The mafic microgranular enclaves consists of plagioclase laths and chlorite. The presence of skeletal augite within chlorite suggests that the latter were altered from augite. The quartz and albitic plagioclase phenocrysts occur in the enclaves, possibly transfered from the host felsic magma. Many quartz and some albite phenocrysts coexist with calcite. The calcite exhibit smooth, regular boundaries with quartz. It is unlikely that the calcite was a post-magmatic alteration products but, rather, was crystallized simultaneously or slightly later than the plagioclase laths.
I'm not an expert on topic, but check out Mancktelow & Pennacchioni (2010) which is discussing the stability of calcite in amphibolite facies conditions from quartz mylonites http://onlinelibrary.wiley.com/doi/10.1029/2009JB006526/full
Thank Gönenç Göçmengil . My samples were collected from the Junggar. The country rocks of the intrusive rocks are volcanic and volcaniclastic rocks of Devonian to Carboniferous. Metamorphism of these country rocks were not higher than prehnite-pumpellyite facies. The intrusive rocks as post-collisional show even weaker metamorphism, if ever. So, the occurrence of calcite and its coexistence with quartz phenocrysts in mafic microgranular enclaves cannot be ascribed to metamorphism. This phenomenon should be considered in connection to magmatism or magmatic fluids instead of mylonitization.
My bet would be that the calcite is secondary, considering the evidence for alteration. The metamorphic grade does not need to be high to get calcite - as long as you have fluids around of the correct composition, you can get calcite. I have seen it in < 1Ma volcanics too...
it is clearly secondary in nature. Probably it was a vug filled by fluids rich in bicarbonates.
It is always difficult to decide from a single photograph, but I agree with Martina and Michele in that calcite is most probably secondary. In contrast, quartz may be not. In particular, quartz crystals at the upper left in the photograph look like broken phenocrysts, so that quartz and calcite did not form altogether by alteration. If so, it could be expected to find quartz phenocrysts unrelated to calcite. Are there?
The assemblage calcite-chlorite-quartz is very common in late- to post-magmatic vugs and clefts formed from late to post magmatic (metasomatic) fluids. The photomicrograph clearly shows the succession from the surface of the microfracture to the middle 1. subhedral quartz, 2. unhedral calcite and 3. unhedral chlorite. Furthermore, the calcite grains contain post crystalline deformation lamellae, and the thin film of chlorite flakes appears foliated.
In conclusion, the photomicrograph shows a slightly foliated microfracture developed after the crystallization of the plagioclase matrix, healed by quartz, calcite and chlorite.
Thank for all colleagues who added an explanation. All the explanations are useful. Yes, Guenter Grundmann got the point. The coarse quartz grains are not phenocryst but are anhedral and occur interstitially. The frequencies of occurrence of the three alteration minerals are different. Chlorite occurs in both the enclave and the host monzonite, calcite in the enclave but much less so in the host rock, and quartz “phenocrysts” (coarse but anhedral without regular boundary) only in the enclave. I wonder whether the alteration has any connection with mixing of the enclave with the host magmas.
Dear Wei Liu,
I would like to propose a hypothetic explanation of the formation of the quartz-calcite-chlorite mineral association in your enclave.
The formation of your mafic enclave in monzonite is probably late- to post magmatic. Your plagioclase lathes are unaltered and they seem to be the border of a king of miarole in the enclave. They even “enter” (are included) into the calcite and the quartz. The mafic magma of the enclave is probably rich in CO2 and H2O. H2O could be partly from the monzonitic magma. During the process of alteration of the clinopyroxene (SiO2 = 48 – 52%) and it replacement by chlorite (SiO2 = 25 -35 %), SiO2 and CaO are released. They are used for the formation of the calcite and the quartz. The rich in fluids “miarole” is probably formed at the late magmatic stage, but the processes of clinopyroxene alteration and the calcite and quartz crystallization are at lower temperature and at the postmagmatic stage.
The explanation is close to the one of Guenter Grundmann
I would like to suggest that one should look at more than a single sample. How does the rock occur? Is there drill core, an outcrop map, a regional map. Starting with a map is usually a good idea. Is the rock layered, are the enclaves aligned or in cross-cutting features? What kind of monzonite is it. What evidence is there of metamorphism?...etc........
Thank Rossen Nedialkov. Your hypothesis is just what I thought. But, CO2 and H2O probably were derived from the host monzonite. The gabro-dioritic dykes that were intruded into granite have unaltered augite and do not contain calcite. Only the mafic magma mixed into the monzonite as enclaves contain significant calcite, anhedral quartz, and chlorite. Relative to the encalves, the fluids may be post-magmatic. However, to the host magma that was solidified later, the fluids were late-magmatic.