Petrography and detrital zircon study of late Carboniferous sequences in the southwestern North China Craton: Implications for the regional tectonic evolution and bauxite genesis

摘要

The North China Craton (NCC) has been flanked by the North Qilian and North Qinling arc-accretionary belts to the south and southwest since ～400 Ma. The part of the NCC to the east of the Alax terrane (E-NCC) experienced a long sedimentary hiatus and tectonic quiescence between the Middle Ordovician and the late Carboniferous. The northern margin of the E-NCC was reactivated and uplifted with contemporaneous volcanism during the late Carboniferous, an event that partly induced the transformation of the E-NCC from an erosional platform to a continental sedimentary basin. However, the factors controlling this transformation are still not fully understood. A series of sedimentary rocks overlying Ordovician carbonates in the southwestern E-NCC contains a lower iron-oxide layer and an upper phyllosilicate layer. Detrital zircons from different parts of the profile, from the base to the top of the two layers, have similar U-Pb ages. These zircons have a minimum age of ca. 300 Ma and a prominent peak at ca. 450 Ma, with subordinate peaks at ca. 1000 and 2500 Ma. The near-identical minimum age for the two layers suggests they were semi-simultaneously deposited in the late Carboniferous after the long hiatus in sedimentation. Detrital zircons with ages of ～450 Ma have initial Hf isotopic compositions that vary from large negative to elevated positive. These data, together with the trace element compositions of these zircons, indicate that these minerals formed in a continental arc environment. Samples from the upper sedimentary layer contain mica group minerals that are weakly buckled and fractured, and have weathered to form clay minerals, including chlorite and illite. This suggests that the protolith of this sedimentary layer was dominated by mica schist or mica-bearing granitoid that most likely located near the adjoined part between the North Qilian and North Oinling arc-accretionary belts. Detrital zircons with the youngest ages (ca. 300 Ma) were considered to derive from the northern marginal region of the E-NCC, an area characterized by contemporaneous volcanism. The co-existence of the 450 and 300 Ma groups of detrital zircons also suggests that the northern and southern marginal regions of the E-NCC underwent almost simultaneous late Carboniferous tectonic uplift and denudation, a tectonic change that led to the formation of the sedimentary basins within the E-NCC. Late Carboniferous phyllosilicate layers within the profile are likely to be the equivalent of economically important diaspore and clay dominated bauxites within the E-NCC, suggesting that the diaspore in these bauxites was derived from mica group minerals by resolution-precipitation processes.