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 KO and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low KO contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (ε = +7.4; ε = +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) = 0.9–1.4], uniform initial Sr/Sr ratios (0.7038–0.7042), and positive ε(t) (+5.9 to +6.8) and ε(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) = 1.6–3.3] with uniform initial Sr/Sr ratios (0.7038–0.7042) and slightly low ε(t) (+3.9 to +5.4) and ε(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.

中文翻译：

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龙目岛东巽他弧的形成和成熟


大洋内俯冲带是壳幔物质交换和地壳生长的主要场所；然而，岩浆演化和新生弧转变为地壳较厚的成熟洋弧的过程仍存在争议。我们提供了印度尼西亚东巽他弧龙目岛新生代火山岩的矿物化学、锆石 UPb 年龄和 Hf 同位素数据以及全岩化学和 Sr-Nd-Hf-Pb 同位素数据，以研究其岩石成因和弧成熟度。对龙目岛南部熔岩的 SIMS 锆石 UPb 分析得出渐新世早期约 30 Ma 的年龄。根据年龄和地球化学特征，熔岩可分为三组：来自龙目岛南部的 I 组（始新世晚期）和 II 组（30 Ma）熔岩，以及来自龙目岛北部的 III 组熔岩（3-0 Ma）。熔岩的特点是 KO 和不相容元素含量以及 NdHf 同位素组成随时间逐渐富集。 I 组熔岩是拉斑玄武岩，具有低 KO 含量（0.15–0.17 wt%）和印度-MORB 型稀土元素（REE）模式和 NdHf 同位素组成（ε = +7.4；ε = +14.5），富集大离子亲石元素（例如，Rb、U 和 K）和典型的弧前玄武岩 (FAB) 地球化学特征。第二组熔岩是低 K 玄武质安山岩到英安岩，产生平坦的球粒陨石标准化 REE 模式 [(La/Sm) = 0.9–1.4]、均匀的初始 Sr/Sr 比率 (0.7038–0.7042) 和正 ε(t) ( +5.9 至 +6.8) 和 ε(t) (+13.9 至 +14.4) 值。 III 组熔岩通常为中至高 K 钙碱性玄武岩至安山岩，富含轻质稀土元素 [(La/Sm) = 1.6–3.3]，具有均匀的初始 Sr/Sr 比率 (0.7038–0.7042) 和略低的 ε (t) (+3.9 至 +5.4) 和 ε(t) (+10.5 至 +12.5) 值。 随着时间的推移，从 FAB 到高 K 钙碱性熔岩，不相容元素含量和 NdHf 同位素组成逐渐富集，反映了东巽他弧的成熟。 Sr-Nd-Pb-Hf 同位素混合模型以及微量元素比率（例如 Ba/Th、La/Yb、Th/Yb）表明，越来越多的沉积物被纳入 I 组岩浆的地幔源中。 (~0%) 到第 II 组 (<0.5%) 和第 III 组 (0.5%–1.0%) 熔岩。部分熔融模型表明，I 组和 II 组熔岩的形成可以通过尖晶石橄榄岩的高度（~10%）部分熔融来再现，而 III 组熔岩是由尖晶石橄榄岩的低度（~5%）部分熔融形成的。尖晶石橄榄岩。我们认为，龙目岛岛弧岩浆的演化主要受俯冲板片物质通量的控制，同时随着时间的推移，地壳增厚导致部分熔融程度降低。