Dear Nirmalya: the 300°C equilibrium isotopic temperature seems to reflect the cooling of the rock after the medium grade metamorphism peak, instead the young K-Ar age, 13-11 Ma (Miocene), is quite probably the age of the cataclasis or mylonitization obtained in the recrystallized muscovite, due to an almost total loss or radiogenic 40Ar. A similar case we found in a 15 m thick pegmatite dike in NE Yaracuy State, in North-Central Venezuela. The pegmatite, composed of large (up to 12 cm) fractured schorlite crystals, minute spessartite garnets, and large muscovite leaves, with a typical graphic texture between microcline and quartz, intrudes a biotite schist of the San Julian Complex, an extensive Neoproterozoic to Early Paleozoic unit in the epidote-amphibolite facies. The first age obtained by the Ar-Ar method in muscovite was a scant 33 Ma, way too young for such a body, so similar to other pegmatites cropping out in the Venezuelan Andes, of Paleozoic age. We couldn't believe such age, it made no sense whatsoever! Fortunately there are also associated granitic dikes to the pegmatite body, which were dated using U-Pb by LA-ICP-MS in zircon, and gave a more "reasonable" age of 450 Ma. We found that the large moscovite leaves of the pegmatite (5-6 cm wide) were crushed and recrystallized, even showing deformed and bent polysinthetic twinning under the microscope, thus having lost a most of its original radiogenic 40Ar, due to the closeness of the pegmatite to the gigantic Boconó Fault Zone, which in this place has a width of more than 15 km. This megafault is a main structural feature in NW South America, it is a dextral strike-slip stretching fault of Miocene age, which strikes N40E. It cuts in two halves the Venezuelan Andes, forming two distinct parallel ranges, which have heights above sea level of 4450 m (Northern Range) and 4992 m (Sierra Nevada of Mérida), beginning in the Colombian border, and ending, after 500 km, right in NE Yaracuy State forming a large graben structure, the Yaracuy Trough.

Both cases are quite similar, even being tens of thousands of km away. I also believe that the original medium grade Barrovian metamorphism (garnet-staurolite) of your metapelitic mylonite is quite possibly of Paleozoic age, an event which we call in America the Taconic Orogeny (equivalent to the Caledonian Orogeny in Western Europe), and the 300°C temperature reflects the orogenic uplift of your metamorphic belt during the Himalayan Orogeny, possibly during the Eocene. Of course, the mylonitization should be a Miocene event, related to some major young faulting. Am I right? 

With regards, Sebastian Grande.

Dr. Sebastian Grande: 1) The Barrovian metamorphism  for my pelites which are part of the Tethyan Sedimentary Sequence (TSS), occurred not before Eocene as earlier works by other authors suggest...also Ar-Ar dating of large porphyroclasts of biotite gave age 35-40 Ma and also as young as 25 Ma. [Although, the protolith of these metamorphics, is of  Mesoproterozoic age] 2) If you mean uplift then there is strong reason to believe that it occurred due to Miocene normal shearing as the activity of South Tibetan Detachment System (STDS) started during that time....So my question is why not the 300 deg C isotopic equilibrium temperature can be considered as the deformation temperature related to mylonitization during STDS activity?

Please highlight your views.

Regards,

Nirmalya

Surely, Nirmalya, the geology of Northen Venezuela is not the same as the Himalayan, I thought there could have been some similarity, but I was utterly mistaken, since your protolith is from the Indian Shield, and Mesoproterozoic and the regional metamorphism is so young (by the way, if it is a metapelite, how did you determined its old age? I suppose it could have been intruded by some igneous bodies, dolerite dikes, perhaps, which could be dated effectively, or something like that). Therefore it is quite possible that the isotopic equilibration temperature of 300°C coincides with the Miocene age of the South Tibetan Detachment System, which is a system of regional thrust faults, obviously.   

Thanks Prof. Sebastian for sharing your experiences.1) Although I have not personally measured the age of protolith, but there are other works to suggest the age of protolith. The Geological Survey of India, for example, reported the fossil "Redlichia noetlingi" (indicating lower Cambrian age)  from a section which has undergone very little metamorphism and have gradational boundary with the metapelites that I'm working on. 2) These metapelites have deformed leucogranites which has shown muscovite Ar-Ar cooling age of c.13 Ma indicating that normal shearing of the STDS played a major role behind the cooling and exhumation of the felsics.

Hope, these will meet your queries. If anything else, let me know...

