吉尔吉斯斯坦北天山二叠纪钾玄岩系的成因及构造背景研究

The Tianshan Orogenic Belt (TOB), as the major part of the Central Asian Orogenic Belt (CAOB), records the important information of the continental crustal growth and lithospheric mantle evolution in the Paleozoic. The study of the tectonic-magmatic evolution of the Tianshan and its adjacent tectonic plates is the principal basis of the reconstruction of the evolutionary history of the TOB. However, the tectonic-magmatic evolution in the Late Paleozoic of the Kyrgyz North Tianshan (KNT), as the western part of the Yili Block, is unclear. And it restricts to understand the evolution of the whole TOB in the Late Paleozoic. Thus, this proposal will investigate the zircon U-Pb and plagioclase 39Ar-40Ar dating, whole-rock geochemistry, in-situ mineral compositions and zircon Hf-O isotopes of the Permian shoshonitic volcanics in the KNT, with aims to recognize their petrogenesis and tectonic setting during the generation. These data, comparison with the Late Paleozoic magmatism in the Yili Block, will be used to summarize their spatial and temporal distribution and reveal the nature and evolution of the lithospheric mantle beneath the two blocks during late Paleozoic, which is important for the tectonic evolution of the whole TOB.

天山造山带记录着中亚造山带古生代陆壳增生与拼合过程以及岩石圈地幔演化的重要信息。对比天山及其邻区不同构造单元古生代构造-岩浆演化特征是重建天山造山带演化历史的基础。然而，作为伊犁地块的西延部分，吉尔吉斯斯坦北天山晚古生代构造-岩浆演化的研究仍十分薄弱，从而制约了对天山造山带晚古生代属性和演化的整体理解。针对此问题，项目拟以新发现的吉尔吉斯斯坦北天山Ak-Kuduk二叠纪钾玄岩系火山岩为研究对象，进行精确的锆石U-Pb以及斜长石39Ar-40Ar定年，开展全岩地球化学、矿物化学、锆石Hf-O同位素等分析，综合探讨其岩石成因及构造背景。并通过与伊犁地块晚古生代岩浆作用的对比研究，总结出两地块晚古生代岩浆作用的时空分布及演化特征，从而约束它们的岩石圈地幔的性质及演化特征，为整体探讨天山造山带的构造演化提供重要信息。

天山造山带记录着中亚造山带古生代陆壳增生与拼合过程以及岩石圈地幔演化的重要信息。对比天山及其邻区不同构造单元古生代构造-岩浆演化特征是重建天山造山带演化历史的基础。然而，作为伊犁地块的西延部分，吉尔吉斯斯坦北天山晚古生代构造-岩浆演化的研究仍十分薄弱，从而制约了对天山造山带晚古生代属性和演化的整体理解。新发现的吉尔吉斯斯坦北天山Ak-Kuduk二叠纪钾玄岩系火山岩可很好地解决这个科学问题。两个精确的SHRIMP锆石U-Pb年龄表明Ak-Kuduk钾玄岩系应喷发于290-292 Ma（早二叠世）。该钾玄岩系主要由钾玄岩-安粗岩-粗面岩-碱玄质响岩组成，并与煌斑岩脉、碱性辉绿岩脉以及碱性正长岩脉共生。SiO2 含量为51.4~68.4%，且富碱 (Na2O+K2O = 6.5~13.5%)，高K2O/Na2O，低钛 (TiO2 = 0.7~1.27%)，具典型钾玄岩系火山岩的岩石化学特征，且和伊犁地块的钾玄岩系具有相似的矿物组合和主量元素组成。同时，它们富集轻稀土元素（LREE），大离子亲石元素 (LILE)，亏损高场强元素 (HFSE)，并显示强烈的Nb-Ta、Ti、Sr负异常和Pb正异常，显示“典型岛弧”的地球化学特征。初始(87Sr/86Sr)t为0.6932-0.7324，εNd(t) 为-1.6 - -5.9， Nd模式年龄TDM2 = 1186-1536 Ma；εHf(t)为-0.2 - -2.7，Hf模式年龄TDM2 = 1314-1476 Ma。(206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t的初始值分别为17.93-18.72、15.58-15.63和38.02-38.6。均显示富集的同位素特征，指示玄武岩-安山岩的地幔源区为富集的古老岩石圈地幔，而粗面岩-响岩很可能是玄武岩-安山岩结晶分异的结果。同时Nd-Hf同位素解耦，εHf(t)数值明显要高于εNd(t)，暗示俯冲物质主要是以俯冲沉积物为主。粗面岩锆石δ18O为6.45-8.3，εHf(t)为 -9.5- -1.5，并且其Hf模式年龄TDM2 = 1402-1745 Ma，稍微老于全岩的二阶段Nd和Hf模式年龄，综合以上地球化学特征，我们倾向于Ak-Kuduk 钾玄岩系应形成于陆下岩石圈地幔拆沉的地球动力学背景，而源区的富集则可能与早期的洋壳俯冲作用有关。