佳木斯地块早古生代镁铁质岩浆作用及其地质意义

Jiamusi Block is the famous late Pan-African high-grade metamorphic microcontinent in the eastern segment of the Xing’an-Mongolia Orogenic Belt (XMOB), the tectonic affinity and late Pan-African evolution history of this block have both been subjects of controversy. This project will conduct a precise study of geochronology, petrology, mineralogy, whole rock major elements, trace elements and Sr-Nd-Hf isotopes, and zircon in situ Hf-O isotopes for the newly discovered early Paleozoic mafic magmatic rocks in the Jiamusi Block, in order to (1) investigate the formation timing, rock association and temporal-spatial distribution of the early Paleozoic mafic magmatism; and (2) determine the source nature, magmatic process and tectonic setting of these mafic magmas. In combination with the regional research results of the early Paleozoic granitic rocks, and the late Pan-African granulite-facies metamorphism, this project will further discuss the late Pan-African metamorphic-magmatic history and crust-mantle interaction processes of the Jiamusi Block and their possible geodynamic mechanism. This project not only provides vital evidence for revealing the tectonic affinity and geological evolution history of the Jiamusi Block, but also has important significances for understanding crustal growth and reworking as well as evolution of the deep lithosphere in the XMOB.

佳木斯地块是兴蒙造山带东段著名的晚泛非期高级变质微陆块，其构造起源及晚泛非期演化历史一直是地学领域研究的热点与争论的焦点问题之一。本项目拟通过对佳木斯地块上新发现的早古生代镁铁质岩浆岩开展精细的年代学、岩石学、矿物学、全岩主量、微量元素和Sr-Nd-Hf同位素、以及锆石原位Hf-O同位素研究，旨在查明该期岩浆作用的形成时代、岩石组合及时空分布规律，确定该期镁铁质岩浆的源区性质、演化过程及构造背景。结合区域早古生代花岗质岩石、晚泛非期麻粒岩相变质作用及其他相关研究成果，探讨佳木斯地块晚泛非期变质-岩浆作用历史、壳-幔作用过程及可能的地球动力学机制。该项研究不仅可以为揭示佳木斯地块构造起源及地质演化历史提供关键证据，而且对理解兴蒙造山带的地壳生长和再造以及深部岩石圈演化具有重要意义。

本项目对佳木斯地块新识别的早古生代镁铁质侵入岩进行了岩石学、矿物学、锆石U-Pb年代学、全岩-锆石元素-同位素地球化学的系统研究，查明了佳木斯地块早古生代镁铁质岩浆作用的时空分布、岩浆源区性质、成岩过程及其形成的地球动力学背景，揭示了佳木斯地块晚泛非期变质-岩浆-构造演化历史。研究结果表明，佳木斯地块早古生代镁铁质岩浆作用可划分为寒武纪（541~495Ma）和志留纪（435~429Ma）两期，岩石组合包括辉长岩-辉长闪长岩和少量闪长岩-石英闪长岩。寒武纪镁铁质侵入岩主要分布在敦密断裂以西，起源于受早期俯冲改造的岩石圈地幔的部分熔融，其原始岩浆在向上运移过程中普遍经历了麻粒岩相下地壳物质的充分混染和强烈的橄榄石+斜长石+单斜辉石的分离结晶，形成于后造山伸展背景，是晚泛非期碰撞造山带伸展垮塌过程中剧烈壳-幔相互作用的产物。志留纪镁铁质侵入岩分布在敦密断裂以东，属于白乃庙岛弧的东延部分，形成于古亚洲洋洋内北向俯冲过程，岩浆源区是受低温蚀变洋壳沉积物和陆源沉积物来源的熔-流体共同交代改造的地幔楔，其原始岩浆在形成后基本未受陆壳混染的影响，并且经历了强烈的单斜辉石-角闪石-锆石分离结晶作用和橄榄石-斜长石堆晶作用。综合区域地质资料，认为晚泛非期佳木斯地块最可能在东冈瓦纳经历了Kuunga造山带碰撞形成至后造山垮塌的全部过程。此外，明确提出大陆碎片普遍存在洋内弧之中，很好地解释了洋-洋起始俯冲和岛弧岩浆岩的多样性。