Site 1137
Site 1137 lies on Elan Bank, a large western salient of the main Kerguelen Plateau, at a water depth of 1016 m (Figs. 3, 4). Elan Bank, flanked on three sides by oceanic crust of the Enderby Basin, had not been sampled before our drilling at Site 1137; therefore, the age and geochemistry of its igneous crust, as well as the feature's relationship to the contiguous central and southern Kerguelen Plateau, were completely unknown. Site 1137 lies on the eastern portion of the crest of Elan Bank; we chose the location as representative of the entire Elan Bank on the basis of its relatively simple structural setting, thin sedimentary section, and the presence of intrabasement seismic reflections (Fig. 21). The major objective at Site 1137 was to obtain igneous basement to characterize the age, petrography, and compositions of the lavas, the physical characteristics of the lava flows, and the environment of eruption (subaerial or submarine). We were especially interested in constraining the age of the uppermost igneous basement at Elan Bank for comparison with the proposed ~110 and ~85 Ma volcanic pulses on the southern and central Kerguelen Plateau, respectively. Sedimentary objectives at Site 1137 were to determine sequence facies, to define the ages of seismic sequence boundaries, to estimate the duration of possible subaerial and shallow marine environments, to obtain minimum estimates for basement age, and to determine the paleoceanographic history of this high-latitude site. As discussed below, we largely achieved our goals at Site 1137. We cored basaltic basement and interbedded sediment from 219.5 to 371.2 mbsf. One of the most significant and unexpected results of the leg was the discovery of garnet gneiss clasts in a fluvial conglomerate interbedded with basaltic basement at this site. This provides unequivocal evidence of continental crust in Elan Bank. In addition, the geochemical characteristics of these basalts clearly indicate a continental crustal component.
We recognize three sedimentary lithologic units (I-III) in the upper 219.5 mbsf (Fig. 22). They rest unconformably on basaltic basement (Unit IV). Unit I (0-9.5 mbsf) consists of Pleistocene foraminifer-bearing diatom ooze with apparent ice-rafted sand and pebbles. Unit II (9.5-199.5 mbsf) is Miocene to uppermost Eocene white nannofossil ooze with rare chert. Some intervals contain diatoms or foraminifers. Units I and II represent marine pelagic deposition and are characterized by compressional (P-) wave velocities of 1564 to 1785 m/s that show little scatter. Porosity in the upper two units clusters between 50% and 63%. Unit III (199.5-219.5 mbsf) is a 20-m-thick sequence of glauconite-bearing sandy packstone with abundant shell fragments that was probably deposited in a neritic environment. In the core overlying basaltic basement, the packstone contains well-preserved late Campanian (72-76 Ma) foraminifers, calcareous nannofossils, and dinoflagellates. P-wave velocities in Unit III vary considerably from 3120 and 4340 m/s, and porosity, which ranges between 4.7% and 24.9%, averages 11.7%. Natural gamma-ray intensities are relatively high in the glauconitic sand.
Basalt and interbedded volcaniclastic sediment comprise the 151.7-m basement sequence in Hole 1137A (Unit IV; 219.5-371.2 mbsf), which we subdivide into basement Units 1-10 (Fig. 22). The 10 units include seven basaltic lava flows, totaling ~90 m in thickness, and three sedimentary units. All basement units are clearly distinguishable in downhole logging data. Relatively good core recovery and high quality logs enable us to constrain true thicknesses of the basement units through core-log integration. All basalts are normally magnetized; in light of biostratigraphic ages in Unit III and thickening of this unit to the east, the basalts likely acquired their magnetization during the long Cretaceous Normal Superchron prior to 83 Ma. Inclinations after thermal demagnetization of basalts from the seven flows range from -55° to -72°, with a mean of -66°. We calculate a paleolatitude of ~48°S, which is 8.5°N of Site 1137. P-wave velocities in basement rocks range from 2648 to 6565 m/s, averaging ~4650 m/s. Velocities within lava flows correlate inversely with the degree of alteration estimated from visual inspection.
