Petrogenesis Of The Late Mesozoic Alkali-enriched Intrusions In Western Zhejiang Province:Enlightenment For The Process Of Regional Tectonic Evolution

Widespread magmatic activities have occurred due to the subduction of palae-Pacific Plate during the Late Mesozoic,which produced large amounts of intermediate-acid granitoids.Some granitoids,such as alkali-enriched intrusions,are closely related to mantle upwelling and magma underplate,which is a good lithoprobe/window to discuss the composition,evolution and dynamic mechanism of subduction-zone-related materials and the interaction relationships between different sphere in the interior of the earth.Some alkali-enriched intrusions from western Zhejiang Province have been studied in geochronological and geochemical to constrain the emplacement age and the feature of the deep magma source of these alkali-enriched intrusions,and the geodynamic process of southeast China during the Late Mesozoic.These alkali-enriched intrusions are located in the southern of Jiangshan-Shaoxing fault,the western of Zhenghe-Dapu fault,and within the Cathaysia Block.They can be divided into two magma activities due to the zircon U-Pb ages,includes:?1?early period alkali-enriched intrusion?renamed MC?with the age of 112?111 Ma;and?2?late period alkali-enriched intrusion?renamed MD?with the age of 101?100 Ma.MC and MD can be clarified to A-type granitoids and I-type granitoids,respectively.Synthesize other geochemical data,we propose that these alkali-enriched intrusions are derived from partial melting of the palae-Pacific plate subduction-related metasomatized mantle.Meanwhile,some crustal-derived felsic materials have also contributed to the MD?simulated results indicate the proportion of crustal materials is about 45?55%?.MD have more enriched Nd-Hf isotope compositions and high Mg#value than MC.In order to preclude the effect of fractional crystallization,we adopt the normalized value Mg#₆₅.The results of MD still display higher Mg#₆₅ values than MC.Also,considering the negative correlation between the Mg#₆₅ values and the?_Nd?t?values and?_Hf?t?values of these alkali-enriched rocks,we propose that the different Mg#values between MC and MD are not attributed to the effect of mantle contamination and/or fractional crystallization.The active correlation between Mg#₆₅ values and Sr/Y ratios indicates the different Mg#values of these alkali-enriched rocks may be caused by crustal thickening.The Mg#₆₅ values could become a prospective qualitative indicator to record the change of crust thickness.In addition,comparing the different calculation methods of magma temperature,we find that the zircon Ti saturation temperature is distinctive high than the whole-rock Zr saturation temperature.Considering the evolution trends of whole-rock Ti and Zr of MC,we proposed that the whole-rock Zr saturation temperature are more close to the real source magma of MC.