Geochronology and Petrogenesis of the Donghe Pt-Pd-Bearing Ultramafic Dykesin the Northern Margin of the Yangtze Block:Constraints from Zircon Geochronology, Geochemistry and Sr-Nd-Hf Isotpopes
Cao Liang
Chen Lin
Duan Qifa
Zhou Yun
Liu Chongpeng
Zhang Liguo
Abstract：The Donghe Pt-Pd-bearing ultramafic dykes are located in the northern margin of the Yangtze block in Fangxian, the Pt-Pd ore bodys are present in the ultramafic dykes of pyroxene veins.The LA-ICP-MS zircon U-Pb dating indicates that the ultrabasic dykes were emplaced with age of 433.2±2.9 Ma, belonging to the early Silurian.The rocks are characterized by low SiO2(39.73％-47.46％, mean 41.41％), moderate Mg# (46.98-67.37) and high alumina (11.25％-15.46％), belonging to high-alumina ultrabasic rocks.The rocks have higher total REE, with slightly enriched LREE and without significant Eu positive anomaly (ΣREE=59.59×10-6 to 375.02×10-6, (LaN/YbN) =3.94-19.13, δEu=0.93-1.13).All rocks are enriched inRb, Ba, Sr, Nb, Ti, etc., depleted in Nb, Hf, P, etc.and have significantly different ratios of incompatible elements to that in the crust.Sr, Nd and Hf isotope studies show that the Donghe ultramafic rocks have low initial 87 Sr/86 Sr ratios (0.703 26-0.704 15), positiveεNd (t) (4.37～5.27) and positiveεHf (t) (7.29-10.26).Based on the above discussion, we propose that the studied ultramafic rocks were derived from a depleted lithospheric mantle (garnet-lherzolite) source which may be enriched lithospheric mantle components.Magma in the rising process was not significantly affected by the crust material mixed.The tectonic discrimination of trace elements shows that it formed in intraplate tectonic environment.The analysis shows the Donghe region should be on the passive continental margin during the Early Silurian and should be in extensional state.The twostage ultramafic bedrocks (veins) exposed in the study area were the products of the early stage of the continental rift in the state of extensional of the lithosphere.
Keywords：ultramafic rockszircon geochronologypyroxeniteSr-Nd-Hf isotopesDonghe areaYangtze blockgeochemistry
Publication Date：2019-01-01
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：21( 366-386 )