Paleoposition and initial rifting of the Yangtze Block in Columbia Supercontinent: Discovery of Paleo- and Mesoproterozoic metasediments in the Ailaoshan Metamorphic Belt

Abstract

Supercontinent cycle is one of the most spectacular processes that govern the assembly and dispersion of continental blocks globally and drives intensive crustal-mantle interactions. Numerous magmatic and sedimentary records documented the involvement of the Yangtze Block and the Ailaoshan Metamorphic Belt (AMB) to the southwest in the Precambrian supercontinent cycles (i.e., Columbia and Rodinia). However, the paleoposition and geodynamic processes of the Yangtze Block in the Columbia cycle remain controversial. Moreover, whether and how the AMB participated in the Precambrian Columbia and Rodinia cycles remain largely disputed. In this paper, we report the first identification of the Paleoproterozoic-Mesoproterozoic meta-sediments within the AMB that was intruded by Neoproterozoic magmatic rocks locally. This provides a prime opportunity to resolve the above issues through sedimentary correlation and comparison. Our comprehensive zircon trace element and U-Pb-Lu-Hf isotopic study on these meta-sediments in the AMB allow us to reach the following conclusions. The youngest magmatic zircon age populations indicate these meta-sediments deposited during the Paleo- to Mesoproterozoic. Therefore, they are the oldest geological records in the AMB that have ever been reported. These Paleo- and Mesoproterozoic meta-sediments in the AMB share uniform zircon U-Pb age peaks at ∼ 2.3 Ga and 1.85 Ga. Circa 1.6–1.4 Ga age population and an increase of the ca. 2.5 Ga population are also detected in the Mesoproterozoic sample. These meta-sediments in the AMB could be most plausibly interpreted as the southwestward extension of the contemporaneous depositions in the SW Yangtze Block given their identical depositional ages, age distribution pattern, and source areas with concurrent strata in the SW Yangtze Block. Accordingly, the Neoproterozoic magmatic rocks in the AMB should document the southward extension of the supra-subduction zone along the northern to western margin of the Yangtze Block. Lines of evidence reveal that the SW Yangtze Block and the NW Laurentia shared an adjacent paleoposition in Columbia. Changes of detrital zircon ε_Hf(t) values, Eu/Eu* and ¹⁷⁶Lu/¹⁷⁷Hf ratios trends through time support the onset of an extensional tectonic at 1.75 Ga in the SW Yangtze Block. Matched magmatic and sedimentary records documented the onset of early rifting processes at 1.75 Ga in the SW Yangtze Block and the NW Laurentia during the breakup of Columbia.