HIGH-MG ROCKS OF A TONGA (TOFUA) ARC SEAMOUNT: IMPLICATIONS FOR EARLY ARC HISTORY

Petrologically immature intra-oceanic island arcs are dominated by basalts and basaltic andesites displaying varied extent of fractionation. Rocks from an arc’s earliest magmatic history are seldom exposed. A 474 Ka old seamount at the northernmost end of the Tofua Arc, near the westward bend of the Tonga Trench, rises to 1700 meters depth from surrounding 2200 meter deep sea floor. It may be the northernmost (and youngest ?) Tofua Arc volcano. It lies at the intersection of the Tofua Arc and the trend of the east limb of the Mangatolu Triple Junction which erupts MORB-like lavas. Seamount lavas are olivine-phyric basalts with higher Mg, Cr, Ni; and lower Si, alkali metals, LILE and HFSE than low-K arc tholeitiic basalts, basaltic andesite, and low-K high-Ca dacites of the Tofua Arc. The mantle source was relatively depleted compared to MORB-source, as for most arc magmas, but had less, or none, of a subduction component. Lavas include vitrophyres, aphyric, and sparsely olivine-phyric, high-Mg to picritic basalts (e.g., glasses with 8.5% MgO and rocks with up to 16.2% MgO). The latter have OL phenocrysts (Fo93 cores) which would have been in equilibrium with melts having Mg #=76. These data indicate the role of "primitive" mantle-derived parental melts in early stages of Tofua Arc volcanism. The Mg-rich lavas indicate a depleted supra-subduction zone mantle in the Tonga forearc and give insight to petrologic processes operative in the earliest stages of island arc evolution. Tofua Arc picritic magmas are similar to western Pacific high-Mg arc lavas from Aoba, Vanuatu (New Hebrides) arc; New Georgia Group, Solomon Islands; and a nascent arc at Hunter Fracture Zone.