Geochemical Characteristics and LA‐ICP‐MS Zircon U‐Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

Abstract

Abstract Geochemical studies on the amphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The amphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N = 0.33–0.55, (La/Sm)N = 0.45–0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N‐MORB. The second group of rocks have (La/Yb)N = 0.63–0.95, (La/Sm)N = 0.69–0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N‐MORB and E‐MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in the highly incompatible elements spider diagram, but the first group of rocks have lower element abundances than the modern N‐MORB, indicating a derivation of their mantle source from more depleted mantle source than the present N‐MORB. The abundances of Th, Ta, Nb, La and Ce in the second group of rocks are slightly higher than those of the present N‐MORB, and other elements, such as Hf, Zr, Sm, Ti, Y and Yb, are close to those of the N‐MORB, indicating that the original magma was derived from depleted mantle but mixed with the enriched mantle. These characteristics, combined with the regional geology and previous studies, provide further evidence that the mafic‐ultramafic rocks have the features of a typical ophiolite.Zircon grains from the amphibolite are generally rounded, and in most of them a distinguishable core‐mantle texture is preserved as shown in the cathodoluminescence (CL) images. The core or core‐mantle parts of the zircon grains are also rounded, same as those in basalts from other regions of the world. The LA‐ICP‐MS trace element and U‐Pb isotopic analyses show that the zircon grains from the amphibolites are similar to the typical magmatic zircon in terms of their very low U and Th contents (62.36–0.10 μg/g and 78.47–0.003 μg/g, respectively). Seven pits from the core and core‐mantle parts of the zircon grains yielded an average weighted 206Pb/238U age of 973±35 (2s̀) Ma with the Th/U ratios range from 0.01 to 8.38 and mostly greater than 0.23. This age is consistent within the error range with the whole‐rock Sm‐Nd isochron age of 1030±46 Ma for the same kind of rocks reported by Dong et al. (1997a). In a combined analysis with the zircon positions on the CL images and the corresponding Th/U ratios, the age of 973±35 Ma is probably the formation age of tholeiite, the protolith of the Songshugou amphibolite. The geochronological determination gives further evidence that the Songshugou ophiolite was formed during the Neoproterozoic. In addition, there is one pit from the rim of a zircon grain giving a 206Pb/238U age of 572±199 (1s̀) Ma with a Th/U ratio of 0.08. It may represent the age of the accretionary zircon in the amphibolite‐facies metamorphism.