The clasts from Sites 1110 through 1113 are highly heterogeneous in terms of mineralogy, consistent with the interpretation that the holes drilled here are located in talus fans from the Moresby Seamount. In hand specimen, the majority are massive to weakly foliated rocks ranging in color from dark gray to greenish, caused by the presence of increasing amounts of epidote. Some are banded, and veins, sometimes folded, are common. Thin-section examination suggests that most of these rocks were originally dolerites that have been subjected to varying degrees of veining, shearing, and hydrothermal alteration under greenschist facies conditions. We refer to them loosely as metadolerites, although the protolith is, in some cases, in doubt.
Other lithologies include small amounts of mica schist, which probably has a pelitic protolith, as well as lamprophyres, granite porphyries, sedimentary rocks (volcanogenic sandstones), and so forth. Figure F7 shows the total recovery from each section and notes where these diverse rock types occur. Small amounts of clay were also recovered, sometimes adhering to the clasts. Such clay is a likely matrix of the heterogeneous clasts described here.
Protoliths of the metamorphic rocks clearly include igneous acidic types (granites and porphyritic granites), basic rocks (mainly dolerites, as indicated by relict igneous textures visible in hand specimen, which resemble those seen in the relatively fresh dolerite at Site 1109), and sedimentary rocks, as inferred by the presence of abundant micaceous minerals in schists. The following is a detailed description of the various rock types and textures within the unit referred to as heterogeneous talus at each of the sites. Because all the sites apparently sampled the same unit--talus from the seamount-- we consider them together here. Similar material from Site 1108 has already been briefly described in the Site 1108 chapter (see "Lithostratigraphy" in the "Site 1108" chapter). In addition, we give more detail on the Site 1108 material in the following sections.
A number of fine-grained mica schist clasts were recovered from 0 to 20 mbsf in Holes 1110B and 1110D, 1111A, and 1112B, whose distribution is shown in Figure F7. They consist of primary muscovite, quartz, and plagioclase with secondary chlorite, calcite, epidote, zoisite, clay minerals, iron oxide, and trace amounts of pyrite. Ductile structures, which range from lepidoblastic to mylonitic, including foliation planes and asymmetric tails around plagioclase and quartz porphyroclasts with quartz and chlorite in pressure shadows, are indicative of shear. The temperature conditions prevailing during the evolution of the mica schists is inferred to have ranged between 300ºC (quartz shows evidence of recrystallization) and 500ºC (plagioclase shows evidence of brittle deformation) (Rubie, 1983). The general aspect of these clasts in thin section shows a foliation plane (Fig. F8A), whereas some samples show subsequent folding of the foliation plane (Fig. F8B). This deformation is thought to have occurred early in the metamorphic evolution, before the retrograde metamorphic evolution under greenschist facies conditions, because calcite and epidote grains are not ductilely deformed.
As deformation proceeded, the epidote-rich layers were boudinaged (Fig. F9A), and fractures filled with quartz and calcite oriented perpendicular to the foliation were created. This indicates that brittle deformation occurred at a late stage of the metamorphic evolution (see also Fig. F10). Shearing is also associated with this late tectono-metamorphic stage because asymmetric pressure shadows with quartz and calcite fibers around pyrite grains occur (Fig. F11). Pyrite, chlorite, epidote, and quartz are indicative of hydrothermal alteration.
Some rocks were classified as carbonate or epidote schist according to the development of an assemblage characteristic of greenschist facies metamorphism, probably associated with strong hydrothermal alteration. Some may also have been dolerites, judging from the large amounts of epidote. It was not possible to deduce the protolith of some of the rocks with greater mylonitization. The highly schistose foliation in both hand-specimen and thin-section examination shows high contents of mica and epidote (Fig. F9B), suggesting that in some examples the primary composition was pelitic and a metasedimentary origin is likely. Fine-grained mylonites of feldspathic composition, apparently metamorphosed under greenschist facies conditions, were recovered at Site 1108, located to the east of Sites 1112 and 1113, and compositionally banded rocks of uncertain origin were found at the other talus sites. Primary mineralogy of these gneissic to mylonitic rocks is almost completely obliterated, but probably consisted largely of quartz and plagioclase (Figs. F12A, F12B). Well-developed foliation planes are crosscut perpendicularly by fractures filled with quartz, calcite, and plagioclase, which indicate late brittle deformation. Shear is indicated by chlorite sigmoidal trails surrounding lenses of quartz.
