Compositionally variable basement and tectonic affinity of the Bainaimiao arc belt: Implications for crustal growth of the Central Asian Orogenic Belt

Abstract

As an important part of the southeastern boundary of the Central Asian Orogenic Belt (CAOB), the Bainaimiao arc belt, particularly its tectonic affinity and Precambrian basement composition, is an important element to provide new insights about the crustal growth of the CAOB. However, these remain poorly understood. This paper presents new geochronological, whole-rock geochemical and zircon Hf isotope data on early Paleozoic intrusive rocks from the Bainaimiao arc belt in the southeastern CAOB. The Ordovician gabbroic diorites in the Zhengxiangbaiqi area of the western Bainaimiao arc belt have low SiO2 contents (51.73–54.82 wt%), high TiO2 (0.99–1.07 wt%) and TFe2O3 contents (9.68–9.89 wt%), depletions in Nb, Ta and Ti, and positive εHf(t) values (6.8–11.3). These rocks generated from partial melting of a mantle wedge that has been metasomatized by subducted-derived fluids. The Silurian monzogranites in the Zhengxiangbaiqi area have high contents of SiO2 (72.61–75.12 wt%) and K2O (2.75–4.12 wt%), positive εHf(t) values (0.7–9.3) and abundant Precambrian inherited zircons. These monzogranites originated from partial melting of the thickened juvenile crustal components with significant ancient crustal contamination. The Silurian granites and granodiorites in the Faku area of the eastern Bainaimiao arc belt are characterized by high Na2O contents (4.00–4.52 wt%), high Sr/Y ratios, low MgO (1.15–1.74 wt%) and K2O contents (0.52–1.42 wt%), low K2O/Na2O ratios, and positive εHf(t) values ranging from 0.6 to 8.1. These rocks were derived from partial melting of a subducted slab interacted with mantle peridotite. New zircon U-Pb-Hf isotope data from these early Paleozoic intrusive rocks combined with previous results indicate that the Precambrian basement of the Bainaimiao arc belt is laterally heterogeneous, with Neoarchean-Paleoproterozoic basement beneath the western Bainaimiao arc belt and Paleo-Mesoproterozoic basement beneath the central Jilin area in the eastern Bainaimiao arc belt. The Bainaimiao arc belt has a tectonic affinity different from the North China Craton (NCC) and represents a composite arc system consisting of an ensialic island arc with a basement composition similar to that of the Precambrian microcontinents in the CAOB in the Bayan Obo-Damaoqi-Bainaimiao-Zhengxiangbaiqi and central Jilin areas and an oceanic island arc in the Ongniud-Faku and southern Jilin areas. The involvement of composite arc systems in the amalgamation of the accretionary orogenic belt could be an important mechanism for crustal growth in the CAOB.