ORIGIN OF POST-VARISCAN GABBRO-DERIVED GRANULITES (NORTHERN APENNINES, ITALY): CRUSTAL CONTAMINATION PROCESSES AND IMPLICATIONS FOR MANTLE SOURCES

The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide blocks of mantle peridotites, MOR-basalts and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of gabbro-derived and felsic granulites, which locally preserve primary contacts. The gabbro-derived granulites show variable degree of recrystallisation related to the granulite-facies re-equilibration. However, rocks which preserve a gabbroic fabric are found locally. The most frequent rock-types are olivine-bearing gabbronorites and Fe-Ti-oxidebearing gabbronorites; troctolites and anorthosites are also present. The felsic granulites include quartzo-feldspathic granulites and rare quartz-poor to quartz-free rocks. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallization under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900~ They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times (Marroni et al., 1998). Permian gabbroic rocks with granulite-facies metamorphic overprint are widespread in the Western Alps, in the remnants of the Adria plate margin. These gabbroic complexes are ascribed to the extensional tectonics which postdate the Variscan orogeny. In particular, striking similarities between the gabbro-derived granulites from Northern Apennines and the Ivrea Mafic Body (Pin, 1990) can be shown, i.e. similar ages of emplacement, close paragenetic and compositional resemblances and comparable retrograde metamorphic evolution. This work shows a geochemical investigation of gabbro-derived and felsic granulites from the Northern Apennines, in order to unravel (i) the affinity of the parental liquids of the gabbroic protoliths, (ii) the role of fractional crystallization and crustal assimilation in the igneous differentiation process, (iii) the nature of the mantle source. Trace element and Sr-Nd isotopic analyses have been performed on selected granulite samples, and clinopyroxene and plagioclase cores from undeformed gabbro-derived granulites were analysed for trace elements by ion microprobe. Relics of igneous textures, mineral and bulk-rock compositional variations indicate a comagmatic origin for the gabbro-derived granulites. They can be generally recognized as cumulitic rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/A1203 ratios (Kempton and Harmon, 1992) and overall low contents of incompatible trace elements. The mg# value ranges from 80 to 52, and points to negative correlations with TiO2 (up to 5 %) and MnO, thus indicating a tholeiitic differentiation trend. Major element variations are consistent with a process of fractional crystallization of plagioclase + olivine as liquidus phases, followed by plagioclase + clinopyroxene + orthopyroxene. In addition, petrographic relations in undeformed gabbroic rocks suggest the growth of AIspinel and Fe-Ti-oxides as intercumulus igneous minerals. The range of incompatible trace element ratios (Zr/ Y = 0.7-2.9, Y/Nb <~ 9, Zr/Nb ~< 10) encompasses that of subalkaline basaltic liquids. The R E E distribution (Fig. 1) is characterized by: (i) slightly LREE-enr iched patterns showing decreasing Eu positive anomaly with increasing absolute R E E

The Limpopo Belt of southern Africa is generally believed to represent the root of a late Archaean continental collision zone, and has been used to demonstrate the validity of the uniformitarian concept in tectonics Large scale tectonic models have been applied in spite of the fact that large portions of the belt yet await the most basic investigations. Here we report the first detailed field and petrographic study of the northernmost part of the Limpopo Belt, the Northern Marginal Zone sensu stricto (NMZ s.s) and conclude, on the basis of relative age relation, nature of PT evolution and deformation, that none of the current models can correctly explain the evolution of the study area. The evolution of NMZ s.s, is complex and includes four metamorphic stages, two major plutonic episodes and at least three deformation events. The oldest rocks, mafic granulites, record all four stages of metamorphic mineral growth. The first two stages predate deposition of sediments and the intrusion of voluminous enderbite and charnockite between 2.72 and 2.62 Ga. The bulk of our observations relate to the late Archaean (stage 3) granulite facies event. Abundant reaction textures are preserved in mafic granulite, metasediments, metamorphosed charnockite and enderbite and allow us to qualitatively reconstruct the PT evolution. Prograde heating occurred in the sillimanite stability held. During peak temperature conditions vapour-absent melting is observed in most felsic lithologies,Temperatures between 800 and 850 degrees C at pressures as low as 0.4-0.5 GPa are indicated by various mineral assemblages. The thermal peak was followed by an increase in pressure. Typical reactions of anti-clockwise PT evolution, like breakdown of cordierite + spinel to sapphirine and orthopyroxene + plagioclase to garnet + quartz, are frequently found. Maximum pressure is constraint to ca. 0.85 GPa by the complete absence of garnet in mafic granulites. Initial cooling was rapid, and is indicated by the back reaction of melt + orthopyroxene to biotite + quartz symplectites preserved in migmatites. This metamorphic event was accompanied by intrusion of porphyritic charnockite and granite, and by coeval compressional deformation. The observed evolution, especially the combination of an anti-clockwise PT loop and compressional tectonics, requires a strong, transient heat-source affecting the base of the crust. Neither the thermal evolution nor the relative timing is correctly predicted by existing collision models. The NMZ s.s. granulites were finally exhumed in a separate event along upper greenschist-facies thrusts, in response to a transpressive orogeny affecting the units further south at 2.0 Ga. In spite of potential ambiguities inherent to a qualitative approach, our observations show that petrography and field work, if used in conjunction with dating of a few key age relations and structural interpretation, are a pre-requisite to the erection of realistic tectonic models. The example of the NMZ s.s. may encourage geoscientists with limited access to analytical facilities to reassess the geological evolution of terrains which lack basic description.

Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

Process-related isotope variability in oceanic basalts revealed by high-precision Sr isotope ratios in olivine-hosted melt inclusions

Investigating the main geochemical characteristics of the lower continental crust is essential to understand its formation and evolution, identifying crustal differentiation processes and possible crust-mantle interactions. We performed bulk rock (major and trace elements), noble gases isotopes (He, Ne, Ar), and fluid inclusions (Raman spectroscopy) analyses on metamorphic rocks from Ivrea-Verbano Zone (Southern Italian Alps). Specifically, we studied various lithologies (metapelite, metagabbro, mafic and felsic granulite, amphibolite, and gneiss) to analyse the continuous metamorphic gradient from amphibolite- to granulite-facies.Bulk rock analyses confirm the mafic nature of the protoliths for metagabbros (MgO = 5.36-10.25 wt.%), mafic granulites (MgO = 8.32-25.80 wt.%) and amphibolite (MgO = 7.98 wt.%) plotting in the metabasite field of the ACF chemographic diagram. Felsic granulite and sillimanite-gneiss fall within metamorphosed quartz-feldspar rocks, except for metapelite, which approaches the metacarbonate field, due to the presence of secondary carbonates. Metagabbros, mafic granulites and amphibolite show low REE concentrations (&amp;#8721;REE between 3 and 25 ppm) and high Cr and Ni contents (up to 1865 and 265 ppm respectively in mafic granulite), reflecting the mafic/ultramafic nature of the protoliths, whereas felsic granulite, sillimanite-gneiss and metapelite show higher REE contents (&amp;#8721;REE between 48 and 197 ppm).3He/4He isotope ratios in metamorphosed quartz-feldspar rocks (0.06-0.30 Ra) and metabasites (0.15 and 0.45 Ra) are significantly radiogenic, although the metabasites show slightly higher values, corroborating a more primitive component in their source. Most samples plot near the air component in the 20Ne/22Ne vs 21Ne/22Ne diagram, except for mafic granulites which show a crustal-air mixing trend. As regards the Ar isotope ratios, all samples appear rich in radiogenic component (40Ar/36Ar up to 2645 in metagabbros).Raman spectroscopy analyses on fluid inclusions in orthopyroxene from mafic granulites show the coexistence of talc, graphite and magnesite with methane, providing direct evidence of a complex history in terms of post-metamorphic reactions and P-T-fO2 conditions.Our preliminary results show the compositional diversity and evolution of the lower continental crust, highlighting the interplay between mafic and sedimentary sources and the importance of fluid interactions and post-metamorphic processes.

Paleoproterozoic metamorphism of high-grade granulite facies rocks in the North China Craton: Study advances, questions and new issues

Geochemical constraints on the Cenozoic, OIB-type alkaline volcanic rocks of NW Turkey: Implications for mantle sources and melting processes

