Geochemical constraints on the provenance and depositional setting of the devonian liuling group, east qinling mountains, central china: Implications for the tectonic evolution of the qinling orogenic belt

摘要

The Qinling orogenic belt of Central China is one of the largest collision orogens in eastern Asia and is considered to mark the site where the eastern part of the Paleotethyan ocean, which separated the North and South China plates, was consumed. Many researchers have described the major units and tectonic framework of the Qinling orogenic belt, but there is little agreement over its Paleozoic history, particularly the tectonic setting of the Devonian assemblages. In this paper geochemistry is used to constrain the provenance and depositional setting of the Middle-Upper Devonian Liuling Group clastic sedimentary rocks from the East Qinling Mountains. Chondrite-normalized REE patterns are uniform with light REE enrichment (La _N/Yb _N c. 9.6), negative Eu anomalies (Eu/Eu c. 0.62), and flat heavy REE patterns (Gd _N/Yb _N c. 1.6), indicating an upper-continental-crustal source and/or juvenile differentiated arc material. Trace-element discriminant diagrams also suggest a continental arc setting, albeit major-element chemistry can show some characters of passive-margin sandstones. Chemical index of alteration and A-CN-K relations, Nb-Ta negative anomalies (PAAS-normalized data), and high Cr-Ni-V-Ti anomalies all suggest that the source area was dominated by non-steady-state weathering regimes indicative of active uplift along an active continental convergent plate boundary, and not a passive margin. This indicates that the Liuling Group was probably related to subduction accretion with rapid uplift of its source areas. The minor recycling of older sedimentary components and oceanic-crust-related mafic inputs could be derived from the basement of the North Qinling arc and ophiolites or an accretionary complex, respectively. We conclude that the Middle to Upper Devonian Liuling Group was deposited at an active continental margin at the southern edge of the North China plate, and hence closure of eastern Paleotethys was postDevonian.