Ion microprobe U-Th-Pb isotopic studies of zircons from three early Precambrian areas in the U.S.S.R.

Abstract

Ion microprobe U-Th-Pb dating of zircons from three polymetamorphic Archaean terranes in the Soviet Union has provided some precise estimates of protolith and metamorphic ages which confirm that the ages inferred previously from conventional zircon U-Pb analysis are broadly correct. The gneisses of the Novopavlovsk Complex, intersected by drilling in the eastern Ukrainian Shield, contain pyroxenite enclaves. Zircons in one pyroxenite give 3450±8 Ma, and in a nearby enderbite give 3441 ± 10 Ma. Two pyroxenites from another drill hole contain zircons as old as 3633 ± 16 Ma and 3640 ± 11 Ma respectively, in addition to clearly metamorphic zircons interpreted to be 2901 ± 34 Ma and 2664 ± 68 Ma old in one, and 2595 ± 92 Ma old in the other. The tonalitic orthogneiss host to these two enclaves is 3609 ± 5 Ma old, and was metamorphosed at 2826 ± 16 Ma. The zircons in the pyroxenites are probably not primary, but grew when the pyroxenites were incorporated into the magmatic precursors of the orthogneisses. Zircons in a leucocratic orthogneiss from the Aulandjha block in the Omolon Massif, Soviet Far East, are interpreted to be mostly 3481 ± 18 Ma old, but a minor metamorphic population gives 2667 ± 29 Ma. A recently reported age of ∼ 3900 Ma from the Daldin Series in the Anabar Shield in central Siberia was not confirmed. The zircon age of an enderbite from the Daldin Series is interpreted to be 3164 ± 32 Ma.