Juxtaposition of Melt Impregnation and High-Temperature Shear Zones in the Upper Mantle; Field and Petrological Constraints from the Lanzo Peridotite (Northern Italy)

Results of a field and microstructural study between the northern and the central bodies of the Lanzo plagioclase peridotite massif (NW Italy) indicate that the spatial distribution of deformation is asymmetric across kilometre-scale mantle shear zones. The southwestern part of the shear zone (footwall) shows a gradually increasing degree of deformation from porphyroclastic peridotites to mylonite, whereas the northeastern part (hanging wall) quickly grades into weakly deformed peridotites. Discordant gabbroic and basaltic dykes are asymmetrically distributed and far more abundant in the footwall of the shear zone. The porphyroclastic peridotite displays porphyroclastic zones and domains of igneous crystallization whereas mylonites are characterized by elongated porphyroclasts, embedded between fine-grained, polycrystalline bands of olivine, plagioclase, clinopyroxene, orthopyroxene, spinel, rare titanian pargasite, and domains of recrystallized olivine. Two types of melt impregnation textures have been found: (1) clinopyroxene porphyroclasts incongruently reacted with migrating melt to form orthopyroxene + plagioclase; (2) olivine porphyroclasts are partially replaced by interstitial orthopyroxene. The melt-rock reaction textures tend to disappear in the mylonites, indicating that deformation in the mylonite continued under subsolidus conditions. The pyroxene chemistry is correlated with grain size. High-Al pyroxene cores indicate high temperatures (1100-1030°C), whereas low-Al neoblasts display lower final equilibration temperatures (∼860°C). The spinel Cr-number [molar Cr/(Cr + Al)] and TiO2 concentrations show extreme variability covering almost the entire range known from abyssal peridotites. The spinel compositions of porphyroclastic peridotites from the central body are more variable than spinel from mylonite, mylonite with ultra-mylonite bands, and porphyroclastic rocks of the northern body. The spinel compositions probably indicate disequilibrium and would favour rapid cooling, and a faster exhumation of the central peridotite body, relative to the northern one. Our results indicate that melt migration and high-temperature deformation are juxtaposed both in time and space. Melt-rock reaction may have caused grain-size reduction, which in turn led to localization of deformation. It is likely that melt-lubricated, actively deforming peridotites acted as melt focusing zones, with permeabilities higher than the surrounding, less deformed peridotites. Later, under subsolidus conditions, pinning in polycrystalline bands in the mylonites inhibited substantial grain growth and led to permanent weak zones in the upper mantle peridotite, with a permeability that is lower than in the weakly deformed peridotites. Such an inversion in permeability might explain why actively deforming, fine-grained peridotite mylonite acted as a permeability barrier and why ascending mafic melts might terminate and crystallize as gabbros along actively deforming shear zones. Melt-lubricated mantle shear zones provide a mechanism for explaining the discontinuous distribution of gabbros in ocean-continent transition zones, oceanic core complexes and ultraslow-spreading ridge

Enriched Basaltic Andesites from Mid-crustal Fractional Crystallization, Recharge, and Assimilation (Pilavo Volcano, Western Cordillera of Ecuador)

