A PETROLOGIC REINVESTIGATION OF THE ADAK VOLCANIC CENTER, CENTRAL ALEUTIAN ARC, ALASKA

The Adak volcanic center is located in the central part of the Aleutia n arc and consists of three main volcanic vents. Andrew Bay Volcano, t he oldest center, has been mostly removed by erosion. The next younges t vent, Mount Adagdak, was built in three major volcanic stages wherea s Mount Moffett, the largest volcanic edifice, consists of a main cone and a parasitic cone each with several magmatic phases. Adak is uniqu e compared to other modem Aleutian volcanic centers in that it contain s two xenolith suites (Conrad and Kay, 1984; Debari et al., 1987). One suite consisting predominantly of mafic xenoliths occurs on Mount Mof fett whereas an assemblage of ultramafic and mafic xenoliths is found on Mount Adagdak. Lavas erupted at Adakspan the compositional range fr om 48.4 to 65.0 wt.% SiO2 and are characterized by significant variati ons in Al2O3, MgO, Sr, Ni and Cr. On Harker diagrams, this variability produces compositional trends with significant scatter. The Adak suit e has total REE contents that vary from 32 to 154 ppm but do not corre late systematically with silica. (La/Yb)n ratios range from 2.41 to 21 .72 with the majority of lavas between 2.41 and 6.06. On process ident ification diagrams, the Adak suite plots as steeply sloping trends tha t contrast with the horizontal patterns of most other Aleutian centers . Measured isotopic ranges are large and nearly equal to those for the entire arc. Although they span similar silica ranges, subtle geochemi cal and isotopic differences distinguish the different volcanic vents of Adak. On Mount Moffett, a geochemically and isotopically distinct g roup of andesites (55.5-57.9% SiO2), the mafic andesites, occur on its NE flank. These lavas have elevated MgO, Ni and Cr but are depleted i n Al2O3 relative to other Mount Moffett andesites with similar silica. They also have more heterogeneous REE abundances and isotopic ratios than most of the other andesites. Significant compositional difference s exist between Adak and the other volcanic centers of the central Ale utian arc. Although these differences are characteristic of all geoche mical systems, they are greatest for major and rare-earth elements and isotopic ratios. The lack of coherent relationships on major- and tra ce-element Harker diagrams, the isotopic variability, as well as the s teeply sloping trends on REE process identification diagrams suggest t hat the Adak volcanic suite was not formed predominantly by closed-sys tem crystal fractionation, but must be the product of a complex open-s ystem process(es). The significant isotopic variability displayed by t he suite suggests that contamination by an isotopically distinct conta minant must also have been an important petrologic component in the ev olution of the suite. REE data are also suggestive of a role for magma mixing. Such a complex petrologic evolution is consistent with an imm ature lithospheric plumbing system. Based on REE systematics, the xeno lith suites of Adak cannot, as previously proposed, be related to the host lavas or the rest of the Adak suite through crystal fractionation schemes. Rather they are probably accidental fragments derived from v arious depths along lithospheric conduits. In light of their relation to xenolith-bearing units, the mafic andesites of Adak presumably repr esent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock. They are, therefore, characteristic of im mature volcanic centers and unlikely to be related directly to the mag matic processes responsible for the generation of primary arc magmas. Because of the close proximity of the vents and the subtle composition al differences between their lavas, the Adak volcanic center was proba bly supplied by a single, deep lithospheric plumbing system that fed s eparate crustal magma chambers. The absence of historic volcanic activ ity on Adak suggests this plumbing system was abandoned before complet e conduit development. This decline in magmatism may reflect a re-adju stment of volcano spacing within this part of the Aleutian arc.