Dipartimento di Georisorse e Territorio, Università Udine, Italy.

The West Philippine Basin (WPB) is a back-arc basin that opened within the Philippine Sea Plate (PSP) between the current position of the Palau-Kyushu Ridge (PKR) and the margin of East Asia. Spreading occurred at the Central Basin Fault (CBF) mainly from 54 until 30 Ma. The PKR was active since ~ 48 to 35 Ma constituting a single volcanic arc with the Izu-Bonin-Mariana (IBM) Arc. At ~ 42 Ma ago spreading rate and direction changed from NE-SW to NS, stopping at ~ 30 Ma. A late phase of spreading and volcanism took place between 30 and 26 Ma. ODP Leg 195 Site 1201 is located in the WPB, ~ 100 km west of the PKR, on 49 Ma crust formed by NE-SW spreading at the CBF. From ~ 35 to 30 Ma, pelagic sedimentation at Site 1201 was followed by turbidite sedimentation, fed mostly by arc-derived volcanic clasts. The geochemical and isotopic features of Site 1201 basement rocks, which represent Eocene WPB oceanic crust, compared with those of Site 1201 volcanics from the turbidite sequence, representing products of the early Mariana Arc (PKR), provide some insights into the early history of the IBM subduction factory. The WPB basement is made up of aphyric to porphyritic basalts with altered olivine, and preserved plagioclase, clinopyroxene and opaques. The PKR volcanics are porphyritic basalts and andesites with plagioclase, clino- and orthopyroxene, hornblende, alkali feldspar and opaques. Variable textures, and degree of alteration suggesting zeolite facies metamorphic grade, characterize both groups of rocks. The mineralogical and geochemical characteristics of the investigated Site 1201 PKR volcanics highlight their calc-alkaline affinity. This feature is at variance with both other PKR rocks, having mostly boninitic and arc tholeiitic affinity, and WPB basement basalt, having tholeiitic affinity, with some characters transitional to arc-like, as expected for a back-arc basin. New Sr and Nd isotope data, coupled with published Sr, Nd, Pb and Hf isotope data (Savov et al., 2006), highlight the Indian Ocean MORB-like character of Site 1201 basement basalts. This suggests that WPB volcanism tapped an upper mantle domain distinct from that underlying the Pacific Plate. The isotopic features of Site 1201 PKR volcanics are more enriched relative to those of basement basalts reflecting higher amounts of subduction-derived component(s) in the source of arc magmas. Th-Nb relationships and isotope geochemistry of the WPB basement and overlaying arc volcanics suggest addition of subducted sediment mostly as siliceous melts, to the mantle source of the arc volcanics. In that respect, Site 1201 PKR volcanics resemble calc-alkaline volcanics of the currently active Mariana Arc. In addition, the calc-alkaline affinity, unradiogenic neodymium, and inferred Middle Oligocene age of PKR volcanics, suggest they might represent an evolved stage of arc volcanism at Palau-Kyushu Ridge, perhaps shortly before the end of its activity.

The Southern Uralides are a collisional orogen generated in the Late Devonian– Early Carboniferous by the collision of the Magnitogorsk island arc (MA) generated in the Early to Middle Devonian by intra-oceanic convergence opposite to the continental margin, and the continental margin of the East European craton. A suture zone of the arc to the continental margin, the Main Uralian Fault (MUF), is marked by ophiolites and exhumed high-pressure–low-temperature metamorphic rocks of continental origin. The preorogenic events of the Southern Urals and their geodynamic setting are traced by means of fluid-immobile incompatible trace elements (rare earth elements and high field strength elements) and Sr–Nd–Pb isotope geochemistry of the MA suites, in particular the protoarc suite with boninites and probably ankaramites, and the mature arc comprised of island arc tholeiitic (IAT) suites, transitional IAT to calc-alkaline (CA), and CA suites. The MA volcanics result in genetically distinct magmatic source components. In particular, depleted normal-mid-oceanic ridge basalt-type mantle sources with various enrichments in a slabderived aqueous fluid component are evident. The enriched component is not involved in significant amounts, as testified by the rather radiogenic Nd isotopes and unradiogenic Pb isotopes. Further information on the pre-orogenic events is provided by the Mindyak Massif metagabbros derived from diverse gabbroic protoliths that were affected by oceanic rodingitization, and subsequently by a high-temperature (HT) metamorphism related to the development of a metamorphic sole. The HT metamorphism has the same age as the protoarc volcanism, and constrains the initiation of subduction at approximately 410 Ma. Consequently, the maximum timespan between initial intra-oceanic convergence and final collision is approximately 31 my, a duration consistent with that of present-day ongoing collisions in the western Pacific. The characteristics of early volcanism and the traces of a metamorphic sole provide useful criteria to attribute most MUF ophiolites to the Tethyan type with a complex pre-orogenic evolution.

Site 1201D of Ocean Drilling Program Leg 195 recovered basaltic and volcaniclastic units from the West Philippine Basin that document the earliest history of the Izu–Bonin–Mariana convergent margin. The stratigraphic section recovered at Site 1201D includes 90 m of pillow basalts, representing the West Philippine Basin basement, overlain by 459 m of volcaniclastic turbidites that formed from detritus shed from the Eocene–Oligocene proto-Izu–Bonin–Mariana island arc. Basement basalts are normal mid-ocean ridge basalt (N-MORB), based on their abundances of immobile trace elements, although fluid-mobile elements are enriched, similar to back-arc basin basalts (BABB). Sr, Nd, Pb and Hf isotopic compositions of the basement basalts are similar to those of basalts from other West Philippine Basin locations, and show an overall Indian Ocean MORB signature, marked by high 208Pb/204Pb for a given 206Pb/204Pb and high 176Hf/177Hf for a given 143Nd/ 144Nd. Trace element and isotopic differences between the basement and overlying arc-derived volcaniclastics are best explained by the addition of subducted sediment or sediment melt, together with hydrous fluids from subducted oceanic crust, into the mantle source of the arc lavas. In contrast to tectonic models suggesting that a mantle hotspot was a source of heat for the early Izu–Bonin–Mariana arc magmatism, the geochemical data do not support an enriched, ocean island basalt (OIB)-like source for either the basement basalts or the arc volcanic section.