Everybody,
I'm studying a Late Cretaceous (~68 Ma) opx-bearing granite in the Qiangtang Terrane, Tibetan Plateau. The two-pyroxene barametry indicates that the melt-formation of the this pluton require pressure ≥ 14.2–18.1 kbar and temperature ≥ 900–1000 ℃. When plotted on the P-T diagram illustrating the partial melting of mafic lower crust and phase relationship, this P-T range corresponds to the “amphibole and plagioclase-out” and “garnet-in” field, implying the breakdown of amphiboles and plagioclases and occurrence of garnets in the source region.
In combination of chemical compositions, we suggest that partial melting of mafic lower continental crust in the stability of garnet (e.g., garnet-granulites or eclogites) was the most plausible scenario for the genesis of the pluton. High Sr and Ba, low Y and heavy rare earth elements (REEs), strong depletion of high-field-strength elements (HFSEs) such as Nb, Ta and Ti, and lack of negative Sr and Eu anomalies (Martin 1986, 1999; Defant and Drummond 1990; Martin et al. 2005) in the rocks indicate that the pluton closely resembles adakites in element compositions. However, peculiarly, it exhibits higher Yb and Y concentrations as well as lower Sr/Y and (La/Yb)N ratios relative to the typical adakites.
It is so peculiar. So, I want to ask partial melting of garnet-granulites or eclogites necessarily produces adakites with high Sr/Y and (La/Yb)n ratios? If not, what geological processes would result into the decrease of these two ratios in the partial melts from the eclogites or garnrt-bearing granulites?
Thanks.
Lu
Dear Lu-Lu: I don´t understand why this opx bearing rock should be called "granite" at all, and not more properly charnockite, due to its mineralogy. I've studied granitic rocks which show restites of very oxidised opx and more or less preserved cpx included inside hornblende clots, but these are restites of partial fusion of possible granulitic source rocks, there are not primary minerals of the rock. Could you send some photomicrographs of this strange rock to have a better opinion? Regards. Sebastian.
Dear Prof. Grande,
These rocks are comprised by Opx, Cpx, Pl and KfsI, with minor Bit and Amp. We have demonstrated that the rocks are of igneous origination based on petrographic and geochemical features. For example, they have porhyaceous and ganitic textures. As you said, from its mineralogy, it is better to call this pluton as "charnockite".
I attached some photographs as a Word File in the attachment. You can download it and have a brief scan with respect to the petrographic features.
The orthopyroxenes that contained in the rocks are mostly anhedral. Tests for equilibrium between pyroxenes and a nominal coexisting melt using the methods of Putirka (2008) indicates that the compositions of the pyroxenes are not in equilibrium with their coexisting “melt” composition (i.e., whole rock composition of the host rocks). So it is likely that even though the "melts" are associated with the pyroxene grains, they are not the melts from which the pyroxene grains actually formed. A better explaination is that, we think, these orthopyroxene may be restites of of partial fusion of possible granulitic or eclogitic source rocks, and entrained into the magmas as anhedral pyroxene xenocrysts. However, we are not very confident about this explain. Can you help confirm ?
Another question is, just as stated before, whether partial melting of garnet-granulites or eclogites necessarily produces adakites with high Sr/Y and (La/Yb)n ratios? If not, what geological processes would result into the decrease of these two ratios in the partial melts from the eclogites or garnrt-bearing granulites?
Hope for your advices.
Thanks. With best regards.
Lu
Dear Lu-Lu: for some srange reason I haven't been able to unload the photomicrographs you attached, a message saying "Netwok error" appears all the time. Perhaps the file is too heavy (21 MB) and doesn't come through. Any way, pyroxenes, both ortho and clino, are silica saturated minerals, so they should be stable in a granitic melt, unlike olivine, for example. The possibility that these minerals could be restites is also quite likely, I've found similar examples in Venezuelan and Colombian granitoid rocks. But eclogites don't contain orthopyroxene, only granulites, so the source rock of the magma should have been granulites, not eclogites. Regards. Sebastian.
P.D.: try to send the pictures one by one, and not as an attached, and heavy, file...
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