Thermotectonic evolution of the Paleozoic granites along the Shangdan suture zone (central China); crustal growth and differentiation by magma underplating in an orogenic belt

The nature of source rocks and the pressure-temperature-hydration (P-T-H (sub 2) O) condition are the two main factors that control the geochemical properties of granites. Therefore, the evolution of P-T-H (sub 2) O conditions can be used to deduce the tectonic setting of granites. In this paper, we report on three Paleozoic granite plutons along the Shangdan suture that revealed increasing melting temperature and decreasing pressure from 437 to 403 Ma, suggesting a crustal thinning process. The Tieyupu granodiorites (437+ or -4 Ma) display Na-rich adakite affinity, i.e., SiO (sub 2) = 69.1-70.1 wt%, Na (sub 2) O/K (sub 2) O = 1.9-2.26, positive zircon epsilon Hf(t) values (+4.29 to +12.04), and high Sr/Y (137-160) and Y/Yb (9.89-10.25) ratios, implying a garnet-rich residue in their source. In combination with moderate zircon saturation temperatures (814-822 degrees C), we infer that the Tieyupu granodiorites were formed by melting of Neoproterozoic metabasites under high-pressure (>1.5 GPa) and moderate-temperature (HP-MT) conditions. The Liangchahe granodiorites (415+ or -8 Ma) also display Na-rich adakite affinity, i.e., higher Na (sub 2) O/K (sub 2) O (2.16-3.11) and lower Sr/Y (77-88) ratios, and higher zircon saturation temperatures (854-874 degrees C), and they are interpreted to have been derived from melting of metabasites under moderate-pressure (>1.0 GPa) and high-temperature (MP-HT) conditions. Their variable zircon epsilon Hf(t) values (-14.97 to +9.80) and the existence of zircon xenocrysts suggest that the primitive adakitic melts were assimilated by evolved crustal components. The Yaogou monzogranites (403+ or -4 Ma) have the highest K (sub 2) O/Na (sub 2) O (0.81-1.00) ratios and total rare earth element (Sigma REE; 105-191 ppm) contents, lowest Sr/Y (14-43) ratios, positive zircon epsilon Hf(t) values (+6.79 to +12.22), and highest zircon saturation temperatures (891-973 degrees C), showing they were formed by high-temperature melting of intermediate rocks under low-pressure conditions (<1.0 GPa). The evolution of P-T conditions revealed by these three granites suggests that crustal growth and differentiation were related to gradual extensional and melting of mafic protoliths in the orogenic belt.