Partial melting of amphibole–clinozoisite eclogite at the pressure maximum (eclogite type locality, Eastern Alps, Austria)
<p>Pristine amphibole–clinozoisite eclogite from within the eclogite type locality (Hohl, Koralpe) of the Eastern Alps in Austria preserves centimetre-thick, concordant, laterally continuous leucocratic segregations of coarse-grained (up to <span class="inline-formula">∼</sp...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2023-09-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/35/715/2023/ejm-35-715-2023.pdf |
| Summary: | <p>Pristine amphibole–clinozoisite eclogite from within the eclogite type locality (Hohl, Koralpe) of the Eastern Alps in Austria preserves centimetre-thick, concordant, laterally continuous leucocratic segregations of coarse-grained (up to <span class="inline-formula">∼</span> 1 cm grain diameter) euhedral amphibole–clinozoisite–quartz and disseminated garnet–omphacite–rutile. The segregations locally show selvedges dominated by coarse-grained amphibole at the interface with their host eclogite. Retrogression is limited to thin films of texturally late plagioclase <span class="inline-formula">±</span> amphibole and minor symplectites of diopside–plagioclase partially replacing omphacite. Mineral compositions are largely homogeneous except for clinozoisite, which is significantly enriched in Fe<span class="inline-formula"><sup>3+</sup></span>, rare-earth and high-field-strength elements in the rock matrix compared to that in segregations. Petrography, mineral chemical data and phase diagram modelling are interpreted in terms of limited melting under high-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>a</mi><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="5bee85d0462f9867d66b5d18182c84a0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-715-2023-ie00001.svg" width="24pt" height="12pt" src="ejm-35-715-2023-ie00001.png"/></svg:svg></span></span> conditions, at or close to the well-established pressure maximum (21 <span class="inline-formula">±</span> 3 kbar and 680–740 <span class="inline-formula"><sup>∘</sup></span>C), followed by melt crystallization near these conditions. Exsolution of melt-dissolved H<span class="inline-formula"><sub>2</sub></span>O led to the formation of the amphibole-rich selvedges at the leucosome–eclogite interface. Plagioclase <span class="inline-formula">±</span> amphibole/clinopyroxene films formed at lower pressure from final melt vestiges adhering to grain boundaries or from secondary fluid–rock interaction. Natural variability in rock composition and the bulk oxidation state leads to variable mineral modes and calculated high-pressure solidus temperatures for compositional end-members sampled at Hohl. Modelling suggests that oxidized conditions (<span class="inline-formula"><i>X</i></span>Fe<span class="inline-formula"><sup>3+</sup><i><</i>0.5</span>) favour hydrated but refractory amphibole–clinozoisite-rich assemblages with a fluid-present solidus temperature of <span class="inline-formula">∼</span> 740 <span class="inline-formula"><sup>∘</sup></span>C at 20 kbar, whereas more reduced conditions (<span class="inline-formula"><i>X</i></span>Fe<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msup><mo>∼</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="40pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="2c93687be32825fca3900b64f6b5c1b1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-715-2023-ie00002.svg" width="40pt" height="13pt" src="ejm-35-715-2023-ie00002.png"/></svg:svg></span></span>) yield “true” eclogites (<span class="inline-formula"><i>></i></span> 80 vol % garnet <span class="inline-formula">+</span> omphacite) that commence melting at <span class="inline-formula">∼</span> 720 <span class="inline-formula"><sup>∘</sup></span>C at the same pressure. The interlayering of such eclogites potentially constitutes a fluid source–sink couple under appropriate pressure–temperature conditions, favouring fluid transfer from neighbouring dehydrating layers to melt-bearing ones down gradients in the chemical potential of H<span class="inline-formula"><sub>2</sub></span>O (<span class="inline-formula"><i>μ</i></span>H<span class="inline-formula"><sub>2</sub></span>O). Phase diagram calculations show that for moderate degrees of fluid-fluxed melting (<span class="inline-formula">≤</span> 10 vol % melt) near the pressure maximum, the observed equilibrium assemblage is preserved, provided the melt is subsequently removed from the source rock. The resulting hydrous melts may be, in part, parents to similar eclogite-hosted pegmatitic segregations described in the eclogite type locality. We suggest that eclogites with a comparable composition and metamorphic history are however unlikely to produce voluminous melts.</p> |
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| ISSN: | 0935-1221 1617-4011 |