Continental growththrough accreted oceanic arc: Zircon Hf-O isotope evidence for granitoids fromthe Qinling orogen
Continental growththrough accreted oceanic arc: Zircon Hf-O isotope evidence for granitoids fromthe Qinling orogen
复制标题
通过增生洋弧的大陆生长：秦岭造山带花岗岩类的锆石 Hf-O 同位素证据
DOI：
10.1016/j.gca.2016.03.016
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发表时间：
2016
影响因子：
5
通讯作者：
Sai-Hong Yang
中科院分区：
文献类型：
--
作者：
Hao Wang;Yuan-Bao Wu;Shan Gao;Zheng-Wei Qin;Zhao-Chu Hu;Jian-Ping Zheng;Sai-Hong Yang
大陆地壳通常被认为是在俯冲带中形成的，但对于大洋弧或大陆弧在大陆地壳形成中的相对作用没有共识。大洋弧模型的主要困难在于如何使大洋弧不被俯冲破坏，如何通过其高硅产物追踪以前的大洋弧，以及大洋弧如何产生类似于上陆壳的高硅富钾花岗岩类成分。东秦岭造山带保存了出露良好的二郎坪洋弧，具有大量的花岗岩类，为解决这些问题提供了最佳场所。本文对二郎坪单元中四个典型的花岗岩类岩体进行了锆石U-Pb年龄和Hf-O同位素的综合研究。原位锆石西姆斯U-Pb测年结果表明，张家大庄、西庄河和桃源岩体的形成年龄分别为472 ± 7、458 ± 6和443 ± 5 Ma，均晚于秦岭微大陆向二郎坪洋弧的俯冲。张家大庄、西庄河和桃源岩体为钠质花岗岩，εHf（t）值为+7.6 ~+12.9，δ 18 O值较低（4.7-5.0‰），可能是二郎坪洋弧增生后热液蚀变下洋壳迅速重熔的结果。满子营二长花岗岩体中锆石的Hf-O同位素组成与西庄河岩体相似，可能是西庄河岩体中英云闪长岩脱水熔融的产物。秦岭微大陆的深俯冲导致了二郎坪洋弧的增生，表明弧-陆碰撞为防止洋弧完全俯冲提供了有效途径。二郎坪单元花岗岩类锆石εHf（t）高正值和δ 18 O相对较低的特征，表明陆壳可通过增生后岩浆作用分异，从增生洋弧中获得高硅、富钾的性质，从而突出洋弧在陆壳生成中的意义。
The continental crust is commonly viewed as being formed in subduction zones, but there is no consensus on the relative roles of oceanic or continental arcs in the formation of the continental crust. The main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted, how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust. The eastern Qinling orogen provides an optimal place to address these issues as it preserves the well-exposed Erlangping oceanic arc with large amounts of granitoids. In this study, we present an integrated investigation of zircon U–Pb ages and Hf–O isotopes for four representative granitoid plutons in the Erlangping unit. In situ zircon SIMS U–Pb dating indicated that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, all of which postdated the deep subduction of the Qinling microcontinent under the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positiveεHf(t) (+7.6 to +12.9) and relatively low δ18O (4.7–5.0‰) values, which were suggested to result from prompt remelting of hydrothermally altered lower oceanic crust of the accreted Erlangping oceanic arc. The zircon grains from the Manziying monzogranitic pluton show similar Hf–O isotopic compositions to those of the Xizhuanghe pluton, and thus the Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the Xizhuanghe pluton. The deep subduction of Qinling microcontinent resulted in the accretion of the Erlangping oceanic arc, which implies that arc–continent collision provides an effective way for preventing oceanic arcs from being completely subducted. The highly positiveεHf(t) and relatively low δ18O values of zircon grains from the granitoids in the Erlangping unit reveal that the continental crust can acquire its high-Si, K-rich nature from accreted oceanic arcs through differentiation by post-accretional magmatism, and thus highlight the significance of oceanic arcs for the generation of continental crust.