Continental Lithospheric Contribution to Alkaline Magmatism: Isotopic (Nd, Sr, Pb) and Geochemical (REE) Evidence from Serra de Monchique and Mount Ormonde Complexes

Isotopic results (Sr, Nd, Pb), as well as concentrations ofmajor and trace elements (REE) are reported for whole-rock samplesand mineral separates from the onland alkaline complex of Serrade Monchique (South Portugal) and the offshore alkali basaltvolcanic suite of Mount Ormonde (Gorringe Bank). These two geneticallyrelated alkaline complexes were emplaced at the east Atlanticcontinent–ocean boundary during the Upper Cretaceous,i.e. 66–72 m.y. ago. Taken together, Serra de Monchiqueand Mount Ormonde may be seen as one of the few examples ofwithin-plate magmatism that straddles the continent–oceanboundary. Major and trace element compositions fail to revealany significant differences between onland and offshore complexes.This is particularly true regarding less differentiated samples(mg-number 0.40) which show the same progressive and continuousenrichment of their trace element patterns, with no specificanomaly (e.g. negative Nb anomaly) being present in samplesfrom the onland complex. Initial Pb and Sr isotopic compositionsalso do not allow any distinction to be made between Serra deMonchique and Mount Ormonde samples. Initial Pb isotope ratiosare moderately high (19.1 < ²⁰⁶Pb/²⁰⁴Pb < 19.8; ²⁰⁷Pb/²⁰⁴Pb= 15.6) in both cases. Moreover, once the effects of Sr contaminationby seawater are taken into account and the most contaminatedsamples discarded using data from fresh clinopyroxene separatesand results of leaching experiments, the initial Sr isotopiccompositions of Mount Ormonde samples are found to be unradiogenic(⁸⁷Sr/⁸⁶Sr = 0.7031±1) and identical to those obtainedat Serra de Monchique (⁸⁷Sr/⁸⁶Sr = 0.7032±1). In contrast,a systematic mean difference of 2 _Nd units is observed betweenSerra de Monchique [_Nd(T) = +4.8] and Mount Ormonde [_Nd(T) =+6.6] whole-rock samples. Surprisingly, a variation is alsoobserved at Mount Ormonde between the whole-rock samples andone of the two analysed clinopyroxene separates. Whereas MountOrmonde whole-rock samples invariably yielded _Nd(T) = +6.6 (meanvalue), a value of +0.5 is obtained for one clinopyroxene separate,whereas another gives +6.0. The above geochemical and isotopicresults make it possible to assign respective roles to the asthenosphere,lithosphere and crust in the petrogenesis of Serra de Monchiqueand Mount Ormonde complexes. We propose that both complexesshare a common mantle source whose isotopic characteristicsare very similar to the source of oceanic island basalts. Continentalmantle lithosphere, already characterized isotopically by studiesof peridotite massifs within the Iberian peninsula, acts asa contaminant which is evident onland on the whole-rock scale,and also present offshore as discrete clinopyroxene xenocrysts.The continental crust appears to play no role in the petrogenesisof the Serra de Monchique alkaline rocks. KEY WORDS: alkaline complexes; continental lithosphere; isotope geochemistry; passive continental margin; within-plate volcanics     

Solar Scintillometry: Calibration of Signals and its Use for Seeing Measurements

Since 1993 it is known that there is a good correlation between the scintillation in the solar irradiance and solar image quality (Seykora, 1993). This effect is now being used in a number of experiments to evaluate solar image quality and to measure site seeing. In this paper we explore further the calibration of this scintillation (_I) in terms of the Fried parameter (_r0) taking into account variations in the refractive index structure constant C _N ² with height (h), zenith distance () dependence and the effects of wind velocities. A variant in the scintillometer setup is proposed which decreases sharply the dependance on C _N ² , , and the wind velocities. It uses an array of scintillometers. The same array can be used to measure theC _N ² profile with height. Some preliminary results of the calibration of current NSO site survey measurements are presented.     

