J. Girardeau

Petrology of the mafic rocks of the Xigaze ophiolite, Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.

Hydrothermal activity in a peculiar oceanic ridge: oxygen and hydrogen isotope evidence in the Xigaze ophiolite (Tibet, China)

The Xigaze ophiolite locally displays a 7-km-thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge. It consists from top to bottom of: n1. n(1) pillow-lavas with δ18O between +9 and +16‰ and δD around −90‰ vs. SMOW; n n2. n(2) rare trondjhemites (+9.7‰ < δ18O < 13.4‰ and δD around −75‰); n n3. n(3) dolerite sills and dikes with δ18O between +7.0 and +9.5‰ and δD between −77 and −40‰; n n4. n(4) isotropic gabbros occurring as screens of the sills and dikes, and rare layered gabbros [these gabbros have δ18O between +4.8 and +11.0‰ and δD around −65‰ and bear high-δ18O albitized plagioclases (δ18O ≈ +10‰) which coexist with clinopyroxenes of mantle values (δ18O ≈ +5.5‰) or having values being slightly depleted (down to +3.0‰]; n n5. n(5) ultramafic rocks are either fresh or serpentinized; they have δ18O mainly around mantle values and δDs are between −250 and −100‰) n n n n nfor the mafic part of the ophiolite, the isotopic compositions correspond to hydrothermal interaction with seawater-derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in D. In particular, the gabbros reacted with seawater-derived fluids under amphibolite-facies conditions (as seen from their secondary mineralogy and their 18O-depleted pyroxenes) and subsequently were pervasively reacted under greenschist-zeolite-facies conditions during which the high-18O albitized plagioclase formed. This occurrence of amphibolite-facies conditions followed by greenschist-zeolite-facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower-temperature hydrothermal interaction if compared with other ophiolites. n nIn contrast, the δDs of the serpentine (< −100‰) from the ultramafics are attributed to fluids of meteoric origin at high elevation. Nevertheless, the particular location of the serpentinized harzburgites just beneath the gabbros and the high-δD dolerite sills and dikes (−60‰ < δD < −40‰) enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction. n nThe Xigaze ophiolite sequence displays very little 18O-depleted rocks (relative to mantle oxygen composition). They cannot balance the amount of 18O-enriched rocks. Consequently the result of the hydrothermal interaction between the Xigaze ophiolite sequence and the seawater is an 18O depletion of the seawater. However, the contribution of Xigaze-type “ridges” to the buffering of δ18O in Cretaceous seawater was probably small.

Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct “magmatic” diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioKf=(XDiopx)/(1 −XDicpx), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularXEncpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.

Evidence for a Heterogeneous Upper Mantle in the Cabo Ortegal Complex, Spain

A well-preserved fragment of a heterogeneous upper mantle is present in the Cabo Ortegal Complex (Spain). This section is made of harzburgite containing a large volume of pyroxenite. The pyroxenite is concentrated in a layer 300 meters thick by 3 kilometers long. In this layer, ultramafic rocks, essentially pyroxenite (massive websterite and clinopyroxenite) and minor dunite, alternate without any rhythmicity. Part of this layering is of primary magmatic origin and possibly resulted from crystallization of magmas in dikes intruded into the host peridotite under mantle conditions.