Age and nature of the Jurassic–Early Cretaceous mafic and ultramafic rocks from the Yilashan area, Bangong–Nujiang suture zone, central Tibet: implications for petrogenesis and tectonic Evolution

The Jurassic–Early Cretaceous Yilashan mafic–ultramafic complex is located in the middle part of the Bangong–Nujiang suture zone, central Tibet. It features a mantle sequence composed of peridotites and a crustal sequence composed of cumulate peridotites and gabbros that are intruded by diabases with some basalts. This article presents new whole-rock geochemical and geochronological data for peridotites, gabbros, diabases and basalts to revisit the petrogenesis and tectonic setting of the Yilashan mafic–ultramafic complex. Zircon laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U–Pb ages of three diabase samples are 169.6 ± 3.3 Ma, 132.5 ± 2.5 Ma, and 133.6 ± 4.9 Ma, respectively. These ages together with previous studies indicate that the Yilashan mafic–ultramafic complex probably formed during the Jurassic–Early Cretaceous. The peridotites exhibit nearly U-shaped REE patterns and are distinct from abyssal peridotites. The diabase and basalt samples show arc features with selective enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILEs; e.g. Rb, U, and Sr) and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti). The gabbro samples display cumulate features with selective enrichment in LILEs (e.g. Rb, Ba, and Sr) but depletion in LREEs and HFSEs (e.g. Nb, Zr, and Ti). Combing the positive <i>ε</i>_Nd(<i>t</i>) values (+6.1 to +10.0) and negative zircon <i>ε</i>_Hf(<i>t</i>) values (–16.5 to –11.7 and –13.6 to –0.4) with older Hf model ages for the mafic rocks, these signatures suggest that the Yilashan mafic and ultramafic rocks likely originated from an ancient lithospheric mantle source with the addition of asthenospheric mantle materials and subducted fluids coupled with limited crustal contamination in a continental arc setting as a result of the southward subduction of the Bangong–Nujiang Tethys Ocean beneath the Lhasa terrane during the Jurassic–Early Cretaceous.

侏罗纪-早白垩世的伊拉山镁铁质-超镁铁质杂岩（mafic–ultramafic complex）位于西藏中部班公湖-怒江缝合带（Bangong–Nujiang suture zone）中段。该杂岩发育由橄榄岩（peridotites）构成的地幔序列，以及由堆晶橄榄岩和辉长岩（gabbros）构成的地壳序列，辉绿岩（diabases）与部分玄武岩（basalts）侵入上述序列中。本文针对橄榄岩、辉长岩、辉绿岩及玄武岩提供了全新的全岩地球化学与年代学数据，以期重新探讨伊拉山镁铁质-超镁铁质杂岩的岩石成因与构造背景。对3件辉绿岩样品的锆石激光剥蚀电感耦合等离子体质谱仪（LA-ICP-MS）U-Pb定年结果分别为169.6 ± 3.3 Ma、132.5 ± 2.5 Ma及133.6 ± 4.9 Ma。结合已有研究成果，这些定年数据表明伊拉山镁铁质-超镁铁质杂岩可能形成于侏罗纪-早白垩世时期。橄榄岩呈现近似U型的稀土元素（REE）配分模式，与深海橄榄岩存在显著差异。辉绿岩与玄武岩样品表现出岛弧地球化学特征：选择性富集轻稀土元素（LREE）与大离子亲石元素（LILEs，如Rb、U及Sr），且亏损高场强元素（HFSEs，如Nb、Ta及Ti）。辉长岩样品则显示堆晶岩特征：富集大离子亲石元素（如Rb、Ba及Sr），但亏损轻稀土元素与高场强元素（如Nb、Zr及Ti）。结合镁铁质岩石的ε_Nd(t)正值（+6.1至+10.0）、锆石ε_Hf(t)负值（-16.5至-11.7及-13.6至-0.4），以及较老的Hf模式年龄，上述地球化学特征指示：伊拉山镁铁质-超镁铁质岩石可能起源于古老岩石圈地幔源区，且有软流圈地幔物质与俯冲流体的加入，并伴随有限的地壳混染，形成于侏罗纪-早白垩世班公湖-怒江特提斯洋向南俯冲至拉萨地体（Lhasa terrane）之下的大陆弧构造环境中。