<p>Mafic&#8211;ultramafic lenses embedded in felsic granulites of the Gf&#246;hl Unit, Moldanubian Zone, are considered to be mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m sized mafic lens mainly formed by garnet pyroxenite. The primary mineral assemblage comprises calcium-rich garnet (X<sub>Grs</sub> = 0.4), kyanite, and sodium-rich clinopyroxene (X<sub>Na_M2</sub> = 0.29) (&#177; quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 &#176;C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine&#8211;spinel&#8211;plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase&#8211;orthopyroxene&#8211;spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine&#8211;spinel&#8211;plagioclase symplectites and the plagioclase&#8211;clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine&#8211;spinel&#8211;plagioclase symplectite, the garnet composition changes from Alm<sub>14</sub>Prp<sub>42</sub>Grs<sub>44</sub> in the pristine garnet to Alm<sub>22</sub>Prp<sub>63</sub>Grs<sub>15</sub> at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm<sub>19</sub>Prp<sub>40</sub>Grs<sub>41</sub> to Alm<sub>30</sub>Prp<sub>54</sub>Grs<sub>16</sub>. The secondary zoning towards the plagioclase&#8211;orthopyroxene&#8211;spinel symplectites is characterized by an increase of X<sub>Alm</sub> from 0.19 to 0.27 and a concomitant decrease of X<sub>Prp</sub> from 0.55 to 0.49 at constant X<sub>Grs</sub> of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase&#8211;orthopyroxene&#8211;spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 &#176;C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic&#8211;ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.</p>
<p>Uncommon Ba-Cl-rich phases including Ba-Cl micas and Cl-phosphates have been found in garnet pyroxenites as a part of the matrix or in polyphase inclusions in garnets. Polyphase inclusions are rich in carbonates (dolomite, magnesite, norshetite), phosphates (Cl-apatite, goryainovite (Ca<sub>2</sub>PO<sub>4</sub>Cl), monazite) and other silicates (spinel, amphibole, orthopyroxene, clinopyroxene, margarite, aspidolite, scapolite, cordierite). The inclusions appear as chains crosscutting garnet crystals and their presence is not linked with any chemical zoning in the host garnet.</p><p>The Ba-Cl-rich mica has composition ranging from Ba-rich phlogopite to chloroferrokinoshitalite and to oxykinoshitalite. The mica present in the matrix correspond to Ba-rich phlogopite with low Cl contents and occur together with celsian and low-Cl hydroxyl apatite. The mica in the polyphase inclusions ranges to almost pure chloroferrokinoshitalite and oxykinoshitalite endmembers and coexists either with Cl-apatite (Cl = 1.2 apfu) or rarely goryainovite containing up to 2.5 wt% of SrO. This is second world occurrence of goryainovite and first evidence that Ca can be partially replaced by Sr in this mineral.</p><p>Special attention was paid to the composition trends of the Ba-Cl-rich micas. These are mainly related to the XFe ratio, which correlates positively with Cl, Ba, and Al and negatively with Si and Na. Positive correlation of Cl with Ba and XFe leads to the formation of mica with composition Ba<sub>0.95</sub>K<sub>0.03</sub>Fe<sub>2.69</sub>Mg<sub>0.37</sub>Al<sub>1.91</sub>Si<sub>2.02</sub>Cl<sub>1.98, </sub>XFe<sub>0.88</sub>, which is the most Cl-rich mica so far described from natural samples (10.98 wt% Cl) and is very close to the theoretical formula of chloroferrokinoshitalite BaFe<sub>3</sub>Al<sub>2</sub>Si<sub>2</sub>O<sub>10</sub>Cl<sub>2</sub>. The positive correlation of Ba with Al and their negative correlation with Si and K is corresponding to the coupled substitution Ba<sub>1</sub>Al<sub>1</sub>K<sub>-1</sub>Si<sub>-1</sub> linking the composition of phlogopite and kinoshitalite. Composition trend related with the Ti-content shows that Ti correlates positively with Ba but negatively with Cl, XFe, and with the sum of Mg and Fe. It implies that Ti is incorporated into mica in coordination with O (Ti<sub>1</sub>O<sub>2</sub>(Mg,Fe<sup>2+</sup>)<sub>-1</sub>(OH)<sub>-2</sub>) and it leads to the formation of oxykinoshitalite (BaMg<sub>2</sub>TiSi<sub>2</sub>Al<sub>2</sub>O<sub>12</sub>). Since the incorporation of either Cl or Ti + O correlates with XFe content of mica, XFe ratio can be the crucial factor controlling the ability of mica to incorporate Cl into its crystal lattice. In some cases, two micas with contrasting composition corresponding closer to chloroferrokinoshitalite or oxykinoshitalite coexist in one polyphase inclusion, demonstrated by distinct content of XFe, Ti and Cl (for example: XFe<sub>0.20:0.77</sub>, Ba<sub>0.48:0.63</sub>, Ti<sub>0.35:0.02</sub>, Cl<sub>0.27:1.45</sub>). This could imply the existence of an immiscibility between the composition trends of chloroferrokinoshitalite and oxykinoshitalite .</p><p>Such Ba, Cl and K-rich phases are atypical for garnet pyroxenite. Their presence may be caused by the injection of fluid/melt of crustal source during subduction and subsequent exhumation processes or may be related to earlier mantle metasomatism. The presence of Cl-rich phases together with carbonates indicates extremely high activity of Cl and CO<sub>2</sub> in the metasomatizing fluid/melt that interacted with garnet pyroxenites.</p>
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