Formation and Evolution of a Magmatic System in a Rifting Continental Margin: Neoproterozoic Arc- and MORB-like Dike Swarms in South China

Abstract

Mafic magmas generated in extensional tectonic settings preserve important information about the nature of their mantle sources, interactions between crust and mantle, and the processes associated with magmatic evolution. However, careful study is needed to interpret the complex petrogenesis and diverse origins of such rock suites. A large volume of c.650 Ma mafic and ultramafic dikes are found in the South Qinling Belt (SQB) along the northern margin of the Yangtze Block, South China. The dikes from the eastern SQB are composed of olivine gabbro and gabbro that have low SiO_2 (42·60 to 49·56 wt %), low K_2O+Na_2O (0·98 to 4·48 wt %), and high MgO (4·96 to 14·41 wt %). MELTS modeling reveals that many of these dikes could have originated from a common primary magma that underwent extensive fractional crystallization of olivine + plagioclase + clinopyroxene followed by accumulation of phenocrysts in most samples. They show arc-like trace element compositions characterized by enrichment of LILE and LREE and depletion of HFSE. They have high initial ^(87)Sr/^(86)Sr (0·705192 to 0·706622), negative eNd_(650) (-7·11 to -4·45), and very low ^(206)Pb/^(204)Pb (16·71 to 16·98), ^(207)Pb/^(204)Pb (15·36 to 15·42) and ^(208)Pb/^(204)Pb (37·17 to 37·48). Their magmatic zircons preserve relatively homogeneous δ^(18)O (+4·95‰ to +6·41‰) and highly variable eHf (-0·5 to +10·0). Both chemical compositions and modeling results suggest that dikes from the eastern SQB were derived from a lithospheric mantle source that had been extensively modified by earlier subduction and further underwent strong contamination by the ancient granulite facies lower crust. In contrast, mafic dikes from the western SQB have relatively high SiO_2 (44·97 to 52·09 wt %) and low MgO (4·73 to 9·40 wt %). They fall into two groups that show N-MORB and E-MORB-like elemental characteristics, respectively. Both types have low initial ^(87)Sr/^(86)Sr (0·703752 to 0·706850), positive eNd_(650) (+2·75 to +5·85) and scattered ratios of ^(206)Pb/^(204)Pb (17·25 to 18·42), ^(207)Pb/^(204)Pb (15·44 to 15·55) and ^(208)Pb/^(204)Pb (37·37 to 38·74). Their zircon δ^(18)O (+3·64‰ to +5·33‰) and eHf (+10·2 to +14·8) values are also significantly different from those of the eastern SQB dikes. The chemical evidence suggests that mafic dikes from the western SQB originated from a heterogeneous asthenospheric mantle, one part of which may have been enriched by OIB/seamount subduction. These mafic dikes underwent only minor modification in the lower crust, but are contaminated by high temperature hydrothermally-altered supracrustal materials. Generation of the voluminous mafic and ultramafic dikes in the SQB occurred in a rifting continental margin after a long period of subduction. Their diverse origins and complex geodynamic setting suggest that magmatism in rifting continental margins is not only controlled by the structure of the lithosphere and upwelling of the asthenospheric mantle, but also by interaction between melts and continental crust. The mafic–ultramafic dikes in this study further indicate that an active magma plumbing system may have evolved beneath a ‘non-volcanic’ passive continental margin.