TOWARD A UNIFIED MODEL FOR THE WEATHERING OF GRANITOIDS: THE CASE FOR K-FELDSPAR DEFICIENT SYSTEMS

We investigated the in-situ mineralogical and chemical patterns resulting from the transformation of tonalitic corestone to saprock in a mild Mediterranean climate at three variably weathered sites located along the slopes of the canyon containing the Santa Margarita River, Santa Margarita Ecological Reserve, SW California. At site I, near the base of the regolith, weakly developed saprock surrounds a corestone and falls apart with a gentle tap of the hammer. In contrast, at site II located 8.8 m higher than site I, well developed saprock envelopes another corestone and easily disaggregates under moderate hand pressure. At site III, lying 3.4 m higher than site II, very well developed saprock adjacent to a partially exposed corestone easily crumbles with weak hand pressure. K-feldspar is present only at site I where it makes up ~<2% of whole-rock mineralogy; otherwise the primary components of the three sites are quartz, plagioclase, biotite, and hornblende. Biotite in saprock is significantly less pleochroic than is biotite in adjacent corestones and in plain light takes on a golden orange color. In contrast, the An-rich cores of plagioclase are uncommonly altered to a high birefringent clay material, while hornblende and quartz appear to be unaffected by the transformation of corestone to saprock. When plotted on A-CN-K diagrams, whole-rock chemical data at site I cluster, but at site II spread slightly forming a weathering trend oriented at a high angle to the CN-K join. At site III, whole-rock chemical data capture the weathering trend identified at site II, and then spread parallel to the A-CN join. Since biotite is the most abundant K-bearing mineral at the three sites, the above trends and observations suggest that weathering is initiated primarily by the alteration of biotite and to a lesser extent plagioclase, and that only after removal of potassium from interlayer positions, is weathering dominated by the leaching and removal of calcium and sodium from plagioclase. Our microprobe data support this idea, and show that biotite at site III is significantly more altered than biotite at site II, and that plagioclase, though weathered, is less altered than is biotite. We conclude that the popular weathering model for granitoids must be modified to incorporate the above results for tonalitic plutons lacking K-feldspar.