Geochemical and zircon U-Pb and Lu-Hf isotopic constraints on the origin of supracrustal rocks from the mid-Qilian terrane: A comparison between supracrustal rocks on the two sides of the eastern segment of the Altyn Tagh Fault

Abstract

Comparisons between the tectonic units on two sides of the sinistral ENE-striking Altyn Tagh Fault (ATF) are important tracers to find out the corresponding relationships among different terranes. This study focuses on the tectonic units along the eastern segment of the ATF. The Paleoproterozoic Beidahe Group in the mid-Qilian terrane, south side of the eastern segment of the ATF, is mainly composed of metasedimentary rocks and amphibolites. These rocks are petrologically and mineralogically similar to those from the Dunhuang Group, north side of the eastern segment of the ATF. The detrital zircon U-Pb ages indicate that the protolith sediments of the Beidahe metasedimentary rocks were mainly sourced from a provenance dominated by magmatic rocks in ages of 2.85–2.55Ga, 1.70–1.55Ga and 1.42–1.03Ga. This sedimentary sequence was deposited no earlier than 1.03Ga as passive continental margin deposits. Geochemical and zircon U-Pb and Lu-Hf isotopic data suggest that the Neoarchean-Mesoproterozoic basement rocks from the Tarim Craton most likely provide the majority of source materials for their protolith. Amphibolites from the Beidahe Group have high total Fe2O3 and Al2O3 contents but low Mg# values, indicating a high degree of compositional evolution and contamination of the older crust. They have flat REE patterns but positive LILE and negative Nb-Ta and Ti anomalies, suggesting a back-arc basin basalts (BABBs) affinity for their protolith. The amphibolites have a crystallization age of 1.61Ga and zircon εHf(t) values ranging from −7.7 to +10.6. The zircon εHf(t) values and whole-rock εNd(t) values indicate an involvement of juvenile and pre-existing old materials in the formation of their parental magmas. Comparisons between the Beidahe and Dunhuang Groups indicate that the metasedimentary rocks from these two groups are both sourced from the Tarim Craton but have different depositional ages, and that the amphibolites have similar crystallization ages but different tectonic settings for their source rocks. The field relationships and geochemical and geochronological characteristics of the metasedimentary rocks and amphibolites from the Beidahe Group support the interpretation that the observed recent proximity of these two rock complexes might be the result of tectonic juxtaposition rather than a primary intrusive contact.