I've been studying a charnockite (silica=60 to 70 wt.%) pluton in the Tibetan Plateau. This pluton is characterized by high Mg (magnesian), K (postassic), Ba and Sr contention. However, it has no adakitic affinity due to the low Sr/Y, very similar to that of normal arc magmatic rocks. The epsilon Nd (t) values are close to zero. I'm now confused on a question what magma source the melts were derived from. The magma formation pressures calculated from coexisting clinopyroxene-orthopyroxene and whole-rock normative Q-An-Ab plotting are high, at about 25 kPa. Base on these, I think the mantle lithosphere is the likely magma source. However, whether subducted oceanic crust can be the magma source?
Usually, the remelting of basalts will give sodic, andesitic melts. Whether the partial melting of basalts can generate potassic, intermediate melts? If it can hanppen, what extent in the nature?
Hope for your answers. With regards.
Lulu
Dear Lu Lu
probably there is a msiunderstanding.
Charnockite is a Felsic Granulite (classified as An+Ab+Qtz+Opx).
You cannot treat it as a plutonic rock.
The presence of Opx (Hypersthene I presume) is a constraint of every granulitic rock.
Moreover, you cannot use the Putirka (2008) (or similar) thermobaromether for Opx-Cpx equilibria. It has been written for subsolidus equilibria in plutonic rocks.
Since you are not looking at a plutonic rocks (in the meaning of crystallised from magma) i think you have to re-evaluate your mineralogy, your petrology, your geochemistry and search a new approach to the understanding of such granulites.
Ciao
F
Dear Shumlyanskyy
I haven't said that opx-granite crystallize in granulite condition.
I've said that CHARNOCKITE IS AN OPX-BEARING GRANULITE.
(see the milestone work of Harris et al. The Journal of Geology 90(5):509-527 · September 1982)
The question is: Is Mr.Lu talking of an opx-granite or an opx-granulite?
Only fabric and mineral chemistry will answer the question.
Ciao
F
Dear Leonid
I know what you mean
IUGS consider Charnockite a Hypersthene Granite and classifies it in the field 3 of QAPF diagram. Following the definition of Holland (1900).
On the other hand, Harris pointed out that the main problem is to discriminate if the Opx-felsic rock is "Plutonic" or "Metamorphic".
Just because the PT conditions are completely different. And, in consequence, the petrological scenario, too.
I really believe that, while we are talking about charnockites, the real problem is defining the fabric and texture just to understand which approach is correct for the unnamed rock.
High #Mg number with very low Sr/Y and again high TiO2 content.
Quite strange for a felsic plutonic rock...
Don't you think?
Ciao!
F
P.S. Nice to have a talk on igneous petrology!, very rare here on RG
Thanks for everybody having a talk on such "strange" rocks! The fabric and texture of my rocks are typical of igenous rocks, i think. the Opx have composition different from those from the metamorphic granulites and "dehydrated zones". In addiiton, iegneous enclaves are present in the rocks. So, it believe that my rocks are of igneous origine. A difficult question now is how to explain the petrogenesis of such "strange" rocks and what magma source the rocks are derived from.
Very thanks Federico Lucci. The method of calculating pressure is a normative Q-Ab-Ob plotting, and a clinopyroxene - plagioclase baromether from Ellis (1980) and McCarthy and Patino-Douce (1998), rather than clinopyroxene-orthopyroxene baromether from Putirka (2008) . I'm sorry having maken a clerical error in the above question description. i think i have a necessary to check the avalability of the baromethers i have used.
P.S. Definition of charnockite is a meaningful scientific question, but difficult to reach a consensus. For this, Frost and Frost (2008) may have given a relatively new summary.
This discussion is getting really interesting and informative, but complicated also. If we reach a consensus on the definition of charnockite, things may remain simpler.
I would like to agree with the IUGS classification and Leonid's view that it is an orthopyroxene granite. Generally, perhaps because of traditional reasons, charnockites are considered to be granulite facies metamorphic rocks of granitic composition (and parentage). But is there any serious geochemical or petrological constraint that would inhibit the formation of orthopyroxene during the crystallization of granitic magma if its H2O content is not high? Let us not forget that the temperature conditions of the granulite facies at moderate to low pressures are not much different than those of of granitic magma at moderate to low depths.
By the way, this issue has been plaguing me for 40 years during the study of the huge Chilas gabbronorite complex of Kohistan arc in NW Himalaya (see Biqees et al, 2016, Island Arc). These rocks have granulitic fabric, lack garnet, and yield moderate P granulite facies temperatures. Suggestions have ranged from magmatic origin, to static recrystallization of the gabbronorites under granulite facies conditions.
Cheers,
Qasim
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