Ferromagnesian silicate association in S-type granites: the Darongshan granitic complex (Guangxi, South China)

摘要

The late Indosinian Darongshan granite complex (Guangxi Province, South China) consists mainly of three plutons (Taima, Jiuzhou and Darongshan), with coarse-grained to subvolcanic rock types. There is a rough mineral evolution from the western to the eastern part of the complex, with ferromagnesian magmatic silicates sequentially distributed : Opx+Crd±Bt (Taima), Opx+Grt+Bt+Crd (Jiuzhou) and Bt+Crd (Darongshan). Restitic, cumulative or xenocrystic minerals (mainly Crd with fibrolite+spinel inclusions, Grt and probably Opx in some cases) are also encountered. Mineralogical, chemical and isotopic compositions of the granites suggest that the three plutons derive from a source dominated by a reduced, immature greywacke-psammite series, which has experienced high-amphibolite to granulite-facies metamorphism. This S-type granite complex is considered to result from various degrees of melting at different crustal levels by biotite dehydration-melting in a high T (800° to 950°C), low P (400–600 MPa) metamorphic environment. Abundant granulite-facies metasedimentary enclaves with refractory compositions are considered as xenoliths entrapped during magma ascent. The presence of scarce mafic microgranular enclaves and the high temperatures needed for melting could suggest that heat may have been partly contributed by mantle-derived magmas.