Evolution of the middle Paleozoic magmatism in the Chinese Altai: Constraints on the crustal differentiation at shallow depth in the accretionary orogen

Abstract

The Chinese Altai underwent intensive mafic and felsic magmatism in the middle Paleozoic, which was the most important magmatic event in the region. It is still unclear for the role of the event in formation of the crust that is thick (∼56 km) and has structure of three-layer stratification. In this study, the data for the middle Paleozoic mafic rocks and granites are comprehensively compiled and integrated with new geochronological, petrological, geochemical and Sr-Nd-Pb isotopic data for gabbroic rocks from Keketuohai area and tonalite and biotite granites from Habahe area. The middle Paleozoic mafic rocks exhibit two evolutionary trends, which can be defined as high-Al and low-Al types because of their evolution at different crustal levels but mainly at amphibole stability field. The middle Paleozoic I-type granites are mainly metaluminous to weakly peraluminous and were possibly generated by dehydration melting of those mafic rocks evidenced by similar Nd isotopic compositions (εNd(t) = −2.7 to +3.8 for granites; εNd(t) = −2.5 to +8.5 for the middle Paleozoic mafic rocks). The strongly peraluminous granites show large range in MgO (0.41–3.85 wt%), Cr (5.32–153 ppm) and Ni (3.69–104 ppm), suggesting that they might be produced by dehydration melting of a mixed source composed of materials from the Habahe Group and the middle Paleozoic igneous rocks. The middle Paleozoic I-type and strongly peraluminous granites mostly have high contents of Y and Yb and show low Sr/Y and La/Yb ratios, indicating insignificant residual garnet in the source. The Habahe tonalites (372 ± 3 Ma) are characterized by positive εNd(t) values (+4.7–+4.8) and high (207Pb/204Pb)t and (208Pb/204Pb)t ratios, similar to those of the middle Paleozoic mafic rocks. These granites have low contents of MgO (0.50–0.88 wt%), flat rare earth elements (REE) patterns ([La/Yb]N = 1.1–1.6) and strongly positive Eu anomaly (1.6–2.5), indicating that they were possibly evolved from the high-Al mafic magma through fractionation of Mg-rich minerals. The Habahe biotite granites (359 ± 4 to 381 ± 4 Ma) are strongly peraluminous (A/CNK = 1.12–1.23), show concaved REE patterns ([Dy/Yb]N = 0.90–1.24), and have low (87Sr/86Sr)i ratios, positive εNd(t) values (+1.9 to +5.5) and high (207Pb/204Pb)t and (208Pb/204Pb)t ratios. These geochemical features suggest that they were possibly derived from hydrous melting of the middle Paleozoic mafic rocks with residual amphibole at middle crustal level. The middle Paleozoic magmatism recorded the most important period for the crustal differentiation in the Chinese Altai. The evolution of mafic magma and formation of granites mainly occurred within amphibole stability field, illustrating a relatively shallow crustal differentiation process (<30 km) although the crust may have been thickened.