Franca Siena

Glasses in mantle xenoliths as geochemical indicators of metasomatic agents

Abstract An extensive comparison between world-wide occurrences of mantle glasses in both continental and oceanic settings is presented here. Several localities were chosen on the basis of the available major (and trace) element data, coupled with a clear identification of the nature of the metasomatizing agents. Xenoliths bearing amphibole and/or phlogopite, which can strongly affect the glass chemistry, were not considered. Despite their considerable geochemical variability world-wide, glasses are rather homogeneous within single xenoliths and their main geochemical features are broadly maintained in the same area. Orthopyroxene always plays a major role in glass genesis, as suggested by the relationship between its presence in the primary assemblage and the silica saturation degree, as well as by the FeO vs. SiO2 decreasing trends. At the same time, the absence of any correlation between mg# and SiO2 rules out the possibility of glass chemistry resulting from (or being affected by) secondary mineral fractionation. Thus, the geochemical features of mantle glasses are a powerful tool for investigating the nature of the metasomatic melts. According to available data, glasses in mantle xenoliths from Gran Comore, Samoa, Spitsbergen, Mongolia and Hoggar have been related to carbonatite metasomatism, whereas glasses in mantle xenoliths from Mt. Lessini, Hawaii, Cape Verde and Yitong have been related to metasomatism by alkali silicate basic melts. Moreover, the distinctly different alkali contents in Mt. Lessini and Hawaii glasses as compared with those of Cape Verde and Yitong allow a sodic signature to be attributed to the alkali silicate metasomatic melt of the former with respect to a potassic characteristic of the latter. Glasses related to carbonatite metasomatism are characterized by high CaO, Na2O, and low SiO2 and K2O contents, with Na2O/K2O ratios usually greater than 2; they tend to have the highest Nb and LREE and the lowest Zr and Ti contents. On the other hand, glasses related to K-alkali silicate metasomatism are mainly characterized by high SiO2 and K2O and low CaO and NaO contents, with Na2O/K2O ratios less than 1; they generally present higher Rb and lower Nb and REE contents. Glasses related to Na-alkali silicate metasomatism are characterized by SiO2 contents comparable to those related to K-alkali silicate metasomatism, but present higher Na2O contents (in any case lower than those of carbonatite metasomatism), with Na2O/K2O ratios usually ranging between 1 and 2. On these bases, diagrams for discriminating between carbonatite- and Na-alkali or K-alkali silicate-related metasomatism are presented.

Differentiation of partial melts in the mantle: Evidence from the Balmuccia peridotite, Italy

The Balmuccia peridotite shows evidence, in the form of a network of dykes, of partial melting and flow crystallization processes. The partial melting processes probably occurred over a fairly long time interval, and seem to have been related to different “melting pulses”. Resultant liquids were broadly picritic. Melting occurred incongruently according to the scheme cpx+opx+(ol+sp)=Mg-richer ol+Cr-richer sp+L.Partial melts tended at first to accumulate in horizontal layers; then, as the critical melting threshold was exceeded, liquids were able to filter slowly towards lower pressure zones. In doing so liquids fractionated initially in situ, via crystallisation of websteritic dykes of the Cr-diopside suite, and later, in the overlying mantle, via crystallisation of transitional dykes and those of the Al-augite suite. This filter-pressing stage, when flow velocities were very low and discontinuous, probably corresponded to the period of maximum deformability of the peridotite.The type of differentiation testified by the dykes of the Balmuccia peridotite, is characterized by a decrease in SiO2, a rapid enrichment in Al2O3 and a mild increase in FeO, and is substantially in accordance with experimental trends from the fo-an-di-SiO2 system in the spinel-peridotite stability field. A close relationship between type of differentiation, flow velocity and mechanical behaviour of the mantle peridotite is a feature of the proposed model.

The southern margin of the Caribbean Plate in Venezuela: tectono-magmatic setting of the ophiolitic units and kinematic evolution

Abstract The southern Caribbean Plate margin in Venezuela consists of a W–E elongated deformed belt, composed of several tectonic units dismembered along the northern part of the South America continental Plate since the Late Cretaceous. The present review, based on petrology and tectono-magmatic significance of each unit, makes it possible to define the main geotectonic elements and to reconstruct the paleogeographic domains from Late Jurassic to Tertiary: (a) Mid-Ocean Ridge Basalt (MORB) proto-Caribbean oceanic basin (Loma de Hierro Unit); (b) oceanic plateau (Dutch and Venezuelan Islands basement); (c) rifted continental margin (Cordillera de La Costa and Caucagua–El Tinaco Units) with Within Plate Tholeiitic (WPTh) magmatism; (d) an intra-oceanic subduction zone represented by Island Arc Tholeiitic (IAT) magmatism (Villa de Cura and Dos Hermanas Units) of Early Cretaceous age; (e) an Early Cretaceous ocean–continent subduction trench filled by melange (Franja Costera); (f) a new intra-oceanic subduction zone, represented by the tonalitic arc magmatism of Late Cretaceous age (Dutch and Venezuelan Islands). Regional tectonic constraints and coherent kinematic reconstruction suggest an original “near-Mid America” location of the Jurassic–Cretaceous “proto-Caribbean” oceanic realm. From Early to Late Cretaceous one sub-continental subduction with melanges (Franja Costera Unit) and two main stages of intra-oceanic arc magmatism are recorded in the so-called “eo-Caribbean” phases. The first consists of generally metamorphosed and deformed volcano-plutonic sequences with IAT affinity (Villa de Cura and Dos Hermanas Units), probably in relation to a southeastward-dipping subduction. The second is mainly represented by generally unmetamorphosed tonalitic intrusives cutting the oceanic plateau in the Dutch and Venezuelan Islands, and related to the new intra-oceanic subduction with reverse lithospheric sinking. The latter probably marked the onset of the Aves/Lesser Antilles arc system in the Late Cretaceous.

