Petrogenesis and tectonic implications of Neoproterozoic, highly fractionated A-type granites from Mianning, South China

摘要

The origin of Neoproterozoic intrusions (ca. 860-750 Ma) along the western part of the Yangtze Craton has been the subject of debate in recent years, with two competing models proposed. The plume model argues for an extensional setting and emphasizes the role of a superplume in the Rodinia breakup, whereas the arc model argues for the presence of a subduction zone in the Yangtze Craton. As a contribution to this animated dispute, geochronologic and geochemical analyses have been carried out on the Mianning granite, which is the largest pluton (similar to 700 km(2)) in the northern Kangdian rift of the western Yangtze Craton. It is shown that the Mianning granites were emplaced at ca. 780 Ma and display highly fractionated feature (i.e., SiO2 > 75 wt%: Eu/Eu* =0.03-0.50: enrichment of K, Rb, Th, U, Zr, Hf, Y and REEs; depletion of Nb, Ta, Ba, Sr, P, Eu and Ti). They are metaluminous to strongly peraluminous (A/CNK = 0.93-1.55) and contain abundant perthite and minor alkali riebeckite and sphene, sharing the petrological and geochemical characters of A(2)-type granites. Positive epsilon(Nd) (t) (2.97-5.24) and zircon epsilon(Hf) (t) (9.2-12.1) values are consistent with a derivation by partial melting of a relatively young crust formed about 1000-900 Ma. Given the general absence of A-type granites in arc settings, the Mianning A-type granites are suggestive of ail anorogenic, crustal extensional environment for the western Yangtze Craton during the Neoproterozoic. The data presented in this Study are therefore consistent with an intracontinental rift model, but are not sufficient to identify plume involvement in the Neoproterozoic magmatism.