ALBERT

Description: U–Pb zircon data on retrogressed eclogites sampled in the Giuncana locality from the Sardinian Medium-Grade Metamorphic Complex yielded a weighted age of 454 ± 6 Ma. This is in agreement with U–Pb zircon ages of 453–460 Ma obtained from eclogites from the High-Grade Metamorphic Complex. The Giuncana eclogites are very similar to the other well-known Sardinian eclogites. All of the Sardinian eclogites show positive K, Rb, Ba, U and Pb anomalies and negative Nb, La, Ce and Sr anomalies. Th is depleted in the Giuncana eclogites and enriched in those from Punta de Li Tulchi and Punta Tittinosu. All these data reveal clear crustal contamination of the Sardinian Ordovician mantle. REE patterns typical of normal mid-ocean ridge basalt (N-MORB) characterize all of the Sardinian eclogites. The supply of crustal and calc-alkaline materials to the Sardinian mantle during the Ordovician is further confirmed by the fact that most Sardinian eclogites plot on the left side and well above the mantle array in a Th/Yb v. Nb/Yb diagram. In the general Variscan framework of northern Gondwana, the Sardinian eclogites are witness to the most recent back-arc basins generated by the northward opening of the Rheic Ocean.

Description: 〈span〉The ideal formulae of two root-names of the hellandite group, 〈span〉i.e.〈/span〉 mottanaite and ciprianiite, were originally given as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(CeCa)〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉Be〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)REE]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉□〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉(OH)〈sub〉2〈/sub〉. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉REE〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)Ca]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)〈sub〉Σ2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉, respectively (IMA-CNMNC vote 18D). Also, the mineral description is provided for the first Fe〈sup〉3+〈/sup〉-dominant species of the hellandite group, ferri-mottanaite-(Ce), ideally 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉Ce〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Fe〈sup〉3+〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[Si〈sub〉4〈/sub〉B〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉. The type specimen was found in an ejectum collected at Tre Croci (Vetralla, Vico volcanic province, Italy). The empirical formula derived from electron microprobe and LA-ICP-MS analyses, and validated by single-crystal structure refinement is: 〈sup〉〈span〉X〈/span〉〈/sup〉(Ca)〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(Ca〈sub〉0.40〈/sub〉REE〈sub〉0.93〈/sub〉(Th,U)0.544+〈sub〉0.13〈/sub〉)〈sub〉Σ2.00〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉(Fe0.503+Al〈sub〉0.23〈/sub〉Mn0.173+Ti0.174+)〈sub〉Σ1.07〈/sub〉〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.04〈/sub〉Li〈sub〉0.04〈/sub〉〈sub〉0.92〈/sub〉)〈sub〉Σ2.00〈/sub〉[Si〈sub〉4.03〈/sub〉B〈sub〉3.89〈/sub〉O〈sub〉22〈/sub〉] (O〈sub〉1.09〈/sub〉(OH)〈sub〉0.38〈/sub〉F〈sub〉0.53〈/sub〉)〈sub〉Σ2.00.〈/sub〉 Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2 〈span〉V〈/span〉 (meas.) = 85.9(5)°, 2 〈span〉V〈/span〉 (calc.) = 82.5°. The unit-cell parameters are 〈span〉a〈/span〉 = 19.0548(9), 〈span〉b〈/span〉 = 4.7468(2), 〈span〉c〈/span〉 = 10.2560(5) Å, β = 110.906(2)°, 〈span〉V〈/span〉 = 866.58(7) Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 2, space group 〈span〉P〈/span〉2/〈span〉a〈/span〉. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [〈span〉d〈/span〉 values (in Å), 〈span〉I〈/span〉, (〈span〉hkl〈/span〉)] are: 2.648, 100, (013,4¯13); 2.857, 50, (411); 1.904, 48, (023,4¯23,6¯21); 2.919, 44, (212); 3.086, 44, (4¯12); 3.246, 43, (4¯10); 3.453, 36, (2¯12); 4.745, 33, (010).〈/span〉

Description: A peculiar feature of the Himalaya is the occurrence of a system of low-angle-normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during syn-convergent tectonics. We describe the STDS-related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body cross-cutting the STDS, occurs. A detailed multidisciplinary study, integrating field-based, microstructural, petrographic and geochronological analyses was carried out on rocks along this valley. We demonstrate how the non-coaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P) – temperature (T) conditions of 0.9-1.1 GPa and ≥ 750°C at ≥ 24 Ma. This migmatite has been reworked structurally upwards leading to the formation of high-T sillimanite-bearing mylonites. Further upward, medium-T shearing deformed the Malari granite and leucogranite dykes, forming medium-T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar-Ar geochronology, between ~20-15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement and deformation along low-angle-normal faults.