Six of the seven basaltic flow units erupted subaerially; one may have erupted into shallow, wet sediment. We interpret each basaltic unit as a single lava flow. The flows are 7 to 27 m thick, and we recovered three flow-top breccias, a pahoehoe surface, and two basal contacts where lava apparently baked the underlying units. Flow-top breccias appear to have formed by breaking up of small pahoehoelike fingers that are repeatedly intruded into older breccia. This style of autobrecciation is atypical of pahoehoe, aa, or Hawaiian transitional lava flows, but is common in western United States flood basalts. The three or possibly four pahoehoe flows in Hole 1137A are inflated; in morphology, they resemble large-volume lava flows forming continental flood basalts. Multiple horizontal vesicular zones within the 27-m-thick flow suggest a complicated inflation history of perhaps four separate pulses of lava. Sediments intercalated with the lowermost flow top breccia suggest that the lava intruded wet sediment. With the exception of this lowermost unit, the flows show oxidation zones and morphologies consistent with subaerial emplacement.
Basement Units 1-4 are aphyric to moderately plagioclase ± clinopyroxene ± olivine-phyric basalt, whereas Units 7, 8, and 10 are moderately to highly plagioclase ± clinopyroxene-phyric basalt. Like all other Cretaceous basement basalt recovered from the Kerguelen Plateau, Site 1137 basalts are tholeiitic to transititional in composition (Fig. 19); they have 50.4 to 52.7 wt% SiO2 and 4.4 to 7.3 wt% MgO. However, abundances of incompatible minor and trace elements (Fig. 23), as well as the ratio of more incompatible to less incompatible elements (e.g., Zr/Y; see Fig. 24), are higher than in other Cretaceous basement tholeiites recovered from the plateau. Consequently, Site 1137 basalts form a distinct geochemical group that may reflect a relatively lower extent of melting or a source more enriched in incompatible elements. A continental lithospheric component, probably crust, in Site 1137 basalts is indicated by their relatively low Nb/Ce and high Zr/Ti ratios, as well as their trend of Nb/Y vs. Zr/Y (Fig. 25). Such a component is also present in Kerguelen Plateau basalts from Sites 738 and 747, but not in basalts from Sites 749, 750, and 1136 (Figs. 24, 25).
Lava flow interiors at Site 1137, as at Site 1136, are relatively unaltered compared to Cretaceous basement lavas previously recovered from the Kerguelen Plateau. For example, LOI for least altered samples is <2.2% and averages 1.2%. The mobility of Rb and K during postmagmatic alteration shows in the poor correlation of Rb and Nb abundances (Fig. 23) and the wide abundance range of these elements compared to other incompatible elements (Fig. 26). Basement rocks vary from slightly to completely altered with low-temperature secondary phases replacing primary minerals and mesostasis and filling veins, fractures, and vesicles. Clay minerals are the dominant secondary minerals in all basement units. More permeable horizons (such as brecciated and/or vesicular flow tops and bases, and zones with high vein and fracture densities) exhibit higher degrees of alteration and more diverse secondary phases that include calcite, zeolite, quartz, and amorphous silica. In downhole logs, highly altered flow tops are characterized by higher potassium content caused by the increased abundance of clay minerals. Alteration of Hole 1137A lavas likely results from both weathering and low-temperature alteration. Unlike postmagmatic submarine alteration of typical oceanic crust, subaerial weathering and low-temperature interaction of basalts and interbedded sediments with groundwater at Site 1137 preceded submarine alteration.
Basement Units 5, 6, and 9 consist of volcaniclastic sedimentary rocks. Basement Unit 5 (286.7-291.0 mbsf) is a succession of interbedded volcaniclastic siltstones and sandstones. Many beds are normally graded, and others show parallel laminations. These sediments overlie Basement Unit 6 (291.0-317.2 mbsf), which consists of volcaniclastic conglomerate. Clasts range from well rounded granules to small boulders. Most intervals are clast supported, but matrix-supported intervals also are present. The depositional environment of Basement Units 5 and 6 appears fluvial, perhaps associated with a braided river. These units represent a significant hiatus of unknown duration between eruptions of basaltic flow Units 1-4 and 7 and 8. Furthermore, fluvial facies of Units 5 and 6 corroborate our interpretation of subaerial lava flow effusion.
