Coexisting calderite and spessartine garnets in eclogite-facies Mn-rich metasediments of the Western Alps

International audienceThe coexistence of a colourless and a yellow garnet was observed in eclogite-facies manganese concentrations of the Mesozoic ophiolitic Zermatt-Saas Unit, at the Praborna mine near Saint-Marcel, Val d'Aoste, Italy, and in the upper Maurienne Valley, France. They occur both in oxidised metachert with hematite and braunite (þ minor Mn-pyroxenoid and tirodite, rare tiragalloite; with ardennite or piemontite in distinct layers), and in more reduced, carbonate-rich boudins included in it. The cooccurrence takes a variety of textural aspects, from coexisting euhedral garnets (10-100 mm in size for the calderite to mm-size for spessartine) to sharp overgrowths of yellow calderitic garnet on colourless spessartine, to yellow cauliflower-like masses (a few hundreds of mm in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resorption stages and resumed growth. Sector zoning and anisotropy are common, although not consistent features. Compositions can be expressed to 95% in the quadrilateral system (Ca, Mn 2þ) 3 (Al, Fe 3þ) 2 Si 3 O 12 , with less than 1.0 wt% MgO and 0.8 wt% TiO 2 in colourless spessartine, and less than 0.2 wt% MgO and 1.6 wt% TiO 2 in yellow garnet. Calcium partitions into the ferric garnet. Coexisting pairs define two compositional gaps, bounded by values of the Fe 3þ =(Al þ Fe 3þ) ratio of 10 and 15% for the first one, of 40 and 65% for the other. The optically obvious discontinuity (colour change and Becke's line) corresponds to the narrower gap, between colourless spessartine and yellow spessartine, whereas the broad compositional gap occurs within yellow garnet, between yellow spessartine and yellow calderite, and is only revealed by back-scattered electron images. Only the latter can be a candidate for a miscibility gap, if any

A snapshot of the Late Jurassic Western Tethys seafloor composition and morphology provided by the geochemistry of pelitic sediments (Corsica, Central Alps and Northern Apennines)

The chemical composition of fine-grained siliciclastic sediments is a powerful tool in provenance studies, either as a complement to other whole rock/single grain methods, or as a stand-alone method when other techniques are not applicable, and particularly in those cases where the coarser sediment fractions are not available or the regional-scale geologic framework is lost due major successive tectonic events. A comprehensive geochemical investigation of pelites from the post-rift deposits of the Ligurian-Piedmont ocean (sampled in tectonic units of the Alpine-Apennine orogen: Balagne Nappe, Corsica; Tuscan Nappe and Internal Ligurian units, Northern Apennines; Err-Platta units, Central Alps) has identified for the first time a major mafic-ultramafic input immediately following rifting. Key trace element ratios (e.g. LaN/YbN < 10; avg. Eu/Eu* = 0.73 ± 0.06, 1SD; Th/(Cr + Ni + V) < 0.03) show that the pelitic siliciclastic layers intercalated in the Radiolarite Formation (the first post-rift deposits) are systematically enriched in a mafic-ultramafic source component compared with the younger post rift sediments (Calpionella Limestone and Palombini Shale). Such a peculiar chemical fingerprint is interpreted as the result of erosion and distribution across the whole basin (even to continental domains) of intraoceanic ophiolitic debris by turbidity and bottom currents sweeping the sea floor at the time of deposition of the Radiolarite Formation. Exhumed mantle and gabbroic-basaltic rocks exposed at the morphologically articulated seafloor of the slow-spreading Ligurian-Piedmont ocean were available to erosion during the whole time-span of the deposition of the Radiolarite Formation, whilst they became progressivey subordinate as a source as the basin floor was progressively covered by the siliciclastic input from the developing passive continental margin