MICROSTRUCTURES AND COMPOSITIONS OF MELT INCLUSIONS FROM JUBRIQUE (S SPAIN): IMPLICATIONS FOR ANATEXIS

Melt inclusions (MI) are small droplets of silicate liquid trapped by minerals growing either with or from the melt [1]. Most MI in anatectic terranes appear partially or totally crystallized due to slow cooling at depth, and have been named “nanogranites” [2]. MI represent a new and powerful method to study anatexis, because they can give information about melt compositions at the source region of crustal granites, including concentrations of H₂O [3]. There are very few studies of nanogranites, and only one has reported the presence of MI along a prograde metamorphic sequence from metatexites to diatexites [4]. We present a new occurrence of nanogranites along the prograde metamorphic sequence of Jubrique, located on top of the Ronda peridotites (S Spain). Jubrique represents a complete though strongly thinned (≤5 km) section of upper to middle-lower continental crust, ranging from carbonates and low-grade phyllites at the top to felsic Grt-bearing gneisses at the bottom and in contact with the peridotites. MI show partially irregular to well facetted negative crystal shapes and occur in cores and rims of Grt porphyroblasts along the sequence of gneisses. They show a variable size, from ~5-10μm to several tens of μm in diameter; some of them reach up to ~200-300μm. Because of the large size, nanogranites at Jubrique have the potential to be remelted and analyzed by EMP but also by LA-ICP-MS. Nanogranites are composed of rare glass, daughter phases Qtz, Pl, Kfs, Bt and Ms, and solid inclusions of Ky and less frequently Gr, Hc, Rt, Ilm, Zrn, and Mnz. Ky was the main solid phase that favored the trapping of MI by poisoning crystal surfaces during Grt growth. Previous studies concluded that anatexis in the gneisses at the contact with the peridotites occurred in the Sil field during decompression and thining. The presence of Ky+Rt within MI, and their occurrence in the high-P cores of Grt, indicate that partial melting in granulites and migmatites initiated at high P conditions, and that most Grt in these rocks crystallized in the presence of melt.

[1] Cesare et al. (2011) J. Virt. Expl., 40, paper 2. [2] Cesare et al. (2009) Geology, 37, 627-630. [3] Bartoli et al. (2013) Geology, 41, 115-118. [4] Bartoli (2012) Ph.D. Thesis, Univ. of Parma, Italy.