TECTONIC NATURE OF CONTINENTAL ARCS CONTROLS THEIR CHEMICAL WEATHERING SIGNATURES

The fluctuation of the Strontium isotope ratios in seawater at 10-100 Myr timescale is conventionally thought to reflect the weathering contributions from the juvenile oceanic and evolved continental crusts. Recent studies show strong correlations between the global length of continental arcs, igneous zircon Hf isotopes, and seawater Sr isotopes since mid-Paleozoic. Particularly, a broad peak in global continental arcs length in the Cretaceous temporally corresponds to a broad trough in ⁸⁷Sr/⁸⁶Sr ratios in the seawater and estimated ⁸⁷Sr/⁸⁶Sr ratios in arc-derived igneous zircons. In this study, I compiled the global database on the tectonic nature of the basements upon which Cretaceous continental arcs were built. The compiled Cretaceous arcs include the North American Cordilleran arcs, central and southern South American Cordilleran arcs, Eastern Asian arcs (Chukchi Peninsula to Japanese islands), and some of the Tethyan arcs. The compilation results show that most Cretaceous arcs were fully or partially built on juvenile basements formed by the accretion of oceanic arcs or terranes of oceanic affinity along the continental margins. Most Cretaceous continental arcs are characterized by juvenile Sr-Hf-Nd isotope signatures, suggesting that the arc igneous rocks inherited the isotope ratios from their basements. Since many arcs were formed in tropical or warm-temperature regions in the Cretaceous, the continental arcs provided influxes of juvenile Sr signatures to the seawater via arc magmatism and subsequent chemical weathering of high-elevation magmatic orogenic belts, promoting element cycling from mantle lithosphere to seawater. Interpretation of seawater Sr isotope ratios should take the contributions of the continental arcs into account.