40-MILLION YEARS OF MAGMATIC EVOLUTION IN THE MARIANA ARC - THE TEPHRA GLASS RECORD

Tephra glasses retrieved from 10 Deep Sea Drilling Project (DSDP) core s around the Mariana are system make up a remarkable record of explosi ve volcanism in the Marianas over the past 40 m.y. Major element compo sitions for approximately 1800 tephra glasses, from basalt through rhy olite, are reported and used to examine the nature and history of this activity. Three maxima of volcanic explosivity, presumably related to especially vigorous volcanism in the Mariana are, are identified, wit h the biggest maximum at around 18-11 Ma and two other maxima at 35-24 and 6-0 Ma; the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western Pacific arcs. Explosive are volcanism has been predominantly tholeiitic since shortly after a re inception; no boninitic glasses were found. The tephra glasses belo ng to the low- to medium-K suite, except during an enigmatic phase of medium- to high-g explosive volcanism during the late Miocene (11-7 Ma ). Even though Mariana tephra glasses are largely similar in compositi on to Mariana are lavas, the tephra show a much larger compositional r ange than that found for subaerial Mariana are lavas, which totally la ck dacitic to rhyolitic volcanic products. The tephra glasses define a bimodal population in terms of silica content, with a pronounced mini mum, or ''Daly gap,'' around 65-66% SiO2. Mariana tephra glasses are f ractionated, with the least evolved glass being Fe-rich and having a m agnesium number (Mg #) (100Mg/Mg+Fe2+) of 55. Basaltic tephra glasses contain < 16% Al2O3 (average 14.3% Al2O3) and are not high-alumina bas alts; this contrasts with the observation that modern Mariana are lava s contain 15-21% (average 17.4%) Al2O3. The high-alumina basalt lavas of the Mariana are probably reflect plagioclase accumulation and not l iquid compositions. All tephra glasses plot near low-pressure cotectic s and reaction curves on the subprojection olivine-clinopyroxene-quart z; mafic and felsic samples define distinct trends. The mafic trend re flects fractional crystallization of mantle-derived basaltic magma, wh ereas the felsic trend may be due either to anatexis of Mariana are cr ust or to fractionation of mafic melts. The tephra glass data reinforc e the model that the magmatic evolution bf the Mariana are has been do minated by low-pressure fractionation, perhaps accompanied by anatexis . Episodic changes in melting regime to generate Miocene potassic teph ra may be related to changing mantle sources and processes related to episodes of back are basin spreading. These episodic changes are super imposed on a long-term increase in potassium that reflects progressive metasomatism of the mantle source. Long-term increases in K2O content s for Mariana are magmas inferred from the tephra glass record are 0.0 04 wt % m.y.(-1) (mafic), 0.011 wt % m.y.(-1) (intermediate), and 0.02 3 wt % m.y.(-1) (felsic).