Discovery and geological significance of the Neoproterozoic bimodal intrusive rocks in Dabie Orogen

Abstract

In Mingshan reservoir of Dabie Orogenic Belt, there is a collection of Neoproterozoic bimodal intrusive rock formations. This study focuses on the zircon U-Pb chronology, rock geochemistry, and Hf isotopes of these formations. The findings reveal that: (1) The bimodal intrusive rocks mainly consist of gneissic monzogranite and biotite-bearing plagioclase amphibolite, with zircon U-Pb ages of 785.0 ± 7.1Ma and 787.3 ± 6.1Ma, respectively. These ages indicate that they were formed during the late Qingbaikou to early Nanhua epoch of the Neoproterozoic era. (2) The monzonitic granite in the peraluminous series displays a strong right deviation of LREE (Light Rare Earth Elements) and relatively flat HREE (Heavy Rare Earth Elements), as well as a negative δEu anomaly. It is enriched in elements like Rb, Ba, and K, while slightly depleted in elements like Nb, Sr, P, and Ti, similar to Al-type granite. The plagioclase-hornblende rock belongs to the metaluminous peraluminous, shoshonitic, and calc-alkaline series. It has a Mg# (molar ratio of Mg to Mg + Fe) of 0.15 ~ 0.25 and is enriched in LIL (Large Ion Lithophile) elements such as Rb, Ba, and K, with a slight positive anomaly of Ba, and depleted in Nb and Sr. (3) The monzogranite exhibits values of εHf(t) (epsilon Hafnium) ranging from − 13.4 to -7.2 (all negative), and tDM2(Hf) (two-stage depleted mantle model age) ranging from 2099 to 2298Ma. The amphibolite shows εHf(t) values ranging from 2.9 to 5.7 (all positive), and tDM2(Hf) ranging from 1257 to 1410Ma. The geochemistry and Hf isotopes indicate that the gneissic monzogranite and biotite-bearing plagioclase amphibolite have different magmatic sources. The mafic rocks originated from a depleted mantle and underwent fractional crystallization and strong crustal contamination. On the other hand, the acidic rocks resulted from partial melting of shallow ancient felsic crust due to the heating or upwelling of the mafic rocks. Taking into account regional correlation, the Middle Neoproterozoic bimodal intrusive rocks were formed in the structural setting of an intraplate extensional zone (incipient rift).