Drilling at Site 900 encountered the top of a series of metamorphosed and brecciated mafic rocks at 748.9 mbsf, beneath sediments of Pleistocene to late Paleocene age. A total of 56.1 m of these mafic rocks was drilled and 27.71 m of material was recovered before the hole was abandoned at a depth of 805 mbsf, giving a recovery in the basement section of the hole of 49.4%.
This succession is composed of mainly fine-grained metamorphosed mafic rocks showing intense deformational features. Many sections are now highly brecciated, separated from nonbrecciated parts by sharp contacts (Fig. 20). The age of the rocks is uncertain beyond the obvious fact that they are pre-late Paleocene.
Core recovery in Core 149-900A-79R was poor, and the nature of the sediments and their contact relationships to the underlying mafic rocks in Core 149-900A-80R are unknown. The first hard-rock sample recovered in Section 149-900A-80R-1 is a fragment of amphibolite (drilling dropstone?). Below this, extending downward to the base of the drilled section, is a succession of fine- and coarse-grained mafic rocks that have been cut by veins and 1-cm to 4-m (Core 149-900-84R) zones of brecciation. The color of the whole section varies from a light to a more frequently dark greenish gray grading toward brownish gray at the bottom.
Intervals 149-900A-82R-5, 0-38 cm, and -83R-2, 50-90 cm, display a marked planar fabric that is the result of alternating discontinuous mafic and felsic bands, each having a maximum thickness of 8 mm. This banding gives these intervals a distinctive "flasered" appearance (Fig. 21). Though less obvious, this same planar fabric characterizes the fine-grained massive-appearing material, in bands less than 1 mm thick (Fig. 22).
The coarser-grained intervals grade rapidly over a few millimeters into finer-grained banded zones, or are limited by brecciated zones or a sharp contact with a matrix that displays differently oriented dipping layers (Interval 149-900A-83R-2, 105-115 cm). These grain-size variations might result from varying degrees of deformation.
Late-stage brecciated zones disrupt approximately 37% of the re covered core. Their distribution is not homogeneous and Cores 149-900A-80R, -81R, and -84R are the most intensely brecciated with, respectively, 68%, 53%, and 70% of their length brecciated (vs. 16% to 38% for the remaining cores). This brecciation may be progressive: the first stage shows little displacement of blocks (Interval 149-900A-84R, 45-80 cm) and corresponds to an increase in veining frequency.
More highly brecciated zones contain sparse clasts set in an ultrafine varicolored matrix (Fig. 23).Veining
The cores are cut by a large number of veins and fractures that in places represent more than 20% of the rock. Although some variation is visible, one can observe the same paragenetic sequence of veins throughout this section of the core. This sequence is listed here, starting with the earliest veins:
The most recent veining and brecciation associated with calcite (Type 4 above) is preferentially developed in the upper part and base of the basement section, but extends more rarely through the entire mafic interval. The other vein-types are more uniformly developed throughout this section.
The breccia is frequently deformed, particularly near the contact of blocks. In that case, the epidote and some chlorite veining clearly has been deformed, sheared, and broken. Rarely, even the calcite veining has been sheared (Interval 149-900A-82R-3, 74-78 cm). No intrusive or crosscutting relationships were observed, other than the late zones of brecciation and fracturing.
Thirty-one thin sections were made of the metamorphosed mafic rocks from Hole 900A. The recovered rocks include granoblastic and cataclastic microgabbro, small amounts of cataclastic norite, plagioclasite, and a single (dropstone?) of garnet-bearing amphibolite. Most of the massive rocks have a porphyroclastic texture and discontinuous foliation bands (Fig. 21). Porphyroclasts of plagioclase (up to 4 mm in size) have been strongly stretched and bent (Fig. 24), and porphyroclasts of clinopyroxene (up to 1.8 mm), and sometimes orthopyroxene, have been bent and sometimes kinked. Smaller (0.1-0.2 mm) granular crystals of the same minerals mark foliation planes. The fine-grained minerals show typical recrystallization textures (i.e., triple junction boundaries and little or no strain). This kind of structure is developed in coarser-banded intervals (149-900A-83R-2, 67-70 cm), as well as in finer-grained intervals (149-900A-82R-3, 11-15 cm). In the latter, porphyroclasts may be absent, giving the rock a granoblastic texture (Sample 149-900A-85R-5, 30-32 cm).
