Provenance and Al/Si Order-Disorder of Detrital Alkali Feldspars

The principle that detailed characterization of minerals enhances petrogenetic interpretation is extended to sedimentary petrology by estimating the Al/Si order-disorder of detrital alkali feldspars from Holocene fluvial sands derived from granite plutons. A weathering-independent characterization of the influence of source rocks on daughter sands is based on structural state determination of 42 detrital alkali feldspars. Structural states were estimated from refined unit cell parameters derived from slow oscillatory difractograms. .

None of the studied feldspars show anomalous cell dimensions as inferred from b-c plots. In addition, estimates of Or content from refined cell volumes correspond very well with microprobe measurements of the Or contents of the same grains. In crystalline rocks, however, many alkali feldspars have anomalous cell dimensions; their estimated and measured Or contents are also often widely different. This suggests that detrital alkali feldspars may be amenable to rapid x-ray methods of structural state determination.

Detrital alkali feldspars in sands derived from pre-Mesozoic plutons are triclinic with a relatively low structural state (average Δ bc=0.975; t₁(o)=0.479).

The age of the parent pluton may be a factor in controlling the relative structural states of detrital feldspars derived from the pluton. Alternatively, the difference in ordering between the two groups of feldspars might reflect the differences in the thermal history of the different zones of the parent batholith supplying feldspars. It is possible that the rapidly cooled outer shells of plutons are quenched at higher temperatures than their relatively slowly cooled cores. Thus the outer shells would tend to produce alkali feldspars with relatively higher structural states than those coming from the core. If so, a vertical sequence of sediments derived from systematic unroofing of a batholith should reflect this change in feldspar structural state.