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: New insights into the origin of high-Mg andesites are inferred from the mineral chemistry and U–Pb geochronology of Tertiary amphibole-rich ultramafic intrusive rocks (hornblendites) and included clinopyroxene-bearing dunitic clots from the southern Adamello batholith (Central Alps). The hornblendites consist mostly of amphibole grains with brown cores (Ti-pargasite) that grade through brownish-green (Mg-hornblende) to light green (edenite) rims. Brown amphibole contains olivine (Fo = 85–87 mol %) and clinopyroxene inclusions with irregular boundaries indicating disequilibrium with the host amphibole. The ultramafic clots are interpreted to represent fragments of older cumulates dismembered by the injection of the amphibole-forming melts, thereby providing evidence for a melt–rock reaction process. Amphibole from the hornblendites shows a marked trace element zoning. From the brown core outward to the brownish-green portion of a single crystal, a significant enrichment is observed in light rare earth elements, Th and U, coupled with a decrease in Ti and heavy rare earth elements. The melt in equilibrium with the brownish-green amphibole has an adakitic trace element signature (e.g. high La N /Yb N and Sr/Y). Based on amphibole/liquid partition coefficients, a fractional crystallization process driven by amphibole could explain most of these chemical variations. However, the outward increase of highly compatible elements in amphibole (e.g. Mg, Ni, Co, and Zn) argues against closed-system fractional crystallization. The assimilation of olivine is considered the most efficient mechanism to supply or buffer the highly compatible elements in the evolving system during amphibole crystallization. In situ U–Pb zircon geochronology of hornblendites and associated amphibole gabbros reveals the occurrence of inherited cores, thereby providing evidence for assimilation of crustal material. We propose that a differentiation process controlled by amphibole crystallization and assimilation of slightly older ultramafic cumulates may produce melts rich in SiO 2 and MgO with adakitic trace element signatures.

Description: Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event. The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

Description: New insights into the role of amphibole in arc magma petrogenesis are provided by the mineral chemistry and U–Pb geochronology of Cretaceous amphibole-rich mafic rocks and associated granitoids from Shikanoshima Island (Kyushu, Japan). In the northeastern part of Shikanoshima Island a relatively large body (about 600 m in length) of amphibole-rich mafic rocks is found within granodiorite host-rocks. The core of the mafic body consists of amphibole-rich gabbrodiorite with a porphyritic texture. Towards the host granodiorite the porphyritic texture is progressively lost and a band of relatively homogeneous medium- to fine-grained mafic rock marks the boundary with the granitoid rocks. The amphibole-rich porphyritic gabbrodiorite consists of large amphibole grains (up to 60 vol. %) characterized by brown cores, occasional inclusions of clinopyroxene, and green rims. These large amphibole grains are dispersed in a fine-grained matrix consisting of green amphibole, clinopyroxene and plagioclase. Literature whole-rock data on the mafic rocks from Shikanoshima Island suggest that they are the intrusive counterparts of high-Mg andesite (HMA). Major and trace element mineral compositions reveal a marked chemical contrast between the brown amphibole (and its inclusions) and the matrix minerals, suggesting that they are not on the same liquid line of descent. The brown amphibole and its clinopyroxene inclusions were inherited from amphibole-rich ultramafic intrusive crustal rocks (e.g. hornblendites) crystallized from a melt with a chemical composition close to that of continental arc basalts. U–Pb geochronological data suggest that the xenocrystic material is about 20 Myr older than the matrix minerals. The matrix mineral crystallized from a parental liquid similar to sanukite-type HMA and with a trace element signature characterized by strong enrichment in elements with high crustal affinity and depletion in heavy rare earth elements. Green amphibole is a common mineral in all the studied lithologies; this allowed us to monitor the compositional variations in the liquid from which it crystallized moving from the core of the mafic complex to the host granodiorite. The data reveal that the interstitial melt had interacted with a melt enriched in elements with a high crustal affinity that, given the close association in the field, is inferred to be the host granitoid. These results favour an origin for sanukite-type HMA not from primary mantle melts but from mantle melts that have been affected by crustal processes and have been contaminated by crustal material. The major and trace element composition of the brown amphibole from the Shikanoshima Island mafic rocks is compared with that of brown amphibole from other amphibolite-rich intrusive rocks in orogenic settings worldwide (Alpine chain and Ross Orogen). The observed similarities suggest that the amphibole-rich mafic rocks are the expression of a magmatic process with a common geochemical affinity that is independent of the age and local geodynamic setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks could be a common feature of all collisional systems and thus represent a ‘hidden’ amphibole reservoir in the arc crust. We show that amphibole plays a major role in the petrogenesis of sanukite-type HMA but is also expected to play a major role in the differentiation of many other arc magmas.

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: This paper presents a winter carbon budget for the Northern Adriatic Sea, obtained through direct measurements during two multidisciplinary cruises and literature data. A box model approach was adopted to integrate estimates of stocks and fluxes of carbon species over the total area. The oligotrophy at the basin scale and the start of primary productivity well before the onset of spring stratification were observed. In winter, the system underwent a complete reset, as the mixing of water masses erased any signal of previous hypoxia or anoxia episodes. The Northern Adriatic was phosphorus depleted with respect to C and N availability. This fact confirms the importance of mixing with deep-sea water for P supply to biological processes on the whole. Despite the abundant prokaryotic biomass, the microbial food web was less efficient in organic C production than phytoplankton. In the upper layer, the carbon produced by primary production exceeded the fraction respired by planktonic community smaller than 200 µm. On the contrary, respiration processes prevailed in the water column below the pycnocline. The carbon budget also proved that the Northern Adriatic Sea can be an effective sink for atmospheric CO 2 throughout the entire winter season.

Description: Tectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 x 10 –14 s –1 within the country rocks during granodiorite intrusion.

Description: The volcano of Tallante (Pliocene) in the Betic Cordillera (Spain) exhumed a heterogeneous xenolith association, including ultramafic mantle rocks and diverse crustal lithologies. The latter include metagabbroids and felsic rocks characterized by quartz-rich parageneses containing spinel ± garnet ± sillimanite ± feldspars. Pressure–temperature estimates for felsic xenoliths overlap (at 0.7–0.8 GPa) those recorded by the mantle-derived peridotite xenoliths. Therefore, we propose that an intimate association of interlayered crust and mantle lithologies characterizes the crust–mantle boundary in this area. This scenario conforms to evidence provided by the neighbouring massifs of Ronda and Beni Bousera (and by other peri-Mediterranean deep crust/mantle sections) where exhumation of fossil crust–mantle boundary reveals that this boundary is not sharp. The results are discussed on the basis of recent geophysical and petrological studies emphasizing that in non-cratonic regions the crust–mantle boundary is often characterized by a gradational nature showing inter-fingering of heterogeneous lithologies. Silica-rich melts formed within the crustal domains intruded the surrounding mantle and induced metasomatism. The resulting hybrid crust–mantle domains thus provide suitable sources for exotic magma types such as the Mediterranean lamproites.