THE PROVENANCE OF ARCHEAN CLASTIC METASEDIMENTS IN THE NARRYER GNEISS COMPLEX, WESTERN AUSTRALIA - TRACE-ELEMENT GEOCHEMISTRY, ND ISOTOPES, AND U-PB AGES FOR DETRITAL ZIRCONS

Clastic metasedimentary rocks of mid-Archean age from the Mt. Narryer and Jack Hills metasedimentary belts have REE patterns resembling those of mid- to late-Archean pelitic-quartzitic cratonic sequences elsewhere, and post-Archean continental rocks in general. Detrital zircons in the metasediments range in age from ca. 3000 to 3700 Ma. This indicates a provenance from mature cratonic sources controlled by K-rich granitic rocks. Additional minor sediment sources were identified as older, mainly chemical sedimentary sequences, ultramafic rocks, and felsic rocks characterized by low HREE contents, perhaps of tonalitic affinity. The association of the near-shore/fluviatile clastic association studied here with extensive turbiditic and chemical sedimentary sequences indicates these sources formed part of a (rifted ?) cratonic margin ca. 3 Ga ago. Differences between sedimentary REE patterns and those in the surrounding 3.73-3.0 Ga orthogneiss terrain, and between detrital zircon ages and the age distribution in the gneisses, suggest that the present association of the metasedimentary belts with the orthogneiss terrain is of tectonic origin. The occurrence of detrital zircons with U-Pb ages > 4 Ga in certain quartzites and conglomerates of the Jack Hills and Mt. Narryer metasedimentary sequences indicates a further, most likely granitic, source. Epsilon-Nd(T(Dep)) values in Jack Hills metasediments vary widely (+5 to -12) but have a smaller range in the Mt. Narryer belt (-5 to -9). The lowest epsilon-Nd values of both sequences are interpreted to reflect the presence of detritus derived from 4.1-4.2 Ga old LREE-enriched continental crust in proportions considerably larger (greater-than-or-equal-to 10%) than estimated previously from the abundance of pre-4 Ga detrital zircons (almost-equal-to 3%). This would imply the former existence of significant volumes of pre-4 Ga continental crust in the provenance of the Mt. Narryer and Jack Hills metasediments.