An Integrated Investigation of Ultrahigh-Pressure (?) Metamorphism, North Qaidam, China

地质学家最近的一项发现是，大陆地壳的埋藏深度可达90公里。这是一个相当惊人的发现，因为大陆地壳通常约30-35公里厚，要将其埋在90公里以下的深度，就需要大陆地壳被更致密和流动的上地幔岩石所包围。这在物理学上是相当困难的，因为它需要将较轻的大陆地壳拖入较致密的地幔物质中。究竟是什么力量使这一不寻常的过程成为可能仍然是一个活跃的辩论。来自加州大学的一组地质学家目前正在调查这个问题。由于有各种可能的过程可能导致大陆岩石下降到很深的地方，因此在地球上尽可能多的地方检查具有如此深埋藏记录的岩石是很重要的。地质学家尽可能多地研究深埋岩石记录的另一个原因是，个别地区可能没有保存完整的埋藏过程记录。因此，地质学家作为侦探，必须用尽可能多的案例来拼凑完整的历史。 目前正在调查的研究区位于青藏高原北方的柴达木盆地北部。这是一个沙漠地区，岩石暴露得很好。他们的研究包括以下内容。(1)绘制详细的岩石地质图，记录深埋过程。地质图允许将自然发生的地质元素置于严格的空间背景中。因此，可以系统地检查不同类型岩石之间的几何关系。(2)取样和岩石学分析。目前正在使用各种最先进的技术和仪器分析从现场收集的许多样品的矿物和化学成分。这种分析确定了岩石被埋的确切深度以及与岩石埋置相关的温度历史。(3)岩石埋藏的年龄、持续时间和速率的测定。目前正在利用离子探针和惰性气体分析进行这项工作。这次调查的主要目的是确定岩石埋藏的速度以及将岩石带回地球表面需要多长时间。该研究项目的最终完成将使地质学家能够确定导致轻质大陆地壳向下俯冲进入更致密的上地幔的力量，这反过来将揭示地球地壳形成和演化的物理和化学的基本方面，这是人类日常生活的核心。

A recent discovery by geologists is that continental crust could be buried as deep as 90 km. This is a rather astonishing revelation because continental crust is typically about 30-35 km thick, and to have it buried below a depth of 90 km would require the continental crust to be surrounded by denser and fluid upper mantle rocks. This is rather difficult in physics because it requires the lighter continental crust to be dragged into the denser mantle materials. Exactly what forces have made the occurrence of this unusual process possible remains an active debate. A team of geologists from the University of California is currently investigating this problem. Because there are a variety of likely processes that may have induced continental rocks to decent to great depths, it is important to examine the rocks that bear the record of such deep burial in as many places on earth as possible. Another reason for geologists to examine as many cases of rock records of deep burial is that individual areas may not preserve a complete record of the burial process. Therefore, geologists as detectives, must use as many cases as possible to piece the complete history together. The study area that is currently under investigation is located in North Qaidam of the northern Tibetan plateau. This is a desert region exhibiting excellent exposure of rocks. Their research includes the following components. (1) Making a detailed geologic map of rocks that record deep-burial processes. The geologic map allows the naturally occurred geologic elements to be placed into a rigorous spatial context. As a result, the geometrical relationships among the different types of rocks can be systematically examined. (2) Sampling and petrologic analysis. Numerous samples collected from the field area are currently being analyzed for their mineralogical and chemical compositions using a variety of the state-of-the-art techniques and instruments. This analysis determines the exact depth that the rocks were buried and the temperature history associated with the rock burial. (3) Determination of age, duration, and rate of rock burial. This task is being carried out using the ion-microprobe and noble gas analyses. The main purpose of this investigation is to determine how fast the rock burial took place and how long it has taken to bring the rocks back to Earth's surface. The final completion of this research project will allow geologists to determine the forces that caused the down plunging of the light continental crust into the denser upper mantle, which in turn will reveal the fundamental aspects of physics and chemistry for the formation and evolution of Earth's crust that is central to Human's day-to-day livelihood.