Petrology geochemistry and tectonic setting of some flysch deposits

Abstract

The Paleozoic flysch sequences of eastern Australia show large variations in their mineralogical and geochemical compositions. On the basis of detrital mineralogy, the following five greywacke suites are recognised : Tamworth, Hill End, Hodgkinson, Bendigo and Cookman. The graywackes exhibit increasing maturity from the Tamworth to Cookman suites, characterised by an increase in quartz content and a decrease in lithic and feldspar grains. The greywacke suites are derived from dominantly andesitic, dacitic, crystalline, meta-sedimentary and sedimentary source rocks, respectively. The associated mudrocks are also characterised by variations in abundances of phyllosilicates and tectosilicates (feldspar and quartz). Three broad groups of mudrocks are recognised : tectic type (Tamworth suite); phylo-tectic type (Hill End and Hodgkinson suite); and phyllic type (Bendigo and Cookman suites). With the increase in mineralogical maturity, increasing geochemical differentiation is observed in the sedimentary suites, in the form of enrichment of Si and Zr in graywackes, and enrichment of large cations (K, Rb, Ba), Al-group (Al, Ga) and ferromagnesian elements (Cr, Ni, Zn) in mudrocks, and the loss of small cations (Ca, Na, Sr) in solution. A close similarity is observed between the Th, U, Nb, La/Y, Zr, Sc/Ni, Ni/Co, La/Yb and rare earth element characteristics of greywacke suites and oregenic andesites from various tectonic settings. On this basis, the Tamworth suite is assigned to an oceanic island arc; the Hill End suite to a continental island arc; and the Hodgkinson suite to an Andean type tectonic setting. The Bendigo and Cookman suite graywackes are characterised by their highly quartzose nature and highly fractionated chemistry, suggesting their recycled nature and possibly a passive margin type of tectonic setting. The major element geochemistry of arenites can be used to infer the provenance type and tectonic setting of sedimentary basins. In general, there is a progressive decrease in total Fe as Fe20/Na20 and A1203 /(Ca0+Na20) in arenites as the tectonic setting changes from oceanic island arc to continental island arc to Andean type to passive margins. The trace element characteristics of sedimentary rocks show excellent signatures f provenance types and tectonic settings. The most useful elements are those which are relatively immobile, fractionate only in the clastics and have low residence times in sea water, e.g., Th, U, Nb, Zr, REE, Y, Sc and Co. Optimum discrimination of oceanic island arc, continental island arc, Andean type and passive margin tectonic settings is achieved by La-Th; Ti/Zr-La/Sc; La/Y-Sc/Cr; La-Th-Sc; Th-Sc-Zr/10; and Rb-V-Zr plots, for arenites. The trace elements in mudrocks also show characteristics of the tectonic setting and the most discriminating parameter are Th, Nb, U, Nb/Y, Th/U, Zr/Th and La/Sc. The bulk oceanic island arc sedimentary composition is similar to the composition of the total crust, whereas the average Andean type-passive margin sedimentary composition is comparable to the upper continental crustal composition. The continental island arc sedimentary composition is intermediate. The change from oceanic island arc to continental island arc to Andean type and passive margin sedimentary compositions is similar to the change in the average compositions of sedimentary rocks from Archean through Proterozoic to Phanerozoic. This suggests a gradual mafic to felsic transition in crustal composition through geographical time.