Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

Tourmaline grains extracted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora. Maira, Western Alps (peakP-Tconditions ~3.7 GPa, 750 °C), has a structural, formula of x(Na0.90Ca0.05K0.01004) Y(Mg1.78Al0.99Fe2+0.12Ti 0.034+0.08)z(Al 5.10Mg0.90)(BO3)3TSi 6.00O18v(OH)3w[(OH) 0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (-2.7-2.9 GPa, 600-630 °C), has a formula of x(Na 0.34Ca0.09K0.010.06) Y(Mg1.64Al0.79Fe0.482+Mn0.062+Ti0.064+Ni 0.02Zn0.01)z(Al5.00Mg 1.00)(Bo3)3T(Si 5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. Oxy-schorl from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at \u3e2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.020.10)Y(Al1.63Fe1.232+Ti0.114+Mg0.03Zn0.01) z(Al5.05Mg0.95)(BO3) 3T(Si5.96Al0.04)O18v(OH)3W[O0.81F0.10(OH) 0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 Å), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P tourmaline, which typically has significant amounts of 141Al. There is an excellent positive correlation (r2 = 1.00) between total [6]A1 (i.e., YA1 + 7Al) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation (r2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YA1 content (r2 = 1.00) that is best explained by the exchange vector YA10(R2+F)-1. This is consistent with, the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature