14. PETROLOGY OF VOLCANIC ROCKS RECOVERED ON DSDP LEG 19 FROM THE NORTH PACIFIC OCEAN AND THE BERING SEA Richard J. Stewart, Department of Geological Sciences, University of Washington, Seattle, Washington James H. Natland, Department of the Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California and William R. Glassley, Department of Geological Sciences, University of Washington, Seattle, Washington ABSTRACT On Leg 19 of the Deep Sea Drilling Project, basaltic volcanic rocks were recovered from Site 183 in the western Gulf of Alaska, Site 191 on the floor of Kamchatka Basin, and Site 192 atop Meiji Guyot, the northernmost of the Emperor Seamounts. At Site 183, alkali-olivine basalt was recovered underlying the thick sedimentary sequence of the Aleutian Abyssal Plain. The rocks strongly resemble Hawaiian alkali basalts, have high K, Sr, Zr, and Ba contents, and may be associated with the formation of nearby Derickson Seamount. They are inferred to postdate the basement age estimated from magnetic anomalies and to be extrusive onto oceanic crust. Tholeiitic basalt recovered at Site 191 beneath the sedimentary fill of Kamchatka Basin has low K2O, Ba, and Sr-87/Sr-86 values similar to midocean ridge tholeiites, but has higher Rb, Sr, and Zr values which follow patterns established for other inter-arc basin basalts. The basalt is probably part of a regionally developed basaltic basement which may have intruded and enveloped older basin deposits in middle Oligocene time. A series of at least five extrusive alkali basalt flows was recovered from near the crest of Meiji Guyot. The rocks are striking in having potassium feldspar cores in most Plagioclase phenocrysts, and in their coarse grain size, reaching sub-gabbroic textures in some flow interiors. Theoretical and experimental data indicate the almost pure potassium feldspar could not crystallize in equilibrium with associated labradorite. Origin as xenocrysts derived from late-stage plutonic differentiation and later inclusion in basaltic lavas is not possible as glassy rinds containing no potassium feldspar xenocrysts. The mineralogy probably results from secondary replacement of calcic Plagioclase cores by potassium-rich alkaline solutions produced during alteration of the basalts. INTRODUCTION Additional studies on seismic velocity determinations by Basaltic basement rocks were recovered from three sites N.I. Christensen and magnetic properties by R. T. Merrill on Leg 19, each of which was located in a different are reported in separate papers in this volume. structural and tectonic setting within the North Pacific- Bering Sea region (see Figure 1). Recovered material RESPONSIBILITY AND ACKNOWLEDGMENTS includes samples of alkali-olivine basalt from the Aleutian The rocks discussed in this report were described on ship Abyssal Plain (Site 183); tholeiitic basalt from Kamchatka by Stewart and Natland. Additional thin section descrip- Basin, a back-arc marginal basin (Site 191); and alkali basalt tions were done on shore by Stewart, microprobe analyses from atop Meiji Guyot, the northernmost of the Emperor were done by Glassley, and X-ray mineralogic studies Seamounts (Site 192). compiled, by Natland. We are indebted to L. Shapiro, P. Relatively complete descriptions of megascopic features Elmore, R. Moore, H. Smith, J. Kelsey, and J. Glenn of the and the thin section petrography of rocks from each site are United States Geological Survey Rapid Rock Analysis included in the respective site chapters. The purpose of this Laboratory in Menlo Park, California, for major element chapter is to summarize these descriptions, provide geochemistry on three basalt samples, and to R. E. Mays of preliminary chemical and electron microprobe analyses, and the same laboratory for minor element concentration data. interpret their petrology along with their lithologic, Analytical work connected with isotopic age dating was structural, and tectonic framework. performed by Teledyne Isotopes and was funded by a grant 615 R. J. STEWART, J. H. NATLAND, W. R. GLASSLEY CCCP ttv 4/ * ufvWßh 1 ü Hf ; /A ALEUTIAN ABYSSAL PLAIN Figure 1. Topographic sketch map of the Aleutian Island Arc and adjacent portions of the North Pacific Ocean and Bering Sea. Site locations shown diagrammatically; for exact locations see respective site description chapters. from the U.S. Geological Survey Office of Marine Geology, appears on seismic reflection records; the knoll at Site 191, Pacific-Arctic Branch. part of this basement relief, appears to be depositionally draped by a palagic sequence, succeeded upward by GEOLOGIC SETTINGS Plio-Pleistocene turbidites (see Figure 2). Site 183 is located on the floor of the North Pacific The most striking basalts recovered on Leg 19 were Ocean near