Boy, would it be wonderful if there was a geologist here for each geologic topic that came up. There have been a lot lately.
The degree of metamorphism seen in the Vishnu and other schists is quite pervasive over long distances and very little of it is the result of igneous intrusions as far as I can tell. The reason for this conclusion is that the degree of metamorphism, which includes both deformation fabric/structure and mineralogy, is present throughout the entire body of schist regardless of the presence of intrusive bodies.
Contact metamorphism is a point source for the heat and so heat is radiated out from the dike/intrusion through the rock. This sort of thermal conductivity results in minralogic and textural changes in the country rock (rock being intruded), which, depending on it's depth in the crust, can be cold, warm, or hot. The higher in the crust, the colder the country rock, the more water it typically has, and (I think) the more pronounced the reaction of the country rock to the intrusion. The presence of water or other fluids is easily detected by the resultant metamorphic minerals and this water can come from the country rock or the intrusion. We can tell the difference using isotopic signatures.
So in contact metamorphic zones, the highest degree of alteration is typically located immediately adjacent to the intrusion (again especially if the rock is cold) and the alteration becomes progressively less pronounced the further away from the heat source we move.
Because I don't see such a pronounced zoning (an alteration halo) around each intrusion, and the degree to which the Vishnu Schist is deformed, the metamorphism event that formed the schists in the Grand Canyon area are not primarily the result of contact-metamorphism.
That doesn't mean there is not any contact metamorphism present in the schists, there very likely is, and this would probably result in increasing the grade of metamorphism in the vicinity of the intrusions. But again, it would be like a halo radiating away from the intrusive bodies unless there was a large concentration of intrusive bodies in one location. The resulting minerals found in a contact metamorphic halo would be different that what is found in the rest of the schist simply because by intruding the schist with an igneous body, you are introducing new fluids and minerals (in other words, new elements/chemistry) into the system.
From what I've read about these intrusions in the paper I linked to on the first page (Ilg et al, 1996), there are two separate types of intrusions that occurred at different times and are not genetically related to each other.
One is an earlier and more mafic group of gabbroic to granoditoritic intrusions probably related to arc magmatism, which may have been the source of the volcanic material (pillow basalts, volcanic seds, flows, etc.) erupted by the island arc. The mafic bodies are highly deformed, though not too alteres as to obliterate primary mineralogy and structure apparently. But the fact that they are deformed suggests they preceded the major deformation event and were deformed during the same event that created the schist.
The other group of intrusives is a later granitic phase that may have been the result of partial melting of the schist or sourced by other processes, but these appear to have intruded the schist at the same time the schist was forming. This is based on various things such as lack of deformation and how dikes tend to follow folds and fraacture/jointing patterns as the schist later cooled and hardened. Basically, they took advantage of incipient weaknesses in the schist while it was forming and after it started cooling.
This message has been edited by roxrkool, 03-15-2006 03:00 PM
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