The Croda Nera (CN) amphibolites belong to the Austroalpine polymetamorphic Merano-Mules-Anterselva basement (MMA). The protoliths were affected by Variscan regional metamorphism, undergoing maximum P-T conditions typical of garnet amphibolite facies. During Oligocene, the MMA basement was intruded by several plutons. The CN body is located close to the Vedrette di Ries intrusion, which caused medium grade contact metamorphism, not exceeding 550° C, overprinting the older regional event. In CN we identified ortho- and para-amphibolites, based on both petrographic and chemical grounds, employing different discriminant diagrams. According to SCMR, the CN metabasites can be classified as amphibolites s.s., with minor garnet amphibolites, biotite+garnet amphibolites and monomineralic amphibolites. The most abundant phase is a subidiomorphic green amphibole. These crystals define the scistosity of the rock, when present. Sometimes they display a decussate microstructure. Garnet occurs as large porphyroblasts which include Amph + Bt + Ttn. Biotite, when present, is abundant (>10% vol.) and displays decussate microstructure. Plagioclase is usually very scarce, so that the rocks are melanocratic (M ≈ 90). Ti-bearing phases are Ttn and Ilm, the latter being often retrogressed into the former; Rt is absent. Calcite is widespread in variable amounts, from less than 1% vol. (ortho-amphibolites) to 15-20% vol. (calc-silicate para-amphibolites). Sporadic mylonitic fabric caused local retrogression to Act + Ep ± Chl greenschist assemblages. Clinopyroxene relics of likely igneous origin are sporadically observed, always bearing an hornblende corona. Amphibole is dominantly Mg-hornblende, with minor tschermakite and Fe-tschermakite. Garnet has a low Mg content (prp<sub>1-10</sub>) and a highly variable Ca (grs<sub>25-44</sub>), Fe (alm<sub>25-60</sub> and Mn (sps<sub>1-15</sub>) contents. Plagioclase composition ranges from an<sub>12</sub> to an<sub>33</sub>; in places, some pure albite crystals occur, especially within retrogressed deformed layers. Pyroxene relics have a composition close to the Di-Hd join (Ca > 0.94 a.p.f.u.); this fact, together with Na and Ti contents and the microstructural occurrence, points to an igneous origin of this phase, by crystallisation from alkaline magmas. Conventional ion exchange geothermobarometers, where applicable, yield pressures and temperatures lower than expected, indicating post-peak reequilibrations. The bulk-rock chemistry of CN ortho-amphibolites highlights a mafic to ultramafic nature of the protoliths, which are classified as alkali basalts, basanites and picro-basalts. In a few samples, pyroxene cumulus phenomena have been detected. Major and trace element geochemistry indicates that many protoliths were primary liquids; possibly, only slight fractionation of olivine occurred sporadically. The REE are highly fractionated (La<sub>N</sub>/Yb<sub>N</sub> between 21 and 36), with no significant difference between LREE and HREE and no detectable Eu anomaly. Discriminant diagrams employing immobile trace elements (e.g., Zr-Ti/100-3*Y, Th-Hf/3-Ta) confirm the alkaline nature of the protoliths and point to a within-plate tectonic setting for their emplacement. Preliminary Nd isotope data on metaderivates from supposedly primary magmas yield ages of about 1100 Ma, calculated employing a linear evolutionary trend of the depleted mantle. They agree within error with other model ages in the MMA (Meli, unpublished); however such ages do not fit any previous model for this sector of pre-Variscan crust, being at the present state of knowledge somewhat controversial.
Pre-Variscan alkaline magmatism in the Eastern Apls: The Croda Nera metabasites and amphibolites
SACCANI, Emilio;
2005
Abstract
The Croda Nera (CN) amphibolites belong to the Austroalpine polymetamorphic Merano-Mules-Anterselva basement (MMA). The protoliths were affected by Variscan regional metamorphism, undergoing maximum P-T conditions typical of garnet amphibolite facies. During Oligocene, the MMA basement was intruded by several plutons. The CN body is located close to the Vedrette di Ries intrusion, which caused medium grade contact metamorphism, not exceeding 550° C, overprinting the older regional event. In CN we identified ortho- and para-amphibolites, based on both petrographic and chemical grounds, employing different discriminant diagrams. According to SCMR, the CN metabasites can be classified as amphibolites s.s., with minor garnet amphibolites, biotite+garnet amphibolites and monomineralic amphibolites. The most abundant phase is a subidiomorphic green amphibole. These crystals define the scistosity of the rock, when present. Sometimes they display a decussate microstructure. Garnet occurs as large porphyroblasts which include Amph + Bt + Ttn. Biotite, when present, is abundant (>10% vol.) and displays decussate microstructure. Plagioclase is usually very scarce, so that the rocks are melanocratic (M ≈ 90). Ti-bearing phases are Ttn and Ilm, the latter being often retrogressed into the former; Rt is absent. Calcite is widespread in variable amounts, from less than 1% vol. (ortho-amphibolites) to 15-20% vol. (calc-silicate para-amphibolites). Sporadic mylonitic fabric caused local retrogression to Act + Ep ± Chl greenschist assemblages. Clinopyroxene relics of likely igneous origin are sporadically observed, always bearing an hornblende corona. Amphibole is dominantly Mg-hornblende, with minor tschermakite and Fe-tschermakite. Garnet has a low Mg content (prp1-10) and a highly variable Ca (grs25-44), Fe (alm25-60 and Mn (sps1-15) contents. Plagioclase composition ranges from an12 to an33; in places, some pure albite crystals occur, especially within retrogressed deformed layers. Pyroxene relics have a composition close to the Di-Hd join (Ca > 0.94 a.p.f.u.); this fact, together with Na and Ti contents and the microstructural occurrence, points to an igneous origin of this phase, by crystallisation from alkaline magmas. Conventional ion exchange geothermobarometers, where applicable, yield pressures and temperatures lower than expected, indicating post-peak reequilibrations. The bulk-rock chemistry of CN ortho-amphibolites highlights a mafic to ultramafic nature of the protoliths, which are classified as alkali basalts, basanites and picro-basalts. In a few samples, pyroxene cumulus phenomena have been detected. Major and trace element geochemistry indicates that many protoliths were primary liquids; possibly, only slight fractionation of olivine occurred sporadically. The REE are highly fractionated (LaN/YbN between 21 and 36), with no significant difference between LREE and HREE and no detectable Eu anomaly. Discriminant diagrams employing immobile trace elements (e.g., Zr-Ti/100-3*Y, Th-Hf/3-Ta) confirm the alkaline nature of the protoliths and point to a within-plate tectonic setting for their emplacement. Preliminary Nd isotope data on metaderivates from supposedly primary magmas yield ages of about 1100 Ma, calculated employing a linear evolutionary trend of the depleted mantle. They agree within error with other model ages in the MMA (Meli, unpublished); however such ages do not fit any previous model for this sector of pre-Variscan crust, being at the present state of knowledge somewhat controversial.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.