Evidence for rutile-bearing eclogite in the mantle sources of the Cenozoic Zhejiang basalts, eastern China

Abstract

Cenozoic intraplate basalts are widely distributed along the southeastern margin of Eurasia. Both carbonated and eclogitic/pyroxenitic components, which are considered related to oceanic crust recycling, have been identified in the mantle source of these basalts. However, the origin of the recycled oceanic crust and the genetic relationship between different source lithologies remain unclear. Here, we assess the source lithologies of late Miocene intraplate basalts from Ninghai, Zhejiang Province, eastern China, using new elemental and Sr–Nd–Hf isotopic compositional data. These data are also compared with those of the nearby Xinchang–Shengzhou flood basalts and other Cenozoic basalts in Zhejiang Province. New Mg isotopic compositions of the Ninghai and Xinchang–Shengzhou basalts are used to evaluate the potential influence of carbonated components during their formation. Nephelinites and basanites from Ninghai and Xinchang–Shengzhou have negative Ti and Hf anomalies, superchondritic Ca/Al ratios, and light Mg isotopic compositions (δ26Mg = −0.35 to −0.38‰), indicating a carbonated component in the mantle source. By comparison, other Ninghai basalts have positive Ti and Nb anomalies, with Sm/Yb and Lu/Hf ratios that generally correlate with Ti/Ti* values. These observations indicate that rutile-bearing eclogite should also play an important role in the formation of the Ninghai basalts, in addition to mantle peridotite. The terrestrial mantle-likeMg-isotopic compositions of the Ninghai basalts further support their formation by high-degree partial melting of such mixed source. In the plots of Sr-Nd isotopes and Ti/Ti*, the nephelinites and basanites from Ninghai and Xinchang–Shengzhou show different trends compared with other Ninghai basalts, and the carbonated component in the mantle source of the former has similar and depleted Sr-Nd isotopes to the rutile-bearing eclogite source of the latter. Additionally, samples of these basalts show negative ΔεHf values similar to those of Pacific MORB. Subducted Pacific oceanic crust is therefore considered an appropriate source for both the carbonated and rutile-bearing eclogitic components. The Ninghai basalts provide an important insight into oceanic crust recycling in eastern China, with data indicating a genetic relationship between the carbonated component and rutile-bearing eclogite in the mantle sources of intraplate basalts.