Arkose, Subarkose, Quartz Sand, and Associated Muds Derived from Felsic Plutonic Rocks in Glacial to Tropical Humid Climates

Abstract

Abstract In the range of climates from high-latitude glacial to equatorial tropical, the mineral and chemical compositions of over 600 modern nonmarine, first-cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied. Under glacial conditions, sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks. Humid tropical climates yield quartz-rich sands, kaolinite-rich muds, along with Si, Mg, Ca, Na, and K in solution. Between these extremes, a wide range of detrital sediment compositions are developed in arid to wet climates. In arid-climate and semi-arid-climate sands, feldspar is commonly more abundant than quartz, and plagioclase > K-feldspar (except in the cases of granite source rocks); the muds are feldspathic with smectite dominant in the clays. With increasing temperature and moisture in temperate to subtropical climates, mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant K-feldspar. Through this range of climate, muds become more kaolinitic and thus more aluminous with increasingly severe weathering. Typically, feldspathic sands are associated with feldspathic muds; with decreasing feldspar in sands, there is less feldspar in muds. With increasingly intense chemical weathering, rare earth elements (REE) are more fractionated into muds. Typically sands contain 150 ppm REE with much of the REE in heavy minerals. Abundance differences are greatest between humid tropical sands and muds. In many samples the REE are mostly in monazite, allanite, sphene, and apatite. More intensely weathered samples contain more xenotime and zircon, evidenced by Yb enrichment.