The origin of andesite is an important issue in petrology because andesite is the main eruptive product at convergent margins, corresponds to the average crustal composition and is often associated with major Cu-Au mineralization. In this study we present petrographic, mineralogical, geochemical and isotopic data for basaltic andesites of the latest Pleistocene Pilavo volcano, one of the most frontal volcanoes of the Ecuadorian Quaternary arc, situated upon thick (30-50 km) mafic crust composed of accreted Cretaceous oceanic plateau rocks and overlying mafic to intermediate Late Cretaceous-Late Tertiary magmatic arcs. The Pilavo rocks are basaltic andesites (54-57·5 wt % SiO2) with a tholeiitic affinity as opposed to the typical calc-alkaline high-silica andesites and dacites (SiO2 59-66 wt %) of other frontal arc volcanoes of Ecuador (e.g. Pichincha, Pululahua). They have much higher incompatible element contents (e.g. Sr 650-1350 ppm, Ba 650-1800 ppm, Zr 100-225 ppm, Th 5-25 ppm, La 15-65 ppm) and Th/La ratios (0·28-0·36) than Pichincha and Pululahua, and more primitive Sr (87Sr/86Sr ∼0·7038-0·7039) and Nd (εNd ∼ +5·5 to +6·1) isotopic signatures. Pilavo andesites have geochemical affinities with modern and recent high-MgO andesites (e.g. low-silica adakites, Setouchi sanukites) and, especially, with Archean sanukitoids, for both of which incompatible element enrichments are believed to result from interactions of slab melts with peridotitic mantle. Petrographic, mineral chemistry, bulk-rock geochemical and isotopic data indicate that the Pilavo magmatic rocks have evolved through three main stages: (1) generation of a basaltic magma in the mantle wedge region by flux melting induced by slab-derived fluids (aqueous, supercritical or melts); (2) high-pressure differentiation of the basaltic melt (at the mantle-crust boundary or at lower crustal levels) through sustained fractionation of olivine and clinopyroxene, leading to hydrous, high-alumina basaltic andesite melts with a tholeiitic affinity, enriched in incompatible elements and strongly impoverished in Ni and Cr; (3) establishment of one or more mid-crustal magma storage reservoirs in which the magmas evolved through dominant amphibole and clinopyroxene (but no plagioclase) fractionation accompanied by assimilation of the modified plutonic roots of the arc and recharge by incoming batches of more primitive magma from depth. The latter process has resulted in strongly increasing incompatible element concentrations in the Pilavo basaltic andesites, coupled with slightly increasing crustal isotopic signatures and a shift towards a more calc-alkaline affinity. Our data show that, although ultimately originating from the slab, incompatible element abundances in arc andesites with primitive isotopic signatures can be significantly enhanced by intra-crustal processes within a thick juvenile mafic crust, thus providing an additional process for the generation of enriched andesite

Quaternary Sanukitoid-like Andesites Generated by Intracrustal Processes (Chacana Caldera Complex, Ecuador): Implications for Archean Sanukitoids

High-Mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction-related rock types that have been found in modern arc settings and in Late Archean sequences, where they are associated with trondhjemite-tonalite-granodiorite (TTG) suites. Archean rocks with these geochemical features are known as sanukitoids and, despite their limited abundance, are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction-related high-Mg andesites and diorites. Understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes. The accepted petrogenetic models for modern, enriched, high-Mg andesites and their Archean equivalents, the sanukitoids, consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting, or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge. We present new data on the petrography, mineral chemistry and whole-rock geochemistry (major and trace elements and Sr-Nd-Pb isotopes) of andesitic rocks from an ∼30 ka lava flow (Yuyos flow) from the Chacana Caldera Complex, Eastern Cordillera of Ecuador. These rocks show a remarkable geochemical affinity with Archean sanukitoids, including high magnesium numbers (0·58-0·63) accompanied by high contents of incompatible elements (e.g. Th 17-23 ppm, U 6-7·5 ppm, Ba 1600-1800 ppm, Sr 1430-1565 ppm, La 74-94 ppm). Additionally, the sanukitoid-like andesites of Yuyos are associated with predominant silica-rich (adakite-like) andesites, which are widespread throughout the Quaternary arc of Ecuador. This makes the Quaternary Ecuadorian magmatic province a close equivalent of the Archean TTG-sanukitoid association. The bulk-rock geochemistry, petrography and mineral chemistry data indicate that the sanukitoid-like features of the andesites of the Yuyos flow derive from intracrustal recycling of the felsic-intermediate to mafic-ultramafic roots of the Quaternary volcanic arc of Ecuador by ‘normal' mantle-derived basaltic magmas with the geochemical characteristics of continental arc basalts or high-alumina basalts. In view of the similarities between the Yuyos andesites and Archean sanukitoids in terms of geochemistry and lithological association, we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both Archean sanukitoids and modern enriched high-Mg andesite

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

Amphibole fractionation in the deep roots of subduction-related magmatic arcs is a fundamental process for the generation of the continental crust. Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes. The Chelan Complex in the western U.S. forms the lowest level of a 40-km thick exposed crustal section of the North Cascades and is composed of olivine websterite, pyroxenite, hornblendite, and dominantly by hornblende gabbro and tonalite. Magmatic breccias, comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes. Phase equilibria, textural observations and mineral chemistry yield emplacement pressures of ∼1.0GPa followed by isobaric cooling to 700°C. The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole. Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3% of olivine websterite, 12% of pyroxene hornblendite, 33% of pyroxene hornblendite, 19% of gabbros, 15% of diorite and 2% tonalite. Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process. Crustal assimilation during fractionation is thus not required to generate peraluminous tonalit

