Early Paleozoic adakite in the Liuyuan area from the Beishan orogenic belt, NW Gansu Province: Petrogenesis and implication for tectonic setting

摘要

The Liuyuan area, which is located on the southern margin of the Beishan orogenic belt, develops abundant Early Paleozic granitoids. SHRIMP zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 421±8 Ma for the Liuyuan granodiorite (Zhao Zehui et al., 2007), implying its Late Silurian intrusion. Geochemical compositions showed that the Liuyuan granodiorite is characterized by high SiO2 (65.01%–67.31%), A12O3 (17.17%–18.05%) and Na2O (Na2O/K2O=1.67–1.87) but low Mg# contents calculated as 100×Mg2+/(Mg2++ΣFe2+) from 28.77 to 31.15, as well as being enriched in Sr (472×10−6–517×10−6) but depleted in Yb (1.2×10−6–1.42×10−6) and Y (12.8×10−6–14×10−6). The REEs are characterized by right-inclined patterns with LREE enrichment, HREE depletion and slightly negative Eu anomalies (Eu/Eu*=0.91–0.97). Major and trace elements indicate that the granodiorite is an adakite. The Nb/Ta values of the granodiorite vary from 10.80 to 18.01 and Nb/U from 6.32 to 10.09, both lying between the values of the crust and the mantle. The rock has low ɛNd(t) values (−2.5–−0.8) and high I Sr (0.706321–0.706495). Geochemical and Sr-Nd isotopic compositions indicate that the Liuyuan granodiorite is possibly derived from partial melting of thickening lower crust, related to mantle underplating. The Yb-Ta and Y+Nb-Rb discriminant diagrams imply the Liuyuan granodiorite intruded in a local extensional tectonic setting during late collision. Combined with previous studies on geochronology, geochemistry and tectonic setting of granitoids, we interprete that the constraint of this adakite in the Liuyuan area indicates that the tectonic setting may have transformed from collision to extension during the Early Devonian.