Melting of Newly Formed Mafic Crust for the Formation of Neoproterozoic I-Type Granite in the Hannan Region, South China

摘要

Neoproterozoic magmatism in the Hannan region at the northwestern margin of the Yangtze Block is characterized by numerous felsic plutons associated with minor mafic-ultramafic intrusions. The felsic plutons are either adakitic or normal-arc granitic in composition. The adakitic plutons are similar to 735 Ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust. Among the normal-arc-related felsic plutons, the Tianpinghe pluton is the largest and has a SHRIMP zircon U-Pb age of 762 +/- 4 Ma, older than the adakitic plutons in the region. Rocks from the Tianpinghe pluton have relatively high SiO2 (67.1-70.1 wt%) and K2O + Na2O (7.8-8.6 wt%) and relatively low MgO (0.7-1.3 wt%) and Al2O3 contents (14.5-15.6 wt%), with Al2O3/(CaO + K2O + Na2O) (A/CNK) values ranging from 0.95 to 1.08. They have arc-affinity trace-element compositions that are characterized by enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with strong positive Pb and negative Ti anomalies. They have a narrow range of epsilon Nd(t) values (+0.15 to -1.76) and relatively high zircon epsilon Hf values (+0.6 to +8.3). These geochemical features are typical of I-type granites. The rocks from the Tianpinghe pluton have relatively young single-stage and two-stage Hf model ages (1.01-1.31 and 1.31-2.01 Ga, respectively), suggesting that the pluton was generated by partial melting of newly formed basaltic rocks. On the basis of its arc-related geochemical affinity and its emplacement before voluminous adakitic magmatism but after mafic-ultramafic intrusions, the Tianpinghe pluton is considered to be Neoproterozoic arc granite formed during a period of crustal growth and reworking. Generation of the later adakitic plutons suggests that the crustal thickness increased to more than 50 km by mafic magma underplating.