Petrology and geochemistry of ultramafic and mafic rocks in the late Silurian-early Devonian Darbut ophiolitic mélange of west Junggar (NORTHWESTERN CHINA): implications for petrogenesis and tectonic evolution

New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.

本次研究整合了西准噶尔达尔布特蛇绿混杂岩中二辉橄榄岩（lherzolite）、方辉橄榄岩（harzburgite）及辉长岩（gabbros）的岩石学与地球化学数据，以约束达尔布特蛇绿岩的地质演化过程。二辉橄榄岩由橄榄石、斜方辉石、单斜辉石及铬尖晶石组成，其铬指数（Cr#）介于34~39之间，与饱满型深海橄榄岩特征相似。方辉橄榄岩以橄榄石、斜方辉石、单斜辉石及铬尖晶石为主要矿物，铬指数（Cr#）相对较高（48~55），与俯冲带上方（SSZ）橄榄岩特征相似。各向同性辉长岩具有平坦型稀土元素（REE）配分模式、低Nb/Yb比值及高Ti/V比值的特征，可与洋中脊玄武岩（MORB）类比。角闪辉长岩表现出流体活动性元素相对富集以及较高的Th/Yb比值，其地球化学特征与弧前玄武岩相近。部分熔融地球化学模拟结果显示，二辉橄榄岩样品形成于原始地幔经11%~16%低程度无水动态熔融的过程，而方辉橄榄岩样品则是在原始地幔已亏损16%的基础上，经历了5%~10%的二期部分熔融作用。上述数据表明，达尔布特蛇绿岩形成于弧前构造环境。准噶尔洋岩石圈俯冲起始引发的上涌软流圈发生低程度、无水减压熔融，于晚志留世形成了二辉橄榄岩及类洋中脊玄武岩熔体。随着俯冲板片持续下沉，俯冲板片析出流体的注入量不断增加，显著增强了亏损地幔的部分熔融程度，进而形成了难熔方辉橄榄岩。