关键词: 燕山晚期, 天柱山, 岩浆起源, 陆内伸展, 软流圈上涌

Abstract:

The Tianzhushan intrusive complex is one of the Late Yanshanian intermediate-felsic complexes in the Dabie orogenic
belt, which was emplaced  in  two stages. The amphibole quartz monzonite of  the earlier stage has  larger SiO2 content variation
ranging  from 59.83%  to 70.12%, with K2O+Na2O  from 7.02%  to 9.56%. The biotite monzogranite of  the  later stage has SiO2
content variation ranging from 72.46% to 78.65%, with K2O+Na2O from 8.05% to 10.22%. They both belong to the calc-alkaline
to alkali-calcic series. The  intermediate-felsic  rocks are enriched  in Rb, Th, K, Pb and U, and depleted  in Nb, Ta and Ti. The
biotite monzogranite also has  remarkable Eu depletion which can be ascribed  to separation of plagioclase. Sr and Nd  isotope
geochemical characteristics suggest  that  the  intermediate-felsic  rocks were derived  from  the old  lower crust which  is similar  in
chemical composition  to  the metamorphic assemblage  in North Dabie. New LA-ICP-MS zircon U-Pb dating suggests  that  the
crystallization age of  the Tianzhushan complex  is  (127±1) Ma~(131±1) Ma,  in agreement with  the ages  (120~135 Ma) of  the
large-scale intrusive intermediate-felsic and mafic-ultramafic rocks in the region. The Tianzhushan granitoids were formed under
extensional tectonic setting, providing an evidence that the Early Cretaceous large scale magmatism was generated by lithosphere
delamination and asthenosphere upwelling. Subduction of  the paleo-Pacific plate  intensified Cretaceous extention of  the Dabieorogen which triggered delamination.

Key words:  Late Yanshanian, Tianzhushan, magmatic origin, intracontinental extention, asthenosphere upwelling