6533b7d1fe1ef96bd125d604

RESEARCH PRODUCT

Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating

subject

description

The authors report precise U-Pb zircon ages for single grains of a metarhyodacite from the Late Archean Dengfeng greenstone belt in Henan Province, China, near the southern margin of the North China craton. Most grains belong to an igneous population whose U-Pb isotopic systematics define a straight line intersecting concordia at 2512 +/- 12 Ma, and this is interpreted as the time of crystallization of the original greenstone volcanics. Several grains are distinctly older, between 2576 +/- 9 and 2945 +/- 44 Ma, and the authors interpret the older grains as xenocrysts of pre-greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava. The xenocrysts suggest evolution of the Dengfeng greenstone belt in a continental environment that may be represented by the Taihua high-grade gneisses bordering the Dengfeng greenstones and for which the authors obtained ages of 2806 +/- 7 and 2841 +/- 6 Ma. The data add evidence to the now widely held concept that most Archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination. In such cases, conventional zircon dating may not always record the precise age of rock formation.