ALBERT

Description: The Galicia margin lies northwest of the Iberian Peninsula and is a passive ocean margin with thin sedimentary cover. Altered peridotite was recovered from ODP Site 637, on the north-trending ridge at the western edge of the margin, near the oceanic/continental crust boundary. The altered ultramafics were originally clinopyroxene-rich upper mantle harzburgites and are now extensively serpentinized (〉85%) and cut by very late-stage carbonate veins. Despite pervasive late, low-temperature alteration, evidence of early, high-temperature alteration remains. Alteration is apparent as (1) amphibole rims on clinopyroxene (〉800°C), (2) hornblende + tremolite (450° to 800°C), (3) breakdown of hornblende to form tremolite + chlorite (〈450°C), (4) zoned Cr-spinels, (5) hydration of orthopyroxene and olivine to serpentine, (6) serpentine veins, (7) replacement of pyroxene and olivine by calcite, and (8) calcite veins and vugs. Both the relict igneous and the high-temperature alteration minerals (amphiboles) show evidence of brittle deformation. Subsequent low-temperature alteration veins and minerals are deformed only in faulted and brecciated zones. This textural evidence suggests that the low-temperature alteration occurred after emplacement of the ultramafics at the surface. Serpentine fills tension fractures in orthopyroxene, and both serpentine and calcite fill tension cracks in olivine. The high-temperature alterations in these samples are similar to those found in oceanic fracture zone and ophiolite ultramafics. This widespread occurrence of high-temperature alteration suggests that hot fluids were pervasive in these ultramafic blocks. Localization of high-temperature alteration close to large carbonate veins suggests channelization of the late, low-temperature fluids. Earlier hydrations (e.g., high-temperature alterations and serpentinization) were pervasive.

Description: The late Cenozoic deposits recovered at ODP Site 637 from the Iberian Abyssal Plain near the continental margin off northwestern Spain include three main facies groups. Turbidites are the dominant facies association (two-thirds of the total thickness), followed by pelagites (one-fourth), and subordinate amounts of contourites (one-tenth). Slump deposits occur locally in the upper Miocene and middle Pliocene. Turbidity currents and pelagic settling were the significant sediment depositional processes from the Pliocene to the Pleistocene, whereas bottom currents predominated during the late Miocene. Fine-grained, base-cut-out turbidites, normally starting with the Td division, are the most abundant sequence type. The pelagites include both carbonate-rich pelagic and hemipelagic facies. The two types of contourites, sandy and calcareous-rich or fine-grained terrigenous, record two types of bottom-current processes. The Cenozoic deposits at Site 637 show a general upward transition from contourites in the upper Miocene to turbidites in the Pliocene-Quaternary. The entire section is rhythmically bedded and has a poorly developed cyclic pattern defined by variations in the total carbonate content. The low sedimentation rates also show the same cyclicity, with lower values for the late Miocene and late Pliocene. This evolution reflects the predominant depositional processes and the dissolution of carbonates by a lower CCD during the late Miocene.

Description: A ridge of strongly serpentinized, plagioclase-bearing peridotite crops out at the boundary between the Atlantic oceanic crust and the Galicia continental margin (western Spain). These peridotites, cored at Hole 637A (ODP Leg 103) have been mylonitized at high-temperature, low-pressure conditions and under large deviatoric stress during their uplift (Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988). After this main ductile deformation event, the peridotite underwent a polyphase metamorphic static episode in the presence of water, with the crystallization of Ti- and Cr-rich pargasites at high-temperature (800°-900°C) interaction with a metasomatic fluid or alkaline magma. Introduction of water produced destabilization of the pyroxenes and the subsequent development of hornblendes and tremolite at temperatures decreasing from 750° to 350°C. The main serpentinization of the peridotite occurred at a temperature below 300°C, and possibly around 50°C, as a consequence of the introduction of a large amount of seawater, which is suggested by stable isotope (d18O and SD) data. Finally, calcite derived from seawater precipitated in late-formed fractures or locally pervasively impregnated the peridotite at low temperature (~10°C).