Pebbles of granite porphyry (phenocrysts of plagioclase in a fine-grained quartz groundmass) were recovered at depths ranging from 20 to 150 mbsf in Holes 1108B, 1110B, 1111A, and 1112A. The precise locations of these samples are as follows: Samples 180-1108B-3R-CC, 4-5 cm; 180-1108B-13R-CC, 0-5 cm; 180-1110B-3X-1, (Piece 3, 17-23 cm); 180-1111A-16R-CC (Piece 4, 16-20 cm); and 180-1112B-1W-1, 12-17 cm. Analysis of one of these porphyries is shown in Table T7 and T8 (Analysis 6), confirming its granitic composition (69% SiO2). The ferromagnesian minerals (Fig. F13) are green hornblende and/or biotite occurring as phenocrysts. From their mineralogy (dominantly feldspar with lesser amount of ferromagnesian), these samples appear to have calc-alkaline to alkaline affinities and the chemistry of the analyzed example is not particularly rich in Zr, but highly enriched in Sr and Ba, which tends to confirm this inference.
Sample 180-1108B-13R-CC, 0-5 cm, contains deep green pleochroic amphibole. Some samples show both ductile and brittle deformation indicated respectively by undulose extinction within plagioclase phenocrysts and by the presence of veins filled with calcite and pyrite. Additional alteration products include chlorite, sericite, quartz, calcite, and iron oxide in all samples. Although these samples have been classified as granitic in composition, the groundmass is too fine to be resolved under the microscope, and the amount of quartz cannot be estimated without a chemical analysis. We assume that the analyses in Tables T7 and T8 are representative, but, in fact, other samples may be more intermediate in composition.
Two pebbles recovered from Hole 1112A (Sample 180-1112A-5R-1, Pieces 5 and 6, 20-36 cm) were identified in hand specimen as porphyries. They appear to be cut by numerous parallel fractures filled with quartz, but were not examined in thin section. All these porphyry types probably occur as dikes on Moresby Seamount.
One holocrystalline quartz trachyte pebble containing alkali feldspar was recovered at interval 180-1108B-47R-1, 45-46 cm, but it has not been studied further.
One pebble of vein quartz was recovered in Hole 1111A (interval 180-1111A-18R-1, 33-43 cm). It consists of 95% quartz with 5% calcite precipitated in veins. Undulose extinction and recrystallization at the margins of the quartz grains is indicative of ductile deformation, whereas calcite veins are indicative of brittle deformation.
One medium-grained granitic gneiss was recovered during Leg 180 (Sample 180-1108B-8R-CC, Piece 3) (see "Lithostratigraphic Unit II" in "Lithostratigraphy" in the "Site 1108" chapter). It has a lepidoblastic texture and consists primarily of feldspar, quartz, and biotite elongated in the foliation plane. Accessory minerals include sphene and zircon. Very slight chlorite and sericite alteration is seen in thin section.
A volcanic or hypabyssal rock with microphenocrysts of clinopyroxene set in a groundmass of flow-oriented (trachytoid) amphibole prisms and felsic mesostasis was recovered from the deepest part of Site 1112 (Core 180-1112B-1W). This rock has been provisionally named "andesite," but the relatively high alkali content, especially K2O (Tables T7, T8; Analysis 5), suggests that it may be related to the shoshonitic rocks of nearby Woodlark Island (Ashley and Flood, 1981), as is the "lamprophyre" discussed in "Other Basic Igneous Rocks".
The basic igneous clasts at Sites 1110-1113 are mainly metadolerite that shows various degrees of alteration to greenschist facies assemblages.
We use the term "dolerite" to refer to clasts where the original mineralogy and texture is sufficiently well preserved to allow an easy recognition of the original protolith, whereas rocks that are so changed as to have lost this texture and mineralogy because of deformation and new mineral growth, are called metadolerites, providing the composition is appropriate.
In general, dolerites were recovered in the deeper levels of the holes from 30 to ~170 mbsf. Between ~35 and 95 mbsf, the metadolerite seems to prevail. Below ~95 mbsf, metadolerite forms the main lithology recovered in Hole 1111A, whereas dolerite forms the main lithology recovered in Hole 1112B. These dolerite clasts are mixed with a few clasts of other lithologies (lamprophyre, porphyries, vein quartz, etc.) as shown in Figure F7.
