The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risör, Norway
The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risör, Norway
复制标题
岩浆反应、扩散和退火在挪威 Risör 变云质辉长岩冠冕岩微结构演化中的作用
DOI：
10.1180/minmag.1986.050.357.08
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发表时间：
1986
影响因子：
2.7
通讯作者：
R. Joesten
中科院分区：
文献类型：
--
作者：
R. Joesten
挪威Risör滑石辉长岩中辛斜长岩[X Mg = 68, X Ca = 69, X Si = 68] +尖晶石[SP45]和正辉石[EN70]的冕状分离积云斜长石[AN64]和橄榄石[FO64]。初生微观结构为低角度晶界的日冕矿物组合层序为：斜长石：斜长石+尖晶石：正长辉石+尖晶石：正长辉石：橄榄石；退火微观结构的日冕矿物组合层序为：斜长石：斜长辉石+尖晶石：斜长辉石：橄榄石。原生共长石由扇形的寄生石颗粒组成，这些颗粒向斜长石方向开放，并由尖晶石杆形成棱纹。在退火到单一晶粒时，低角度晶界消失，尖晶石棒旋转到平行位置。原生正辉石，最初呈放射状到层状接触，退火成单一晶粒，边缘有橄榄石。正辉石+尖晶石的不规则不连续层和羽状体在退火过程中消失，在正辉石和角闪孔正辉石之间形成一单矿物的共生层。初生日冕中同相积云间辉长岩和正辉石组成的一致性，初生和退火日冕围绕积云钛铁矿的出现，以及分离积云橄榄石颗粒和分离积云橄榄石与斜长石的径向正辉石的出现，说明初生日冕是在大于5 kbar的压力下由饱和尖晶石的橄榄石岩浆分晶形成的。不可逆热力学计算表明，初生日冕的矿物组合层序是扩散不稳定的，这种不稳定性为其自发转变为退火日冕的稳定矿物组合层序提供了动力。
Abstract Coronas of symplectic pargasite [X Mg = 68, X Ca = 69, X Si = 68] + spinel [SP45] and orthopyroxene [EN70] separate cumulus plagioclase [AN64] and olivine [FO64] in troctolitic gabbro from Risör, Norway. Coronas with a primary microstructure characterized by low-angle grain boundaries have the mineral assemblage layer sequence, PLAGIOCLASE : PARGASITE + SPINEL : ORTHOPYROXENE + SPINEL : ORTHOPYROXENE : OLIVINE, whereas those with an annealed microstructure have the layer sequence, PLAGIOCLASE : PARGASITE + SPINEL : PARGASITE : ORTHOPYROXENE : OLIVINE. Primary symplectite consists of fan-shaped grains of pargasite that open toward plagioclase and are ribbed by rods of spinel. On annealing to a single grain, low-angle grain boundaries disappear and spinel rods rotate into parallel positions. Primary orthopyroxene, initially radial to layer contacts, anneals to a single grain, rimming olivine. Irregular discontinuous layers and plumes of symplectic orthopyroxene+spinel disappear on annealing and a monomineralic layer of pargasite forms between orthopyroxene and the amphibole symplectite. The identity of composition of intercumulus pargasite and orthopyroxene with the same phases in primary coronas, the occurrence of primary and annealed coronas rimming cumulus ilmenite and the occurrence of radial orthopyroxene separating grains of cumulus olivine and separating cumulus olivine and plagioclase argue for the origin of primary coronas by fractional crystallization of spinel-saturated troctolitic magma at a pressure greater than 5 kbar. Irreversible thermodynamic calculations show that the mineral assemblage layer sequence of the primary coronas is diffusionally unstable and that this instability provides the driving force for their spontaneous transformation to the stable mineral assemblage layer sequence of the annealed coronas.