EXPLOSITE: A TYPE OF MANTLE-DERIVED MAGMATIC-HYDROTHERMAL LACUSTRINE LAMINATED SEDIMENTARY ROCKS

Sublacustrine volcanic and hydrothermal processes and products are complex and affected by special physical, chemical and biological factor. A new type of sedimentary rocks, named explosite, is discovered in Permian Lucaogou Fm. in lacustrine Santanghu rift basin, NW China. Core and outcrop samples were studied using transmitted and reflective-light, CL, and SEM microscopy, microprobe and XRD analyses. Element composition and Sr-Nd isotopic compositions were analyzed to interpret the origin of minerals and diagenetic fluids. Four types of lithofacies are composed of magmatic, hydrothermal, and sedimentary minerals and grains of diopside, dolomite, peralkaline feldspars, analcime, quartz, ankerite, serpentine, montmorillonite, and calcite. Grains are 0.001-2 mm in size and mostly 0.01-0.1 mm. The rocks are generally well laminated. Laminae are mostly 0.5-3 mm thick and alternate with tuffaceous dolomicrite. The small grain size suggests that the framework and matrix grains were originated from fine-grained sources. Embayed boundaries are common in individual minerals as well as aggregated grains, suggesting multiple episodes of melting and welding under a high temperature condition. The sediments were sourced mainly from underground magma and hydrothermally-altered country rocks, and transported through volcanic and hydrothermal conduits into the lake water. Hot fluids formed by interaction of magma and groundwater near or in the magma chamber. Liquid magma, magmatic minerals, hydrothermal metamorphic minerals, country rock fragments, and steam and water probably comprised the hot fluids before eruption. As a result, the sediment grains supplied by the hot fluids have complex compositions. The hot fluids entered the lake to pulverize a variety of solid phases. The powers were transported on the lake floor by turbidity currents and deposited as laminated sediments. They were cemented and further hydrothermally altered on the lake floor or during shallow burial. Explosive hydrovolcanic process is key to forming these unique sedimentary rocks. Thus, the rocks can be classified as explosite. The depositional model of these rocks may spur new concepts on the origins of metallic ore and hydrocarbon deposits and paleogeography in the Santanghu region.