甘肃野马南山地区北大河岩群变质演化及其大地构造意义

Widely outcropping in the Mts. Yema Nanshan area, at the western segment of central Qilian Mountains, northwest China, the Beidahe metamorphic complex (Beidahe Group) of upper greenschist to amphibolite facies is conventionally considered as basement rocks of the Central Qilian block. Recently, we firstly found the high-pressure (HP) mafic granulite in the complex, advocating HP metamorphism had occurred. Metabasites, including (garnet-bearing) amphibolite and HP granulite, exposed as puddingstones or lenses within metapelitic schist/gneiss or marble. The formation age and tectonic attribution of the Beidahe complex are still in great debate, and the metamorphic history remains unknown. Therefore, based on detailed field investigation, studies of metamorphism and high resolution geochronology, we intend to clarify the composition and nature, reconstruct reliable metamorphic P-T-t paths of the Beidahe complex, and finally place constraints on its tectonic implication and attribution.

甘肃祁连山西段野马南山地区广泛出露一套变质杂岩系—北大河岩群，变质程度为高绿片岩相至角闪岩相，传统上认为其代表祁连地块前寒武纪结晶基底。我们在北大河群中首次发现了高压基性麻粒岩，表明部分北大河岩群经历了高压变质作用。斜长角闪岩和高压基性麻粒等普遍保留有丰富的变质反应结构，多呈布丁或透镜体的形式，产于变泥质片岩/片麻岩或大理岩组成的基质围岩中。北大河岩群时代和构造属性还存在很大争议，变质演化过程尚不明确。本项目拟在详细的野外地质调查、变质演化及高精度年代学研究基础上，查明北大河岩群的组成、结构及性质；反演北大河岩群变质作用P-T-t轨迹，精确限定其变质时代；探讨北大河群变质杂岩系在区域造山系演化中的具体作用及其大地构造归属。

青藏高原北部中祁连地体在原特提斯洋构造演化中起着关键作用，但其组成和构造属性一直存在争议。通过对中祁连地体西段北大河岩群进行野外详细的大比例尺地质填图及剖面解剖，识别出高压基性麻粒岩、石榴角闪岩、角闪片麻岩和变余辉长岩、（石榴）云母石英片岩、（石榴）黑云斜长片麻岩、绿泥石石英片岩、大理岩、变质火山沉积岩等以构造岩片逆冲叠置或呈“基质夹岩块”的形成产出。详细的显微岩相观察和矿物成分测试识别出进变质、峰期变质和退变质三个阶段的变质作用，并通过变质热力学相平衡模拟及矿物温压计方法，恢复出北大河群变质杂岩记录的“发卡状”顺时针型变质P-T轨迹。通过高精度SHRIMP和LA-ICP-MS锆石、榍石、金红石U-Pb定年方法，将变质杂岩的峰期变质时代限定为~554-516 Ma，峰后（减压）折返阶段时代限定为~515-506 Ma，退变质冷却时代限定为~495-455 Ma，同时揭示出北大河岩群中含有古老的继承锆石年龄信息（~2.8-0.7 Ga）。结合区域资料，本研究提出北大河岩群可能代表晚新元古代-早古生代期间发育的日本型弧前增生杂岩体系，很可能是北祁连洋主洋盆闭合过程中洋壳向南俯冲作用的结果。