ABSTRACT

Coexisting alkalic and tholeiitic basalt lavas has been identified in a seamount chain located near the Australian–Antarctic spreading ridge. The KR1 Seamount Trail (KR1 ST) is a series of volcanic seamounts extending to the southeast in the spreading direction of the Australian–Antarctic Ridge (AAR). We herein report Sr, Nd and Pb isotopic compositions and (U–Th)/He and K–Ar geochronology for dredge samples from the KR1 ST in order to evaluate mantle processes and the role of enriched components for alkalic to tholeiitic magma generation in this region. The KR1 ST is a medium-sized seamount chain that extends for ~60 km, has a maximum height of ~1600 m above the seafloor, and consists of alkaline basalts and tholeiites with formation ages of ~0.4 Ma to ≤1.3 Ma. The isotopic characteristics of the alkaline basalts (²⁰⁶Pb/²⁰⁴Pb = 19.52–19.91; ⁸⁷Sr/⁸⁶Sr = 0.7030–0.7033; ¹⁴³Nd/¹⁴⁴Nd = 0.5128–0.5130) from the KR1 ST reflect a dominant ‘PREMA (or FOZO)’ mantle component represented by radiogenic Pb and mildly enriched Sr and Nd isotopic compositions. On the other hand, the weak PREMA (FOZO)-affinity (²⁰⁶Pb/²⁰⁴Pb = 18.89–18.93; ⁸⁷Sr/⁸⁶Sr = 0.7028–0.7029; ¹⁴³Nd/¹⁴⁴Nd = ~0.5130; ³He/⁴He = 7.64 ± 0.13 (R/R_A)) coupled with their enriched mid-ocean ridge basalt (E-MORB) characteristics of tholeiites from the KR1 ST largely overlap with the KR1 MORB composition. The potential source materials for the alkaline basalts are considered to be ancient, recycled oceanic crust (i.e. eclogite) as well as sub-KR1 depleted MORB mantle (DMM). Whereas the main source materials for the KR1 ST tholeiites are presumed to be the DMM-dominant lithology with minor recycled material. We interpret the KR1 ST as a submarine hotspot chain that was formed by asthenospheric upwelling and spreading processes that delivered fertile blobs of recycled oceanic crust to the sub-KR1 region. The fundamental reason for sub-KR1 upper mantle enrichment might be attributed to a mantle plume event that possibly occurred prior to the formation of the KR1 ST.

摘要

在位于澳大利亚-南极扩张脊附近的海山链中发现了共存的碱性玄武岩和拉斑玄武岩熔岩。KR1 海山小径 (KR1 ST) 是一系列火山海山，沿澳大利亚-南极海脊 (AAR) 的扩展方向向东南延伸。我们在此报告了来自 KR1 ST 的疏浚样品的 Sr、Nd 和 Pb 同位素组成以及 (U-Th)/He 和 K-Ar 年代学，以评估地幔过程以及富集组分在该过程中碱性到拉斑岩浆生成中的作用。地区。KR1 ST 是一条中等大小的海山链，延伸约 60 公里，海底最大高度约 1600 米，由碱性玄武岩和拉斑岩组成，形成年龄为约 0.4 Ma 至≤1.3 Ma。碱性玄武岩的同位素特征（²⁰⁶Pb/ ²⁰⁴ Pb = 19.52–19.91；⁸⁷ Sr/ ⁸⁶ Sr = 0.7030–0.7033；来自 KR1 ST 的¹⁴³ Nd/ ¹⁴⁴ Nd = 0.5128–0.5130) 反映了主要的“PREMA（或 FOZO）”地幔成分，以放射性铅和轻度富集的 Sr 和 Nd 同位素组成为代表。在另一方面，弱PREMA（FOZO） -亲和（²⁰⁶ PB / ²⁰⁴ PB = 18.89-18.93; ⁸⁷ SR / ⁸⁶ SR = 0.7028-0.7029; ¹⁴³钕/ ¹⁴⁴的Nd =〜0.5130; ³他/ ⁴赫= 7.64 ± 0.13 (R/R _A)) 加上来自 KR1 ST 的拉斑岩的富集大洋中脊玄武岩 (E-MORB) 特征，与 KR1 MORB 的成分在很大程度上重叠。碱性玄武岩的潜在来源被认为是古老的、再生的洋壳（即榴辉岩）以及亚 KR1 耗尽的 MORB 地幔（DMM）。而 KR1 ST 拉斑沸石的主要来源材料被认为是 DMM 主导的岩性，具有少量回收材料。我们将 KR1 ST 解释为海底热点链，它是由软流圈上升流和扩散过程形成的，这些过程将循环的海洋地壳的肥沃团块输送到亚 KR1 区域。sub-KR1 上地幔富集的根本原因可能归因于可能发生在 KR1 ST 形成之前的地幔柱事件。