COLLABORATIVE RESEARCH: Formation and Exhumation of UHP and HP Terranes in the Greenland Caledonides - Understanding Processes at the Base of Continental Collisions

格陵兰和斯堪的纳维亚喀里多尼德都暴露出巨大的大陆地壳板块，这些板块在劳伦西亚和波罗的海碰撞期间经历了高压变质作用。在过去的15年里，人们已经在挪威西部发现了一个超高压（UHP）地层，而几个月前，人们刚刚在格陵兰岛发现了超高压变质作用的第一个证据。总的来说，这两个连贯的大陆榴辉岩地体构成了地球上最广泛的深而古老的山根，给了我们一个山带解剖学的视图，通常隐藏在像喜马拉雅山这样的活跃造山带的底部。该项目的最终目标是了解这种厚或深的造山根是如何形成的，以及它是如何被带回地球表面的。超厚地壳的伸展塌陷是一种经常被引用的挖掘机制，其中高压和超高压岩石沿着表状正断层无顶；然而，在根兰榴辉岩省未发现主要的伸展构造。Gilotti早期由美国国家科学基金会支持的研究发现，峰值压力大约在410至370 Ma之前达到；而在榴辉岩省以南的峡湾地区，工作人员发现了同步熔融、浅花岗岩生成和10-70 Ma的伸展断裂的广泛证据。要么在南部观测到的伸展分离与榴辉岩的挖掘无关，要么PI之前的年龄记录的是挖掘路径末端的冷却而不是峰值压力条件。本研究的主要目标是：1)确定超高压变质作用的范围和时间；2)通过比较榴辉岩荚果逆行边缘的锆石种群与保存完好的荚果中心的锆石种群，检验前人对HP变质作用年龄的稳健性；3)建立这块含榴辉岩的地壳板块回到地球表面时的压力-温度-时间历史；4)确定可能成为挖掘构造的收缩和走滑构造的几何形状和时间。在这次重要的大陆-大陆碰撞中，PI的数据集对于制定榴辉岩形成和挖掘的造山带范围模型至关重要。虽然挪威正在进行必要的研究，但如果没有对格陵兰的高压和超高压变质作用的相应知识，就无法将其结果置于背景中。初步结果也有可能为高压/超高压变质作用和随后的减压过程中U-Pb同位素系统的行为提供新的见解。

Both the Greenland and Scandinavian Caledonides expose huge slabs of continental crust that have experienced high-pressure (HP) metamorphism during the collision between Laurentia and Baltica. An ultrahigh-pressure (UHP) terrane has been recognized in western Norway for the past fifteen years, while the first evidence for UHP metamorphism has just been found in Greenland a few months ago. Taken together, these two coherent continental eclogite terranes comprise the most extensive exposure of a deep, ancient mountain root on the planet - giving us a view of mountain belt anatomy that is generally hidden at the base of active orogens like the Himalayas. The ultimate goal of this project is to understand how this thick or deep orogenic root formed, and how it was brought back up to the earth's surface. Extensional collapse of overthickened crust, whereby HP and UHP rocks are unroofed along listric normal faults, is an oft-cited mechanism of exhumation; however, major extensional structures are not exposed in the Geenland eclogite province. Gilotti's earlier NSF supported research found that the peak pessure was attained approximately 410 to 370 Ma ago; whereas, workers in the Fjord Region, to the south of the eclogite province, are finding widespread evidence of synchronous melting, leucogranite generation and extensional faulting that is 10-70 Ma older. Either the observed extensional detachments in the south have nothing to do with eclogite exhumation, or the PI's previous ages are recording cooling at the end of the exhumation path instead of peak pressure conditions. The main objectives of the proposed research are: 1) to identify the extent and timing of UHP metamorphism; 2) to test the robustness of our previous ages for HP metamorphism by comparing zircon populations from the retrograde margins of eclogite pods to zircon populations from the well-preserved centers; 3) to establish the pressure-temperature-time history of this slab of eclogite-bearing crust as it returns to the earth's surface; and 4) to determine the geometry and timing of contractional and strike-slip fabrics that might be possible exhumation structures. The PI's data set is essential for formulating an orogen-wide model of eclogite formation and exhumation in this important continent-continent collision. Although the necessary studies are underway in Norway, their results cannot be put into context without the corresponding knowledge of HP and UHP metamorphism in Greenland. The preliminary results also have the potential to provide new insight on the behavior of U-Pb isotope systems during HP/UHP metamorphism and subsequent decompression.