Melt-rock reactions in the mantle peridotites from Tuscany ophiolites

Here we present mineralogical and geochemical data on mantle peridotites from the “Monti Rognosi” Jurassic ophiolite from southern Tuscany. The ophiolites are included in Cretaceous-Eocene flysch formations and are mainly composed of serpentinitized peridotites containing small gabbroic bodies. Locally the serpentinites preserve lenses of relatively fresh mantle assemblages. The samples selected for the present study are clinopyroxene-poor porphyroclastic lherzolites (Cpx = 5-7 vol%). The clinopyroxene mainly occurs as large exsolved porphyroclasts, frequently corroded most likely in response to reaction with a melt that crystallised thin gabbroic (Pl+Opx) veins and patches. The orthopyroxene occurs as large deformed porphyroclasts characterized by undulose extinction and irregular outlines. Olivine typically forms corrosion loops on orthopyroxene. Small amount of dark brown Cr-rich spinel with irregular shapes are generally rimmed by plagioclase. Olivine has forsterite contents between 88.7-89.6 mol%. The cores of (exsolved) orthopyroxene porphyroclasts have Al2O3 and CaO contents up to 2.6 and 1.6 wt% respectively. Neoblasts have lower Al2O3 and CaO contents. Ca-in-Opx thermometry (Brey & Kohler, 1990) provides T of 1070 ± 50°C and 960 ± 40°C for the porphyoclastic and neoblastic Opx, respectively. The clinopyroxene (Mg# 88-90) has relatively high Al2O3 (about 7 wt%), TiO2 (0.5-0.8 wt%) and Na2O (0.2-0.5 wt%) contents. Plagioclase is highly calcic (An = 81-90 mol%). Mg# and Cr# composition of olivine and spinel plot off the mantle array. The highest Cr# values of the spinel (43-46) are associated with high TiO2 contents (0.5-0.8 wt%). Similar chemical variations are commonly observed in plagioclase peridotites that experienced by melt/rock reactions. Major element whole-rock compositions depart from the trends of residual peridotites, being characterized by higher Al2O3 and FeOtot, and lower CaO and SiO2. Hence, these chemical compositions are not compatible with products of simple partial melting. Bulk rock REE compositions apparently record small degrees (2-4%) of fractional melting in the spinel stability field, thereby arguing against clinopyroxene compositions. In fact, clinopyroxenes show a wide REE heterogeneity, ranging from markedly to slightly LREE-depleted (CeN/SmN = 0.02-0.40), with nearly flat and variable HREE contents (YbN = 10-22). We propose that the clinopyroxene compositional variations were related to infiltration and impregnation of the Monti Rognosi residual peridotites by a MORB-type (Opx-saturated) melt in the plagioclase stability field.