Provenance versus weathering control on sediment composition in tropical monsoonal climate (South China) - 2. Sand petrology and heavy minerals

Abstract

Together with the companion article dedicated to sedimentary geochemistry and clay mineralogy, this study investigates the interplaying controls on the generation and composition of river sediments across South China. In the Pearl River and southern Yangtze catchments, dominantly sedimentary and basaltic rocks of the Yangtze block shed quartzo-lithic sedimentaclastic sand, whereas mostly granitic and sedimentary rocks of the Cathaysia block generate feldspatho-quartzose sand. Rivers of the Fujian and Zhejiang Provinces draining the SE Coast Magmatic Belt carry feldspatho-litho-quartzose volcaniclastic sand containing epidote with minor clinopyroxene and olivine locally. Within the Pearl River catchment, 35–40% of the sand is derived from the Yangtze block, and 60–65% from the Cathaysia block. Erosion rates are fairly evenly distributed across the basin, reaching higher values in the Hongshui headwaters where topographic relief is greater. South China hosts one of the largest and most spectacular karst areas on Earth, where two landscape types are characterized by different intensity of chemical dissolution. In fengcong (cone) karst, carbonate detritus attacked by carbonic and sulfuric acids is partly preserved, whereas carbonate grains are completely dissolved in fenglin (tower) karst developed in wetter regions to the east. The strong climatic gradient from the dry Tibetan plateau in the west to monsoon-drenched coastal areas is faithfully reflected in illite-rich vs. kaolinite-rich clay-mineral assemblages, but in sand the weathering effect is much harder to isolate from the dominant effects of source-rock lithology and recycling. The different durability of tectosilicates (quartz > microcline > orthoclase > plagioclase) and the degree of corrosion displayed by detrital minerals offer essential qualitative information but cannot represent robust proxies to quantify weathering intensity. The sand generation index SGI reveals whether lithologies are under- or over-represented in detrital assemblages and proved to be most useful to trace different weathering regimes across South China.