Magmatism in the Bransfield Basin: Rifting of the South Shetland Arc?
Magmatism in the Bransfield Basin: Rifting of the South Shetland Arc?
复制标题
布兰斯菲尔德盆地的岩浆活动：南设得兰岛弧的裂谷？
DOI：
10.1029/2004jb003046
复制
发表时间：
2004
影响因子：
--
通讯作者：
P. Stoffers
中科院分区：
文献类型：
--
作者：
S. Fretzdorff;T. Worthington;K. Haase;R. Hékinian;L. Franz;R. Keller;P. Stoffers
布兰斯菲尔德盆地是一个活动伸展的边缘盆地，将不活跃的南设得兰弧与南极半岛北部隔开。与裂谷有关的火山活动在整个布兰斯菲尔德盆地中部普遍存在，但更广泛的东部布兰斯菲尔德盆地以前没有取样。从东部次盆地中回收的熔岩形成三个不同的群：(1)Bransfield群相对于正常的大洋中脊玄武岩(NMORB)具有中等程度的大离子亲石元素(LILE)富集性；(2)Gibbs群具有较强的LILE富集性，并局限于被解释为南设得兰弧一部分的残留海山；(3)新鲜碱性玄武岩是从西班牙隆起附近的盆地东北部恢复的。Bransfield和Gibbs群熔岩中与俯冲有关的成分是一种富含LILE的流体，其放射性成因的锶、钕和铅同位素组成主要来自俯冲沉积物。这些熔岩可以模拟为来自太平洋MORB源地幔的熔体，受到高达5%的俯冲相关成分的污染。它们进一步揭示，自3 Ma以来，布兰斯菲尔德盆地一直处于太平洋地幔之下，而不是南大西洋地幔。岩浆生产力在布里奇曼隆起以东突然下降，俯冲成分最少的熔岩在这一端露头。俯冲成分向东的减少和年轻碱性玄武岩的产出都要求南设得兰弧期产生的俯冲变质地幔从东北向西南方向逐渐移出。这与之前提出的南设得兰海沟持续缓慢俯冲的模型不一致，而是有利于南斯科舍海脊断层自3 Ma以来向西传播的模型，该模型产生了横跨盆地的伸展。
Bransfield Basin is an actively extending marginal basin separating the inactive South Shetland arc from the northern Antarctic Peninsula. Rift-related volcanism is widespread throughout the central Bransfield Basin, but the wider eastern Bransfield Basin was previously unsampled. Lavas recovered from the eastern subbasin form three distinct groups: (1) Bransfield Group has moderate large-ion lithophile element (LILE) enrichment relative to normal mid-ocean ridge basalt (NMORB), (2) Gibbs Group has strong LILE enrichment and is restricted to a relic seamount interpreted as part of the South Shetland arc, and (3) fresh alkali basalt was recovered from the NE part of the basin near Spanish Rise. The subduction-related component in Bransfield and Gibbs Group lavas is a LILE-rich fluid with radiogenic Sr, Nd, and Pb isotope compositions derived predominantly from subducting sediment. These lavas can be modeled as melts from Pacific MORB source mantle contaminated by up to 5% of the subduction-related component. They further reveal that Pacific mantle, rather than South Atlantic mantle, has underlain Bransfield Basin since 3 Ma. Magma productivity decreases abruptly east of Bridgeman Rise, and lavas with the least subduction component outcrop at that end. Both the eastward decrease in subduction component and occurrence of young alkali basalts require that subduction-modified mantle generated during the lifetime of the South Shetland arc has been progressively removed from NE to SW. This is inconsistent with previous models suggesting continued slow subduction at the South Shetland Trench but instead favors models in which the South Scotia Ridge fault has propagated westward since 3 Ma generating transtension across the basin.