The first of two questions
Back to The Pluton
In Prof paper 731, Hietanen described the occurrence of the pyroxene diorite in the Bucks Lake pluton in two distinct phases, fine grained and coarse. Here are my photos of this occurrence:
Passing upward from the contact, small patches of coarse rock begin to appear within the finer, increasing until what remains is coarse rock with blobs and blocks of finer within. Then these disappear before the whole sequence repeats itself.
It occurred to me one day while working out these details, to see how far I could follow the fine/coarse contact laterally. Here is a shot of that.
The coarse rock, on the right, is more massive. The finer rock, to the left, is more scabby, and broken up by the weathering along the hornblende stringers.
The result was this map:
Seemingly the fine grained material formed against the roof of the magma chamber, and peeled off , sometimes in quite broad sheets, and sank down into the magma. Or perhaps just floated around in the attic of the chamber- the density difference between magma and rock may not have been very great.
Was there one period of fine-grained crystallization or multiple? Is there some difference in the modality of the sheets that might answer the question?
Hietanen noted, and I confirm, the plagioclase in the finer rock, and the coarse, is remarkably unzoned according to the microscope. More advanced techniques could refine that observation. But it raises in my mind the question of how the crystallization was prompted. Was it cooling against the wall rock, which should have produced some kind of zoning. Perhaps it was pressure drop, the magma losing water vapor pressure to volcanic eruption at the surface. Hence the question posed: attic of a magma chamber or basement of a volcano.
As an historical note to all you Plate Tectonic generation folk, at the time I was doing my work, it was very unfashionable to mix volcanism and plutonism in a field way. Lots of theoretical speculation, but nobody expected to see connections in the field.
Next The Second Question: Paragenesis of the magnetic minerals