María Teresa Gómez-Pugnaire

Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle

We present first trace element analyses of the fluid produced during breakdown of antigorite serpentine, a major dehydration reaction occurring at depth within subducting oceanic plates. Microinclusions filled with crystals+aqueous liquid are disseminated within olivine and orthopyroxene grown at pressures and temperatures beyond the stability field of antigorite. Despite hydrogen loss and significant major element changes that have affected the analyzed inclusions, their trace element composition still reflects characteristics of the subduction fluid released during serpentinite dehydration. The fluid is enriched in incompatible elements indicating either (1) interaction with fluids derived from crustal slab components, or (2) dehydration of altered (serpentinized) oceanic mantle previously enriched in incompatible elements. Several features of the analyzed fluid+mineral inclusions (high Pb/Th, Pb/U and Pb/Ce) are in agreement with available experimental work, as well as with the geochemical signatures of most arc lavas and of several ocean island basalt mantle sources. The trace element patterns of the fluid+mineral inclusions do not display relative enrichment in large ion lithophile elements compared to high field strength elements, thus suggesting that the latter elements may become soluble in natural subduction fluids. fl 2001 Elsevier Science B.V. All rights reserved.

The amphibolites from the Ossa–Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation

Abstract Basic rocks included in the Ossa–Morena/Central Iberian Variscan suture of SW Iberia have been studied in order to decipher the pre-orogenic evolution along this major boundary. These rocks appear as garnet-free amphibolites and garnet-bearing amphibolites. Previous geochronological data on these basic rocks suggest the existence of two different protolith ages: Upper Precambrian and Lower Palaeozoic (Ordovician). Upper Precambrian amphibolites form small-size (up to several metres thick), lense-, dike- or elongated-shaped bodies, while Lower Palaeozoic ones form elongated bodies with thicknesses up to several hectometres. The chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas. As for the originally basaltic rocks, their geochemical characteristics indicate a shallow melting with different proportions of at least three end-members, namely (i) a MORB-like asthenospheric source, (ii) a more enriched (plume-derived or subcontinental lithosphere) mantle source, and (iii) a continental crust component. Moreover, Upper Precambrian and Lower Palaeozoic amphibolites have different geochemical affinities. In the former group, we envisage an advanced intra-continental rifting situation or an intra-continental back-arc setting related to the Cadomian orogeny, while for the latter, we propose an environment of oceanic crust formation predating Variscan subduction and collision. The recognition of these remains of oceanic-affinity basic rocks along the OMZ/CIZ boundary allows us to propose that some sort of ocean may well have existed between the two zones during Early Palaeozoic times, and that this ocean was presumably closed at a later stage, probably during the Devonian, by oceanic subduction under the CIZ crust.

Primary melt inclusions in andalusite from anatectic graphitic metapelites: Implications for the position of the Al2SiO5 triple point

Anatectic crustal xenoliths in the Miocene volcanic rocks of Mazarron (southeast Spain) contain andalusite with melt inclu- sions, an unprecedented finding. Microstructures indicate that the melt inclusions were trapped during andalusite growth. The vola- tile content of the peraluminous inclusions is too low (Cl , 0.4 wt%, F , 0.3 wt%, P 1500 ppm, B , 850 ppm) to have caused a significant reduction of the wet solidus temperature. Moreover, the presence of graphite, as observed, during partial melting is expected to have raised the temperature of the wet solidus. Melting temperatures for the inclusions—obtained from quartz-albite-or- thoclase haplogranite system (680-790 8C), Zr (620-705 8C), and light rare earth element (615-725 8C) thermometry—indicate that the stability of andalusite 1 melt is incompatible with the position of the most commonly used andalusite 5 sillimanite equilibrium, and that the Al2SiO5 triple point must be placed at higher tem- peratures and pressures.

Phase diagram sections applied to amphibolites: a case study from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif)

