metamorphic belts: field examplescourses.washington.edu/ess212/lecture_files/2012... · the alps!...

Metamorphic belts: field examples!

Just a few more definitions…!

ü  Index minerals: distinctive minerals that characterize increasing grades of regional metamorphism!

ü  Isograd (“same grade”) : line on a map indicating the location where a particular mineral first appears or an existing mineral disappears from the metamorphic mineral assemblage!ü  Area in the field over which this distinctive assemblage

occurs is called a zone!

Orogenic Regional Metamorphism of the Scottish Highlands!

Barrow’s Area (Studied 1893- 1912)

Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

Barrowian Area!

Metamorphic zones!

P-T relations in progressive metamorphism of pelites!

A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district

Pelitic compositions are similar, but the sequence of isograds is: chlorite biotite cordierite andalusite sillimanite

The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite → sillimanite at the sillimanite isograd only above this pressure

Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

Figure 21-13. Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.

Coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt

Paired Metamorphic Belts

Change of protolith!

ü Mafic lithologies!

Fig. 25-3. Temperature-pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Examples of large orogenic belts!

ü Continental collisions!

The Alps!

Tectonics of the Mediterranean!

The Alps!

Himalayas!

Himalayas!

Dezes, U. of Lausanne

http://www.youtube.com/watch?v=loFxYSHxTf0&feature=related

North Cascades Geology

Washington Geology!

ü  Terrane accretion!

Terrane Accretion!

Washington Geology: last 40 Ma!

1.  Cascades continental volcanic arc begins (40-35 million years ago to present)!

2.  Columbia River basalts erupt (17-6 million years ago, mostly around 15 Ma) !

3.  Cordilleran glaciation (2 million years – 11,000 years ago) !

4.  Lake Missoula floods (13,000 years ago to present)!

North Cascades!

Formation of The Olympics!1.  About 50 Ma: Accretionary prism

material thrust underneath NA!2.  Large anticline / dome formed due to

subduction-related shortening!

Formation of Olympics!ü  Faulting & Folding related to subduction

zone!1.  About 50 Ma: Accretionary prism

material thrust underneath NA!

Prominent features of Cascades!ü Mt. Baker?!

ü less than 30,000 years old, most lava less than 10,000!

Prominent features of Cascades!ü Mt. Shuksan?!

ü ocean crust subducted about 120 Ma, then uplifted during terrane accretion!

Prominent features of Cascades!ü Yellow Aster Meadows (N Nooksack

river)?!ü piece of metamorphosed continental

crust caught up during terrane accretion!

Go rest…!

Almost…!

And welcome to the Real world!!!