Initial breakup of supercontinent Rodinia as recorded by ca 860-840 Ma bimodal volcanism along the southeastern margin of the Yangtze Block, South China
Initial breakup of supercontinent Rodinia as recorded by ca 860-840 Ma bimodal volcanism along the southeastern margin of the Yangtze Block, South China
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华南扬子地块东南缘约 860-840 Ma 双峰式火山活动记录的罗迪尼亚超大陆的初始裂解
DOI：
10.1016/j.precamres.2017.04.039
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发表时间：
2017
影响因子：
3.8
通讯作者：
Li Xian-Hua
中科院分区：
文献类型：
--
作者：
Lyu Pu-Liang;Li Wu-Xian;Wang Xuan-Ce;Pang Chong-Jin;Cheng Jin-Xiong;Li Xian-Hua
地幔热柱在超大陆解体中起着重要作用，但大陆裂谷作用及其伴生的双峰式火山作用往往先于地幔热柱岩浆作用和超大陆解体的主要阶段。因此，这些双峰式火山岩的成因对于理解岩浆作用与超大陆解体之间的因果关系至关重要。本文报道了扬子地块东南缘出露的黄山和梅岭双峰式火山岩的新的SIMS锆石U-Pb年龄、Nd同位素数据和全岩主量元素和微量元素数据。黄山和梅岭双峰式火山岩分别形成于860 Ma±9.9 Ma和840 Ma±6.5 Ma，比中国南部记录的新元古代热柱事件早35~15 Ma。黄山玄武岩具有弧形特征，富含水(喷发前岩浆水含量为∼3.6%)和流体活动元素，亏损高场强元素，具有均一的Nd同位素组成(εNd(T)=1+4.4~+5.3)。尽管这些地球化学特征表明黄山玄武岩为含水的软流圈地幔来源，但它们不相容的微量元素分布在板内构造环境中，与真正的弧玄武岩背离。而较年轻的梅岭玄武岩则具有板内型(类OIB型)的地球化学特征和均一的Nd同位素组成(εNd(T)x=+4.3~+4.7)，表明其源区为正常的软流圈地幔。地球化学分析表明，黄山玄武岩在岩浆上升过程中经历了橄榄石、→、橄榄石+角闪石+单斜辉石+磁铁矿、斜长石分离结晶作用。尽管镁铁质和长英质端元具有不同的化学成分，但这两个端元具有相似的Nd同位素组成。这说明长英质火山岩是玄武岩经分离结晶形成的。分离结晶矿物组合可能以角闪石为主，其次为斜长石，与玄武岩的分离矿物组合一致。相反，梅岭长英质岩石的εND(T)值与硅质含量呈负相关。地球化学分析表明，前驱岩浆经历了角闪石和斜长石与少量钛铁矿的分离结晶。这意味着梅岭长英质岩石是由含水的年轻玄武岩部分熔融形成的，可能是860 Ma的底侵玄武岩，然后是不同的地壳混染作用。860~840 Ma双峰式火山岩的岩石成因表明，在约825 Ma的地幔热柱岩浆作用之前，发生了与裂谷有关的多期非造山岩浆作用。可能在地幔过渡带内部或顶部的湿上升流可能是浅层软流圈地幔水化的主要原因，这推动了地幔部分熔融，从而产生了这些双峰式火山岩。我们得出结论，地幔水化作用是早期陆内裂谷相关岩浆作用的触发因素，可能对应于Rodinia超大陆的最初解体。
It is considered that mantle plumes play an important role in the breakup of supercontinents, but continental rifting and associated bimodal volcanism often predate mantle-plume magmatism and the major stage of supercontinent breakup. Therefore, the petrogenesis of those bimodal volcanic rocks is crucial for understanding the causal relationship between magmatism and supercontinent breakup. In this paper, new SIMS U–Pb zircon ages, Nd isotopic data, and whole-rock major and trace element data are reported for Huangshan and Meiling bimodal volcanic rocks that crop out along the southeastern margin of the Yangtze Block. The Huangshan and Meiling bimodal volcanic rocks were formed respectively at 860 ± 9 Ma and 840 ± 5 Ma, about 35 to 15 Mys earlier than the recorded Neoproterozoic plume event in South China. The Huangshan basalts have arc-like geochemical signatures with enrichment of water (pre-eruption magma water content of ∼3.6%) and fluid-mobile elements, and depletion of high-field-strength elements (HFSE), as well as homogeneous Nd isotopic compositions (εNd(t) = +4.4 to +5.3). Although such geochemical characteristics suggest a hydrous asthenospheric mantle source for the Huangshan basalts, their incompatible trace elements plot within an intraplate tectonic setting, offset from true arc basalts. In contrast, the younger Meiling basalts have intraplate type (OIB-like) geochemical and homogeneous Nd isotopic compositions (εNd(t) = +4.3 to +4.7), implying a normal asthenospheric mantle source. Geochemical analyses indicate that the Huangshan basaltic rocks underwent olivine → olivine + amphibole + clinopyroxene + magnetite and then plagioclase fractional crystallization during magma ascent. Although mafic and felsic end-members have distinctive chemical compositions, the two end-members share similar Nd isotopic compositions. This implies that the felsic volcanic rocks were derived from basaltic rocks through fractional crystallization. The assemblage of fractional crystallization minerals may be amphibole-dominated and then plagioclase, coincident with fractioned mineral assemblage of the basaltic rocks. In contrast, Meiling felsic rocks have varied εNd(t) values that are correlated negatively with silica contents. Geochemical analyses indicate that the precursor magma underwent fractional crystallization of amphibole and plagioclase with minor titanite. This implies that Meiling felsic rocks were produced by partial melting of hydrous juvenile basaltic rocks, possibly ca 860 Ma underplated basaltic rocks, and then followed by variable crustal contamination. The petrogenesis of the 860–840 Ma bimodal volcanic rocks suggests that multi-stage rift-related anorogenic magmatism occurred before ca 825 Ma mantle-plume magmatism in South China. Wet upwelling, probably within or on top of the mantle transition zone, may have been the main cause of the hydration of the shallow asthenospheric mantle, which drove the mantle partial melting that generated these bimodal volcanic rocks. We conclude that mantle hydration was the trigger for early intracontinental rift-related magmatism, probably corresponding to the initial break-up of the Rodinia supercontinent.