Hi Denise

I hope the old saying "Better late than never" applies...

Blueschists will probably trip me up (not something found around here) but I think I can shed some light on greenstone belts and ophilites.

"Greenstone" is a sort of generic field term for any "compact dark-green ... metamorphosed basic igneous rock that owes its color to chlorite, actinolite, or epidote" (from the Dictionary of Geological Terms.) They are frequently found in long, relatively narrow, bands of folded rocks in mountain ranges - hence the use of the term "belt." The term "basic" referes to the chemistry - not the ABCs. As opposed to acid. Essentially take any basaltic rocks, or rocks derived from the erosion of basaltic rocks and deposition of the sediments, and put them through low to medium grade metamorphic processes, and you end up with a "greenstone." There are actually quite a few different rock types - which can be determined by close examination or lab work - which fall under this generic term. They range in grade from slates to schists.

An "ophilite" is not a single rock type, but rather an "assemblage of mafic to ultramafic igneous rocks ... whose origin is associated with an early phase of the development of a geosyncline" (also from the DGT.) That one's a little mis-leading, the bit about early development of a geosyncline. It's true, but it doesn't really tell you the whole story. They are essentially suites of ocean floor rocks - volcanic and sedimentary - that have been bulldozed up and "welded" to the edge of a continent by plate collision - sea-floor to continent. There is also at least some low grade metamorphism involved - and there can be medium to high grade as well. (I've seen a gneiss-grade ophilitic assemblage in western Mass. That's pretty high.)

"Blueshist" is another broad term, not a specific rock type. They are high temperature, low pressure, metamorphic rocks. (Oddly enough, my copy of the DGT doesn't include it...) The characteristic blue-grey color associated with them is due to glaucophane - a blue Amphibole Group species. [Note that in the "mainstream" metamorphic sequence the temperatures involved produce chlorite and actinolite - green Amphibole Group minerals; while under high-pressure, low temperature conditions it's glaucophane that results.]

Essentially all three terms are from metamorphic geology and refer to broad groups of rocks - "shorthand" used by geologists to semi-define them when in the field.

Hope that all clarified rather than confused... :~}

KOQ(uestioning)

Al Plante

From Alan Plante - May 06, 2002 at 20:43:48
Email: sheral[ ]ncia.net