Zircon U-Pb ages and Hf isotopic compositions of fluvial sediments from the Huangshui, Beichuan, and Xichuan rivers, Northwest China: Constraints on the formation and evolution history of the Central Qilian Block

摘要

In this paper, 219 concordant detrital zircons from the main stem and tributary rivers of the Central Qilian Block (CQB) have been analyzed using excimer laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine their U-Pb ages and Lu-Hf isotopic compositions. The detrital zircons from three samples show four major age groups: 246-509 Ma, 899-1176 Ma, 1620-2089 Ma, and 2131-2610 Ma. These age populations indicate that prominent magmatic events occurred at 0.5 Ga, 0.9 Ga, 1.8 Ga, and 2.5 Ga. Archean basement components are rare in these river sands and likely derived from Paleoproterozoic metamorphic complexes. The crustal model ages from Hf isotopic analyses show age peaks at -2.5 and 1.8 Ga. The crustal accretion of material derived from the depleted mantle of the CQB occurred in two stages at 3.2-1.8 Ga and 1.8-0.6 Ga. The crustal accretion curve based on the Hf model ages indicates that approximately 10% of the present crustal volume of the CQB formed at 3.2 Ga, while -60% formed at 1.8 Ga. Few crustal components have accreted from the depleted mantle since 0.6 Ga in the CQB. The reworking rate calculation shows that continental crustal accretion also occurred at 2.5 Ga in the CQB; the most intensive crustal reworking occurred at 0.8 Ga. Comparisons of Precambrian continental growth patterns of the African continent and CQB indicate that the zircon Hf isotopic compositions can be used to accurately constrain the growth patterns of continental crust and that a microcontinent can serve as window into the crustal growth of supercontinents during certain periods.