Insight into the upper mantle beneath an active extensional zone: the spinel-peridotite xenoliths from San Quintin (Baja California, Mexico)
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Many of the peridotite xenoliths included in the San Quintin (Baja California Norte, Mexico) quaternary alkali-basalts have undergone a very intense shear deformation (deviatoric stresses up to 0.1 GPa), hence a first-order classification into coarse-grained lherzolites and deformed peridotites (porphyroclastic and mosaic textures) has been applied. All of these rocks show a very limited compositional variability in the Mg/(Mg+Fe2+) ratios (olivine: 0.894–0.905±0.005; orthopyroxene: 0.899–0.9105±0.005), and the observed trends in the Cr/(Cr+Al) spinel ratios (from 0.1 to 0.6) can be interpreted as resulting from gradual partial melting followed by homogenization of the bulk phases. A later and less accentuated melting event is also evidenced by internal core-rim variations in the spinels from a few samples and ascribed to the thermal effect of the host lava.
Simultaneous application of exchange geothermometers which give the latest equilibrium temperatures (i.e. at the time of eruption: Fe-Mg exchange between olivine and spinel) and of pyroxene transfer thermobarometers yields two distinct behaviours: the porphyroclastic and mosaic peridotites record an event of deformation and recrystallization and were equilibrated at 800°–950° C and P≲-1 GPa at the time of eruption, but have also retained evidence of higher temperatures (1000°–1050° C) and pressures; the coarsegrained lherzolites, which yield conditions of 1000°–1050° C and P<-2 GPa at the time of eruption, were originally equilibrated at higher temperature and pressure conditions and were subsequently re-equilibrated to 1000°–1050° C by solid-state bulk diffusion, without exsolution.
Clinopyroxenite veins provide evidence of magma injection into the host-peridotite, before deformation but after the major melting event.
To explain the simultaneous sampling of both groups of peridotites by the San Quintin alkali basalts, we suggest that the ascending magma reached the critical limit for hydraulic fracturing in the coarse-grained lherzolites. At shallower depth, the magma cross-cut an active shear zone, sampling prophyroclastic and mosaic samples of the strained peridotites.
Our model is consistent with the regional tectonic context: upwelling of the mantle by isostatic re-equilibration after the end of the subduction processes and subsequent opening of the California Gulf. The only questionable parameter of the model remains the geometry of the shearzone, high or low angle orientation.
KeywordsOlivine Subduction Hydraulic Fracture Bulk Diffusion Alkali Basalt
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- Allegre CJ, Shimizu N, Rousseau D (1982) History of the continental lithosphere recorded by ultramafic xenoliths. Nature 296:732–735Google Scholar
- Ave Lallemant HG, Mercier JCC, Carter NL, Ross JV (1980) Rheology of the upper mantle: Inferences from peridotite xenoliths. Tectonophys 70:85–113Google Scholar
- Basu AR (1977) Textures, microstructures and deformation of ultramafic xenoliths from San Quintin, Baja California. Tectonophys 43:213–245Google Scholar
- Benoit V (1987) Etat d'équilibre de péridotites du manteau supérieur: application au plateau du Colorado. Thèse Doctorat. Paris VII-I.P.G. ParisGoogle Scholar
- Bertrand P, Sotin C, Gaullier JM, Mercier JCC (1987) La solubilité de l'aluminium dans l'orthopyroxène: inversion globale des données experimentales du systeme chimique MgO-Al2O3-SiO2. Bull Soc Geol Fr 8:821–832Google Scholar
- Bertrand P, Mercier JCC (1986) The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system? Earth Planet Sc Lett 79:109–122Google Scholar
- Cabanes N (1988) Etude de zones de cisaillement mantellique. Les péridotites de Montferrier (France) et de San Quintin (Mexique). Analyse texturale, pétrologique et géochimique. Thèse Doctorat. Documents et Travaux du Centre Géologique et Géophysique de Montpellier n∘16Google Scholar
- Cabanes N, Briqueu L (1987) Hydration of an active shear-zone. Interactions between deformation, metasomatism and magmatism: the sp-lherzolites from Montferrier (Southern France) oligocene basalts. Earth Planet Sc Lett 81:233–244Google Scholar
