Arc-related high-K magmatism in the Ceuta Peninsula (Internal Rif, Spain): discovery and consequences
Emmanuelle HomonnayJean‐Marc LardeauxMichel CorsiniAsmae El BakiliDelphine BoschOlivier BruguierMohamed Ouazzani-Touhami
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Abstract We document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated at c . 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions at c . 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pb in situ analyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.Keywords:
Diorite
Sill
Greenschist
Greenschist
Actinolite
Hornblende
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<p>Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO&#8217;s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.</p>
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Greenschist
Greenstone belt
Chromite
Protolith
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A subcircular area of about 650 km 2 in northern California and southwestern Oregon is occupied by rocks of the greenschist metamorphic facies called the Condrey Mountain Schist. This greenschist terrane is bordered on the east and west by rocks belonging to the amphibolite metamorphic facies that structurally overlie and are thrust over the Condrey Mountain Schist. The amphibolite facies is succeeded upward by metavolcanic and metasedimentary rocks belonging to the greenschist metamorphic facies. The Condrey Mountain Schist is composed predominantly of quartz-muscovite schist and lesser amounts of actinolite-chlorite schist formed by the metamorphism of graywacke and spilitic volcanic rocks that may have belonged to the Galice Formation of Late Jurassic age. Potassium-argon age determinations of 141?4 m.y. and 155?5 m.y. obtained on these metamorphic rocks seem to be incompatible with the Late Jurassic age usually assigned the Galice. The rocks that border the amphibolite facies are part of an extensive terrane of metavolcanic and metasedimentary rocks belonging to the western Paleozoic and Triassic belt. The metavolcanic rocks include some unmetamorphosed spilite but are mostly of the greenschist metamorphic facies composed of oligoclase (An15-20) and actinolite with subordinate amounts of chlorite and clinozoisiteepidote. The interbedded sedimentary rocks are predominantly argillite and slaty argillite, less commonly siliceous argillite and chert, and a few lenticular beds of marble. On the south, high-angle faults and a tabular granitic pluton separate the greenschist metavolcanic terrane from the amphibolite facies rocks; on the east, nonfoliated amphibolite is succeeded upward, apparently conformably, by metasedimentary rocks belonging to the greenschist metavolcanic terrane. In the southern part of Condrey Mountain quadrangle, an outlier of a thrust plate composed of the Stuart Fork Formation overlies the metavolcanic and metasedimentary rocks. The Stuart Fork in this region is composed of siliceous phyllite and phyllitic quartzite and is believed to be the metamorphosed equivalent of rocks over which it is thrust. In the Yreka-Fort Jones area, potassium-argon determinations on mica from the blueschist facies in the Stuart Fork gave ages of approximately 220 m.y. (Late Triassic) for the age of metamorphism. Rocks of the amphibolite facies structurally overlie the Condrey Mountain Schist along a moderate to steeply dipping thrust fault. The amphibolite terrane is composed of amphibolite and metasedimentary rocks in approximately equal amounts accompanied by many bodies of serpentinite and a number of gabbro and dioritic plutons. Most of the amphibolite is foliated, but some is nonfoliated; the nonfoliated amphibolite has an amphibolite mineralogy and commonly a relict volcanic rock texture. The nonfoliated amphibolite occurs on the southern and eastern borders of the amphibolite terrane between the areas offoliated amphibolite and the overly ing metavolcanic and metasedimentary rocks. Hornblende and plagioclase (An30-35) are the characteristic minerals, indicating that the rocks are of the almandine-amphibolite metamorphic facies. The metasedimentary rocks interbedded with the amphibolites include siliceous schist and phyllite, minor quartzite, and subordinate amounts of marble. Potassium-argon age dates obtained on hornblende from foliated amphibolite yield ages of 146?4 and 148? 4 m.y., suggesting a Late Jurassic metamorphic episode. Mafic and ultramafic rocks are widespread in the amphibolite terrane but are almost entirely absent from the area of greenschist facies metavolcanic and metasedimentary rocks. The ultramafic rocks, predominantly serpentinite, occur as a few large bodies and many small tabular concordant bodies interleaved with the foliated rocks. The ultramafic rocks include harzburgite and d1lIlite and their serpentinized equivalents. In the Condrey Mountain quadrangle, probably more t
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Abstract The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea‐floor alteration, has a strong influence, via fluid‐rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na 2 O‐CaO‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐CO 2 ‐H 2 O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T – X CO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist‐amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P – T conditions, the proportion of carbonate minerals and the X CO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low X CO2 . Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO 2 ‐rich fluid ( X CO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher X CO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at X CO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of X CO2 = 0.2–0.3 into the upper crust.
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In the Archean Dharwar craton of southern India N-S trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2.5 Ga ago. The progressive changes in the assemblages and mineral chemistry of metabasites was studied in a N-S traverse covering the Chitradurga and Nagaman-gala belts, the Sargur area, and the Nilgiri Hills. Towards the south with increasing metamorphic grade greenschists ($$chl + act + ab + ep \pm carb$$, qtz) give way to amphibolites ($$hbl + plag \pm qtz$$, gar, cumm) and mafic granulites ($$pyx + plag + gar \pm hbl$$, qtz). The amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic-hastingsitic hornblende in the granulite zone. The Ti content of the amphiboles systematically increases in this direction. The plagioclase composition changes from albite in the greenschist zone to oligoclase/andesine in the amphibolite zone and andesine/labradorite in the granulite zone. Almandine-rich garnets first occur in the high-grade amphibolite zone whereas garnets in the granulite zone are higher in pyrope and grossularite components. Pyroxenes, which occur only in the granulite zone, are hypersthenes and salites with distinctly lower tschermak components in the pyroxene-granulite zone than in the hornblende-granulite zone, suggesting higher pressures of metamorphism in the latter zone. Geothermobarometry, mineral stability data, and mineral isograds demonstrate that P-T conditions along the N-S traverse increase continuously from about 500°C/4-5 kbars in the greenschist zone to about 600°C/5-7 kbars in the amphibolite zone. Further south there is a more pronounced temperature increase to about 700-750°C/8 kbars in the hornblende-granulite facies zone. This transition virtually coincides with the change from Dharwar to Sargur metabasites. South of the Moyar shear zone, which is characterized by higher pressures (9.5 kbars), metamorphic conditions of 700-750°C/6-7 kbars have been determined in the pyroxene-granulite zone of the Nilgiri Hills. The increase of metamorphic grade toward the south is also documented by the systematic partitioning of Na between plagioclase and the A-site of coexisting amphibole. The progressive development of the metabasites not only reflects increasing temperature and pressure of metamorphism but also a change in the fluid regime from hydrous in the greenschist and amphibolite zones to water-deficient in the granulite zone.
Greenschist
Dharwar Craton
Hornblende
Geothermobarometry
Amphibole
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