Two 1993 Kamchatka earthquakes

Two earthquakes occurred in 1993 off southern Kamchatka. They have similar surface wave magnitudes, focal mechanisms, and depths, but have distinctly different characteristics. The November earthquake is a standard or impulsiveM7 underthrusting event. The June earthquake is a tsunamigenic or low-stress-drop event with several unusual characteristics, including a large, diffuse aftershock zone, directivity, and a long source time function. The 1993 earthquakes ruptured a segment of the Kamchatka Arc which has not ruptured since 1904. The 1993 earthquakes seem to signal the midpoint in the southern Kamchatka seismic cycle.     

Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships

Island arcs, active and passive margins are the best tectonic settings to generate fertile reservoirs likely to be involved in subsequent granitoid genesis. In such environments, greywackes are abundant crustal rock types and thus are good candidates to generate large quantities of granitoid magmas. We performed a series of experiments, between 100 and 2000 MPa, on the fluid-absent melting of a quartz-rich aluminous metagreywacke composed of 32 wt% plagioclase (Pl) (An₂₂), 25 wt% biotite (Bt) (X _Mg45), and 41 wt% quartz (Qtz). Eighty experiments, averaging 13 days each, were carried out using a powder of minerals (5m) and a glass of the same composition. The multivariant field of the complex reaction Bt+Pl+QtzGrt/Crd/Spl+ Opx+Kfs+melt limited by the Opx-in and Bt-out curves, is located between 810–860°C at 100 MPa, 800–850°C at 200 MPa, 810–860°C at 300 MPa, 820–880°C at 500 MPa, 860–930°C at 800 MPa, 890–990°C at 1000 MPa, and at a temperature lower than 1000°C at 1500 and 1700 MPa. The melting of biotite+plagioclase+ quartz produced melt+orthopyroxene (Opx) +cordierite (Crd) or spinel (Spl) at 100, 200 and 300 MPa, and melt+orthopyroxene+garnet (Grt) from 500 to 1700 MPa (+Qtz, Pl, FeTi Oxide at all pressures). K-feldspar (Kfs) was found as a product of the reaction in some cases and we observed that the residual plagioclase was always strongly enriched in orthoclase component. The P-T surface corresponding to the multivariant field of this reaction is about 50 to 100°C wide. At temperatures below the appearance of orthopyroxene, biotite is progressively replaced by garnet with increasing P. At 850°C, we observed that (1) the modal proportion of garnet increases markedly with P; (2) the grossular content of the garnet increases regularly from about 4 mol% at 500 MPa to 15 mol% at 2000 MPa. These changes can be ascribed to the reaction Bt+Pl+Qtz Grt+Kfs+melt with biotite +plagioclase+quartz on the low-P side of the reaction. As a result, at 200 MPa, we observed the progressive disappearance of biotite without production of orthopyroxene. These experiments emphasize the importance of this reaction for the understanding of partial melting processes and evolution of the lower continental crust. Ca-poor Al-metagreywackes represent fertile rocks at commonly attainable temperatures (i.e. 800–900°C), below 700 MPa. There, 30 to 60 vol.% of melt can be produced. Above this pressure, temperatures above 900°C are required, making the production of granitoid magmas more difficult. Thin layers of gneisses composed of rothopyroxene, garnet, plagioclase, and quartz (±biotite), interbedded within sillimanite-bearing paragneisses, are quite common in granulite terrains. They may result from partial melting of metagreywackes and correspond to recrystallized mixtures of crystals (+trapped melt) left behind after removal of a major proportion of melt. Available experimental constraints indicate that extensive melting of pelites takes place at a significantly lower temperature (850°C±20) than in Al-metagreywackes (950°C±30), at 1000 MPa. The common observation that biotite is no longer stable in aluminous paragneisses while it still coexists commonly with orthopyroxene, garnet, plagioclase and quartz, provides rather tight temperature constraints for granulitic metamorphism.Abbreviations Ab albite - alm almandine component in garnet - Als aluminum silicate - An anorthite - Ap apatite - Bt biotite - Cal calcite - Crd cordierite - Crn corundum - En enstatite - Fl fluid phase - Fs ferrosilite - Ged gedrite - Gl glass - Grs Grossular - grs grossular component in garnet - Grt garnet - Hc hercynite - Hem hematite - Ilm ilmenite - Kfs K-feldspar - M melt - Mag magnetite - Ms muscovite - Opx orthopyroxene - Or orthoclase - Phl phlogopite - Pl plagioclase - Po Pyrrhotite - Prp pyrope - prp pyrope component in garnet - Otz quartz - Rt rutile - Sa sanidine - Sil sillimanite - Spl spinel - St staurolite - Ti-Mag titano-magnetite - W water     