The Northern Apennine ophiolites crop out in different stratigraphic settings. The petrological and geochemical features of the mantle-gabbro association are well known for the ophiolites of the Internal Liguride Units (Rampone et al., 1998). The mantle ultramafics are mainly clinopyroxene- poor plagioclase-bearing lherzolites, residual after the extraction in Permian times of MORB-type liquids, which were intruded in Middle Jurassic times by N-MORB-type gabbros. The mantle ultramafics of the ophiolites from the External Liguride Units dominantly consist of rather fertile plagioclase-bearing lherzolites with pyroxenite bands (Rampone et al., 1995), and they are considered as slices of subcontinental mantle, probably accreted to the lithosphere in the Proterozoic. Little is known about the rare gabbroic rocks from the External Liguride ophiolites, as well as about the mantle ultramafics from the ophiolites which crop out in Southern Tuscany. In this work, we have carried out traceelement and Nd-Sr isotopic investigations of i) the gabbroperidotite association from Southern Tuscany ophiolites (Lanciaia Unit), and ii) the gabbroic rocks from the External Liguride ophiolites. This has allowed us to depict a complete overview of the gabbro-peridotite association of the Northern Apennine ophiolites. The gabbroic rocks from the different Units contain clinopyroxenes with similar trace element compositions, that yield N-MORB equilibrium liquids. Moreover, clinopyroxenes show marked trace element zonings, similarly to what was observed for the gabbroic rocks from ODP Leg 153 in the MARK area (Ross and Elthon, 1997), most likely related to the entrapment of interstitial liquid (Tribuzio et al., 1999). The gabbroic rocks considered in this study give Sm/Nd mineral isochron ages ranging from 179 to 170 Ma, interpreted to date the primary magmatic crystallization. These ages are significantly older than those recently found (163 ± 2 to 166 ± 2 Ma) on the basis of U/Pb and Ar/Ar geochronological determinations on zircons and amphiboles for the gabbroic rocks of the Western Alps ophiolites (Bill et al., 1997; Rubatto et al., 1998). 87Sr/86Sr of clinopyroxenes and initial eNd (8.5 to 8.9) are consistent with a formation from N-MORB. The gabbroic rocks from Internal Liguride ophiolites and the basalts (Rampone et al., 1998) from both Internal and External Liguride ophiolites show closely similar initial eNd. The incompatible trace element compositions of basalts from the different Units resemble those calculated for the parental liquids of the gabbroic rocks. The studied peridotites from Southern Tuscany ophiolites are porphyroclastic spinel-lherzolites. These rocks have a residual geochemical fingerprint, as shown by the marked depletion in incompatible trace elements of clinopyroxenes, similarly to what was observed for the Internal Liguride lherzolites. The clinopyroxenes from the southern Tuscany lherzolites indeed have slightly lower Sm/Nd values and slightly higher Sr contents than those from the Internal Liguride lherzolites. Major element mineral compositions of Southern Tuscany lherzolites yield relatively low-temperature estimates for the spinel-facies equilibration (ca. 1000 °C), similarly to those found for the External Liguride lherzolites. On the other hand, the latter rocks cleary differ from the Southern Tuscany lherzolites in the occurrence of accessory Ti-rich amphibole and in the fact that clinopyroxenes have markedly higher concentrations of incompatible trace elements. Nd and Sr isotopic compositions of clinopyroxenes from the Southern Tuscany lherzolites calculated at 170 Ma are respectively higher (eNd ca. 11.0) and lower than those of associated gabbros. Therefore, there is not a genetic meltresidue relationship for this gabbro-peridotite association, as inferred by Rampone et al. (1998) for the Internal Liguride ophiolites. The clinopyroxenes from the External Liguride lherzolites are characterized by markedly heterogeneous Nd and Sr isotopic compositions, coupled with a relatively constant Sm/Nd value (Rampone et al., 1995). Similar geochemical relations have been also observed for the Lanzo lherzolitic massif in the Western Alps (Bodinier et al., 1991). In the External Liguride Units, the clinopyroxenes from the Monte Nero lherzolite body, which is locally intruded by gabbroic to basaltic dykes, display Nd and Sr isotopic compositions that are consistent with those of analyzed gabbroic rocks. As a whole, the ophiolites of the Northern Apennine/Western Alps system are characterized by a homogeneous composition of the gabbroic complex and by a marked heterogeneity of the associated mantle section. The compositional spread of the mantle lherzolites can be related to two major events: 1) partial melting at 300-270 Ma, in relation to the post-Variscan extensional regime; 2) partial re-equilibration with N-MORB liquids derived from deeper astenospheric sources at 180-160 Ma.

