THE KUIQI PERALKALINE GRANITIC COMPLEX (SE CHINA) - PETROLOGY AND GEOCHEMISTRY

The Mesozoic volcano-plutonic belt of SE China is characterized, in the Kuiqi area, by acid volcanics followed by the Yanshan granites. The Kuiqi granitic complex, which belongs to the latter unit, is made up of a calc-alkaline and a peralkaline group. The calc-alkaline group consists of two intrusions, the Danyang monzogranite and the Fuzhou syenogranite, emplaced 103 +/- 10 Ma and 104 +/- 5 Ma ago, respectively (Rb-Sr whole-rock isochrons). Formation of the Danyang monzogranite can be explained by a three-stage model: (1) partial melting of a metasomatized mantle generated a dioritic magma known in the area as the Nanyu diorite; (2) the magma was contaminated (approximately 25%) by lower continental crust; (3) large amounts (70-80%) of fractional crystallization of hornblende and plagioclase at depth gave rise to the magmatic suite. The Fuzhou syenogranite is more fractionated and its formation involved crystallization of plagioclase + biotite + K-feldspar + apatite. Intrusion of the peralkaline group is dated at 93 +/- 1 Ma (Kuiqi peralkaline granite) and at 91.8 +/- 0.9 (Bijiashan peralkaline granite). These units are homogeneous and their petrogenesis is less constrained than for the calc-alkaline suite. Nevertheless, a multistage process can be proposed: (1) partial melting of a metasomatized mantle produced a dioritic magma; (2) fractional crystallization began with segregation of hornblende + plagioclase +/- ilmenite and/or magnetite; subsequently, hornblende no longer crystallized; (3) the last stage of fractionation corresponded to the crystallization of K-feldspar + plagioclase + REE-rich accessory phases. Mineralogical study indicates that during the last stage, fluids played a prominent role and controlled the nature of the crystallizing minerals. The magma evolved from F- and S-rich, and water-undersaturated to water-oversaturated, leading to the exsolution and dissociation of an H2O vapour phase and to the loss of H-2. The change from calc-alkaline to peralkaline magmatism is related to inferred changes in the tectonic environment. The calc-alkaline granites were generated in a subduction setting in which water was supplied by dehydration of the downgoing slab. The peralkaline granites were produced in a crustal thinning environment where little water was available, thus necessitating high temperatures to initiate partial melting. On the other hand, petrogenetic modelling shows that both calc-alkaline and peralkaline granites could have been derived from the same source which is metasomatized mantle. This possibly indicates that the volcanic arc source, active during subduction, persisted beneath the continent and was reactivated during the post-orogenic magmatism. The reactivation was caused by a deep crustal fault.