Tracing provenance and sediment fluxes in the Irrawaddy River basin (Myanmar)

This study illustrates the petrographic, heavy-mineral, geochemical and geochronological signatures of sand transported by various branches of the Irrawaddy (Ayeyarwadi) River, one of the first in the world for sediment flux. Intrasample and intersample compositional variability, weathering and hydraulic-sorting controls are also discussed. Feldspatho-quartzose sand in Irrawaddy headwaters is largely derived first-cycle from mid-crustal metamorphic and plutonic rocks of the Mogok Belt and Lohit complex, whereas feldspatho-litho-quartzose Chindwin sand is largely recycled from supracrustal, sedimentary and very low-grade metasedimentary units. Additional mafic to ultramafic detritus is derived from ophiolites and blueschists exposed from the Indo-Burman Ranges to the Jade Mines and Myitkyina belts, linked northward to the Yarlung-Tsangpo suture of the Himalaya. Volcanic detritus derived from the Popa-Wuntho arc or recycled from forearc-basin strata also occurs. Decreasing concentration of most chemical elements along the Irrawaddy reflects progressive addition of detritus recycled from sedimentary rocks, most evident downstream of the Chindwin confluence. REE patterns with LREE enrichment and negative Eu anomaly reflect the occurrence of allanite, largely derived from granitoid rocks in the Mali catchment. Chemical indices indicate moderate weathering in the monsoon-dominated climate of Myanmar. Young U-Pb ages (15–170 Ma) represent 85% of detrital zircons in Irrawaddy headwater branches, reflecting long-lasting subduction-related magmatism along a ring of fire connecting with the southern and central Lhasa batholiths in Tibet and polyphase metamorphism in the Mogok belt. Chindwin sand contains larger amounts of finer-grained, recycled pre-Mesozoic zircons, also yielding early Mesoproterozoic to Archean ages. Such different petrographic, heavy-mineral, geochemical and geochronological fingerprints characterizing sand in different river branches allowed us to calculate bulk-sediment and zircon-provenance budgets that converge to indicate equivalent sand supply from the Nmai and Mali Rivers to the upper Irrawaddy, and from the Chindwin and upper Irrawaddy to the lower Irrawaddy. This implies that despite of higher erosion potential indicated by stream-profile analysis in high-relief Irrawaddy headwaters, sediment yields and erosion rates are detectably higher in the Chindwin catchment, which is mainly ascribed to higher erodibility of widely exposed siliciclastic rocks. Quantifying sediment provenance and defining erosion patterns based on an integrated compositional database in a big-river system such as the mighty Irrawaddy allows us to expand our understanding of sediment-generation processes with the ultimate goal to increase our capacity to read into the stratigraphic record.