Geochronology-geochemistry of the Cida bimodal intrusive complex, central Emeishan large igneous province, southwest China: petrogenesis and plume-lithosphere interaction

摘要

The Cida complex is situated in the Panxi region and is predominantly composed of mafic-ultramafic and syenitic rock units; minor amounts of intermediate rocks occupy the contact zone between the two major rock types. The intermediate unit is mineralogically heterogeneous and typically exhibits a mottled structure. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon dating shows that the mafic-ultramafic rocks and syenitic rocks formed almost coevally (243 ± 0.77 Ma and 240.5 ± 0.76 Ma, respectively). These ages may represent the end phase of the Emeishan large igneous province (ELIP) magmatism. Most of these three rock types possess alkaline and metaluminous affinities. The mafic-ultramafic, syenitic, and intermediate units have K_2O + Na_2O contents of 1.85-5.16, 6.55-10.46, and 9.55-11.54 wt.%, and SiO_2 contents of 40.06-46.70,61.74-68.54, and 51.57-54.13 wt.%, respectively. The mafic-ultramafic unit displays ocean-island basalt (OIB)-like primitive-mantle-normalized incompatible element patterns, coupled with low initial (87)~Sr/~(86)Sr ratios (0.7048-0.7064), positive ε_(Nd)(t) (0.32-2.23), and zircon (eHf)~(t) (4.53-14.17) values, consistent with a mafic plume-head origin, whereas one exceptional sample with negative ε(Nd)~(t)(-0.22) can be interpreted as due to the involvement of considerable amounts of enriched subcontinental lithospheric mantle. The relatively low (La/Yb)_N ratios (3.40-7.69) reflect a spinel-facies lherzolite source. The syenitic unit is characterized by enrichment in large ion lithophile elements (e.g. Rb, K, Pb) and light rare earth elements (LREEs), relative to high field strength elements (e.g. Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), respectively. These features, together with their metaluminous affinities, low SiO_2 contents, lower initial (87)~Sr/~(86)Sr ratios (0.7043), positive (εNd)~(t) (0.18), and zircon( εHf)~(t) (2.63-10.09) values as well as modelling of REEs, can be plausibly explained by crustal partial melting of juvenile basic materials beneath the Yangtze Block. In contrast, the field, petrographic observations, and geochemical signatures (e.g. the linear correlations between FeO* and MgO, K/Ba and Rb/Ba ratios) suggest that the intermediate unit may have resulted from magma mixing between the syenitic and basaltic magmas that in turn had evolved from a parental mafic-ultramafic liquid. Thus, the formation of the Cida complex can be attributed to the plume-lithosphere interaction plus partial melting of juvenile basic lower crust in response to heating of underplated plume-derived basaltic magma.