Zircon SHRIMP U-Pb dating and geochemistry of the Laojunshan Intrusion, western Henan Province

Abstract：The Laojunshan Intrusion is situated in northern Qinling orogenic belt, which is located in southern Heigou-Luanchuan faulted zone in the Qinling orogenic belt. Based on the contact relationship, the evolution of the intrusion can be divided from early to late into three stages, i.e., small-porphyry fine-medium-grained biotite monzonitic granite, mid-porphyry medium-coarse grained biotite monzonitic granite and big-porphyry medium-coarse grained biotite monzonitic granite. They are characterized by concentric zoning. All the three stages of granites have similar mineralogical and geochemical characteristics. They are composed of potash feldspar, plagioclase, biotite and quartz, but there are obvious differences in the rock texture. They are characterized by high Si (>70%) and alkali (8.28-9.53 %) and low Ca (0.68%-2.42%) . They also have relatively high Ba, Sr, Nb, Zr, Th and Rb concentrations and comparatively low Co, Ni, Cr, V and Sc concentrations, belonging to I-type granite. The SHRIMP U-Pb zircon dating of the second and third stage rocks yielded ages of 111±1Ma（MSWD=0.41） and 108±1Ma（MSWD=0.39） respectively, suggesting Late Mesozoic. It is inferred that the Laojunshan Intrusion resulted from large-scale late Mesozoic magmatism in eastern China.