Crust-Mantle relationships close at hands. Walking through the Ulten-Nonsberg orogenic lower crust

This two-day excursion focuses on the Variscan crystalline basement of the Ulten-Nonsberg Zone, Eastern Italian Alps. The Ulten-Nonsberg Zone, also known as the Ulten Zone or unità d’Ultimo in the new geological map of Italy (1:50 000 scale), mainly consists of high-pressure paragneisses and migmatites containing disseminated lenses of mantle peridotites. This lithological association originated in a continent-continent collision zone that reached its metamorphic peak at 340-330 Ma. What makes the Ulten-Nonsberg special is the opportunity to see in the field the transition from spinel- to garnet-spinel-peridotites. This transformation occurred during the influx of C-O-H fluids derived from a submerging crustal slab. The fluids were responsible for the blastesis of abundant amphibole ± phlogopite and trace amounts of dolomite and apatite. This field trip aims to make a long tradition of field and petrological studies accessible to a wider audience and to showcase our increased knowledge on how a continent-continent collision works and what are the metasomatic processes driven by the crust-to-mantle mass transfer. The first part of this guide provides a summary of the Ulten-Nonsberg Zone geology and of some of the petrogenetic models proposed. The second part describes the garnet-spinel lenses occurring in the Samerberg area (1st day) and the exciting traverse through the Maddalene range, from the Auerbergtal (Ulten valley) to the Lavazzè valley (Non valley), to address the field relation among mantle peridotites, pyroxenites and migmatites (2nd day)

Amphibole and phlogopite in "hybrid" metasomatic bands monitor trace element transfer at the interface between felsic and ultramafic rocks (Eastern Alps, Italy)

Ultramafic rocks in contact to gneisses in the Mt. Hochwart HP mélange (Eastern Italian Alps) preserve a series of metasomatic mineral zones. A phlogopitite with minor tremolite and accessory zircon and apatite forms close to the gneiss (Zone 1). Zone

Amphibole and phlogopite in “hybrid” metasomatic bands monitor trace element transfer at the interface between felsic and ultramafic rocks (Eastern Alps, Italy)

Ultramafic rocks in the Mt. Hochwart HP m\ue9lange (Eastern Italian Alps) preserve a series of metasomatic mineral zones generated by infiltration of hydrous fluids at T of 660-700\ub0C and P<1.2 GPa, which occurred at the gneiss-peridotite interface. We present the results of in situ LA ICP-MS trace element analysis of minerals from contact lithologies (metasomatic zones and phlogopitite). Trace element composition of hydrous phases (phlogopite and amphibole) in different metasomatic zones indicate mobility of many elements, including elements such as Ta, which are considered to show scarce mobility in fluids. Trace element composition of accessory minerals such as apatite in the phlogopite-rich zone suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. The progressive depletion in some trace elements (LREE and LILE) and the enrichment in Li from the gneiss towards the peridotite suggest a strong influence of bulk composition on the trace element budget of hydrous minerals. Since these metasomatic zones represent geochemical processes occurring at the slab-mantle interface, we can infer that metasomatic reactions between slab derived fluids and ultramafic mantle wedge will follow a specific series of reactions, creating mineral zones similar to those observed in this study. We stress the importance of phlogopite crystallization at the slab-mantle interface as an efficient mechanism to filter LILE from the fluid. Our results further favour the evidence that the primary composition of subduction zone fluids reaching the source region of arc magmas is substantially modified by metasomatic reactions occurring in the mantle wedge

