越南昆嵩地体三叠纪花岗岩岩石成因及其特提斯构造意义

Petrogenesis of Triassic Granites from Kontum Massif in Vietnam and Its Tethyan Tectonic Implications

摘要

昆嵩地体位于印支陆块的核部，记录了大量的印支期岩浆作用和构造热事件，是了解古特提斯洋演化及印支与华南陆块碰撞拼合过程的关键区域，但目前对该期岩浆事件的成因及其与北部长山带的关系未能得到有效厘定.对昆嵩地体绥安（Huy n Tuy An）和胶寮（Chu Loan）地区的Van Canh花岗岩开展了岩相学、锆石U-Pb年代学、锆石原位Hf同位素和全岩地球化学分析，以限定其形成时代、岩石成因及构造环境.锆石U-Pb定年显示花岗岩样品的结晶年龄为244~239 Ma.该套样品包括了二长花岗岩和钾长花岗岩，均属于高钾钙碱性系列.它们的A/CNK值为1.03~1.21，为S型花岗岩.花岗岩均显示明显的Rb、Th和U富集，以及Nb、Sr、Zr和Ti的亏损，并具有强烈的Eu负异常（Eu/Eu* = 0.24~0.56）.锆石具有富集的原位Hf同位素组成（ε_Hf （t） = -11.2~-0.7）以及古-中元古代的Hf二阶模式年龄（T _DM2 = 1.98~1.31 Ga）.研究表明该套中三叠世花岗岩是古元古代-中元古代变沉积岩部分熔融的产物，并伴有少量变火成岩的加入.研究结合区域地质资料表明昆嵩地体中三叠世Van Canh花岗岩的成因与马江古特提斯分支洋闭合之后的印支与华南陆块的碰撞拼合有关，形成于后碰撞阶段，进而证实了长山带向南可以延伸至昆嵩地体内部.

Abstract

Kontum massif in the central Indochina records abundant Indosinian magmatism and tectonic thermal events, and is a key area for investigating the tectonic evolution of Paleotethyan ocean and subsequent collision between the Indochina and South China blocks. However, the origin of this Indosinian magmatism and its relationship with the Truong Son zone in the north are poorly constrained. In this study, it presents a set of petrographic, zircon U-Pb geochronological, zircon in-situ Hf isotopic, and whole-rock geochemical data for the granites from the Huyen Tuy An and Chu Loan area in the Kontum massif to constrain their crystallization ages, petrogenesis, and tectonic setting. Zircon U-Pb dating shows that the granites formed at 244-239 Ma. These samples contain monzogranites and K-feldspar granites and belong to the high-K calc-alkaline series. They are S-type granites with A/CNK values ranging from 1.03 to 1.21. These samples show enrichment of Rb, Th, and U, and depletion of Nb, Sr, Zr, and Ti, with obvious negative Eu anomalies (Eu/Eu* = 0.24-0.56). Their negative zircon ε_Hf (t) values ((-11.2)-(-0.7)) and Paleo- to Meso-Proterozoic Hf model ages (T _DM2 = 1.98-1.31 Ga) indicate that these granites were derived from the partial melting of the Paleo- to Meso-Proterozoic metasedimentary rocks with a small amount of metaigneous component. Our new data along with the regional geological observations suggest that the Middle Triassic Van Canh granites in the Kontum massif were formed in a post-collisional setting in response to the collision between the Indochina and South China blocks following the closure of Song-Ma Paleotethyan branch. Our studies further suggest that the Truong Son zone can southerly extend to the central Kontum massif.

关键词

印支陆块 / 越南昆嵩地体 / 中三叠世 / S型花岗岩 / 古特提斯洋 / 后碰撞 / 地球化学

Key words

Indochina block / Kontum massif / Middle Triassic / S-type granite / Paleotethyan ocean / post-collision / geochemistry

基金资助

国家自然科学基金项目(41830211;42072256)

广东省基础与应用基础研究基金项目(2018B030312007;2019B1515120019)

中山大学高校基本业务费(22lgqb14)