Geodynamic evolution in the post-collisional stage of the East Kunlun Orogenic Belt: constraints from the Late Triassic intermediate–felsic igneous rocks

The Late Triassic igneous rocks associated with post-collision are widely distributed in the East Kunlun Orogenic Belt (EKOB), but their specific dynamic mechanism and evolutional process are still controversial. To address these key issues, we investigated the geochronology and geochemistry of intermediate–felsic igneous rocks from the EKOB, including trachyandesite, rhyolite, porphyritic monzogranite, syenogranite, granite porphyry and crystalline tuff. Zircon U–Pb isotopes of these rocks indicate that they were formed between 231.1 and 215.8 Ma (i.e. Late Triassic). These igneous rocks have moderate to high SiO2 (60.29–79.56 wt%) and low mantle compatible element contents (e.g. Co: 0.37–12.62 ppm; Ni: 1.55–15.59 ppm), suggesting that continental crustal-derived material played an important role in their formation. Porphyritic monzogranite (231.1 Ma), syenogranite (228.1 Ma), trachyandesite (227.4 Ma) and rhyolite (215.8 Ma) have Nb/U (1.41–4.71) and Ce/Pb (1.48–6.19) ratios like that of the crust, with εHf(t) values (–1.31 to 2.26) and old two-stage model (TDM2) ages of 1340–1119 Ma, suggesting that they originated from the partial melting of a Mesoproterozoic crust with minor mantle material involved in their source. Crystalline tuff (224.6 Ma) and granite porphyry (222.3 Ma) have Nb/U (1.93–3.81) and Ce/Pb (0.30–3.18) ratios, negative εHf(t) values (–7.04 to −5.12) and old TDM2 ages (1703–1581 Ma) closer to those of crust, suggesting that they were derived from the partial melting of a Paleo–Mesoproterozoic continental crust without addition of mantle material. Based on our new data and published data, the Late Triassic igneous rocks from the EKOB can be divided into three stages, 236–227, 226–218 and 216–208 Ma, corresponding to slab break-off, lithospheric mantle delamination and thickened lower crust delamination, respectively. The intermediate–felsic igneous rocks from the East Kunlun Orogenic Belt are dated from 231.1 to 215.8 Ma (i.e. Late Triassic).These igneous rocks were derived from the partial melting of ancient continental crust.The Late Triassic igneous rocks from the East Kunlun Orogenic Belt can be divided into three stages, including slab break-off, lithospheric mantle delamination and thickened lower crust delamination. The intermediate–felsic igneous rocks from the East Kunlun Orogenic Belt are dated from 231.1 to 215.8 Ma (i.e. Late Triassic). These igneous rocks were derived from the partial melting of ancient continental crust. The Late Triassic igneous rocks from the East Kunlun Orogenic Belt can be divided into three stages, including slab break-off, lithospheric mantle delamination and thickened lower crust delamination.

东昆仑造山带（East Kunlun Orogenic Belt, EKOB）广泛分布有与碰撞后构造背景相关的晚三叠世火成岩，但其具体动力学机制与演化过程仍存在争议。为解决这一关键科学问题，我们对东昆仑造山带内的中酸性火成岩开展了年代学与地球化学研究，研究对象包括粗安岩、流纹岩、斑状二长花岗岩、正长花岗岩、花岗斑岩及晶屑凝灰岩。对这些岩石的锆石U-Pb同位素测试结果显示，其形成时代为231.1~215.8 Ma，即晚三叠世。这些火成岩具有中等至较高的二氧化硅（SiO₂）含量（60.29~79.56 wt%），且幔源相容元素含量较低（如钴（Co）：0.37~12.62 ppm；镍（Ni）：1.55~15.59 ppm），指示陆壳来源物质在其形成过程中发挥了重要作用。斑状二长花岗岩（231.1 Ma）、正长花岗岩（228.1 Ma）、粗安岩（227.4 Ma）与流纹岩（215.8 Ma）的Nb/U比值（1.41~4.71）与Ce/Pb比值（1.48~6.19）均与地壳特征相似；其εHf(t)值为-1.31~2.26，两阶段模式年龄（TDM2）为1340~1119 Ma，指示这些岩石起源于中元古代地壳的部分熔融，且源区有少量幔源物质参与。晶屑凝灰岩（224.6 Ma）与花岗斑岩（222.3 Ma）的Nb/U比值（1.93~3.81）、Ce/Pb比值（0.30~3.18）、负εHf(t)值（-7.04~-5.12）以及1703~1581 Ma的古老TDM2年龄均更接近地壳特征，指示其起源于古-中元古代大陆地壳的部分熔融，且源区未受幔源物质混染。基于本次新获得的数据与已发表资料，东昆仑造山带内的晚三叠世火成岩可划分为三个阶段：236~227 Ma、226~218 Ma与216~208 Ma，分别对应板片断离、岩石圈地幔拆沉与加厚下地壳拆沉过程。东昆仑造山带内的中酸性火成岩形成时代为231.1~215.8 Ma，即晚三叠世，其起源于古老大陆地壳的部分熔融。东昆仑造山带内的晚三叠世火成岩可划分为三个阶段，分别对应板片断离、岩石圈地幔拆沉与加厚下地壳拆沉过程。东昆仑造山带内的中酸性火成岩形成时代为231.1~215.8 Ma，即晚三叠世，其起源于古老大陆地壳的部分熔融。东昆仑造山带内的晚三叠世火成岩可划分为三个阶段，分别对应板片断离、岩石圈地幔拆沉与加厚下地壳拆沉过程。