Marbles from Castagneto Carducci Area (Tuscany, Italy)

In this work, marbles from Castagneto Carducci (Livorno province, Tuscany), which originated by contact metamorphism of the Calcare Massiccio Fm., an Early Jurassic limestone belonging to the Tuscan Sequence, were studied for determining their chemical, mineralogical and petrographic characteristics, and the main physical and mechanical properties. Forty marble samples were sampled and analysed; they are from two inactive quarries on the NW and NE slopes of the Mt. Romitorino, and from natural outcrops in the vicinity of the quarries. The analysed rocks are marbles with high calcite content (> 98% by weight). Optical microscopy observations showed Castagneto Carducci marbles generally have a heteroblastic/granoblastic texture with crystal boundaries from curved-right to lobate. The maximum grain size of the calcite crystals ranges from 0.2 mm to 0.6 mm. The determination of the main physical and mechanical properties of the analysed marbles showed that these rocks are characterised by low porosity and, in general, good physical and mechanical properties.

Geochemical Characterization of Boula Ibi Granitoids and Implications in Geodynamic Evolution

Petrographical and geochemical study, consistent with observed field relations show that the Boula Ibi syn- and post-kinematic granitoids in north Cameroon, occurred in banded gneisses. These syn- and post-kinematic granitoids consist of deformed monzonites typified by its granoblastic texture, diorites, syenites, granites and basic xenoliths of dioritic and monzonitic composition. They are calc-alkaline, hyperpotassic, metaluminous to slightly peraluminous and I-Type granitoids. They display high content in Fe2O3 + MgO + CaO (2.16 – 23.24 %) that reveals their intermediate affinity, magnesian and metaluminous character whilst the low A/CNK (< 1.1) content indicates their mantle origin. Harker diagrams and La/Sm vs La define the fractional crystallization and partial melting as the two main processes that led the geodynamic evolution of the Boula Ibi syn- and post-kinematic granitoids. These are consistent with low-content of Cs, Ta, Nb, Tb and Hf, supporting high melting rates ranging between 20 and 40% as well as molar Al2O3/(MgO + FeOt) vs CaO/(MgO + FeOt) plot showing magmatic evolutions from metabasaltic and metagreywackes sources.

A retrogressive sapphirine-cordierite-talc paragenesis in a spinel-orthopyroxenite from southern Karnataka, India

Within amphibolite facies Peninsular gneisses in the south of the Dharwar craton, units of Sargur supracrustal rocks contain ultrabasic enclaves. One of these enclaves is an orthopyroxenite which comprises bronzite, spinel and minor phlogopite preserving coarse-grained, relic textures of probable igneous origin. After incorporation into the gneisses the enclave evolved through several distinct stages, elucidation of which allow an assessment of its metamorphic history.Firstly, deformation during closed system, anhydrous recrystallisation caused the coarse-grained textures to be partially overprinted by similar mineral assemblages but with a granoblastic texture. Secondly, open system hydration caused retrogression of the bronzite to alumino-gedrite at the margins of the enclave. Subsequently, the penetration of these fluids along grain boundaries caused reactions between spinel and bronzite to produce reaction pockets carrying assemblages of peraluminous sapphirine associated with cordierite and talc. The differences in the mineral assemblages in each pocket coupled with slight variations in their chemistry, suggest that equilibrium did not develop over the outcrop. Because sapphirine + magnesite is present in some pockets, it is evident that CO2 was also a component of the fluid.Phase relations from the MASH portion of the FMASH system, to which the chemistry of the reaction pockets approximates, suggest that the hydrous metamorphism causing the changes depended upon the assemblage enstatite + spinel + vapour which exists at PT conditions above the position of I16, ∼760°C at 3 kbar and below I21 at ∼765°C at 5.6 kbar (Seifert, 1974, 1975), where sapphirine is replaced by kornerupine. The suggested path of reaction occurred between I18 and I21. Subsequent reactions related to I20 cause the formation of cordierite. Talc formation has to be modelled in a different reaction grid.The metamorphism recorded by these reactions is thus at a maximum of amphibolite facies and is interpreted to have formed during the uplift and cooling history of the gneiss complex when hydrous fluids were free to migrate. Given the complex high-grade metamorphic history of this part of the Dharwar craton this event is likely to be late Archaean or Palaeoproterozoic in age.