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

Citation
H. Martin et al., THE KUIQI PERALKALINE GRANITIC COMPLEX (SE CHINA) - PETROLOGY AND GEOCHEMISTRY, Journal of Petrology, 35(4), 1994, pp. 983-1015
Citations number
90
Categorie Soggetti
Geology
Journal title
ISSN journal
00223530
Volume
35
Issue
4
Year of publication
1994
Pages
983 - 1015
Database
ISI
SICI code
0022-3530(1994)35:4<983:TKPGC(>2.0.ZU;2-J
Abstract
The Mesozoic volcano-plutonic belt of SE China is characterized, in th e Kuiqi area, by acid volcanics followed by the Yanshan granites. The Kuiqi granitic complex, which belongs to the latter unit, is made up o f a calc-alkaline and a peralkaline group. The calc-alkaline group con sists of two intrusions, the Danyang monzogranite and the Fuzhou syeno granite, emplaced 103 +/- 10 Ma and 104 +/- 5 Ma ago, respectively (Rb -Sr whole-rock isochrons). Formation of the Danyang monzogranite can b e explained by a three-stage model: (1) partial melting of a metasomat ized mantle generated a dioritic magma known in the area as the Nanyu diorite; (2) the magma was contaminated (approximately 25%) by lower c ontinental crust; (3) large amounts (70-80%) of fractional crystalliza tion 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 + apati te. Intrusion of the peralkaline group is dated at 93 +/- 1 Ma (Kuiqi peralkaline granite) and at 91.8 +/- 0.9 (Bijiashan peralkaline granit e). These units are homogeneous and their petrogenesis is less constra ined than for the calc-alkaline suite. Nevertheless, a multistage proc ess can be proposed: (1) partial melting of a metasomatized mantle pro duced a dioritic magma; (2) fractional crystallization began with segr egation of hornblende + plagioclase +/- ilmenite and/or magnetite; sub sequently, hornblende no longer crystallized; (3) the last stage of fr actionation corresponded to the crystallization of K-feldspar + plagio clase + REE-rich accessory phases. Mineralogical study indicates that during the last stage, fluids played a prominent role and controlled t he nature of the crystallizing minerals. The magma evolved from F- and S-rich, and water-undersaturated to water-oversaturated, leading to t he exsolution and dissociation of an H2O vapour phase and to the loss of H-2. The change from calc-alkaline to peralkaline magmatism is rela ted to inferred changes in the tectonic environment. The calc-alkaline granites were generated in a subduction setting in which water was su pplied 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 m elting. On the other hand, petrogenetic modelling shows that both calc -alkaline and peralkaline granites could have been derived from the sa me source which is metasomatized mantle. This possibly indicates that the volcanic arc source, active during subduction, persisted beneath t he continent and was reactivated during the post-orogenic magmatism. T he reactivation was caused by a deep crustal fault.