Rapid exhumation of lower crust during continent-continent collision and late extension: Evidence from 40Ar/39Ar incremental heating of hornblendes and muscovites, Caledonian orogen, western Norway
Rapid exhumation of lower crust during continent-continent collision and late extension: Evidence from 40Ar/39Ar incremental heating of hornblendes and muscovites, Caledonian orogen, western Norway
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大陆-大陆碰撞和晚期伸展过程中下地壳的快速折返：来自挪威西部喀里东造山带角闪石和白云母的 40Ar/39Ar 增量加热的证据
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发表时间：
1996
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通讯作者：
A. Halliday
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作者：
T. Boundy;E. Essene;C. Hall;H. Austrheim;A. Halliday
解决卑尔根弧区深部地壳岩石年龄关系的神秘方面，为挪威西部加里东造山带有争议的构造演化提供了关键的见解。在卑尔根弧区，下地壳Lindas推覆体（上板块）的榴辉岩在结构上覆盖在西部片麻岩区（下板块）含榴辉岩的基底片麻岩上，穿过卑尔根弧剪切带。激光40 Ar/ 39 Ar增量加热数据的角闪石和白云母的矿物分离，与详细的热压和结构观测，允许评价构造冷却和折返的历史，地壳深部岩石以及偏移和运动的历史跨越构造边界。在Lindas推覆体中，榴辉岩形成于1700 °C和>17 kbar的压力。同一榴辉岩样品中的角闪石和三个独立的白云母颗粒分别在447.7 ± 4.2、432.6 ± 0.5、429.2 ± 0.9和429.5 ± 0.6 Ma（此处和下文给出了2σ误差）处显示出明确的40 Ar/ 39 Ar同位素平台，表明高压岩石相对快速冷却（10-15 °C/m.y.）。白云母和其它榴辉岩的坪年龄分别为451.3 ± 1.1Ma和462.6 ± 0.8Ma，这可能与不明显的过剩氩有关。Lindas推覆体中的高级斜长角闪岩（温度为690 °C，压力为8-12 kbar）的年龄分别为439.3 ± 3.5和455.4 ± 1.5 Ma。与此相反，从邻近的西部片麻岩区（约670 °C，8 kbar）向东穿过卑尔根弧剪切带的片麻岩中的角闪石显示出409.3 ± 2.7和394.8 ± 2.2 Ma的高原年龄，与紧邻北部的西部片麻岩区的其他角闪石和白云母40 Ar/ 39 Ar年龄相似。在Lindas推覆体以西的Oygarden片麻岩杂岩的异地基底片麻岩（温度为670 °C，8 kbar），其坪年龄为408.1 ± 3.4 Ma。从所有的样品分析的温度估计以上的角闪石和白云母的封闭温度，从而表明，氩年龄反映冷却后的变质条件。Lindas推覆体高压榴辉岩的Ar年龄表明，在约50 km之前，地壳曾极度增厚（>50 km）。450毫安这些数据共同表明，最低大陆地壳的俯冲，可能与大陆-大陆碰撞有关，发生在早期（约200年）。450 ~ 430 Ma，导致了Lindas推覆体内下地壳榴辉岩和高级角闪岩的发育和折返。相比之下，西部片麻岩地区的高压榴辉岩，可能形成得很晚（约200年）。425 Ma），在造山带历史晚期（约425 Ma）与基底片麻岩一起被挖出并沿着冷却。400Ma）。40 M. Y。跨越卑尔根弧剪切带的偏移，将Lindas推覆体（上部板块）和西部片麻岩区域（下部板块）分开，与沿着断层的伸展运动一致，导致该区域在造山带构造演化后期（约1000年）快速折返和冷却。400Ma）。因此，深地壳岩石在上覆加里东推覆体在挪威西部似乎已经冷却和折返迅速有关的大陆-大陆碰撞在一个主要的收缩阶段早期的构造演化的造山带，而下地壳岩石的基础似乎已经折返非常迅速，与晚期造山后的伸展一致。在陆-陆碰撞造山带的构造演化过程中，深部地壳折返的机制在时间上和可能的构造上是不同的。
Resolving enigmatic aspects of the age relations of deep crustal rocks in the Bergen Arcs area provides key insights into the controversial tectonic evolution of the Caledonian orogen in western Norway. In the Bergen Arcs area, eclogites of the lower crustal Lindas nappe (upper plate) structurally overlie eclogite-bearing basement gneiss of the Western Gneiss region (lower plate) across the Bergen Arcs shear zone. Laser 40 Ar/ 39 Ar incremental heating data for hornblendes and muscovites on mineral separates, in tandem with detailed thermobarometry and structural observations, allow evaluation of the tectonic cooling and exhumation histories of the deep crustal rocks as well as offset and movement history across tectonic boundaries. In the Lindas nappe the eclogites formed at ≈700 °C and pressures >17 kbar. Hornblende and three separate