Petrogenesis of the Linxi granitoids, northern Inner Mongolia of China: constraints on basaltic underplating

Five representative granitoid plutons in the Linxi area, northern Inner Mongolia of China, divided into two suites, are mainly granodiorite and syenogranite and correspond to I- and A-type granites, respectively. Zircon U-Pb dating yields an age of ∼ 230 Ma, but most ages range from 149 Ma to 111 Ma for intrusion of the five plutons. Suite 1 granitoids show lower SiO2 (66.1-72.51%) and K2O, higher Al2O3, CaO, MgO, FeO, and TiO2, higher Sr, Ba, and Eu, and lower Sm + Gd, Nb, Y, and Yb than Suite 2 granitoids (71.73-78.14% SiO2). Modeling of major and trace elements shows that fractional crystallization of hornblende and lesser amounts of feldspars for Suite 1 and plagioclase and alkali feldspar for Suite 2, with overprint of magmatic hydrothermal fluids, can explain the geochemical variations. The Linxi granitoids have initial εNd(t) values varying between +5.60 and −2.89, and initial (87Sr/86Sr)t ratios ranging from 0.701764 to 0.707654. Geochronology of inherited zircons, coupled with the Nd, Sr isotopic comparison between granitoids, Late Jurassic basalts, and Proterozoic gneisses, indicates that the Linxi granitoids derive from a mixed source of dominantly underplated material or mafic magma with various amounts of the Proterozoic gneiss. Basaltic underplating provides both material and heat to produce the Late Jurassic to Early Cretaceous granitoids. Variation of εNd(t) and (87Sr/86Sr)t with age suggests involvement in the melting of increasingly greater amounts of the old lower-crustal rock. Our refined equation yields less scattered Nd isotope depleted mantle model ages. The least-fractionated granitoids from each suite have low Ca/Sr, Rb/Sr, Rb/Ba, and K/Ba ratios and, hence, represent parental magmas. Suite 1 parent magma is produced by dehydration melting or derives principally from the fluid-metasomatized mantle. Melting of the underplated tholeiites and their differentiated products generates Suite 2 parent magma.