Post-kinematic lithospheric delamination of the Wuyi–Yunkai orogen in South China: Evidence from ca. 435 Ma high-Mg basalts
Post-kinematic lithospheric delamination of the Wuyi–Yunkai orogen in South China: Evidence from ca. 435 Ma high-Mg basalts
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DOI：
10.1016/j.lithos.2012.06.033
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发表时间：
2012-12
期刊：
影响因子：
3.5
通讯作者：
W. Yao;Zheng‐Xiang Li;Wuzhi Li;Xuan‐Ce Wang;Xian‐Hua Li;Jin-Hui Yang
中科院分区：
文献类型：
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作者：
W. Yao;Zheng‐Xiang Li;Wuzhi Li;Xuan‐Ce Wang;Xian‐Hua Li;Jin-Hui Yang
了解华南陆块早古生代武夷-云开造山（> 460 Ma-420- 415 Ma）的板内作用过程，对于推断当时该陆块与其他大陆的相互作用，以及东亚的构造演化具有重要意义。造山带的一个显著特征是，尽管造山带中广泛存在同造山期至晚造山期（440 Ma至420- 415 Ma）花岗岩，但没有任何类型的同造山期至晚造山期火山岩或基性岩的报道。这些基性岩可以提供在这样一个大造山运动期间任何幔壳相互作用的线索，从而有助于了解造山事件的动力学。本文首次报道了位于广东北方造山带变质核边缘的一个中基性火山岩系的年代学、同位素和地球化学资料。火山岩不整合覆盖在强烈变形的寒武-奥陶系地层之上，但与上覆的造山后中泥盆世地层呈低角度不整合接触。对两个安山质和英安岩样品的锆石LA-ICP-MS和SHRIMP U-Pb测年结果表明，它们的结晶年龄一致，约为1000年。435 Ma，晚于造山带460- 445 Ma高峰期变质作用，但与广泛分布的晚造山期（约435 Ma）变质作用同步。440- 415 Ma）花岗质侵入体。9个地壳污染程度最低的玄武岩样品具有高MgO（12.3-19.2wt.%）的特征，Ni（214- 715 ppm）和Cr（724- 1107 ppm），但低TiO 2（0.6-0.8wt.%），氧化铝（10.2-12.8wt.%）和Fe 2 O 3 T（总Fe以Fe 2 O 3计）（8.7- 11.4重量%）内容.玄武岩还表现出低的Nb/La比值（0.4-0.8）和恒定的εNd（t）值（−8.0-−8.4），SiO2（44.8-51.5wt.%）变化内容，建议可能的次大陆岩石圈地幔起源。这些高镁玄武岩的化学成分与它们的原始岩浆相似，这是使用SiO2 ≤ 50 wt.%的地球化学模型估计的，MgO ≤ 14wt.%和FeOT为0.9wt.%。估计熔融的潜在温度为>1300°C，远高于正常的次大陆岩石圈。这意味着岩浆可能是由热上涌的软流圈加热岩石圈橄榄岩部分熔融而成。高镁安山岩的锆石εHf（t）值为负值（−21.7 ~ −6.3）和锆石δ 18 O值较高（7.3-9.0‰），说明其为同一玄武质岩浆源分异和AFC作用的产物。总的来说，我们解释，这后运动玄武岩，再加上安山岩和英安岩的差异，导致晚造山岩石圈拆沉，导致造山带的崩溃，熔融的SCLM，和广泛的晚造山花岗岩侵入造山带。
Understanding the processes responsible for the intra-plate early Paleozoic Wuyi–Yunkai orogeny (>460Ma to 420–415Ma) in the South China Block (SCB) is important for deducing the interactions of this block with other continents at that time, as well as the tectonic evolution of East Asia. One salient feature of the orogen is that despite the wide occurrence of syn- to late-orogenic (440Ma to 420–415Ma) granites in the orogen, neither syn- to late-orogenic volcanic rocks nor mafic rocks of any type have been reported. Such mafic rocks could shed clues about any mantle–crust interaction during such a major orogeny, thus help to understand the dynamics of the orogenic event. We present here, for the first time, geochronological, isotopic and geochemical data for a mafic-intermediate volcanic succession in northern Guangdong, near the edge of the metamorphic core of the orogen. The volcanic rocks unconformably overlie strongly deformed Cambro-Ordovician strata, but are in low-angle unconformable contact with overlying post-orogenic mid-Devonian strata. LA-ICP-MS and SHRIMP U–Pb dating of zircons from two andesitic and dacitic samples gives a consistent crystallization age of ca. 435Ma, younger than the 460–445Ma peak metamorphism of the orogeny but synchronous with the widespread late-orogenic (ca. 440–415Ma) granitic intrusions. Nine least crustally-contaminated basaltic samples are characterized by high MgO (12.3–19.2wt.%), Ni (214–715ppm) and Cr (724–1107ppm), but low TiO2(0.6–0.8wt.%), Al2O3(10.2–12.8wt.%) and Fe2O3T(total Fe as Fe2O3) (8.7–11.4wt.%) contents. The basalts also exhibit low Nb/La ratios (0.4–0.8) and constant εNd(t) values (−8.0 to −8.4) with variable SiO2(44.8–51.5wt.%) contents, suggesting a likely sub-continental lithospheric mantle origin. These high-magnesian basalts have chemical compositions similar to their primary magma, which was estimated using geochemical modeling at SiO2≈50wt.%, MgO≈14wt.% and FeOT≈9wt.%. The estimated potential temperature for the melts is >1300°C, much higher than that of a normal sub-continental lithosphere. This implies that the magma was likely generated from partial melting of lithospheric peridotite heated by hot upwelling asthenosphere. The high-magnesian andesites are interpreted as the products of differentiation and AFC processes from the same basaltic magma source, as supported by their negative zircon εHf(t) values (−21.7 to −6.3) and high zircon δ18O values (7.3–9.0‰). Overall, we interpret that this post-kinematic basalts, plus andesites and dacites as differentiates, resulted from a late-orogenic lithospheric delamination which led to an orogenic collapse, melting of the SCLM, and widespread late-orogenic granitic intrusions in the orogen.