Day #120 Deskcrop: Bear Valley Springs Tonalite

We’ll close out the week and the month back in California with a deskcrop and thin section of the Bear Valley Springs Tonalite, one of the more abundant granitoids from the southern end of the Sierra Nevada Range in the Tehachapi-Bakersfield region. I’m not at all certain of the exact locality of this sample, but I strongly suspect that I collected somewhere along the Kern River Canyon between Caliente and Lake Isabella. It’s a run-of-the-mill Sierran tonalite, and a classic I-type granite with both hornblende and biotite as varietal minerals. The age of the BVS tonalite is right around 100Ma.

Bear Valley Springs Tonalite

Plane Light


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Day #119 Deskcrop: Powell Kyanite Schist (Thin Section)

Sticking with the metapelites, today’s thin section “deskcrops” are from the Powell kyanite schist featured on April 16th. How many minerals can you identify in this thin section? Can you narrow down the range of P,T at which it last equilibrated? Bonus points for applying the phase rule – show your work! (Assume the rock is described by the components KFMASH.)

Powell Kyanite Schist, Plane Light

Powell Kyanite Schist, X-Polars

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Day #118 Deskcrop: Staurolite Schist (Thin Section)

Today’s thin section “deskcrop” revisits the Black Hills staurolite schist featured back on March 24th. And those honkin’ big staurolite porphyroblasts don’t just look good in hand sample! In the thin sections below you’ll note the distinctly poikiloblastic texture of both the large staurolite porphyroblasts and the somewhat smaller garnet porphyroblasts, as well. You may also not the pleochroism displayed by both the staurolite and biotite crystals going from the first to the third images below. Um wa!

Zoomed-out, Plane Light

Zoomed-out, X-polars

Zoomed-out, Rotated, Plane Light

Zoomed-out, Rotated, X-polars

Zoomed-in, Plane Light

Zoomed-in, X-polars

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Day #117 Deskcrop: Dunite with Cumulate Chromite

Another of my well-loved teaching samples comes from the Del Puerto Ophiolite in central California (though my recollection of the specific locality is somewhat vague). The sample is dunite, an ultramafic rock made of >90% olivine, with layers of cumulate chromite. This is a great sample for discussing crystal settling in magma chambers and is one of my favorite thin sections for introducing students to the mineral olivine, in all of its technicolor glory. In addition to the hand sample shot below, there are also standard photomicrographs of the sample in plane and cross-polarized light and MicroGigaPans of the same two lighting schemes. None of these is going to replace the experience of actually viewing the sample in a petrographic microscope, where one can turn the sample to see different optical properties, but with a bit of work and a little more technological innovation I think there’s the possibility to make a web-deliverable experience that’s a reasonable approximation.

Dunite with Cumulate Chromite

Photomicrograph in Plane Light

Photomicrograph in X-Polarized Light

Dunite in Plane-Polarized Light
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Dunite in Cross-Polarized Light
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Day #116 Deskcrop: Half Dome Granodiorite (Thin Section)

I’m kicking this week of thin section deskcrops with a rock you’ll already be quite familiar with if you’ve been following my blog this year – that’s right, it’s another look at the Half Dome Granodiorite! We’ve previously seen it pulverized, in deskcrop, and outcrop. Today’s views are of thin sections (30 micrometer thick slices of the rock) as seen under a petrographic microscope (or technically, a petrographic macroscope, in this case).

The plane light view is nothing spectacular, since the minerals that make up the bulk of this rock (quartz and plagioclase) are low relief, uncolored minerals in plane light. Near the center of the plane light view there is a grain of biotite with secondary chlorite oriented parallel to cleavage in a NE-SW orientation in this grain. Just to the right of it is a large green crystal of hornblende. Both the biotite and hornblende are pleochroic (they change colors as the stage is rotated in plane light), but I’ve only photographed this orientation.

Half Dome Granodiorite (plane light)

In cross polarized light the plagioclase grains really steal the show. There are prominent grains that show both Carlsbad and albite twinning, as well as a couple that show concentric compositional zoning (check out the NE quadrant). To get a closer look at any of these photomicrographs click through to the image on Flickr and then click the link to “All Sizes” above the photo there.

