Geochronology and geochemistry of the dioritic rocks from the Inexpressible Island, Northern Victoria Land, Antarctica and their geological implications

Abstract

东南极北维多利亚地沿威尔逊、鲍尔斯地体边缘发育广泛的花岗质岩浆作用。本文选取了罗斯造山带北维多利亚地难言岛古生代闪长质岩石开展了锆石U-Pb年代学、Sr-Nd-Hf同位素地质学及全岩地球化学研究，分析探讨其岩浆的源区特征和岩石成因。锆石U-Pb同位素定年结果显示，难言岛闪长质岩石形成于约500Ma，属于晚寒武世罗斯造山后岩浆活动产物。岩石地球化学特征显示的SiO₂含量变化范围为54.03%~60.10%，总碱值为5.92%~6.91%，里特曼指数σ为2.67~3.68，A/CNK值为0.77~0.85，属于高钾钙碱性、准铝质岩石。在正常洋中脊玄武岩标准化蛛网图中，样品富集大离子亲石元素（LILE），亏损Nb、Ta、Ti；在球粒陨石标准化稀土元素配分图中，难言岛闪长质岩石显示轻稀土相对富集，（La/Yb）_N=12.99~14.60，Eu弱负异常（0.62~0.71）。样品的锆石ε_Hf（t）值为-10.7~-6.3；（⁸⁷Sr/⁸⁶Sr）_i范围0.7095~0.7097，（¹⁴³Nd/¹⁴⁴Nd）_i范围0.511502~0.511629，ε_Nd（t）范围-7.1~-10.3。结合前人资料，显示难言岛闪长质岩石来自于先前富集的陆下岩石圈地幔部分熔融，在罗斯汇聚的过程中被俯冲组分进一步交代。晚寒武世罗斯造山带处于后碰撞环境，熔融可能与后碰撞伸展环境下俯冲板块回卷、撕裂过程中软流圈上涌或其他减薄机制有关。;Extensive Early Paleozoic granitic magmatism developed along the margins of the Wilson and Bowers terranes in Northern Victoria Land, Antarctica. Based on zircon U-Pb geochronology, Sr-Nd-Hf isotope geology and geochemistry of the Paleozoic Inexpressible Island dioritic rocks in the Northern Victoria Land, Ross Orogenic Belt, this paper analyzed the initial time of the post-collisional extensional environment and the characteristics of magma source region. Zircon U-Pb isotope dating reveals that the Inexpressible Island diorites were formed at about 500Ma, representing Late Cambrian Ross magmatic activity. The samples show characteristics of a high potassic and magmatic arc affinity. The contents of SiO₂ and total alkali (K₂O+Na₂O) range within 54.03%~60.10% and 5.92%~6.91%, respectively. The σ and A/CNK values vary from 2.67 to 3.68 and 0.77 to 0.85, respectively, indicating that the rocks belong to metaluminous rocks. On the normal mid-ocean ridge basalt-normalized diagram, the samples are enriched in large ion lithophilic elements (LILE) and depleted in Nb, Ta and Ti. In the chondrite-normalized REE diagram, the Inexpressible Island diorite shows a relatively rich LREE with (La/Yb)_N=12.99~14.60 and a slight negative Eu anomaly (0.62~0.71). Furthermore, zircon ε_Hf(t) and ε_Nd(t) values of the measured samples range from -10.7 to -6.3 and -7.1 to -10.3, respectively, with correspondent (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i ratios of 0.7095 to 0.7097 and 0.511502~0.511629. Combined with previous data, the Inexpressible Island diorites can be ascribed to the melting of previously enriched subcontinental lithospheric mantle, which further metasomatized by a subduction component before. The mechanism that triggers partial melting could be slab rollback and convective thinning and/or delamination of overthickened lithosphere during Ross orogenic belt post-collision.