Petrogenesis of S-type granites : the example of the Cape Granite Suite

[摘要] ENGLISH SUMMARY: S-type granite intrusions are extremely common in the continental crust and formfrom the partial melting of metasediments. Compositions of S-type granite rangefrom leucogranite to granodiorite and have trace element contents that globallyincrease with increasing maficity (Fe + Mg). Models proposed for the formationof S-type granite do not answer satisfactorily all petrological and compositionalrequirements. In this study, S-type granite of the Cape Granite Suite (CGS), SouthAfrica is used to discriminate between potential sources of compositional variation.Experimental studies show that melt produced from the partial melting of sedimentis exclusively leucocratic. On this basis, the entrainment of up to 20 wt.% ofperitectic garnet within S-type melt can be established to produce the observedmajor element variations. S-type CGS locally contains garnet. This garnet is inequilibrium with granite composition at P-T conditions (5kb and 750 C for thecore of the garnet and 3kb and 720 C for the rim) well below conditions recordedby xenoliths from the same granite (10 kb and 850 C from a metabasite). Fromthis result it seems that the originally entrained garnet no longer exists in the StypeCGS and it have been replaced by newly formed minerals (garnet, cordieriteand biotite). Considering the short time necessary to emplace granites (about100 000 years), it appears that garnet has been compositionnally re-equilibrated through a dissolution-precipitation process. The study of trace element variationsin S-type CGS shows that most leucocratic compositions are undersaturated inZr and Ce compared to predictions from experimental models for dissolution ofaccessory zircon and monazite in their source regions. Thus, S-type melts arelikely to be formed in disequilibrium with respect to accessory phase stability. Asa result the observed increase in trace element content with increasing maficityindicates that accessory minerals such as zircon and monazite are co-entrainedwith peritectic garnet in melt to produce the observed trace element variation inS-type granite. Trace element disequilibrium in the CGS S-type granitoids requiresparticularly short times of residence of melt within the source region. Together,these results provide for the first time, a fully comprehensive model for majorand trace elements variations. Compositional variation in CGS S-type graniteresults from source processes by a selective entrainment of peritectic and accessoryminerals. After entrainment, these minerals are likely to be re-equilibrated withinthe magma, through a dissolution-reprecipitation process. In addition, it appearsthat the construction of large S-type granitic bodies occurs through successiveaddition of magma batches of different composition that originates directly fromthe source region.

[发布机构] Stellenbosch University