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Gao Y.,Shijiazhuang University of Economics | Santosh M.,Kochi University | Santosh M.,China University of Geosciences | Wei R.,Shijiazhuang University of Economics | And 3 more authors.
Journal of Asian Earth Sciences

A number of porphyry Cu deposits have been described from east China which occur in association with Mesozoic high Sr/Y rocks within the continental interior rather than in an arc setting. However, the origin of these high Sr/Y rocks remains controversial. In this study we report precise zircon U-Pb age, as well as major-trace element and Sr-Nd-Pb isotope compositions from the Mujicun Cu mineralized porphyries in the northern Taihang orogen of eastern North China Craton (NCC). LA-ICP-MS zircon U-Pb dating yields an emplacement age of 143±2Ma, identical to the molybdenite Re-Os isochron ages of 142.5±1.4Ma for this intrusion. Like most of the Mesozoic adakitic rocks from the eastern NCC, the ore-bearing porphyries and associated volcanic lavas from northern Taihang orogen are rich in large ion lithophile elements and light REE, and have highly differentiated REE patterns. The porphyries and associated volcanic lavas have Sr-Nd-Pb isotopic compositions showing EM1-like isotopic signatures. Such geochemical and isotopic features confirm that the parental magma for these rocks originated from melting of an enriched sub-continental lithospheric mantle source. In comparison to the associated lavas, the ore-bearing porphyries have pronounced low Fe2O3T, TiO2 and P2O5 contents, and middle-heavy REE (and Y) and Zr concentrations, indicating fractional crystallization of amphibole with the observed accessory mineral assemblage such as Fe-Ti oxides, titanite, zircon and apatite. On the other hand, most of the porphyries exhibit relatively high Al2O3, Ba and Sr concentrations and pronounced positive Eu anomalies, excluding significant plagioclase fractionation due to suppression of the high water content in the magmas. The presence of the contemporary amphibole cumulates regionally exposed in the study area strongly support significant amphibole fractionation during the formation of the Mujicun porphyries. Thus, fractionation of a water-saturated magma is proposed as a plausible mechanism through which the high Sr/Y magmas possessing a high potential to generate porphyry Cu deposits formed. The identification of high Sr/Y magmas associated with porphyry Cu systems during the craton destruction of the NCC has important implications in formulating strategies for prospecting porphyry Cu deposits. © 2012 . Source

Gao Y.,Shijiazhuang University of Economics | Santosh M.,Kochi University | Hou Z.,Chinese Academy of Geological Sciences | Wei R.,Shijiazhuang University of Economics | And 3 more authors.
Gondwana Research

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 206Pb/ 238U 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 87Sr/ 86Sr ratios ranging from 0.7060 to 0.7062, and relatively low initial 143Nd/ 144Nd values from 0.5116 to 0.5117, defining a tight cluster in 87Sr/ 86Sr versus 143Nd/ 144Nd plots. They show low initial 206Pb/ 204Pb (16.590-16.807), 207Pb/ 204Pb (15.247-15.269) and 208Pb/ 204Pb (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. © 2011 International Association for Gondwana Research. Source

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