is a widely continuous carbonate succession deposited in a mostly middle to outer neritic setting. Of particular interest are the well-winnowed grain to packstones of the middle Miocene Bryozoan Limestone, where occurrences of authigenic phosphate grains coincide with the prominent carbon isotope excursion of the Monterey event. This unique setting allows the analysis of orbital forcing on phosphogenesis, within a bio, chemo, and cyclostratigraphically constrained age-model. LA-ICP-MS analyses revealed a significant enrichment of uranium in the studied authigenic phosphates compared to the surrounding carbonates, allowing natural gamma-radiation (GR) to be used as a qualitative proxy for autochthonous phosphate content. Time series analyses indicate a strong 405 kyr eccentricity forcing of GR in the Bryozoan Limestone. These results link maxima in the GR record and thus phosphate content to orbitally paced increases in the burial of organic carbon, particularly during the carbon isotope maxima of the Monterey event. Thus, phosphogenesis during the middle Miocene in the Mediterranean was controlled by the 405 kyr eccentricity and its influence on large-scale paleoproductivity patterns. Rare earth element data were used as a tool to reconstruct the formation conditions of the investigated phosphates, indicating generally oxic formation conditions, which are consistent with microbially mediated phosphogenesis.