The formation of garnet in olivine-bearing metagabbros from the Adirondacks
The formation of garnet in olivine-bearing metagabbros from the Adirondacks
复制标题
阿迪朗达克山脉含橄榄石变辉长岩中石榴石的形成
DOI：
10.1007/bf00371301
复制
发表时间：
1982
影响因子：
3.5
通讯作者：
E. Essene
中科院分区：
文献类型：
--
作者：
C. Johnson;E. Essene
对阿迪朗达克山脉中含橄榄石变辉长岩的区域研究，可以测试石榴石形成反应的P（压力）-T（温度）-X（成分）依赖性，并提供额外的区域变质压力数据。六个相，橄榄石，斜方辉石，单斜辉石，石榴石，斜长石和尖晶石，这可以相关的反应：斜方辉石+单斜辉石+尖晶石+钙长石=石榴石，镁橄榄石+钙长石=石榴石一起出现在冠状和equant结构指示平衡。各种矿物的组成通常为Fo 25 -72、En 44 -75、En 30 - 44 Fs 9 - 23 Wo 47 -49、Pp 13 - 42 Alm 39 - 63 Gr 16 -20、An 29 -49和Sp16-58。当它们出现在同一块岩石中时，赤道石榴石和日冕石榴石是同质的，成分相同，这表明尽管有日冕纹理，化学平衡可能已经达到。将简单CMAS系统中的反应外推到麻粒岩温度，并对固溶体进行热力学校正，得出橄榄石-斜长石-石榴石组合的平衡压力（使用Bohlen等人1980 b的测温法）范围从低地和南部阿迪朗达克山脉的约6.5 kb到高地的7.0-8.0 kb。这些结果与推断的阿迪朗达克峰变质条件一致（Valley和Bohlen 1979; Bohlen和Boettcher 1981）。因此，Whitney和McLelland（1973）和Whitney（1978）提出的橄榄石变辉长岩中日冕石榴石形成的等压逆行路径可能是不必要的。应用相同的平衡得到苏格兰的South Harris>8.7 kb，Nain Complex为0.9 kb。后一个值与斜方辉石橄榄石-石英气压计（Bohlen和Boettcher 1981）的不一致表明，使用富铁岩石（橄榄石| Fa 50）导致计算压力的错误。
A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pressure)-T(temperature)-X(composition) dependence of garnet-forming reactions as well as providing additional regional metamorphic pressure data. Six phases, olivine, orthopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene+clinopyroxene+spinel +anorthite=garnet, and forsterite+anorthite=garnet occur together both in coronal and in equant textures indicative of equilibrium. Compositions of the respective minerals are typically Fo25–72, En44–75, En30–44Fs9–23Wo47–49, Pp13–42Alm39–63Gr16–20, An29–49 and Sp16–58. When they occur in the same rock, equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures. Extrapolating reactions in the simple CMAS system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adirondacks to 7.0–8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be required. Application of the same equilibria gives >8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Complex. Disagreement of the latter value with orthopyroxeneolivine-quartz barometry (Bohlen and Boettcher 1981) suggests that the use of iron-rich rocks (olivines ≧Fa50) results in errors in calculated pressures.