Trace-Element Geochemistry of Volcanic Rocks from Site 786: The Izu-Bonin Forearc

Abstract

Eocene-Oligocene volcanic rocks drilled at Site 786 in the Izu-Bonin forearc cover a wide range of compositions from primitive boninites to highly evolved rhyolites. K-Ar dating reveals at least two distinct episodes of magmatism; one at 41 Ma and a later one at 35 Ma. The early episode produced low-Ca boninites and bronzite andesites that form an oceanic basement of pillow lavas and composite intrusive sheets, overlain by flows and intrusive sheets of intermediate-Ca boninites and bronzite-andesites and a fractionated series of andesites, dacites, and rhyolites. The later episode produced high-Ca boninites and intermediate-Ca boninites, exclusively as intrusive sheets.Trace element data indicate that all of the evolved chemical groups at Site 786 can be related by fractionation and/or accumulation of the observed mineral phases back to the three boninite groups, which represent distinct parental magmas. The boninites have very low abundances of Ti, Y, and HREE relative to MORB, consistent with an origin from a depleted source, and consideration of Cr-Y-Ti melting systematics and major element data indicates that the low-Ca boninites came from a source more depleted than the high-Ca boninite source. The boninites show enrichment in LIL elements, LREE and selected HFS (Zr, Hf) relative to Ti, Y, HREE which reflect the addition of a "subduction" component to the boninite source region. The distinctive enrichment of Zr is a feature not found in typical arc-related volcanics, but it has been recognized in several other boninite suites. The fractionation of Zr from Sm and Ti suggests an important role for amphibole in any petrogenetic model to explain the genesis of these boninites. Possibilities include the addition of a melt derived from subducted amphibolitized ocean crust and the interaction of an OIB-like melt with amphibole stabilized in the mantle wedge.The multiple episodes of boninite magmatism at Site 786 imply a recurrence of conditions for boninite genesis over an extended period of time (at least 7 m.y.). This refutes the idea that boninites are generated solely at the initiation of subduction. However, the predominance of LREE and Zr, Ta, Nb enrichment in the Eocene boninite groups does imply a genetic relationship with the onset of subduction and may be explained by early hydrous melting of amphibolitized forearc lithospheric mantle combined with mobilization of a pre-existing OIB-like component. 1 Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125: College Station, TX (Ocean Drilling Program).

A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

“…The rare-earth elements (REE) and trace elements Ba, Th, Hf, Ta, U, Pb, Rb and Cs were also analyzed at Washington State University by a Sciex Elan model 250 ICP-MS equipped with a Babington nebulizer, watercooled spray chamber and Brooks mass £ow controller. The precision on in-house standards [8,13], basalts from the Quaternary arc [14,15] and basalts from the Sumisu Rift [17].…”

“…This was followed by Nd puri¢cation from other REE on cation resin using 2-methyllactic acid. [13,18,19], (E) Sumisu Rift [17] and (F) active arc basalts from the Shichito Ridge and Torishima knoll [15,17]. Normalization values are from Taylor and McLennan [39].…”

“…3. They are compared to forearc basement recovered at Site 786 of ODP Leg 125 [8,13], basalts from the Quaternary IzuB onin arc [14,15], Paci¢c plate MORB [16] and basalts from the Sumisu Rift [17] Chondrite-normalized REE patterns for the inner trench slope samples (Fig. 4) show depletion of the light REE and Ba, with values of La/Yb CN from 0.4 to 0.6.…”

“…The patterns for each of the samples are strikingly parallel, even for the samples dredged from Myojin forearc seamount. The samples do not have the typical enrichment of largeion lithophile elements (such as Ba) that is so characteristic of island arc magmas, including basalts from the Quaternary Izu^Bonin arc [14,15], boninites from the forearc basement [13,18,19] and Neogene sills from the Mariana and Izu^Bo-nin forearcs [7]. They also do not share REE characteristics with backarc basin basalts of the Sumisu Rift [17] or the Shikoku Basin [5,20].…”

Hydrothermal Alteration of Oman Ophiolite Extrusives in Ghuzayn Area

“…The sulfide deposits show semi-graben structures ( Ewart et al (1994), Izu-Bonin fore-arc lavas from Murton et al (1992) and lavas in the East Pacific Rise at 10°30´ from Regelous et al (1999 Intense silicification and argillization S li g h t s il ic if ic a ti o n , e p id o ti z a ti o n a n d s u lf id e d is s e m in a ti o n s S li g h t s il ic if ic a ti o n , e p id o ti z a ti o n a n d s u lf id e d is s e m in a ti o n s S l i g h t s i l i c i f i c a t i o n a n d s u l f i d e d i s s e m i n a t i o n s S li g h t s il ic if ic a ti o n a n d s u lf id e d is s e m in a ti o n s S li g h t s il ic if ic a t io n a n d s u lf id e d is s e m in a t io n s S li g h t s il ic if ic a ti o n a n d rite, sphalerite and rarely pyrrhotite. Chalcopyrite fills the cracks, fractures and interstices between the pyrite grains.…”