Biotite-controlled linear compositional weathering trends in tonalitic to quartz dioritic saprock, Santa Margarita Ecological Reserve, southern California, USA

Geochemical data derived from saprock produced by the weathering of K-feldspar-absent or K-feldspar-poor tonalite to quartz diorite yield linear compositional weathering trends pointed away from the p(K)-apex in p(A)–p(CN)–p(K) ternary diagrams. In contrast, published geochemical data derived from the classical granodioritic Toorongo weathering profile of south Australia yields a linear compositional weathering trend pointed away from the p(CN)-apex. The latter plagioclase-dominated weathering trend has been recognized since about 1984 and is due to the removal of CaO and Na2O from plagioclase. In contrast, to our knowledge, the biotite-dominated weathering trend derived from K-feldspar-absent to K-feldspar-poor tonalite to quartz diorite has not been documented or recognized previously, and is likely due to the removal of K2O from the interlayer position of biotite as it is transformed into vermiculite or mixed-layer biotite/vermiculite.

► We modeled A–CN–K linear compositional weathering trends in saprock at three sites.
► On ternary diagrams linear compositional trends point away from the p(K) apex.
► Biotite is only mineral containing significant K in studied quartz diorite to tonalite.
► Clay minerals include vermiculite ± mixed-layer biotite/vermiculite ± kaolinite.
► During weathering K is removed from the interlayer position in biotite.