High-temperature granulites
High-temperature granulites
复制标题
高温麻粒岩
作者：
S. Harley
先生-Hayob等人。1论证了玄武岩岩浆底侵作用在下地壳形成年轻的高温麻粒岩相变质岩。尽管我们不否认现代或现代玄武岩底侵和麻粒岩相变质作用的可能性，但我们认为这些作者未能证明它的存在。S等人的争论围绕着他们在麻粒岩相包体中发现了高温出溶的长石成分，这些包体是在墨西哥中部的第四纪火山作用下与玄武质凝灰岩一起喷发的。这些长石(中钙长石)形成现有片层和宿主成分的温度估计为800-950℃，形成原始长石的综合成分的温度估计为950-1,125℃。Hayob等人。声称这些温度估计开创了一系列先例，并做出了几个误导性的陈述，例如：中玄武岩产生的最低重新整合温度(950-1,125 AQ)远远高于通常从暴露的麻粒岩中获得的温度(第266页)。这些中玄武岩提供了T>；950-1,125℃的区域变质作用的证据，高于以前记录的任何区域变质岩石的温度(P.267)。但至少有两个众所周知的地体，其变质条件和中玄武岩的条件与Hayob等人所描述的相似。有记载的有：苏格兰的刘易斯杂岩和南极洲的纳皮尔杂岩；在这两个例子中，相关的岩石都暴露在地表，形成了区域太古代麻粒岩相地体的一部分。
SIR-Hayob et al. 1 claim to demonstrate the formation of young high-temperature granulite-facies metamorphic rocks in the lower crust as a result of underplating by basaltic magma. Although we would not deny the possibility of recent or presentday basaltic underplating and granulitefacies metamorphism, we feel that these authors have failed to demonstrate its occurrence. Hayob et al.'s arguments revolve around their finding of hightemperature exsolved feldspar compositions in granulite-facies xenoliths, which have been erupted with basanitic tuffs by Quaternary volcanism in central Mexico. These feldspars (mesoperthites) yield temperature estimates of 800-950 oc for formation of the existing lamellae and host compositions, and temperature estimates of 950-1,125 oc for formation of the integrated compositions of the original feldspars. Hayob et al. claim that these temperature estimates set a series of precedents, and make several misleading statements, such as: mesoperthites yield minimum reintegrated temperatures (950-1,125 aq that are considerably higher than those typically obtained from exposed granulites (p. 266). The mesoperthites provide evidence for regional metamorphism at T> 950-1,125 C, higher than any temperatures recorded previously in regionally metamorphosed rocks of undoubted sedimentary parentage (p. 267).But there are at least two well-known terrains where both the conditions of metamorphism and mesoperthites similar to those described by Hayob et al. have been documented: the Lewisian complex of Scotland and the Napier complex of Antarctica; in both cases the relevant rocks are exposed on the surface, forming parts of regional granulite-facies terrains of Archaean age.