敦煌地区变质作用演化及其大地构造意义

Exposed at the SE margin of the Tarim Basin, the Dunhuang Metamorphic Complex (DMC) is ascribed to be part of the crystalline basement of the Tarim Craton. Preliminary studies show that: (a) at least four episodes of early deformational events had occurred although their original orientations should be uncovered through careful investigations; (b) diverse metamorphic reaction textures and various mineral assemblages formed at different metamorphic stages are preserved in the garnet-bearing metapelites, garnet-bearing amphibolites and garnet-bearing mafic granulites; and (c) our preliminary geochronologic studies and geochronologic researches of other geologists imply that there occurred at least three episodes of geological events, i.e., ~1.83Ga, ~1.55Ga and ~0.43Ga. Thus the DMC is an appropriate terrane through which the tectono-metamorphic evolution of the basement of the Tarim Craton can be deciphered through metamorphic and chronologic studies, but hitherto such researches are still scare. In this program comprehensive investigations including metamorphic, chronologic, structural, thermodynamic and petrochemical studies should be implemented, in order to clarify the polymetamorphism, deformation, metamorphism-deformation relationships and metamorphic P-T-t paths, and thus to present detailed scientific data in discovering the tectono-metamorphic evolution of the crystalline basement of the Tarim Craton.

敦煌变质杂岩出露于塔里木盆地东南缘。传统上"敦煌地块"被认为(可能)是塔里木克拉通的结晶基底。初步研究表明：(1)该区早期可能经历了四期变形事件，其变形形迹的原始方位需深入研究才能恢复；(2)该区变质岩石种类复杂，其中含榴变泥质片麻岩、含榴斜长角闪片麻岩、石榴辉石麻粒岩中，大多保留了多个阶段的变质矿物组合，变质反应结构清楚；(3)申请者对变质锆石的U-Pb测年初步结果及前人的年代学研究表明，该区可能记录了约1.83Ga、约1.55Ga、约0.43Ga等区隔明显的三个阶段的热事件信息。该区是适宜于从变质地质学角度，深入探讨塔里木克拉通基底形成演化的理想地区。本项目拟通过实地地质调查，综合采用变质演化、热力学、年代学等多种研究手段，查明该地区不同时代变质地质体的空间分布、变质期次、变形期次、变质-变形关系、变质作用P-T-t轨迹，为塔里木克拉通基底的形成与演化研究，提供高质量的第一手科学资料。

传统上，敦煌地区被认为是形成于前寒武纪的古老稳定地块，本项目推翻了这个既往认识。本项目首次在敦煌地区发现了古生代变质的榴辉岩。根据我们目前的认识，敦煌造山带是一个南西－北东走向的造山带, 长约660 Km, 宽约100 Km。由于新生代断裂的错动, 敦煌造山带被肢解为一个个不连续的离散的构造块体, 形成今天所见的地垒－地堑组合。榴辉岩、高压基性麻粒岩、中压基性麻粒岩、含榴斜长角闪片麻岩等变质岩岩块, 呈构造布丁、构造透镜体的形式, 被夹持于变质泥质片岩/片麻岩、大理岩构成的基质中, 呈现构造混杂带中典型的“基质夹岩块(block-in-matrix)”特征。变质岩岩块中均保留有2-4个世代的变质矿物组合。榴辉岩岩块、高压基性麻粒岩岩块、中压基性麻粒岩岩块、含榴斜长角闪岩岩块的变质作用P-T-t轨迹全部为典型造山带的顺时针型，它们近于平行，但其变质高峰压力差异巨大。榴辉岩岩块、高压基性麻粒岩岩块、中压基性麻粒岩岩块、斜长角闪岩岩块的变质高峰P-T条件分别为~24.2 kbar / 830 ℃、~16.5–17.7 kbar / 735–860 ℃、~ 9.3–11.6 kbar / 750–850 ℃、~6.8–16.0 kbar／630–770 ℃。变质泥质岩基质变质高峰P-T条件为~6.0–10.7 kbar / 635–710 ℃。这说明，这些岩石形成于同一俯冲隧道，但俯冲深度差异较大，它们是在构造折返阶段才穿插、混杂到一起的。红柳峡构造块体、东巴兔－蘑菇台构造块体中，变质岩岩块退变质阶段的P-T-t轨迹为Western Alpine型，说明构造抬升速率相对较快。青石沟构造块体中，变质岩岩块退变质阶段的P-T-t轨迹为Franciscan型，说明构造抬升速率相对较慢。采用SIMS CAMECA方法，测得敦煌造山带不同构造块体的大量变质锆石U-Pb年龄介于430–365 Ma。常规40Ar/39Ar测年结果表明，变质矿物角闪石40Ar/39Ar年龄为407–397 Ma。据此可以肯定，敦煌地区并非前寒武纪期间形成的古老稳定地块，而是一个早志留世-晚泥盆世活动的古生代造山带。