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）代表原特提斯弧前大洋岩石圈的残留体。两件辉长岩样品的锆石²⁰⁶Pb/²³⁸U年龄为510~512 Ma。这套岩浆岩可划分为三个亚类：（1）类洋中脊玄武岩（Mid-ocean Ridge Basalt, MORB）型弧前玄武岩（forearc basalts, FABs）；（2）低钛拉斑质辉长岩（Low-titanium tholeiitic gabbros, LTGs）；（3）标准钙碱性英安岩（Normal calc-alkaline dacites, CADs）。

弧前玄武岩（FABs）的二氧化钛（TiO₂）含量相对较高（1.13~1.42 wt%），稀土元素（Rare Earth Element, REE）配分模式近乎平坦。在正常洋中脊玄武岩（Normal Mid-ocean Ridge Basalt, NMORB）标准化的微量元素配分图中，FABs的高场强元素（high field strength element, HFSE）分布平坦。FABs的εNd(t)值介于+4.0至+4.8之间，这些地球化学特征与伊豆-小笠原-马里亚纳弧前玄武岩的组成相似。

低钛拉斑质辉长岩（LTGs）以较高的氧化镁（MgO）含量（8.85~9.95 wt%）和较低的二氧化钛（TiO₂）含量（0.29~0.50 wt%）为特征。其稀土元素配分模式表现为轻稀土元素（Light Rare Earth Element, LREE）亏损，且稀土元素总含量相对低于FABs。LTGs的εNd(t)值介于+7.4至+8.4之间，这些特征表明其岩浆起源于逐渐亏损的地幔源区。

与之相反，标准钙碱性英安岩（CADs）具有较高的二氧化硅（SiO₂）含量（63.58~70.92 wt%），且稀土元素总含量更高。在NMORB标准化的微量元素配分图中，CADs以大离子亲石元素（Large Ion Lithophile Element, LILE）富集和高场强元素（HFSE）亏损为特征。CADs的εNd(t)值为负值，介于-9.8至-10.5之间，这暗示其源区为地壳来源。

FABs与LTGs的岩石组合，结合构造判别图解与区域数据，表明阿西特蛇绿混杂岩形成于俯冲带之上（Supra-Subduction Zone, SSZ）的弧顶环境。