The Neoproterozoic granitoids from the Qilian block, NW China: Evidence for a link between the Qilian and South China blocks

Abstract

The field occurrence, petrography, geochemistry, Sm–Nd isotopes, and geochronology of the Neoproterozoic granitoids exposed at Tuole, Huangyuan, Gahai, Haiyan, Riyueshan, Baokuhe, and Maxianshan in the Qilian block were studied. The Neoproterozoic granitoids in the Qilian block form two age groups: ca. 800Ma and ca. 900Ma. The ca. 800Ma group granitoids consist of granodiorite and granite, whereas the ca. 900Ma group granitoids quartz diorite, granodiorite, and potassic granite. They have intruded the Huangyuan Group in the basement sequence of the Qilian block. The foliations are moderately to well-developed and are concordant with those of the country rocks. The granitoids of both age groups all show enrichment in LREEs, but this enrichment is more pronounced in the ca. 800Ma group than in the ca. 900Ma group. All granitoids show negative europium anomalies, but the ca. 800Ma group granitoids are more varied in Eu/Eu* values (0.23–0.83) than most of the ca. 900Ma group granitoids (0.54–0.98). Spider diagrams of both groups also exhibit enrichment in large ion lithophile elements, i.e., Rb, Th, U, and K, and negative anomalies in Nb–Ta, Sr, P, Ti, and, with exception of the Maxianshan quartz diorite, Ba. The ɛNd(1Ga) and TDM are −6.7 to −12.7 and 2.2–3.0Ga for the ca. 800Ma granitoids and are −4.3 to −5.2 and 2.0–2.3Ga for the ca. 900Ma granitoids. The granitoids of the ca. 900Ma group are interpreted to have formed in an arc tectonomagmatic environment on an active continental margin, whereas those of the ca. 800Ma group are thought to form in a continental rift environment. The ca. 900Ma granitoids are I-type and could be formed from partial melting of K-rich metabasalt or eclogite with significant amount of mantle components at a pressure of 1–4GPa in the lower crust. The strongly peraluminous or peraluminous Gahai granites and Haiyan granodiorite are S-type and could be derived from melting of clay-poor, mature psammitic sources.Similarities in age and tectonic environment of magma genesis suggest a correlation of Neoproterozoic granitoid magmatism between the Qilian and South China block. This correlation gives a strong support to the consideration that the two blocks might be unified in the Neoproterozoic. In other words, the Qilian block might be a part of the South China block or, more precisely, the Sibao orogen, in the Neoproterozoic time.