Notice of advances made in the Science and Mineralogy during the year 1837. From the report of Berzelius to the Swedish Academy

Abstract

Crustal xenoliths can provide new insights into the unexposed crust, and those from the northeastern Yangtze Block have rarely been studied. This paper reports U–Pb–Hf isotopes and trace-element compositions of zircons from six felsic xenoliths hosted by the Neogene alkali basalts in the Donghai region (i.e. Anfengshan and Pingmingshan) of the Sulu orogen in central eastern China. The xenoliths are mainly composed of orthoclase and quartz, or orthoclase and natrolite, with accessory minerals of Fe–Ti oxides and zircon. Most zircon grains show core-rim structures, with the cores and rims being magmatic and metamorphic in origin, respectively. The zircon cores mainly yield ages of ca. 827–794 Ma, while the zircon rims give ages of ca. 232–212 Ma. We interpret the zircon core ages as the time of an early Mid-Neoproterozoic magmatic event in the northeastern Yangtze Block and the zircon rim ages as the time of collision between the Yangtze and North China Blocks. Our data suggest that much more ca. 830–800 Ma magmatic records are possibly preserved in the unexposed deep crust, and the early Mid-Neoproterozoic is an important era for the crust evolution of the northeastern Yangtze Block. The new zircon Hf isotopic analyses show that the Anfengshan sample (south of Donghai) has zircon εHf (820 Ma) values ranging from −15.3 to −9.4, and two-stage Hf model ages of 2.66–2.30 Ga; the Pingmingshan sample (southeast of Donghai) has zircon εHf (820 Ma) values ranging from −1.4 to +3.8, and two-stage Hf model ages of 1.80–1.47 Ga. These data suggest that ancient crust as old as Neoarchean to Mesoproterozoic was involved in the early Mid-Neoproterozoic magmatism. Combined with the previously reported zircon U–Pb–Hf results of the exposed rocks, it is highlighted that crustal recycling was dominant in the early Mid-Neoproterozoic (ca. 830–800 Ma) magmatism, whereas both crustal recycling and addition of mantle-derived melts were significant in the late Mid-Neoproterozoic (ca. 800–720 Ma) magmatism in the northeastern Yangtze Block.