Saturday, January 28, 2012

Ophiolites exploration, Mount Diablo, California, USA.

This post is about my short field trip to Mt Diablo that is located in Contra Costa Co., California , in San Francisco Bay Area (view on wikimapia). Mount Diablo is the part of The Diablo Range (Devil Range) which is a mountain range in the California Coast Ranges subdivision of the Pacific Coast Ranges.
The geology of this area is significant and a you can find a lot of information in the web.The simplified geological map of Mt Diablo State Park area is show on Fig. 1.
I was most interested in ophiolite part of structure and particularly in glaucophane schists that could be observed here.
Ophiolite complex includes deep water sediments, pillow lavas, parallel diabase dikes layer, gabbros and serpentinized peridotite blocks.
Fig. 1. Simplified geological map of Mt Diablo State park. Taken from
Fig. 2. The process of ophiolite and Franciscan Complex formation. Taken from

Franciscan complex actually is a tectonic melange and consist of may different rock types such as mafic rocks and deep-sea radiolarian cherts (ophiolite formation), greywacke sandstones, limestones, shales, and high-pressure metamorphic rocks, all of them faulted and mixed in a seemingly chaotic manner.

28.01.12  Mt Diablo State Park, Clayton, CA

The most abundant rocks here were gabrro and diabase, rarely pillow lavas. Evidently it was part of ophiolite formation.
Fig 3. Diabase quarry.
Fig 4. Typical diabase rock.
Fig 5. Basalt lava pillow.

 Several minerals of low grade metamorphism or postmagmatic processes could be found here:
Fig 6. Calcite in basalt amygdules.
Zeolite-like mineral was found (Fig. 7). X-ray analys shows stilbite structure.

Fig 7. Stilbite.
February 12th. Mt Diablo State Park, Walnut Creek, CA

Fig 8. The road in Mt Diablo State Park
Fig 9. The view form Mt Diablo peak. Diabase quarry.
Fig 10. The boulder of blueschist rock.
Fig 11. Blue glaucophane schist.

Fig 12. Piece of blueschist rock. Sample is about 5cm.

Fig 13. Folded red radiolarian chert.


Brocker, M.,  Day, H. W. "Low-grade blueschist facies metamorphism of metagraywackes, Franciscan Complex, Northen California" J. Metamorphic Geol. 1995

Erickson, R. C. "The Geology of the Franciscan Complex in the Ward Creek-Cazadero Area, Sonoma County, Californa" California Geol. 1995

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