华北克拉通南缘蚌埠地区高压基性麻粒岩变质演化的相平衡模拟及成因机制研究

The garnet-bearing high-pressure mafic granulites of the Wuhe Group in the Bengbu area, southern margin of the North China Craton preserved mineral assemblages and various metamorphic reactions during multiple metamorphic stages which record complex metamorphic evolution history and are suitable to investigate the metamorphic evolution, petrogenesis as well as dynamic setting. The high-pressure mafic granulites of the Wuhe Group are therefore treated as the main study object. Phase equilibria modeling combined with ternary feldspar thermometry, and Zr-in-rutile thermometry is firstly applied to precisely constrain the P-T conditions of metamorphic mineral assemblages with multi-stages illustrating whether these rocks experienced ultrahigh-temperature metamorphism as well as linking the observed microstructures with possible metamorphic reactions, indicating petrogenetic mechanism, genetic relationship between melts behavior and mineral evolution. On the basis of Laser Raman spectroscopy, cathodoluminescence and backscattered image analyses, (in-situ) isotopic dating of zircon, baddeleyite, and rutile, and Ar-Ar dating of amphibole are applied to restrict the timing of protolith and multi-stage metamorphism involving peak and post-peak retrograde evolution, and subsequently construct the P-T-t path. The study shows scientific significance of depicting the metamorphic evolution of the high-pressure mafic granulites of the Wuhe Group and reconstructing the regional orogenic evolution and geodynamic processes.

位于华北克拉通南缘的蚌埠五河群石榴高压基性麻粒岩保存多期/多阶段矿物组合和变质反应结构信息，记录了复杂的变质-热演化历史，是探究变质作用、成因机制及动力学背景的理想窗口。拟以五河群石榴基性麻粒岩为重点研究对象，采用相平衡模拟并配合三元长石温度计、金红石Zr温度计等手段精确限定各变质阶段矿物组合形成的P-T条件，明确该岩石是否达到了超高温变质条件，并模拟矿物相转变结构与变质反应间的关系，揭示其成因机制、熔体与矿物组合演化的成因联系。在矿物包体激光拉曼、阴极发光和背散射图像分析的基础上，对锆石、斜锆石和金红石开展同位素定年，并结合薄片尺度下原位微区定年和角闪石Ar-Ar定年，精确构筑其原岩形成时代-峰期变质时代-峰后退变质时代的年代格架及完整的变质演化P-T-t轨迹。该项研究对于精细刻画其变质-热演化历史、岩石成因机制、重塑大地构造背景及动力学过程具有重要的科学意义。

在我国华北克拉通东南缘安徽蚌埠地区识别出了五河群高压-超高温石榴基性麻粒岩，以透镜状产出于大理岩中，主要组成矿物为石榴石、单斜辉石、斜长石、紫苏辉石、角闪石、钾长石、石英及少量磁铁矿、钛铁矿和金红石，部分岩石还保留少量残留熔体的显微结构证据。该石榴基性麻粒岩经历了峰期高压-超高温和峰后的中压变质阶段，显示近等温减压型顺时针P-T轨迹，其中峰期高压-超高温阶段矿物组合为石榴石+单斜辉石+斜长石+钾长石±角闪石±石英±金红石，温压条件为840-980°C和12-17kbar。峰后近等温减压阶段矿物组合为角闪石+单斜辉石+斜方辉石+斜长石+钛铁矿+石榴石+钾长石，温压条件为850-960°C和7-10kbar。相平衡模拟进一步得出超高温条件下的降压（14kbar降至8kbar）过程中，岩石发生部分熔融产生少量熔体（<5 mol.%），成分主要与（石英）二长岩一致，其SiO2、Al2O3和K2O/Na2O比值均较高，而MgO和XMg值较低，熔体中的Si含量受控于石英的稳定域。在高压阶段产生的熔体富Na2O而贫FeO和MgO，随着降压过程产生熔体的FeO、MgO和K2O含量均有升高。降压后熔体摩尔含量趋于近一致，并随角闪石、斜方辉石和斜长石的生成而显著降低。注意到部分岩石中的粗粒辉石和长石具出溶结构，应为早期岩浆结晶产物并残留，对两者进行成分恢复并分别获得1100-1200°C和1055-1087°C（10kbar条件下），综合限定其原岩最低结晶温度为~1055°C。该岩石的变质时代集中在1930-1840Ma。蚌埠基性麻粒岩的峰期温压条件明显高于前人报道和胶-辽-吉带其它地区的麻粒岩相变质岩石。综合表明蚌埠地区基底岩石在1950-1900Ma陆壳间的碰撞增厚作用下被带入到中-下地壳深度，经历了高压-超高温变质并伴随部分熔融作用。后期~1850Ma快速折返抬升过程中经历了近等温减压的变质阶段并产生少量熔体，具有顺时针P-T轨迹，反映了碰撞构造过程。蚌埠的基性麻粒岩与胶-辽-吉带其它地区（包括胶北、辽南和吉林南部地区）的麻粒岩相变质岩石的变质演化过程和变质时代近一致，也进一步证实蚌埠地区应为胶-辽-吉古元古代造山带西南部的延伸。