the northern edge of the Aleutian Abyssal Plain, drilled at Site 192 near the summit of Meiji Guyot, the in the western Gulf of Alaska (Figure 1). The location is northwestmost of the Emperor Seamounts (Figure 1). The approximately midway between anomalies 25 to the south Emperor Seamounts extend north-northwesterly from west and 24(?) to the north, as identified by Erickson and Grim of Midway Island to the vicinity of the confluence of the (1969). The site lies about midway between Derickson and Aleutian and Kuril trenches. Near its northwest terminus, Sirius seamounts, and is about 15 km north of Seamap the volcanic core of the chain loses its ruggedness because Channel, a fossil feeder channel for the sediments of the it is buried beneath as much as 1500 meters of Aleutian Abyssal Plain (Hamilton, 1967 in press; Grim, topography-subduing deposits; even the summit area is 1969; Mammerickx, 1970). Overlying the basalt, in mantled with as much as 1100 meters of sediments, apparent stratigraphic succession, are turbidites of the dominantly mudstones (see Figure 2). Site 192 is located at Aleutian Abyssal Plain, a sequence in turn capped by a the northwest end of the chain, at a water depth of 3000 thick pelagic blanket (see Figure 2). meters, and over its sediment-mantled crest. In contrast to Site 183, the basalt recovered at Site 191 is from the floor of Kamchatka Basin, a back-arc NATURE OF EMPLACEMENT depositional trough lying north of the main Aleutian chain Although interpretations are hindered by incomplete (Figure 1). Shirshov Ridge, a submerged north-south- recovery of material at the sediment-basalt interfaces of trending mountain range, projects outward from Cape Sites 183 and 191, local and regional geologic considera- Olyutorskiy, eastern Siberia, and divides the deep-water tions suggest that all three sites discussed in this report region of the Bering Sea into Kamchatka Basin and the encountered normal extrusive basalt. larger Aleutian Basin to the east. Throughout Kamchatka At Site 183, recovery was limited to a few rounded Basin, a rather rough, "oceanic-like" acoustic basement pieces and chips of alkali-olivine basalt, and the critical 616 PETROLOGY OF VOLCANIC ROCKS, NORTH PACIFIC OCEAN AND BERING SEA deposited at low temperatures directly from sea water (see 5 Natland, this volume). Furthermore, evidence from Site < s 192 indicates that, during this period of the early Tertiary, x u < sedimentation rates in the North Pacific were exceedingly ε low; thus sediments representing the 12 m.y. age < discrepancy may be very thin and were simply washed through in coring operations. Also, the alkali basalts of Site 192 191 183 183 could well be related to the nearby seamounts; normal Φ oceanic crust as old as predicted from the magnetic anomaly patterns could well underlie the alkali basalts. At Site 191, the critical basalt-sediment interface again was not cored. However, Plagioclase microlites in the basalt U. MIOCENE have a K-Ar age of middle Oligocene age (approximately // / M. MIOCENE 30 m.y. See Table 3), which presumably means that the ; "-• :-; L. MIOCENE basalt is extrusive lava overlain unconformably by lower V / Pliocene or perhaps late Miocene mudstone. The grada- / 8 tional textural change observed from variolitic basalt downward to sub-diabasic basalt suggests that the margin of 1 a flow or a large pillow was drilled. Variolitic textures commonly result from rapid freezing of basaltic liquids and are usually reported from the pillowed portions of submarine extrusions. However, variolitic textures have / Φ / been reported from intrusive sills and even from pillows intrusive into soft marine sediments (Wilson, 1960; Garrison, 1972) and thus are unreliable indicators of Diαtomαceous Mud underwater extrusion. On a regional scale, the basalt layer underlying Kamchatka Basin does not give an impressive magnetic signature (see Figure 2, Site 191 report, this volume) and local intrusive-like peaks are associated with fairly weak (200-300 gammas) anomalies. Acoustic reflection records (see Figure 3, Site 191 report, this volume) demonstrate that the basalt layer encountered at Site 191 is regional in - L. MIOCENE ç ^ : _OR OLIGOCENE extent. It has a buried subsurface relief that can amount to - L. OLIGOCENE" as much as 800 meters in 10 to 12 km, but typically is more subdued. The reflection records suggest that the 1000 basalt layer beneath Kamchatka Basin may be equivalent to 1AESTRICHTIAN the volcanic core of nearby Shirshov Ridge. These same records suggest that the relief of the basalt layer is Ash Horizons depositionally draped by pelagic or hemipelagic deposits Φ (Figure 2). Only locally, especially near the southern end of Ice- Rafted Debris Shirshov Ridge, are there uplifted