Abstract:

This paper reports a detailed study of UPb zircon dating, geochemical and SrNd isotopic compositions for the Maozangshi granodiorite and Huangyanghe Kfeldspar granite in Lenglongling area, North Qilian Mountains. UPb zircon dating by LAICPMS shows magma crystallization ages of 424±4 Ma for the Maozangshi granodiorite and 402±4 Ma for the Huangyanghe Kfeldspar granite. The Maozangshi granodiorite has high Mg# (~55), K2O/Na2O=0.77-0.91, A/CNK=0.92-0.94. It is rich in LILE and depleted in HFSE. REE data for the granodiorite show strongly fractionated REE patterns, with (La/Yb)N = 16.9-19.5 and Eu/Eu*=0.75-0.83. The granodiorite has ISr=0.7063-0.7065 and εNd(t) =1.5-1.1, with TDM=1.10-1.16 Ga. The geochemical characteristics and SrNd isotopic composition indicate that the magma of the granodiorite was derived from partial melting of basaltic rocks in lower crust, but with minor contribution from mantle material. The Huangyanghe Kfeldspar granite has K2O+Na2O=6.91%-7.66%, K2O/Na2O >1 and A/CNK=0.97-1.05. The Kfeldspar granite is rich in LILE and HFSE. It has moderately fractionated REE patterns with (La/Yb)N =10.6-17.8 and Eu/Eu*=0.43-0.68. These features show that the Kfeldspar granite has an affinity with aluminous Atype granite. The Kfeldspar granite shows ISr=0.7103-0.7113 and εNd(t)=6.7-6.0, with TDM =1.46-1.55 Ga, suggesting that the magma resulted from partial melting of felsic materials in crust. The diverse petrogenesis and evolutionary history of the granitoids in Lenglongling area reveal a tectonic transformation from compression to extension for North Qilian orogen. Both the Maozangshi granodiorite and Huangyanghe Kfeldspar granite formed in postcollisional tectonic setting. And the petrogenesis of the postcollisional granitoids in North Qilian Mountains may have close relation with the previously subducted North Qilian oceanic slab.

Key words: granitoids, petrogenesis, tectonic implications, Lenglongling area, North Qilian Mountains