Abstract
Cenozoic alkali basalts in Southeast (SE) China generally are genetically related to intracontinental rifting. Hence, they can be used to probe the nature of their underlying mantle sources and aid studies of the tectonic background in this region. This paper focuses on the Shanhoujian alkali basalts located in Bailing County, northeastern Fujian, SE China. We herein report their petrology, whole-rock major, and trace element geochemistry, and Sr-Nd isotopic composition and provide a new zircon U-Pb age for the basalts (~40 Ma, Eocene). These data help to constrain the petrogenesis of alkali basalts, their mantle source, and tectonic settings. The basalts are characterized by high Mg# (58.21–63.52) with Na2O/K2O > 1. MgO content is weakly correlated with CaO and Cr content but shows no correlation with Ni and Fe2O3 (total). Such features suggest that fractionation of clinopyroxene rather than olivine was important. In terms of trace elements, the alkali basalts display: (1) enrichment in La, Ce, Rb, Ba, Nb, and Ta and depletion in K, Pb, Zr, Hf, and Ti and (2) notable fractionation of light rare earth elements from heavy rare earth elements. Determined (87Sr/86Sr)i is in the range of 0.7041–0.7040 and εNd (t) is between +3.2 and +3.3. The Shanhoujian alkali basalts show a notable affinity to oceanic island basalts (OIBs) with little assimilation of crustal materials. They were derived from a pyroxenite and carbonated peridotite mantle source metasomatized by sediments carried by the subduction plate at different depths. The primary magmas of these basalts were derived from partial melting of this metasomatized mantle source during upwelling of the asthenospheric mantle as an intracontinental rift formed through extension in this part of SE China.
Highlights
Alkali basalts are predominantly distributed within intraoceanic and intracontinental plates.There are two main opinions about the formation of oceanic alkali basalts: (1) they were formed from recycled materials of subducted oceanic crust [1,2] and (2) they were derived from partial melting of the Minerals 2020, 10, 770; doi:10.3390/min10090770 www.mdpi.com/journal/mineralsMinerals 2020, 10, 770 metasomatized mantle source [3,4,5]
This paper focuses on the Shanhoujian alkali basalts in the northeastern part of the province and explores their petrogenesis using petrological and geochemical methods
After removing analyses showing strongly discordant ages, the dating result exhibited two age groups, one showing a weighted mean age of 117.5 ± 5.9 Ma (MSWD = 14) (Figure 4a) and the other an age of 40.10 ± 0.45 Ma (MSWD = 0.15) (Figure 4b). The latter was assumed to be the formation age of the alkali basalts, whereas the former probably represents the age of early Cretaceous magmatic rocks, consistent with late Mesozoic magmatism
Summary
Alkali basalts are predominantly distributed within intraoceanic and intracontinental plates.There are two main opinions about the formation of oceanic alkali basalts: (1) they were formed from recycled materials of subducted oceanic crust [1,2] and (2) they were derived from partial melting of the Minerals 2020, 10, 770; doi:10.3390/min10090770 www.mdpi.com/journal/mineralsMinerals 2020, 10, 770 metasomatized mantle source [3,4,5]. There are two main opinions about the formation of oceanic alkali basalts: (1) they were formed from recycled materials of subducted oceanic crust [1,2] and (2) they were derived from partial melting of the Minerals 2020, 10, 770; doi:10.3390/min10090770 www.mdpi.com/journal/minerals. There are several arguments regarding the formation and genesis of continental alkali basalts, such as that they were derived from an asthenosphere mantle source [6], a mixture of asthenospheric and lithospheric materials [7], or source mantle containing recycled crustal materials [8,9]. Experimental results demonstrated that the mantle source for alkali basalts contained significant pyroxenites, hornblendites, and carbonated peridotites [4,10,11,12,13,14]. It is important to investigate the components of the mantle source of alkali basalts
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