Multiple sources for the origin of granites: Geochemical and Nd/Sr isotopic evidence from the Gudaoling granite and its mafic enclaves, northeast China

摘要

Geochemical and Sr- and Nd-isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China. The rocks include monzogranite, porphyric granodiorite and quartz diorite. Monzogranites have relatively high (87) Rb/Sr-86 ratios (0.672-0.853), low initial Sr-87/(86) Sr ratios (0.7052-0.7086) and epsilon (Nd)(t) values (-18.5 to -20.9) indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time. Quartz diorites have high initial Sr-87/Sr-86 ratios (0.7118-0.7120) and negative epsilon(Nd)(t) values (- 13.2 to - 18.1) coupled with high Al2O3 and MgO contents, indicating they were derived from enriched lithospheric mantle with contributions of radiogenic Sr from plagioclase-rich metagreywackes or meta-igneous rocks, i.e., ancient lower crust. Two groups of enclaves with igneous textures and abundant acicular apatites are distinguished: dioritic enclaves and biotite monzonitic enclaves. Dioritic enclaves have low AI(2)O(3) (13.5-16.4 wt%) and high MgO (Mg# = similar to72.3) concentrations, low initial Sr-87/(86) 16 Sr ratios (0.7058-0.7073) and negative epsilon(Nd)(t) values (similar to-7.2), and are enriched in LILEs and LREEs and depleted in HFSEs, suggesting they were derived from an enriched lithospheric mantle source. Biotite monzonitic enclaves have Sr and Nd isotopic compositions similar to the monzogranites, indicating they were crystal cumulates of the parental magmas of these monzogranites. Granodiorites have transitional geochemistry and Nd- and Sr-isotopic compositions, intermediate between the monzogranites, quartz diorites and the enclaves.Geochernical and Sr- and Nd-isotopic compositions rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mafic mantle-derived and juvenile crustal-derived magmas, coupled with crystal fractionation and assimilation of ancient lower crust, is compatible with the data. This example shows that at least some calc-alkaline granitoids are not produced by pure intracrustal melting, but formed through a complex, multi-stage hybridization process, involving mantle- and crustal-derived magmas and several concomitant magmatic processes (crystal fractionation, crustal assimilation and crustal anatexis). Copyright (C) 2004 Elsevier Ltd.