The recycling of chromitites in ophiolites from southwestern North America

Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61–0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS)-erlichmanite (OsS), osmium and irarsite (IrAsS) in chromite, that yield T ≈ T model ages peaking at ~ 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [ɛ(t) = − 18.7 to + 9.1 and O values < 12.4‰] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and ~ 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago. The T ages at ~ 325 Ma record a partial melting event in the mantle prior to the construction of the Vizcaino intra-oceanic arc, which is probably related to the Permian continental subduction, dated at ~ 311 Ma.Funding for this research was provided through the CONACyT research project number 155662 and internal budget of the Instituto de Geología (UNAM) to AC, the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ, and by the project CGL2015-65824-P granted by the Spanish “Ministerio de Economía y Competitividad” to JAP. Additional funding for chromite microanalyses were provided by the DGAPA-UNAM postdoctoral fellowship to VC. This is also a contribution from the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents ( www.es.mq.edu.au /GEMOC) and the ARC Centre of Excellence for Core to Crust Fluid Systems. The SHRIMP facility lab and technical staff at Curtin University are also thanked for their support in the data acquisition. We also thank to Carlos Linares (Petrology Laboratory of the, UNAM, Mexico) for his help with the EMPA analyses on chromite. Michelangelo Martini, Luis Abel Jiménez-Galindo and all the personnel at the Fishermen's Cooperative Society at Bahía Tortugas (Baja California Sur) are wholeheartedly thanked for their kind assistance during our fieldwork

かんらん捕獲岩の微細構造と岩石学的特徴 : 背弧拡大域及びリフト帯における最上部マントル構造発達過程と地震波特性への応用

博士(理学)doctoral創造科学技術大学院静岡大学甲第711号ET

Melt-rock interaction, High-temperature deformation, and seismic properties in the continent-ocean transition mantle, and in the oceanic lithosphere

Cette thèse regroupe deux études distinctes, qui documentent le contrôle des microstructures sur les propriétés sismiques des roches. La première partie traite du développement des orientations préférentielles cristallographiques (OPC) dans le manteau supérieur, associé aux interactions liquide/magma-roche, enregistré dans des xénolites de péridotites du bassin d'arrière-arc de la mer du Japon. Les caractéristiques microstucturales et géochimiques des échantillons étudiés montrent que l'ouverture arrière-arc active est associée à une déformation du manteau supérieure similaire à celle observée dans l'ophiolite d'Oman. L'initiation de l'extension d'arrière-arc n'est pas associée à de fortes interactions entre percolation magmatique et déformation, en comparaison avec les zones de rifting continentales, probablement en raison des taille et durée relativement petites de l'épisode d'ouverture. La seconde partie présente une base de données unique d'OPC de plagioclase de roches mafiques plus ou moins déformées. Les OPC sont classées en 3 types principaux; leurs caractéristiques en fonction du régime de déformation (magmatique ou plastique) sont présentées et discutées. Les propriétés sismiques calculées des roches gabbroiques montrent que l'anisotropie tend à croitre avec l'intensité des fabriques, bien qu'elle soit généralement faible, en raison des effets opposés des olivines/clinopyroxènes et du plagioclase.This thesis compiles two distinct studies that both document the control of microstructures on rock seismic properties. The first part deals with the development of crystallographic preferred orientations (CPO) in the uppermost mantle associated with melt/fluid-rock interactions, recorded in peridotites xenoliths from the Japan sea back-arc basin. The microstructural and geochemical characteristics of the studied samples reveal that active spreading is associated to uppermost mantle deformation similar to that observed in the Oman ophiolite. At the onset of back-arc spreading, there are no strong interactions between melt percolation and deformation in comparison to continental rift zones, probably due to the relatively small size and short duration of the spreading event. The second part presents a unique database of plagioclase CPO from variously deformed mafic rocks. CPO are grouped in three main types; their characteristics as a function of deformation regime (magmatic or crystal-plastic) are outlined and discussed. Calculated seismic properties of gabbroic rocks show that anisotropy tends to increase as a function of fabric strength, although it is generally weak, due to the competing effect of olivine/clinopyroxene and plagioclase

Interactions magma-roche, déformation à haute température et anisotropie sismique dans le manteau de la transition continent-océan et dans la lithosphère océanique