A rough estimate of the ratio of plagioclase to pyroxene in these rocks is 60:40, but may reach 40:60. In most rocks, this mineralogy has been partially overprinted by amphibolite or greenschist facies metamorphic minerals, mainly fibrous tremolite and/or hornblende, chlorite, epidote, and zoisite or clinozoisite. The greenish cast of most of these rocks reflects this metamorphism. However, a striking feature of nearly all these rocks is the freshness of the recrystallized plagioclase, which is unaltered except in brecciated zones and in the bottom of the hole (Sections 149-900A-85R-6 and -86R-1).
Green hornblende has partially or totally replaced clinopyroxene crystals with a clear obliquity of the cleavage to the compositional banding. Because no granulation or deformation of the amphiboles is seen, this replacement post-dates the strain recrystallization (Interval 149-900A-82R-5, 0-15 cm). At the base of the recovered section, fibrous green and colorless amphiboles become major constituents of the rock (Intervals 149-900A-85R-6, 135-139cm, and-86R-1, 1-4 cm). Only in these intervals does the amphibole show some deformation.
Chlorite often has replaced what may have been orthopyroxene, surrounds the plagioclase grains, and occurs in veinlets that crosscut these layers. It clearly post-dates the green hornblende. Epidote and clinozoisite or zoisite are also ubiquitous, but more often restricted to veins. Prehnite occurs in the interval at 149-900A-84R-1, 131-132 cm. Calcite is the latest mineral to crystallize, occurring mainly in veins. It may have been preceded by zeolite (phillipsite), as in Interval 149-900A-85R-2, 60-64 cm.
Major element determinations for nine metamorphosed mafic rocks are given in Table 7 and confirm the mafic nature of these rocks. However, the abundances of TiO2 and P2O5 are unusually low and abundances of Al2O3 are high, with four samples containing more than 20% Al2O3. The loss-on-ignition values reflect the generally hydrated nature of the rocks and the presence of some carbonate veining.
Results of trace element analyses for 21 samples, given in Table 8, are particularly revealing in that they show the rocks contain very low concentrations of incompatible elements zirconium and yttrium. Basalts having comparable low concentrations of these elements are very rare. For example, typical mid-ocean ridge basalts contain approximately 74 ppm zirconium, whereas these rocks contain an aver age of 16 ppm zirconium.
The relative consistency of trace elements within the suite of analyzed samples suggests that large-scale mineralogical layering is generally absent and elemental mobility during metamorphism has been limited. The only significant vertical variation in the composition of the rocks is in the concentrations of yttrium, vanadium, and TiO2. These elements and TiO2 have higher abundances in Core 149-900A-81R (Fig. 25) that corresponds to an enrichment in opaque minerals noted in thin section.
Petrographic, mineralogical, and geochemical studies suggest that these rocks are mafic in nature and have had a complex history. The earliest minerals observed are porphyroclasts of clinopyroxene, ortho pyroxene, and plagioclase, without spinel or magnetite (the rocks are very weakly magnetized, see "Paleomagnetism" section, this chapter).
The earliest pyroxene/plagioclase mineralogy, preserved in some porphyroclasts, has suffered high-temperature ductile deformation accompanied by dynamic recrystallization of these two minerals. This ductile deformation was followed by a mainly static retrograde metamorphism in the amphibolite or greenschist facies with amphibole or chlorite replacing pyroxenes. A late brittle deformation event brecciated these rocks and filled voids and fractures with chlorite, epidote and clinozoisite, zeolite, then calcite.
At present, and without radiometric dates to constrain the events indicated, it is not possible to differentiate between an oceanic or continental origin for these rocks. They might be:
Microprobe analyses and isotopic studies are obviously required to constrain pressure, temperature, and ages to differentiate among these models.