Geochemical discrimination of elastic sedimentary rock sources

Factors controlling the geochemistry of a elastic sedimentary rock can include: (1) composition of source terrain, (2) chemical weathering, (3) hydraulic sorting, (4) diagenesis, (5) metamorphism, and (6) hydrothermal alteration. A linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present. The use of graphical analysis of element pairs circumvents the problem. Chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements. This results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin, if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner. As chemically mobile element plots will produce a scatter of points, this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history. The constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin. A second technique, using SiO2 plots, has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements. SiO2-immobile element plots result in a linear arrangement of points extending towards either 0% or 100% SiO2. The 0 intercept position occurs for elements with major mineral phases concentrated in sand; the 100 intercept for those concentrated in clays. Plotting chemically mobile elements produces different patterns, and this can be used to gain information on alteration and sorting history. Elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material. This provides a powerful tool for deducing source terrain from sediment geochemistry. Techniques outlined above were tested on Archaean metasandstones from Superior Province, Canada. Immobile element ratio diagrams for Nb-Al-Ti and Zr-Al-Ti indicate that a calc-alkaline extrusive-intrusive suite lying to the north of the study area was the source, and not five other volcanic suites in the region. This conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques.