Proterozoic evolution of the Alxa block in western China: A wandering terrane during supercontinent cycles

Abstract

The tectonic affinity of the Alxa block in the western North China craton remains controversial because of the widespread Cenozoic sediments and limited Proterozoic geochronological data, especially in the western Alxa block. To establish the tectonic affinity of the Alxa block with neighbor units, we systematically investigated U-Pb age and Hf isotope composition of zircon and U-Th-Pb age of monazite from Proterozoic rocks in the southwestern Alxa block. Ages of zircon and monazite from granitic gneiss and mica schist indicate that the Paleoproterozoic Longshoushan Complex recorded a magmatic event at ∼2.0 Ga and two metamorphic events at ∼1.95–1.90 and ∼1.85–1.80 Ga. Detrital zircons from quartz schist of the Longshoushan Complex show an age peak at ∼2.0 Ga with εHf(t) values between −4.048 and 6.432. By contrast, detrital zircons from the Late Paleoproterozoic Dunzigou Group yield different Paleoproterozoic age peaks and εHf(t) values, suggesting a provenance change. Detrital zircons from the Neoproterozoic Hanmushan Group exhibit three age peaks at ∼2.5 Ga, ∼1.85 Ga and ∼0.8 Ga, and two types of εHf(t) values with wide variations. The depositional ages of the Dunzigou Group and Hanmushan Group are constrained as 1.8–1.7 Ga and 845–830 Ma, respectively. The detrital zircon age populations suggest that the Longshoushan area experienced a transition from a convergent setting (less polymodal age spectra) at ∼1.98–1.95 Ga to a divergent setting (polymodal age spectra) at 1.8–1.7 Ga and then intracontinental rifting at ∼830 Ma. Compared with previous zircon studies, we propose that the Alxa block may be connected with the Khondalite belt at ∼1.98–1.90 Ga during assemblage of the Columbia supercontinent, and located between the North China craton and the northern Siberian craton at 1.8–1.7 Ga in a divergent setting. The Alxa block probably separated from the North China craton in the Late Mesoproterozoic and moved next to the northwestern Yangtze craton in the Early Neoproterozoic during the assemblage and breakup of the Rodinia supercontinent.