Rock of the Week #10

1. What are the sedimentary structures in this rock?
2. What can they tell you about the depositional environment?
3. Why is this rock reddish in colour?

Solution to Rock of the Week #9:

This is a SKARN – that is, a contact metamorphic rock produced when magma intruded low-grade country rock. This sample comes from the Birch Creek Pluton in the White-Inyo Mountains of California. The older rock here was a Cambrian (530Ma) carbonate and shale marine platform sequence. The intrusion of granitic plutons (80Ma) transferred heat and Si-rich fluids into the sedimentary rocks, causing metasomatism and metamorphism. This was a moderate-sized pluton and the zone of contact metamorphism around it is about 600m thick. This sample was taken ~200m from the pluton where temperature reached about 500°C. It contains garnet, dolomite, calcite, fluorite, epidote, and quartz and is classified as a Ca-Fe-Si skarn.

1. (1 pt) 2 minerals?
: 1 point if 2 of the above listed minerals are given.

2. (1 pt) Rock name?
: 1 point for skarn (only).

3. (2 pt) How did it form?
: 1 point for contact metamorphism or metasomatism.

1 comment:

Lockwood said...

Looks like sandstone infilling dessication cracks in a mudstone. Environment would be drying mudflats near a pool, pond, lake, playa, etc. Though this structure doesn't necessarily imply an arid climate, it does imply a stretch of arid weather. I would guess the red color implies oxidation and hydration (thus prolonged atmospheric exposure) of iron rich minerals.