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Decompressuonal melting is common for UHP metamorphic rocks during their exhumation from mantle depths to crustal depths (see reviews by Zheng et al., 2011; Chen et al., 2017). This has been found not only in UHP metamorphic rocks (Zhao et al., 2007; Xia et al., 2008; Gao et al., 2012; Liu et al., 2013) but also in the magmatic rocks of syn-collisional age (Zhao et al., 2012, 2017). As documented by these publications, it is a kind of dehydration melting due to muscovite breakdown during the decompressional exhumation of UHP metamorphic slices. It has only produced small volumes of felsic melts during collisional orogeny. The dehydration melting would take place at a time when the tectonic regime has been switched from compression to extension, though there are only short timescales of 5 to 20 Myr for collisional orogeny. In the postcollisional stage, nevertheless, much larger volumes of felsic melts can be produced by muscovite dehydration melting in response to the extension of orogenic lithosphere. Although the tectonism for this episode of dehydration melting is independent of the collisional orogeny, it results from reworking of the preexisting orogens and thus shows inheritance in both structure and composition (Zheng and Chen, 2017).

References

Gao, X.-Y., Zheng, Y.-F., Chen, Y.-X., 2012. Dehydration melting of ultrahigh-pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet. Journal of Metamorphic Geology 30, 193-212.

Liu, Q., Hermann, J., Zhang, J.F., 2013. Polyphase inclusions in the Shuanghe UHP eclogites formed by subsolidus transformation and incipient melting during exhumation of deeply subducted crust. Lithos 177, 91-109.

Xia, Q.-X., Zheng, Y.-F., Zhou, L.G., 2008. Dehydration and melting during continental collision: constraints from element and isotope geochemistry of low-T/UHP granitic gneiss in the Dabie orogen. Chemical Geology 247, 36-65.

Zhao, Z.-F., Zheng, Y.-F., Chen, R.-X., Xia, Q.-X., Wu, Y.-B., 2007. Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision. Geochimica et Cosmochimica Acta 71, 5244-5266.

Zhao, Z.-F., Zheng, Y.-F., Zhang, J., Dai, L.-Q., Li, Q., and Liu, X., 2012. Syn-exhumation magmatism during continental collision: Evidence from alkaline intrusives of Triassic age in the Sulu orogeny. Chemical Geology 328, 70–88.

Zhao, Z.-F., Zheng, Y.-F., Chen, Y.-X., Sun, G.-C., 2017. Partial melting of subducted continental crust: geochemical evidence from syn-exhumation granite in the Sulu orogen. Geological Society of America Bulletin 129, 1692-1707.

Zheng, Y.-F., Xia, Q.-X., Chen, R.-X., Gao, X.-Y., 2011. Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision. Earth-Science Reviews 107, 342–374.

Zheng, Y.-F. and Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences 145, 46-73.

-----原始邮件-----
发件人:"Frank Spear" <[log in to unmask]>
发送时间:2018-06-27 00:21:58 (星期三)
收件人: [log in to unmask]
抄送:
主题: Re: Crustal anatexis

The amount of melt generated by muscovite dehydration melting is volumetrically around 50% of the amount of muscovite that melts (by simple mass balance). Additionally, the reaction is very nearly univariant, so you only need to increase T by a few degrees (obviously accounting for the latent heat of melting required is also important). But in my experience, rocks either cross the reaction or don’t. I haven’t heard of any occurrences where the peak conditions straddle the muscovite dehydration melting curve.

Cheers,

Frank

On Jun 26, 2018, at 11:22 AM, Marcos García Arias <[log in to unmask]> wrote:

Hello again,

My personal point regarding muscovite breakdown during decompression is that it cannot produce large amounts of melt. The muscovite breakdown reaction is very steep, so if decompression occurs after crossing it, the P-T path will be (more or less) subparallel to that reaction. So, even under an adiabatic ascent of the source due to high rates of exhumation, the P-T path will not depart significantly from the muscovite breakdown reaction, leading to rather small amounts of melt. Consequently, large amounts of melt can only be produced if the prograde P-T path goes beyond the muscovite reaction, crossing the biotite breakdown reaction, before undergoing decompression.

Kind regards,

Marcos

--

Dr. Marcos Garcia-Arias
Assistant Professor in Metamorphic Petrology

University of Los Andes, Colombia

Links:

SCOPUS: https://www.scopus.com/authid/detail.uri?authorId=36124834200
ORCID: http://orcid.org/0000-0002-3585-2364
ResearcherID: http://www.researcherid.com/rid/A-7595-2011
Google Scholar: https://scholar.google.com/citations?user=IWdJLncAAAAJ&hl=es
ResearchGATE: https://www.researchgate.net/profile/Marcos_Garcia-Arias

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