Neoarchean DTTG gneisses in southern Liaoning Province and their constraints on crustal growth and the nature of the Liao-Ji Belt in the Eastern Block

Abstract

The Neoarchean metamorphic basement block (NBB) in the southern Liaoning Province (SLP) is located to the southeast of the Liao-Ji Belt and is an important part of the Eastern Block of the North China Craton (NCC). The NBB consists mainly of quartz diorite-tonalite-trondhjemite-granodiorite (DTTG) gneisses. Based on the petrological and chemical compositions, the DTTG gneisses are divided into two types: quartz dioritic gneisses and tonalite-trondhjemite-granodiorite (TTG) gneisses. LA-ICPMS zircon U-Pb isotopic dating reveals that the magmatic precursors of the TTG gneiss samples emplaced at 2548Ma, and that the magmatic precursors of the quartz dioritic gneiss samples emplaced at 2551–2512Ma, except for sample 15DL10-1 from Liangjiadian, which was formed earlier at 2669Ma. Petrogenetic studies indicate that the TTG magmas were formed by partial melting of the juvenile lower crust and that the magmatic precursors of the quartz dioritic gneiss samples were formed by the mixing of magmas derived from partial melting of both the depleted mantle and the juvenile lower crust. Combined with previous chronological data, our newly obtained zircon U-Pb dating data suggest that the NBB in the SLP experienced magmatism at∼2.68–2.63Ga and∼2.56–2.51Ga and metamorphism at∼2.50–2.47Ga. Integrating the zircon U-Pb chronological data with Lu-Hf isotopic data from the DTTG gneisses, we suggest that the NBB in the SLP was subjected to two major episodes of crustal growth at∼2.7Ga and∼2.5Ga. These two episodes of crustal growth at∼2.7Ga and∼2.5Ga are extensive in the Eastern Block (EB) of the NCC. Based on the Archean geological events in the two flanks of the Paleoproterozoic Liao-Ji Belt (LJB), a unified late Neoarchean crystalline basement is proposed in the east of the EB prior to the LJB, which indicates that the LJB was a Paleoproterozoic continental rift belt.