Abstract The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

中文翻译：

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华南早泥盆世岩心杂岩体记录的古特提斯洋后造山扩张与开放洞察

摘要 古特提斯洋在泥盆纪形成，是由于地块与冈瓦纳大陆北部分离的结果。古特提斯洋的记录横跨欧亚大陆，从欧洲阿尔卑斯山向东延伸到西南华南地块（SCB）。SCB的构造圆顶记录了古特提斯洋拆离断层和构造演化引起的深部地壳收缩、伸展、减压和部分熔融。本研究基于构造分析和 40Ar/39Ar 年代学，研究了南海西南部摩天岭核心杂岩所记录的古特提斯洋的造山后（广东）扩张和开放。位于江南褶皱带最南端的摩天岭核心杂岩由低品位变沉积岩包围的三方片麻质花岗岩组成。核心复合体至少记录了三个变形 (D) 事件：前 D1、D1 和 D2。D1 之前的事件产生了具有轴向表面解理的大规模 NE 走向的褶皱，并且与三方基岩的就位同时发生。D1 变形由顶部到 NW 的韧性-脆性正断层系统表示，具有糜棱片理，并且与绿片岩相变质作用一致，表明核心复合体在 NW-SE 伸展过程中形成。D2 变形以新生代 NW-SE 向正断层为主。D1 变形和随后冷却的时间受到 40Ar/39Ar 地质年代学的限制。同运动白云母产生 40Ar/39Ar 年龄为 417 ± 2。3 Ma (1σ) 表明圆顶最初是在早泥盆世通过韧性剪切带发育的。来自花岗岩糜棱岩的岩浆黑云母产生的加权平均 40Ar/39Ar 年龄为 363.0 ± 3.1 Ma。因此，摩天岭核心杂岩体至少记录了构造演化的两个主要阶段：（1）光县（加里东）造山带崩塌和随后的第一阶段冷却（约 417 至 363 Ma）；(2) 新生代区域扩张和最终冷却。45 Ma，根据已发表的磷灰石裂变轨迹数据推断。目前的结果结合以往的研究表明，D1变形事件导致了古特提斯洋开放期间大陆裂谷边缘的摩天岭核杂岩体的形成和剥露。