A. Montanini 1
(ales01@ipruniv.cce.unipr.it)
S. Meli 2
(sandrom@dmp.unipd.it)
F. Castorina 3
(fax: 39 6 446482)
M. Thöni 4
(Martin.Thoeni@univie.ac.at)
1 Istituto di Petrografia, Università di Parma, (Italy)
2 Dipartimento di Mineralogia e Petrologia, Università di Padova (Italy)
3 Dipartimento di Scienze della Terra, Università La Sapienza, Roma, (Italy)
4 Institut für Geologie, Geozentrum, Universität Wien (Austria)
Widespread magmatic activity developed in Western Sardinia during Tertiary and Quaternary times: (1) a Oligo-Miocene calc-alkaline cycle and (2) within-plate volcanic activity of Pliocene-Pleistocene age. Xenoliths of mafic metaigneous rocks equilibrated in granulite-facies conditions have been carried up to surface by basanitic eruptions of Pliocene age in the Montiferro volcanic complex which was one of the most active site of Tertiary volcanism. Two varieties of xenoliths can be distinguished: (1) meta-gabbronorites and (2) spinel-rich meta-gabbros sometimes interlayered with pyroxenites.
Xenoliths of gabbronoritic composition consist of plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides and minor amounts of alkalifeldspar, biotite, apatite. They are foliated rocks commonly characterized by relatively large (2-4 mm) orthopyroxene porphyroclasts with clinopyroxene exsolution in a recrystallised granoblastic matrix composed of pyroxenes, plagioclase and opaques; clinopyroxene is more rarely observed as porphyroclast. Subhedral plagioclase grains of likely magmatic origin have been frequently preserved. Both neoblastic and porphyroclastic pyroxenes have moderate Al2O3 contents (2.4-4.9 wt. % in clinopyroxene and 1.9-3.4 wt. % in orthopyroxene) and low to negligible TiO2 and Cr2O3. Composition of relic igneous plagioclase varies between An50 and An66; complex K-feldspar exsolution occur in plagioclase with An < 54. Neoblastic plagioclase is slightly enriched in Ca (An56-72). The meta-gabbronorites are generally quartz-normative and show a relatively narrow range of SiO2 (48.5-53.2 wt.%). Although whole-rock chemistry is sometimes affected by cumulus of orthopyroxene or plagioclase, the composition of most xenoliths reflect variably evolved mafic melts. The geochemical features of melt-like samples, e.g. iron enrichment in AFM diagram and REE patterns with LREE enrichment (LaN/SmN = 1.2-3.3, LaN/YbN = 2.0-5.8) and flat HREE patterns, point to tholeiitic magmas with orogenic affinity as parental magmas of their igneous protoliths. Likewise, the occurrence of orthopyroxene + moderately Ca-rich plagioclase as liquidus phases suggest that magmatic crystallisation occurred at pressure > 0.5-0.6 GPa.
Spinel-rich metagabbros are medium-grained, granoblastic rocks composed of Ca-rich plagioclase, aluminous clino- and orthopyroxene and Al-spinel or Ti-magnetite. Very low contents of incompatible elements (K, Rb, Nb, Zr, P, LREE) together with extreme enrichment in FeOtot (up to 19 wt. %) and CaO (up to ~ 16 wt. %) suggest that the igneous protoliths were cumulitic rocks dominated by Ca-rich plagioclase, clinopyroxene and spinel which probably crystallised from magmas with calc-alkaline affinity (e.g. high-alumina basalts) at P ~ 0.7-1 GPa.
Two-pyroxene geothermometry applied to neoblastic opx-cpx pairs of the gabbronorites gives mean equilibration temperature of ~ 880°C; consistent values (~ 860 °C) have been obtained for the spinel-rich gabbros.
Present-day 87Sr/86Sr (0.70578-0.70647) and 143Nd/144Nd WR ratios (0.51240-0.51256) of the xenoliths are relatively uniform; however, a weak correlation occurs in a Sm/Nd isochron plot. Sm-Nd isotopic data obtained on mineral separates of two gabbronorite xenoliths indicate 143Nd/144Nd values close to the WR present-day ratios suggesting either a very young age of the protoliths (< 100 Ma: underplating of mafic magmas related to the Oligo-Miocene calc-alkaline cycle?) or recent re-equilibration of older rocks (thermal event in the lower crust related to the Cainozoic magmatic activity). A Rb/Sr internal isochron at 6 ± 0.6 Ma obtained for one of these xenoliths is consistent with the age of eruption of the host lavas.