A metasomatized mantle wedge origin for low-δ 18O olivine in late Cretaceous Junan and Qingdao basalts in the Sulu orogen

Abstract

The North China Block (NCB) experienced extensive lithospheric thinning and subsequent mantle accretion in the Mesozoic to Cenozoic, but their mechanism remains controversial. This paper reports in situ chemical and oxygen isotope analyses on olivine xenocrysts and porphyroclastic olivines in mantle xenoliths hosted in the late Cretaceous basalts from Junan and Qingdao, the Sulu orogen. The results show that all these olivines have low  18 O values relative to normal mantle peridotite. The olivine xenocrysts and porphyroclastic olivines from Junan have Fo=87.589.8,  18 O=4.1‰-5.2‰ with an average of 4.8‰; the porphyroclastic olivines from Qingdao also have Fo=89.089.9,  18 O=4.1‰-5.2‰ with an average of 4.8‰. These peridotite xenoliths have petrological and geochemical affinities similar to newly accreted MORB-type mantle, we hence consider such low- 18 O features in the olivines to inherit from a mantle wedge that was metasomatized by melts derived from the subducted oceanic crust, which had experienced high-temperature hydrothermal alteration to acquire the low- 18 O signatures. Combined the existence of Cenozoic low- 18 O basalts and garnet pyroxenite xenoliths (relicts of recycled oceanic crust) hosted in Cenozoic basalts in the NCB, the subducted oceanic crust likely played an important role in the lithospheric evolution of the NCB during the Mesozoic to Cenozoic.

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

“…[] (the data set with Δ 18 O cpx‐ol < 0 are filtered). The data of δ 18 O ol for 86–67 Ma and late Cenozoic NCC lithospheric mantle are from Guo et al ., and Hao et al ., .…”

“…The δ 18 O of olivine as xenocrysts or in peridotite xenoliths hosted by these basanites have been measured by Guo et al . [] with secondary ion mass spectrometer (SIMS).…”

“…Cpx phenocrysts are usually zoned, with the core as mantle-or crustal-derived xenocrysts and rim growing from the magma (see section 4). The δ 18 O of olivine as xenocrysts or in peridotite xenoliths hosted by these basanites have been measured by Guo et al [2013] with secondary ion mass spectrometer (SIMS).…”

“…Journal of Geophysical Research: Solid Earth 10.1002/2016JB013156 mantle [Ying et al, 2006;Zhang et al, 2011]. Guo et al [2013] reported the low δ 18 O (<5.4‰) of olivine as xenocrysts or in the peridotite hosted in Jünan and PSK basalts. These olivines with Mg# ranging from 87 to 89 indicate these samples would come from a fertile part of the lithospheric mantle [Ying et al, 2006].…”

“…On the other hand, the secular changes in δ 18 O anomaly for the lithospheric mantle and the contemporary basalts (Figure 7) could be best explained by that the components from the subducted Pacific slab (e.g., upper ROC versus lower ROC) were dynamically supplied to the upper mantle beneath [Liu et al, 2015b] and those from Zhang et al [2009], Wang et al [2011], and Z. Guo et al, 2013 andHao et al, 2015. Journal of Geophysical Research: Solid Earth Figure 8 illustrates the variation of δ 18 O cpx for the 106-60 Ma basalts. In Figure 9, companying with the published data of the Cenozoic basalts in the Central NCC and Southeastern NCC, the average δ 18 O cpx within each sample are compared with the bulk rock Ce/Pb, 87 Sr/ 86 Sr, Ba/Th, and Nb/La ratios.…”