Are the glasses in mantle xenoliths witness of the metasomatic agent composition?

Glass veins and pockets in mantle xenoliths are often considered as indicators of the composition of metasomatic agents affecting the mantle. Here we demonstrate that infiltration of, and reaction with, the host basalt may produce glassy veins and pockets whose composition encompasses that of different potential metasomatic agents. The xenoliths studied are 4-19 cm large, equigranular, spinel-facies harzburgites and lherzolites from the Patagonia lithospheric mantle. A reaction rim occurs at the contact with the basalt. Peridotite orthopyroxene is in reaction with the basalt and a glassy pocket (up to 600 μm in diameter) is thereby formed. New crystals of euhedral olivine (Fo = 84) and Ti-oxides crystallise in the glass pocket close to the basalt, while euhedral crystals of clinopyroxene (mg# = 85-89) and olivine crystallise close to orthopyroxene. The reaction-crystallisation processes induce dramatic compositional variations in theglass pocket from phonotephryte to trachyte. Trace element concentration and patterns and the element anomalies are controlled by the reaction-crystallisation process. Orthopyroxene dissolution has mainly a dilution effect, whereas clinopyroxene crystallisation and the crystallization of Ti oxides (and apatite) largely controls the trace element fractionation and element anomalies. The largest trace element variations are documented in the veins. The studied glasses obviously do not represent metasomatic agents that affected the lithospheric mantle, but the large compositional variations they document encompass those of glasses quoted in literature (Wulff-Pedersen et al., 1996; Coltorti et al., 2000) and believed to represent metasomatic agents, thus suggesting caution in drawing inferences on mantle processes and components from glass pockets and veins in xenoliths

Equivocal carbonatite markers in the mantle xenoliths of the Patagonia backarc: the Gobernador Gregores case (Santa Cruz Province, Argentina)

Amphibole ± phlogopite ± apatite-bearing mantle xenoliths at Gobernador Gregores display modal, bulk-rock and phase geochemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO₂/Al₂O3 and those displaying the most pronounced carbonatitic geochemical markers modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na₂O and high field strength elements (HFSE) concentrations. Steady variation trends between xenoliths which have and do not have carbonatitic characteristics suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling [Plate Model (Vernieres et al. in J Geophys Res 102:24771–24784, 1997)] gives results consistent with the observed geochemical features by assuming the presence of loveringite. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation in the garnet-facies.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Results of melt migration in the lithospheric mantle and in continental mafic-ultramafic layered complexes.

We have studied the following topics: 1) metasomatism of the lithospheric mantle atop a plume (NE Brazil and Fernando de Noronha); 2) metasomatism of the mantle wedge by slab-derived components (Cerro de los Chenques, Patagonia); 3) origin of dunite and wehrlite regions in the subcontinental lherzolitic mantle (Balmuccia); 4) reactions between diorite dikes and subcontinental mantle and consequent geodynamic implications (Baldissero massif).Isotopic and geochemical results indicate that in the case study 1 the mantle lithosphere underwent thermochemical and mechanical erosion by infiltration of alkali basalts during Cenozoic and that this process affected a mantle recording previous, Mesozoic, mantle processes. The similarity of processes and geochemical characteristics in NE Brazil and Fernando de Noronha supports the hypothesis that the lithosphere under the Island is a detached portion of the continental one and that the respective lithospheres interfered with the same plume.Geochemical and isotope evidence indicate that the wedge beneath Chenques (case 2) interfered with slab-derived fluids rather than melts. Since the study locality is 400 km east from the trench and the slab beneath Chenques is about 200 km deep, a main problem is how to explain the long travel of slab-released fluids in order to reach the observed spinel-facies region of the wedge. We have modelled this process as break-down of phengite in the slab and percolation of the derived fluids in the wedge under increasing temperature and water activity. Reaction during the long fluid migration produces phlogopite, K-richterite, garnet, and, in the lower-P region, amphibole. The geochemical effects of these reaction products are recorded in the fluids reaching the observed spinel-facies region. Besides already known decimetric dunite at websterite dyke contacts, decametric dunites regions have been discovered into the subcontinental Balmuccia Peridotite (case 3). The latter dunites (± wehrlites) are of two types: a) very high Mg# values in bulk-rock and minerals and characterised by massive Cr-spinel layers; b) Mg# values in bulk rock and minerals lower than the ambient lherzolite and characterised by amphibole, phlogopite and plagioclase pods. As discussed in more detail in a specific abstract of this session, these two dunite types are related with infiltration of different melts and different processes, of increasing and decreasing melt mass, respectively, during reactive flow.The reaction between diorite dykes and the Baldissero peridotite (topic 3) has already been previously discussed. However, new isotopic data provide constraints on the age of dyke emplacement which have important geodynamic implications, specifically illustrated in a work presented in this session. Finally, the mafic-ultramafic Niquelandia complex have been re-interpreted in the light of isotopic evidence in terms of energy-controlled assimilation and fractional crystallisation. Thermal balance indicate that the complex underwent large crystallisation before heating the crust to melting. Heat production was related to the intrusion volume and resulted higher in the largest lower sequence with respect to the smaller upper sequence. The results were production of different anatectic melts, different residual metasediments, and different interference results between anatectic melts and mafic magmas