Regards,

Nirmalya

Thanks Prof. Sebastian for sharing your experiences.1) Although I have not personally measured the age of protolith, but there are other works to suggest the age of protolith. The Geological Survey of India, for example, reported the fossil "Redlichia noetlingi" (indicating lower Cambrian age)  from a section which has undergone very little metamorphism and have gradational boundary with the metapelites that I'm working on. 2) These metapelites have deformed leucogranites which has shown muscovite Ar-Ar cooling age of c.13 Ma indicating that normal shearing of the STDS played a major role behind the cooling and exhumation of the felsics.

Hope, these will meet your queries. If anything else, let me know...

Regards,

Nirmalya

Dear Nirmalya:

Yo stated that the protolith of the staurolite-garnet metapelite was Mesoproterozoic, but in the last letter you say it is related to a Cambrian unit of low metamophism. So it cannot be so old, perhaps Neoproterozoic or Lower Cambrian, but even this is impossible, as the contrast in metamorphic grade is too strong. All of this doesn't make any sense! The metapelites don't "have" deformed leucogranites, they could have been intruded by them. The only logical conclusion is that the Ar-Ar Miocene age reported is due to the general deformation of the whole Detachment Nappe which caused a massive loss of radiogenic 40Ar. With regards. Sebastián Grande.

No Prof. Grande, I said or what I meant that :1) protolith of the metapelites is of mesoproterozoic age, but at the extreme north they have undergone low grade metamorphism (Eocene or later age) so that Redlichia noetlingi (of lower Cambrian age) is well preserved. The Barrovian sequence is evident from this spot towards south (upto staurolite grade)....that's why I called a gradational change. The entire STDS (Miocene age) is located within this sequence. 2) Obviously, the deformed leucogranite have intruded metapelites prior to the ductile deformation  of both (folding and later STDS normal shearing).....

Hope this is clear now...otherwise let me know..:)

regards,

Nirmalya

Dear Nirmalya Chatterjee,

The staurolite-garnet metapelite would be much higher temperature. Along the STDS, there are some of them would be related to the leucogranite intrusions. For the STDS, I have dated lots of Ar-Ar ages, indicate that it occurred duing ~17-15 Ma. Thus if you confirm that the metamorphic temperature is ~300 degree, and related to the ductile shear then it can be deformation age. Please read my recent papers.  

Dear Nirmalya, 

I think it is valid to consider the Ar-Ar muscovite age as representative of the mylonitization. Here some ideas about this:

-the ages you obtained (I assummed they are plateau ages) are clearly younger than the Mesoproterozoic protolith as well as the Ar-Ar bt ages, indicating that they represent a different event. Furthermore, the closure temperature of the K-Ar system for muscovite is

Dear Sebastian in my case 1) Biotite are large porphyroclast (>400 micron) whereas muscovite ( max.30-60 micron) form the matrix of the mylonite. Both of them are part of the same metamorphic event as textural evidence suggests. The biotite showing Ar-Ar cooling age 22-38 Ma underwent ductile grain-size reduction by basal (001) parallel slip---they are not recrystallized as they show undulose extinction. The muscovite on the other hand are dynamically recrystallized (and NOT crystallized ) during mylonitization and devoid of any undulose extinction. They are showing Ar-Ar age 11-13 Ma.

2) Textural evidence like undulose extinction, BLG recrystallization etc within quartz also suggests that deformation condition are low (300-400 deg C)

----I think biotite closure temp will be little higher than muscovite because of larger grain size of the former.

Do you now think that O18 isotopic composition represents equilibrium between muscovite and biotite? ( I saw that muscovite are enriched in del O18 compared to biotite if equilibrium conditions are met (Sharp, 2007)

Dear Nirmalya,

deformation conditions seem to be thus well constrained. Consequently, I think grain size could be a possible explanation to understand the obtained ages. One interesting point that you could evaluate is whether the so called "closure temperatures" are valid, specially for mylonitic rocks (e.g., have a look on Igor Villa´s papers). I´ve been working on this topic for the last year and I think it´s not as easy as it´s thought due to the complexity and interaction of different processes during mylonitization (deformation, metamorphism, magmatism, fluids).

In any case, I am still not pretty sure whether both micas are contemporaneous or not. For example, the biotites >400 microns, do they represent the grain-size reducted biotites or the remnants of "underformed" biotites? Based on what you mention, I would still think that biotite nucleated prior to muscovite, although both could be afterwards homogeneised in terms of 18O. In that case, this event shouldn´t have affected K-Ar isotopy.