SMVC: Sheet Dikes EarthCache

Smartville Complex: Sheet Dikes

Notes on this cache:

• This road cut was extensively studied as part of the Masters Thesis (Day 1977). 
• The best parking is on the west side of the road just north of the road cut (south bound side).  If you heading northbound, turn right on Collins Lake Rd, make a safe U-turn, and then you can turn left head southbound, and safely pull into the shoulder. 

 The Smartville Complex is a geologic terrane (set of related rocks) formed in the ocean from a volcanic island arc that was accreted onto the North American Plate during the late Jurassic (~160–150 million years ago, Jurassic age). The collision created sufficient crustal heating to drive mineral-laden water up through numerous fissures along the contact zone. When these cooled, among the precipitating minerals including gold and copper (creating deposits in our gold country). Associated with the Western Metamorphic Belt of the Sierra Nevada foothills it extends from the central Sierra Nevada mountain range, due west, under a section of the Central Valley and California Coast Ranges, in northern California. The ophiolitic sequence found in this terrene is one of several major ophiolites found in California. Ophiolites are crustal and upper-mantle rocks from the ocean floor that have been moved on land.

To think of an example in current terms: 

Phase #1:   Think of modern Japan as a set of island.  Ocean crust was subducting (sliding under) other crust where two oceanic plates are converging.  As the descending plate melts in the in the mantle, blobs of magma buoyantly migrate upwards.  As the magma erupts it builds an island arc. 

Phase #2:  The oceanic-oceanic crust convergence seizes up.  The plates break at the continental margin, then convergence continues until the island arc “crashes” into the continent.  It’d be like Japan crashing into the Korean and Chinese costs.  This island arc “accretes” and gets lifted up.  That is what the Smartville Complex is, portions of an accreted island arc.

Terms:

Basalt is a dark, fine-grained volcanic rock that sometimes displays a columnar structure. It is typically composed largely of plagioclase with pyroxene  and olivine.

Dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies.  Sheet Dikes are when a series of dikes form usually in a cross cutting relationship, that is the younger dikes cut through older earlier formed dikes.

Gabbro is a dark, coarse-grained plutonic rock of crystalline texture, consisting mainly of pyroxene, plagioclase feldspar, and often olivine.

Greenschist Facies, lets break that down.  Greenschist is a metamorphic rock that has undergone low-grade metamorphosis.  Greenschist have undergone a typical temperature-pressure regime during metamorphosis.  During metamorphosis a range of secondary minerals, but notably epidote and chlorite which give the green hue to greenschists.  Although there are some mineralogical changes and foliation may be introduced, typically greenschists retain the morphology of the original rocks, whether igneous or sedimentary origin.  Facies is a term used in geology to describe a group of related rocks origin, whether they be sedimentary, igneous, or metamorphic. 

Pillow Basalts are mounds of elongate lava pillows formed by repeated oozing and quenching of hot basalt in a submarine environment.  The lave flows out like oozing toothpaste as it is quenched by the sea water, there are great YouTube videos you can look up.

Plutonic Rock is formed with coarse grained minerals from slowly cooling magma at depth

Outcrop

At an oceanic spreading center you end up with a very distinct set of rocks.  As the two plates move apart magma upwells at the spreading center and extrudes onto the surface of the ocean. 

• At the ocean floor, the lava extrudes, often in pillows basalts.  These extrusive basalts cool rapidly often having only very fine crystals (often carried up for depth), or may even be glassy (with no visible minerals). 
• In the middle near surface the magma when it cools forms sheet dikes.  These sheet dikes don’t cool quite as quickly, giving time for small crystals to form.
• At depth the magma can cool very slowly (thousands to millions of years) allowing large crystals to form giving them a granite-like texture with large crystals.  These iron rich granite-like-textured rocks that have a high iron content and dark minerals are referred to as a gabbro.

• Because of the subduction these dikes experience low-grade metamorphosis to the greenschist facies. Although the rocks are pretty dark because of the relatively high iron content, you may see an imprint of green associated with the greenschist facies minerals epidote and chlorite.  There are other greenschist minerals, but they do not contribute to the green hue.

• Oceanic crustal rocks (like at this location) typically relative high iron contents and relative less silica than crustal rocks.  The iron and silica control the darkness of the rocks and the mineral assemblage.

At this outcrop the sheet dikes at this location formed over a series of eruptive events and vary from low, medium, and high iron contents.  There are a whole series of eruptive events with cross cutting dikes that are too complicated to discuss here and actually quite difficult to see here.   To get credit for this cache, please look at the broken faces of the rocks.

As with all EarthCaches you will need to send the Cache Owner (me) some answers to the questions below.  Go ahead and log you cache immediately, but send me the answers in a timely manner, or I’ll be forced to delete your log.  The questions below are a bit tougher, hence the higher D rating, but I do expect answers to all questions.  Just do your best.  I honor all good faith attempts regardless of the accuracy of the answer.  If you just try, I’ll give you credit.  I’m easy that way

1. Name of the EC and the GC code
2. These dikes were described as having undergone metamorphosis to a greenschist facies, from the information above:   What two minerals contribute to the green hue in greenschists?
3. Look at 3 different exposures a few 10s of feet apart to be sure you’re looking a different dikes.  Each dike is only a few meters wide. 
   1. Describe color of the matrix rock at each of your 3 locations
   2. Describe the fine minerals, size, color, and shape.

References:

Day, S.D. 1977. The petrology of a mafic dike complex near Smartville, Yuba County, California.  [M.S. Thesis]; University of California, Davis.

Deierdorfer, R.E. and Day H.D (1992) Metamorphic Features of the Smartville Complex, Northern Sierra Nevada, California  (see stop 4 page 44) In Field Guide to Geology and Metamorphism of the Franciscan Complex and Wester Metamorphic Belt of Northern California by Schiffman, P and Wagner D.L..  California Divisions of Mines and Geology.  Special Publication 114.