It is a hard (steel nail will not scratch it), probably silicified green rock with euhedral plagioclase phenocrysts (some showing twinning), other elongated phenocrysts now altered to chlorite and peculiar rounded fragments with thick silicified rims. Most rounded fragments are circular, some egg-shaped. Quartz and chlorite make up most of the visible parts of the rounded fragments.
This rock has been transported by ice and was found in glacial till.
Hi, Michael, what a fantastic rock have you found this time!
Orbicular Porphyry, apparently.
There is an oldish publication that tackes the issue of orbicular structures http://www.geologinenseura.fi/bulletin/Volume46/sgs_bt_046_1_pages_053_074.pdf
I surely simplify by saying that your rock originates by some convective movements in the magmatic chamber before the melt was erupted.
It would be fascinating to find where the boulder originates from (bed-rock outcrop of the same rock).
Best regards
Hello Michael,
Could you give some indication of scale please, and the location of the host till - there's always a chance there may be a more local source candidates for this rock type.
Ciao Roberto, thanks for the interesting paper!
Best regards
Graham
Sorry Graham, I forgot the scale. The large egg-shaped fragment is 3 cm by 4 cm large.
The location is about 53rd parallel in Manitoba close to Saskatchewan border and the glacial striations point to the NE (towards Thompson, Manitoba). There are volcanic rocks everywhere up-ice.
There is no mention of orbicular rocks in the literature that I am aware of.
Thank you Roberto for the interesting paper and your identification!
I was wrong about no-literature. I found a reference of an orbicular porphyry with quartz and plagioclase phenocrysts and fragments up to 4 cm in the Flin Flon area and another reference from the Snow Lake area of Manitoba. Both these areas are N - NE of the location of the boulder.
Thank you Roberto and Graham for your input!
I do not believe that the spherical structures in your pictures are orbicules. Your second picture looks like part of a volcanic ignimbrite sheet. Note the laminated and layered fabric in the lower part of your second photo. My colleagues and I found small spherical lapilli in such sheets and associated with porphyritic cone-sheets in a sub-volcanic complex. All orbicules I have studied occur in coarse-grained (>2mm) matrices of plutonic intrusive complexes, most frequently in diorites, but I have seen them in gabbros. Some ignimbrites we studied contain cm scale fragments, some spherical, but I haven't seen any like yours.
Article Subterranean Origin of Accreted Lapilli in Cone-Sheets of th...
It looks to me as a xenolith with a reaction corona. In other words something not related to the bulk of the magma.
michele
Thank you Roger and Michele for your input.
When I get home I will take additional pictures of the boulder so you can a view of the other sides.
I can't be very sure from the pictures, but there seem to be some lithic clasts in the rock (brownish color) in addition to the crystals. This makes it an explosive deposit, rather than effusive. Some of the crystals look anhedral and that is again normal for explosive deposits (some crystals will be splintered). I've seen similar rocks on a Miocene and on a Pleistocene volcano in Romania, and we interpreted them to be produced by hydromagmatic interaction (lateral phreatomagmatic blasts in both cases, small case). You should check whether you find lithic clasts with "baked" rims: those would attest for heating.
The rounded features, do they contain crystalline quartz or an amorphous SiO2 phase (chalcedony or something else)?. I've seen such nodules in explosive and in effusive units that were deposited underwater in small, land-locked basins (without much communication and water exchange). The water can become saturated in silica via hydrolysis reactions with the volcanic material and it will precipitate amorphous silica phases, forming nodules. Give them enough time and the structure of the nodules will become more crystalline (an XRD analysis might help you here). Hydrolysis involving volcanic material can also promote diatom blooms, so you can check for frustules in thin sections; finding them would suggest such a process. Interesting rocks!
This rock is really interesting! I agree with Michele and also think that it looks like a xenolith. Do you have other pictures, of the whole outcrop or you just found this feature on these blocks?
Razvan-Gabriel,
Thank you so much for your excellent contribution towards explaining formation of this rock.
With a hand lens the rounded features appear to enclose crystalline quartz mixed with other parts rich in chlorite.
Rafael,
This rock is not from an known outcrop. It is a boulder that has been transported by a glacier many hundreds of km (probably) away from its original outcrop and down a river where I found it. During its long transportation journey it has become rounded in shape.
The area up-ice from its discovery location is underlain by Precambrian rocks of the Superior and Churchill tectonic Provinces ranging in age from about 1.7 b. y. ago to 3.5 b. y. ago. These Precambrian rocks contain granite-gneiss and numerous volcanic rocks. The geology of this area has been mapped and additional information is obtained from exploration drilling looking for base metals and gold.
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