Contrasting mantle-crust melting processes within orogenic belts: Implications from two episodes of mafic magmatism in the western segment of the Neoproterozoic Jiangnan Orogen in South China

Abstract

Partial melting of crust and mantle is the dominant mechanism controlling Earth’s differentiation. The two melting processes are commonly associated because mantle-derived magmas could simultaneously provide heat and volatiles to promote partial melting of crust. To explore how the two melting regimes interact during the evolution of orogenic belts, we evaluated the petrogenesis and possible relationships with the Rodinia supercontinent of the two episodes of Neoproterozoic mafic magmatism in the western segment of the Jiangnan Orogen in South China. The first episode of mafic magmatism took place at ca. 830Ma in the Baotan area and was associated with widespread peraluminous granitoids. In contrast, the second formed at ca. 770Ma in the Longsheng area without significant granitic magmatism. The ca. 770Ma Longsheng mafic-ultramafic rocks show radiogenic and coupled Nd [εNd(t) values of −1.3 to +3.1)] and Hf isotopes [εHf(t) values of +2.6 to +6.7], whereas the ca. 830Ma Baotan mafic rocks show unradiogenic and decoupled Nd-Hf compositions [εNd(t)=−8.3 to −0.9, εHf(t)=−6.3 to −2.0]. Crustal contamination cannot result in the enriched isotopic features and the arc-like (such as strong deletions of Nb and Ta) geochemistry for the Baotan rocks because of the absence of correlations of Nd isotopes with incompatible elements and SiO2 contents, but crust-derived components may have been incorporated into the mantle source (<60km) by oceanic subduction. Geochemical modeling suggests that the Longsheng mafic-ultramafic rocks with steep rare earth element patterns formed from a deeper (deep to 120km) mantle source with little crustal contamination. We suggest that the Baotan mafic rocks formed in an active continental margin setting where a lot of volatiles can be released from the underplating mafic magmas to facilitate further crustal melting to generate the associated granitoid rocks. In contrast, the Longsheng rocks formed in a post-orogenic extensional setting where mafic magmas were apt to ascend so that extensive crustal melting was prohibited. The transition between the two episodes of mantle-derived magmas shed light on the secular evolution of lithospheric mantle in the western segment of the Jiangnan Orogen. The diverse mantle-crust melting processes in orogenic belts can serve as an indicator of tectonic transition of Precambrian orogenic belts in the history of the earth.