Slab-controlled elemental–isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy
Slab-controlled elemental–isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy
复制标题
俯冲起始岩浆作用期间板片控制的元素同位素富集和弧前化学地层学的变化
DOI：
10.1016/j.epsl.2020.116217
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发表时间：
2020
影响因子：
5.3
通讯作者：
Chi-Yue Huang
中科院分区：
文献类型：
--
作者：
Mengming Yu;Yildirim Dilek;Graciano P. Yumul Jr;Yi Yan;Carla B. Dimalanta;Chi-Yue Huang
形成于弧前环境和Izu-Bonin-Mariana(IBM)原地弧前环境的超俯冲带蛇绿岩表现出从正常的大洋中脊玄武岩(MORB)向博尼基亲和作用的转化，这被广泛解释为俯冲起始岩浆作用(SIM)的化学地球动力学演化规律。然而，俯冲启动所产生的弧前层序并不一定总是包括波尼质岩石。本文以菲律宾西部巴拉望中部和安奈(CPA)蛇绿岩为研究对象，重建了不含蛇绿岩亲缘关系的弧前化学地层学，以显示SIM熔体演化的多样性。MORB型安奈辉长岩的锆石U-Pb定年显示，33~23 Ma之间有10 My的海底展布。这些年龄与全岩元素和Nb-Hf同位素特征([La/Yb]N=0.19~3.45；Nb/Yb=0.0 7~5.2 4；εND=2.1~10.2；εHf=11.2~2 4.1)一起表明，安奈蛇绿岩亏损的MORB型源区早期板片流体的熔融作用，后期加入埃达克质熔体和富Nb板岩熔体，使安奈蛇绿岩亏损的地幔氧同位素(DMM18O=1.6 2~10.1‰)和全岩元素及Nb-Hf同位素特征([La/Yb]N=0.19~3.45；Nb/Yb=0.0 7~5.2 4；εND=2.1~10.2；εHf=11.2~2 4.1)显示了安奈蛇绿岩亏损的地幔来源。这种独特的地幔演化，加上安奈镁铁质岩石样品在给定的εNd处高于同时代的南中国海的ε-Hf，不支持安奈蛇绿岩是南海的一个碎片的解释。然而，33~23 Ma的Amnay蛇绿岩可能是34 Ma弧前型中央巴拉望蛇绿岩的年轻部分，两者(即CPA蛇绿岩)都代表了西北苏鲁海的陆上碎片。我们认为，34-23 Ma CPA蛇绿岩和苏鲁海西北部构成了晚渐新世-中中新世岛弧--卡加延岭-苏鲁海东南部的弧后盆地，伴随着34 Ma以来白垩纪至始新世原南海的俯冲。CPA蛇绿岩缺乏玻安岩，但具有从正常到富MORB亲和力的地球化学演化过程，其熔体演化与IBM SIM不同。我们将SIM的差异和弧前化学地层学的变化归因于原始南海较年轻的陆源沉积盖层较厚的板块与太平洋板块较薄的远洋沉积盖层的较老板块的俯冲作用。
Supra-subduction zone (SSZ) ophiolites formed in forearc settings and the Izu–Bonin–Mariana (IBM) in-situ forearc show conversions from normal mid-ocean ridge basalt (MORB) to boninitic affinities, which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism (SIM). Nevertheless, forearc sequence generated by subduction initiation need not always include boninitic rocks. Here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the Central Palawan and Amnay (CPA) ophiolites of the west Philippines to show diversity in melt evolution of SIM. Zircon U–Pb dating on the MORB-type Amnay gabbros reveal a 10 my of seafloor spreading from 33 to 23 Ma. These ages together with the progressively enriched zircon in-situ oxygen isotopes (δ 18 O= 1.62–10.1‰) and whole rock elemental and Nd–Hf isotopic signatures ([La/Yb] N= 0.19–3.45; Nb/Yb= 0.07–5.24; εNd= 2.1–10.2; εHf= 11.2–24.1) indicate that the depleted MORB mantle (DMM)-type source of the Amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and Nb-enriched slab melts in the later stage. This distinct mantle evolution together with higher εHf at a given εNd of the Amnay mafic samples than the contemporaneous South China Sea (SCS) oceanic samples do not support previous interpretation that the Amnay ophiolite was a fragment of the SCS. However, the 33–23 Ma Amnay ophiolite can be the younger part of the 34 Ma forearc-type Central Palawan ophiolite, and both (ie the CPA ophiolites) represent on-land fragments of the NW Sulu Sea. We propose that the 34–23 Ma CPA ophiolites and the NW Sulu Sea constitute forearc basin of the late Oligocene–middle Miocene island arc–backarc basin duo of the Cagayan Ridge and SE Sulu Sea, associated with subduction of the Cretaceous to Eocene Proto-SCS since 34 Ma. The CPA ophiolites lack boninite but show geochemical progression from normal to enriched MORB affinities, whose melt evolution differs from that of the IBM SIM. We attribute the differences in SIM and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the Proto-SCS versus an older slab with thin pelagic sediment cover of the Pacific Plate.