Diverse clasts within the conglomerate (Unit 6), and to a lesser extent similar lithic clasts within the underlying crystal-vitric tuff (Unit 9), constitute a greater variety of rock types than usually recovered from a single drill hole into igneous basement. In particular, the predominant clast lithologies in Units 6 and 9 include porphyritic trachyte, flow-banded rhyolite, plagioclase phyric basalt, and a variety of small, highly altered, sparsely phyric and aphyric basalts. The most unexpected clasts, however, are rounded cobbles of garnet-biotite-gneiss and granitoid. We also find single grains of garnet and perthitic alkali feldspar, presumably weathered from sources similar to those of the gneiss and granitoid cobbles, in the sand fraction of Unit 5 and as xenocrysts in parts of the Unit 9 crystal-vitric tuff. It is difficult to imagine anything but an originally continental source for such material.
Basement Unit 9 (344.0-360.7 mbsf) consists of altered crystal-vitric tuff composed of ~40% coarse (1-2 mm) angular crystals of sanidine and <5% lithic clasts enclosed within a light- to dark green dense matrix. Cuspate and tricuspate glass shards, now partially to completely altered to clay minerals, and sanidine phenocrysts (<5 mm) are the principal components of the tuff; minor components include amphibole, plagioclase, quartz, and opaques. Broken bubble-wall shards and abundant embayed and broken crystals indicate that the tuff formed in an explosive volcanic eruption. Suspended within the tuff are 1%-2% subangular to rounded, granule- to pebble-sized lithic clasts of varying lithologies, mostly basalt. The coarse grain size of both the enclosed pebbles and the primary sanidine crystals precludes deposition of this material by settling from an ash cloud; transport in a pyroclastic flow is more likely. However, we infer that the material was reworked because of the absence of (1) internal stratification, (2) a basal breccia or fine flow top, or (3) normal grading of lithics and crystals. The even distribution of crystals and pebbles throughout the tuff and the massive internal texture of the deposit provide evidence for mass flow redeposition of these sediments to their present locations. Small fault zones with offsets of <=5 cm are highly altered and contain locally abundant (<=5%) pyrite and possibly chlorite. Native copper surrounds lithic fragments in more intense alteration zones (<1 cm wide).
In summary, significant results bearing on the origin and evolution of Elan Bank include the following:
1. Subsidence of the Kerguelen Plateau is recorded by the paleoenvironments of volcanic rocks and sediment that range from subaerial or fluvial (basalt and interbedded sediment) to neritic (packstone) to pelagic (ooze).
2. The volcaniclastic conglomerate contains clasts of trachyte, rhyolite, granitoid, and garnet-biotite gneiss; the garnet-biotite gneiss, in particular, indicates continental crust at this south Indian Ocean location.
3. The sanidine-bearing crystal-vitric tuff, as well as the trachyte and rhyolite clasts in the conglomerate, indicates that highly evolved magmas erupted, in some cases explosively, during the final stages of the volcanism that formed Elan Bank.
4. Most of the seven basement flows erupted in a subaerial environment. These inflated pahoehoe and transitional rubbly flows are typical of continental flood basalts, such as the Columbia River Basalt.
5. Like other Cretaceous igneous basement rocks of the Kerguelen Plateau, the seven basement lava flows are tholeiitic to transitional basalts; however, Site 1137 basalts are more enriched in incompatible elements, perhaps a result of lower extents of partial melting or derivation from a source more enriched in incompatible elements. Also, we infer a continental crust component in Site 1137 basalts from their less-than-primitive mantle ratios of Nb/Ce and Zr/Ti and their Nb/Y vs. Zr/Y trends.

Leg 183 Principal Results - Site 1138

Leg 183 Table of Contents