Guatemala, Jade and Tectonics
Monday, 3. August 2009, 13:08:53
Jade is a cultural term for two rare metamorphic rocks known as jadeitite and nephrite that are both extremely tough and have been used as tools and talismans throughout the world. Jadeitite is rare, with less than 10 identified deposits worldwide. The jadeitite (or jadeite jade) is a sort of scar tissue from some collisions between Earth's plates. As ocean crust is pushed under another block, or subducted, pressure increases with only modest rise in temperature, squeezing and drying the rocks without melting them. Jade precipitates from fluids flowing up the subduction channel and into the chilled, overlying mantle that becomes serpentinite. The serpentinite assemblage, which includes jade and has a relatively low density, can be uplifted during subsequent continental collisions and extruded along the band of the collision boundary, such as those found in the Alps, California, Iran, Russia, and other parts of the world.
Jadeite rocks (jadeitites) were discovered of in Guatemala in 1954, and the discovery led to the recognition of the Motagua River Valley as a Maya jade source. Guatemala is second only to Myanmar (Burma) as a modern jadeite jade source and is the most important archaeological source.
A new analysis of jade found along the Motagua fault, where the North American Plate and the Carribean Plate slide past each other underline the fact that this region has a more complex geologic history than previously thought. Because jade and other associated metamorphic rocks are found on both sides of the fault, and because the jade to the north is younger by about 60 million years, a team of geologists argue in a new research paper, published in Earth and Planetary Science Letters, that the North American and Caribbean plates have done more than simply slide past each other: they have also collided - not only once, but twice.
This is not the first paper on Guatemalean jade combined with Guatemalean tectonics. In an earlier paper, the authors found evidence of two different collisions by dating mica found in collisional rocks (including jade) from the North American side of the fault to about 70 million years ago and from the southern side (or the Caribbean plate) to between 120 and 130 million years ago. But mica dates can be "reset" by subsequent heating (See my post on fission track dating). Now, the authors have turned to eclogite, a metamorphic rock that forms from ocean floor basalt in the subduction channel. Eclogite dates are rarely reset, and the authors found that eclogite from both sides of the Motagua dates to roughly 130 million years old.
A possible scenario is that a collision 130 million years ago created a serpentinite belt that was subsequently sliced into segments. Then, after plate movement changed direction about 100 million years ago, a second collision between one of these slices and the North American plate reset the mica clocks in jadeitite found on the northern side of the fault to 70 million years. Finally, plate motion in the last 70 million years juxtaposed the southern serpentinites with the northern serpentinites, which explains why there are collisional remnants on both sides of the Motagua.
Jadeite rocks (jadeitites) were discovered of in Guatemala in 1954, and the discovery led to the recognition of the Motagua River Valley as a Maya jade source. Guatemala is second only to Myanmar (Burma) as a modern jadeite jade source and is the most important archaeological source.
A new analysis of jade found along the Motagua fault, where the North American Plate and the Carribean Plate slide past each other underline the fact that this region has a more complex geologic history than previously thought. Because jade and other associated metamorphic rocks are found on both sides of the fault, and because the jade to the north is younger by about 60 million years, a team of geologists argue in a new research paper, published in Earth and Planetary Science Letters, that the North American and Caribbean plates have done more than simply slide past each other: they have also collided - not only once, but twice.This is not the first paper on Guatemalean jade combined with Guatemalean tectonics. In an earlier paper, the authors found evidence of two different collisions by dating mica found in collisional rocks (including jade) from the North American side of the fault to about 70 million years ago and from the southern side (or the Caribbean plate) to between 120 and 130 million years ago. But mica dates can be "reset" by subsequent heating (See my post on fission track dating). Now, the authors have turned to eclogite, a metamorphic rock that forms from ocean floor basalt in the subduction channel. Eclogite dates are rarely reset, and the authors found that eclogite from both sides of the Motagua dates to roughly 130 million years old.
A possible scenario is that a collision 130 million years ago created a serpentinite belt that was subsequently sliced into segments. Then, after plate movement changed direction about 100 million years ago, a second collision between one of these slices and the North American plate reset the mica clocks in jadeitite found on the northern side of the fault to 70 million years. Finally, plate motion in the last 70 million years juxtaposed the southern serpentinites with the northern serpentinites, which explains why there are collisional remnants on both sides of the Motagua.
-
http://www.eurekalert.org/pub_releases/2009-07/amon-jsl072709.php
-
http://www.physorg.com/news167936004.html
-
http://www.scienceblog.com/cms/jade-sheds-light-guatemalas-geologic-history-23427.html
-
http://www.amnh.org/science/papers/jade_guatemala.php
-
http://my.opera.com/nielsol/blog/2009/04/24/north-and-south-islands-new-names
-
http://research.amnh.org/eps/staff/georgeharlow/jade
WIDGETIZE