High Sr/Y magmas generated through crystal fractionation: Evidence from Mesozoic volcanic rocks in the northern Taihang orogen, North China Craton

摘要

Among the Phanerozoic magmatic pulses that substantially modified the structure and composition of the subcontinental lithosphere of the North China Craton (NCC), the Mesozoic suite includes a wide variety of high Sr/Y and La/Yb rocks. The spatio-temporal distribution and characterization of these rocks are fundamental to evaluating the various models of decratonization of the NCC. Here we report petrologic, geochemical, Sr-Nd-Pb and U-Pb zircon isotopic data on the Mesozoic volcanic rocks from northern Taihang orogen in the eastern NCC. The magmatic zircons in these rocks display high Th/U values (0.4-1.7) and yield a ~(206)Pb/ ~(238)U age range of 152Ma to 138Ma, with a weighted mean of 145.6Ma. The cores of inherited zircon xenocrysts yield discordant ages of 1840Ma and 2013Ma suggesting derivation from ancient crustal sources. Most of the volcanic rocks of our study posses moderately high initial ~(87)Sr/ ~(86)Sr ratios ranging from 0.7060 to 0.7062, and relatively low initial ~(143)Nd/ ~(144)Nd values from 0.5116 to 0.5117, defining a tight cluster in ~(87)Sr/ ~(86)Sr versus ~(143)Nd/ ~(144)Nd plots. They show low initial ~(206)Pb/ ~(204)Pb (16.590-16.807), ~(207)Pb/ ~(204)Pb (15.247-15.269) and ~(208)Pb/ ~(204)Pb (36.656-36.820) values. The associated hornblendites have isotopic compositions identical to those of the volcanic rocks and define continuous variation trends reflecting common parent magma. Geochemical modeling shows that the high Sr/Y and La/Yb signatures of the Mesozoic volcanic rocks from the northern Taihang orogen are primarily the result of fractional crystallization of mantle-derived melts. We evaluate the magma tectonics through the MASH (melting, assimilation, storage, homogenization) model with subsequent evolution of the hybrid magmas and their emplacement controlled by structural conduits. Our interpretation is in deviation from previous models involving partial melting in a thickened lower crust, and crustal delamination, but provides a more robust mechanism to account for the field relations including the occurrence of a wide compositional variety of lithologies in the same region, distinct petrological features such as reverse zoning of some phenocryst phases, geochemical characteristics including smooth fractionation trends, as well as the presence of multiple age populations of inherited zircons.