Alkalic to tholeiitic magmatism near a mid-ocean ridge: petrogenesis of the KR1 Seamount Trail adjacent to the Australian-Antarctic Ridge

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.

研究在澳南极扩张脊（Australian–Antarctic Spreading Ridge, AAR）附近的一条海山链中，识别出共存的碱性玄武岩（alkalic basalt）与拉斑玄武岩（tholeiitic basalt）熔岩。KR1海山链（KR1 Seamount Trail, KR1 ST）是沿澳南极扩张脊（AAR）扩张方向向东南延伸的一系列火山海山。本文报道了KR1 ST拖网采样样品的锶（Sr）、钕（Nd）、铅（Pb）同位素组成，以及（铀-钍）/氦[(U–Th)/He]和钾-氩（K–Ar）地质年代学数据，旨在评估该区域的地幔过程，以及富集组分在碱性至拉斑质岩浆生成过程中的作用。KR1 ST属于中型海山链，延伸长度约60 km，最大相对海底高度约1600 m，由形成年龄介于~0.4 Ma至≤1.3 Ma的碱性玄武岩与拉斑玄武岩组成。KR1 ST碱性玄武岩的同位素特征（²⁰⁶Pb/²⁰⁴Pb = 19.52–19.91；⁸⁷Sr/⁸⁶Sr = 0.7030–0.7033；¹⁴³Nd/¹⁴⁴Nd = 0.5128–0.5130）反映出以放射成因铅、弱富集锶和钕同位素组成为特征的主导“普雷马地幔端元（PREMA，或称FOZO）”信号。与之相对，KR1 ST拉斑玄武岩表现出弱的PREMA（FOZO）亲和性（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)），且具备富集型洋中脊玄武岩（Enriched Mid-Ocean Ridge Basalt, E-MORB）的地球化学特征，其同位素组成与KR1洋中脊玄武岩（KR1 MORB）的组分高度重合。研究认为，KR1 ST碱性玄武岩的潜在源区物质包括古老的再循环洋壳（即榴辉岩）以及KR1下方的亏损型洋中脊地幔（Depleted MORB Mantle, DMM）；而KR1 ST拉斑玄武岩的主要源区则以亏损型洋中脊地幔为主，仅含少量再循环物质。我们将KR1 ST解释为一条海底热点链，其形成机制为软流圈上涌与扩张作用将再循环洋壳的富集团块输送至KR1下方区域。KR1上方地幔富集的根本原因，可能归因于KR1 ST形成之前发生的一次地幔柱事件。