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 ϵNd(T) < − 9, TDM model ages between 1.67 and 2.02 Ga, and 87Sr86Sr0 > 0.71. The plutonic rocks in the central Zone 2 exhibit a broad spectrum of isotope characteristics with ϵNd(T) values of −4.2 to −7.0, TDM between 1.30 and 1.51 Ga, and 87Sr86Sr0 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 ϵNd(T) between −5.5 and −6.5 and 87Sr86Sr0 ratios in the range 0.7067–0.7109; TDM model ages are between 1.39 and 1.47 Ga. ntrasting 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 TDM 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.