Description: We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser ablation–inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kinematics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonic-metamorphic discontinuity in the Greater Himalayan Sequence (High Himalayan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

Description: A geochronological study was performed on zircon grains from a middle-lower crustal shear zone exposed in the northern sector of the Ivrea-Verbano Zone (Southern Alps, Italy) for the first time. The shear zone developed at the boundary between mafic rocks of the External Gabbro Unit and ultramafic rocks of the Amphibole-Peridotite Unit. It is ~10–20 m wide and can be followed along a NE strike for several km and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin section and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks, and are generally equant, rounded to sub-rounded with dimensions up to 500μm. U-Pb data are mainly discordant and the apparent 206 Pb/ 238 U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high-temperature conditions during late Triassic – early Jurassic. High-temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels, and is inferred to be related to Mesozoic rifting processes at the Adriatic margin.

Description: 〈span〉The ideal formulae of two root-names of the hellandite group, 〈span〉i.e.〈/span〉 mottanaite and ciprianiite, were originally given as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(CeCa)〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉Be〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)REE]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉□〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉(OH)〈sub〉2〈/sub〉. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉REE〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)Ca]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)〈sub〉Σ2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉, respectively (IMA-CNMNC vote 18D). Also, the mineral description is provided for the first Fe〈sup〉3+〈/sup〉-dominant species of the hellandite group, ferri-mottanaite-(Ce), ideally 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉Ce〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Fe〈sup〉3+〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[Si〈sub〉4〈/sub〉B〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉. The type specimen was found in an ejectum collected at Tre Croci (Vetralla, Vico volcanic province, Italy). The empirical formula derived from electron microprobe and LA-ICP-MS analyses, and validated by single-crystal structure refinement is: 〈sup〉〈span〉X〈/span〉〈/sup〉(Ca)〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(Ca〈sub〉0.40〈/sub〉REE〈sub〉0.93〈/sub〉(Th,U)0.544+〈sub〉0.13〈/sub〉)〈sub〉Σ2.00〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉(Fe0.503+Al〈sub〉0.23〈/sub〉Mn0.173+Ti0.174+)〈sub〉Σ1.07〈/sub〉〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.04〈/sub〉Li〈sub〉0.04〈/sub〉〈sub〉0.92〈/sub〉)〈sub〉Σ2.00〈/sub〉[Si〈sub〉4.03〈/sub〉B〈sub〉3.89〈/sub〉O〈sub〉22〈/sub〉] (O〈sub〉1.09〈/sub〉(OH)〈sub〉0.38〈/sub〉F〈sub〉0.53〈/sub〉)〈sub〉Σ2.00.〈/sub〉 Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2〈span〉V〈/span〉 (meas.) = 85.9(5)°, 2〈span〉V〈/span〉 (calc.) = 82.5°. The unit-cell parameters are 〈span〉a〈/span〉 = 19.0548(9), 〈span〉b〈/span〉 = 4.7468(2), 〈span〉c〈/span〉 = 10.2560(5) Å, β = 110.906(2)°, 〈span〉V〈/span〉 = 866.58(7) Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 2, space group 〈span〉P〈/span〉2/〈span〉a〈/span〉. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [〈span〉d〈/span〉 values (in Å), 〈span〉I〈/span〉, (〈span〉hkl〈/span〉)] are: 2.648, 100, (013,4¯13); 2.857, 50, (411); 1.904, 48, (023,4¯23,6¯21); 2.919, 44, (212); 3.086, 44, (4¯12); 3.246, 43, (4¯10); 3.453, 36, (2¯12); 4.745, 33, (010).〈/span〉