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

Abstract

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 2 O+Na 2 O = 10.56%－11.96%), low percent K 2 O/Na 2 O 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 ( δ Eu = 0.63－1.82), relatively low ( 87 Sr/ 86 Sr) i (0.70412－0.70543), and relatively high e 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 2 O+Na 2 O = 8.10%－9.80%), high percent K 2 O/Na 2 O 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 ( δ Eu = 0.27－0.33), relatively high ( 87 Sr/ 86 Sr) i (0.70805－0.70912), and relatively low e 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.