Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

U and Th content in the Central Apennines continental crust: a contribution to the determination of the geo-neutrinos flux at LNGS

The regional contribution to the geo-neutrino signal at Gran Sasso National Laboratory (LNGS) was determined based on a detailed geological, geochemical and geophysical study of the region. U and Th abundances of more than 50 samples representative of the main lithotypes belonging to the Mesozoic and Cenozoic sedimentary cover were analyzed. Sedimentary rocks were grouped into four main "Reservoirs" based on similar paleogeographic conditions and mineralogy. Basement rocks do not outcrop in the area. Thus U and Th in the Upper and Lower Crust of Valsugana and Ivrea-Verbano areas were analyzed. Based on geological and geophysical properties, relative abundances of the various reservoirs were calculated and used to obtain the weighted U and Th abundances for each of the three geological layers (Sedimentary Cover, Upper and Lower Crust). Using the available seismic profile as well as the stratigraphic records from a number of exploration wells, a 3D modelling was developed over an area of 2^{\circ}x2^{\circ} down to the Moho depth, for a total volume of about 1.2x10^6 km^3. This model allowed us to determine the volume of the various geological layers and eventually integrate the Th and U contents of the whole crust beneath LNGS. On this base the local contribution to the geo-neutrino flux (S) was calculated and added to the contribution given by the rest of the world, yielding a Refined Reference Model prediction for the geo-neutrino signal in the Borexino detector at LNGS: S(U) = (28.7 \pm 3.9) TNU and S(Th) = (7.5 \pm 1.0) TNU. An excess over the total flux of about 4 TNU was previously obtained by Mantovani et al. (2004) who calculated, based on general worldwide assumptions, a signal of 40.5 TNU. The considerable thickness of the sedimentary rocks, almost predominantly represented by U- and Th- poor carbonatic rocks in the area near LNGS, is responsible for this difference.Comment: 45 pages, 5 figures, 12 tables; accepted for publication in GC

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

Mesozoic Magmatism in the Ivrea-Verbano Zone and its geodynamic implications.

abs. 10.1474, 01-0560.In the Southern Alps, Mesozoic magmatism is well documented in the Dolomite region and in the Lombard Pre-Alps. In the western portion of the Southern Alps, the Mesozoic magmatic activity has received much less attention.In the present study we report a review of geochronological evidences of the existence of Mesozoic magmatism in the portion of the Southern Alps west of the Lugano lake. The Mesozoic events are exclusively located: a) south of the Cremosina fault system; b) in the northeasternmost portion of the Ivrea-Verbano Zone.a) A suite of diorite-norite dykes occurring into the Baldissero mantle peridotite (Southern Ivrea-Verbano Zone) has been recently recognized. The age of dyke intrusion is currently constrained by: a) a two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data which yields a slope corresponding to an age of 18026 Ma, with a Ndi = 0.512804 and Ndi = 7.8 (Obermiller, 1984, PhD Thesis Un. Mainz, Germany); b) Re-Os model ages (Re depletion model age) between 140 and 190 Ma calculated on whole rock samples of the ambient peridotite (Mazzucchelli et al., 2004, EGU04 Geophys. Res. Abs., 6, 03966). Moreover a number of acid tuff layers from mm to several meters in thickness are present in the Mesozoic sedimentary cover of the Southern Alps from the Lugano to Biella area. They occur in the Crevacuore and Sostegno sedimentary succession, in the Villafortuna-Trecate oil field, and in the sedimentary cover of Monte San Giorgio (Ticino, Switzerland). In the Monte San Giorgio occurrence, high-resolution U-Pb zircon age gives 241±0.8 Ma (Mundil et al., 1996, Earth Planet. Sci. Letters, 141, 137-151). b) In the Basic Complex cropping out in the Finero and Val Grande area, most of the isotopic data invariably give Triassic or Early Jurassic ages. The Basic Complex in this area shows an antiformal structure, constituted by various cumulus rocks and gabbroic lithotypes (Internal Gabbro, Hornblende Peridotite and External Gabbro Units). At the core of the antiform, in tectonic contact with the rocks of the Complex, a mantle phlogopite-bearing peridotite occurs, whose metasomatic imprint was attributed to crustal components, deriving from a subducting slab. The age of metasomatism is Mesozoic [207 Ma - U/Pb on zircons from chromitites (Von Quadt et al., 1992, Ivrea-Verbano Zone Workshop, U.S. Geol. Survey Circular 1089, Abs., 20.); 226-177 Ma - Rb/Sr internal isochrons on amphibole and phlogopite pairs (Hartmann & Wedephol, 1993, Geochim. Cosmochim. Acta, 57, 1761-1782); 220 Ma - Ar/Ar on phlogopite (Hartmann & Wedephol, 1993)]. Magmatic, subeuhedral, pink crystals with oscillatory zoning in CathodoLuminescence (CL) from the External Gabbro Unit has been recently dated with SHRIMP (Peressini et al., 2005, this session). Magmatic growth of the zircons took place at 232±2 Ma and was overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. These ages are well in accordance with the literature data reported for the Basic Complex. In spite of the similar ages, the Basic Complex does not record any evidence of the metasomatic agent which affected the mantle peridotite. The Finero Basic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian (Peressini et al., 2005, this session) relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola.Presently, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are confined by tectonic lineaments to the southernmost and northernmost portion, respectively. This put new constraints for the comprehension of the geodynamic reconstruction of the whole Southern Alps

