A variety of fresh to highly altered igneous rocks and sediments with minor to abundant volcanic components was cored at seven sites during Leg 17. Diagenetic alteration of the igneous rocks is ubiquitous and is particularly intense at Sites 167, 170, and 171. Zeolite facies metamorphism affected the lowest rocks at Hole 165A. Diagenesis is slow in the absence of high CO2 activity. However, in a Cθ2-rich environment it leads to rapid replacement of olivine, glass, and mesostasis, and slow replacement of pyroxene by montmorillonite. Plagioclase is only slightly to moderately attacked in the CO2-ri.cn environment and calcium is lost from pyroxene. Sodium, potassium, and water are gained in the montmorillonite, and iron is oxidized to the extent that available oxygen permits.The best methods of deriving original rock and magma types of altered specimens are through "refractory" (i.e., alterationresistant) trace elements, but even these appear to be lost when alteration is so intense that pyroxene is destroyed, such as at Sites 167, 170, and 171. Taking all available data into account, the following conclusions are drawn.True basement (ocean ridge basalts of Layer 2 of the oceanic crust, accreted at a spreading center) was cored at Sites 164 and 166 and possibly at the bottom of the hole at Site 169. At all other sites and higher in the hole at Site 169, igneous rocks were recovered that are more alkaline than ocean ridge basalts. They probably were emplaced at a distance from a spreading center above a section of older sediments and do not represent true basement. The evidence for this is conclusive at Site 169, where a sill of uncertain petrologic affinities was penetrated, and is very strong at Sites 167 and 171, which are located on large seamounts (Magellan Rise and Horizon Guyot, respectively). The evidence is weaker, but persuasive, at Site 165A and Site 170. Sills were definitely (Site 169) or probably (Sites 167 and 170) cored at three sites, and seem to be a common mode of emplacement when the true basement is covered by unconsolidated sediments.In addition to flows and sills, volcanic material appears in the sedimentary section as turbidites, hyaloclastites, pyroclastites, and dispersed detritus in pelagic sediments. Like the relatively alkaline rocks discussed above, the volcanic components in the sediments record activity that postdates the time of accretion of the underlying crustal plate at a spreading center. In general, the volcanic sediments were derived from nearby seamounts and ridges which yielded rubble and turbidites not only by slumping of unstable slopes but also by explosive activity as peaks approached sea level. Where the peaks emerged as islands, as at Site 171, subaerially eroded detritus and perhaps forereef calcarenites and calcirudites also were supplied.
