Hello everyone. In my research area, monzonite and granite with MMEs (mafic microgranular enclaves) included was analyzed. They show indistinguishable whole-rock Sr-Nd isotope compositions and zircon in-situ Hf isotope compositions and indicate a similar parental magma. Thier major elements and trace elements follow the magma evolution (fractional crystallization) trend, while the monzonite is the least evolved and the granite sample is the most evolved with MME samples lined between them.
However, the compositions of minerals in them have told a different story. The mafic minerals in them were analyzed. The compositions of magnesiohornblende-actinolie in monzonite and MMEs (not found in granite) were different. The amphiboles in mozoniete have higher MgO and SiO2 content but lower TiO2, Al2O3, MnO, alkali, F and REE content, which may indicate the early stage of crystallization. However, they have lower Ni, Cr, V contents than MMEs.
For the composition of biotite in MMEs and granite (biotites in monzonite were altered to chlorite), they show similar major elements content, but granite have higher Cr, Ni contents than MMEs.
That is to say: it looks like that Ni, Cr content concentrating in the late stage of the magma evolution. But they are compatible elements and should be incorporated to early formed crystals. I'm confusing and would like to know if someone have ideas.
Dear Ruoxi Zhang
The petrography you report maybe is incomplete.
What is the kind of opaque/spinel minerals present in monzonite, granite and MME?
Which are the bulk contents in Cr and Ni in monzonite, in MME and in granite?
Is it possible that granite is enriched also in micrometric enclaves that produce the Cr-Ni enrichment?
Cr-Co-Ni are compatible elements. It is difficult they remain in melt during fractionation.
Thank you for your kind reply, Federico. Federico Lucci
The mineral assemblages of these rocks are similar but with different proportion. They are plagioclase+alkali feldspar+quartz+amphibole+biotite+titanite+magnetite+ilmenite+accessory minerals (apatite+zircon).
Amphibole is not found in granite but some amphibole pseudocrystal (has turned to biotite composition) was recognized. Almost all the biotites in monzonite were altered to chlorite, and some remnant was found with compositions between biotite and chlorite.
Bulk content in Ni in monzonite, MMEs and granite are 3.44~3.61, 2.42~3.59,2.6~3.2, respectively. While Cr contents are 6.2~9.1, 5.83~9.23, 5.67~29.7, respectively. The biotite compositions in high Cr granite sample (29.7 ppm in bulk) and low Cr granite sample (5.67 ppm in bulk) are similar, with higher Cr contents than that in MMEs.
"Is it possible that granite is enriched also in micrometric enclaves that produce the Cr-Ni enrichment?" That's good question. However, 20 points at least of biotites in each rock types were selected for LA-ICP-MS analysis. Most of them has higher Cr and Ni contents (0~4.49, 5.29~18.15) than that in MME samples (0~1.96, 0.99~5.95). So, I tend to consider the differences are existed between them, rather than the elevated contents caused by the minor inclusions.
Dear Ruoxi Zhang
I think that composition of some elements could be changed during metamorphic or hydrothermal alterations.
Second option is influx of new magma with other composition to the magma chamber (for example link betwen two defferent magma chambers).
These are just ideas, there are probably more possibilities.
dear Ruoxi Zhang
thank you for your detailed mail.
The fact there is a different behavior between Ni and Cr suggests that maybe their contents are not directly controlled by pure Fractional crystallization.
Moreover, you have metaluminous calcic melts (amphibole-titanite-plagioclase-biotite).
Carr and Turekian (1962) Article Chromium in granitic rocks
many decades ago investigated the Cr behaviors in Ca-poor and Ca-rich granites. Ca-rich granites are prone to be more enriched in Cr content.Lastly, it is known since the old work of Simon and Rollinson (1976)Article Chromium in rocks and minerals from the southern California batholith
that Cr partitioning between biotite and hornblende (biotite/hornblende) is favouring biotite. it means that the progressive and more efficient fractionation of amphibole to generate granite could locally promote enrichment of Cr then hosted by biotite.However, if biotite is not primary because it is the consequence of amphibole replacement, it means that granites were open systems opened to fluids or to interaction with other materials (maybe digestion/assimilation) producing variations in the Cr budget.
However, these are only suggestions. It is very difficult to propose a reasonable interpretation from the very few data you presented to us.
The great variability of Cr in granites is compatible with open system.
cheers
f
Dear Ruoxi Zhang
We had recently published a paper about amphibole recycling on which more evolved magmas have higher Ni and Cr contents
https://doi.org/10.1016/j.jsames.2023.104569
Regards
Benigno
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