Triassic U-Pb SHRIMP Ages on magmatic Zircons from the External Gabbro unit of the Finero mafic complex, Ivrea Zone, Western Italian Alps.

abs. 236-38The northern part of the mafic-ultramafic Ivrea Verbano complex, the Finero region, differs from the rest of the complex in many features: petrology and geochemistry of mantle peridotites, stratigraphy, lithology and geochemistry of igneous bodies, relationships with the metamorphic sequence (Kinzigite Formation) into which the complex intruded. We provide evidence for a substantial difference also in the age of emplacement. The Mafic Complex (MC) in the Val Sesia area has been recently proved to have intruded between 283 and 289 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol.140, 223-235, and ref. therein).We performed a zircon study on 5 samples from the External Gabbro unit (EG) of the complex. Three events are revealed by the SHRIMP U-Pb results on the 2 most representative samples, one of which has a composite population of magmatic and detrital zircons, clearly distinguished for grain-morphology, color and CL-pattern. Primary crystallization of the pink magmatic zircons was dated at 232±2 Ma; these were overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, but poorly constrained by the colorless zircons (U<20 ppm), proving that older ages are preserved, but must be considered detrital. This is further confirmed by the ages recorded in the second sample, which yielded no magmatic zircons, but only metamorphic grains. As for the significance of the older age peak, zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples suggest that zircons and zirconium were inherited/digested from the country rock. This latter underwent a major event that opened and reset the U-Pb system in zircon around 300 Ma, compatible with ages in the MC of the Balmuccia sector.In the EG, though, no magmatic Carboniferous or Permian zircon was found: the EG was emplaced in Anisian-Ladinian time at 232Ma, and a separate thermal event took place in Norian time at 214 Ma (Staehle et al., SMPM 70, 1990; von Quadt et al., SMPM 73, 1993). The clearly documented Mesozoic igneous activity, distinct from the predominant Permo-Carboniferous magmatism in the Ivrea-Verbano, and the possibly Mesozoic age of mantle metasomatism at Finero (Grieco et al, 2001, J. Pet.52, 89-101) suggest revisiting the current interpretations of Mesozoic magmatism in the Southern Alps of Lombardy and Trentino regions

Triassic emplacement of the External Gabbro unit of the Finero mafic complex: U-Pb SHRIMP zircon ages and their implications for the Ivrea-Verbano Zone, Western Italian Alps.

