Dynamic contribution of recycled components from the subducted Pacific slab: Oxygen isotopic composition of the basalts from 106 Ma to 60 Ma in North China Craton

Abstract

How the materials derived from the stagnant Pacific slab contributed to the mantle sources of the mafic rocks in east China is still in hot debate. In this work, δ18O (Vienna standard mean ocean water) values of clinopyroxene phenocrysts in the oceanic island basalts (OIB)‐type mafic rocks from 106 Ma to 60 Ma in the east North China Craton (NCC) were measured by secondary ion mass spectrometry. Our data show that for all of the samples, the δ18Ocpx are dominantly higher than that of the clinopyroxene from normal mid‐oceanic ridge basalt (5.4–5.8‰), which confirms the role of recycled oceanic crust (ROC) in their mantle sources. Combining the δ18O data of basalts and the lithospheric mantle in the literature, we found that in the southeast NCC, upper and lower ROC components alternately appeared in the mantle sources of basalts, but these ROC components are consistently different from that in the lithospheric mantle, while in the northern NCC, the recycled components in the sources seem to be persistently from upper ROC. These observations suggest that (1) these mafic OIB‐type rocks are most likely derived from the convective asthenosphere and (2) the contribution of components from the Pacific oceanic slab into the NCC upper mantle was dynamic, without a simple temporal trend. This new knowledge calls for the reconsideration of the existing models of the thinning process of the NCC lithospheric mantle, and it warns against simply using the chemical composition of basalts to infer the evolution of lithosphere.

Light Mg Isotopic Composition in the Mantle Beyond the Big Mantle Wedge Beneath eastern Asia

“…All elemental and stable isotopic compositions of the Cenozoic basalts (sensu lato) in eastern China thus highlight the importance of carbonated crustal material in their mantle sources, which is typically interpreted to derive from the subducted Pacific slab (e.g., Li, Ma, et al, ; Liu, Xia, Deloule, et al, , Liu, Xia, Ingrin, et al, ; Liu, Wang, et al, ; Liu, Xia, et al, ; Sakuyama et al, ; Xu et al, , ; Xu et al, ; Yu et al, ). The present‐day subducted Pacific slab has been seismically detected in the mantle transition zone beneath eastern Asia, with its western edge extending to about 120°E longitude, where it roughly coincides with the eastern margin of the Daxing'anling‐Taihang Gravity Lineament (DTGL; Figure ; Huang & Zhao, ; Wei et al, ).…”

“…Low Ca and high Ni contents of their olivine phenocrysts (e.g., Hong et al, ; Li, Xu, et al, ; Liu, Ren, et al, ; Qian et al, ; Wang et al, ; Xu et al, ), high whole‐rock FC3MS values (here FC3MS = FeO/CaO – 3 * MgO/SiO 2 ; >0.65; Yang & Zhou, ; Yang et al, ), and high Fe/Mn (>60) and high 10,000 * Zn/Fe (>14) ratios (Liu et al, ; Xu et al, ; Zhang et al, ) suggest that these basalts (sensu lato) have pyroxenitic/eclogitic mantle sources. The distinct δ 18 O values of phenocrysts relative to typical mantle indicate that such pyroxenitic/eclogitic components may derive from subducted oceanic crusts and sediments (e.g., Liu, Xia, Deloule, et al, , Liu, Xia, Ingrin, et al, ; Liu, Wang, et al, ; Xu et al, ). Heavy δ 66 Zn signatures (δ 66 Zn = 0.30‰ to 0.63‰; Liu et al, ) of the basalts, compared to peridotite mantle values (δ 66 Zn = 0.16 ± 0.06‰; Sossi et al, ), reflect contributions from isotopically heavy subducted carbonated material (average δ 66 Zn = 0.91 ± 0.47‰; Pichat et al, ).…”