Tectonic setting and element geochemistry of the Aikendaban Formation volcanic rocks in west Tianshan

Abstract

The Aikendaban Formation is a red-colored, continental volcanic sequence, unconformably overlying on the Dahalajunshan Formation of Early Carboniferous age. It shows no regional deformation and metamorphism. Its main lithologies include trachy basalt, basaltic trachyandesite, trachyandesite and trachyte. The SiO2 contents range from 41. 69 to 65. 99%, lower than the average of upper continental crust (66%). Along with increasing SiO2 contents, w(Na2O + K2O) values increase, whereas SI values decrease, with TiO2 generally lower than 1. 3% and Al 2O3 ranging from 12.82 to 18. 37%. These petrochemical characters show that the Aikendaban Formation is a typical volcanic sequence of shoshonite series. SiO2 contents in basalts and basaltic trachyandesites are lower than the average of lower continental crust (54. 4%), suggesting that basalts and basaltic trachyandesites were originated from the mantle rather than the continental crust. On the contrary, the trachytes and trachyandesites, with SiO2>54. 4%, were likely to be resulted from the crust or related to intracrustal differentiation. ∑REE, LREE, Zr, Hf, Nb, Ta, Ba, Sr, Pb and Y contents and LaN/YbN ratios in basalts and basaltic trachyandesites are higher than their averages in rocks of the same kinds worldwide, whereas Cr, Co and Ni contents are lower than their averages in same kinds of rocks worldwide. This indicates that mantle source was enriched in large ion lithosphile elements (LILE) and incompatible elements. With Eu/Eu * <0.94 and variable Sr contents (from depletion to enrichment), the basalts and basaltic trachyandesites were probably derived from a heterogeneous mantle source resulted from return of upper crustal compositions related to subduction. Eu/Eu * values in trachytes average 0. 59, lower than 0. 65 which is supposed as average of upper continental crust. Sr is strongly depleted in trachytes. Meanwhile, the ∑REE, LREE, LREE /HREE, LaN/YbN and Ce/Ce * values in trachytes are lower than those in trachyandesites. Sm/Nd ratios average 0. 25, same as the average of lower continental crust (0.25). La, Ce, Nd and Sm are enriched relative to Y and Yb. Ba shows positive anomaly, while Nb, Ta and Hf show depletion relatively. All these characteristics indicate that the trachytes are originated from an intracrustal differentiation with plagioclase-crystalline fractionation. Among various lithologies, trachyandesites have highest ∑REE, LREE, Ba, Sr, Zr and Nb contents, highest LREE/HREE, LaN/Yb N, Ce/Ce * ,Zr/Y and Hf/Yb ratios; and lowest Sm /Nd ratios. This shows that LILE and HFSE have enriched in trachyandesites, suggesting that magma has experienced intense crystalline fractionation. With Eu/Eu *=0. 63, lower than the upper continental crust (0. 65), the trachyandesites have higher HREE contents than basalts and basaltic trachyandesites, suggesting that trachyandesites have evolved from basaltic and/or basaltic-trachyandesitic magmas by intracrustal differentiation. Study on lithologic sequence and geological synthesis shows that the Aikendaban Formation developed in extension-collapse stage of late collision, i. e. Early Permian in Tianshan Orogen. Extension of a collisional-orogenic lithosphere generally evolved from shallow crust-level to deep root-level (lower crust and lithospheric mantle). Eruption of basalts, probably coeval with underplating of mantle-derived magmas, can be used as indicator of the end of a collisional-orogenic event.