Tectonic transition from oceanic subduction to continental collision: New geochemical evidence from Early-Middle Triassic mafic igneous rocks in southern Liaodong Peninsula, east-central China
Tectonic transition from oceanic subduction to continental collision: New geochemical evidence from Early-Middle Triassic mafic igneous rocks in southern Liaodong Peninsula, east-central China
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DOI：
10.1130/b35278.1
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发表时间：
2020-07
影响因子：
4.9
通讯作者：
W. Fang;Li‐Qun Dai;Yong‐Fei Zheng;Zi‐Fu Zhao;Li-Tao Ma
中科院分区：
文献类型：
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作者：
W. Fang;Li‐Qun Dai;Yong‐Fei Zheng;Zi‐Fu Zhao;Li-Tao Ma
与大洋俯冲期间广泛发生的基性弧岩浆活动相比，大陆俯冲期间普遍缺乏这种岩浆活动。这一模式受到了华北地块东南缘早中三叠世基性火成岩的挑战，该地块在三叠纪期间被华南地块（SCB）俯冲。这些基性岩的锆石U-Pb测年结果表明，它们的侵位时间为247±2 ~ 244±5 Ma，与两个大陆块体的初始碰撞时间一致。这些三叠系基性岩的微量元素分布模式基本呈洋岛玄武岩（OIB）状，中间（87Sr/86Sr）i比值为0.7057 ~ 0.7091,εNd(t)值为-1.2 ~ -3.8,εHf(t)值为-1.3 ~ -3.2。这样的地球化学特征表明，它起源于交代地幔源，并参与了由先前俯冲的古特提斯海洋地壳脱水融化而产生的长英质熔体。三叠纪同岩浆锆石Hf-O同位素组成变化，表明其地壳成分既有蚀变玄武岩洋壳，也有陆源沉积。较高的Fe/Mn和Zn/Fe比值表明地幔源主要由超镁铁质辉石岩组成。在辉石岩部分熔融过程中，熔体可移动不相容的微量元素相对于熔体不可移动的微量元素进一步分馏，形成了具有oib类地球化学特征的玄武岩熔体。基性岩浆活动可能是早三叠世NCB与SCB初始碰撞后，古特提斯洋板块俯冲回退的构造伸展所致。因此，同俯冲性岩浆活动为中国中东部由洋俯冲向大陆碰撞的构造转变提供了新的地球化学证据。
In contrast to the widespread occurrence of mafic arc magmatism during oceanic subduction, there is a general lack of such magmatism during continental subduction. This paradigm is challenged by the discovery of Early-Middle Triassic mafic igneous rocks from the southeastern margin of the North China Block (NCB), which was subducted by the South China Block (SCB) during the Triassic. Zircon U-Pb dating for these mafic rocks yields 247 ± 2–244 ± 5 Ma for their emplacement, coeval with the initial collision between the two continental blocks. These Triassic mafic rocks generally exhibit ocean island basalt (OIB)-like trace element distribution patterns, intermediate (87Sr/86Sr)i ratios of 0.7057–0.7091, weakly negative εNd(t) values of –1.2 to –3.8, and εHf(t) values of –1.3 to –3.2. Such geochemical features indicate origination from a metasomatic mantle source with involvement of felsic melts derived from dehydration melting of the previously subducting Paleo-Tethyan oceanic crust. The syn-magmatic zircons of Triassic age show variable Hf-O isotopic compositions, indicating that the crustal component was composed of both altered basaltic oceanic crust and terrigenous sediment. High Fe/Mn and Zn/Fe ratios suggest that the mantle source would mainly consist of ultramafic pyroxenites. The melt-mobile incompatible trace elements were further fractionated relative to melt-immobile trace elements during partial melting of these pyroxenites, giving rise to basaltic melts with OIB-like geochemical signatures. The mafic magmatism may be caused by tectonic extension due to rollback of the subducting Paleo-Tethyan oceanic slab in response to the initial collision of the NCB and SCB in the Early Triassic. Therefore, the syn-subduction mafic magmatism provides new geochemical evidence for tectonic transition from oceanic subduction to continental collision in east-central China.