Geochemistry, Tectonics and the Peridotites of the Northeastern Solomon Islands

Ian J. Parkinson1, R. J. Arculus1, E. McPherson1, R. A. Duncan2, and R. L. Stanton3

1GEMOC, Department of Geology, Australian National University, ACT 0200, AUSTRALIA

2College of Oceanography, Oregon State University, Corvallis, OR 97331, USA

3Department of Geology & Geophysics, University of New England, Armidale, NSW 2351, AUSTRALIA

The Solomon Islands have a complicated history of subduction zone polarity reversals, oceanic plateau collision and obduction, and active arc magmatism. In the Paleogene, subduction occurred of the Pacific Plate westward beneath an eastward-facing arc ("the "Vitiaz" arc-trench system (VATS)). During the Neogene, collision, docking and obduction of the Ontong Java Plateau (OJP) occurred along the Solomons portion of this arc-trench system, triggering polarity reversal. The OJP is one of the largest flood basalt outpourings known, and at least part of this magmatic pulse occurred during the so-called "Cretaceous Super-Plume" event. The Deep Sea and Ocean Drilling Projects have recovered two major age groupings of pillow basalts (~120 and 90 Ma) from relatively shallow penetrations at geographically widely distributed locations on the Plateau. In the past few years, international collaborative efforts together with the Solomons Survey have targeted the northeastern Solomons (e.g., Malaita, Makira, and Santa Isabel) as a region where potentially more extensive (spatial and temporal) sequences of the OJP might be sampled compared with the drilling recovery. The collision event also seems to have exhumed deeper portions (e.g., plutonic and upper mantle) of the OJP and the old forearc of the VATS.

Pillowed basalt slices of the Ontong Java Plateau (OJP) are exposed in extensive river sections on the northeastern flanks of the island of Santa Isabel (SI). New 39Ar/40Ar results for the basalts give ages of 122 Ma and 90 Ma consistent with the two major groups recovered by ocean drilling of the OJP. On SI, the older basalts outcrop southwest of the younger sequence. Within the 2 major age-groupings however, we recognise 4 geochemical groups compared with 3 groupings recognised in the drilling recovery - these occur as discrete stratigraphic groups in the field. For the drilled recovery, a relatively tight grouping of eNd and 87Sr/86Sr exists (+4 to +6, and 0.7035 - 0.7042 respectively, J. Mahoney pers. comm.) which are less and more radiogenic than Pacific MORB respectively. Preliminary data for the samples from SI range to slightly higher eNd of ~ +7. On Vitora island off the southeastern tip of SI, a sample of the plutonic sequences of the Plateau are exposed as troctolites (~Fo81, An75) and gabbros.

A wide range of ages have been determined for basalts outcropping on the southwestern flank of SI - including ~ 63 and 45 Ma sequences, and transitional MORB-backarc pillow basalts with an age of 34 Ma at Tanabusu along the west coast of SI. A NW-SE-striking major fault running most of the length of SI (Kaipito-Korigole Fault) is clearly a terrane boundary.

Interleaved with the volcanic sequences on SI are fault-bound slices of ultramafic rocks, some of which are virtually unserpentinised. Fresh ultramafic blocks also occur in fault-bounded serpentine diapirs on San Jorge (off the south coast of SI), analogous to occurrences in the Izu-Bonin-Mariana forearc. Whole rock trace element and ƒO2 calculations indicate that individual fault slices contain ultramafic rocks with affinities to variously mid-ocean ridge, island arc, or plume-related settings. A melt-metasomatised abyssal peridotite thrust sheet outcrops on southeastern Choiseul (Siruka Ultramafics) to the northwest of SI.

We conclude that the mantle section of the OJP and fragments of a subarc mantle - possibly formed in a forearc setting during the pre-Pliocene, westerly-directed subduction of the Pacific Plate - outcrop on SI and San Jorge.