Cg. Skilbeck et Pa. Cawood, PROVENANCE HISTORY OF A CARBONIFEROUS GONDWANA MARGIN FORE-ARC BASIN,NEW-ENGLAND FOLD BELT, EASTERN AUSTRALIA - MODAL AND GEOCHEMICAL CONSTRAINTS, Sedimentary geology, 93(1-2), 1994, pp. 107-133
The Visean-?Stephanian Myall Trough, eastern Australia, formed part of
a forearc basin to the east of a magmatic are situated along the pala
eo-Pacific margin of Gondwana. Petrographic, modal, whole-rock geochem
istry and mineral phase chemistry of sandstones, siltstones, conglomer
ates and associated igneous rocks provide a consistent and coherent da
ta set which indicate sediment derivation from a magmatic are of predo
minantly silicic volcanic composition. The low-quartz modes of the san
dstones, the largely volcanic origin of lithic fragments and the overa
ll high SiO2 content of the siliciclastic sediments and igneous rocks
require the are source to be largely dacitic to rhyolitic in compositi
on. The predominance of vitric type-a (highly siliceous volcanic rocks
) lithic fragments suggests a source composed largely of silicic pyroc
lastics. The presence of potassium feldspar, as well as the presence o
f biotite as the principal detrital ferromagnesian phase in the sandst
ones, suggest a source of medium- to high-K tholeiite composition. The
high trace element abundances of relatively immobile elements such as
Zr, La, Ce, V, Nb, Y and Th and corresponding high Th/U, Zr/Y, La/Y L
a/Sm ratios and low Zr/Th, Zr/Nb, La/Th and Ti/Zr ratios for both the
igneous rocks and derived siliciclastic sediments in the Myall Trough
also require a high-K and/or strongly medium-K are source. A lack of t
emporal or spatial trends in the data indicate that the magmatic are r
emained of uniform composition throughout the 25 Ma interval of strata
preserved within the trough. The provenance history of the Myall Trou
gh requires accumulation along an Andean-type continental margin, rath
er than an intra-oceanic are outboard of the Gondwana margin.