Late Ordovician regional high-pressure metamorphism in Scotland: Caledonian metamorphic climax predated the closure of Iapetus

Comment #231 J Faithfull @ 2025-08-07 13:44

I enjoyed reading your manuscript - good to see the ideas in the earlier talk presentation developed and taken forward, and some of the obvious issues addressed (eg predicted cpx in one of the models not seen in any of the assemblages).

As you state, the Mull "Moine" complex poses a lot of questions which are currently unresolved. The assemblage as a whole records a range of metamorphic events, but the detailed chronology and event/assemblage "stratigraphy" remains largely unknown, apart from the later (448 Ma regional growth, and ca. 418 Ma (?) granite aureole overprint)

One of the long-standing questions is whether some/most the rocks in upper units (Scoor Pelitic Gneiss and Ardalanish SBF) have experienced melting or not.

While there is plenty of field evidence for felsic veins, "lits" and segregations, not all of these look like unambiguous partial melts, and they almost certainly reflect a range of processes, and probably time: there are a wide range of assemblages in various host lithologies, with varied relationships to host deformation. There *may* be some granitic veins in some of the amphibolites, but I don't think its been definitively demonstrated that they are partial melts of their host. it would be good to see detailed field info on the host-vein relationships, and thin sections of the felsic lithologies. I think that most of the "siliceous veins" in amphibolites which are associated with coarse euhedral garnet growth seem to be pure quartz veins, rather than granitic, and I suspect that they represent fluids rather than granitic melts. The garnet-quartz veins look like they are probably coeval with the main episode of garnet growth in the host amphibolites: some garnets appear to grow across the vein boundaries.

It's also hard to correlate textural and paragenetic information between relatively competent low-strain hosts like the thicker amphibolites, and enclosing foliated mica-rich rocks, where older structures are readily overprinted by deformation, and perhaps fluid ingress.

There are pre-deformational boudinaged quartz veins as well as later planar quartz veins, some with K-fsp margins, others without. There are also a range of pods and segregations with diverse chemistries in different hosts whose chronologies and significance are unknown.

Many are very quartz-rich, and would be hard to explain as partial melts. But some are unambiguously granitic (eg in the Scoor Pelitic Gneiss especially, and near Assapol in the Ardalanish SBF) and might be be in equilibrium with their host rocks. Or might just be externally-sourced intrusives. These would be really interesting to date, along with pegmatitic veins and segregations in the Lower Shiaba Psammite, and Shiaba Pelite. How much of the metamorphic history is shared between the "Morar" and "Glenfinnan" units on Mull? when were they juxtaposed?

One issue for the hypothesis of widespread melting in the Ardalanish SBF rocks might be the calc-silicate assemblages. These almost invariably have epidote group minerals (usually clinozoisite/zoisite) and lack diopside. They also sometimes contain calcite. On the mainland, where similar Moine rocks show melting at sillimanite grade, similar calc silicates almost always lack epidote, and contain diopside. Perhaps at the higher pressures seen in Mull, hydrous phases might be stabilised? Or maybe there has been subsequent retrogression? But if so, I'd be surprised that the calc-silicates would consistently be more responsive to retrogressive mineral growth than pelites or metabasites.

The location of sample WMAR007 is not very precisley given, but the western half of Ardalanish Bay is pretty affected by granite-related low-P metamorphism, and other, later retrogression. (eg there's a lot of hydration, with later post-deformational muscovite) so I suspect that there's a good chance this sample doen't repreresent an equilibrium assemblage. Regional effects might be best examined using amphibolite and pelite samples from further east. There's no shortage of nice materials...

I suspect we're some way from a definitive statement on the P-T path(s) represented by the Moine assemblages on Mull. We need a robust stratigraphy of events and assemblages, with more robust dates, and we need a larger range of samples from diverse lithologies and assemblages. Even within the garnet amphibolites, there is a lot of complexity: plagioclase coronas around garnets, post-garnet foliation zones, deformed and undeformed vein assemblages and segregations, as well as late pervasive greenschist retrogression in places, often associated with quartz veining. All these are post-garnet, so might represent Scandian, or even later Acadian events... Or not :) Outside the garnet amphibolites, there are rare assemblages with corundum and spinel, which *might* represent relict earlier HT (Knoydartian?) events. But mostly, it does look as though the 450 Ma event is probably the dominant episode of bulk mineral growth and equilibration, at least in the "Glenfinnan" part of the Mull Moine succession.

Good to see new data on these lovely rocks - thanks! Much to think about...

Comment #214 J Faithfull @ 2025-05-26 15:49

I enjoyed reading this manuscript - good to see the ideas in the earlier talk presentation developed and taken forward, and some of the obvious issues addressed (eg predicted cpx in one of the models not seen in any of the assemblages).

As the authors state, the Mull "Moine" complex poses a lot of questions which are currently unresolved. The assemblage as a whole records a range of metamorphic events, but the detailed chronology and event/assemblage "stratigraphy" remains largely unknown, apart from the later (448 Ma regional growth, and ca. 418 Ma (?) granite aureole overprint)

One of the long-standing questions is whether some/most the rocks in upper units (Scoor Pelitic Gneiss and Ardalanish SBF) have experienced melting or not.

While there is plenty of field evidence for felsic veins, "lits" and segregations, not all of these look like unambiguous partial melts, and they almost certainly reflect a range of processes, and probably time. It's also hard to correlate textural and paragentic information between relatively competent low-strain hosts like the thicker amphibolites, and enclosing foliated mica-rich rocks, where older structures are readily overprinted by deformation, and perhaps fluid ingress.

There are pre-deformational boudinaged quartz veins as well as later planar quartz veins, some with K-fsp margins, others without. There are also a range of pods and segregations with diverse chemistries in different hosts whose chronologies and significance are unknown.

Many are very quartz-rich, and would be hard to explain as partial melts. But some are unambiguously granitic (eg in the Scoor Pelitic Gneiss especially, and near Assapol in the Ardalanish SBF) and could be be in equilibrium with their host rocks.

One issue for the hypothesis of widespread melting in the Ardalananish SBF rocks might be the calc-silicate assemblages. These almost invariably have epidote group minerals (usually clinozoisite/zoisite) and lack diopside. They also sometimes contain calcite. On the mainland, where similar Moine rocks show melting at sillimanite grade, similar calc silicates almost always lack epidote, and contain diopside. Perhaps at the higher pressures seen in Mull hydrous phases might be stabilised? Or maybe there has been subsequent retrogression of many/most assemblages? I'm not sure we know enough to say anything definite just now...

This is a very sketchy drafty comment - get in touch if you want a fuller discussion!