Provenance signatures of sandstone-mudstone suites determined using discriminant function analysis of major-element data

Abstract

Sandstones and argillites from selected New Zealand greywacke terranes represent four differing provenance groups: P1 (mafic) — first-cycle basaltic and lesser andesitic detritus, represented by the Maitai terrane and part of the Caples terrane; P2 (intermediate) — dominantly andesitic detritus (Waipapa and Pelorus terranes); P3 (felsic) — acid plutonic and volcanic detritus (Torlesse terrane); and P4 (recycled) — mature polycyclic quartzose detritus (Greenland Group). Contrasts in major-element composition occur between the groups due to differing provenance. SiO 2 Al 2 O 3 and K 2 O Na 2 O ratios increase, and Fe 2O 3T + MgO decreases from P1 to P4, as a result of petrologic evolution (P1-P3) and sedimentary maturation (P4). Some parts of oxide or ratio variation diagrams are distinctive for each group, but considerable overlap occurs due to bulk compositional variation with decrease in grain size. This overlap is almost eliminated by discriminant function analysis using Al 2O 3, TiO 2, Fe 2O 3T, MgO, CaO, Na 2O and K 2O, and a plot of the first two functions gives effective separation between the four groups. Tests of the functions with analyses of volcanic and plutonic rocks from New Zealand and the Lau volcanic arc, additional sedimentary terranes of New Zealand, and published data of sedimentary suites from Australia and the Pacific margin give good results, suggesting that the method is a viable technique for provenance determination which is largely independent of grain-size effects. The functions are applicable only to rocks which lack significant biogenic fractions, or to those where analyses can be corrected for these inputs. A second set of functions using oxide Al 2 O 3 ratios are designed for samples influenced by biogenic sedimentation. Although classification and test results are adequate, results are mixed for suites of Tertiary and modern sediments with substantial calcareous or siliceous components from New Zealand, the Japan Trench and the Solomon Islands.