Description: A ridge of peridotite was drilled off of the Galicia margin (Hole 637A) during ODP Leg 103. The ridge is located at the approximate boundary between oceanic and continental crust. This setting is of interest because the peridotite may be representative of upwelling upper mantle beneath an incipient ocean basin. The composition of the Galicia margin peridotite is compared with those of other North Atlantic peridotites. Hole 637A ultramafic lithologies include clinopyroxene-rich spinel harzburgite and lherzolite, as well as plagioclase-bearing peridotites. Variations in mineral modal abundances and mineral compositions are observed but are not systematic. The peridotites are broadly similar in composition to other peridotites recovered from ocean basins, but the mineral compositions and abundances suggest that they are less depleted in basaltic components than other North Atlantic peridotites by about 10%. In particular, the peridotites are enriched in the magmaphilic elements Na, Al, and Ti, as compared with other abyssal peridotites. The high abundances of these elements suggest that the Hole 637A peridotites had experienced, at most, very small amounts of partial melting prior to their emplacement. The presence of plagioclase rimming spinel in some samples suggests that the peridotite last equilibrated at about 9 kbar, near the transition between plagioclase- and spinel-peridotite stability fields. Temperatures of equilibration of the peridotite are calculated as 900°-1100°C. The relatively undepleted composition of the peridotite indicates that it was emplaced at a shallow mantle level under a relatively cool thermal regime and cooled below solidus temperatures without having participated in any significant partial melting and basalt production. This is consistent with the emplacement of the peridotite during incipient rifting of the ocean basin, before a true spreading center was established.

Description: Mineral compositions of the plagioclase-bearing ultramafic tectonites dredged and cored seaward of the continental slope of the Galicia margin (Leg 103, Site 637) were compared to mineral compositions from onshore low-pressure ultramafic bodies (southeastern Ronda, western Pyrenees, and Lizard Point), on the basis of standardized (30-s counting time) probe analyses. The comparison was extended to some plagioclase-free harzburgites related to ophiolites (Santa Elena in Costa Rica, north Oman, and the Humboldt body in New Caledonia) on the basis of new analytical data and data from the literature. The behavior of Cr, Na, Al, Mg, Fe, Ni, and Ti in olivine, pyroxenes, and spinel was examined in order to distinguish between the effects of partial melting and mineral facies change, from the spinel to plagioclase stability fields. The peridotite from the Galicia margin appears slightly depleted in major incompatible elements and experienced a minor partial melting. However, it experienced large scale but heterogeneous recrystallization in the plagioclase field. These features are very similar to those observed in Ronda, whereas in the western Pyrenees the minerals exemplify a very minor partial-melting event (or none at all) and have retained compositions corresponding to those of the relatively high-pressure Seiland sub facies. The minerals from the Lizard Point peridotite have characteristics (low Mg/(Mg + Fe) ratio; high Cr/(Cr + Al) ratio in spinel) more related to cumulate from a differentiated tholeiitic melt than related to ophiolitic tectonite. Diffusion profiles of Al and Cr across pyroxenes and spinel show that recrystallization features occurred at different speeds or temperatures in the different bodies. The pyroxenes from Ronda would have experienced recrystallization about 14 times faster than the peridotite from the Galicia margin. The western Pyrenean lherzolites also experienced rapid recrystallization; nevertheless, because they are of a different mineral facies, the data are not directly comparable to that from Ronda and Galicia. The harzburgite at Santa Elena as well as a xenolith from alkali basalt exemplify rapid cooling characterized by very weak re-equilibration. Recrystallization speed is related to emplacement speed in the present geological environment. The slow-rising Galicia margin peridotite was emplaced by thinning of the lithospheric subcontinental mantle near an incipient mid-oceanic ridge. The fast-rising peridotites from Ronda and the western Pyrenees were hot diapirs emplaced from the asthenosphere along transcurrent faults, possibly related to the opening of the Atlantic Ocean.

Description: The peridotite recovered from Ocean Drilling Program Hole 637A, Galicia margin, has suffered extensive low-temperature alteration that includes serpentinization, calcite veining, and calcite replacement. This note presents textural and geochemical data on the serpentine and calcite. Such data indicate that the serpentinization, serpentine veining, and calcite veining of the peridotite occurred in several stages late in the history of the peridotite emplacement, probably after the peridotite was emplaced at crustal levels. It is also apparent that some deformational events (evidenced by faulting and brecciation of both serpentine and calcite veins) continued after the main phase of low-temperature alteration. The geochemistry and petrology, structure, and high-temperature alteration of the peridotite are discussed in separate papers in this volume (Evans and Girardeau, 1988, doi:10.2973/odp.proc.sr.103.138.1988; Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988; Kimball and Evans, 1988, doi:10.2973/odp.proc.sr.103.140.1988; Agrinier et al., 1988, doi:10.2973/odp.proc.sr.103.136.1988).