Neoproterozoic anatexis of Archean lithosphere: Geochemical evidence from felsic to mafic intrusions at Xiaofeng in the Yangtze Gorge, South China

Abstract

An integrated study of geochronology and geochemistry was carried out for Neoproterozoic intrusions at Xiaofeng in the Yangtze Gorge. The results are used to provide constraints on their origin, with potential resolution to hotly debated models concerning the source and process of contemporaneous igneous rocks in South China. The Xiaofeng intrusions are composed of felsic–mafic dykes and their wall-rock granitoids in an area of <10 km 2. They show intrusion relationship to TTG rocks of the Archean Kongling Complex. Zircon U–Pb dating indicates that the dykes and granitoids were crystallized almost contemporaneously at 800 ± 3 Ma. They share similar distribution patterns of REE and trace elements, i.e. enrichment in LILE and LREE but depletion in HFSE. The dykes have whole-rock ɛ Nd( t) values of −9.9 to −6.4 and zircon ɛ Hf( t) values of about −12.9 to −9.0, and the granitoids have whole-rock ɛ Nd( t) values of −10.6 to −9.2 and zircon ɛ Hf( t) values of −10.5 to −8.3. Furthermore, their zircon Hf model ages are similar to each other, from 2.8 to 3.2 Ga. These results indicate that they both are derived from reworking of Mesoarchean lithosphere, with the difference only in major element composition. Zircon has δ 18O values of 5.4–6.8‰ for them, typical for I-type granite. Mineral O isotopes indicate small-scale high-T water–rock interaction during the intrusion of the dykes into the granitoids. No growth of juvenile crust during the middle Neoproterozoic magmatism is identified to occur in the Xiaofeng intrusions. Thus, neither mantle plume nor oceanic arc is responsible for their origin, ruling out the plume-rift and slab-arc models for their petrogenesis. Instead, the Neoproterozoic anatexis of the Archean lithosphere can be explained by the tectonic collapse of a thickened intracontinental orogen that formed by Paleoproterozoic arc-continent collision to the Archean Kongling Complex, but it became molten due to lithospheric extension in response to plate reorganization prior to breakup of the supercontinent Rodinia. This provides a positive test to the plate-rift model for their petrogenesis.