A ∼700-km-Long Fossil Tonian Magmatic Arc Belt Hidden Within the Yangtze Block's Interior, South China

Abstract

Fossil magmatic arcs preserve important archives of juvenile magmatism, crustal growth and differentiation, thereby aiding in our understanding of continental evolutionary histories. However, some ancient arcs are situated in continental interiors, where they are buried beneath thick cover sequences, making their identification challenging. Here, we present a data set of deep boreholes, geochronology, geochemistry and geophysics to investigate the basement properties and evolution of the Sichuan Basin in the Yangtze Block, South China. These results provide evidence for a large Tonian magmatic arc belt hidden within the Yangtze Block's interior. Seven deep boreholes (∼3,600–6,500 m) penetrating into basin basement, together with geochronology and 3D seismic reflection profiles, show extensive Tonian (ca. 820–770 Ma) rhyolite/granites overlain by Ediacaran sediments. Aeromagnetic data delineate a NE-SW-trending, ∼700-km-long positive magnetic anomaly belt (PMAB) across the central basin, indicating the presence of (ultra-) mafic rocks in lower crust. Petrogenetic analyses indicate that the Tonian felsic rocks drilled within PMAB range were likely derived from a juvenile arc basaltic source. Thus, we interpret the PMAB recorded input of mantle-derived melts and their subsequent evolution into granitic magmas, associated with a Tonian arc magmatic front that once occurred in the Yangtze Block's interior. Our finding broadens the range of the Tonian continental arc system in the region, reaching at least 400–900 km landward from block's northwestern margin. Such a broad orogenic system played a vital role in continental evolution, element recycling and basin formation of the Yangtze Block.