Secular evolution of the Neoproterozoic lithospheric mantle underneath the northern margin of the Yangtze Block, South China

The 820-Ma Wangjiangshan, 814-Ma Beiba, 780-Ma Bijigou and 746-Ma Luojiaba mafic-ultramafic intrusions in the Hannan region of the northern margin of the Yangtze Block, South China, are used to constrain the secular evolution of the lithospheric mantle. These intrusions are mainly composed of gabbros, but the Beiba and Wangjiangshan intrusions also contain diorite. Rocks from these four intrusions have arc-like geochemical characteristics with high Ba/Nb (13-733), Ba/La (2-150), Sr/Nb (28-2320), Pb/Nb (0.2-13) and Ba/Th (107-7150) ratios relative to the primitive mantle. The Bijigou and Wangjiangshan intrusions have relatively constant initial Sr-86/Sr-86 ratios (0.703267 to 0.703729) and epsilon Nd(t) values (+2.95 to +3.87). The Luojiaba intrusion has initial Sr-87/Sr-86 ratios (0.703690 to 0.703914) and epsilon Nd values (+ 0.93 to + 1.34) similar to those of the Beiba intrusion (Sr-87/Sr-86=0.703768 to 0.704586; epsilon Nd=+0.23 to +1.98). The average zircon epsilon Hf values of the Bijigou and Wangjiangshan intrusions (+7.38 to +9.58) are higher than those of the Luojiaba (+5.73) and Beiba intrusions (+4.76). The Wangjiangshan, Bijigou and Luojiaba intrusions do not show crustal contamination, whereas the Beiba intrusion shows evidence of crustal contamination. Modeling calculations reveal that the Wangjiangshan, Luojiaba and Beiba intrusions were produced by partial melting of garnet-spinel lherzolite to spine] lherzolite mantle, whereas the Bijigou intrusion was derived from a garnet lherzolite to garnet-spinel lherzolite mantle source. The arc-related geochemistry of these intrusions suggests that the lithospheric mantle sources were modified by slab fluids. The Luojiaba intrusion with high Th and high-field-strength-elements (HFSE) was derived from a mantle source enriched by sediment melts. Combined with 950 Ma arc volcanic rocks in the region, these intrusions support a model in which the lithospheric mantle beneath the northern margin of the Yangtze Block was enriched by slab-derived fluids since 950 Ma, and then depleted by melt extraction at similar to 780 Ma, eventually further enriched by slab-derived melts at similar to 746 Ma. (c) 2008 Elsevier B.V. All rights reserved.