Foglio 026 Appiano - Guida ai percorsi Geologici

La pubblicazione delle carte geologiche dell'Alto Adige in scala 1:25.000 da parte dell'Amministrazione provinciale ha come scopo quello di fornire una moderna cartografia estesa a tutto il territorio provinciale in cui sia possibile trovare le pi\uf9 aggiornate conoscenze geologiche. In questo modo esse possono rappresentare un valido supporto per altri tipi di ricerche ed applicazioni ed un indispensabile strumento di base per la pianificazione territoriale ed urbanistica. Per consentire a tutti i fruitori della cartografia geologica di vedere e toccare con mano gli oggetti descritti, \ue8 nata l'idea di accompargare le carte geologiche a scala 1:25.000 con una "Guida geologica" in cui sono illustrati alcuni itinerari particolarmente rappresentativi ed interessanti, nonch\ue8 belli anche dal punto di vista geoturistico. La "Guida" si rivolge, oltre ai geologi (comprese le universit\ue0 che spesso utilizzano il nostro territorio per escursioni con studenti), a tutti gli appassionati della montagna e della natura in generale, sia residenti sia turisti. Il linguaggio usato e l'impostazione con approfondimenti per gradi speriamo consenta un'agevole fruizione sia agli esperti, sia agli appassionati

A petrographic study of carbonate phases in the Ulten Zone ultramafic rocks: Insights into carbonation in the mantle wedge and exhumation-related decarbonation

We present a comprehensive petrographic study on carbonates in orogenic mantle wedge-derived peridotites from the Ulten Zone (UZ) in the Eastern Italian Alps representing a fragment of the Variscan belt. These peridotites are found in ultramafic bodies incorporated in high-grade crustal rocks from a former continental slab and are characterized by highly variable mineral assemblages and microstructures, which reflect their evolution from spinel peridotite in a hot mantle wedge, to garnet peridotite after incorporation into the subducting slab and finally exhumation accompanied by retrogression. Carbonate phases in UZ peridotites are observed in diverse textural sites and can be related to particular petrographic peridotite types. Inclusions of dolomite and dolomite-breakdown products in primary spinel from coarse-grained protogranular peridotites indicate that carbon-bearing liquids were introduced into the mantle wedge at a time before achieving garnet stability. Discrete dolomite grains occur in fine-grained deformed garnet-bearing peridotite and are suggested to have formed simultaneously with hydrous phases (amphibole, apatite) from a carbon-bearing aqueous crustal slab-derived fluid during garnet stability. Intergrowths of calcite and brucite occur mainly in serpentinized fine-grained garnet-amphibole peridotites and are interpreted to be products of dedolomitization (CaMg(CO3)2 + H2O \ue2\u86\u92 CaCO3 + Mg(OH)2 + CO2), thus decarbonation, during exhumation. Veins of dolomite and calcite-brucite indicate secondary dolomite formation from carbon- bearing fluids due to interaction with slab-derived crustal fluids during retrogression on the exhumation path of the UZ peridotite. Magnesite veins and calcite veins that are texturally linked to alteration features are probably of low-temperature origin. In summary, this petrographic study reveals multi-stage carbonation of the UZ peridotites during residence in the mantle wedge and decarbonation with the release of a carbon species during low-temperature peridotite reaction with aqueous fluids leading to serpentinization accompanied by dolomite breakdown, thus providing important constraints on the carbon budget and carbon cycling in collisional settings. Overall, the ubiquity of carbonates in a variety of textural settings in the UZ peridotites suggests that the supra-subduction zone mantle in continental settings represents an efficient carbon trap

Polymineralic inclusions as tracers of multistage metasomatism in a paleo mantle wedge

