Spinels of cortlandtites related to the Montnegre granitoids revisited (Hercynian belt, NE Spain)



Olivine hornblendites (cortlandtites) form part of the Montnegre mafic complex related to late-Hercynian I-type granitoids in the Catalan Coastal Ranges. Two generations of spinel exist as accessory minerals in these hornblendites: Spl1 forms euhedral crystals included by both olivine and the other spinel; Spl2 forms euhedral to anhedral crystals associated with phlogopite and fibrous colourless amphibole forming pseudomorphs after olivine. Compositions of Spl1 are picotite-Al chromite (Fe#: 77.78-66.60; Cr#: 30.12-52.22; Y3+: 6.99-21.89; 0.10< TiO2%< 0.62). Compositions of Spl2 are pleonaste (Fe#: 37.86-52.12; Cr#: 1.00-15.45; Y3+: 0.31-5.21; TiO2% <0.10%). The two types of spinel define a CrAl trend, mainly due to the substitution (Fe2+)-1Cr-1= MgAl, which is interpreted as the result of mixing between two different mantle-derived melts. The compositions of earlier Spl1 crystals included by olivine are characteristic of Al-rich basalts, related either to a subduction zone or to a post-collisional extensional setting at the end of the Hercynian orogenesis. More aluminous Spl2 would result from crystallization after the refilling of the magma chamber or channel with a less evolved, Al and K-richer mantle-derived melt. As a whole, spinels from similar examples of Hercynian cortlandtites also define a CrAl trend, but starting at much higher Cr# and evolving at higher Fe# than other typical trends of this type. The trend of cortlandtite spinels would be inherited from the heterogeneity of the mantle sources involved in the genesis of the parental mafic melts of these cortlandtites and their subsequent differentiation in an open system.


Catalan Coastal Ranges. Olivine hornblendites. Spinel compositions. Mantle-derived melts.

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DOI: http://dx.doi.org/10.1344/%25x


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