Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes, Northeastern Oman

Late Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO₂), with low MgO (4.7–10.2 wt. %) and high Al₂O₃ (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO₂, Al₂O₃, and Na₂O abundances, and their higher MgO, CaO, and P₂O₅ contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70435–0.70646, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0 · 512603 and 0 · 512716 (εNd_i 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: ²⁰⁸Pb/²⁰⁴Pb_i = 38.97–39.39 and ²⁰⁷Pb/²⁰⁴Pb_i = 15.35–15.58, ²⁰⁶Pb/²⁰⁴Pb_i = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.

晚侏罗世超镁铁质煌斑岩（ultramafic lamprophyre, UML）岩床与岩脉以长3km的侵入体形式产出，赋存于阿曼东部巴塔因推覆体的异地瓦拉组（Whara Formation）中。该类岩床与岩脉由斑晶金云母、含尖晶石的阿利基岩（aillikite）以及达姆提尔岩（damtjernite）组成。阿利基岩为浅灰色块状细粒凝灰质岩石，发育自形云母板状晶体；而达姆提尔岩则呈深灰色中-粗粒结构，富含球粒状火山砾与球状分异结构。两种岩性均由方解石、金云母、磷灰石、磁铁矿、尖晶石、透辉石以及利透闪石构成，仅正长石见于达姆提尔岩中。该类岩石整体呈强硅不饱和特征，二氧化硅（SiO₂）含量为17.6~33.7 wt.%，氧化镁（MgO）含量较低（4.7~10.2 wt.%），氧化铝（Al₂O₃）含量较高（3.5~8.6 wt.%）。阿利基岩与达姆提尔岩的区别在于：前者二氧化硅、氧化铝以及氧化钠（Na₂O）的丰度更低，而氧化镁、氧化钙（CaO）以及五氧化二磷（P₂O₅）的含量更高。两种岩石的稀土元素（rare earth element, REE）配分模式相似，均表现出显著的轻稀土元素（light REE, LREE）富集特征。二者均富集钡（Ba）、钍（Th）、铀（U）、铌（Nb）以及钽（Ta），其标准化浓度最高可达原始地幔的1000倍。二者相对亏损重稀土元素（heavy REE, HREE）、钾（K）、铷（Rb）、铅（Pb）、锶（Sr）、磷（P）、锆（Zr）以及铪（Hf）。该类岩石的初始⁸⁷锶/⁸⁶锶比值介于0.70435~0.70646之间，而初始¹⁴³钕/¹⁴⁴钕比值则为0.512603~0.512716（初始εNd值为2.6~3.2）。阿利基岩与达姆提尔岩的铅同位素比值差异更为显著：²⁰⁸铅/²⁰⁴铅初始比值为38.97~39.39，²⁰⁷铅/²⁰⁴铅初始比值为15.35~15.58，²⁰⁶铅/²⁰⁴铅初始比值为18.08~18.96。Sal UMLs中富含金云母、磷灰石与金红石，且轻稀土元素、钡、钍、铀、铌以及钽均发生富集，这表明该类岩石在亏损源区发生低度部分熔融后，经历了交代富集作用。对阿利基岩与达姆提尔岩中金云母斑晶开展的氩-氩年代学测年结果分别为154 Ma与162 Ma，该时限与原印度洋在140~160 Ma期间记录的大规模构造事件相吻合。