There is a typical assemblage of garnet + kyanite + microperthite + quartz + rutile in high-pressure (HP) felsic granulite of Qinling complex in Songshugou area. East Qinling. The HP granulite was formed at 800 -900℃ and 1.3 -1.6GPa and has experienced two stages of retrograde metamorphism at 600- 650 ℃, 0.8-1.0GPa and 500-600℃, 0.3-0.6GPa, forming two retrograde metamorphic assemblages of margarite + plagiodase (PlI)+quartz and sillimanite + biotite + plagioclase(PlII) + microdine+quartz, respectively. They construct a two-stage clockwise P-T path which shows down-pressure cooling in both early and late stage.

The petrological record of the MOHO transition zone (MTZ) of the Oman ophiolite indicates complex multi-stage magmatic processes for the accretion of the deep oceanic crust at fast-spreading ridge systems. For the genesis of specific lithologies within the Oman ophiolite, some models include a “second” partial melting of harzburgites triggered by water-rich fluids, e.g. for the formation of a special type of chromitite/dunite associations within the MTZ (Ahmed and Arai, 2002) or for the genesis of so-called depleted gabbronorites (Benoit et al. 1999). Moreover, a recently in the Oman ophiolite observed new type of hydrothermal circulation operating at very high temperatures has the potential to reach the sub-MOHO level causing interaction between seawater and harzburgite at magmatic temperatures (Boudier et al., 2004). In order to understand the water-triggered partial melting behavior of typical harzburgites within the MTZ level of the oceanic lithosphere, we started an experimental study. Water-saturated partial melting experiments using a typical Oman harzburgite as starting material were performed under pressures of 100, 200, and 500 MPa at reducing (corresponding to the FMQ buffer) and at oxidizing conditions (FMQ+4) in specially designed internally heated pressure vessels enabling the control of oxygen fugacity. Temperatures varied between 980 and 1220°C, run durations were up to 82 hours. Solidus and the clinopyroxene-out curve vary strongly with pressure. For example, at 100 MPa first melts and the breakdown of clinopyroxene were detected at 1100/1160 °C, respectively, while the corresponding temperatures at 500 MPa were 1060/1100 °C. As expected, the generated melts are generally SiO2-rich varying between 54 and 65 wt% SiO2. The compositions of these melts show boninitic affinities with high Ca/Na ratios but generally with much lower MgO contents (< 10 wt% MgO) than those “high-Ca boninites” experimentally studied by Falloon and Danyushevsky (2000). According to these authors, the petrogenesis of “high-Ca boninites” requires temperatures as high as ~1480°C at depths of ~45 km (~ 1.5 GPa pressure) in the mantle wedge above an intra-oceanic subduction zone. Here we show that hydrous partial melting of harzburgites at MOHO level pressures has also the potential to produce boninitic melts with very high Ca/Na ratios, but at significant lower temperatures. Due to the residual character of the starting material, the experimental melts are highly depleted in incompatible trace elements showing extremely high Ca/Na ratios. A potential fractionation of such melts could produce cumulate rocks resulting in highly depleted gabbronorites with An contents of plagioclase generally above 90 mol%. Thus, such cumulates would correspond well to those “depleted gabbronorite” dikes typically observed in the Oman ophiolite intruding the layered gabbros and the MTZ (Benoit et al. 1999). At temperatures above the breakdown of clinopyroxene, the amount of residual olivine and chromite increases while that of orthopyroxene decreases. Thus, the residual mineral paragenesis shows characteristics of typical chromite-bearing dunites observed in many ophiolites. Interestingly, the compositional features of the experimental products fit in detail with natural dunite-harzburgite relations observed in some ophiolites where dunite bodies crosscut the harzburgite host. For example, Suhr et al. (2003) presented very accurate detailed traverses across dunites and neighboring harzburgites of the Bay of Islands ophiolite in Newfoundland. Main compositional features are: olivine in dunite shows an increase in Mg# and CaO but a decrease in NiO, and spinel in dunite shows an increase in Cr# relative to olivine and spinel of the associated harzburgite. Exactly the same features can be observed in our experiments when comparing olivines and spinels produced in our experiments with those of the harzburgitic starting material. In summary, our experimental results show that the formation of distinct lithologies of the oceanic lithosphere like high-Ca boninitic rocks, depleted gabbronorites, and chromite-bearing dunites, could, in principle, be attributed to a single process of fluid-induced partial melting of harzburgite at MOHO level or within the uppermost kilometers of the sub-oceanic mantle below spreading centers. The temperatures for the heating process ranging between 1060 and 1200°C for such a model could be provided by ascending MORB magmas. The presence of water-rich fluids at MOHO level which are necessary for such a model could be derived from different sources. One possible scenario would be that seawater has the potential to reach the sub-MOHO level, due to a special type of hydrothermal circulation operating at very high temperatures (Boudier et al., 2004), triggering the partial melting of harzburgite. Another possibility would be that seawater may penetrate the deep oceanic crust due to the special tectonic conditions at segment boundaries resulting in the partial alteration of harzburgites to serpentinites. In a later stage, these could be reheated by ascending magmas causing dehydration providing the water-rich fluids needed for a hydrous partial melting process. It should be emphasized that both scenarios mentioned above may occur in normal mid-ocean ridge settings away from any subduction zone. Another mechanism providing water-rich fluids in a shallow mantle level is conceivable in a supra-subduction zone setting (SSZ). Hirai and Arai (1987) reported that olivines in harzburgites of many SSZ ophiolites often contain characteristically high amounts of water-rich fluid inclusions. Thus, in a SSZ setting water can be transported to the sub-MOHO mantle trapped within olivines providing water-rich fluids after the dehydration of olivines due to hot ascending magmas.

Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Transdanubian Volcanic Region (TVR) is composed mainly of Pliocene alkali basalts, basanites, olivine basalts and olivine tholeiites, as well as rare nephelinites. The partial melting and genesis of alkali basaltic liquids is a consequence of an upwelling of the upper mantle which also caused thinning of the lithosphere and recent sinking of the Pannonian Basin.Four different types of lower crustal and upper-mantle xenoliths are found within the TVR: garnet-free and garnet-bearing granulites, clinopyroxenites and spinel lherzolites. We present mineralogical and geochemical data on granulite facies and clinopyroxenite xenoliths from three localities in the Hungarian part of the TVR (Bondoróhegy, Szentbékálla and Szigliget). It is concluded that, whilst the protoliths of the granulite facies xenoliths were tholeiitic igneous rocks and could be part of an ancient crust, the clinopyroxenite xenoliths represent recent underplating and may have formed from an alkali basaltic liquid similar to the host lava. Planar contact relations between clinopyroxenites and spinel lherzolites as observed in composite xenoliths, as well as high Al-contents in clinopyroxenes, point to a high-pressure genesis in the upper mantle for these rocks. In contrast, geobarometrical estimates yielded only a moderate pressure range characteristic of lower crustal levels for the garnet-free granulite xenoliths (7–9 kbar). Nevertheless, two-pyroxene geothermometry yielded high temperatures of equilibration (&gt;900°C) for these xenoliths, probably caused by advective heat transfer connected with underplating and in agreement with the high present-day geothermal gradient of this region. In the Central Range localities only garnet-free granulite xenoliths occur, whereas at the border of the TVR both garnet-free and garnet-bearing granulite facies nodules are found. It is possible that the incoming of garnet is retarded by higher temperatures in the lower crust below the Central Range.It is also suggested that the difference in seismically measured crustal thickness between the Central Range and adjacent basin areas may be connected with different thermal conditions below these regions and that the seismically defined Moho and the petrological Moho do not necessarily coincide.

O Complexo Acaiaca abrange uma região composta, predominantemente, por rochas de fácies granulito e essa região está localizada próximo à cidade de Acaiaca, Minas Gerais. O Complexo se estende por, no mínimo, 36 km na direção norte-sul e atinge cerca de 6 km de largura na porção central. Granulitos félsicos (biotita granulitos e charnockitos), granulitos máficos (piroxênio ± hornblenda granulitos), granulitos ultramáficos (olivina-piroxênio granofels) e granulitos aluminosos (granada-sillimanita ± cordierita granulitos e granada-ordierita-cianita granulitos) são litotipos encontrados nesse Complexo. Os granulitos félsicos são derivados de rochas de composição riolítica. Os máficos são derivados de rochas quimicamente semelhantes a basaltos de ambientes de arco de ilhas. Os granulitos aluminosos apresentam protólito sedimentar pelítico a grauvaquiano. O olivina-piroxênio granofels possui composição química semelhante a harzburgitos. As paragêneses minerais indicam metamorfismo de fácies granulito de pressão intermediária para formação das mesmas. Os gnaisses de fácies anfibolito encontrados na área do Complexo Acaiaca comumente apresentam microestruturas miloníticas e evidências de geração por retrometamorfismo a partir dos granulitos em zonas de cisalhamento.