Equilibrium and Fractional Crystallization Experiments at 0·7 GPa; the Effect of Pressure on Phase Relations and Liquid Compositions of Tholeiitic Magmas

Two series of anhydrous experiments have been performed in an end-loaded piston cylinder apparatus on a primitive, mantle-derived tholeiitic basalt at 0·7 GPa pressure and temperatures in the range 1060-1270°C. The first series are equilibrium crystallization experiments on a single basaltic bulk composition; the second series are fractionation experiments where near-perfect fractional crystallization was approached in a stepwise manner using 30°C temperature increments and starting compositions corresponding to that of the previous, higher temperature glass. At 0·7 GPa liquidus temperatures are lowered and the stability of olivine and plagioclase is enhanced with respect to clinopyroxene compared with phase equilibria of the same composition at 1·0 GPa. The residual solid assemblages of fractional crystallization experiments at 0·7 GPa evolve from dunites, followed by wehrlites, gabbronorites, and gabbros, to diorites and ilmenite-bearing diorites. In equilibrium crystallization experiments at 0·7 GPa dunites are followed by plagioclase-bearing websterites and gabbronorites. In contrast to low-pressure fractionation of tholeiitic liquids (1 bar-0·5 GPa), where early plagioclase saturation leads to the production of troctolites followed by (olivine) gabbros at an early stage of differentiation, pyroxene still crystallizes before or with plagioclase at 0·7 GPa. The liquids formed by fractional crystallization at 0·7 GPa evolve through limited silica increase with rather strong iron enrichment following the typical tholeiitic differentiation path from basalts to ferro-basalts. Silica enrichment and a decrease in absolute iron and titanium concentrations are observed in the last fractionation step after ilmenite starts to crystallize, resulting in the production of an andesitic liquid. Liquids generated by equilibrium crystallization experiments at 0·7 GPa evolve through constant SiO2 increase and only limited FeO enrichment as a consequence of spinel crystallization and closed-system behaviour. Empirical calculations of the (dry) liquid densities along the liquid lines of descent at 0·7 and 1·0 GPa reveal that only differentiation at the base of the crust (1·0 GPa) results in liquids that can ascend through the crust and that will ultimately form granitoid plutonic and/or dacitic to rhyodacitic sub-volcanic to volcanic complexes; at 0·7 GPa the liquid density increases with increasing differentiation as a result of pronounced Fe enrichment, rendering it rather unlikely that such differentiated melt will reach shallow crustal level

Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids

To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2GPa, 800-1,000°C and variable H2O contents. At water undersaturated conditions and fO2 established around QFM, garnet has a wide stability field. At 1.2 GPa garnet+amphibole are the high-temperature liquidus phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H2O-contents ≤9wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in $${\text{T{\text{-}}X}}_{{{\text{H}}_{2} {\text{O}}}}$$ space. At 0.8GPa, garnet is stable at magmatic H2O contents exceeding 8wt% and is replaced by spinel at decreasing dissolved H2O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1.2GPa series, but show substantial enrichment in FeO/MgO for the 0.8GPa series related to the contrasting roles of garnet and amphibole in fractionating Fe-Mg in derivative liquids. Our experiments indicate that the stability of igneous garnet increases with increasing dissolved H2O in silicate liquids and is thus likely to affect trace element compositions of H2O-rich derivative arc volcanic rocks by fractionation. Garnet-controlled trace element ratios cannot be used as a proxy for ‘slab melting', or dehydration melting in the deep arc. Garnet fractionation, either in the deep crust via formation of garnet gabbros, or in the upper mantle via formation of garnet pyroxenites remains an important alternative, despite the rare occurrence of magmatic garnet in volcanic rock

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt-mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW-SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., εNd=+1.5 to +6, 87Sr/86Sr=0.7040-0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN=5.7-34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt-mantle interactions. The isotopic signatures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic Carnegie Ridg