Abstract Amphibolites included in the Ossa-Morena/Central Iberian suture of SW Iberia offer the opportunity of investigating the subduction and collision events, followed by exhumation, which affected this major boundary during the Variscan orogeny. This suture contact is marked by a band of highly deformed and metamorphosed rocks, namely, the Central Unit. The main deformation affecting this unit is a ductile shearing that produced an intense planar–linear fabric. This shearing has two components: a dominant left lateral one and a subordinate dip-slip normal one, which can be taken to be responsible for the exhumation of this unit in Devonian–Carboniferous times. The rocks studied in the present paper are garnet-free amphibolites and garnet-bearing amphibolites intercalated between gneisses. They appear as lens-, dike- or elongated-shaped bodies, their thickness ranging between a metre and several hectometres. The investigation of their metamorphic evolution has been carried out by computing phase diagram sections (pseudosections) for specified bulk rock compositions. This method enables us to achieve a twofold goal: (i) to model the P – T path undergone by the rocks during the metamorphism in question, and (ii) to assess the influence of the fluid phase abundance on the formation and the preservation of the mineral assemblages as well as on the density decrease of the rocks during exhumation. The metabasites of the Central Unit underwent a metamorphic evolution, characterized by an initial eclogite facies event with peak pressures around 1.9 GPa and temperatures about 550 °C, during which grossular-rich garnet and jadeite-rich clinopyroxene coexisted with chlorite, glaucophane, and paragonite. The former two hydrous minerals disappeared during the subsequent stage, characterised by a slight decompression with strong heating. At the peak temperature (≈725 °C and 1.6 GPa), important amounts of water were released owing to paragonite breakdown. This water produced (i) the formation of large amounts of amphibole coexisting with jadeite-rich clinopyroxene and sodium-rich plagioclase, and (ii) a sharp density decrease of the rocks, which must have contributed to their exhumation. Fast uplift is consistent with the deduced sharp isothermal decompression (1.18 GPa). This was, in turn, followed by simultaneous strong cooling and decompression (≈455 °C and 0.3 GPa), which produced the calcium enrichment observed in plagioclase and the jadeite decrease and diopside enrichment in clinopyroxene. This final retrograde evolution occurred at fluid absent conditions, thus allowing the preservation of previously formed high-pressure assemblages.

Retrograde formation of NaCl-scapolite in high pressure metaevaporites from the Cordilleras Béticas (Spain)

AbstractA Permo-Triassic pelite-carbonate rock series (with interacalated metabasitic rocks) in the Cordilleras Béticas, Spain, was metamorphosed during the Alpine metamorphism at high pressures (Pmin near 18 kbar). The rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc, of kyanite-phengitetalc-biotite, and of quartz after sulfate minerals, and relicts of baryte, anhydrite, NaCl, and KCl, indicating a salt-clay mixture of illite, chlorite, talc, and halite as the original rock. The evaporitic metapelites have a whole rock composition characterized by high Mg/(Mg+Ca) ratios>0.7, variable alkaline and Sr, Ba, contents, but are mostly K2O rich (<8.8 wt%). The F (<2600 ppm), Cl (<3600 ppm), and P2O5 (<0.24 wt%) contents are also high. The pelitic member of this series is a fine grained biotite rock. Kyanite-phengite-talc-biotite aggregates in pseudomorphs developed in the high pressure stage. Albite-rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift. Scapolite, rich in NaCl (Ca/(Ca+Na) mol% 24–40) and poor in SO4, with Cl/(Cl+CO3) ratios between 0.6 and 0.8, formed as porphyroblasts, sometimes replacing up to 60% of the rock in a late stage of metamorphism (between 10 and 5 kbar, near 600°C). No reaction with albite is observed, and the scapolite formed from biotite by:

Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarrón, SE Spain

\begin{gathered} Al - biotite + CaCO_3 + NaCl + SiO_2 \hfill \\ = Al - poor biotite + scapolite + MgCO_3 + KCl \hfill \\ + MgCl_2 + H_2 O \hfill \\ \end{gathered}

Kyanite, margarite and paragonite in pseudomorphs in amphibolitized eclogites from the Betic Cordilleras, Spain

Calculated fluid composition in equilibrium with scapolite indicates varying salt concentrations in the fluid. Distribution of Cl and F in biotite and apatite also indicates varying fluid compositions.

What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks

Abstract At Mazarrón, SE Spain, dacitic lavas of the Neogene Volcanic Province contain numerous xenocrysts and xenoliths with abundant andalusite that displays variable degrees of transformation to both fibrolite and coarse sillimanite. At the onset of replacement, andalusite dissolves along grain boundaries and (110) cleavage planes, probably assisted by fluids or melts. At the same time, fibrolite crystallizes together with plagioclase, cordierite, and graphite in newly formed embayments or in the adjacent matrix. With increasing reaction progress, fibrolite needles coalesce into coarser sillimanite prisms, and direct topotactic replacement of andalusite is observed. The mutual crystallographic orientation of andalusite and sillimanite obtained from TEM investigation deviates slightly from the topotactic relationship proposed in the literature (cAnd || cSil, aAnd || bSil, bAnd || aSil). The two lattices are rotated by ~2.5° around aAnd (= bSil). With this misorientation, the structurally equivalent {032}And and {302}Sil planes, which exhibit the smallest misfit between the two lattices, become parallel. Macroscopic interfaces with such orientations are rare. Microscopically, however, decomposition of faces into {032}And || {302}Sil and {110} facets are common. The mutual crystallographic orientation of the reactant and the product phases is, therefore, controlled by lattice misfit minimization. The prismatic shape of the final coarse sillimanite crystals, however, is controlled by kinetic factors. The reaction seems to proceed fastest parallel to the octahedral Al chains resulting in the development of crystals elongated along the c axis. The high activation energy and the large overstepping of the equilibrium temperature required for the transformation are probably responsible for the large differences in reaction progress observed in the samples from Mazarrón.