Raven, J.G.M.  <br/><br/>
Metamorphic fluids and transtension in the cantabrian mountains of northern Spain : An application of the conodont colour alteration index 

 - 673-681pp.  Illustration 
<br/><br/> Conodont colour alteration index (CAI) values from Upper Paleozoic rocks in the Cantabrian zone of northern Spain show that temperatures during Hercynian metamorphism locally exceeded 300 °C. Various temperature domains have been defined, which are generally separated by fundamental structures. These domains do not correspond with the tripartite subdivision based on stratigraphic analysis.<br/><br/>The observed CAI values of conodonts are in general agreement with the mineral paragenesis. Areas with high CAI values display extensive alteration and mineralization, and where CAI values exceed 4–4.5 (>200 °C) slaty cleavage has developed.<br/><br/>The Cantabrian zone is an area of very low grade metamorphism, where peak conditions were reached in Upper Carboniferous to Lower Permian times. The characteristics of the metamorphism and its spatial relationship with major faults suggest that fluids were the main source for regional heating and that fluid transport was focussed along crustal-scale structural features.<br/><br/>The overall deformation regime in this part of the Variscan orogen of western Europe is interpreted to be large-scale transtension. This is in agreement with earlier proposed models for the formation of Upper Palaeozoic basins in this area. 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Structural geology<br/>Variscan orogeny<br/>Iberian peninsula<br/>Cantabrian mountains - Spain<br/>Conodont color alteration index<br/>Regional metamorphism