Titanite petrochronological data across the continental crust section exposed in Val d'Ossola (Ivrea-Verbano Zone, Italy)

Titanite-bearing calc-silicates and mafic gneisses, metamorphosed under amphibolite- to granulite-facies conditions, crop out in Val d'Ossola area (Ivrea-Verbano Zone, Italy). The Ivrea-Verbano Zone represents an exhumed section of the pre-Alpine middle to lower continental crust which escaped the Alpine subduction, thus provides a unique opportunity to study continental crustal processes and evolution. Among several samples, three, collected from different locations, were chosen for detailed analyses of titanite. Petrochronology of titanite was performed with Laser ablation split-stream (LASS) technique on petrographic thin sections. Petrochronological results on titanite do not define clear correlations with chemistry except for one sample. Rare earth elements (REE) patterns of titanite from the three samples are apparently different in terms of average concentration (i.e., lower or upper 1000 times CI), shapes and occurrence or absence of Eu negative anomaly. Al/Fe vs ΣLREE and Fe content vs Zr/Y plots show that the studied samples coincide with metamorphic rock field deriving from calc-silicates and mafic protoliths, as previously demonstrated in literature. Any compilations of petrochronological data on titanite from the metamorphic volcano-sedimentary sequence of Val d'Ossola can be found in literature. Therefore, these data represent a new insight on an accessory mineral phase whose significance and scientific interest are rising in the last years. Future studies of the evolution of these kinds of rock, widespread in the high-grade metamorphic basements, will benefit from these data as a term of comparison.

a b s t r a c t Titanite-bearing calc-silicates and mafic gneisses, metamorphosed under amphibolite-to granulite-facies conditions, crop out in Val d'Ossola area (Ivrea-Verbano Zone, Italy).The Ivrea-Verbano Zone represents an exhumed section of the pre-Alpine middle to lower continental crust which escaped the Alpine subduction, thus provides a unique opportunity to study continental crustal processes and evolution.Among several samples, three, collected from different locations, were chosen for detailed analyses of titanite.Petrochronology of titanite was performed with Laser ablation split-stream (LASS) technique on petrographic thin sections.Petrochronological results on titanite do not define clear correlations with chemistry except for one sample.Rare earth elements (REE) patterns of titanite from the three samples are apparently different in terms of average concentration (i.e., lower or upper 10 0 0 times CI), shapes and occurrence or absence of Eu negative anomaly.Al/Fe vs LREE and Fe content vs Zr/Y plots show that the studied samples coincide with metamorphic rock field deriving from calcsilicates and mafic protoliths, as previously demonstrated in literature.Any compilations of petrochronological data on titanite from the metamorphic volcano-sedimentary sequence of Val d'Ossola can be found in literature.Therefore, these data represent a new insight on an accessory mineral phase whose significance and scientific interest are rising in the last years.Future studies of the evolution of these kinds of rock, widespread in the high-grade metamorphic basements, will benefit from these data as a term of comparison. ©

Value of the Data
• Titanite provides constraints on the genesis, metamorphism and age of the host rock [2][3][4].
• Petrologists dealing with the evolution of the fossil passive Adriatic margin of the Alpine Tethys can found interesting information on titanite chemistry and ages.• Regional-scale studies could benefit from these data for comparison with similar rocks.
• Any data about titanite in Val d'Ossola are available in literature.
• Petrological interest for Ti-bearing mineral phases such as titanite is growing in the last years [ 4 ].• Titanite petrochronological data from different compositional rocks are very few in literature.

Background
Titanite is a useful accessory mineral that, in the last decades, has gained great interest in the petrological community since is considered as a powerful petrochronometer that allow the comprehension of Earth's continental crustal processes [ 4 ].Moreover, titanite is a widespread accessory mineral that crystallizes over a wide range of crustal pressures and temperatures in many rock types, including metamorphosed mafic rocks, calc-silicates as well as felsic calc-alkaline igneous rocks [3][4][5].Recently, titanite petrochronology provided unique information about the timing of shear zones [e.g., 2 ].Nevertheless, since the strong reactiveness of titanite during metamorphic reactions [ 3 ], it can experience and record multi-stage tectono-metamorphic events not always easily distinguishable.
The aim behind this dataset is to provide new petrochronological data of the best preserved (and studied) section of middle to lower continental crust of the world, i.e., the Ivrea-Verbano Zone (IVZ) in the Southern Alps.In particular, we focus on the titanite-bearing rocks outcropping in Val d'Ossola showing progressively higher temperature conditions with increasing crustal depth [ 6 ].
Although numerous geochronological and thermochronological data relative to zircon, monazite, mica, hornblende and rutile are available for the IVZ [see 7 for an extensive review]; titanite dating are still rare [ 2 ].
The spatially progressive switch from granulite to amphibolite facies is marked by a transition zone characterized by abundant migmatites that were involved in several high-temperature (granulite-amphibolite facies) mylonitic shear zones, including the Anzola shear zone ( Fig. 1 B; [ 1 , 6 , 7 ]).These structures are thought to have accommodated, since the Triassic, crustal thinning in the mid-low crust during the early Tethyan rifting [ 14 ] and reference therein.At the same time, brittle-ductile shear zones and faults developed in the upper crust (e.g., the Pogallo Line, PL).
Both the activity of the Pogallo Line and the Anzola shear zone is constrained between Triassic and Jurassic times through Ar-Ar and K-Ar dating on micas [ 7 ].