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivine and clinopyroxene. Some of these macrocrysts might represent fragments of the source region of the host magma transported to the Earth's surface. If this hypothesis is correct these fragments can be used to characterize the composition of the source region and to put constraints on the magma generation process. In this study, we investigate the origin of macrocrysts and mineral aggregates from an olivine nephelinite from the Kaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), which can be divided into three groups. Cpx I is relict Cpx from aggregates with deformed olivine that is depleted in Ca and characterized by strong light rare earth element (LREE) fractionation, low Ti/Eu and negative high field strength element (HFSE) anomalies. Its geochemical signature is consistent with formation by carbonatite metasomatism and with equilibration in the presence of orthopyroxene. Cpx II is Ca-rich Cpx, forming both aggregates with deformed olivine and individual macrocrysts. The LREE, as for Cpx I, are strongly fractionated. Convex REE patterns may be present. The depletion in HFSE is less pronounced. Cpx III is oscillatory zoned Cpx phenocrysts showing enrichment in Ca, convex REE patterns and no HFSE anomalies. The transition in the trace element abundances between the Cpx of the three groups is gradual. However, Cpx I and II did not crystallize from the host magma, as demonstrated by the presence of kink-bands and undulose extinction in the associated olivine and by the composition of alkali aluminosilicate glass inclusions in Cpx II. Based on the Cpx relationships, we interpret the studied suite of macrocrysts and mineral aggregates as a mixture of disintegrated fragments of the source region of the host olivine nephelinite. The process of melt generation was multi-stage. A primary carbonatite melt ascending from deeper levels in the mantle, probably from the dolomite-garnet peridotite stability field, reacted with mantle peridotite along the solidus ledge in the system lherzolite-CO2 (< 20-22 kbar) and started to crystallize carbonate minerals. Because of its low solidus temperature, the resulting carbonate-wehrlite assemblage melted incongruently with the formation of additional clinopyroxene. The carbonatite melt evolved during crystallization of carbonate minerals and concomitant incongruent melting of the carbonate-wehrlite, accompanied by the segregation of incipient alkali aluminosilicate melts. As a consequence of fast reaction rates in the presence of a carbonatite melt, this process probably took place under disequilibrium conditions. Further melting of the assemblage wehrlite + alkali aluminosilicate melt led to the generation of the olivine nephelinite magma. It entrained fragments of the wehrlite and brought them to the surfac

Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere

The existence of different mantle domains exposed in ocean-continent transition zones provides a framework for understanding the generation of ultramafic seafloor along magma-poor rifted margins. In this study we present detailed petrological and geochemical data on peridotites from the Eastern Central Alps ophiolites in Switzerland and Italy to identify different mantle domains, to estimate the extent of refertilization, and to test whether refertilization is associated with a thermal signature that has important implications for geophysical interpretations of ocean-continent transitions. The compositions of clinopyroxene, orthopyroxene and spinel clearly reflect the different mantle domains. Relative to clinopyroxenes from spinel peridotites, clinopyroxenes from plagioclase peridotites have lower Na2O and Sr contents, but higher middle to heavy rare earth element ratios and Zr concentrations, and different Sc-V relationships. Spinels in plagioclase peridotites have higher TiO2 and lower Mg-numbers compared with those in spinel peridotites. Mineral-mineral trace element partitioning suggests that spinel peridotites equilibrated at substantially lower temperatures than plagioclase peridotites. The temperature difference between the spinel and plagioclase peridotites indicates an important thermal boundary between the two. The geochemical data show systematic spatial variations. A heterogeneous, ‘subcontinental domain' with no syn-rift melt imprint is separated from a ‘refertilized domain' that exhibits a complex history of regional-scale melt infiltration and melt-rock reaction, which has erased most of the ancient history. Simple calculations suggest that up to 12% of mid-ocean ridge basalt-type melt can be stored in plagioclase peridotite, relative to a depleted residue. Such a ‘lithospheric sponge' provides an explanation for the fertile compositions of the peridotites and the rare occurrence of volcanic rocks in magma-poor rifted margins. We suggest that magma-poor vs magma-rich margins are largely determined by the efficiency of melt extraction and not so much by melt generation processes, given a similar initial composition of the upwelling mantle. It is proposed that refertilization increases textural diversity and chemical heterogeneity related to shallow crystallization in the mantle lithospher