Abstract Two magnesite‐bearing impure dolomitic marbles from the Dabie–Sulu ultrahigh‐pressure (UHP) region have been investigated to clarify if they had actually attained P–T conditions outside the dolomite stability field, limited by the reaction dolomite = aragonite + magnesite, and to test their potential for recording (U)HP conditions. In both cases, the silicate mineral assemblage records conditions around the terminal amphibole breakdown reaction: amphibole + aragonite ± quartz = clinopyroxene + talc, which is a good geobarometer between at least 2.0 and 2.6 GPa. At higher pressures, the terminal breakdown of talc to clinopyroxene + coesite is the last P–T milestone below 4 GPa recordable by the silicate assemblage, but evidence of former coesite is ambiguous. The dolomite dissociation curve becomes strongly divariant in Fe‐bearing marbles and may thus be attainable during cold subduction near the 5 °C km −1 ‘geotherm’. At least one of the samples (from Xinyan village, near Taihu, Dabie Shan) preserved relicts of both magnesite and aragonite and most likely attained conditions within the aragonite + magnesite stability field. For the second sample from Sanqingge village in the Sulu terrane, no certain evidence has been found in this study. Impure dolomitic marbles have considerable potential to preserve (ultra)high‐pressure relicts. Particularly, massive impure marbles that have not been pervasively infiltrated by fluids during exhumation are considered to preserve high‐pressure relicts better than metapelites, paragneisses and orthogneisses, and perhaps almost as well as eclogites or metaperidotites. The inconspicuous mineral assemblage clinopyroxene + talc or quartz (after former coesite) may in fact record UHP conditions.
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The Ligurian-Piedmont Ocean (LPO) is inferred as a relatively narrow oceanic basin in palaeogeographic restorations, but the actual amount of oceanic lithosphere generated and the timing of magmatic accretion are still debated. Magmatic ages obtained from LPO intrusive rocks predominantly range between 165 and 160 Ma, supporting the interpretation of the LPO as a magma-poor ocean. However, this relatively short timespan of magmatic accretion may also suggest that the orogen sampled older sectors of the oceanic lithosphere, while younger (and more oceanward?) sectors could have been deeply subducted without returning.We therefore focus on studying a poorly-known stack of oceanic lithosphere (i.e., the Susa and Lanzo Valley Ophiolites; SLVO), which is exposed in the inner-central sector of the Western Alps and tectonically juxtaposed with the Gran Paradiso and Dora-Maira massifs. The SLVO were metamorphosed under eclogite-facies peak conditions and consist of large volumes of serpentinite hosting up to kilometer-sized metagabbro bodies, with Fe-Ti-rich differentiated masses and rare metaplagiogranite dykes. The metaophiolite sequence also includes widespread metabasaltic rocks and a metasedimentary cover consisting of minor quartzite and marble levels overlain by calcschist.Two pairs of Fe-Ti metagabbro and metaplagiogranite s.l. sampled close to the Avigliana (lower Susa Valley) and Mondrone (middle Ala Valley) localities have been selected for zircon U-Pb dating. In each sample, the dated zircons yield magmatic ages falling within the uppermost Jurassic Period (~150 Ma). The common age, along with similar major and trace element compositions, suggests a cogenetic origin within differentiation trends for the two pairs of metagabbro-metaplagiogranite (De Togni et al., 2024). Consequently, the SLVO were sampled from a sector of the LPO characterized by magmatic activity at ~150 Ma, significantly younger than most of previously reported ages for the LPO magmatism. We argue that the SLVO represent the youngest oceanic lithosphere accreted in the Western Alps and they may provide new constraints on the structural architecture of the LPO. De Togni, M., Balestro, G., Rubatto, D., Castelli, D., Gattiglio, M., & Festa, A. (2024). Late Jurassic magmatism in the Ligurian-Piedmont Ocean constrained by zircon ages of mafic and felsic meta-intrusives. Terra Nova, 00, 1–11. doi.org/10.1111/ter.12723
The impure marbles of the internal Sesia‐Lanzo Zone underwent a multi‐stage metamorphic evolution of Alpine age and retain early‐Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high‐ P assemblages consist of carbonates, phengite, quartz, omphacite, grossular‐rich (locally spessartinic) garnet, zoisite and Al‐rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen‐rich biotite and amphibole are also present. P‐T estimates of the early‐Alpine metamophism have been calculated from these unique high‐ P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt‐Cpx calibrations and the phengite geobarometer give results ( T = 575 ± 45° C at 15 kbar for the eclogitic climax and T ≤ 500° C at P H 2 O ≤ 9 kbar for early‐Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P‐T‐X CO 2 space indicate that mineral assemblages in the impure marbles coexisted with H 2 O‐rich fluids ( X CO 2 <0.03) during their entire Alpine evolution.
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