How theMarianaVolcanicArc ends in the south

Abstract

AbstractThe southernMarianaArc–Trench system is rapidly deforming, resulting in unusual interactions between arc and back‐arc basin (BAB) magmatic systems. New geochemical data for volcanoes in this region are presented and explored.TraceySeamount, an extinct submarine volcano about 30 km northwest ofGuam, is the southernmost stratovolcano of theMarianaArc.Tracey is built about 125 km above the subducted slab and has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as 0.527 ± 0.023 Ma (40Ar/39Ar age). An unusual cluster of small basaltic volcanoes, informally termed theAlphabetSeamountVolcanicProvince (ASVP), is found about where the next arc volcano to the southwest ofTraceySeamount should have grown. Samples from six of these volcanoes were studied here. At least twoASVPvolcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where theASVPis situated reflects strong extension in theBAB. Construction of northernASVPvolcanoes is controlled by east–west extension accompanying opening of theMarianaTrough. In contrast, southernASVPvolcanoes are affected by north–south extension due to rapid rollback of a narrow slab ofPacific seafloor that is subducted along the east–west trendingChallengerDeep segment of theMarianaTrench to the south.ASVPlava compositions are distinct fromTraceySeamount and otherMarianaArc lavas, instead showing affinities withMarianaTroughBABbasalt (BABBs): they are mafic, tholeiitic, low‐K2O andLREE‐depleted, with low87Sr/86Sr, but show a subtle gradient from somewhat more arc‐like lavas closer to the trench toBABBs farther west. The unusual tectonic setting ofASVPprovides a unique perspective on how different arc magma batches reflect melting of mantle with strong compositional gradients which are mixed together beneath long‐lived arc volcanoes but here rise to form scattered smallASVPvolcanoes.