Dolerite clasts were recovered in the deeper levels of Holes 1111A, 1112A, and 1112B. This dolerite is medium grained, generally granular, although occasionally ophitic, and relatively unaltered. It is similar to the small portion of the dolerite recovered at Site 1109 (see "Igneous and Metamorphic Petrology" in the "Site 1109" chapter) that also had a granular texture rather than the typical ophitic texture. An example of a moderately altered dolerite with ophitic texture is shown in Figure F14. At Site 1109 the dolerite contains similar proportions of plagioclase and clinopyroxene, with rare opaque minerals and green alteration products that appear to be replacing olivine or interstitial glass. Feldspar shows various degrees of sericitization.
Many of the dolerite clasts are cut by veins filled with angular rock and mineral fragments in an opaque groundmass of highly milled material, which is unresolvable by optical methods. There is no fabric to these bodies, and clasts make up about 30% of them. They, therefore, fall under the term "cataclasite." The cataclastic veins displace an earlier generation of quartz veins (Fig. F15) and probably formed late in the history of the rock. Similar material occurs abundantly at Site 1117 and is depicted in Figure F5, in the "Site 1117" chapter.
Similar pebbles of dolerite with altered feldspars and sporadic epidote, probably representing incipient greenschist facies metamorphism, were recovered in Hole 1108B (e.g., interval 180-1108B-47R-CC, 1-3 cm; an analysis of this rock is given in Table T4, in the "Site 1114" chapter).
A pebble of amphibole-plagioclase rock was recovered in Hole 1110D in the wash core (Core 180-1110D-1W) (Fig. F16). Mineralogy of this rock includes primary amphibole (green hornblende prisms), plagioclase, clinopyroxene, and quartz with secondary biotite (replacing amphibole) and sericite (replacing plagioclase). It may be interpreted as an amphibolite, but the slightly porphyritic character and the lack of any obvious planar or linear structures in thin section lead us to think that it may be a hornblende-rich igneous rock, perhaps of lamprophyric affinities. Limited deformation of thin biotite flakes is not thought to be significant in this context.
Metadolerites from Hole 1111A are fine grained, with lepidoblastic to mylonitic texture, and consist of plagioclase, amphibole (tremolite?), quartz, and iron oxide with secondary epidote, chlorite, quartz, calcite, and clay. These rocks have been ductilely deformed, leading to the formation of a foliation plane, which was subsequently folded. The latest metamorphism in the greenschist facies is static because both chlorite and epidote are fibrous and nonoriented. Late brittle deformation is indicated by the presence of fractures filled with quartz, calcite, chlorite, and iron oxide, although some fractures are reworked (Fig. F8A) and must belong to an earlier phase of brittle deformation.
Epidote-rich pebbles were recovered together with the dolerite from several holes. The rocks can be described mostly as epidosite or metadolerite because of the abundance of epidote alteration throughout previous dolerite rocks. These rocks are similar to those described earlier, but are more altered, showing pervasive hydrothermal alteration and veins filled with calcite, quartz, epidote, and pyrite, indicating brittle deformation. It is possible that these rocks represent areas of more intense alteration proximal to veins in the dolerite that have channeled hydrothermal fluid flow.
The question of the extent of chemical alteration during the transition from dolerite to metadolerite and ultimately to epidosite is addressed in the chapter on Site 1114 (see "Igneous and Metamorphic Petrology" in the "Site 1114" chapter).
A pebble associated with the metadolerites and given the field term "lamprophyre" was recovered in Core 180-1111A-4R at about 29 mbsf. One pebble of diorite was recovered from near the base of the same hole at about 155 mbsf.
The lamprophyre has a few percent euhedral clinopyroxene phenocrysts and pseudomorphs after olivine phenocrysts, which may have made up about 10% of the rock originally. The groundmass is fresh with a panidiomorphic texture (i.e., euhedral ferromagnesian minerals in a felsic mesostasis). It consists of about 25% clinopyroxene and 35% amphibole needles. The felsic mesostasis could not be identified. Chemically, the rock is quite unlike the dolerites recovered at other sites (e.g., Site 1109), as shown in Tables T7 and T8. This difference is not likely to be caused by alteration because the rock is rather fresh, apart from the replacement of olivine. Most marked are the very high contents of K, Sr, and Ba, which suggest affinities to the shoshonitic rocks of the area. No completely satisfactory equivalent could be found from the "high-K" suite of Woodlark Island (Ashley and Flood, 1981), but this does not rule out the comparison, because such rocks, which are often highly porphyritic, show considerable variability. In particular, we note that the Site 1111 sample has about twice the K2O content of any from Woodlark Island, which suggests that it is distinct. However, the very elevated Sr and Ba contents can be matched. In many ways, it resembles the subrecent Madilogo volcano near Port Moresby (Blake, 1976), an analysis of which is given for comparative purposes in Tables T7 and T8. This small volcano is probably an outlier of the Papuan high-K province. We tentatively conclude that this rock is a representative of the widespread shoshonitic (also called the "high-K" suite) activity of the Papuan Peninsula and adjacent islands but cannot suggest a more localized provenance.
