Massimo Tiepolo

In situ Pb geochronology of zircon with laser ablation–inductively coupled plasma–sector field mass spectrometry

Abstract The in situ Pb geochronological capabilities of a laser ablation–inductively coupled plasma–mass spectrometer (LA–ICP–MS) coupling a magnetic sector ICP–MS with a Nd:YAG laser probe working at 213 nm have been tested on three zircon populations with different age (150–294–577 Ma) and radiogenic Pb contents (0.7–10–40 ppm). The influence of scan mode and spatial resolution on age precision and accuracy has also been evaluated. All the signals necessary to independently determine the 206 Pb/ 238 U, 207 Pb/ 235 U, 207 Pb/ 206 Pb and 208 Pb/ 232 Th ratios have been acquired. The external standardization approach has been used to correct for laser induced U/Pb elemental fractionation, instrumental mass bias and sequential acquisition of transient signals. The efficiency of the external standardization correction has been carefully evaluated in each analytical session and residual error with a spot size of 40 μm has been estimated to be in the 0.1–1.5% range. Geologically meaningful ages can be achieved in zircons with less than 1 ppm of radiogenic Pb and with a spatial resolution down to 20 μm. With a spatial resolution of 40 μm, the E-scan mode is more efficient and offers, for zircon with about 40 ppm of radiogenic Pb, an internal precision (2 σ ) on the apparent age to better than 1.1% for all isotope ratios. At lower radiogenic Pb contents, internal precision decreases and, for zircons with radiogenic Pb contents lower than 1 ppm, is better than 7% and 2% on the 207 Pb/ 235 U and 206 Pb/ 238 U ages, respectively. Accuracy is strictly related to Pb* in zircon and ranges between 1% and 5%. At a 20 μm of spot size, internal precision and accuracy are approximately 1.5–2 times lower than at 40 μm.

Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): Comparison with the lower oceanic crust from modern slow-spreading ridges

Field and petrographic relationships together with major and trace element mineral chemistry have allowed us to determine the igneous to high-temperature metamorphic evolution of the gabbroic rocks of the Northern Apennine ophiolites. Gabbroic rocks formed by the intrusion of liquids of normal mid-oceanic-ridge (N-MORB) type in a heterogeneous mantle section under low-pressure conditions. These liquids underwent an igneous-differentiation process controlled by fractional crystallization, most likely associated with late percolation in the gabbroic cumulate pile of a volatile-bearing igneous agent, possibly a trondhjemite-type liquid. Such an igneous-differentiation process produced highly evolved liquids that gave rise to Fe-rich rocks (mostly Fe-Ti oxide-bearing diorites). The gabbroic rocks underwent high-temperature (T 900 °C) recrystallization in ductile shear zones in the absence of seawater-derived fluids. The gabbroic rocks of the Northern Apennine ophiolites bear striking similarities to those recovered from modern slow-spreading ridges, such as the Southwest Indian Ridge, the Mid-Atlantic Ridge (at its intersection with the Kane Fracture Zone, i.e., the MARK area), and the Mid-Cayman Rise. The Northern Apennine ophiolites are interpreted to represent a fossil analogue of a magma-poor slow-spreading center that formed as a result of continental lithospheric extension.

U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons ( c . 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

Reactive flow as dominant evolution process in the lowermost oceanic crust: evidence from olivine of the Pineto ophiolite (Corsica)

The Jurassic Pineto ophiolite from Corsica exposes a ~1-km-thick troctolite–olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow spreading system. To constrain the petrogenesis of the olivine-gabbros, minor and trace element analyses of olivine (forsteritexa0=xa084–82xa0mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompatible element compositions are also shown by olivine (forsteritexa0=xa086xa0mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating through an olivine–plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in composition as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine-gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing lithological variations classically attributed to fractional crystallization processes.

