Petrology, Geochemistry and Geochronology of Kaua‘i Lavas over 4·5 Myr: Implications for the Origin of Rejuvenated Volcanism and the Evolution of the Hawaiian Plume

Abstract

Kaua'i lavas provide a unique opportunity to examine over 4·5 Myr of magmatic history at one location along the Hawaiian chain. New field, geochronological, petrological and geochemical results for a large suite of shield, post-shield and rejuvenated lavas are used to examine models for the origin of rejuvenated volcanism, and to evaluate the composition and structure of the Hawaiian plume. Kaua'i has the most voluminous (∼58 km3 based on new field and water well interpretations) and longest-lived suite of rejuvenated lavas (∼2·5 Myr) in Hawai'i. New K–Ar ages and field work reveal an ∼1 Myr gap (3·6–2·6 Ma) in volcanism between post-shield and rejuvenated volcanism. Isotopic and trace element ratios, and modeling of major elements of Kaua'i's rejuvenated lavas require low-degree melting (0·02–2·6%) at ∼1525 ± 10°C and 3·5–4·0 GPa of a heterogeneous, peridotitic plume source. High-precision Pb, Sr, Nd and Hf isotopic, and inductively coupled plasma mass spectrometry trace element data show substantial source variations with a dramatic increase in the depleted component in younger lavas. Some shield, post-shield and rejuvenated lavas (4·3–0·7 Ma) have high 208Pb*/206Pb* (radiogenic Pb produced since the formation of the Earth) values (>0·947) indicative of Loa-type compositions, the first reported Loa values in rocks >3 Ma, questioning previous models for the emergence of the Loa component in Hawaiian lavas. The timing, long duration, temporal variation in rock types and voluminous pulse of rejuvenated volcanism (58 km3), and the synchronous eruption of compositionally similar rejuvenated lavas, indicating tapping of common components along 350 km of the Hawaiian chain, are inconsistent with current models for this volcanism. Combining the lithospheric flexure and secondary zone of melting models provides a physical mechanism to initiate and focus the melting at shallower levels within the plume (flexural uplift) with a means to extend the duration of Kōloa volcanism at higher degrees of partial melting.