The Middle Triassic evolution of the Bangong–Nujiang Tethyan Ocean: evidence from analyses of OIB-type basalts and OIB-derived phonolites in northern Tibet
The Middle Triassic evolution of the Bangong–Nujiang Tethyan Ocean: evidence from analyses of OIB-type basalts and OIB-derived phonolites in northern Tibet
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DOI：
10.1007/s00531-017-1570-x
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发表时间：
2018-07
影响因子：
2.3
通讯作者：
Jian‐Jun Fan;Cai Li;Jin-Heng Liu;Ming Wang;Yiming Liu;Chao-Ming Xie
中科院分区：
文献类型：
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作者：
Jian‐Jun Fan;Cai Li;Jin-Heng Liu;Ming Wang;Yiming Liu;Chao-Ming Xie
本文报道了藏北北方班公湖-怒江缝合带中段纳热洋岛段玄武岩和响岩的主量元素和微量元素化学数据，以及纳热洋岛段响岩的锆石U-Pb年龄和Hf同位素组成。我们的目的是评估这些岩石的成因和重建班公湖-怒江特提斯洋（BNTO）的中三叠世演化。NaOI保留了洋岛型双层结构，包括玄武岩基底和海洋沉积盖层序列（砾岩和石灰岩，后者伴随着靠近序列顶部的喷发响岩层）。NaOI中的玄武岩富含轻稀土元素和高场强元素（Nb、Ta、Zr、Hf和Ti），具有与洋岛玄武岩相似的稀土元素配分模式和原始地幔标准化微量元素配分模式。考虑到它们的高Dy/Yb，Sm/Yb和La/Sm比值，我们认为NaOI玄武岩来自地幔中的石榴橄榄岩的部分熔融。NaOI响岩具有富集轻稀土元素、Eu负异常（Eu/Eu* = 0.41-0.43）的辉石岩标准化稀土配分模式和富集Nb、Ta、Zr和Hf、亏损Ba、U、Sr、P和Ti的原始地幔标准化微量元素配分模式。由于铌含量高（172-256 ppm），钽（11.8-16.0 ppm），锆（927-1117 ppm）和Hf（20.8-26.9 ppm），MgO含量很低（0.11- 0.25wt%）、非常低的Mg#值（5-10）和接近零的Cr含量（1.27-7.59 ppm），镍（0.43-7.19 ppm），以及Co（0.11-0.38 ppm），εHf（t）值较小且均为正值（+ 4.9 ~+ 9.5），推测NaOI响岩是由OIB源基性母岩浆分离结晶形成的。NaOI的锆石U-Pb年龄为239和242 Ma，表明NaOI形成于中三叠世。这些数据，结合现代海洋岛屿的数据（例如，加那利群岛、佛得角、大西洋的费尔南多-迪诺罗尼亚群岛、特里斯坦-达库尼亚和高夫群岛，以及太平洋的社会群岛和南库克群岛），使我们推断BNTO在中三叠世之前已经开放了很长一段时间，并且至少在中三叠世之前，海洋已经发展成为一个成熟的海洋，具有厚的海洋岩石圈。
In this paper, we present new major and trace element chemical data for the basalts and phonolites of the Nare ocean island fragment (NaOI), as well as zircon U–Pb age data and Hf isotope compositions for the NaOI phonolites in the middle segment of the Bangong–Nujiang Suture Zone, northern Tibet. Our aim is to assess the genesis of these rocks and to reconstruct the Middle Triassic evolution of the Bangong–Nujiang Tethyan Ocean (BNTO). The NaOI retains an ocean island-type double-layered structure comprising a basaltic basement and an oceanic sedimentary cover sequence (conglomerate and limestone, the latter accompanied by layers of erupted phonolite near the top of the sequence). The basalts in the NaOI are enriched in light rare earth elements and high field strength elements (Nb, Ta, Zr, Hf, and Ti), and they exhibit chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns similar to those of ocean island basalts. Taking into consideration their high Dy/Yb, Sm/Yb, and La/Sm ratios, we conclude that the NaOI basalts were derived from the partial melting of garnet peridotite in the mantle. The NaOI phonolites have LREE-enriched chondrite-normalized REE patterns with negative Eu anomalies (Eu/Eu* = 0.41–0.43) and primitive mantle-normalized trace element patterns with enrichments in Nb, Ta, Zr, and Hf, and depletions in Ba, U, Sr, P, and Ti. Given the high contents of Nb (172–256 ppm), Ta (11.8–16.0 ppm), Zr (927–1117 ppm), and Hf (20.8–26.9 ppm), and the very low contents of MgO (0.11–0.25 wt%), the very low Mg#values (5–10), and the near-zero contents of Cr (1.27–7.59 ppm), Ni (0.43–7.19 ppm), and Co (0.11–0.38 ppm), and the small and homogeneously positiveεHf(t) values (+ 4.9 to + 9.5), we infer that the NaOI phonolites were formed by the fractional crystallization of an OIB-derived mafic parent magma. The phonolites of the NaOI contain zircons that yielded U–Pb ages of 239 and 242 Ma, indicating that the NaOI formed during the Middle Triassic. These data, combined with data from modern ocean islands (e.g., Canary Islands, Cape Verde, Fernando de Noronha, Tristan da Cunha, and Gough in the Atlantic Ocean, and Society and Austral–Cook in the Pacific Ocean), lead us to infer that the BNTO was open for a long time before the Middle Triassic, and that the ocean had already developed into a mature ocean with a thick oceanic lithosphere by at least the Middle Triassic.