Low-to-medium pressure crustal melting and granite formation in the Variscan High-Grade Metamorphic Complex of the Asinara Island (Italy)

Here we report the first results of the petrological investigation performed on the high-grade, partially melted rocks belonging to the so-called High-Grade Metamorphic Complex (HGMC) cropping out at Punta Scorno area, north Asinara Island (Sardinia, Italy). We focus on banded amphibolites, ortho- and paragneisses and diatexite/granites. The gradual transition from amphibolites to gneisses to diatexite/granites is clearly visible on the field and makes this area an excellent target for tracing melt production processes in the medium-lower continental crust.

The amphibolite forms a large lens (200 x 50m) in the SE part of the studied area. The more massive and darker portions show under microscope observation a slightly foliated structure given by iso-oriented hornblende and elongated plagioclase. K-feldspar and Fe-sulphides are common throughout the rocks and align to the foliation. This rock also contains randomly oriented biotite, quartz, zoisite and white mica. Plagioclase is Ca-rich (An_78-96), whereas biotite has X_Fe of 0.48-0.51. In the samples where the cm-scale banding is more visible, the amphibole is more randomly oriented. In these portions, biotite has higher X_Fe (0.55-0.60), the most abundant amphibole is grunerite (Fe-rich) which is overgrown by hornblende, and plagioclase is poorer in Ca (An_43-58). Melt pseudomorphs of quartz are developed in contact with plagioclase and biotite. Strongly resorbed garnet porphyroblasts of almandine-rich garnet (Alm_67-68Grs_13-16Sps_9-10Prp_7-10) are associated with amphiboles and biotite.

Moving toward north the amphibolites transition to amphibole-bearing orthogneisses. The orthogneiss is foliated at the outcrop scale, although this feature is barely noticeable under the microscope. This rock has a similar assemblage to the banded portion of the amphibolite. However, the matrix contains more quartz and plagioclase, and garnet forms anhedral porphyroblasts. Locally garnet preserves a euhedral shape when in contact with biotite. Grunerite is present as anhedral crystals and it is overgrown by hornblende. Ca dominates the plagioclase composition (An_48-81), whereas biotite and garnet are richer in Fe (Alm_71-74Grs_12-14Prp_7-13Sps_3-6) than in amphibolites.

Paragneisses occur both in the N and S parts of the studied area and show variable grain size and schistosity. They are characterized by iso-oriented biotite and white mica flakes, quartz-plagioclase elongated lenses, a large amount of apatite and melt pseudomorphs in the more felsic layers. Plagioclase has variable composition, and few grains of garnet (Alm_~69Sps_~22Prp_~7Grs_~3) are also present.

Leucogranite is composed of fine-grained quartz, feldspar and plagioclase with scattered biotite flakes and skeletal garnet richer in Mn (Alm_66-76Sps_10-26Grs_2-11Prp_3-8). The contact of leucogranite with coarser-grained paragneisses is marked by granophyric intergrowths and myrmekites. Preliminary geochronological data on magmatic zircons from leucogranites yielded an age of 295 ± 3.5 Ma. Coarse-grained granite contains euhedral crystals of feldspar, plagioclase, quartz and garnet with interstitial biotite and white mica. Garnet composition is similar to the one from leucogranite (Alm_66-76Sps_10-24Grs_5-11Prp_3-4).

This work provides the first petrological insights in the area and constitutes an introduction to a more detailed study of P-T evolution and re-melting processes in the HGMC at Asinara Island.