Cambrian (~510 Ma) ophiolites of the East Kunlun orogen, China: A case study from the Acite ophiolitic tectonic mélange

The Acite ophiolitic mélange represents a remnant of the Proto-Tethys forearc oceanic lithosphere. Two gabbros yield zircon ²⁰⁶Pb/²³⁸U ages of 510-512 Ma. The magmatic rocks are divided into three subtypes: (1) Mid-ocean ridge basalt (MORB) -like forearc basalts (FABs), (2) Low-titanium tholeiitic gabbros (LTGs), and (3) Normal calc-alkaline dacites (CADs). The FABs have comparatively higher TiO₂ concentrations (1.13–1.42 wt%) and show almost flat REE patterns. In the NMORB normalized trace element patterns, the FABs display flat distributions of high field strength element (HFSE). The <i>ε</i>_Nd(t) values of FABs range from +4.0 to +4.8. These features are similar to the composition of Izu-Bonin-Mariana forearc basalts (FABs). LTGs are characterized by higher contents of MgO (8.85–9.95 wt%) and lower concentration of TiO₂(0.29–0.50 wt%). They show LREE-depleted patterns, however having comparatively lower total REE contents than those of FABs. The <i>ε</i>_Nd(t) values of LTGs range from +7.4 to +8.4. These features show that LTG magmas originated from a progressively depleted mantle source. In contrast, the CADs having higher SiO₂ contents (63.58–70.92 wt%) have higher total REE contents. In the NMORB normalized trace element patterns, CADs are characterized by enrichment of LILEs and depletion of HFSEs. CADs have negative <i>ε</i>_Nd(t) values ranging from -9.8 to -10.5, which are likely suggestive of a crust-derived source. The rock association of FABs and LTGs, together with tectonic discrimination plots and regional data, suggest a forearc setting above the SSZ for Acite ophiolitic mélange.

阿西特蛇绿混杂岩（Acite ophiolitic mélange）代表原特提斯洋（Proto-Tethys）弧前大洋岩石圈的残余体。两件辉长岩样品的锆石²⁰⁶Pb/²³⁸U年龄为510~512 Ma。
该套岩浆岩可划分为三个亚型：(1) 类洋中脊玄武岩型弧前玄武岩（Mid-ocean ridge basalt-like forearc basalts，FABs）；(2) 低钛拉斑玄武岩质辉长岩（Low-titanium tholeiitic gabbros，LTGs）；(3) 标准钙碱性英安岩（Normal calc-alkaline dacites，CADs）。
类洋中脊玄武岩型弧前玄武岩具有相对较高的二氧化钛（TiO₂）含量（1.13~1.42 wt%），稀土元素（REE）配分模式近乎平坦。在以正常洋中脊玄武岩（NMORB）标准化的微量元素配分图中，该类岩石的高场强元素（HFSE）分布平坦。其ε_Nd(t)值介于+4.0至+4.8之间，上述地球化学特征与伊豆-小笠原-马里亚纳弧前玄武岩的组成特征高度相似。
低钛拉斑玄武岩质辉长岩以较高的氧化镁（MgO）含量（8.85~9.95 wt%）与较低的二氧化钛含量（0.29~0.50 wt%）为典型特征。其稀土元素配分模式表现为轻稀土元素（LREE）亏损，但总稀土元素含量低于类洋中脊玄武岩型弧前玄武岩。该类岩石的ε_Nd(t)值介于+7.4至+8.4之间，指示其岩浆起源于持续亏损的地幔源区。
与之相对，标准钙碱性英安岩具有较高的二氧化硅（SiO₂）含量（63.58~70.92 wt%）与总稀土元素含量。在以正常洋中脊玄武岩标准化的微量元素配分图中，该类岩石以富集大离子亲石元素（LILEs）、亏损高场强元素为典型特征。其ε_Nd(t)值介于-9.8至-10.5之间，为负值，指示其岩浆起源于壳源物质。
类洋中脊玄武岩型弧前玄武岩与低钛拉斑玄武岩质辉长岩的岩石组合，结合构造判别图解及区域地质数据，表明阿西特蛇绿混杂岩形成于俯冲带之上（Supra-Subduction Zone，SSZ）的弧前构造环境。