ABSTRACT

Despite numerous years and avenues of investigation, the timing, evolution, and mechanism of lithospheric thinning of the North China Craton (NCC) are still highly debated. Late Triassic igneous rocks especially mantle derived mafic rocks could provide critical information related to the post-collisional lithospheric thinning of the Liaodong Peninsula in the eastern NCC. Using newly acquired geochronological data points, we determined that the intrusive rocks have emplacement ages of 219–209 Ma, which are typical of Late Triassic magmatism. The studied Qinghekou gabbros have low SiO₂ (49.43–51.86 wt.%), Na₂O+K₂O (3.59–7.20 wt.%), high MgO (3.70–8.54 wt.%) and transition metal element contents, with Mg^# values ranging from 49.2 to 69.3. While these samples are enriched in large ion lithophile elements such as Rb, Ba, and K and light rare earth elements, they are depleted in high field strength elements such as Nb, Ta, Ti, and P and heavy rare earth elements. We conclude that these gabbros samples were formed in a subduction-related tectonic setting, where the primary magmas were derived from partial melting of the lithospheric mantle that experienced metasomatism via slab-related fluids. Our Tongyuanpu granite samples have I-type granite affinity, as indicated by their high SiO₂ (73.60–75.30 wt.%) and low MgO (0.08–0.32 wt.%) contents, as well as the A/CNK values of 0.99–1.25 and the zircon saturated temperature values of 757–832°C, the parental magmas for these granites were generated by partial melting of the lower crust. Combined with the spatio-temporal distribution and rock-assemblages of the Late Triassic intrusive rocks, we conclude that these rocks were formed in an extensional environment characterized by lithospheric thinning caused by lithospheric delamination after the collision between the NCC and the Yangtze Craton. Furthermore, we infer that the initial lithospheric thinning of the eastern NCC occurred during the Late Triassic.

摘要

尽管经过多年的研究和研究，华北克拉通（NCC）岩石圈减薄的时间、演化和机制仍然存在很大争议。晚三叠世火成岩，尤其是地幔衍生的基性岩可以为华北克拉通东部辽东半岛的碰撞后岩石圈减薄提供关键信息。利用新获得的年代学数据点，我们确定侵入岩的侵位年龄为 219-209 Ma，这是晚三叠世岩浆作用的典型特征。研究的清河口辉长岩具有低 SiO ₂ (49.43–51.86 wt.%)、Na ₂ O+K ₂ O (3.59–7.20 wt.%)、高 MgO (3.70–8.54 wt.%) 和过渡金属元素含量，镁^#值范围从 49.2 到 69.3。虽然这些样品富含 Rb、Ba 和 K 等大离子亲石元素和轻稀土元素，但它们富含 Nb、Ta、Ti 和 P 等高场强元素和重稀土元素。我们得出结论，这些辉长岩样品是在与俯冲相关的构造环境中形成的，其中原始岩浆来自岩石圈地幔的部分熔融，该地幔通过与板片相关的流体发生交代作用。我们的通源堡花岗岩样品具有 I 型花岗岩亲和力，其高 SiO _2表明(73.60–75.30 wt.%) 和低 MgO (0.08–0.32 wt.%) 含量, 以及 0.99–1.25 的 A/CNK 值和 757–832°C 的锆石饱和温度值, 母岩浆这些花岗岩是由下地壳部分熔融形成的。结合晚三叠世侵入岩的时空分布和岩石组合，我们得出结论，这些岩石形成于以华北克拉通与扬子克拉通碰撞后岩石圈剥离导致岩石圈减薄为特征的伸展环境中。此外，我们推断华北克拉通东部岩石圈的初始减薄发生在晚三叠世。