Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

<p>An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz <span class="inline-formula">+</span> phengite <span class="inline-formula">+</span> K-feldspar <span class="inline-formula">±</span> garnet <span class="inline-formula">+</span> rutile <span class="inline-formula">+</span> zircon, whereas the paragneiss hosts garnet <span class="inline-formula">+</span> jadeite <span class="inline-formula">+</span> phengite <span class="inline-formula">±</span> glaucophane <span class="inline-formula">+</span> epidote <span class="inline-formula">+</span> rutile <span class="inline-formula">+</span> quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 <span class="inline-formula">±</span> 50 <span class="inline-formula">^∘</span>C and 18 <span class="inline-formula">±</span> 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean <span class="inline-formula">²⁰⁶</span>Pb <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6f00d13d95b9183e3e2526db4298e27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-645-2023-ie00001.svg" width="8pt" height="14pt" src="ejm-35-645-2023-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">²³⁸</span>U age of 457 <span class="inline-formula">±</span> 5 Ma. The cores have <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Th</mi><mo>/</mo><mi mathvariant="normal">U</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="66336532033b9fae90efac8c3cb90ed2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-645-2023-ie00002.svg" width="29pt" height="14pt" src="ejm-35-645-2023-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">=</span> 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 <span class="inline-formula">^∘</span>C, based on Ti-in-zircon measurements. Zircon rims yield a range of <span class="inline-formula">²⁰⁶</span>Pb <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="265e2a7d42d09da6c1e252e5649f9787"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-645-2023-ie00003.svg" width="8pt" height="14pt" src="ejm-35-645-2023-ie00003.png"/></svg:svg></span></span> <span class="inline-formula">²³⁸</span>U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean <span class="inline-formula">²⁰⁶</span>Pb <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6ab83fe49e60fc98c8f39a665eb0d5b9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-645-2023-ie00004.svg" width="8pt" height="14pt" src="ejm-35-645-2023-ie00004.png"/></svg:svg></span></span> <span class="inline-formula">²³⁸</span>U age of 78 <span class="inline-formula">±</span> 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Th</mi><mo>/</mo><mi mathvariant="normal">U</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="9fc4a0c98ca077ea326c889213881ea1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-645-2023-ie00005.svg" width="29pt" height="14pt" src="ejm-35-645-2023-ie00005.png"/></svg:svg></span></span> (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at <span class="inline-formula">&lt;600</span> <span class="inline-formula">^∘</span>C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.</p>