Missing Sr[sbnd]Nd isotopic decoupling in subduction zone: Decoding the multi-stage dehydration and melting of subducted slab in the Chinese Altai

Abstract

Some island arc basalts (IAB) show NdHf isotopic decoupling, which can be attributed to higher mobility of Nd over Hf in melts released from subducted slab and subsequent metasomatism in the mantle wedge. If the same mechanism applies, SrNd isotopic decoupling should be common in typical IAB, but in fact it is not the case. In order to investigate this problem, we carried out a systematic study on the Habahe mafic dykes at ~360 Ma, which emplaced in the south margin of the Chinese Altai due to the northward subduction of the Junggar Ocean. These dykes can be subdivided into four types based on distinct geochemical compositions and spatial distribution. Type-I mafic dykes in the southern area are enriched in light rare earth elements (LREE) and Th with positive Pb anomaly. They have high initial 87Sr/86Sr (0.7088–0.7095), 206Pb/204Pb (17.971–18.034), 207Pb/204Pb (15.526–15.551) and 208Pb/204Pb (37.860–37.991) ratios and exhibit NdHf isotopic decoupling with high εHf(t) (+9.5 − +12.1) and low εNd(t) values (+3.5 − +3.9). Thus Type-I mafic dykes are interpreted as originating from a depleted mantle source metasomatized by melts from subducted sediments. Type-II mafic dykes in the western segment of central area are depleted in LREE and Th but enriched in Ba, Sr and Pb. They show high positive εNd(t) (+6.5 − +8.1) and εHf(t) (+13.4 − +15.4) values with relatively low initial 87Sr/86Sr (0.7040–0.7049), 206Pb/204Pb (17.715–17.839), 207Pb/204Pb (15.484–15.495) and 208Pb/204Pb (37.739–37.797) ratios, which would be derived from the partial melting of a depleted mantle source with input of fluids from subducted sediments. Type-III mafic dykes in the eastern segment of central area are enriched in LREE with weakly positive Pb anomaly. They show low initial 87Sr/86Sr (0.7036–0.7038), 206Pb/204Pb (17.871–18.001), 207Pb/204Pb (15.485–15.491) and 208Pb/204Pb (37.610–37.642) ratios and have high positive εNd(t) (+6.7 − +6.9) and εHf(t) (+14.6 − +14.8) values, which are interpreted as products of partial melting of a depleted mantle source refertilized by melts from the subducted oceanic crust. Type-IV mafic dykes in the northern area are also enriched in LREE but show significantly positive Pb anomaly with intermediate (87Sr/86Sr)i ratios (0.7068–0.7070), low εNd(t) values (+0.7 − +1.0), high εHf(t) values (+5.5 − +7.8), and high initial 206Pb/204Pb (18.001–18.402), 207Pb/204Pb (15.522–15.553) and 208Pb/204Pb (38.163–38.299) ratios, thus interpreted as originating from a depleted mantle source with the involvement of melts released from subducted sediments. Type-I mafic dykes have similar Nd-Hf-Pb isotopic compositions with Type-IV mafic dykes but show higher (87Sr/86Sr)i ratios, indicating that the melts and fluids from subducted sediments at shallower depth possess high Sr/Nd ratios because of breakdown of plagioclase. Involvement of such melts and fluids can explain SrNd isotopic decoupling for the southern mafic dykes of this study, as the modern analogue of fore-arc basalts and boninites in the Izu-Bonin-Mariana arc and Western Java, which have higher 87Sr/86Sr ratios than those in typical IAB. Consequently, melting of the residual subducted sediments at the sub-arc depth will generate low Sr/Nd fluids and melts, which can account for the SrNd isotopic coupling of the typical IAB and Type-IV mafic dykes of this study.