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Abstract
Crustal components may be incorporated into continental basalts by either shallow contamination or deep mixing. While the former proceeds at crustal depths with common preservation of refractory minerals, the latter occurs at mantle depths with rare survival of relict minerals. Discrimination between the two mechanisms has great bearing to subcontinental mantle geochemistry. Here we report the occurrence of relict zircons in Cenozoic continental basalts from eastern China. A combined study of zircon U-Pb ages and geochemistry indicates that detrital zircons were carried by terrigenous sediments into a subcontinental subduction zone, where the zircon were transferred by fluids into the magma sources of continental basalts. The basalts were sampled from three petrotectonic units with distinct differences in their magmatic and metamorphic ages, making the crustal contamination discernible. The terrigenous sediments were carried by the subducting oceanic crust into the asthenospheric mantle, producing both soluble and insoluble materials at the slab-mantle interface. These materials were served as metasomatic agents to react with the overlying mantle wedge peridotite, generating a kind of ultramafic metasomatites that contain the relict zircons. Therefore, the occurrence of relict zircons in continental basalts indicates that this refractory mineral can survive extreme temperature-pressure conditions in the asthenospheric mantle.
Highlights
Crustal components may be incorporated into continental basalts by either shallow contamination or deep mixing
Zircon rarely occurs in mafic volcanics such as basalts because the crystallization of zircon from silicate melts requires the oversaturation of both ZrO2 and SiO23,4. This is not justified in basaltic melts that were derived from partial melting of the depleted MORB mantle, the ordinary asthenospheric mantle
They were collected from east-central China, close to the Tan-Lu Fault (Fig. 1). They are located at Donghai in northern Jiangsu (Subei), at Dingyuan, Mingguang and Lai’an in eastern Anhui (Wandong), at Hefei in central Anhui, and at Changle in western Shandong (Luxi). These localities are geographically close to the Dabie-Sulu orogenic belt that is characterized by the occurrence of Triassic ultrahigh-pressure (UHP) metamorphic rocks, only the basalts at Donghai are tectonically located inside the Sulu orogen
Summary
Crustal components may be incorporated into continental basalts by either shallow contamination or deep mixing. This involves both chemical and physical transports of crustal materials from the subducting slab to the mantle, which is illustrated by the occurrence of relict zircons in Cenozoic continental basalts from east-central China.
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