Geochronological and geochemical of igneous rocks from the Jilin area

We present new geochronological, whole-rock geochemical data, and zircon Hf isotopic data from middle Permian to Middle Jurassic igneous rocks that occur near the Changchun-Yanji Suture in the area of Jilin city. These new data can (1) constrain the timing of magmatism and elucidate the spatio-temporal distribution of magmatism; (2) characterize the petrogenesis of the igneous rocks and their possible sources; and (3) provide new insights into the tectonic setting during magmatism. Together with geological evidence from this area allow us to better understand the transition in the tectonic regime during the Late Paleozoic and Mesozoic. Based on zircon U–Pb ages and geochemical data, the following conclusions are drawn. 1. Middle Permian to Middle Jurassic igneous rocks in the area of Jilin were emplaced during four major phases, at ca. 261 Ma, 253–244 Ma, 183–175 Ma, and 173–164 Ma. 2. The earliest phase of magmatism, at ca. 261 Ma, was generated in an active continental margin by partial melting of juvenile mafic lower crustal material. Magmatism at 253–244 Ma was generated in a continental arc environment by partial melting of juvenile mafic subducted oceanic crust. At 183–175 Ma, monzogranitic and dioritic magmas were generated in a continental arc environment via melting of juvenile lower continental crust and mixing of basaltic magma with crustal melt, respectively. The final stage of magmatism, at 173–164 Ma, formed in an active continental margin, generated by melting of juvenile lower continental crust. 3. Integrated evidence suggests that the closure of the Paleo-Asian Ocean could occur at 244–227 Ma, whereas the timing of tectonic regime transition from the influence of Paleo-Asian Ocean subduction to the subduction of the Paleo-Pacific Ocean occurred between Late Triassic and Early Jurassic (223–185 Ma). 4. The Changchun-Yanji Suture experienced multiple tectonic mode switches from Late Paleozoic to Mesozoic, and was controlled by subduction of the Paleo-Pacific Ocean since Early Jurassic.本研究报道了采自吉林市区长春-延吉缝合带（Changchun-Yanji Suture）附近的中二叠世至中侏罗世火成岩的新地质年代学、全岩地球化学以及锆石Hf同位素数据。上述新数据可实现以下三方面研究目标：(1) 限定研究区岩浆作用的时限，并阐明其时空分布特征；(2) 阐明火成岩的岩石成因机制及其可能的物质来源；(3) 为岩浆作用时期的区域构造背景提供新认识。结合本区域的地质证据，可更深入地理解晚古生代至中生代期间的构造体制转变。 基于锆石U-Pb定年数据与地球化学数据，本研究得出以下结论： 1. 吉林市区的中二叠世至中侏罗世火成岩可划分为四期主要的侵位事件，时限分别约为261 Ma、253~244 Ma、183~175 Ma以及173~164 Ma。 2. 最早一期岩浆作用（约261 Ma）形成于活动大陆边缘环境，源于新生基性下地壳物质的部分熔融；253~244 Ma时期的岩浆作用形成于大陆弧环境，源于新生基性俯冲洋壳的部分熔融；183~175 Ma时期，二长花岗岩浆与闪长岩浆分别通过新生下地壳熔融以及玄武质岩浆与地壳熔体的混合作用，在大陆弧环境中形成；最后一期岩浆作用（173~164 Ma）形成于活动大陆边缘环境，源于新生下地壳的熔融。 3. 综合各项证据表明，古亚洲洋的闭合时限应为244~227 Ma；而构造体制从受古亚洲洋俯冲控制转变为受古太平洋俯冲控制的时间节点，处于晚三叠世至早侏罗世（223~185 Ma）之间。 4. 长春-延吉缝合带自晚古生代至中生代经历了多期构造模式转换，且自早侏罗世以来受古太平洋俯冲作用控制。