Sedimentology, geochemistry, geochronology, and tectonic implication of the matrix in Hongliuxia melange, southern Dunhuang Orogenic Belt

The integrated investigation of the matrix in Hongliuxia melange, southern Dunhuang Orogenic Belt (DOB), provides new evidences for understanding the formation and evolution of the DOB. In this contribution, sedimentological, geochemical, and geochronological studies were undertaken on the matrix for constraining its tectonic setting formed. The matrix in Hongliuxia melange dominantly consists of sheared metapelite (mica-quartz schists) and meta-sandstones. Locally, some of the matrix preserves the original sedimentary structures (such as T-ab, T-de, T-bde divisions of Bouma sequence). This indicates the protolith of the matrix is turbidite flysch. In detrital composition, the matrix are composed of predominate feldspar, quartz, and lithic fragment. High proportion of feldspar (47%) and lithic fragment (27%) indicate abundant magmatic-derived clasts were involved. The clasts are poorly sorted and rounded, suggesting a short distance before deposited. Geochemically, Low CIA values (49 similar to 67) of the matrix indicate low-degree of weathering in the provenance. High ICV values (mostly >0.8) and the Zr/Sc-Th/Sc diagram reflect the sediments were not significantly recycled before deposited. The matrix show low Sc, Cr, Co, Ni contents, and similar Eu/Eu*, La/Sc, Th/Sc, La/Co, Th/Co, and Cr/Th ratios with the clasts shed from the felsic rocks provenance. This indicates the source rocks comprise dominant intermediate-acidic rocks, though few mafic rock clasts involved. The La/Sc-Ti/Zr and Th-Sc-Zr/10 plotting of the matrix suggest they form in the continental arc or active continental margin tectonic setting. Framework modes of slightly deformed and low-grade metamorphosed sandstones indicate the clasts of matrix predominantly derive from the "dissected-transitional arc" provenance. To some up, the composition of low recycled, near sourced "dissected-transitional arc" derived clasts, accompanied by the sedimentary structure of mass-transport deposit (MTD), manifest the matrix deposit in the trench associated with the subduction zone along continental arc or active continental margin. The detrital zircons from the matrix show three age groups : 2300Ma, 1850Ma, and 423Ma. Combined with the regional geological background, we preliminary speculate the clasts probably shed from the Paleozoic granitoid and exhumed metamafic rocks in the Sanweishan arc and Dongbatu-Mogutai arc, north to the Hongliuxia melange. The meta-sandstones of the flysch matrix yield the youngest magmatic detrital zircon age of 389Ma, indicating the protolith (sandstone) formed after Middle Devonian. This further indicates the ocean basin of the Hongliuxia area had not been closed at Middle Devonian and the collision hadn't initiated at that time.