[1] Less than 30 Ma tephra glasses from the Izu-Bonin (IB) arc consist dominantly of low-K tholeiites that are highly depleted in high field strength elements (HFSE), light rare earth elements (LREE), and large ion lithophile elements (LILE). Collectively, these characteristics indicate minimal contributions from the subducted sediment, a dominantly aqueous fluid subduction component transfer mechanism, and highly depleted mantle sources. Subtle temporal variations are evident throughout the Oligocene-Neogene. Late Oligocene tephras are geochemically heterogeneous, incorporating low-K, LREE-and HFSE-depleted tephras, as well as low-medium-K glasses with flat-to super-chondritic REE patterns. The latter, have higher Nb/Zr, Th/Nb ratios, possibly indicating small contributions from a slab-derived melt. More LILEand LREE-enriched tephras, with higher Nb/Zr and Th/Nb are also evident during the 18-15 Ma interval, coincident with the cessation of backarc spreading in the Shikoku Basin. Extremely HFSE-depleted glasses are only apparent at the IB volcanic front at the end of backarc spreading, indicating a significant delay between the initiation of extension, and the appearance of more depleted HFSE characteristics at the volcanic front. Similar long time frames are required for replenishment, demonstrating long residence time of material within the mantle wedge. Oligocene tephras are geochemically distinct from coeval turbidites and from Eocene to Oligocene boninitic and tholeiitic series lavas, which are overall characterized by lower HREE and Sm/Zr, and commonly have more enriched HFSE characteristics. Lower Sm/Zr are interpreted to be generated in highly depleted source rocks within the mantle wedge, where orthopyroxene is the dominant residual mineral controlling HFSE-REE distribution of the arc magma. Elevated Nb/Zr and Zr/Hf in Eocene-Oligocene boninites and to a lesser extent some tholeiites arise from an additional melt component that is potentially sourced from the subducted slab. LREE-enrichments in Eocene-Oligocene IB boninitic and tholeiitic are in most cases correlated with lower 143 Nd/ 144 Nd, and are consistent with the mobilization of LREE in a slab-derived fluid and/or melt. The Late Oligocene represents a major turningpoint in the geochemical evolution of the IB arc, possibly involving a shift from high-degrees of partial melting (dominantly decompressional) within a shallower, laterally extensive, tensional environment to dominantly fluid-fluxed melting of deeper mantle components. Resultant cooling of the subducted slab resulted in a cessation in slab-melting.Components: 17,227 words, 20 figures, 3 tables, 1 data set.Keywords: Izu-Bonin; arc; tephra; geochemistry; high field strength elements; boninite. Bryant, C. J., R. J. Arculus, and S. M. Eggins, The geochemical evolution of the Izu-Bonin arc system: A perspective from tephras recovered by deep-sea drilling,
: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H2O.
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