Zircon U-Pb geochronology, Hf isotopic composition and geological implications of the rhyodacite and rhyodacitic porphyry in the Xiangshan uranium ore field, Jiangxi Province, China

Abstract

The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China. The host rock is a volcanic intrusive complex, including rhyodacite, porphyroclastic lava and late stage sub-volcanic rocks. In this study, zircons from an early stage rhyodacite and a late stage rhyodacite porphyry were dated by SHRIMP and LA-ICP-MS U-Pb methods, and their Hf isotopic compositions were measured by LA-MC-ICP-MS. 206 Pb/ 238 U ages of 135.1±1.7 and 134.8±1.1 Ma were obtained for the rhyodacite and rhyodacitic porphyry, respectively. These accurate ages indicate that the Xiangshan volcanic-intrusive complex formed in the Early Cretaceous rather than in the Late Jurassic, as concluded in some previous studies. By the Early Cretaceous, the tectonic setting of the area has evolved into a back-arc extensional setting, possibly related to subduction of the paleo-Pacific plate. The close ages of the (early) eruptive rhyodacite and the (late) hypabyssal rhyodacitic porphyry shows that the Xiangshan volcanism was intensive and concentrated in a short time. Zircons from the rhyodacite show negative ε Hf (t) values of -5.7 to -8.5, with Hf depleted mantle model ages between 1550 and 1720 Ma, whereas zircons from the rhyodacitic porphyry yield ε Hf (t) values of -6.9 to -10.1 and Hf model ages between 1621 and 1823 Ma. These zircon Hf model ages are similar to the whole rock Nd model ages (1486( to 1911. Combined with other geochemical characteristics, the Xiangshan rhyodacite and rhyodacitic porphyry may have been derived from partial melting of the Paleo-Mesoproterozoic metamorphic rocks from the Xiangshan basement, without any significant addition of mantle-derived magma. Contribution of basement of this age is also supported by finding a Paleoproterozoic xenocrystic zircon core in the rhyodacite sample. rhyodacite, rhyodacitic porphyry, U-Pb zircon age, Hf isotope, Xiangshan, Jiangxi Province Citation: Yang S Y, Jiang S Y, Jiang Y H, et al. Zircon U-Pb geochronology, Hf isotopic composition and geological implications of the rhyodacite and rhyodacitic porphyry in the Xiangshan uranium ore field,

Geochemical, zircon U–Pb dating and Sr–Nd–Hf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan, Southeast China

“…The stratigraphically oldest rhyodacite yielded a U-Pb zircon age of 135±1 Ma. The overlying rhyodacitic porphyry was dated at 135±1 Ma (Yang et al 2010). Our new ages for three porphyritic lava samples from Yunji (Sample XS-05), Jurong'an (Sample XS-29-1), Youfangcun (Sample XS-59) area are 136±1 Ma, 132±1 Ma and 135±1 Ma, respectively.…”

“…Previous studies of the regionally extensive volcanicintrusive units at Xiangshan gave a large age variation (e.g. Chen et al 1999;Yu 2001;Fan et al 2005;Zhang and Li 2007) and it is difficult to link the age data with information on the magma evolution of individual systems (Yang et al 2010). Previously, the volcanic activity at Xiangshan was considered to take place mainly in the Late Jurassic and continued to the Early Cretaceous (158 to 135 Ma) , and two magmatic cycles were distinguished (e.g.…”

“…Xia et al 1992;Wu 1999). Recently, ages of 135±1 Ma and 135±1 Ma were obtained for a rhyodacite and a rhyodacitic porphyry, respectively (Yang et al 2010), indicating that the large-scale Xiangshan volcanic-intrusive activity took place in the Early Cretaceous rather than in the Late Jurassic. New data presented here indicate that the Xiangshan volcanicintrusive activity was rather short-lived and that some of the ages obtained by previous researchers are inaccurate and need to be reinvestigated.…”

Geochronology, geochemistry and tectonic significance of two Early Cretaceous A-type granites in the Gan-Hang Belt, Southeast China

“…In these volcanic basins, detailed investigations of the mineralogy, petrology, geochemistry, in situ zircon U-Pb geochronology and Hf isotopic compositions of the Xiangshan volcanic-intrusive complex have been carried out (e.g. He et al, 2009;Jiang et al, 2005;Yang et al, 2010Yang et al, , 2011a.…”

“…A number of A-type granites have been recognized along the GanHang Belt, including the Xiangshan volcanic-intrusive complex (135-137 Ma, He et al, 2009;Yang et al, 2010Yang et al, , 2011a, Damaoshan granite (122-126 Ma, Jiang et al, 2011), Baijuhuajian granite (126 Ma, Wong et al, 2009), Yangmeiwan granite (135 Ma, this study), Daqiaowu granitic porphyry (133-136 Ma, this study) and Tongshan granite (129 Ma, Jiang et al, 2011). In situ zircon U-Pb dating results indicate that these A-type granitic rocks are all Cretaceous and have a relatively long duration from 137 Ma to 122 Ma.…”