Paleozoic magma evolution at the eastern end of Northern Qilian orogenic belt: Evidence from the zircon U-Pb ages, trace elements and Hf isotopic composition of Changgouhe dioritic gneiss

Outcropped at the eastern end of the Northern Qilian orogenic belt in Tianshui area of Gansu Province, Changgouhe dioritic intrusion has traditionally been considered as Precambrian intrusive rocks due to their gneiss structure. The results of LAICP-MS zircon U-Pb dating in this study show two groups of magmatic zircon crystallization ages at 463.3±2.3Ma (MSWD=0.52, n=11) and 443.8 ±2.6Ma (MSWD=0.44, n=9). Zircon trace element analyses show that these two groups share the similar trace element compositions, indicating that they crystallized in a closed magmatic system. t-Eu/Eu^* diagram shows that the magmatic zircons of about 460Ma have no Eu anomalies, similar to the REE patterns of mantle-derived zircons. The negative Eu anomaly of about 440Ma magmatic zircons indicates that the parent magma began to crystallize plagioclase, consistent with the process of the anatexis. Therefore, about 460Ma is the formation age of Changgouhe dioritic gneiss, and about 440Ma is the crystallization age of anatexis magma of Changgouhe dioritic gneiss. Hf isotope analysis shows that ε_Hf(t) values of about 460Ma and about 440Ma zircons are 8.23~11.57 and 6.36~8.03 respectively, indicating that mantle-derived juvenile crustal magma was developed at about 460Ma and that more involvement of crustal remelting took place at about 440Ma.