Basalt was a major constituent of the clasts recovered from the upper part of Hole 1108B, largely present in Cores 180-1108B-2R through 8R, although a few came from below this, perhaps having fallen from higher levels of the hole. Primary mineralogy consists of micropheno-crysts of plagioclase and olivine within a glassy groundmass. Well-preserved quench textures, such as lanterns and swallow tails, are visible in thin section (Fig. F17). These glassy groundmasses vary from tachylitic (fresh but opaque and glassy) to pilotaxitic (glassy, containing aligned feldspar needles) to variolitic (devitrified glass with a spherulitic structure). One sample had complexly zoned plagioclase xenoliths (Fig. F18). The majority of basalts recovered are fresh and generally sparsely vesicular, which implies recent submarine extrusive origin within relatively shallow water (<500 m; see Lackschewitz et al., 1994; Fisher and Schmincke, 1984), although several factors affect the vesicularity. If this interpretation is correct, it is surprising because the only source of basalt known at the present time is at the nearby spreading center, which is everywhere deeper than 2000 m (Taylor et al., 1995). One basalt pebble was crystalline with vesicles filled with epidote and chlorite, and clearly has a completely different provenance. The basalts recovered in Hole 1108B were not found at Sites 1110 through 1113.
These are described under "Lithostratigraphy" in the "Site 1108" chapter. An abundance of types are recognized, including serpentinized ultramafics (with chromite grains), volcanics with plagioclase and/or hornblende phenocrysts, and metamorphic rocks. One rock had a remarkable, complexly zoned plagioclase clast with a great many narrow oscillatory zones (Fig. F19), a type commonly found in andesites, which suggests that these lithic clasts, at least in part, are of calc-alkaline affinities (e.g., MacKenzie et al., 1982).
Most of the metamorphic rocks recovered at Site 1108 and Sites 1110-1113 were originally igneous, and some relict igneous textures are preserved. They are thought to be similar to the relatively fresh dolerite recovered at Site 1109, although we do not suggest that they belong to this body. There are occasional mica schists and gneisses, the former preferentially in the upper part of the holes. Both the mica schists and gneisses deposited as talus seem to record at least two tectonic and metamorphic stages, including the development of an early foliation plane followed by folding of the foliation, possibly prior to massive retrogression into lower greenschist facies conditions and extensive hydrothermal alteration.
Rocks deposited at Sites 1110 through 1113 come from a variety of sources, and it is likely that metamorphic, igneous, and sedimentary rocks are talus deposits from nearby Moresby Seamount. Metasedimentary rocks are most abundant from 0 to 20 mbsf. A mixture of igneous, metamorphic, and sedimentary rocks occurs from 20 to ~90 mbsf. The basic igneous rocks are most abundant from 90 mbsf to the bottoms of Holes 1111A and 1112B. The distribution observed in the talus deposits may indicate that the igneous basic rocks (dolerite and metadolerite) were the first talus deposits (Fig. F8). They most likely came from the top of the seamount, where in situ dolerite was recovered at Site 1114, when extensional tectonics started. As extension proceeded, deeper crustal levels of the basement were uplifted by normal faulting, unroofing the metamorphic rocks. Clasts of mica schist were subsequently shed as talus to the upper parts of the holes at Sites 1108 and 1110 through 1113.
The presence of numerous dolerite clasts suggests that the upper parts of Moresby Seamount (at least its northern flank) are composed largely of dolerite and metadolerite variably deformed and altered. The mixture of relatively fresh and altered material (e.g., epidosites) suggests that the protolith is traversed by zones, perhaps originally faults, which have channelled hydrothermal fluid flow, with consequent wall rock reaction.
Talus material at Site 1108 is more variable than at the other sites discussed here, reflecting that the site is further away from the slope of Moresby Seamount and has received material from other sources, including recent submarine basalts, probably from the nearby Woodlark spreading center tip. The submarine basalts may have been transported from the active spreading tip, which lies only a short distance to the east, but the mode of transport is obscure.
We think that most of the rocks described above are present on Moresby Seamount. Experience gained from the holes drilled in largely sedimentary successions nearby show that rafting of pebbles must be a very rare event. Perhaps the more exotic igneous types are present as dikes cutting the dolerites and metadolerites, which apparently make up the bulk of Moresby Seamount.