The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region - The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The (187)OS/(188)Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the (117)OS/(188)Os-AI(2)O(3) alumichron, the samples yield a model age of 2.5 +/- 0.1 Ga (data of USTC) and 1.9 +/- 0.1 Ga (data of CRPG), late Archean to early Proterozoic. The two samples with the lowest (117)Os/(188)Os ratio (0.119 and 0.117) have the T(RD) (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle but residual fractions of Proterozoic mantle.