muscovite grains from a single eclogite sample reveal well-defined 40 Ar/ 39 Ar isotope plateaus at 447.7 ± 4.2, 432.6 ± 0.5, 429.2 ± 0.9, and 429.5 ± 0.6 Ma (2σ errors given here and below), respectively, indicating relatively rapid cooling of the high-pressure rocks (10–15 °C/m.y.). Apparently older plateau ages from muscovites from other eclogites at 451.3 ± 1.1 and 462.6 ± 0.8 Ma may be related to indiscernible excess argon. High-grade amphibolites (≈690 °C, 8–12 kbar) of the Lindas nappe yield ages of 439.3 ± 3.5 and 455.4 ± 1.5 Ma. In contrast, hornblende from gneiss in the adjacent Western Gneiss region (≈670 °C, 8 kbar) to the east across the Bergen Arcs shear zone reveal plateau ages of 409.3 ± 2.7 and 394.8 ± 2.2 Ma, similar to other hornblende and muscovite 40 Ar/ 39 Ar dates in the Western Gneiss region immediately to the north. To the west of the Lindas nappe allochthonous basement gneiss of the Oygarden Gneiss complex (≈670 °C, 8 kbar) yields a plateau age of 408.1 ± 3.4 Ma. The temperature estimates from all the samples analyzed are above the closure temperatures for hornblende and muscovite and thus indicate that the argon ages reflect cooling after peak metamorphic conditions. The argon ages for the high-pressure eclogites of the Lindas nappe imply extreme crustal thickening (>50 km) prior to ca. 450 Ma. Together the data indicate that subduction of lowermost continental crust, possibly related to continent-continent collision, occurred early (ca. 450 Ma) in the tectonic evolution of the orogen, leading to the development and exhumation of the lower crustal eclogites and high-grade amphibolites within the Lindas nappe (between 450 and 430 Ma). In contrast, the high-pressure eclogites of the Western Gneiss region, which may have formed significantly later (ca. 425 Ma), were exhumed and cooled along with the basement gneiss late in the history of the orogen (ca. 400 Ma). The 40 m.y. offset across the Bergen Arcs shear zone, which separates the Lindas nappe (upper plate) and the Western Gneiss region (lower plate), is consistent with extensional movement along the fault resulting in the rapid exhumation and cooling of the region late in the tectonic evolution of the orogen (ca. 400 Ma). Thus, the deep crustal rocks in the overlying Caledonian nappes in western Norway appear to have been cooled and exhumed rapidly related to continent-continent collision during a predominantly contractional phase early in the tectonic evolution of the orogen, whereas lower crustal rocks of the underlying basement appear to have been exhumed extremely rapidly, coincident with late orogenic to postorogenic extension. Temporally and possibly tectonically distinct mechanisms are illustrated for the exhumation of deep crust within the tectonic evolution of a continent-continent collisional orogen.