Carboniferous mantle-derived felsic intrusion in the Chinese Altai, NW China: Implications for geodynamic change of the accretionary orogenic belt

This paper reports whole-rock Sr–Nd and zircon U–Pb and Hf isotopic compositions, as well as major and trace element data, for the granodiorites and their mafic microgranular enclaves (MMEs) in the Tuokesalei area, northwestern Chinese Altai. The granodiorites are characterized by peraluminous compositions (A/CNK = 1.01–1.45), with relatively high SiO2 (64.4 to 70.9 wt.%) and low MgO (0.75 to 1.97 wt.%) contents. These rocks possess low total rare earth element (REE) contents (67–187 ppm) with weakly negative Eu anomalies and mantle-like whole-rock Nd–Sr isotopic compositions (εNd(t) = + 2.5 to + 4.5; (87Sr/86Sr)i = 0.7038 to 0.7048) and zircon εHf(t) values (+ 5.93 to + 12.9). The MMEs contain abundant magmatic amphiboles and needle-like apatites, have low SiO2 (51.7 to 54.3 wt.%) and high MgO (2.86 to 3.75 wt.%) and REE contents (162–298 ppm), and are characterized by moderately negative Eu anomalies (Eu/Eu * = 0.40–0.60) and slightly depleted to enriched LREE ((La/Nd)N = 0.74–1.41) and flat HREE ((Gd/Yb)N = 1.16–1.39) patterns. The MMEs have whole-rock Nd–Sr isotopic compositions (εNd(t) = + 2.6 to + 2.7, (87Sr/86Sr)i = 0.7034–0.7050) similar to those of the host granodiorites. Their similarities in crystallization age, mineral assemblage and isotopic composition document that the MMEs may represent an early crystallized phase, which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement. Their precursor magmas were probably derived from an oceanic lithospheric mantle, which was characterized by pronounced Nd–Ta depletions. Mineral compositions of MMEs suggest that amphibole was a major early fractional mineral, which effectively differentiated the mantle-derived basaltic liquids to silica-rich melts. Zircon U–Pb dating yielded an age of 313 ± 5 Ma for the host granodiorites and 313 ± 13 Ma for their MMEs. The geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction-related regime to a post-tectonic extension. Results of this study suggest that the accretionary orogenic process did not completely terminate until the Late Carboniferous to Permian in this region.