The pluton

Back to The Mature Years

The Bucks Lake pluton has a proper name now, bestowed on it appropriately by an established field geologist with impeccable credentials, Anna Hietanen, in her publication “Geology of the Pulga and Bucks Lake Quadrangles, Butte and Plumas Counties, California” Professional Paper 731(1973). I met Anna on at least two occasions, and she was always considerate and friendly, in spite of the fact that I was actually an interloper into her domain. That is, when I began my work, she had already published on parts of the area, so the proper protocol would have been for me to seek her out and ask her OK for me to proceed. This I didn’t do, not so much out of arrogance but out of timidity. But she never even commented to me on my impropriety.

I can affirm her observations of the Bucks Lake pluton, not that there is any need for that. I say it only to point you to her paper for the typical details of the geology. In the course of my work on the pluton, I did work one summer (1966),  ostensibly as one of Anna’s field assistants and I first met her in the field in that context. As a consequence of that work, the original Bucks Batholith of H W Turner was separated into the three plutons shown on her map and discussed in her report. I focused on the Bucks Lake pluton, whereas Anna’s work was much more comprehensive.

The pluton is located in Northern California, at the north-west corner of the Sierra Nevada mountains. Just north of here, the Sierran block is cut-off and shifted further northwesterly in the form of the Klammath mountains. Cascade range volcanics lie immediately north.
It is an elliptically shaped mass trending NW-SE, roughly 12 mile long and 9 mile wide. It is oddly zoned, actually reversed from the typical compositional zoning in plutons. The more mafic rocks  (two-pyroxene diorite) are surrounded by the more sialic rocks (hornblende-biotite quartz diorite). But the whole pattern is asymmetric, offset to the NW. The North Fork of the Feather River cuts its deep canyon NE to SW through the NW margin of the pluton, exposing the entrails of the pluton from an elevation of nearly 7200 feet at Bald Eagle Mountain to about 2000 feet on the river, less than 4 miles away.

As an aspiring but naive graduate student, I was looking for a “good” problem, not just to fill in another new map sheet. So I took on the proposition: How to explain reverse zoning in a pluton using only standard mechanisms. No strange chemistry or outlandish stuff. Given this state of mind, I looked for the evidence that the zoning in the pluton was, in fact, exactly inside-out.  Not surprisingly, that’s what I found, continuous variation from a sialic circumference to a mafic center.  Anna offered proper criticisms, but when I read her report now, so many years later, it seems she pretty much agreed. She did conclude that there had been but one magma, and that odd distribution was the result of a complex history. I basically tried to fill in the details of the history.

Potassium-argon dates from the margin and the center of the pluton, done in connection with some paleomagnetic work came in at 129 my (margin) and 142 my (center), indicating a two-magma history. More recent work has revised that to a uniform 140 my.

My own explanation had the magma crystallizing against a synformal depression in the roof of the chamber while the whole body was elongating vertically during emplacement. Early formed mafic rock formed against the lateral margin, as opposed to the roof, would be displaced vertically downward relative to the roof rocks as the surface area of contact increased due to the change in shape. This would result in a kind of growing collar zone of country rock newly exposed to the differentiating, more sialic, magma. An erosional surface cutting across this structure at just the right place would produce a reverse zoned map.

So, a process generally considered normal, when applied to unusual but not unexpected circumstances can lead to apparently contradictory results. It was a first hand experience with exceptions proving the rule, a theme that became part of my teaching about science.

The main thing that was going on in the 60’s and 70’s in geology was the revolutionary developments in Plate Tectonics/ Continental Drift. I departed into teaching just as this process was coming to resolution. The question raised by Hietanen’s field work-and my own, though being unpublished mine doesn’t count- did not attract attention, as the research community was thoroughly absorbed in testing and establishing the new ideas, and using the new technologies. The detailed history of one pluton became less compelling than more esoteric work on whole groups of plutons and how they could be made to yield information about what was where at different whens. I am embarrassed to say that I kept up with the advances in the field mainly by reading the new textbooks that publishers sent in a steady stream.
If you Google up “Bucks Lake pluton” today, I found, mostly it is referred to as the location of a date or a magnetic data point in some more regional Plate Tectonic study. It makes me feel fuddy-duddy and old fashioned- a geosynclinal geologist speaking an obsolete dialect. But I soldier on, anyway.

I’m going to stop this piece here, so that I can isolate the specifics of what I want to state in a shorter piece, so that someone who is actually interested in them doesn’t have to plow through all the extraneous verbiage.

Here is a link to the Chico Sheet of the California Geologic Atlas. The pluton is at the northern edge.

Next The First of Two Questions

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