The Wuqbah peridotite, central Oman Ophiolite: Petrological characteristics of the mantle in a fossil overlapping ridge setting

The Wuqbah peridotites (Wuqbah massif, central Oman Ophiolite) constitute the mantle part of a complete ophiolitic sequence and their field deformation geometry is thought to reflect mantle dynamics in a fossil overlapping ridge settings (Girardeau et al., 2002). These peridotites comprise dominantly residual harzburgites and dunites. Nearly 70% of the harzburgites are clinopyroxene-free, and the rest contains less than 1%. The mineral chemistry of olivine, pyroxenes and spinel, and whole rock major and rare-earth element data, indicate that the Wuqbah peridotites are all strongly refractory and that they record a major percolation event, marked by strong enrichments in incompatible elements. At the massif scale, the Central Zone contains rocks with the most refractory features (20% melt extraction), as expected in an area of mantle upwelling. In the overlapping ridge senario, it corresponds to the overlap zone whose formation is discussed.     

W–Au skarns in the Neo-Proterozoic Seridó Mobile Belt,Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit

The Seridó Mobile Belt (SMB) is located in the Borborema Province in northeastern Brazil and consists of a gneiss basement (Archean to Paleo-Proterozoic), a metasedimentary sequence (marble, quartzites, and schists), and the Brasiliano igneous suite (both of Neo-Proterozoic age). In this region, skarns occur within marble and at the marble–schist contact in the metasedimentary sequence. Most of the skarn deposits have been discovered in the early 1940s, and since then, they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns, which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10- to 15-km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite–zoisite–sericite, calcite, and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble–schist contact. The main ore body is 5–120 cm wide and contains an average of 4.8-wt.% WO₃, which occurs in the basal marble–schist contact. Fold hinges appear to control the location of high-grade scheelite. The late-stage gold mineralization contains bismite (Bi₂O₃), fluorine-bearing bismite, native bismuth, bismuthinite (Bi₂S₃), and joseite [Bi₄(Te,S)₃], and also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold–bismuth–tellurium mineralization exhibits a typical late character and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that crosscut not only the banding in prograde skarn but also the retrograde alkali feldspar and clinozoisite–zoisite–sericite assemblage. The Au–Bi–Te-bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits.     

Progressive degradation of rock properties and time-dependent behavior of deep tunnels

Time-dependent response of deep tunnels is studied considering the progressive degradation of the mechanical properties of the rock mass. The constitutive model is based on a rock-aging law for the uniaxial strength of the rock and for the Young’s modulus. A semi-analytical solution is developed for the stresses and displacements around a deep circular tunnel taking into account the face advance. The evolution of the plastic and damage zones over time is determined. Numerical examples are presented for the case of Saint-Martin-La-Porte access adit in France of the Lyon–Turin Base Tunnel. The computed results which are compared with the field data in terms of the convergence of tunnel wall and of the displacements inside the rock mass monitored by multi-point extensometers show the efficiency of the approach to simulate the time-dependent deformation of a tunnel excavated in squeezing ground. Simple relationships are proposed to evaluate the parameters of the constitutive model directly from those of the empirical convergence law presented in previous work.