Geochemical and Nd Isotopic Characteristics of Some Rocks from the Paleoarchaean Chentaigou Supracru

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The Chentaigou supracrustal of Paleoarchaean age is located in the Chentaigou-Baijiafen area, Anshan City, Liaoning Province, and composed mainly of amphibolite, quartzite and biotite-plagioclase schist, with a small amount of meta-acid volcano-sedimentary rock. They have undergone several episodes of metamorphism. Zir- con ages of the trondhjemitic veins and the leptite (Song et al., 1994) suggest that the supracrustal rocks in the area were formed around 3.35 Ga ago. Our geochemical and Sin-Nd isotopic results indicate that the trondhjemitic veins intruding the Chentaigou supracrustal may be derived from the 3.8 Ga Baijiafen trondhjemite. On the other hand, although the calculated initial ε_(Nd) value (-2.11) of the leptite, the meta-acid volcanosedimentary rock constituting one part of the supracrustal, is similar to that of the trondhjemitic veins, their element distributions, REE patterns in particular, are quite different. This suggests that the parent rock of the leptite was formed by the melting of some ancient continental material which is distinct from the 3.8 Ga Baijiafen trondhjemite. The amphibolite shows a geochemical composition similar to young island arc tholeiite. Eight samples of the amphibolite give a Sin-Nd isochron age of 2979 ± 384 (2σ) Ma, which is considerably younger than the zircon ages of the trondhjemitic veins and leptite. These dais are interpreted as recording a late metamorphic event. The rock association and geochemical compositions further indicate that the Chentaigou supracrustal was formed upon an old continental basement, probably in a tectonic environment similar to a modern island arc system. The Chentaigou supracrustal of Paleoarchaean age is located in the Chentaigou-Baijiafen area, Anshan City, Liaoning Province, and mainly composed of amphibolite, quartzite and biotite-plagioclase schist, with a small amount of meta-acid volcano-sedimentary rock. They have undergone Several episodes of metamorphism. Zir-con ages of the trondhjemitic veins and the leptite (Song et al., 1994) suggest that the supracrustal rocks in the area were formed around 3.35 Ga ago. Our geochemical and Sin-Nd isotopic results indicate that the trondhjemitic veins intruding the Chentaigou supracrustal may be derived from the 3.8 Ga Baijiafen trondhjemite. On the other hand, although the calculated initial ε_ (Nd) value (-2.11) of the leptite, the meta-acid volcanosedimentary rock formed one part of the supracrustal , is similar to that of the trondhjemitic veins, their element distributions, REE patterns in particular, are quite different. This suggests that the parent rock of the leptite was formed by the melting of some ancient continental material which is distinct from the 3.8 Ga Baijiafen trondhjemite. The amphibolite shows a geochemical composition similar to young island arc tholeiite. Eight samples of the amphibolite give a Sin-Nd isochron age of 2979 ± 384 (2σ These are desert younger than the zircon ages of the trondhjemitic veins and leptite. These dais are interpreted as recording a late metamorphic event. The rock association and geochemical compositions further indicate that the Chentaigou supracrustal was formed upon an old continental basement, probably in a tectonic environment similar to a modern island arc system.
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