Growth and differentiation of continental crust in the accretionary orogen : a case study of the Chinese Altai, Central Asian Orogenic Belt

Abstract

The Chinese Altai experienced intensive mafic and felsic magmatism during the Middle Paleozoic, which is the most important tectonothermal event in the history of this region. Geochronological, petrological, geochemical and Sr-Nd-Hf-Pb isotopic analyses were conducted on the Middle Paleozoic mafic and felsic rocks in the region to constrain the tectonic setting and the crustal growth and differentiation processes at this time period. The Middle Paleozoic mafic rocks show wide ranges in Sr-Nd-Hf-Pb isotopes compositions due to widespread metasomatism in the lithospheric mantle source by hydrous melt and fluid from subducted slab. The large scale dehydration and melting of subducted slab suggest subduction of an oceanic ridge beneath the Chinese Altai in the Middle Paleozoic. Addition of fluid and hydrous melt facilitated melting of the mantle wedge and resulted in voluminous emplacement of hydrated mafic magmas, transferring large amount of juvenile materials and heat into the crust. The hydrous mafic magma intruded into variable crustal level, producing high-Al mafic rocks in the middle to lower crust and low-Al type mafic rocks in the upper crust. The high-Al mafic magma experienced significant fractionation of Mg-rich minerals, resulting in formation of the Habahe plagiogranites which show depleted Nd-Hf-Pb isotopes compositions and pronounced positive Eu anomaly. The Middle Paleozoic mafic rocks were remelted under variable content of water to form the Middle Paleozoic I-type granites. The intense basaltic underplating also resulted in the partial melting of sedimentary rocks possibly equivalent to the Habahe Group mainly at middle crustal level, generating strongly peraluminous S-type granites. The Middle Paleozoic felsic rocks all show significant Nd-Hf isotopic decoupling manifested by relatively high εHf(t) and low εNd(t) values. Such isotopic decoupling cannot be explained by the involvement of an ancient basement with elevated Lu/Hf ratios, but can be ultimately resulted from selective enrichment of Nd over Hf in the mantle due to the metasomatism by hydrous melts from subducted sediments, as shown by the Middle Paleozoic mafic rocks. Thus, the Chinese Altai represents a magmatic arc which was made up of relatively juvenile materials in the Paleozoic. The large scale felsic magmatism in the Middle Paleozoic is the most important period for the crustal differentiation in the region. Most of these granites show flatten heavy REE (HREE) pattern and have high Y contents and low Sr/Y ratios, suggesting that the differentiation process mainly occurred within the amphibole stability field (<30 km) although the crust may have been thickened.