Precambrian tectonic evolution of the Songliao–Xilinhot block in the context of a supercontinent cycle: Evidence from Neoarchean granitoid rocks in the Longjiang area, NE China

Abstract

The Songliao–Xilinhot block (SXB) in the eastern part of the Central Asian Orogenic Belt (CAOB) is a Precambrian continental block that provides clues to the evolution of the CAOB. However, its structural relationship with adjoining terranes and cratons, and its role in past supercontinent cycles remain unclear. Here, we describe the whole-rock geochemistry, zircon U–Pb ages, and Sr–Nd–Hf isotopic data of Neoarchean granitoid rocks in the Longjiang area of the SXB, NE China. Zircon U–Pb dating indicates that tonalite–trondhjemite–granodiorites (TTGs) and monzogranites were formed at 2.56–2.55 and 2.55–2.51 Ga, respectively. The TTGs have low MgO, Y, and Yb, and high Sr contents. Their Sr/Y and (La/Yb)_N ratios and Yb_N values vary widely, with εHf(t) values of –2.8 to + 3.1 and a εNd(t) value of + 2.4. Thus, the TTGs are medium- and low-pressure types, possibly formed by partial melting of middle Archean basic lower crust. The monzonitic granites have high K₂O contents, high K₂O/Na₂O, Sr/Y, and (La/Yb)_N ratios, low MgO contents and Mg^# values, and variable ε_Hf(t) and ε_Nd(t) values (–3.1 to + 6.3 and –0.9 to + 2.8). The geochemical characteristics and Hf–Nd isotopic compositions of then monzogranites indicate that their parental magmas were formed by partial melting of thickened lower crust derived from heterogeneous sources. Thickening of the lower crust during the late Neoarchean may have promoted the formation of the TTGs and monzogranites. U–Pb and Lu–Hf isotopic data for magmatic zircons from Precambrian intrusive rocks, and detrital zircons from Meso-Neoproterozoic metasedimentary rocks in the SXB were studied to elucidate the origin of the Precambrian SXB. Results indicate a close affinity of the SXB with Laurentia in the Precambrican, while geological evidence and the presence of 1.4 Ga A-type granite in the SXB preclude most cratons as being the source of the Precambrian SXB. Precambrian magmatic events, sedimentary sequences, and significant changes in zircon Hf isotopic compositions within the Precambrian SXB indicate that the block responded to the assembly of the Nuna supercontinents. It underwent a change in tectonic environment from long-term subduction and extension to short-term collision during the breakup of Nuna and subsequent amalgamation with Rodinia.