In-situ chemical, U–Pb dating, and Hf isotope investigation of megacrystic zircons, Malaita (Solomon Islands): Evidence for multi-stage alkaline magmatic activity beneath the Ontong Java Plateau

Abstract

Previous investigations of pipe-like intrusions of alnöite within northern Malaita (Solomon Islands) have detailed the chemical and isotopic nature of the alnöite and entrained megacrysts/xenoliths. Alnöite emplacement is poorly constrained since available ages include an Ar–Ar date of 34Ma (phlogopite) from a mantle xenolith, and a 206Pb/238U date of 33.9Ma for a zircon megacryst. Hence, we report chemical data, in-situ U–Pb age determinations and Hf isotope compositions for megacrystic zircons recovered from alnöite-derived, ilmenite-rich gravels in the Auluta, Kwainale, and Faufaumela rivers of Malaita.The Zr/Hf ratio (39 to 50) is variable for zircons from Auluta and Faufaumela, whereas it is relatively uniform (40 to 42) in most zircons from Kwainale. Chemical imaging reveals the homogeneous nature for all of the 16 grains analyzed. Trace element compositions obtained by LA-ICP-MS indicate parallel chondrite-normalized REE patterns at variable levels of enrichment; these patterns combined with their low abundances (<1 to 10ppm) of U, Th, and Pb confirm their mantle origin. In-situ U–Pb dating conducted by LA-ICP-MS (n=94 analyses) define a total range in weighted mean (WM) 206Pb/238U ages between ∼35 and ∼52Ma. The zircons from Auluta define a range of WM 206Pb/238U ages between 34.9±2.0Ma and 45.1±2.5Ma (2σ) that correlate negatively with Zr/Hf ratios and total REE contents. Conversely, the chemically homogeneous zircons from Kwainale define a uniform age spectrum yielding a WM 206Pb/238U age of 36.7±0.5Ma (2σ). In-situ Hf isotope analyses (n=30) are uniform and define a WM 176Hf/177Hf value of 0.282933±0.000013 (2σ), which is identical to the previously reported whole rock value for the Malaitan alnöite (0.282939±0.000007). Correlations between ages and chemical compositions (i.e., Auluta zircons), and the uniform Hf isotope compositions are consistent with zircon formation from a common Ontong Java Plateau (OJP)-like mantle undergoing progressive CO2-dominated metasomatism over a ∼17Ma interval. This unique example of prolonged highly alkaline magmatism within an intraplate oceanic setting mimics that defined by cratonic kimberlite provinces and suggests that the Malaitan upper mantle section of the OJP represents an analogy to continental lithosphere.