Evidence of multiple intrusion, possible resetting of U--Pb ages, and new crystallization of zircons in the post-tectonic intrusions ('rapakivi granites') and gneisses from South Greenland
Evidence of multiple intrusion, possible resetting of U--Pb ages, and new crystallization of zircons in the post-tectonic intrusions ('rapakivi granites') and gneisses from South Greenland
复制标题
多次侵入的证据，U--Pb年龄可能重置，以及构造后侵入体（“拉帕基维花岗岩”）和格陵兰岛南部片麻岩中锆石的新结晶
DOI：
10.1016/0016-7037(75)90185-4
复制
发表时间：
1975
期刊：
影响因子：
--
通讯作者：
T. Krogh
中科院分区：
文献类型：
--
作者：
B.L Gulson;T. Krogh
来自南格陵兰Kap Farvel-Prins Christian Sund地区的构造后二长岩、正长岩和苏长岩以及围岩片麻岩和混合岩的锆石U-Pb数据表明，在1740百万年前有两个不同的侵入期。和公元1755年诺利特的年龄与较老的花岗岩相同。沿东南海岸向北沿着的类似侵入体具有相同的年龄（17.55亿年）。Kap Farvel-Prins Christian Sund地区的地球化学特征。我们的U-Pb年龄约为1亿年。K-Ar年龄比以前测定的年龄要老，区域性Ketilidian变质作用最年轻的高峰期约为1800百万年。基于混合岩的同构造花岗岩部分的七个磁性和尺寸分数的线性阵列。这个年龄明显不同于后构造侵入体，并且沿着其他数据排除了通过围岩重熔就地形成侵入体的可能性。在一个后构造侵入体附近采集的紫苏辉石混合岩样品中，发现了明显的过度生长和分离的低铀浓度的清晰颗粒。锆石的清晰颗粒与侵入体具有相同的年龄，表明在麻粒岩相重结晶过程中有新的锆石生长。与此相反，圆形的红色锆石从早期的亚整合花岗岩片有明确的富铀过度生长，可能形成于区域变质作用。如果在一个构造后侵入体中观察到的深红色锆石来自周围的变质岩，则它们的U-Pb系统被重置到侵入时间，与观测结果雅阁（Krogh，1971年）关于火山岩中的非磁性锆石部分，格陵兰岛锆石含有非常低量的普通铅，范围从后构造侵入体中最小磁性部分的<0.01ppm，到6.5 ppm的最磁性部分的片麻岩。这两个岩石群可以根据它们共同的铅含量来区分。
U-Pb data for zircons from post-tectonic monzonite, syenite and norite, and country rock gneiss and migmatite from the Kap Farvel-Prins Christian Sund area of South Greenland indicate two distinct intrusive episodes at ∼1740 m.y. and ∼1755 m.y. The norites have the same age as the older granitic rocks. Similar intrusions further north along the southeast coast have the same age (∼1755 m.y.) and geochemical character as those of the Kap Farvel-Prins Christian Sund area. Our U-Pb ages are about 100 m.y. older than previously determined K-Ar ages.The youngest peak of regional Ketilidian metamorphism in this area is placed at about 1800 m.y. based on a linear array of seven magnetic and size fractions from a syntectonic granitic part of a migmatite. This age is distinctly different from the post-tectonic intrusions and, along with other data, precludes the possibility ofin situformation of the intrusions by remelting of the country rocks.In one hypersthene migmatite sample collected near a post-tectonic intrusion, clear overgrowths and separate clear grains with low uranium concentrations were identified. The clear grains of zircon have the same age as the intrusion, indicative of new zircon growth during the granulite facies recrystallization of the gneiss. In contrast, rounded red zircons from an early subconcordant granitic sheet have clear uranium-rich overgrowths which probably formed during regional metamorphism. If the deep-red zircons observed in one of the post-tectonic intrusions were derived from the surrounding metamorphic rocks, they have had their U-Pb systems reset to the time of intrusion.In accord with observations (Krogh, 1971) on non-magnetic zircon fractions from volcanics, the Greenland zircons contain very low amounts of common lead ranging from <0.01 ppm in the least magnetic fractions in the post-tectonic intrusions, to 6.5 ppm in the most magnetic fractions of the gneisses. These two rock groups can be differentiated on the basis of their common lead content.