In order to constrain the tectonic evolution of the eastern part of the Jiao-Liao-Ji Orogenic Belt,the authors carried out a study on the petrogenesis and diagenetic tectonic background of the metamorphic gabbrocarbonatite complex in the Daxing iron mining area by using petrology, petrochemistry, petrogeochemistry and zircon U-Pb dating.The results show that the metamorphic gabbro-carbonatite complex in the Daxing iron mining area has the rock types of meta-gabbro, carbonatite and magnet ore body, and the mata-gabbro is the main rock type.Petrochemical study of the meta-gabbro showed that the w (SiO2) is from 44.23% to 48.80%, w (Na2O) is from 0.94% to 2.86%, w (K2O) is from 0.51% to 1.90%, w (TiO2) is from 0.42% to 0.93%, w (CaO) is from 8.73% to 14.78%, and the Na2O/ K2O ratio ranges from 1.12 to 2.27, MgO/ (FeO+MgO) ratio ranges from 0.54 to 0.60, (Mg2++Ni2+) / (Fe2++Fe3++Mn2+) ratio ranges from 1.44 to 2.02, Mg/ (Mg+Fe2+)ratio ranges from 0.60 to 0.68, and the differentiation index (DI) ranges from 11.76 to 31.79.The meta-gabbro rocks are basic-ultrabasic magma which is rich in iron and sodium, poor in potassium and titanium, and weak in differentiation.Geochemical studies show that the rocks are rich in large ion lithophile elements such as Rb, Ba [w (Rb) (4.80×10-6-74.82×10-6), w (Ba) (83.09×10-6-430.10×10-6)], and relatively rich in high field strength elements such as Hf, Th, Ta [w (Hf) (1.41×10-6-6.20×10-6), w (Th) (0.51×10-6-28.91×10-6),w (Ta) (0.47×10-6-9.91×10-6)], and ΣREE=(29.56×10-6-812.59×10-6 ); while, Nb is obviously depleted [ w (Nb) (2.94× 10-6-36.01× 10-6 ) ].The (Na2O+K2O) / TiO2 ratio is 0.10-5.54, Zr/ Y ratio is 1.93-5.67, La/ Nb ratio is 0.54-1.73, La/ Ta ratio is 5.53-11.13, Nb/ U ratio is 1.51-14.23.The geochemical characteristics are similar to continental tholeiitic basalts, and having the properties of intra-plate alkaline basalts.The carbonatite and meta-gabbro are interbedded.The average CaO/ (CaO+MgO+Fe2O3+FeO+MnO) ratio is 0.98, mean w (MgO) is 0.62, and mean w (FeO+Fe2O3+MnO) is 0.41.It is calcareous metamagnesic carbonatite which obviously rich in Sr [w (Sr) (1 727×10-6-1 794×10-6)].The complex magma may come from the partial melting of the asthenosphere enriched mantle, formed in the tectonic background of the stretching of the deep fault zone in the continental plate, and assimilated and mixed with crustal materials in a certain extent during the ascent.Most of the meta-gabbro zircons have the core-rim structure, the color of the core is mostly black.It is difficult to observe its internal structure, a few of the core can be seen wide oscillating bands.The Th/ U ratio is 0.23-4.19, which shows the characteristics of basic magmatic zircon.The weighted average age of 207Pb/ 206 Pb of the core is (2 098 ± 11) Ma (MSWD=1.2, n=18), which represents the condensation crystallization age of the gabbro.The color at the rims of the zircons is relatively light and uniform, without zonation,and the Th/ U ratio is 0.03-3.22, indicating metamorphic origin.The weighted average age of 207Pb/ 206Pb of the rims is (1 892 ±23) Ma (MSWD=2.2, n=14), which represents the metamorphic age of gabbro.Combined with regional geological data, it is considered that the eastern part of the Jiao-Liao-Ji Orogenic Belt was in a extensional tectonic environment in the Middle Paleoproterozoic, the formation of metamorphic gabbro-carbonatite complex in the Daxing iron mining area may be related to the extension during the tectonic evolution of the Songjiang-Chongshan fault zone.
To explore the geodynamic mechanisms of the large-scale magmatic activity during Early Cretaceous in Da Hinggan Ling (Mts.), the authors focused on diabase in Huoluotai area in northern segment of Erguna Block,Da Hinggan Ling (Mts.), through the study of petrogeochemical characteristics and zircon U-Pb dating of diabase in Huoluotai area, aimed to determine its formation age, petrogenesis, and tectonic setting.Zircon LA -ICP -MS U –Pb dating results show that the weighted average age of diabase in Huoluotai area is (135.1 ±3.6) Ma, indicating that the crystallization age of diabase is Early Cretaceous.Whole rock geochemical data reveal that diabase in Huoluotai area is characterized by low w (SiO2) (48.84%-52.11%), high w (MgO) (5.13%-6.85%) and w (Fe2O3) (7.45%-8.60%).The Eu anomaly of diabase is insignificant or exhibits a weak negative anomaly (δEu =0.83-1.06).The diabase is enriched in large ion lithophile elements (such as Ba, Sr, and K), and depleted in high field strength elements (such as Zr, Ta, Nb, and Th).Diabase in Huoluotai area is only minimally influenced by fractional crystallization, crustal contamination, and accumulation processes.The Nb/ Yb -Th/ Yb diagram shows that diabase in Huoluotai area falls within the area of primitive mafic melts derived from subduction-modified lithospheric mantle.The Th/ Nb-Ba/ Th and Th/ Yb-Ba/ La diagrams indicate that the magma source of diabase in Huoluotai area was modified by fluids derived from subducted slabs.The La/ Nb-La/ Ba diagram reveals that the magma source of diabase in Huoluotai area is from the lithospheric mantle.The above geochemical characteristics indicate that diabase in Huoluotai area originated from a lithospheric mantle source metasomatized by fluids derived from subducted slabs.Combined with previous research, it is concluded that the formation of diabase in Huoluotai area in northern segment of Erguna Block, Da Hinggan Ling (Mts.) is closely related to the rollback process of the Paleo-Pacific Plate.
