闽西北天井坪变质杂岩物源的年代学及其对华夏陆块构造演化的制约

The Cathysia Block is an important part of the South China Block, which is located in its southeast, and playing a key role in its formation and evolution. The Precambrian basement of the Cathaysia has experienced Paleoproterozoic Columbia/Nuna, and Neoproterozoic Rodinia supercontinents, and Paleozoic Gondwanaland collision-breakup events since its formation. The Tianjingping metamorphic complex consists of schists, gniesses, amphibolites and migmatites, formed by complicated igneous-sedimentary series that has undergone greenschist-amphibolite facies metamorphism and local modification by anatexis. The Cathaysia evolution during supercontinents collision-breakup events would be well recorded in the Tianjingping Complex, for it is one of the important typical exposure areas of the precambrian basement and also contain Precambrian-Early Paleozoic sediments. This application proposed a systematic research on the Tianjingping metamorphic complex via detailed petrography, geochemistry combined with zircon LA-ICP-MS U-Pb dating, trace elements, and LA-MC-ICP-MS Hf isotopes analyses. This project would help to define the nature and environment of the ~1.77 Ga crystal basement and the sedimentary timing of different source rocks in the Tianjingping Complex, and would help tracing the sedimentary source area(s) of Cathaysia by comparison of the Hf isotope compositions for zircons with same ages in possible adjacent blocks in the supercontinents reconstruction models, including Paleoproterozoic Columbia supercontinent, Neoproterozoic Rodinia supercontinent and Early Paleozoic Gondwanaland. That would provide significant constraints on the tectonic evolution of Cathaysia and Precambrian-Early Paleozoic paleocontinents configurations.

华夏陆块前寒武纪基底自形成以来经历了古元古代、新元古代及古生代超大陆拼合-裂解事件。天井坪变质杂岩原岩为复杂的岩浆-沉积系列，既是华夏陆块古元古代基底重要代表性出露之—，又包含前寒武纪-早古生代多期沉积物源。本申请拟通过详细的岩相学、岩石地球化学结合锆石U-Pb定年、微量元素及Hf同位素组成分析，对天井坪前寒武纪-早古生代变质杂岩进行系统研究，确定其1.77 Ga结晶基底的物源及形成环境、不同源岩的沉积时限以及源区的地壳演化信息，并通过与古元古代Columbia/Nuna超大陆、新元古代Rodinia超大陆和早古生代Gondwana大陆重建模型中华夏陆块可能邻区的锆石年龄谱同时代锆石的Hf同位素特征横向对比，示踪华夏陆块的沉积来源。本项目将为华夏陆块的构造演化及前寒武纪-早古生代古大陆重建提供重要制约。

华夏地块北东段的闽西北天井坪地区零星的出露基底变质杂岩系，天井坪变质杂岩原岩为复杂的岩浆-沉积系列，既是华夏陆块古元古代基底重要代表性出露之—，又包含前寒武纪-早古生代多期沉积物源，自古形成以来，至晚古生代遭受强烈的变质变形，经历高角闪岩相—混合岩化作用。本项目主要运用详细的野外地质观察、系统的采样、岩相学研究、全岩地球化学、锆石U-Pb和Lu-Hf同位素及微量元素等研究方法，旨在揭示华夏地块闽西北天井坪地区混合岩中新成体、长英质副变质岩以及基性变质岩的年代学特征和存在的相互关系，从而讨论本区变质岩系演化过程及其在前寒武纪超大陆重建中的响应。. 获得主要认识如下：. 1. 华夏地块副变质岩原岩中锆石U-Pb年龄图谱中，位于~1800Ma处明显的峰值表明其与北美基底物质存在一定联系，而不同于东印度和东南极年龄图谱。尽管在西澳大利亚的锆石年龄图谱中也存在~1800Ma的峰值，然而其176Hf/177Hf比值显然低于华夏地块南西段的比值和此次对华夏北东段研究结果。另外，碎屑锆石的Hf同位素研究结果表明华夏地块和扬子地块西北部在古-中古元古代时期存在存在显著的新生地壳物质，这一特征完全不同于西澳大利亚。这些证据表明，华夏地块同北美大陆存在着相似性，从而佐证在晚中元古代Rodinia超大陆聚合之前以及聚合过程中，华夏地块同西Laurentia大陆相邻。. 2. 闽西北地区长英质副片麻岩的原岩为不成熟沉积物，包括硬砂岩、长石砂岩以及岩屑砂岩等。沉积物源主要来自于发生在新元古代（820±6Ma，780±6Ma，776±6Ma，758±3Ma，740±8Ma和722±9Ma）的几次岩浆事件，其沉积时间上限可能不早于中新元古代（~680Ma）。. 3.在闽西北地区测得的La-ICPMS锆石U-Pb深熔年龄新数据表明，该区存在强烈而广泛的早古生代构造-热事件，并可能与造山运动相关。此次造山运动起始于中寒武纪，持续至中泥盆纪，其间存在三个主要期次（~473Ma、~445Ma、~423Ma），并且其主要造山事件（包括同造山和后造山熔融）在华夏地块北东段发生于~473-~407Ma，在其南西段则发生于~468-415Ma。