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

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone). 2. Geochronology and isotope geochemistry

Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORE-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 +/- 20 Ma, which is consistent with a Pb-207/Pb-206 evaporation age of 549 +/- 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter bring the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this ease, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite). Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 +/- 23, 226 +/- 7,223 +/- 10, 214 +/- 17, and 203 +/- 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 +/- 15 Ma ago. Initial epsilon(Nd)(533) values average + 6.3 +/- 0.4 for six samples of the Amphibole Peridotite unit and + 6.0 +/- 1.2 for ten samples of the External Gabbro unit. Sr-87/Sr-86 ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between Sr-87/Sr-86 and K2O and weaker correlations between initial epsilon(Nd) and K2O imply a comparatively minor (less than or equal to 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative epsilon(Nd) and high Sr-87/Sr-86 values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially \ub4pristine\ub4 and the magmatic body has been extensively contaminated by assimilation of crustal rocks

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

The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (La-N/Yb-N = 8-17) and depleted in Nb. It has high radiogenic Sr (Sr-87/Sr-86((270)) = 0.7055-0.7093), low radiogenic Nd (epsilon Nd-(270) = -1 to -3) and EMII-like Pb isotopes. Two pyroxenite - peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (La-N/Yb-N similar to 16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration similar to 1.9 (Sr) to similar to 7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg# and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of carbonatite metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (D-amph/cpx(REE),(Sr) = 0.7-0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone) .1. Chemistry of MORB-like magmas

Geochemical data are presented for the meta-igneous, mafic-ultramafic complex near Finero. This complex is in contact with a phlogopite-bearing mantle peridotite and is subdivided into the Internal Gabbro unit, the Amphibole Peridotite unit, and the External Gabbro unit. The Internal Gabbro and the Amphibole Peridotite units consist of coarse-grained, chemically heterogeneous cumulates, whereas the External Gabbro unit is generally massive, chemically more uniform and approximately representative of the residual melt with MgO contents between 6.6 and 9.1% and Mg numbers between 38 and 58. Both whole-rock and mineral contents of Ni and Cr are significantly higher (at similar Mg numbers) in the Amphibole Peridotite unit than in the Internal Gabbro, unit. The most straightforward interpretation of this is that the Amphibole Peridotite unit accumulated after the influx of fresh mafic (or ultramafic) magma into the magma chamber. Major-element chemical trends are continuous from the Amphibole Peridotite unit to the External Gabbro unit and are consistent with closed-system fractionation with no further addition of magma or contamination by wall or roof rock assimilation. In the External Gabbro unit, total FeO and TiO2 contents are strongly correlated with each other (and with P2O5 and Zr) and reach values as high as 19 and 4%, respectively, indicating an advanced degree of crystal fractionation along a tholeiitic trend. The External Gabbro samples have generally smooth normalized trace element patterns, which are consistent with being representative of a liquid composition. The residual nature of the External Gabbro magma is also indicated by negative Eu and Sr anomalies, clear evidence for prior feldspar-fractionation. REE patterns are otherwise indistinguishable from N-type MORE, but Th and U an significantly more depleted than in MORE. This Th and U depletion is similar to that found in olivine basalts and picrites on Iceland and Hawaii; its origin is not well understood. No evidence is seen for any assimilation of crustal material, in sharp contrast with the situation of the igneous complex in Val Sesia near Balmuccia, where the magma composition is dominated by assimilation of crust. We suggest that the heat provided by at most two injections of magma near Finero was insufficient to induce crustal anatexis, in contrast with the excess heat supplied by multiple magma injections at Balmuccia

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