We are studying some plutonic rocks of gabbroic to tonalitic composition of calc-alkaline affinity, intruded within meta-mafic rocks of upper amphibolite metamorphic facies. U-Pb zircon data in the calc-alkaline rocks and Ar-Ar geochronology in meta-mafic rocks are very similar suggesting coetaneous metamorphic and magmatic event.
There are a lot of literature suggesting that partial melting of amphibolite lower crust in continental subduction zone usually result in melts with felsic characteristic.
If some of the tonalitic plutonic rocks are products of partial melting of amphibolite lower crust, what are the most reliable geochemistry evidence of this? We have some whole rock geochemistry data of the calc-alkaline and wall rocks, also we have some zircon trace elements analyses from the felsic rocks. Unfortunately we do not have access to specialized geochemical analysis as Sm-Nd.
Thanks for your help.
Regards
Hi Cesar Alfredo Tarazona Velasquez:
There is no specific answer to your question, although such petrogenesis for the gabbroic to tonalitic suite is possible. First, conducting detailed bedrock mapping is a critic way to establish temporal and spatial relationships of the felsic pluton(s) in the field with the assistance of geochronology age determination. Second, determining alkalinity and aluminous saturation state of each phases. Third, building up possible petrogenitic linkage between these phases, e.g., partial melting, or fractionation, or absolutely unrelated. With these work done, then you may have better idea to interpret your geolochemical data and origin of the felsic plutonic rocks.
Best regards,
Xue-Ming Yang
Mr. Xue-Ming Yang, thank you very much for your reply.
Best regards.
Cesar Tarazona
Dear Tarazona,
From the perspective of major element geochemistry, it is difficult to assign a felsic granite to the S or I type (granites from partial melting of amphibolites will have an I-type signature). If there are more mafic rocks in the suite, try plotting your whole-rock analyses in the A-B diagram of Debon and Le Fort (1983), with the modification of Villaseca et al (1998, Trans R. Soc. Edin. 89, 113-119). If your rocks display a negative trend with increasing maficity (B value), your granites will surely come from a source with a bulk amphibolitic/andesitic composition, not necessarily an amphibolite s.s. but a rock with that bulk composition.
If, however, all your analyses are very felsic and do not display a trend (i.e., all your granites are leucogranites), then you need to use trace element signatures, isotopic analyses, zircon morphology, etc. And of course, as Xue-Ming Yang has stated, field relationships with the other mafic rocks give also important clues.
Kind regards,
Marcos
Jorge Costa-de-Moura and Marcos García-Arias I'm very greatfull with your answers.
Regards
Cesar
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