Post-collisional plutonism with adakitic signatures:The Triassic Yangba granodiorite (Bikou terrane,northern Yangtze Block)

摘要

The post-collisional Yangba granodiorite intruded into the Bikou metasedimentary-volcanic group, southern Mianlue Suture, central China. The host granodiorites contain many mafic microgranular enclaves which have acicular apatite, phenocrysts of host granodiorites, implying that the enclaves have been incorporated as magma globules into host granodioritic magma and undergone rapid cooling. The variation trends of major and trace ele- ments between enclaves and host rocks suggest a mixing and mingling process with respect to their petrogenesis. The mafic microgranular enclaves are characterized by shoshonite with SiO2≤63%, σ (4.54–6.18)>3.3, high K2O content (4.22%–6.04%), K2O/Na2O>1; in the K2O-SiO2 diagram, all the samples plot in the shoshonite field, which are enriched in LILE and LREE, with obvious Nb, Ta negative anomalies, indicating a subducting fluid-metasomatised mantle source. Zircon LA-ICP-MS dating of the granodiorites yielded an age of 215.4±8.3 Ma, indicating they were formed during the late-orogenic or post-collisional stage (≤242±21 Ma) of the South Qinling Mountain Belt. The host granodiorites have many close compositional similarities to high-silica adakites from su- pra-subduction zone setting, but tend to have a higher concentration of K2O (3.22%–3.84%) and Mg#. Chon- drite-normalized rare-earth element patterns are characterized by high ratios of (La/Yb)N, the extreme HREE deple- tion and a lack of significant Eu anomalies. In conjunction with the high abundances of Ba and Sr as well as the low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionation mineral assemblage. High Mg# values demonstrate that the host granodiorites were contaminated by enclave magma. On a whole, integrated geological and geochemical studies suggested the Yangba granodiorites and their mafic micro- granular enclaves resulted from mixing of enriched mantle-derived shoshonitic magma and thickened lower crust-derived felsic magma. In combination with previous studies we consider that the Yangba granodiorites were likely to represent underplating activities and delamination of the lower crust during the late orogenic stage in west- ern Qinling Orogenic Belt.