Nd and Sr isotope geochemistry of plutonic rocks from Hong Kong: implications for granite petrogenesis, regional structure and crustal evolution

Combined Nd and Sr isotope data allow the plutonic rocks of Hong Kong to be grouped into three NE-trending domains. Those of Zone 1 in the northwestern part of the Territory have epsilon(Nd)(T) < -9, T-DM model ages between 1.67 and 2.02 Ga, and 87Sr/Sr-86(0) > 0.71. The plutonic rocks in the central Zone 2 exhibit a broad spectrum of isotope characteristics with epsilon(Nd)(T) values of -4.2 to -7.0, T-DM between 1.30 and 1.51 Ga, and Sr-87/Sr-86(0) ratios in the range 0.7061 - 0.7102. Granites of Zone 3 in the southeastern part of the Territory display relatively homogeneous isotope signatures with epsilon(Nd)(T) between -5.5 and -6.5 and Sr-87/Sr-86(0) ratios in the range 0.7067 - 0.7109; T-DM model ages are between 1.39 and 1.47 Ga. The contrasting isotope signatures between Zone 1 granites and those in Zones 2 and 3 suggest there are inherent differences in the source regions and point to the existence of a basement terrane boundary in this part of southeast China. The presence of a deep crustal discontinuity transecting Hong Kong would also account for the distribution and compositional variation of the granites and the dominant northeast structural control on dyke emplacement. The variation in isotope signatures and T-DM model ages observed in the Hong Kong rocks are consistent with a model involving the progressive interaction of mantle-derived melts with two distinct crustal sources during a transition from compressional to extensional tectonics. The boundary between these two crustal sources is marked by Zone 2 in which isotope characteristics are more variable and a stronger mantle influence is preserved. (C) 1997 Elsevier Science B.V.