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) hydro thermal alteration. A linear solution inferring source terrain composi tion from geochemistry of the sediment is impossible in this multivari ate system as several unknowns will commonly be present. The use of gr aphical analysis of element pairs circumvents the problem. Chemically immobile elements will maintain invariant ratios during rock mass chan ge caused by either addition or depletion of mobile elements. This res ults in chemically immobile element scattergrams exhibiting linear tre nds along radians extending from the origin, if the major mineral phas es 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 sedime nt well mixed prior to delivery to the basin. A second technique, usin g SiO2 plots, has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elemen ts. SiO2-immobile element plots result in a linear arrangement of poin ts extending towards either 0% or 100% SiO2. The 0 intercept position occurs for elements with major mineral phases concentrated in sand; th e 100 intercept for those concentrated in clays. Plotting chemically m obile elements produces different patterns, and this can be used to ga in 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 sour ce 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 eleme nt ratio diagrams for Nb-Al-Ti and Zr-Al-Ti indicate that a calc-alkal ine extrusive-intrusive suite lying to the north of the study area was the source, and not five other volcanic suites in the region. This co nclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques.