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 n orthwestern 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 chara cteristics 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 Ch ina. The presence of a deep crustal discontinuity transecting Hong Kon g 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 o bserved in the Hong Kong rocks are consistent with a model involving t he progressive interaction of mantle-derived melts with two distinct c rustal sources during a transition from compressional to extensional t ectonics. The boundary between these two crustal sources is marked by Zone 2 in which isotope characteristics are more variable and a strong er mantle influence is preserved. (C) 1997 Elsevier Science B.V.