U-Th-Pb “multi-phase” approach to the study of crystalline basement: application to the northernmost sector of the Ivrea-Verbano Zone (Alps)

In situ U-Pb geochronology was carried out on amphibolites and siliciclastic metasediments of the Kinzigite Formation exposed in the northernmost sector of the Ivrea-Verbano Zone (Finero area). The aim is to shed light on the tectono-metamorphic evolution of this intermediate-lower crustal section and its bearing with the evolution of the southern and better known sectors of the IVZ. Based on field observation and petrography a metamorphic gradient gently increasing from amphibolite to upper amphibolite facies (from SE to NW) characterizes the whole metamorphic sequence. Metapelites consist mainly of biotite, quartz, plagioclase, garnet, and sillimanite; muscovite progressively disappears as K-feldspar appears and becomes abundant. Amphibolites are made of green-brown hornblende and plagioclase and may contain clinopyroxene defining thin layers together with plagioclase and titanite. Both metapelites and amphibolites show mylonitic deformation which is more intense towards NW, i.e. towards the lower structural levels. The mylonitic deformation strongly affected the lower crustal metabasic rocks of External Gabbro unit (Finero Mafic Complex). Zircon, monazite and titanite U-Pb geochronology was carried out with laser ablation (LA)-ICP-MS on amphibolites, migmatitic paragneiss and mafic granulites occurring as slivers of the Kinzigite Formation in the External Gabbro unit of the Finero Mafic Complex. The multi-chronological approach allowed recognizing three discrete tectono-metamorphic events, at Permian, Triassic and Jurassic. Zircon and monazite yielded Permian ages suggesting (re)crystallization during an high temperature event characterized by both metamorphism and magmatism. Titanite dating provided Triassic and Jurassic ages that were interpreted as U-Pb resetting ages. A Triassic perturbation of the U-Pb system was also recorded by zircon and monazite as rare domains. The tectono-metamorphic reconstruction of the evolution of the northernmost IVZ, as revealed by the new geochronological data, is only partially in agreement with the temperature-time evolutions depicted for the southern sectors of the IVZ. Permian ages indicating magmatism and high temperature metamorphism are common throughout the IVZ, as well as the Jurassic ages related to local thermal pulses and tectonic activity. Conversely, the occurrence of well-constrained Triassic ages is in fact peculiar of the Finero area. Two possible explanations may account for this Triassic event: Triassic ages are possibly related to the thermal effect and fluid circulation during the emplacement of the External Gabbro unit; or alternatively, they are the response to the ductile deformation largely recognized in the whole area. This study is a further evidence of the necessity of approaching crystalline basement with multiple geochronometers in order to unravel the complete tectono-metamorphic evolution.

Zircon U-Pb dating of a lower crustal shear zone: a case study from the Northern sector of the Ivrea-Verbano Zone (Val Cannobina, Italy)

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, can be followed along a NE strike for several kilometers, and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin sections 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 subrounded with dimensions up to 500 μm. U‐Pb data are mainly discordant, and the apparent ²⁰⁶Pb/²³⁸U 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 to 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.

Zircon U–Pb geochronology of lower crust and quartzo-feldspathic clastic sediments from the Balagne ophiolite (Corsica)

The Balagne ophiolite from central-northern Corsica represents a continent-near paleogeographic domain of the Jurassic Liguria-Piedmont ophiolitic basin. Pillow and massive basalt lavas are primarily associated with Middle–Upper Jurassic pelagic sediments (mostly radiolarites at their base), continental-derived quartzo-feldspathic clastic sediments and ophiolitic breccias containing clasts of gabbros and basalts. The basalt-sedimentary succession is tectonically associated with a slice composed of an intrusive sequence overlain by basalt lavas. A “plagiogranite” from the intrusive sequence was dated by U–Pb zircon geochronology. Although affected by some uncertainty, mainly reflecting common Pb contamination, the U–Pb zircon data suggest a crystallization age of 159xa0±xa03xa0Ma (MSWDxa0=xa06.3), which is coeval with the formation of oceanic lower crust in the Schistes Lustrés units from Alpine Corsica. The predominance of quartz grains preserving typical volcanic shape, the prevalence of prismatic zircons and the arkose whole-rock composition indicate that the continental-derived quartzo-feldspathic clastic sediments have a low degree of textural maturity. U–Pb zircon geochronology carried out on two distinct levels of quartzo-feldspathic clastic sediments identified the predominance of zircons with within error U–Pb dates at ~280xa0Ma; minor components at ~457, ~309 and ~262xa0Ma were also obtained. The U–Pb date distribution is consistent with a source magmaticxa0material mostlyxa0developed during the Variscan orogenic collapse.

Dating of ultramafic rocks from the Western Alps ophiolites discloses Late Cretaceous subduction ages in the Zermatt-Saas Zone

The Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.