Carboniferous adakites and Nb-enriched arc basaltic rocks association in the Alataw Mountains, north Xinjiang: interactions between slab melt and mantle peridotite and implications for crustal growth

Abstract

Adakites and Nb-enriched arc basaltic rocks (NEABs) are identified to occur within the Carboniferous arc volcanic sequence in the Alataw Mountains, Xinjiang. The adakites, which consist of calc-alkaline dacites and rhyolites, are characterized by strong depletion of heavy rare earth elements (HREEs) (e.g., Yb) and Y, high Sr contents and Sr/Y ratios, either with no Eu anomalies or obvious positive Eu anomalies, apparent positive Sr anomalies, and depleted Nb and Ti. The Alataw adakites are very geochemically similar to the adakites that were presumably derived from partial melting of subducting oceanic crust. The rhyolitic adakite in the Alataw Mountains shows low MgO contents of 0.35% and Mg# values of about 17. However, the dacitic adakite shows high MgO contents of 2.67% to 3.32% and Mg# values of 53 to 58, suggesting that the adakite was possibly contaminated by mantle peridotite. On the other hand, the NEABs are characterized by Na-rich (Na2O/K2O > 2.0), high P2O5 and TiO2 contents, positive to weakly negative Nb anomalies, and non-negative Ti anomalies, suggesting that the NEABs were probably derived from partial melting of mantle peridotite that interacted with slab melt under high geothermal gradient. The Alataw adakites were probably derived from partial melting of oceanic crust on the southern margin of the Junggar plate that was subducted beneath the Bole block in the Carboniferous. The Alataw adakites-NEABs association implies that the partial melting of the subducting oceanic crust and the succedent interactions between the slab melt and peridotite in the mantle wedge possibly took place under the Bole arc in Carboniferous. On the southern margin of the Junggar plate, the Carboniferous subduction of oceanic crust (basin) was possibly extensive in the late Paleozoic era. In the Alataw area, high geothermal gradient possibly occurred in Carboniferous, and partial melting of subducting oceanic crust was a probable mechanism of Carboniferous regional crust growth.