Petrogenesis and significance of early Yanshanian syenite-granite complex in eastern Nanling Range

摘要

In the eastern Nanling Range there exists the early Yanshanian syenite-granite assemblage. A representative example is the Pitou-Tabei complex composed of the Tabei syenite pluton and the Pitou syenogranite pluton in southern Jiangxi Province. U-Pb zircon dating yields ages of 188.6+2.2 Ma for the Tabei pluton and 186.3+1.1 Ma for the Pitou pluton. The Tabei syenite is characterized by relatively low SiO_2 content (62.40% - 68.75%), high alkalis (K_2O+Na_2O = 10.56%-11.96%), low percent K_2O/Na_2O ratios (0.56-0.93), metaluminous feature (A/CNK = 0.80-1.00), enrichment in LILE (Rb, Ba, K) and HFSE (Th, U, Nb, Ta, Zr, and so on), weakly negative to positive Eu anomalies (δEu = 0.63-1.82), relatively low (~(87)Sr/~(86)Sr)_i (0.70412-0.70543), and relatively high ε_(Nd)(t) (3.14-3.52). The Pitou syenogranite is characterized by high silicon content (SiO_2 = 71.06%-76.28%), relatively low alkalis (K_2O+Na_2O = 8.10%-9.80%), high percent K_2O/Na_2O ratios (1.22-1.94), metaluminous feature (A/CNK = 0.94-1.07), enrichment in Rb, Th (U), K, depletion in Ba, Nb, Ta, Sr, P , Zr, Ti, high ΣREE (averaging 451.03 μg/g), strong negative Eu anomalies (δEu = 0.27 - 0.33), relatively high (~(87)Sr/~(86)Sr)_i (0.70805-0.70912), and relatively low ε_(Nd)(t) (-5.35 - -6.29). The Tabei syenite and Pitou syenogranite both have the characteristics of A-type granites. The former is considered to be from the asthenosphere mantle, and the latter was the product of the crust-mantle mixture. They both were probably formed in a rifting setting.