Geochronology and geochemistry of the polyphase mafic rocks in the North Liaohe Group, Jiao-Liao-Ji Belt, North China Craton: implications for petrogenesis and tectonic evolution
Lixiang Zhao, Minxia Ma, Ye Qian, Haoran Li, Lingyun Sun, Jinlei Sun, Yanjie Shen, Fengyue Sun
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引用次数: 0
Abstract
The Jiao-Liao-Ji Belt (JLJB) is one of the Paleoproterozoic orogenic belts within the North China Craton (NCC), whose tectonic evolution is still controversial due to multiple magmatic/metamorphic events after its formation. To tackle this controversy, we conducted petrological, geochronological, geochemical and zircon Hf isotopic studies for the meta-mafic rocks from the North Liaohe Group (NLG) in the central JLJB. 207Pb/206Pb weighted average ages of 1849 ± 36 Ma and 1853 ± 13 Ma indicate that the protoliths of the amphibolites in the Shisixian area formed at ca. 1.85 Ga. Geochemically, the amphibolites belong to tholeiitic series; while the meta-gabbros can be divided into the low-Ti and high-Ti types: the former belongs to calc-alkaline series, whereas the latter belongs to tholeiitic series. The high-Ti rocks are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g., Ba, K and Pb), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti). These rocks display slightly positive εHf(t) values (+1.40 to +2.02). The low-Ti meta-gabbros were most likely derived from the partial melting of the enriched lithospheric mantle in the spinel stability field, which was metasomatized by subduction-related fluids and/or melts with significant contamination of crustal material, while the amphibolites and high-Ti meta-gabbros were derived from partial melting of depleted asthenospheric mantle in the spinel stability field, which was metasomatized by limited subduction-related fluids and/or melts, coupled with fractional crystallization. The lithological and geochemical characteristics show that the low-Ti meta-gabbros formed in a magmatic arc environment, the high-Ti rocks formed in the later stage of a back-arc basin; while the amphibolites formed in a post-collisional extensional environment. Combined with literature data, we suggest that the JLJB experienced a process from subduction, collision, to extension: At ca. 2.2–2.1 Ga back-arc basin opened via southward subduction of an oceanic plate followed by northward subduction at ca. 2.1–1.91 Ga, and subsequently closed to form the JLJB at ca. 1.91 Ga, which led to the Longgang-Nangrim continental collision, and post-collision extension occurred at ca. 1.85 Ga along with the collapse of the collisional orogen.
焦辽冀带是华北克拉通(NCC)中的古新生代造山带之一,由于其形成后发生了多次岩浆/变质事件,其构造演化至今仍存在争议。针对这一争议,我们对金陵拗陷中部北辽河组的元杂岩进行了岩石学、地质年代学、地球化学和锆石Hf同位素研究。207Pb/206Pb加权平均年龄分别为1849±36Ma和1853±13Ma,表明石西县地区闪长岩的原岩形成于约1.85Ga。1.85 Ga。在地球化学上,闪长岩属于透辉岩系列;而元辉长岩可分为低钛和高钛类型:前者属于钙碱性系列,后者属于透辉岩系列。高钛岩石富含轻稀土元素(LREEs)和大离子亲岩元素(LILEs;如Ba、K和Pb),而贫含重稀土元素(HREEs)和高场强元素(HFSEs;如Nb、Ta、P和Ti)。这些岩石的εHf(t)值略微偏正(+1.40 至 +2.02)。低钛元辉长岩很可能来自于尖晶石稳定场中富集的岩石圈地幔的部分熔融,这些地幔在与俯冲有关的流体和/或熔体的作用下发生了变质作用,并受到大量地壳物质的污染;而闪长岩和高钛元辉长岩则来自于尖晶石稳定场中贫乏的星体层地幔的部分熔融,这些地幔在有限的与俯冲有关的流体和/或熔体的作用下发生了变质作用,再加上碎裂结晶作用。岩石学和地球化学特征表明,低钛元辉长岩形成于岩浆弧环境,高钛岩石形成于后弧盆地后期,而闪长岩则形成于碰撞后延伸环境。结合文献数据,我们认为JLJB经历了从俯冲、碰撞到延伸的过程:在约 2.2-2.1 Ga 时,通过一个大洋板块向南俯冲打开了弧后盆地,随后在约 2.1-1.91 Ga 时向北俯冲,随后在约 1.91 Ga 时闭合形成了 JLJB。约 1.91 Ga 时,龙岗-南岭大陆发生碰撞,碰撞后的延伸发生在约 1.85 Ga 时,同时发生了塌陷。随着碰撞造山带的塌陷,碰撞后的延伸发生在约1.85 Ga。
期刊介绍:
Geosciences Journal opens a new era for the publication of geoscientific research articles in English, covering geology, geophysics, geochemistry, paleontology, structural geology, mineralogy, petrology, stratigraphy, sedimentology, environmental geology, economic geology, petroleum geology, hydrogeology, remote sensing and planetary geology.