DISCUSSION
The set of data included in this chapter holds a meaning in a more comprehensive frame than that depicted at the regional scale by Robertson et al. (in press); however, the petrographic detail on lithic fragments allows significant considerations.
Serpentinites and tholeiitic basalts, dolerites, and gabbros are ascribed to the Paleogene Papuan ophiolite belt (Taylor et al., 1999; Robertson et al., in press). Moreover, dolerites represent the drilled basement of the northern margin (Site 1109) and footwall (Site 1114) of the Moresby Seamount. Gabbros, at places foliated, are present at Site 1117 (Shipboard Scientific Party, 1999).
In the different environments of the basin, ophiolitic clasts show a dishomogeneous distribution; in the northern margin (Sites 1109, 1115, and 1118), basalts, dolerites, and gabbros are the main constituents in conglomerates and are present in sandstones and limestones since middle to late Miocene, but are lacking in the Pliocene sediments. Serpentinite clasts were exceptionally recovered only in an upper Miocene carbonatic level (Site 1115). At the footwall, basalts, dolerites, gabbros, and serpentinites are relatively common in the Pliocene sediments at Site 1116, but lacking at Site 1114. This suggests that already since Pliocene the two sites had a different position relative to the ophiolitic source.
In the rift basin (Site 1108), basalts and gabbros appear associated with metamorphic rocks in the lower levels of middle Pliocene age, whereas serpentinites are diffuse since Pliocene to Pleistocene times.
All over the basin, basalts, dolerite, and gabbro clasts commonly exceed 3-10 mm in size; serpentinite clasts are present as fine- to medium-grained sands. Moreover in the northern margin, the tholeiitic clasts show frequent weathering that constrains a prolonged exposure under subaerial conditions. Such evidence is uncommon in the clasts from the footwall and the rift basin, and rare oxidized serpentinite clasts were observed. On the whole, it is likely to envisage that at the northern margin sites femic clasts experienced relatively short-range transport and originate from sources internal to the basin as a consequence of erosion of the ophiolitic basement. The footwall and rift basin ophiolitic clasts are present in relatively proximal turbidites associated with metamorphic rocks and granites. Conversely, a transport from farther sources can be envisaged at least for part of the serpentinite clasts.
The prevalence of basalt and dolerite clasts and the evolved composition of most gabbros likely originated from uppermost levels of oceanic layer 3, in accordance with erosion restricted to shallow portions of the ophiolitic basement. The greenschist facies alteration of some gabbroic clasts associated with schistosity is not in contrast and can be correlated with early tectonic events developed under high thermal gradient conditions during the build up of the ophiolite sequence, also consistent with shear textures at Site 1117. On the other hand, restricted to Site 1116, rare clasts affected by prehnite-pumpellyite facies overprint support pressures of ~0.2-0.3 GPa (Bucher and Frey, 1994). This requires that portions of the ophiolitic basement attained a lithostatic load of some (
5) kilometers, for instance, in an incipient intra-oceanic subduction prior to uplift and erosion. On the whole, the petrographic features and stratigraphic position of the ophiolite clasts at the northern margin sites are evidence of origins by reworking of subaerial deposits during the early Miocene phases of basin development. In contrast, the ophiolite clasts from the footwall and rift basin suggest that portions of the ophiolitic basement, eventually affected by older tectonics, were uplifted by extensional phases already active since the Pliocene in adjacent areas.
This hypothesis can also account for the presence and distribution of metamorphic rocks and granite clasts. Metamorphic rocks could either (1) represent continental lithosphere associated with granites, (2) represent equivalents of the metamorphosed accretionary complex underlying the Papuan ophiolitic thrust (Davies, 1980a), or (3) be assimilated with the metasediments outcropping at the D'Entrecasteaux Islands (Davies, 1980b; 1990). The significance of metamorphic rocks as substrate unroofed together with the overlying ophiolites is likely and consistent with data from Sites 1111 and 1113 (Taylor et al., 1999). Talc schists, chlorite schists, serpentine schists, and epidosites interpreted as fault gauge clasts can be considered as the evidence for exhumation of ophiolitic and metamorphic materials along tectonic discontinuities.
Tephrite and basanite clasts (middle Miocene sediments in the northern margin, Site 1115) are significant, although localized. The clasts (a few millimeters to 1 cm in size) are poorly rounded and show unaltered glassy groundmass and therefore did not undergo the weathering processes affecting the ophiolitic clasts from the same stratigraphic levels. An early to middle Miocene age and moderate transport can therefore be envisaged. The clasts constrain an alkaline volcanism that cannot be compared with the middle Pliocene-Holocene shoshonitic volcanism including the trachyte clasts (Site 1116) also reported from Sites 1110-1113 (Taylor et al., 1999) and well represented in the area (Smith et al., 1977; Smith and Milsom, 1984; Stolz et al., 1993; Ashley and Flood, 1981). More unlikely, the shoshonitic volcanism, generally subduction-related volcanic sequences including basanites and tephrites, are reported from a continental rift environment (Wilson, 1991).
The remarkable compositional range of arc-related volcanic products including basalt andesites to rhyolites and medium- to high-K calc-alkaline to rare shoshonite compositions is consistent with the provenance of fragments from several volcanic centers and with a temporal evolution of the volcanism, although a space and time volcanic zoneography has not been evidenced.
Clasts reworked in the turbidity currents include a wide range of textural features from porphyritic lavas, glassy to microcrystalline variably vesicular groundmass, pumices, glass shards, and single-grain phenoclasts. Rarer millimeter-sized clasts with granophyric texture could result from the blast of subvolcanic felsic bodies. Mostly in northern margin sites through the lower to middle Pliocene, the occurrence of tephra layers interbedded with turbidites evidences episodic contribution from air fall ashes.
