New U-Pb isotope constraints on timing of metasomatism in a subcontinental mantle: evidence from zircon at Finero, Italy

Malitch K.N.*, Knauf V.V.**, Badanina I.Yu.*, Garuti G.***, Presnyakov S.L.*

* A.P. Karpinsky Russian Geological Research Institute (VSEGEI), St. Petersburg, Russia; ** NATI Research JSC, St. Petersburg, Russia; *** University of Modena and Reggio Emilia, Italy.

 

Introduction. The Finero phlogopite-peridotite represents a metasomatized residual mantle harzburgite, exposed at the base of the lower-crust section in the Ivrea Zone, Western Alps (Hartmann and Wedepohl, 1993). It forms the core of a concentrically zoned sequence of internal layered gabbro, amphibole-rich peridotite and external gabbro (Fig. 1). The phlogopite peridotite contains small-size chromitite bodies, with a suite of accessory minerals such as phlogopite, apatite, Ca-Mg carbonates, zirconolite, zircon, thorianite and uraninite, proposed to form during alkaline-carbonatitic metasomatism process within the mantle (Zaccarini et al., 2004). Various aspects of the metasomatism at Finero are currently debated including the provenance of contaminant fluids (mantle or crust), the geodynamic environment (subcontinental mantle plume vs. subduction setting) and the extent of multiple metasomatic events, which seemingly span about 85 Ma (29313 Ma, Voshage et al., 1987 and 2082 Ma, Grieco et al., 2001). In this study, the combined application of a non-destructive technique to separate zircon from their host rocks and in-situ analytical techniques for compositional and isotopic analysis has provided new, more detailed and more precise age data on the formation of chromitite and related metasomatic events within a mantle tectonite at Finero.

 

Fig. 1. Simplified geological map of the Finero complex, showing location of the occurrences of chromitite investigated (open triangles).

 

Samples and analytical methods. Chromitite samples were collected from the dump in the prospecting trenches of Alpe Polunia and Rio Creves (Fig. 1). In thin sections, zircon at Rio Creves occurs as relatively large (up to 200 μm) grains characterized by subhedral to euhedral shapes, whereas at Alpo Polunia zircon mostly show subhedral to anhedral shapes. It has been commonly observed in contact with chromian spinel and olivine. About two hundred grains of zircon were extracted using a ppm-mineralogy technique at NATI Research JSC (St. Petersburg, Russia), reaching concentration factors between 40000 and 100000. Grains of zircon were hand picked from each concentrate, imaged by SEM and subsequently mounted in epoxy blocks together with grains of the TEMORA (Middledale Gabbroic Diorite, New South Wales, Australia) and 91500 (Geostandard zircon) reference zircons. Transmitted and reflected light photomicrographs and CL images were made in order to select grains and choose sites for analyses, avoiding cracks and inclusions. The Sensitive High-Resolution Ion Microprobe (SHRIMP-II) at the VSEGEI was used to perform 50 in-situ U-Pb analyses by applying a secondary electron multiplier in a peak-jumping mode following the procedure described by Williams (1998). Concordia age calculations were performed with ISOPLOT v. 3.00 (Ludwig, 2003) software.

Results. Separated zircon grains at Alpe Polunia are subhedral and colorless. They are characterized by dominant smoky cathodeluminescense, with faint zoning or almost no internal pattern. Zircon grains at Rio Creves form two distinct populations. Dominant zircon population is pale pink showing different shape features (subhedral, subrounded or elongated, frequently with slightly rounded edges). In cathodoluminescense, the main set of population is represented by complex grains, which show development of core-rim relationship (most likely recrystallized rim on a preserved core). Diffusive zoning patterns, which do not follow the grain outlines, has also been observed in cathodoluminescense images. Subordinate zircon population show similar features revealed for zircons at Alpo Polunia.

Nine zircon grains from Alpo Polunia used for the U-Pb determinations yielded two groups of concordant U-Pb ages (205.32.0 Ma, with mean square of weighted deviates (MSWD) = 0.018, probability (P) = 0.89, n=6 and 194.72.0 Ma, MSWD=0.3; P=0.58, n=6, respectively), being identical to age estimates as revealed in a relative probability plot (Fig. 2A). At Rio Creves, three main age clusters has been recognized. The youngest age cluster, typical for subordinate colorless zircon population and rims in complex grains of dominant pale pink population, show two concordant 206Pb/238U ages (e.g., 208.64.0 Ma, MSWD=2.0; P=0.16, n=8 and 194.93.4 Ma, MSWD=0.45; P=0.50, n=3, respectively). These ages are almost identical to those of zircon at Alpo Polunia. Two older age clusters are characterized by the cores and core overgrowths from composite grains. They yielded concordant 206Pb/238U ages of 288.3 7.3 Ma (MSWD=3.3, n=6) and 248.63.3 Ma (MSWD=0.13, P=0.72, n=8), respectively. These ages are reproduced within uncertainty in a relative probability diagram (Fig. 2B). The significant time span represented by distinct zircon age groups is interpreted as representing the timing of multiple events during zircon growth.

Fig. 2. Relative probability histogram for ion-probe U-Pb zircon ages at Alpe Polunia (A) and Rio Creves (B).

 

Discussion and concluding remarks. Since the pioneering work of Exley et al. (1982), the complex metasomatic history at Finero has received much attention. New U-Pb results are consistent with the age range obtained for mantle rocks, the phlogopite peridotite (29313 Ma, Voshage et al., 1987) and chromitite (2082 Ma, Grieco et al., 2001). The former age estimate, based on a Rb-Sr whole-rock isochron for six phlogopite-bearing peridotites and one phlogopite pyroxenite, has been interpreted as time of K metasomatic enrichment of the harzburgite. This event has been coeval with the intrusion of alkaline ultramafic magmas into the deep crust of the Ivrea Zone during the late Carboniferous (2873 Ma, Garuti et al., 2001). The U-Pb age of 2082 Ma for zircon at Alpe Polunia is corroborated by our ion-probe U-Pb results for zircons of the same locality. It has been attributed (Grieco et al., 2001) to one of the major metasomatic episodes. The U-Pb zircon ages identified at Rio Creves show notable differences. The U-Pb SHRIMP data do not concur with the assumption of a single metasomatic event during chromitite formation. Therefore, we suggest a prolonged formation and multistage evolution of zircon growth, as mirrored by multiple U-Pb ages. Firstly presented U-Pb results for zircons from two chromitite localities (Alpe Polunia and Creves) place tight constraints on their distinctly different temporal evolution, representing previously unknown stages of magmatic activity at Rio Creves. We thus assume that different models suggested for the Ivrea Zone could be updated in light of new U-Pb constraints that indicate the complex geological history of mantle rocks at Finero.

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