The Neoproterozoic high-Mg dioritic dikes in South China formed by high pressures fractional crystallization of hydrous basaltic melts

Numerous ∼830 Ma dikes intruded into the Neoproterozoic Tongde mafic-intermediate intrusion at the western margin of the Yangtze Block, providing an ideal opportunity for examining petrogenesis of high-Mg diorites during the breakup of the supercontinent Rodinia. Rocks from the dikes are composed of fine to medium grained diorites which have high Mg# (40-69), low SiO2 (50.98-61.01 wt%), and variable Na2O contents (1.94-5.50 wt%). They show right inclined REE patterns with slightly negative to positive Eu anomalies (Eu/Eu∗ = 0.90-1.35). Their primitive mantle-normalized trace element patterns are characterized by enrichment of Ba and Pb and depletion of Nb, Ta and Ti. They also show high and variable Sr/Y (12-115) and (La/Yb)N ratios (3-21). The high-Mg diorites have relatively constant initial 87Sr/86Sr ratios (0.704862-0.706320) and positive εNd values (+0.6-+2.0). These geochemical features reveal that the Tongde high-Mg dioritic dikes were not produced by partial melting of subducted oceanic slab or enriched mantle source. Rather, they were more likely formed by fractional crystallization of garnet and hornblende-bearing mineral assemblages from hydrous mafic magmas under high pressure conditions. The dikes intruded along the fractures resulted from magma thermal contraction after emplacement of the Tongde intrusion. The new model proposed for the petrogenesis of the high-Mg dioritic dikes suggest that the western margin of the Yangtze Block was in an active continental arc setting during the early Neoproterozoic.