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/RA)) 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.在澳大利亚-南极扩张脊（Australian–Antarctic Spreading Ridge, AAR）附近的海山链中，已发现共存的碱性玄武岩与拉斑玄武岩熔岩。KR1海山步道（KR1 Seamount Trail, KR1 ST）是沿澳大利亚-南极扩张脊（AAR）扩张方向向东南延伸的一系列火山海山。本文报道了采自KR1 ST的拖网样品的锶（Sr）、钕（Nd）、铅（Pb）同位素组成，以及(U–Th)/He和钾-氩（K–Ar）年代学数据，以期评估该区域的地幔作用过程，以及富集组分在从碱性到拉斑系列岩浆生成过程中的作用。KR1 ST为中等规模海山链，延伸长度约60 km，最大高出海底约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、轻度富集的Sr和Nd同位素组成为代表的主导型"PREMA（或FOZO）"地幔端元。与之相对，KR1 ST拉斑玄武岩具有弱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/RA)），同时兼具富集型洋中脊玄武岩（Enriched Mid-Ocean Ridge Basalt, E-MORB）的特征，其同位素组成与KR1洋中脊玄武岩（MORB）基本重合。研究认为，KR1 ST碱性玄武岩的潜在源区物质为古老的再循环洋壳（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物质则以DMM为主，仅含少量再循环物质。本文将KR1 ST解读为受软流圈上涌与扩张作用控制的海底热点链，该过程将富含再循环洋壳的物质输送至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。