Experimental constraints on amphibole stability in primitive alkaline and calc-alkaline magmas

Equilibrium crystallization experiments were carried out on two primitive basaltic rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to investigate the amphibole stability in the differentiation processes at deep crustal level, of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization occurs in both compositions at H2O in the melt >7wt% while the temperature of their occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O ratio rather than the absolute Na2O content in the melt. This is evident when experimental results on the APR16 and CM42 are compared with experimental data obtained from a primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows that amphibole never saturates the leucite-basanite at any of the investigated/modelled conditions, even when an extended crystallization increases the Na2O of melts up to contents like those of calc-alkaline experimental glasses. We conclude that, at pressure of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9 are more prone to crystallize amphibole

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb)N ratios, and YbN that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce-Nb and Ce-Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantl

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 andesites

Fresh, pseudotachylyte-bearing mantle peridotites from the lawsonite eclogite-facies san petrone unit, alpine corsica

Mantle peridotites exhumed in mountain belts provide important insights on the composition and evolution of the upper mantle, and additionally inform on metamorphic, geochemical, and tectonic processes-including seismic activity-at convergent margins. In this contribution, we present field, microstruc-tural, and mineralogical data of fresh, pseudotachylyte-bearing mantle peridotites from the lawsonite eclogite-facies San Petrone unit, Alpine Corsica, France. The present case study represents the first example of subducted fresh peridotite associated with fresh lawsonite eclogite-facies assemblages. Two bodies of fresh peridotite are embedded in fully serpentinized ultramafic rocks forming the substratum of a subducted ocean-continent transition of the Piemonte-Liguria Basin. Clinopyroxene and spinel mineral chemistry indicates that the investigated peridotite samples were part of a refertilized mantle and, therefore, the San Petrone unit likely belonged to the more distal part of the ocean-continent transition. Because small bodies of fresh peridotite embedded in fully serpentinized rocks can hardly be identified by means of geophysical investigations, this finding suggests that small, yet disseminated bodies of fresh mantle peridotite can potentially be more abundant than previously supposed at ocean-continent transition and, potentially, at mid-ocean ridges and in subduction zones. The preservation of fresh mantle peridotite bodies in subducting slabs is also discussed with respect to its potential implications on intermediate depth seismicity and geochemical cycling-including production of natural energy sources-from rifting to subduction

Trace element chemistry and U-Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161±2, 158±2 and 163±1Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean-continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithospher

The Nidar Ophiolite and its surrounding units in the Indus Suture Zone (NW Himalaya, India): new field data and interpretations

The Nidar Ophiolite is located between the North Himalayan nappes and the Indus Suture Zone in NW Himalaya in eastern Ladakh (India). Based mainly on geochemical argument, this ophiolite is classically interpreted as a relic of an intra-oceanic arc (Mahéo et al. 2000; Mahéo et al. 2004), which developed at around 140 Ma, prior to the collision between the Indian and Eurasian plates (Ahmad et al. 2008). From top to bottom, this ophiolite is composed of various sedimentary rocks (radiolarites, polygenic conglomerates and carbonates), volcanic rocks (pillow lavas, basaltic to andesitic in composition), gabbros (Fe- and layered gabbros, pegmatites and minor troctolites), serpentinites, dunites, pyroxenites and peridotites (mainly harzburgites). The Nidar Ophiolite underwent an anchizonal metamorphism with preservation of primaries structures (layering) and volcanic textures (pillow lavas). This study is mainly focused on new field observations across the ophiolite and the surrounding units. A new detailed geologic map of the ophiolite between the Nidar village and Kyun Tso area is presented. The upper part of the ophiolitic complex is an alternation of volcanic and sedimentary rocks (500- 1000 m thick) and the lower part consists of large outcrops of gabbros (3000m thick). These mafic rocks are separated from the serpentinized ultramafic rocks by a 200m thick ophiolitic breccia and continental Indus Molasse slices. The Nidar Ophiolite is made up of the classical rock type succession (ultramafites, gabbros, pillow basalts, radiolarites), but the internal structure is far more complex than previously suggested. New field data (geologic and structural maps, lithologic sections, etc.) coupled with new geochemical analysis will help to constrain the geodynamic context and deformation history. Ahmad, T., T. Tanaka, H.K. Sachan, Y. Asahara, R. Islam, et P.P. Khanna. 2008. « Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone ». Tectonophysics 451 (1–4): 206‑ 24. Mahéo, Gweltaz, Hervé Bertrand, Stéphane Guillot, Georges Mascle, Arnaud Pêcher, Christian Picard, et Julia De Sigoyer. 2000. « Témoins d’un arc immature téthysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde) ». Comptes Rendus de l’Académie des Sciences - Series IIA - Earth and Planetary Science 330 (4): 289‑ 95. Mahéo, Gweltaz, Hervé Bertrand, Stéphane Guillot, Igor M. Villa, Francine Keller, et Paul Capiez. 2004. « The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys ». Chemical Geology 203 (3–4): 273‑ 303

Thinning mechanisms of heterogeneous continental lithosphere

The mechanisms responsible for the formation of extremely thinned continental crust (<10 km thick) and lithosphere during rifting remains debated. Observations from present-day and fossil passive margins highlight the role of deep-seated deformation, likely controlled by heterogeneities within the continental lithosphere, such as changing lithologies, mechanical anisotropies and inherited structures. We investigate the mechanisms of lithospheric thinning by exploring the role of pre-existing heterogeneities on the architecture and evolution of rifted margins. We estimate pre-rift pressure conditions (P0) vs. depth diagrams of crustal to lithospheric sections, to quantify rift-related modifications on inherited lithostatic pressure gradients. Two field examples from the Alpine Tethys margins in the Eastern and Southern Alps (SE Switzerland and N Italy) were selected to characterize: (1) the pre-rift architecture of the continental lithosphere; (2) the localization of rift-related deformation in distinct portions of the lithosphere; and (3) the interaction between pre-existing heterogeneities of the lithosphere and rift-related structures. These observations are compared with high-resolution, two-dimensional thermo-mechanical numerical models. The design of the models takes into account pre-existing mechanical heterogeneities representing the initial pre-rift architecture of the continental lithosphere. Extensional structures consist of high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. Such structures accommodate the lateral extraction of mechanically stronger levels derived from the middle to lower crust. As a result, the extremely thinned continental crust in Tethyan passive margins represents the juxtaposition and amalgamation of distinct strong levels of the crust separated by major extensional structures identified by sharp pressure gradients. Future work should determine the applicability of these results to other present-day and fossil rifted margins