Abstract
The southern M ariana A rc– T rench 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. T racey S eamount, an extinct submarine volcano about 30 km northwest of G uam, is the southernmost stratovolcano of the M ariana A rc. T racey 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 ( 40 A r/ 39 A r age). An unusual cluster of small basaltic volcanoes, informally termed the A lphabet S eamount V olcanic P rovince ( ASVP ), is found about where the next arc volcano to the southwest of T racey S eamount should have grown. Samples from six of these volcanoes were studied here. At least two ASVP volcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where the ASVP is situated reflects strong extension in the BAB . Construction of northern ASVP volcanoes is controlled by east–west extension accompanying opening of the M ariana T rough. In contrast, southern ASVP volcanoes are affected by north–south extension due to rapid rollback of a narrow slab of P acific seafloor that is subducted along the east–west trending C hallenger D eep segment of the M ariana T rench to the south. ASVP lava compositions are distinct from T racey S eamount and other M ariana A rc lavas, instead showing affinities with M ariana T rough BAB basalt ( BABB s): they are mafic, tholeiitic, low‐ K 2 O and LREE ‐depleted, with low 87 S r/ 86 S r, but show a subtle gradient from somewhat more arc‐like lavas closer to the trench to BABB s farther west. The unusual tectonic setting of ASVP provides 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 small ASVP volcanoes.