U-Pb LASS-ICP-MS petrochronology
3.3.1.Calc-silicate (MV04B) -Ornavasso -amphibolite-facies A total of 59 in-situ analyses were performed on 27 titanite grains ( Table 2 ).The U-Pb data do not show good alignments and are delimited by lower intercepts at about 274 and 228 Ma ( Fig. 3 A).For this sample, U-Pb data show a weak correlation with Fe, Y and Sc.The common Pb (Pb C ) is broadly inversely correlated with Y, whereas Fe and Sc looks lower for the U-Pb data defining younger intercepts ( Table 2 ; Fig. 3 A, B, C).The REE pattern show a flattened trend with average concentrations < 10 0 0 times CI, a weak depletion in LREE and a slight Eu negative anomaly ( Fig. 3 F; Table 5 ).

Mafic gneiss (AN10) -anzola -upper amphibolite-to granulite-facies
A total of 60 analyses were performed on 23 grains ( Table 3 ).The U-Pb data do not show any correlation with chemistry and textural position.All the isotopic ratios are delimited by lower intercepts at about 253 and 195 Ma ( Fig. 3 D).The REE pattern mostly mimic the REE pattern of calc-silicate (MV04B) at higher concentration, exceeding 10 0 0 times CI in most cases ( Fig. 3 F; Table 6 ).

Calc-silicate (MV05) -Anzola -upper amphibolite-to granulite-facies
A total of 60 analyses were performed on 17 grains ( Table 4 ).The U-Pb data define a cluster close to the Concordia curve without apparent alignments ( Fig. 3 E).However, the 207 Pbcorrected age spread from 253 to 196 Ma ( Table 4 ).Also, for this sample we do not observe significant correlations between U-Pb data and textural position (core vs rims).We rather observed a correlation between U-Pb data and Sr concentrations ( Fig. 3 E).The REE pattern shows a strong fractionation of LREE ( > 10 0 0 times CI) over HREE ( < 10 0 0 times CI) and an apparent Eu negative anomaly ( Fig. 3 F; Table 7 ).

Material and methods
Titanite grains from studied samples were first selected after preliminary observations of polished thin sections under a polarization microscope and then producing detailed images by using a scanning electron microscope (SEM-BSE) Tescan Mira3 XMU-series FESEM equipped with an EDAX-EDX at the "Arvedi" laboratory at the University of Pavia.

U-Pb isotopic and trace-element data using LASS-ICP-MS analysis
U-Pb isotopic concentrations and trace-element compositions in titanite grains were collected simultaneously from the same spot using Laser ablation split-stream (LASS) technique at the University of California Santa Barbara following methods of [ 18 , 19 ].Instrumentation consists of a Photon Machines 193 nm ArF Excimer laser and 'HelEx' ablation cell coupled to a Nu Instruments HR Plasma high-resolution ICPMS (U, Th, and Pb isotopes) and an Agilent 7700X

Fig. 2 .
Fig. 2. A, B, C) thin section scans of studied samples; C, D, E) BSE image of the titanite locations and textures within the different samples; F, G, H, I, L, M) Detailed BSE images of titanite showing the main textural features.

Fig. 4 .
Fig. 4. Representative trace-elements concentrations diagrams of titanite from calc-silicates and mafic gneiss (see the legend for details).A) Al/Fe versus LREE cross plot.B) Fe (ppm) versus Zr/Y cross plot.Data are represented as cloud and colored differently to distinguish the samples.
2024 The Authors.Published by Elsevier Inc.

Table 1
[ 15 ]f locality, coordinates and mineral assemblages of the studied samples from Val d'Ossola transect from SE to NW. Mineral abbreviations are after[ 15 ].

Table 2
LASS U-Pb results for titanite from calc-silicate sample MV04B.

Table 3
LASS U-Pb results for titanite from mafic gneiss sample AN10.

Table 5
LASS trace element concentration results for titanite from calc-silicate sample MV04.

Table 6
LASS trace element concentration results for titanite from mafic gneiss sample AN10.

Table 7
LASS trace element concentration results for titanite from calc-silicate sample MV05.