The most conspicuous feature of the Hole 900A rocks is the poorly developed and discontinuous foliation (Fig. 4, Fig. 5, Fig. 6) of recrystallized plagioclase and clinopyroxene. This foliation is typical of high-temperature metamorphism near ridges on the ocean floor, but is not typical of foliation produced by continental orogeny in high-grade metamorphic rocks. Indian Ocean gabbros recovered from Hole 735B on the Atlantis II Fracture Zone metamorphosed to granulite-facies temperatures display deformational and compositional features (Cannat, 1991) similar to those at Hole 900A. These features appear to be characteristic of metamorphosed cumulate gabbros from slow-spreading ridges (Mével and Cannat, 1991). Approximately 30% of the 435 m of Hole 735B gabbros recovered are plastically deformed and recrystallized to amphibolite- and granulite-facies temperatures, compared to 100% of the 27.7 m of metamorphosed mafic rock recovered at Hole 900A, allowing preservation of primary minerals and textures in the Hole 735B rocks. Metamorphosed Hole 735B gabbros contain areas with coarsely recrystallized plagioclase, clinopyroxene, olivine, orthopyroxene, oxides, and apatite, a mineral assemblage that would be considered indicative of granulite-facies continental metamorphism (Yardley, 1989). The Hole 900A rocks consist largely of bands of medium- to fine-grained recrystallized plagioclase and bands of clinopyroxene partially retrograded to fine-grained amphibole that often is identified from XRD powder patterns. The formation of amphibole from the anhydrous plagioclase-clinopyroxene assemblage requires the addition of hydrous fluids.
The Hole 900A rocks have gained K2O and other LILE plus U, while other major oxides and trace elements appear to have remained unchanged. The Hole 900A metamorphosed mafics have major element compositions similar to cumulate gabbros from a variety of sources (Gunn and Roobol, 1977; Tiezzi and Scott, 1980; Meyer et al., 1989) in all respects except for high LILE and U. The average K2O content in the Hole 900A rocks is about 20 times higher than that of a typical cumulate gabbro. Experimental studies of the alteration patterns produced in mafic rocks by seawater at temperatures of 500°C and below (Hajash, 1975; Humphris and Thompson, 1978; Mottl and Holland, 1978; Seyfried and Bischoff, 1979; Hajash and Archer, 1980) indicate that, in addition to a gain or loss of K2O, MgO is consistently gained and CaO is consistently lost. A gain of K2O, such as in the Hole 899B basalt and diabase clasts (Seifert and Brunotte, chapter 29, this volume), combined with the gain of MgO and loss of CaO, is regarded as indicative of seafloor weathering at temperatures below 200°C (Seyfried and Bischoff, 1978). At temperatures above 200°C and up to at least 500°C, experimental studies found that K2O is lost. However, hydration of the Hole 735B gabbros was typically accompanied by increases in CO2, ferric iron, and K2O (Robinson et al., 1991).
The addition of K, and probably the other LILE and U, appears to have occurred during hydration and retrograde metamorphism of the Hole 900A mafic rocks. Only approximately one-quarter of the K2O in the Hole 900A rocks resides in the structure of the abundant metamorphic recrystallized plagioclase, with up to 0.48% K2O (Table 1). The other three-quarters of the K2O is concentrated in small scattered patches of alteration within plagioclase porphyroclasts (Fig. 14) and along grain boundaries between recrystallized plagioclase and clinopyroxene (Fig. 15). The soft K2O-rich alteration patches are marked by small irregular pits on the surface of polished microprobe sections that make accurate analysis difficult. Nevertheless, it appears that K2O concentrations reach at least 15% locally. The association of K with fibrous amphibole in alteration patches (Fig. 14) would seem to indicate it was introduced by hydrous solutions during the retrograde metamorphism. The brown iron stain typical of many alteration patches may indicate the event was also one of increased oxidation.
Cumulate gabbros should be expected to have low K2O, P2O5, TiO2, and incompatible trace element contents because these elements are not incorporated within the structures of early cumulus minerals from basaltic magma. Cumulate and noncumulate (or isotropic) gabbros, that are similar in mineralogy and major element geochemistry, can typically be distinguished by differences in incompatible trace element abundances. Noncumulate or isotropic gabbros are compositionally equivalent to basalts in that they represent solidified magma and have trace element abundances similar to basalts. Cumulate gabbros, with mineralogies and major element compositions similar to isotropic gabbros, consist of minerals that have separated from all or part of the magma from which they crystallized. Consequently, they have lost the majority of incompatible elements, which stay in the magma. Consequently, the incompatible element content of a cumulate rock depends largely on the amount of magma that has been trapped between accumulating crystals, plus some small amount within the cumulus minerals. An example of the difference between related isotropic and cumulate gabbros was illustrated by Leterrier (1985) and has been observed in many other regions. The difference in incompatible element content is often especially obvious on a REE diagram, with isotropic gabbro having a REE abundance near 10X chondrite or above, similar to basalt, whereas cumulate gabbro typically has a REE abundance between 1X-5X chondrite. The lack of incompatible elements is typical of cumulate rocks, as well as rocks from which magma has been removed by partial melting, because, by definition, these elements travel with the magma.