This thesis compiles two distinct studies that both document the control of microstructures on rock seismic properties. The first part deals with the development of crystallographic preferred orientations (CPO) in the uppermost mantle associated with melt/fluid-rock interactions, recorded in peridotites xenoliths from the Japan sea back-arc basin. The microstructural and geochemical characteristics of the studied samples reveal that active spreading is associated to uppermost mantle deformation similar to that observed in the Oman ophiolite. At the onset of back-arc spreading, there are no strong interactions between melt percolation and deformation in comparison to continental rift zones, probably due to the relatively small size and short duration of the spreading event. The second part presents a unique database of plagioclase CPO from variously deformed mafic rocks. CPO are grouped in three main types; their characteristics as a function of deformation regime (magmatic or crystal-plastic) are outlined and discussed. Calculated seismic properties of gabbroic rocks show that anisotropy tends to increase as a function of fabric strength, although it is generally weak, due to the competing effect of olivine/clinopyroxene and plagioclase.Cette thèse regroupe deux études distinctes, qui documentent le contrôle des microstructures sur les propriétés sismiques des roches. La première partie traite du développement des orientations préférentielles cristallographiques (OPC) dans le manteau supérieur, associé aux interactions liquide/magma-roche, enregistré dans des xénolites de péridotites du bassin d'arrière-arc de la mer du Japon. Les caractéristiques microstucturales et géochimiques des échantillons étudiés montrent que l'ouverture arrière-arc active est associée à une déformation du manteau supérieure similaire à celle observée dans l'ophiolite d'Oman. L'initiation de l'extension d'arrière-arc n'est pas associée à de fortes interactions entre percolation magmatique et déformation, en comparaison avec les zones de rifting continentales, probablement en raison des taille et durée relativement petites de l'épisode d'ouverture. La seconde partie présente une base de données unique d'OPC de plagioclase de roches mafiques plus ou moins déformées. Les OPC sont classées en 3 types principaux; leurs caractéristiques en fonction du régime de déformation (magmatique ou plastique) sont présentées et discutées. Les propriétés sismiques calculées des roches gabbroiques montrent que l'anisotropie tend à croitre avec l'intensité des fabriques, bien qu'elle soit généralement faible, en raison des effets opposés des olivines/clinopyroxènes et du plagioclase

Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan

Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spinel grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl}[100]. The angles between the foliation and the olivine slip planes decrease with increasing values of the J-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31-4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higher J-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1-26.6. km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata peridotites record a long and complicated tectonic history; they preserve deformation 'frozen in' during back-arc spreading and this deformation also has an effect on present-day mantle flow.16 page(s

Determination of slip system in olivine based on crystallographic preferred orientation and subgrain-rotation axis : examples from Ichinomegata peridotite xenoliths, Oga peninsula, Akita prefecture

In this study of peridotite xenoliths from Ichinomegata volcano, Oga peninsula, NE Japan, we used electron backscattered diffraction to assess the validity of two methods of identifying the olivine slip system that operated during deformation. The xenoliths are harzburgite in composition and contain a distinct foliation and lineation defined by aligned spinel and pyroxene grains. Using the first method, the dominant slip system in olivine was estimated based on the crystallographic preferred orientation (CPO) of olivine with respect to the foliation and lineation. The second method was based on an analysis of subgrain rotation: given that intracrystalline subgrain boundaries were observed within several olivine grains, we measured the misorientation angles across these boundaries to define sets of slip planes, slip directions, and rotation axes. The olivine slip systems estimated using the two methods are essentially identical. The CPO pattern provides an important constraint on the dominant slip system that operated during deformation, whereas the analysis of subgrain rotation yields the slip system responsible for the development of individual subgrain boundaries composed of edge dislocations. The two methods are therefore complementary.4 page(s

Seismic anisotropy in the uppermost mantle, back-arc region of the northeast Japan arc: petrophysical analyses of Ichinomegata peridotite xenoliths

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Solution-precipitation of K-feldspar in deformed granitoids and its relationship to the distribution of water

We have investigated K-feldspar recrystallisation in granitoid mylonites within a ductile shear zone from the Ryoke metamorphic belt, SW Japan. Fine-grained K-feldspar (20μm on average) occurs in the matrix and in pull-apart areas within fractured K-feldspar porphyroclasts. These fine grains are elongated and oriented parallel to the main foliation in the matrix, and their grain surfaces, observed with the scanning electron microscope, are not smooth, but rough due to the development of very fine (<1μm) round grains of K-feldspar on the surface of each grain. In pull-apart areas, the crystallographic orientation of fine-grained K-feldspar, as measured by electron backscatter diffraction (EBSD), is strongly controlled by that of the host porphyroclast, and shows rotations with shear components parallel to fractures. In the matrix, the crystallographic orientation of fine-grained K-feldspar is not consistent with intracrystalline plasticity, but rather with a growth rate that is slightly controlled by nearby porphyroclasts. All this, together with the growth features on grains, suggests that solution-precipitation of K-feldspar from K-rich aqueous fluid occurred during progressive deformation. Infrared (IR) mapping was performed to evaluate the distribution of water in pull-apart areas and the matrix. Water is heterogeneously distributed within K-feldspar porphyroclasts, which contain 150-2200ppm H₂O. In contrast, the water content is low (150-300ppm H₂O) and homogeneously distributed in fine-grained K-feldspar in the matrix and pull-apart areas, even though included in these analyses are grain boundaries that can generally contain abundant aqueous fluid. The results of EBSD analysis and IR mapping indicate that water is released during solution-precipitation of K-feldspar under mid-crustal conditions. The solution-precipitation process under a water-rich environment in the middle crust results in the formation of fine grains, possibly deforming dominantly by grain-size-sensitive creep, and with the release of aqueous fluid involved in the process.11 page(s