Origin of high-Mg adakitic magmatic enclaves from the Meichuan pluton, southern Dabie orogen (central China): Implications for delamination of the lower continental crust and melt-mantle interaction

摘要

Field observation, petrography and geochemistry of mafic enclaves/dikes and their host felsic rocks from the Meichuan pluton are used to propose a geodynamic model for the southern Dabie orogen, central China. The similar Sr-Nd isotopic ratios [ε_(Nd)(t) =-15, (~(87)Sr/~(86)Sr)_i = 0.70550-0.70597] and zircon Hf isotopic ratios [ε_(Hf)(t) = - 25 to - 16] indicate that the two types of magmatic rocks were formed by coeval felsic and mafic magmas during the Early Cretaceous (132 ± 2 Ma, zircon U-Pb age). The adakitic signatures of both the felsic and mafic rocks, such as very high Sr (770-1400 ppm), high Sr/Y ratios (40-130), low Y (3.5-21 ppm) and HREE concentrations are supposed to be features of the primary magmas, indicating that both of them were generated by partial melting of basaltic protoliths at great depths (>15 kbar). The distinctive major element compositions of the felsic and mafic primary magmas could be attributable to different melting temperatures and melting degrees. The mafic enclaves/dike have distinctively high concentrations of MgO (4.4-5.8 wt%), Cr (229-374 ppm) and Ni (75-163 ppm), indicating a melt-mantle interaction in which olivine is partly consumed while orthopyroxene and/or pyrope are formed under high-pressure. Modeling suggests that 14% of peridotite relative to melt could have been consumed to elevate the Mg~# of melt to the observed values (55-60), and that transformation from orthopyroxene to pyrope in mantle peridotite could have decreased the Al2O3 content from 18-19 wt% in the initial melts to ~ 15 wt.% in the resultant mafic melts. Moderate negative zircon ε_(Hf)( t) and bulk ε_(Nd)( t) values also suggest contributions from both enriched lithospheric mantle and ancient lower crust The results can best be explained by assuming that a block of amphibolite-composition lower continental crust was delaminated into the lithospheric mantle, leading to the formation of mafic magmas. This delamination is attributed to lithospheric extension and asthenospheric upwelling along the Yangtze River fault zone in the late Mesozoic which could have heated up the lithospheric mantle underneath the neighboring southern Dabie orogen. The strong input of heat triggered the delamination of the overlying thickened lower crust into lithospheric mantle by weakening the uppermost mantle and resulted in melting reactions in the delaminated crustal block to produce hot (~ 1100 °C) intermediate adakitic magmas (SiO2 = 55-60 wt%). The ascent of these hot magmas and their underplating below the lower crust induced the generation of high-silica melts (SiO2 = ~70 wt%) at lower temperature (-925 °C or less). Hybridization processes between the two magma types occurred during further ascent and emplacement in the crust, which could have led to the high concentrations of MgO (2.2-3.5 wt%), Cr (56-226 ppm) and Ni (33-99 ppm) in the host felsic rocks.