晚古生代华北地块北缘中地壳热状态及构造变形：对认识安第斯型大陆边缘地壳加厚的意义

As the world's largest non-collisional active mountain belts, the Central Andes is characterized by extremely thick continental crust, high-relief topography with 350~400 km wide plateau, strong earthquakes, large volumes of calc-alkaline magmatism and metallic mineralization. Due to difficulties in obtaining the mid-lower crustal magmatic, metamorphic and tectonite samples in the Andes moutains, the mid-crustal thermal regime, deformation and their roles in the crustal thickening and surface uplift processes of the Andes orogenic belt are still poorly understood. As a typical Andean-type orogenic belt during the Late Paleozoic, the mid-crustal plutons and metamorphic rocks are well exposed in the Inner Mongolia Paleo-uplift in the northern margin of the North China block as revealed by recent geothermobarometrical results. Because the past is the key to the present and the future, this proposal uses the northern margin of the North China block as an example for studying the mid-crustal thermal regime and deformation beneath the Andean-type orogenic belt. In this research, multiple methods including structural geology, igneous petrology, Ar-Ar thermochronometry and geothermobarometry will be used to study Late Carboniferous to Permian mid-crustal dioritic-granitic plutons and metamorphic basement rocks along the three structure-lithology sections across the continental arc and the northern part of the retroarc forland basin. The results are of great significance for understangding the mid-crustal deformation, thermal regime, rheology and mechanism and processes of crustal thickening and surface uplift Andean-type continental margins.

作为全球最大规模的非碰撞型活动造山带，现代安第斯造山带以其巨厚的陆壳、高耸的地貌、强烈的地震、大量的钙碱性岩浆活动及多金属成矿作用为特征。但由于目前可获得中-下地壳侵入岩、变质岩及构造岩样品很少，对于安第斯造山带中-下地壳构造变形、热状态及其在地壳加厚及地表抬升方面作用的认识还不清楚。本申请选择中地壳侵入岩及变质岩出露较为广泛的华北地块北缘晚古生代安第斯型造山带为例，采用“以古论今”的思路，拟通过横穿陆缘弧及弧后前陆盆地的构造-岩石剖面，采用构造解析、岩石学、Ar-Ar热年代学与温压计算等多学科方法，研究华北地块北缘晚石炭世-三叠纪早期中地壳岩石（包括晚古生代侵入岩及早前寒武纪基底变质岩）的构造变形及冷却历史。这些问题的解决对于正确认识安第斯型活动大陆边缘演化深部构造变形、热状态、流变学特征及地壳加厚及抬升机制均具有非常重要的科学意义。

作为全球最大规模的非碰撞型活动造山带，现代安第斯造山带以其巨厚的陆壳、高耸的地貌、强烈的地震、大量的钙碱性岩浆活动及多金属成矿作用为特征。但由于目前可获得中-下地壳侵入岩、变质岩及构造岩样品很少，对于安第斯造山带中-下地壳构造变形、热状态及其在地壳加厚及地表抬升方面作用的认识还不清楚。本项目选择中地壳侵入岩及变质岩出露较为广泛的华北地块北缘晚古生代安第斯型造山带为例，通过横穿陆缘弧及弧后前陆盆地的构造-岩石剖面，采用构造解析、岩石学、Ar-Ar热年代学与温压计算等多学科方法，研究了华北地块北缘晚石炭世-三叠纪早期中地壳岩石的构造变形及冷却历史。.研究结果显示，晚古生代-早中生代期间，内蒙古隆起中地壳具有较高的地温梯度，表现为晚石炭世-早二叠世岩体较慢的冷却速率。远离岩体的基底TTG片麻岩虽然锆石U-Pb年龄为25亿年或18亿年，但其黑云母及角闪石40Ar-39Ar冷却年龄却很年轻（290~250 Ma），显示这些TTG片麻岩在晚古生代期间经历了强烈的热改造。但这一时期，处于华北北缘陆缘弧南侧的马兰峪复背斜核部近中地壳深度的基底TTG片麻岩（锆石U-Pb年龄为25亿年）则具有古元古代-中元古代早期的角闪石和黑云母40Ar-39Ar冷却年龄（1820 Ma和1750 Ma），显示晚古生代期间，位于华北北缘陆缘弧南侧克拉通的中地壳没有受到华北北缘俯冲热体制的影响，具有低的地温梯度。因此晚古生代期间，内蒙古隆起中地壳由于受古亚洲洋俯冲过程中大陆边缘弧岩浆作用的影响相对较热，属于热造山带（hot orogens），这一特征与现今安第斯造山带及侏罗-白垩纪期间北美西侧俯冲型造山带特征非常相似。在华北北缘陆缘弧形成过程中，陆缘弧中部比其南侧（靠近陆缘弧与弧后前陆盆地边界）具有更高的中-上地壳地温梯度，这种垂直于陆缘弧走向的古地温梯度差异可能与古亚洲洋俯冲板片的后撤密切相关。这些研究成果对于正确认识安第斯型活动大陆边缘演化深部构造变形、热状态、流变学特征及地壳加厚及抬升机制均具有非常重要的科学意义。