From Subduction Initiation to Polarity Reversal: Zircon Age and Geochemical Constraints from Solomon Islands

Intra-oceanic arc’s collision with an oceanic plateau plays a crucial role in the development of complex tectonic setting and induce subduction polarity reversal. Despite several studies and investigations, the origin and timing of subduction initiation in Solomon Island Arc (SIA) is still ambiguous. This study presents first robust zircon U-Pb ages and in-situ Hf isotope data from Choiseul, Santa Isabel (SI) and New Georgia Group (NGG), three major islands of SIA. Magmatic zircons and Hf isotope data from one gabbro sample, geochemically identical to N-MORB with Nb, Ta depletion in spidergram yielded 46 ± 1 Ma, which we decipher as the timing of Stage I magmatism by subduction of Pacific plate and subduction initiation in Choiseul. Six Choiseul andesites gave a mean age ²⁰⁶Pb/²³⁸U of 0.7 Ma, with ε_Hf(t) values from +9 to +15 which represents the youngest crystallization age of Stage II magmatism with typical island arc-like signatures and a depleted mantle source. Detrital zircons from two sand sample yielded a population of mean age ranging from 0.3-0.7 Ma, 10 Ma and 48-46 Ma with ε_Hf(t) values +9 to 15, +11 to +12 and +11 to +14 respectively and third sample has yielded a mean age ²⁰⁷Pb/²⁰⁶Pb 2.6 Ga and 500-1600 Ma with ε_Hf(t) values -8 to +9, probably associated with Australian-type source indicating presence of a continental fragment beneath SIA. 

Similar ages of ca. 2.6 Ma have been obtained from inherited zircons from three gabbroic dyke sample from Santa Isabel with ε_Hf(t) values +1 to +9 whereas one gabbroic dyke sample yields 110 ± 1 Ma, with ε_Hf(t) values +14 to + 16 which we interpret as the basement age of SI.

U-Pb dating of zircons from mafic to felsic rocks along NGG, covered mostly by Quaternary eruptive lavas. The youngest age population indicate Late Pliocene-Pleistocene ²⁰⁶Pb/²³⁸U ages, 2.5-1.5 Ma, interpreted as  zircon crystallization ages of Stage II arc magmatism resulting from subduction of the Solomon Sea plate, as those of Choiseul Andesite. The first U-Pb age from ca. 36.8±0.5 Ma granite on Ghizo Island in New Georgia Group, revealing Late Eocene-aged magmatic zircon. This age represents the magmatic emplacement as the basement of plutonic rock from NGG that has not been reported before.

We conclude that, (i) The Solomon Islands has a Cretaceous basement preserved in SI. (ii) The timing of subduction initiation and Stage I N-MORB type tholeiitic magmatism in SIA is 46 Ma followed by episodic eruptions from the early Eocene to late Eocene. (iii) Oligocene (30-20 Ma) magmatic hiatus, probably the time of subduction polarity reversal from subduction of Pacific plate to subduction of Solomon Sea plate. (iv) Stage II island arc magmatism initiated at 20-18 Ma in NGG to youngest emplacement age of Pliocene to Pleistocene in Choiseul as well as in NGG. (v)Abundant Archean zircons are present in samples from all three islands, indicate presence of micro-continent beneath Islands of Solomon. We are still working on the whole rock isotopic analysis to better constrain the tectonic and magmatic evolution of SIA.