In glaucophane al occurs in which side and a occurs in which side
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Blueschist-facies metamorphism is important in subduction zones, where high-pressure, relatively low-temperature mineral assemblages form. Glaucophane and lawsonite, both of which have a bluish color, are common minerals in this setting. In subduction zones, crustal fragments can be carried to great depths (>50 km) yet remain at rather low temperatures, usually less than 400° C (Fig. 2.10). A major unanswered question is how these rocks return to the surface. One possibility is by continual underplating of the accretionary prism with low-density sediments, resulting in fast, buoyant uplift during which high-density pieces of the slab are dragged to the surface (Cloos, 1993). Another possibility is that blueschists are thrust upward during later collisional tectonics.One of the most intriguing fields of research at present examines how far crustal fragments are subducted before returning to the surface. Discoveries of coesite (high-pressure silica phase) and diamond inclusions in pyroxenes and garnet from eclogites from high-pressure metamorphic rocks in eastern China record astounding pressures of 4.3 gigapascals (GPa, about 150-km burial depth) at 740° C (Schreyer, 1995). Several other localities have reported coesite-bearing assemblages recording pressures from 2.5 to 3.0 GPa. Also, several new, high-pressure hydrous minerals have been identified in these assemblages, indicating that some water is recycled into the mantle and that not all water is lost by dehydration to the mantle wedge. Perhaps the most exciting aspect of these findings is that for the first time we have direct evidence that crustal rocks (both felsic and mafic) can be recycled into the mantle.