Nd–Hf isotopic mapping of Late Mesozoic granitoids in the East Qinling orogen, central China: Constraint on the basements of terranes and distribution of Mo mineralization

Abstract

Voluminous Late Mesozoic granitoids and the world’s largest Mo deposits occur in the East Qinling. This paper presents the results of Nd–Hf isotopic mapping for the Late Mesozoic granitoids (155–105Ma) and demonstrates their constraint on the basements and distribution of the Mo deposits in the East Qinling. This isotopic map, made by 98 (21 new and 77 published) whole-rock Nd isotopic and 29 (7 new and 22 published) average zircon Hf isotopic data, shows large variations of whole-rock εNd(t) values from −22.1 to −1.5, and the correspondingly Nd model ages (TDM(Nd)) from 2.83 to 0.79Ga, and zircon εHf(t) values from −26.3 to +0.1 and two-stage Hf model ages (TDM2(Hf)) from 2.86 to 0.96Ga. Three regions of variations have been identified from north to south: (a) εNd(t) values range from −22.1 to −10.9 with TDM(Nd) of 2.82–1.47Ga, and εHf(t) values 26.3 to −13.5 with TDM2(Hf) 2.86–2.04Ga; (b) εNd(t) values −13.9 to −1.5 with TDM(Nd) 2.02–0.79Ga, and εHf(t) values −16.2 to +0.1 with TDM2(Hf) 1.96–0.96Ga; and (c) εNd(t) values −6.3 to −4.5 with TDM(Nd) 1.28–1.12Ga, and εHf(t) values −1.0 to −0.3 with TDM2(Hf) 1.25–1.22Ga, respectively. The three regions approximately correspond to the three different terranes, the southern margin of the North China Block (NCB), the North Qinling Belt (NQB) and the South Qinling Belt (SQB), respectively. These demonstrate that the granitoids in the different terranes have distinct sources and their sources change from old to more juvenile from the north (southern margin of the NCB) to the south (SQB). These also reveal the distinct basements for the terranes in Late Mesozoic. The southern margin of the NCB contains widespread Neoarchaean to Paleoproterozoic basement, the NQB comprises Archaean to Neoproterozic basement and the SQB Mesoproterozic to Neoproterozic basement. All these suggest that the three terranes underwent different tectonic evolution and the continental crust of the East Qinling were mainly formed during Archaean to Neoproterozic, different from a typical accretion orogen. The old sources of the granitoids and basements of the terranes constrain the distribution, scale and number of the Mo mineralization and deposits. Mo mineralization is closely related to the small granitic bodies with old continental component sources and Mo deposits are mainly hosted by the terranes with oldest basement. The scale and number of the Mo mineralization and deposits decreased from the southern margin of the NCB to SQB.