As a result, cumulate gabbros from all tectonic provinces have similar elemental compositions because the incompatible elements used to distinguish between basaltic magmas from different sources (see Seifert and Brunotte, chapter 29, this volume) are largely lost with the escaping magma. Cumulate gabbros formed at mid-ocean ridges have elemental compositions similar to the cumulate gabbros at Hole 900A (Thompson, 1973; Engel and Fisher, 1975; Tiezzi and Scott, 1980; Miyashiro and Shido, 1980; Meyer et al., 1989; Hekinian et al., 1993), as do cumulate gabbros from other tectonic provinces. Cumulate gabbros can be either oceanic, those formed at a divergent ridge, back-arc basin, or even an island arc (Gerlach et al., 1981; Beard, 1986), or continental, those associated with a layered igneous complex. Ophiolitic cumulate gabbros are believed to have formed in all of the distinct oceanic tectonic provinces, although most may represent back-arc basins (Serri, 1981). Only isotopic data allow a distinction to be made between the various tectonic provinces in the oceans and continents (Coish et al., 1982).
On an AFM diagram the Hole 900A metamorphosed gabbroic rocks plot below the Skaergaard trend and N-MORB but with cumulate gabbros from a variety of tectonic provinces (Fig. 16). Part of the scatter is related to differences in the mineralogy of different cumulate gabbros, part relates to the amount of trapped magma, and part may be caused by later alteration. A lack of oxide minerals in all parts of the Hole 900A rocks, except Core 900A-81R which plots next to N-MORB as expected, offers mineralogical evidence for a cumulate origin for most of these rocks. Note that one of the ophiolitic cumulate gabbros also plots near N-MORB and would also be expected to have retained considerable trapped magma. The wide variation in plagioclase composition (An43 to An89) in the Hole 900A rocks, ignoring the retrograde albite, suggests compositional differences, in the magma from which primary plagioclase precipitated, that are probably best explained by multiple magma injections.
No textural information remains to determine if the large strained porphyroclasts represent primary igneous minerals or a previous generation of metamorphic minerals. The Hole 900A samples are characterized by discontinuous foliation bands of recrystallized plagioclase and clinopyroxene containing large isolated strained porphyroclasts of plagioclase (Fig. 7C) or clinopyroxene, respectively. Because of their large size, it seems likely that the porphyroclasts represent relicts of the primary igneous mineralogy that have survived the later metamorphism. The similar composition of recrystallized clinopyroxene and plagioclase and adjacent porphyroclasts, from which the recrystallized minerals were probably derived, suggests recrystallization in a closed system with only two phases involved. Alternatively, the porphyroclasts were compositionally re-equilibrated with recrystallizing minerals during the recrystallization event without being totally destroyed or recrystallized. If the first explanation is correct, the trace element composition of the parental magma can be estimated from the appropriate partition coefficients.
The trace element composition of the parental magma can be estimated from the bulk trace element composition of a cumulate rock using the appropriate mineral partition coefficients assuming certain conditions are met. First, it must be assumed that little or no trapped magma is present so that most of the incompatible elements reside in the cumulus minerals. The depletion of the more immobile incompatible trace elements indicates this to be the case for all parts of the Hole 900A section except Core 900A-81R. Second, as mentioned above, it has to be assumed that, to a first approximation, the Hole 900A cumulate gabbros primary mineralogy was dominated by plagioclase and clinopyroxene with all other minerals being minor. No relicts of olivine or orthopyroxene have been found and opaque minerals are confined to Core 900A-81R. On the basis of these assumptions, the trace element composition of the parental magma can be estimated from literature partition coefficients for plagioclase and clinopyroxene (Drake and Weill, 1975; Arth, 1976; Baitis and Lindstrom, 1980; Henderson, 1982; Rollinson, 1993). When this exercise is carried out for the seven REEs for which instrumental neutron activation analysis has provided good mineral separate data, and thus good partition coefficients, a nearly flat chondrite normalized REE pattern is obtained that resembles transitional MORB (Fig. 17). The transitional MORB pattern is obtained from average values taken from all Hole 900A samples except Sample 149-900A-81R-1, 96-101 cm; this sample was omitted because of its distinct mineralogy and relatively enriched trace element composition. A transitional MORB parental magma for these cumulate gabbros would be consistent with the environment in which these rocks have been found, and agrees with our isotopic data indicating a MORB origin.