Architecture of ophiolitic mélanges in the Junggar region, NW China

Ophiolitic mélanges, units that contain components of an ophiolite suite, provide crucial information on earth history and orogenic evolution. In this paper, four ophiolitic mélanges are characterized, including the Baijiantan-Yeyagou, Hebukesair, Zhaheba and Hongguleleng mélanges in the Junggar region (NW China), southern Altaids. Detailed geological mapping, structural, geochronological, and geochemical analyses constrain ages, geochemical affinities and relationships of magmatic with clastic rocks. MORB-/OIB-type (meta)gabbros and plagiogranite are the oldest mélange components, slightly older or coeval with associated chert and MORB-/OIB-type basalt; these rocks collectively constitute an ophiolite suite. Ophiolitic rocks predate associated clastic sedimentary rocks (conglomerate and turbidite) by ∼90-25 My, and associated SSZ-type hornblende gabbro, basaltic andesite, diabase, diorite and rhyolite by ∼78-37 My, except in the Hongguleleng mélange, where ophiolitic rocks predate latter units by ∼9-25 My. The ophiolitics are repeated by imbricate thrusts and duplexes, and folded. Ophiolitic blocks in mélanges locally preserve similar structures. Such blocks commonly have MORB and/or OIB geochemical affinities. Significantly older ages of MORB/OIB igneous rocks compared to ages of associated clastic sedimentary/SSZ-type igneous rocks shows that the former rocks formed as part of the crust of a large ocean, far from a convergent margin. The far-traveled oceanic crustal slices were imbricated and disrupted into block-in-matrix structures during accretion and incorporation into a subduction complex. SSZ-type magmatic rocks locally intrude into and extrude onto clastic rocks, demonstrating that a mélange contains multi-stage magmatic rocks. Folds, tilted structures and shear band cleavages are locally cross-cut by dikes, and these rocks are themselves have been dismembered into blocks. An intruded conglomerate in the Hongguleleng mélange contains pebbles of gabbro and basaltic andesite, the latter of which overlies sandstone. Superimposed folds in clastic rocks and chert record the polydeformation of the mélanges. Determination of the complex relationships of multi-stage magmatism and deformation illuminates the tectonic history of ophiolitic mélanges in the Junggar region. This history includes formation and subduction-accretion of the crust of a large ocean and post-subduction intracontinental deformation.