SRef-ID: 1607-7962/gra/EGU04-A-05072The Mafic Complex (MC) of the Ivrea Zone displays distinctive features in different sectors. Apart from petrology and geochemistry of the mantle peridotites (e.g. the strong metasomatism of Finero is absent in Balmuccia), the Finero region differs from the rest of the complex in stratigraphy, lithology and geochemistry of the mafic intrusives, and their relationships with the Kinzigite Formation, the metamorphic sequence into which the MC intruded. We give new evidence for a substantial difference also in the age of magmatism. The age of emplacement of the MC in the Val Sesia area has been recently proved to be homogeneous, between 283 and 289 Ma, after a thermal, upper-amphibolite event that affected the country rock at 320-to-310 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol. 140, 223-235, Grieco et al, 2001, J. Pet. 52, 89-101). The more composite nature of the Finero sector is possibly hiding a more articulated geological evolution of the MC, as suggested by the presence of numerous high-T shear-zones (e.g., Kenkmann, 2000, J.Struc.Geol. 22, 471-487, Manckeltow etal., 2002, J.Struc.Geol. 24, 567-585, and ref. therein). We performed a detailed zircon study on samples from the External Gabbro unit of the complex, most of which have a homogeneous population of colorless zircons. The key for interpretation is one sample, bearing both magmatic, subeuhedral, pink crystals with oscillatory zoning in CL, and detrital zircons, rounded, small, colorless grains with blurred CL-patterns. The three events recorded by these zircons are clearly recognizable in CL, and were dated with SHRIMP U-Pb analyses. Magmatic growth of the zircons took place at 232+/-2 Ma and was overprinted at 214+/-5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, though imprecisely constrained by the colorless zircons, proving that older ages are preserved, but must be considered detrital. Two problems arise: reason and significance of the older age peak. Together with the positive correlation between zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples, the U-Pb data allow to conclude that zircons and zirconium were inherited/digested from the country rock. The latter underwent a major event that reset the U-Pb system in zircon around 300 Ma, compatible with both magmatic and metamorphic ages in the MC of the Balmuccia sector. In the External Gabbro, though, no magmatic Carboniferous or Permian zircon was found: emplacement took place at 232 Ma, and a separate thermal event is dated at 214 Ma. The new data extend as far west as the Ivrea zone the evidence of the important Mesozoic magmatism of the central and eastern South Alpine. Mesozoic igneous activity in the Finero region is clearly distinct from the predominating Permo-Carboniferous magmatism in the Ivrea-Verbano Zone. Also, as for most gabbroic intrusions in the alpine-appenine system, ages in the Ivrea Zone cluster in two ranges, at either 285-310 Ma, or at 200-250 Ma. This distribution is thoroughly represented in the zircons from one sample only, the ages of which represent distinct episodes of heating, melting and metasomatism

Origin and significance of late noritic dykes in the Baldissero Peridotite Massif (Ivrea-Verbano zone)