- Cabanes N, Mercier JCC (1988) Chimie des phases minérales et conditions d'équilibre des enclaves de lherzolite à spinelle de Montferrier (Hérault; France). Bull Mineral 111:65–77Google Scholar
- Dick HJB, Bullen T (1984) Chromian spinel as a petrogenetic indicator in abyssal and alpine type peridotites and spatially associated lavas. Contrib Mineral Petrol 86:54–76Google Scholar
- Fabries J (1979) Spinel-olivine geothermometry in peridotites from ultramafic complexes. Contrib Mineral Petrol 69:329–336Google Scholar
- Fabries J, Figueroa O, Lorand JP (1987) Petrology and thermal history of highly deformed mantle xenoliths from the Montferrier basanites (Languedoc, Southern France). Comparison with ultramafic complexes from the North Pyrenean Zone. J Petrol 28:887–919Google Scholar
- Gueguen Y, Darot M (1980) Microstructures and stresses in naturally deformed peridotites. Rock Mechanics (Suppl) 9:159–172Google Scholar
- Haggerty SE, Tompkins LA (1983) Redox state of earth's upper mantle from kimberlitic ilmenites. Nature 303:295–300Google Scholar
- Hagstrum JT, McWilliams M, Howell DG, Gromme S (1985) Mesozoic paleomagnetism and northward translation of the Baja California Peninsula. Geol Soc Am Bull 96:1077–1090Google Scholar
- Harmand C, Moukadiri A (1986) Synchronisme entre tectonique compressive et volcanisme alcalin: exemple de la province quaternaire du Moyen Atlas (Maroc). Bul Soc Geol Fr 8:595–603Google Scholar
- Hart SR, Zindler A (1986) In search of a bulk earth composition. Chemical Geol 57:247–267Google Scholar
- Kawasaki T, Matsui Y (1983) Thermodynamic analyses of equilibria involving olivine, orthopyroxene and garnet. Geoch Cosmoch Acta 47:1661–1679Google Scholar
- Macgregor ID (1974) The system MgO-Al2O3-SiO2: solubility of Al2O3 in enstatite for the spinel and garnet peridotite compositions. Am Mineral 59:110–119Google Scholar
- Mattioli GS, Wood B (1986) Upper mantle oxygen fugacity recorded by spinel lherzolites. Nature 322:626–628Google Scholar
- Mercier JCC (1977) Natural peridotites: chemical and rheological heterogeneity of the upper mantle. Ph D Thesis SUNY Stony Brook: 718 ppGoogle Scholar
- Mercier JCC (1980a) Magnitude of the continental lithospheric stresses inferred from rheomorphic petrology. J Geophys Res 82:6293–6303Google Scholar
- Mercier JCC (1980b) Single pyroxene thermobarometry. Tectonophys 70:1–37Google Scholar
- Mercier JCC, Carter NL (1975) Pyroxene geotherms. J Geophys Res 80:3349–3362Google Scholar
- Mercier JCC, Anderson AD, Carter NL (1977) Stress in the lithosphere: inferences from steady state flow of rocks. Pageoph 115:199–226Google Scholar
- Mercier JCC, Benoit V, Girardeau J (1984) Equilibrium state of diopside bearing harzburgites from ophiolites: geobarometric and geodynamic implications. Contrib Mineral Petrol 85:391–403Google Scholar
- Mysen BO, Kushiro I (1977) Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle. Am Mineral 62:843–865Google Scholar
- Nicolas A (1986) A melt extraction model based on structural studies in mantle peridotites. J Petrol 27:999–1022Google Scholar
- Nixon PH, Boyd FR (1973) Petrogenesis of the granular and sheared ultramafic nodule suite in kimberlites. In: Lesotho kimberlites. Nixon, pp 48–56Google Scholar
- Ross JV, Ave Lallemant HG, Carter NL (1980) Stress dependence of recrystallized grain and subgrain size in olivine. Tectonophys 70:39–61Google Scholar
- Shimizu N (1975) Rare earth elements in garnets and clinopyroxenes from garnet lherzolite nodules in kimberlites. Earth Planet Sc Lett 25:26–32Google Scholar
- Vannier M (1977) Modélisation de la solution solide spinelle naturelle. Bull Soc Fr Mineral Critall 100:239–245Google Scholar
- Webb SAC, Wood BJ (1986) Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio. Contrib Mineral Petrol 92:471–480Google Scholar
- Wilshire HG, Schwarzman EC, Trask NJ (1971) Distribution of ultramafic xenoliths at 12 north american sites. US Geol Survey Interagency Rep Astrogeol 42:47–52Google Scholar
- Witt G, Seck HA (1987) Temperature of sheared mantle xenoliths from the West Eifel, West Germany: Evidence for mantle diapirism beneath the Rhenish massif. J Petrol 28:475–493Google Scholar