The role of HIMU metasomatic components in the North African lithospheric mantle: Petrological evidence from the Gharyan lherzolite xenoliths, NW Libya

Abstract The Neogene–Quaternary alkali-basalt–hawaiite lavas of the Gharyan volcanic field (NW Libya) contain mantle xenoliths. These mostly consist of protogranular spinel lherzolites with superimposed metasomatic textures represented by reaction patches where primary orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) are the main reacting phases. The secondary parageneses include clinopyroxene (cpx2), olivine (ol2) and feldspar (feld) as reaction rims around opx, spongy-textured clinopyroxene with recrystallized portions (cpx2±feldspar), and brown spinel destabilized in a higher Cr/(Cr+Al) black vermicular aggregate (sp2) generally associated with feldspar microlites. Cpx2 are typically depleted in Na2O and Al2O3 relative to cpx; feldspar includes both alkali-feldspar (Or 17–51) and plagioclase (An 23–64). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.2–2.3 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (8–14.5 times chondrite) and variable LREE enrichment with LaN/YbN up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene Sr–Nd isotopic composition (87Sr/86Sr down to 0.7023, 143Nd/144Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lithospheric mantle beneath the area underwent a long-term depletion probably by pre-Palaeozoic extraction of basic melts. The remaining samples approach 87Sr/86Sr c. 0.7030, 143Nd/144Nd c. 0.5130, with 206Pb/204Pb up to 19.66. These data imply that the causative agents of metasomatism were Na-alkali silicate melts with a clear HIMU affinity, in accordance with the isotopic signature of the host lavas (87Sr/86Sr=0.7032, 143Nd/144Nd=0.5130, 206Pb/204Pb=19.60). This prevalent HIMU geochemical signature is comparable with that recorded in Cenozoic alkaline basic lavas and associated mantle xenoliths from other occurrences of the northern–central African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3He/4He of the Gharyan xenoliths (5.3–6.5 Ra) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the Ethiopian–Yemen plateau basalts. Therefore, the Cenozoic volcanic districts of the Saharan belt could be related to smaller-scale shallow mantle upwellings (also referred to as ‘hot fingers’) triggered by intraplate reactivation of regional tectonic lineaments within the Pan-African cratonic basement, as a foreland reaction of the African–Europe collisional system.

Mantle metasomatism by melts of HIMU piclogite components: new insights from Fe-lherzolite xenoliths (Calatrava Volcanic District, central Spain)

Abstract Mantle xenoliths from the Calatrava Volcanic District (CLV), central Spain, are characterized by a wide compositional range that includes lherzolites (prevalent), as well as minor amounts of wehrlite, olivine (ol)-websterite and rare dunites. They generally have a bulk-rock Mg# of less than 89, lower than any primordial mantle estimates. Intra-suite variations in modal proportions are inconsistent with those predicted by melting models irrespective of the starting composition; mineral and bulk-rock variation diagrams show inconsistencies between the CLV compositions (anomalously enriched in Fe–Ti) and those predicted from the partial melting of primordial mantle material. Processes other than pure melt extraction are confirmed by the whole-rock REE (rare earth element) budget, typically characterized by LREE enrichments, with LaN/YbN (up to 6.7), probably related to pervasive metasomatism. CLV mantle clinopyroxenes (cpx) generally display fractionated REE patterns with upwards-convex shapes, characterized by low HREE (Tm–Lu) concentrations (typically <6× chondrite) and enrichments in middle–light REE (MREE–LREE) (NdN/YbN up to 7, LaN/YbN up to 5). These ‘enriched’ cpx compositions either result from re-equilibration of primary mantle cpx with an incoming melt, or represent cpx crystallization directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials). Separated cpx have homogeneous 87Sr/86Sr compositions between 0.7031 and 0.7032; 143Nd/144Nd ranges from 0.51288 to 0.51295 (ϵNd 4.74–6.07) and 176Hf/177Hf is in the range 0.28302–0.28265 (ϵHf −3.6 to 9.0). Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, which range in composition from the depleted mantle (DM) to the enriched mantle (EM), the CLV mantle cpx approach the composition of the HIMU mantle end member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via ancient subduction. A model is proposed for the mantle evolution under central Iberia, where sublithospheric convective instabilities – possibly triggered by the neighbouring subduction along the Betic collisional belt – could have remobilized deep domains from the mantle ‘transition zone’ (410–660 km), which may include relicts of older subducted slabs. Within these remobilized domains, characterized by the coexistence of peridotite and eclogite and referred to as a ‘piclogite’ association, the eclogites melt preferentially generating Fe–Ti rich melts characterized by a HIMU isotopic signature that infiltrates and metasomatizes the shallower lithospheric mantle.

Geotectonic Significance of the Metabasites of the Kinzigitic Series, Ivrea-Verbano Zone (Western Italian Alps)

Geochemical investigations have been carried out on the metabasites of the Ivrea-Verbano Zone (NW-Italy). The Ivrea-Verbano Zone is commonly considered to be a section through the lower continental crust. It is constituted by a steeply dipping sequence of metamorphic rocks (Kinzigitic Series), intruded by a mafic-ultramafic complex. The metabasites of the Kinzigitic Series have a meta-igneous origin. Their protoliths show tholeiitic affinity in the NW and central part of the zone and alkalic affinity in the SE. This fact together with the rock associations suggests that a variation of the paleogeographic environment from NW to SE occurs in the Ivrea-Verbano Zone.