High-Mg diorites were discovered in the southern part of the ca. 830Ma Dongma Pluton, northern Guangxi Province of southern China. The diorites (SiO2=59-65wt%) are characterized by high MgO (6.7-8.9wt%) contents and Mg-number [Mg#=100×Mg/(Mg+Fe)] (69-73), in contrary to the associated medium-Mg (MgO=3.4-3.8wt%, Mg#=59-63) granodiorites in the Dongma main body and the low-Mg (MgO=1.4-1.9wt%, Mg#=46-51) granodiorites in the Bendong Pluton to the north. Moreover, the high-Mg diorites show surprisingly high Cr (595-640ppm) and Ni (171-194ppm) concentrations, which are beyond the ranges of most coeval mafic rocks in the study area. Correspondingly, chromite crystals were separated from the high-Mg diorites and some of the medium-Mg granodiorites, and they show high Cr# [100×Cr/(Cr+Al)] (average of 75), but low Mg# (0.34-2.51) and low Fe3+. The decoupling of Cr# and Mg# and the existence of quartz+apatite mineral inclusion in chromites suggest Mg-Fe exchange that may be facilitated by the disequilibrium resulted from magma mixing. The high-Mg diorites show low La/Yb (6.8-8.5) and Sr/Y (2.1-3.1) ratios, significant negative anomalies of Nb and Ti and positive anomaly of Pb, resembling the Setouchi high-Mg andesites, despite of their relatively low Sr (71-100ppm). All of the studied diorites and granodiorites show enriched Nd isotope compositions, with εNd(t) values (-3.2 to -5.9) a bit higher than some of the associated mafic rocks. Some of the high-Mg diorites show whole-rock εHf(t) (-6.0 to -6.2) coupled with Nd isotopes, similar to the associated mafic-ultramafic rocks in northern Guangxi, suggesting the metasomatism by melts of subducting sediments in the mantle source. Whereas, others show decoupled Nd-Hf isotopes that are similar to the medium- and low-Mg granodiorites [εHf(t)=-1.8 to +0.05], probably indicating the late magma mixing with granitic magmas at a crustal level for the dioritic magmas. We propose a two-stage model for the petrogenesis of the high-Mg diorites: 1) the mantle source was firstly metasomatized by melts from partial melting of subducting terrigenous sediments to form the enriched Nd-Hf isotopic characteristics; and then 2) the mantle-derived high-Mg mafic melts mixed with the crust-derived low-Mg granitic melts to form the high-Mg diorites and medium-Mg granodiorites. The occurrence of high-Mg diorites implies the existence of Neoproterozoic subduction-related metasomatism in the western part of the Jiangnan orogen.
Bibliographical noteA Corrigendum to this article was published in Lithos vol. 208-209, p. 486.
- high-Mg diorite
- Jiangnan orogen