The Neoarchean-Paleoproterozoic volcanic-sedimentary rocks in the Zanhuang Complex, North China Craton: Petrogenesis and implications for tectonic evolution

Abstract

The Zanhuang Complex is located in central section of the Trans-North China Orogen (TNCO) in the North China Craton (NCC) with complicated tectonic history. The formation timing (Neoarchean or Paleoproterozoic) of the Guandu Group or Central Zanhuang Domain (CZD) is still under debate, while this question is critical for understanding the tectonic evolution of the Zanhuang Complex and the TNCO.The mafic rocks in the Guandu Group (or CZD) can be subdivided into three different types by petrological, geochronological, structural and petrogenetic differences. Type 1 consists of “pillow” epidosites and amphibolites, located at the west part of the Guandu Group; type 2 dominated by amphibolites and interbedding with various sedimentary rocks, located at the east part; and type 3 consists of mafic dikes beyond the scope of this paper.Type 1 mafic rock yielded zircon U-Pb age of 2501 ± 12 Ma which constrains it formed at Neoarchean with older Nd TDM ages (2.8–3.0 Ga). Interbedded sedimentary rocks with younger than 2.1 Ga detrital zircon ages, and the much younger Nd TDM model ages (2.2–2.3 Ga) of the amphibolites from type 2, together suggest that the type 2 mafic rocks and associated sedimentary rocks may have formed at Paleoproterozoic. Thus, we propose that the so-called Guandu Group is not a single and unitary tectonic-lithological unit but consists of at least two different volcanic-sedimentary successions formed at different times and pieced together by later tectonic event (s).A new two-period tectonic events model is proposed here. Before 2.5 Ga, the western margin of the Eastern Block deposited a series of volcanic-sedimentary rocks (type 1). At ∼2.5 Ga, the Fuping arc (including the Western Zanhuang Domain) amalgamated with the Eastern Block by arc-continent collision. After a quiescent period, it underwent ∼2.1 Ga extension and rifting. Later, the rift evolved into a small ocean basin with the deposition of volcanic-sedimentary sequences (type 2), then closed through west-dipping subduction and collision at 1.9–1.85 Ga, resulting in the final amalgamation of the NCC and the widespread deformation and metamorphism in the Zanhuang Complex as well as the whole TNCO.