Half Dome Granodiorite (X-polarized light)

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Day #115 Outcrop: Putnam Station Nonconformity

Today’s “outcrop” is, in fact, yet another roadcut – it seems all the best “outcrops” in the northeast are roadcuts, but who’s quibbling. This time we’re on the eastern margin of the Adirondacks near Putnam Station, NY. The contact here, between Proterozoic gneisses (below) and Cambrian Potsdam Sandstone (above), is extremely well preserved near the top of this small cut. The Potsdam even has a thin conglomerate bed just above the contact before grading upwards into more common sandstone layers. Zoom in and you’ll note that my field assistants came out to examine this exposure, as well. Berti is waving at you with his feet planted firmly on the Great Unconformity and Edi is below him examining the vertically foliated gneisses. Needless to say, a good time was had by all.


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Day #114 Outcrop: Alexandria Bay Nonconformity

As promised back on Tuesday, this weekend I’m going to visit a pair of outcrops (well, roadcuts, actually) on opposite sides of the Adirondack Mountains that expose the Great Unconformity where the Cambrian Potsdam Sandstone lies depositionally atop a nonconformity developed on Late Proterozoic gneisses of the Adirondacks.

We’ll begin on the west side of the Adirondacks in the St. Lawrence River Valley. Today’s “outcrop” is actually a long roadcut just northeast of Alexandria Bay, NY and just west of Cranberry Creek. The unconformity surface is near the base of this cut and there is a considerable thickness of Potsdam Sandstone overlying it. The contact itself is somewhat weathered, so it’s not the best place to put your finger on a gap in time of hundreds of millions of years, though that is what it represents. If you look carefully you’ll note there’s some paleotopography on this unconformity surface.


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Day #113 Deskcrop: Orange Mountain Basalt

The Great Falls of the Passaic River at Paterson, New Jersey cascade over the Jurassic Orange Mountain Basalt, the lowest of the basalt flows that make up the Watchung Mountains in the Triassic-Jurassic Newark Basin. The hydropower generated by these falls was important to the industrial development of the newly independent United States in the late 1700s and throughout the 1800s. Today’s deskcrop is from a low roadcut in the park just downstream of the falls. Its diabasic texture is nothing to write home about – it would be a lot more interesting to look at in thin section, but I’ve only got this hand sample for now. (I do think that some thin sections would make for a nice change of pace for the deskcrop series – sounds like a theme for next week!)

Orange Mountain Basalt

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Day #112 Deskcrop: Nepheline Syenite, Stettin Complex

I feel much more confident of the locality of today’s deskcrop. This is due in no small part to the fact that an ILSG field guide to this particular locality is online in PDF format. The sample is a piece of nepheline syenite from the southern edge of the ~1520 Ma Stettin Syenite complex, located not far to the northwest of the Wausau Syenite featured back in March. This sample is one of my favorites for teaching about differential weathering of minerals – the large crystals of nepheline (a variety of feldspathoid), abundant in this sample, weather much more rapidly than the feldspars, giving the weathered surface of the sample a distinctive pitted texture (best seen in the second photo below).

Nepheline Syenite, Stettin Complex

Nepheline Syenite – note the pitted nepheline crystals

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Day #111 Deskcrop: Mylonite, Eau Pleine Shear Zone

The Eau Pleine Shear Zone marks the boundary between the Archean Marshfield Terrane and the Proterozoic Wausau-Pembine Terrane in central Wisconsin. Today’s deskcrop is a mylonite from the Eau Pleine Shear Zone. I have only the vaguest recollection of the actual locality where this rock was collected, and it’s entirely possible I’ve badly mislocated the quarry from which it was retrieved, but the quarry near Dancy, WI where I’ve located the placemark is within the mapped boundaries of the EPSZ and seems plausible based on my weak recollection of the particular field excursion when I collected this deskcrop. I really need to spend a couple of weeks reacquainting myself with Wisconsin’s outcrops…

Mylonite, Eau Pleine Shear Zone

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