The goldfields district, Nevada

Abstract

The Paleo-Mesoproterozoic Xiong’er volcanic rocks along the southern margin of the North China Craton are lithologically and geochemically similar to those formed in subduction-related, continental margin volcanic arcs. The volcanic rocks are primarily composed of basaltic andesites and andesites, with minor dacites and dacitic rhyolites. Traditionally, the Xiong’er volcanic rocks have been divided from lower to upper into the Xushan, Jidanping and Majiahe Formations, but the ages of volcanic rocks in these formations have not been well constrained, which has hindered further understanding the tectonic significance of the Xiong’er volcanic belt at the southern margin of the North China Craton. SHRIMP and LA-ICP-MS U-Pb zircon analyses, combined with cathodeluminescence (CL) images, have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the Xiong’er volcanic rocks. SHRIMP and LA-ICP-MS U-Pb analyses on magmatic zircons from two basaltic andesite samples, one dacite sample and one rhyolite sample of the Xushan Formation, known as the lowest sequence of the Xiong’er volcanic rocks, indicate that the volcanic eruption of this Formation occurred at ∼1.78 Ga, but most xenocrystic/inherited zircons in these samples yielded 207Pb/206Pb ages ranging from 2.55 Ga to 1.91 Ga. Of three samples collected from the Jidanping Formation, two rhyolite samples (05XE015 and 05XE100) yielded weighted mean 207Pb/206Pb ages of 1778 ± 5.5 Ma and 1751 ± 14 Ma, respectively, similar to the ages of the volcanic rocks in the Xushan Formation, whereas one dacite sample (05XE066) gave a weighted mean 207Pb/206Pb age of 1450 ± 31 Ma, which is the youngest age obtained from the Xiong’er volcanic rocks. One andesite sample (06XS012) collected from the Majiahe Formation yielded two major age populations, with the older one at 1850 ± 5.9 Ma, interpreted as the age of the xenocrystic/inherited zircons, and the younger one at 1778 ± 6.1 Ma, interpreted as the age of the volcanic eruption to form the Majiahe andesite, coeval with the formation of most volcanic rocks from the Xushan and Jidanping Formations. These new SHRIMP and LA-ICP-MS U-Pb zircon data indicate that the traditional stratigraphic subdivision of the lower, middle and upper sequences of the Xiong’er volcanic rocks is not viable and that most of the Xiong’er volcanic rocks formed at 1.78–1.75 Ga, with minor felsic volcanic rocks erupting at ∼1.45 Ga. Similar-aged arc-related volcanic belts have also been found in the southern margin of North America, Greenland and Baltica, the western margin of the Amazonia Craton, the southern and eastern margins of the North Australia Craton, and the eastern margin of the Gawler Craton, which are considered to represent long-lived (1.8–1.3 Ga), subduction-related growth via accretion at key continental margins of the Paleo-Mesoproterzoic Columbia (Nuna) supercontinent.