| Tectonic position of the diabase from Naduonong area located in Eastern Bangong-Nujiang suture zone. It consists of diabase No. Ⅰ and diabase dikes group No. Ⅱ. System studies suggest that there are large differences between the two in the diagenetic age, rock types, geochemistry, magma source, geodynamic setting and so on.Ddiabase of No. Ⅰ is Palimpsest diabase,rocks experienced a high degree of alteration. Rittman Index (σ) of these rocks varies from0.86-2.56, an average of1.78, similar to ocean tholeiitic series. Comparative study of major elements showed that Al2O3, TiO2, P2O5, Na2O, K2O, TFeO and the total alkalis (ALK) content close to the mid-ocean ridge basalts MORB. Overall, the trace element display a relatively flat distribution pattern, all incompatible element has no obvious abnormality. Average ratios of Ce/Zr, Zr/Nb, Th/Yb, Zr/Y, Ti/Y is0.15,15.3,0.26,2.20,327.56, respectively, similar to E-MORB. LREE/HREE is0.49-0.64,(La/Yb)N is0.65～1.21, show relatively loss of light REE.δEu(0.96～1.11) and δCe(0.74～1.04), show no obvious abnormalities;(La/Sm)Nis0.85～1.26, showing a flat-type REE pattern that transition from the normal (N-MORB) to enriched mid-ocean ridge (E-MORB) and similar to enriched mid-ocean ridge (E-MORB).(143Nd/144Nd) t=0.510662～0.510709, an average0.5106855,(εNd) t=3.9～4.8, an average of4.35;(87Sr/86Sr)t=0.702414-0.704642, an average of0.703528, suggests the magma Originated from mantle zone which shows varied degrees of mixed melting between the depleted mantle DMM and the abnormally high U/Pb ratios mantle HIMU. The diagenetic age of diabase No. I is1682.4Ma, belongs to Mesoproterozoic, Combined with tectonic evolution of Tibetan Plateau and regional geological data, The author speculate that it may belong to metamorphic residual part of the crystalline basement which Formed in the Proterozoic.Gabbro diabase from dikes group No. Ⅱ experienced Weak alteration, and it Contains some quartz. Rittman Index (σ) of these rocks varies from0.86～2.56, an average of3.10, similar to calc-alkaline series series. With enriched content of LILE(Rb, Th, U) and loss of HFSE(Nb, Sr, P, Ti)and Ba, trace elements proforms a rightist distribution, like arc volcanic rocks of Zenong Group (Kiz) and Duoni (K1d), it implies crustal contamination during diagenesis. LREE/HREE is1.34～2.75,(La/Yb)N is2.47-6.54, display as LREE enrichment type.(La/Sm)Nis1.90～3.29, similar to Mantle plume or pathogenic isoform.δEu is0.60～0.92,δCe is0.85～1.05, the moderate negative Eu anomalies indicate some degree of crystallization differentiation during diagenesis.(87Sr/86Sr), is0.709107～0.709771, average of0.709439, lower (143Nd/144Nd) t initial ratio of0.512207-0.512215and (εNd) t of～5.4～-5.6, indicating magma may suffered contamination crustal materials. Isotopic characteristics show its source region close to typeⅡ enriched mantle EMⅡ. The diagenetic age of diabase dikes group No. Ⅱ is112.6Ma, belongs to Early Cretaceous, Geochemistry of the diabase dikes group is similar to arc volcanic rocks. Combined with the evolution of Bangong-Nujiang ocean basin, The author suggests that these rocks generated from magmatic activity after the closure of Bangong-Nujiang ocean basin which related to subduction and collision. |
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