| Lesser Khingan Range is located in the east section of Xing’an Mongolian orogenic belt, belonging to the Songnen massif. This Range has successively influenced by the superposition of the Paleo-Asian and Paleo-Pacific domains from late Paleozoic to Mesozoic, and has a long evolution and complex geological history, the characteristics of multistage hyperplasia and stitching. The Taoshan granite complex rock mass is located in the middle of the Lesser Khingan Range and is compositionally diverse, consisting of monzogranite-syenogranite diorite and gabbro. It is an ideal location to unravel the magma evolution and Mesozoic geodynamic process of the Lesser Khingan Range. However, the lack of precise geochronological and geochemical data for this complex, especially the basic intrusive rocks, restricts the deep understanding of their petrogenesis. Therefore, systematic geological, petrographical, geochronological, geochemical and Hf isotopic study will be conducted in this essay in order to investigate the rock association, geochronology and petrogenesis of the Taoshan granite complex.The Taoshan granite complex is mainly composed of syenogranite and monzogranite. Several intermediate to mafic dykes and the syenogranite are mutually intrusive, the lithology of intermediate to mafic dykes is augite diorite and gabbro. 1. The age of Taoshan granitic complexThe LA-ICP-MS zircon U-Pb isotopic dating indicates the Taoshan granitic complex was formed in the Early Jurassic（177±2 Ma for syenogranite, 175±1 Ma for monzogranite, 175 Ma for gabbro）. These rocks were emplaced at the same time within the error range, all belong to the early Jurassic magmatic rocks. 2. Geochemical characteristics of rocksGeochemical analysis shows that the syenogranite is characterized by high contents of Si O2 and low concentrations of Fe, Mg, Ca, belong to quasi-aluminous, The Na2O/K2 O ratios range from 0.64 to 0.92 and the differentiation index DI is 9597. The syenogranite is K-rich, mainly belong to the high-K calc-alkaline series and shoshonite series. Mg#=711, ∑REE=133.1×10-6173.1×10-6, LREE/HREE=5.5613.15, δEu =0.160.45, has obvious europium negative anomaly, low Sr high Yb content, Sr=11.2×10-612.2×10-6, Yb=3.16×10-63.4×10-6. They are characterized by enrichment in light rare earth elements, large ion lithophile elements,（LILE）, K, Rb, LREE, and depletion in high field strength elements（HFSE） such as Nb, Ta, P and Ti, consistent with the igneous rocks generated from an active continental margin setting.Monzogranite have w（Si O2）=63.7966.75%, and fall in the middle-K and high-K calc-alkaline series. ∑REE=87.9×10-6181.9×10-6, LREE/HREE= 6.1217.48. Unlike syenogranite, monzogranite is Na-rich with Na2O/K2 O ratio of 1.172.53, and shows no abnormalities and weak positive anomalies（δEu=0.931.30）, high Sr low Yb content, Sr = 418×10-6653×10-6, Yb = 1.31×10-62.03×10-6, Mg# =3139. Monzogranite were strongly enriched in large ion lithophile elements（ LILE） and LREE, such as K, Rb, depletion in high field strength elements Nb, Ta, P, Ti, also shows that comes from the crust.Gabbro exhibit the low-Si and high Fe, Mg, Ca characters. Have w（Si O2）=50.12%, w（Na2O） + w（K2O） = 6.42%, Na2O/K2 O = 1.09, Mg# = 65,∑REE = 236.3 x 10-6, LREE/HREE is 9.33, δEu = 0.80. They are enriched in large ion lithophile elements（LILEs） and light rare earth elements（LREEs） and display moderate negative Eu anomalies, depletion in Nb-Ta-Ti, Zr and Hf high field strength elements.Augite diorite have w（Si O2）=54.05%54.24%, w（Al2O3）=15.53%15.94%,w（Mg O）=5.67%5.78%,w（TFe2O3）=7.32%7.51%,Na2O/K2O=1.54%2.25, Na-rich, metaluminous, Mg# = 6061. ∑REE=143.9×10-6152.0×10-6, LREE/HREE = 5.846.91, δEu = 0.73 0.78, display moderate negative Eu anomalies. Under the influence of surrounding rock of syenogranite, rare earth element distribution curve is flat.The study indicates that syenogranite is high differentiation I-type granites, monzogranite is I-type granite, their depth of magma source area is different, separately by two independent origin of magmatic crystallization, but they have the same geodynamic background and formation mechanism, both generated by partial melting of the lower crust basic igneous rocks. Syenogranite and intermediate to mafic dykes formed independently, the former is the product of partial melting of crust, the latter may experience the aqueous fluid metasomatism, but within a limited extent. 3. petrogenesisIn situ Hf analyses of zircons show that the zircon εHf（t） values of the syenogranite range from 1.5 to 4.2 with the two-phase model age between 953 1124 Ma, the zircon εHf（t） values of the monzogranite is higher than syenogranite, its εHf（t） values range from 2.4 9.0, and its two-phase model is younger than syenogranite, between 644 1067 Ma. All generated by partial melting the Neoproterozoic lower crust basic igneous rock. Geochemical characteristics show that they derived from different parts of the lower crust, and have none petrogenetic connection.The study indicates that was mainly induced by lithospheric extension of subduction of the paleo-Pacific ocean. Mantle is metasomaticed by a subduction fluid to produce the gabbroic and dioritic rocks. Mantle magma rise then trigger different depth of crust melting to form the Taoshan granitic magma. |
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