Arc building and maturation of the Lombok Island, East Sunda Arc

Abstract

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.