Neoproterozoic subduction tectonics of the northwestern Yangtze Block in South China: Constrains from zircon U–Pb geochronology and geochemistry of mafic intrusions in the Hannan Massif

Abstract

The Neoproterozoic mafic intrusions of the Hannan massif at the northwestern margin of the Yangtze Block in South China are composed mainly of OIB-type gabbros and subduction-related mafic intrusions. Here we present the petrology, geochemistry and zircon U–Pb geochronology of these intrusions. The gabbros are characterized by enrichment of LILE and highly differentiated HFSE patterns, suggesting their deviation from an OIB mantle source. The gabbroic melts were generated by about 5–10% partial melting of a garnet + spinel lherzolite, and crystallized at ca. 898 ± 10 Ma. The subduction-related intrusions include the Wangjiangshan gabbro-diorite suite, Xijiaba gabbros, and Youshui diabases and the gabbros associated with ultramafic rocks. The diorites and gabbros in Wangjiangshan are characterized by a depletion of Nb–Ta, P and Ti with a flat distribution of most of the LILE and HFSE except for the markedly large variations in the positive anomalies displayed by Ba, K, Pb and Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process. The melts of the Wangjiangshan intrusions were derived from a spinel lherzolite mantle source by diverse degrees (5–20%) of partial melting, and crystallized at ca. 784.3 ± 6.0 Ma. The geochemical signatures including depletion in Nb–Ta, Zr, P and Ti, and slight differentiation of HFSE without crustal contamination suggest that the Xijiaba gabbros were formed in a mantle wedge setting at ca. 798.7 ± 4.6 Ma. These rocks were derived from ca. 10% partial melting of a garnet + spinel lherzolite. The Youshui diabases display enrichment of Rb, Ba, U, Pb and Sr, and depletion of Nb, Ta, P and Ti, suggesting that their magma source was significantly modified by subduction material. These diabases crystallized at ca. 794.7 ± 5.2 Ma from melts produced by about 20% partial melting of a garnet + spinel lherzolite mantle source. The associated gabbros crystallized at 751.5 ± 6.3 Ma and show variable enrichments of Rb, Ba, U, Pb and Sr, depletion of Nb and Ta, and positive Eu and P anomalies. These characters reflect variable degrees (5–30%) of partial melting from a garnet + spinel lherzolite in a mantle wedge source. The OIB type intrusions suggest the possible involvement of a mantle plume at ca. 900 Ma related to the breakup of South China from the Neoproterozoic Rodinia supercontinent. In contrast, the other intrusions suggest the existence of a subduction system along the western or northwestern margin of the Yangtze Block around 840–750 Ma. Such an active subduction system developed immediately after the breakup of Rodinia, implying that either the Yangtze Block was located at the edge of the Rodinia supercontinent, or that the breakup of Rodinia did not occur simultaneously in all the regions.