Paleosubduction zones are key areas where slab-derived fluids interact with the overlying mantle wedge. Slab fluids play therefore an important role in the deep element cycle and can metasomatize ultramafics showing particular mineral assemblages. The nature of slab fluids is usually investigated from fluid inclusions and/or multiphase solid inclusions, as well as from incompatible elements in mantle minerals. We have made a study on a garnet-peridotite from the Variscan Ulten Zone (Eastern Alps, Italy) in which peculiar polymineralic inclusions (PI) in garnet provide a wealth of information on the interaction between mantle wedge and slab fluids. The selected sample shows a fine-grained amphibole-olivine-orthopyroxene-spinel-clinopyroxene matrix and a fractured large garnet porphyroclast, surrounded by a kelyphitic corona. Garnet core and rim host randomly distributed PI, which may be as large as 284,000\u202f\u3bcm2. The PI show irregular, fracture-controlled shapes and often contain unusual mineral associations. Many PI are directly associated with Mg-hornblende- or chlorite-sealed micro-fractures crosscutting garnet. The most abundant PI contain spinel and amphibole as the main minerals. Spinel-bearing PI consists of high-Cr spinel (Cr#\u202f=\u202f0.31\u20130.41) surrounded by amphiboles (magnesio-hornblende), locally associated with dolomite, sapphirine, calcite, sulphides and kinoshitalite (i.e. a Ba-rich phlogopite). Amphibole-bearing PI contain zoned amphiboles (tschermakitic cores surrounded by Mg-hornblende, which is rimmed by tremolite and Mg-amphibole), dissakisite (an REE-bearing epidote), apatite, dolomite, calcite, spinel, sapphirine, chlorite, kinoshitalite, and sulfides. The retrieved mineral reaction history and the major and some incompatible elements (Cl, Ba, Sr) of PI minerals provide clues about the composition of metasomatic fluids. The studied PI formed in garnet fractures during exhumation by reactions between garnet and (1) slab-derived COH fluids and (2) late-stage saline brines. Metasomatic fluids/brines were rich in volatile elements and had a crustal signature (enrichment in LILE and LREE) with variable Cl contents and CO2/H2O ratios. Products of early garnet-fluid reactions became reactants of later reactions, thus generating complex textures, including replacement textures of sapphirine over spinel, calcite over dolomite and chlorite over Mg-hornblende. Textures and compositions of the PI testify to a multistage metasomatic overprint at peak P-T conditions above 2\u202fGPa (M1), during the first exhumation stage at 775\u20131000\u202f\ub0C, below 2\u202fGPa (M2) and finally at 775\u202f\ub0C and 1\u202fGPa (M3). In this study we demonstrate that Gibbs free energy minimization may aid in distinguishing between multiphase solid inclusions and polymineralic inclusions; the latter do provide valuable information on the evolution of retrograde P-T paths in the paleosubduction channel, the relative chronology of metasomatic events and clues on the chemical signature of the fluids

High-pressure tectono-metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly-deformed basements, the loss of information on pre-kinematic phases of mylonites hinders large-scale correlations based on tectono-metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P) – temperature (T) – deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine-grained K-white mica and chlorite enveloping mica-fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg-rich cores and Mg-poor rims. The amphibolite contains green amphibole+ plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre- to centimetre-scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P-T constraints that are linked to the reconstructed metamorphic-deformational stages. For the first time an early high-pressure stage at > 11 kbar and 510 °C was constrained, followed by a temperature peak at 550-590 °C and 9-10 kbar and a retrograde stage (< 475 °C, < 7 kbar), during which ductile shear zones developed. The inferred clockwise P-T-D path was most likely related to crustal thickening by continent-continent collision during the Variscan orogeny. A comparison of this P-T-D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono-metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established

Zircon and titanite recording 1.5 million years of magma accretion, crystallization and initial cooling in a composite pluton (southern Adamello batholith, northern Italy)

The southern part of the Adamello batholith (the so-called "Re di Castello unit") is an example of a composite pluton, ranging from gabbro to granodiorite in composition. U-Pb dating of single-zircon crystals from four tonalitic to gramodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies, showing apparent durations of growth between 90 and 700 ka. The youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite. The formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization, in contrast to antecrystic zircons present in the same sample, which had formed earlier in the magmatic column or were derived from re-mobilized earlier magma. The autocryst-derived "emplacement" ages range from 42.43 0.09 Ma to 40.90 +/- 0.05 Ma, recording 1.5 Ma of intrusion and crystallization history. We anticipate that extended periods of zircon crystallization may be common in silicic rocks, whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0.1-0.2% in (206)Pb/(238)U age. Decreasing Th/U ratios of dated zircons within one melt batch document the depletion of the residual melt portion in Th due to the contemporaneous crystallization of titanite. Preliminary Hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern Re di Castello unit represent hybrid melts with an important crustal component (epsilon(Hf) between -2.8 and +3.0). Subsequently emplaced melts are more juvenile at epsilon(Hf) values at +6.4 to +8.9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system