敦煌造山带造山过程及其大地构造意义

The Dunhuang area is traditionally considered as an ancient stable terrane formed in the Precambrian and its tectonic attribution is in great debate. Our recent studies show that this area was in fact a Paleozoic orogenic belt and the metamorphic event occurred between the Silurian to Devonian. The Dunhuang Orogenic Belt was dismembered by Cenozoic faults to discrete tectonic blocks formed at different metamorphic grades. Mafic granulite and amphibolite puddingstones are intercalated with the metapelite, marble and quartzite matrices which typify tectonic mélange. For the first time, we have found eclogite puddingstone in the Hongliuxia block. Remarkable differences of metamorphic peak pressure conditions of the Hongliuxia and Qingshigou blocks indicate the mixing of metamorphic rocks from significantly different depths to create a tectonic mélange in a subduction channel, possibly juxtaposed during the uplift stage. Metamorphic evolution and geochronology of the other blocks remain unknown. Therefore, we intend to resolve the following problems based on detailed field investigation, large-scale geological mapping and various high resolution experiments / measurements: (1) if other tectonic blocks are mélanges; (2) differences of metamorphic P-T-t evolution of the different tectonic blocks; (3) orogenic duration of the Dunhuang Orogenic Belt; (4) polarity of subduction of the Dunhuang Orogenic Belt; and (5) tectonic attribution of the Dunhuang Orogenic Belt.

传统上认为敦煌地区是形成于前寒武纪的古老稳定地块，其大地构造归属还存在争议。我们近年来的研究表明，敦煌地区是一个形成于古生代的造山带，其变质作用时段为志留纪-泥盆纪。敦煌造山带已被新生代断裂错断为变质程度不等的一个个离散的构造块体。基性麻粒岩、斜长角闪岩等常呈布丁或透镜体的形式，被夹持于变质泥质片岩/片麻岩、大理岩、变质砂岩等构成的基质中，呈现典型的构造混杂岩特征。我们在敦煌造山带首次发现了榴辉岩，发现红柳峡、青石沟地区不同变质岩块的变质高峰期压力都存在巨大差异。但是，其他众多构造块体的情况如何，尚需深入研究。本项目拟在详细的野外地质调查、大比例尺地质填图、各种高质量实验测试的基础上，尝试回答如下几个重要的科学问题：(1)其它构造块体是否也是构造混杂带；(2)不同构造块体变质作用P-T-t演化史的差异；(3)敦煌造山带的造山时限；(4)敦煌造山带的俯冲极性；(5)敦煌造山带的大地构造归属。

敦煌及若羌、肃北、瓜州、玉门等地，长期被误认为是前寒武纪期间就已稳定的古老地块(“敦煌地块”)。十年来，人们发现这些地区岩石普遍于古生代期间发生了变质改造，造山带变质演化特征显著，我们将之命名为“古生代敦煌造山带”。本项目进行了野外地质调查、大比例尺填图、岩相学、矿物与岩石化学、热力学、年代学、大地构造解析等研究，研究增量归纳如下：(1)该造山带有三种构造混杂岩：基质与岩块完全无序的交织状混杂岩；变质岩岩块沿围岩片麻理折返，呈“基质夹岩块”的构造混杂岩；来自不同深度构造岩片叠置而成的堆叠体；(2)副变质岩中碎屑锆石年龄最小峰值、火山灰夹层中岩浆锆石年龄，限定了原岩最老沉积时代介于早奥陶世~早泥盆世。这些古生代海相沉积地层卷入了古生代俯冲带；(3)三危山岩浆弧大泉地区首次发现被裹携于未变质未变形花岗岩体中的石榴单斜辉石岩捕掳体，变质高峰期达超高压变质(790–920°C/2.8–4.1 GPa)，变质时代为~389–370 Ma；(4)造山带东南红柳峡构造块体新发现三处退变榴辉岩露头，变质高峰期达高压变质(~680–820 °C/1.7–2.4 GPa)，变质时代为~418‒393 Ma；(5)造山带西端卡拉塔什塔格构造块体由来自不同深度(~0.9–1.6 GPa)的变质-构造岩片叠置而成，变质时代为~430–390 Ma；(6)造山带中部蘑菇台构造混杂岩带，来自不同深度的岩块和基质变质高峰期压力(~0.6–1.7 GPa)差异巨大，变质时代为~420–372 Ma；(7)造山带东南部水峡口地区变质程度达高压角闪岩相(~720–750 °C/1.3–1.5 GPa)，变质时代为~438‒398 Ma；(8)敦煌造山带不同构造块体变质时代介于~440−365 Ma，中压、高压变质相系并存，均记录了顺时针型P-T轨迹(含快速抬升的“西阿尔卑斯型”、较慢速抬升的“弗兰西斯科型”)；(9)敦煌造山带古生代期间构造极性为“向北俯冲、向南折返”；(10)造山带西南的阿克塔什塔格地区(“米兰群”)变质岩记录了顺时针型P-T轨迹，东部(~1.98‒1.96 Ga)与西部(~1.88‒1.84 Ga)变质时代不同，未卷入敦煌造山事件，将之命名为“古元古代阿克塔什塔格造山带”，是敦煌造山期间的南侧大陆；(11)首次建立了变质泥质岩系GBAQ压力计、二云母压力计、石榴子石压力计。