| 摘要: | 摘要
俯冲极性反转(Subduction Polarity Reversals, SPRs)在岛弧岩浆记录中留下了独特的
地球化学印记,成为记录岩浆源演化及控制岩浆成因构造过程的可靠档案。自晚白垩世以
来,太平洋板块与印澳板块之间的相互作用导致了一系列长短不一的俯冲事件、微板块的
复杂参与以及西南太平洋地区伸展盆地的反复形成。安托亚瓦高原(Ontong Java
Plateau, OJP)与所罗门群岛弧(Solomon Islands Arc, SIA)的碰撞及其与微陆块的相
互作用进一步加剧了该地区的构造复杂性。尽管已有研究记录了极性反转与碰撞事件,但
SIA在西南太平洋的确切起始时间与成因仍不明确。本研究通过对SIA岩浆样品的锆石U-Pb
定年、Hf同位素数据、全岩地球化学以及Sr-Nd-Hf-Pb同位素组成的综合分析,识别出三
个不同阶段的岛弧岩浆作用。
阶段1(约46 Ma) 包括来自圣伊莎贝尔岛(Santa Isabel)和乔伊索尔岛(Choiseul)
的低钾拉斑玄武质(tholeiitic, TH)岩石。这些岩石表现出N-MORB型稀土元素(REE)
分布特征,其全岩同位素比值表明其来源于原始地幔。阶段1样品在Hf–Nd同位素空间中显
示出太平洋地幔域的特征,被解释为在东北向俯冲的印澳板块之上,太平洋地幔楔部分熔
融所形成的岛弧岩浆。结合区域地质证据,该阶段被认为标志着SIA的第一次俯冲极性反
转(SPR),可能早在约65 Ma即已开始,与太平洋与澳大利亚板块运动方向变化有关。
阶段2(约35 Ma) 表现为来自吉佐岛(Ghizo)和新乔治亚群岛(New Georgia Group,
NGG)的低钾钙碱性(calc-alkaline, CA)花岗岩的侵位。这些花岗岩显示出相对未分异
的REE分布及典型的岛弧地球化学特征,在蛛网图上表现为高场强元素(HFSEs,如Nb、Ta
和Ti)的显著亏损。其同位素特征显示出与印度型地幔源的亲缘关系。全岩同位素组成及
锆石εHf(t)值(+16.9至+13.2)表明其来源于经由上地幔上涌而生成的原始岩浆,该过
程发生在东北向板片断离事件之后。随后,路易西亚德高原(Louisiade Plateau)到达
Pocklington海槽,引发了向西南方向的俯冲,即第二次俯冲极性反转事件。
阶段3(约19 Ma至0.7 Ma) 包括来自新乔治亚群岛(NGG)和乔伊索尔岛的中至高钾钙碱
性(CA)岩石。这些岩石具有典型的岛弧地球化学特征,即大离子亲石元素(LILEs)和
轻稀土元素(LREEs)富集,同时高场强元素(HFSEs)亏损。其εNd(t)值(+7.2至+5.8
)和εHf(t)值(+13.3至+10.4)较前期阶段更低,表明地幔源区的异质性增强。我们推
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断,第三次SPR发生于约19 Ma,在约25–20 Ma安托亚瓦高原(OJP)抵达北所罗门海沟(
North Solomon Trench, NST)之后,触发了OJP与印度型地幔域之间的相互作用。综合来
看,这三个阶段记录了多次重复发生的SPR事件,并限定了OJP–SIA碰撞的时间,该碰撞引
发了最新一次俯冲极性反转。
;Abstract
Subduction polarity reversals (SPRs) leave distinctive geochemical imprints in arc magmatic
records, serving as robust archives for tracing magma source evolution and the tectonic processes
controlling magma genesis. Since the Late Cretaceous, interactions between the Pacific and Indo-
Australian plates have resulted in alternating long- and short-lived subduction episodes, complex
involvement of microplates and the repeated formation of extensional basins across the southwest
Pacific. The collision of the Ontong Java Plateau (OJP) with the Solomon Island arc (SIA) and its
interaction with micro continental fragments have further complicated the tectonics. Although the
polarity reversal and collision events have been documented, the precise timing and origin of the
SIA in the southwest Pacific remain ambiguous. This study reports zircon U-Pb ages, Hf isotopic
data, and whole-rock geochemistry as well as Sr-Nd-Hf-Pb isotopic compositions of magmatic
samples from the SIA, revealing three distinct stages of arc magmatism.
Stage 1 (~ 46 Ma) comprises a set of low-K tholeiitic (TH) rocks from Santa Isabel and Choiseul.
These rocks exhibit N-MORB-like rare earth element (REE) patterns and their whole-rock Sr-Nd-
Hf-Pb isotopic ratios suggest derivation from a depleted mantle source. Stage 1 samples display an
affinity to the Pacific mantle domain in a Hf-Nd and 206Pb/204Pb versus 208Pb/204Pb isotopic space,
and are interpreted as arc magmas generated by partial melting within the Pacific mantle wedge
above the northeast-directed subduction of the Indo-Australian plate at the Pocklington Trough.
Together with regional evidence, this stage is interpreted as marking the first reversal from SW-
directed to NE-directed subduction, which may have initiated as early as ~65 Ma, accompanied by
the changes in the Pacific and Australian plate motions. Stage 2 (~ 35 Ma) is marked by the
emplacement of low-K calc-alkaline (CA) granites from Ghizo, New Georgia Group (NGG). These
granites display relatively unfractionated REE patterns and arc-like geochemical signatures,
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marked by depletion in high field strength elements (HFSEs) such as Nb, Ta, and Ti in spider
diagram. Their isotopic signatures show affinity with an Indian-type mantle source. Whole-rock
isotopic compositions, together with zircon εHf(t) values ranging from +16.9 to +13.2, suggest
derivation from a juvenile crustal source, generated by asthenospheric upwelling following a NE-
directed slab break-off event. This event was subsequently followed by the arrival of the Louisiade
Plateau at the Pocklington Trough, which triggered a southwest-directed subduction, the second
reversal in subduction polarity.
Stage 3 (~19 Ma to 0.7 Ma) comprises a set of medium- to high-K CA rocks from the NGG and
Choiseul. These rocks exhibit typical arc-like geochemical signatures characterized by enrichment
in large ion lithophile elements (LILEs) and light REEs, along with depletion in high field strength
elements (HFSEs). Their less positive εNd(t) (+ 7.2 to + 5.8) and εHf(t) (+ 13.3 to + 10.4) values
relative to earlier stage of magmas suggest involvement of enriched component. We infer a third
SPR occurred ~ 19 Ma, following the ~25-20 Ma arrival of the OJP at the NST, which initiated
interactions between OJP and Indian-type mantle domains. Collectively, these stages document
recurrent SPR events and constrain the timing of the OJP–SIA collision that triggered the latest
SPR. |