The Baldissero Peridotite Massif (Southern Ivrea-Verbano Zone) is a depleted lherzolite (ol 65/70, opx 16/20, cpx 10/11, sp 2/2.5, La(N)/Yb(N) 0.003/0.26, ε(Nd)270 = 6.4/14, 87Sr/86Sr = 0.7021/0.7035). It is cross-cutted by a swarm of 5/15 cm thick, fine grained (&lt;1mm), NNE trending noritic dykes. Contacts peridotite-dyke are sharp. Dyke texture is hypidiomorphic and their mode consists of dominant plagioclase (31/45 %, An 40/48), orthopyroxene (25/37%, En = 78.9/87.8, Wo = 0.6/1.1), subordinate clinopyroxene (14/18%, En = 42.7/44.0, Wo = 47.9/52.4), interstitial pargasitic amphibole (10/15%) and opaques (1/2%, Fe-Ni sulfides). Re-equilibration conditions are 0.8/1 GPa and 800/900°C, like those of the ambient peridotite. Major element (wt%) dyke composition is characterized by high MgO (10/16) and high Mg values (0.81/0.89). In the observed MgO range, SiO2, CaO and K2O vary negligibly (52/53, 9/10 and &lt;0.2, respectively). With decreasing MgO, TiO2 varies randomly from 0.8 to 0.4, Al2O3 increases from 15 to 21, Na2O from 3 to 4.5 and FeO decreases from 5 to 2. Bulk rock incompatible trace element patterns (Fig. 1) vary from weakly LREE depleted to weakly LREE enriched (La(N)/Yb(N) = 0.5/2.4) and have marked positive Sr anomaly (Sr/Sr(*) = 5.6/10.4), smaller Eu anomaly (Eu/Eu(*) 1.3/1.5) and negative Nb anomaly (La(N)/Nb(N) = 1.2/2.2). Incompatible element concentration increases, and the magnitude of the negative Nb and of the positive Sr anomalies decrease, with increasing MgO or Mg. Main incompatible trace element carriers are clinopyroxene and hornblende, which display patterns varying from slightly LREE depleted to enriched. Orthopyroxene is LREE depleted and plagioclase is LREE and Sr enriched. Clinopyroxene is Nb-depleted (La(N)/Nb(N) = 1.0/5.9) and amphibole is Nb enriched (La(N)/Nb(N) = 0.09/0.35). The latter phases have negative Zr spikes. Incompatible element concentration in the mafic phases show correlations with Mg similar to those of bulk rock (Fig. 2). The only available isotope determination refers to a Sm-Nd internal isochron which results in an age of 180Ma and ε(Nd) = 7.8. In spite of the contacts being sharp, peridotite mineral assemblage and composition varies over a distance of 7 cm. In the peridotite, toward the dyke contact, modal orthopyroxene increases and clinopyroxene decreases, whereas trace element concentration and La(N)/Yb(N) increase (from 0.003/0.017 to 0.1/0.26) and Mg decreases from 0.9 to 0.8. The dyke at the peridotite contact does not show sizeable chemical or modal variations. There is no clear relationship between the Mg variation of the dyke and the chemical trends observed in the contact peridotite. The high Mg numbers of the dykes and the Mg value of orthopyroxene, lower than that of bulk-rock, indicate that these norites are not melts, but rather represent mineral segregates possibly retaining some interstitial liquid. Estimates of the Mg values of the parental melts, made assuming (opx/liq)D(Mg) = 1.2, indicates a Mg range from 0.66 to 0.74, which is consistent with that of primary mantle-derived magmas. Since olivine does not occur even in the more MgO-rich dyke composition, the parent melt had to be, furthermore, SiO2-rich and hydrous, as indicated by the amphibole abundance. Whatever the set of solid/melt partition coefficient used for estimating trace elements in the parent melts, the calculated REE profiles would vary from slightly LREE depleted to variably LREE enriched. The HFSE characteristics of the primary melts cannot be precisely constrained because of the strong dependence of their partition coefficient on the melt composition. For example, using available partition coefficients for basaltic compositions, the estimated parental melts would have a weak Nb and Zr anomaly, which disappears using D sets for intermediate compositions and become positive if the D's are referred to high-silica compositions. In any case, the geochemical trends shown by bulk rock and mineral phases (i.e. the increase of incompatible element concentration with increasing Mg) are qualitatively maintained. We propose that this peculiar feature can be explained either by fluid-assisted melting of a depleted peridotite source, where melting degree increases with the increasing supply of hydrous fluid (which is the incompatible element carrier), or by fluid-peridotite reaction during porous flow percolation in a mantle column at a deeper and higher temperature level than that actually observed. If confirmed by new isotopic data, the occurrence of post-Hercynian gabbroic rocks in the Australpine and South Alpine domains will support the involvement of the marginal units of the Adria plate in the pre-oceanic extensional regime (Piccardo et al., 1994)