云南六合地区新生代正长斑岩和包体的起源及其大地构造与成矿意义

Origin of Cenozoic Syenitic Porphyries and Enclaves in Liuhe,Yunnan Province,SW China,and its Tectonic and Metallogenic Significance

新生代以来,印度-欧亚大陆碰撞致使青藏高原发生大规模隆升和岩石圈的东南向挤出。金沙江-哀牢山构造带和喜马拉雅构造带作为调整高原内部挤压应力的两个边界,发育有两条新生代富碱岩浆带,该带内富碱岩浆的成因及其成岩机制是了解印度-欧亚大陆碰撞过程深部动力学的关键钥匙。本文针对哀牢山-红河富碱岩浆岩带的云南六合正长斑岩及各类深、浅源包体开展了系统的锆石U-Pb年代学和Lu-Hf同位素,全岩主量、微量元素和Sr-Nd同位素研究。年代学结果表明,六合正长斑岩形成于37 Ma,略早于金沙江-哀牢山带富碱岩浆作用峰期时间(36 Ma)。地球化学结果显示,六合正长斑岩具富碱、高钾、低镁特征,富集轻稀土元素(LREE)和大离子亲石元素(Rb、Ba、Sr),亏损重稀土元素(HREE)和高场强元素(Ta、Nb、Ti),并具有高Sr含量和Sr/Y值,弱Eu负异常的特征。结合六合正长斑岩和部分深源包体的同位素组成与滇西下地壳起源包体相一致的特征,说明正长斑岩应该起源于加厚下地壳含石榴石斜长角闪岩的部分熔融,而非前人普遍认为的富集岩石圈地幔起源。正长斑岩内大部分浅变质包体是富碱岩浆上升过程从浅部地壳围岩所捕获,它们并未明显改变其寄主富碱岩浆原始化学成分。综合本文研究结果和区域上的地质资料表明,始新世早期金沙江-哀牢山-红河构造带富碱岩浆作用是印度-欧亚大陆碰撞过程中青藏高原岩石圈向东南方向(滇西地区)扩展的结果,而非前人普遍认为的岩石圈地幔拆沉的产物。此外,沿着金沙江-哀牢山-红河构造带展布的新生代富碱岩浆作用在始新世早期(38~45 Ma)并不存在由北向南逐渐变年轻的趋势,滇西地区早期富碱岩浆岩可能保存在地壳深部尚未剥露至地表。

Since the Cenozoic,the India-Eurasia continental collision has caused large-scale uplift and lithospheric extrusion of the Tibetan Plateau.As the response to this collision,two Cenozoic alkaline magmatic belts developed along the Jinshajiang-Ailanshan and Himalayan tectonic belts which have been considered to be two boundaries for adjusting the compressional stress in the interior of the plateau.The genesis and rock-forming mechanism of these alkali-rich magmatic rocks are the key to explore the deep geodynamics during the India-Eurasia collision.In this paper,we present zircon U-Pb geochronology and Lu-Hf isotopic compositions,whole-rock major and trace elemental,and Sr-Nd isotopic results of the Liuhe syenite porphyries and their enclaves.Our dating results together with previous data indicate that the syenite porphyries formed at ca.37 Ma,which is slightly earlier than the peak alkali-rich magmatism(36 Ma)along the Jinshajiang-Ailaoshan belt.The Liuhe syenite porphyries are geochemically rich in alkali and potassium,and low in magnesium,coupled with the enrichment of Light Rare Earth Elements(LREE)and Large Ion Lithophile Elements(Rb,Ba,Sr),and depletion of Heavy Rare Earth Elements(HREE)and High Field Strength Elements(Ta,Nb,Ti).Their high Sr contents and Sr/Y ratios along with weak negative Eu anomalies(δEu=0.84-0.85)show an affinity to adakitic rocks.Their overall geochemical signatures and the similarities in Hf-Nd isotopic compositions to those of the hosted garnet-bearing amphibolite enclaves indicate that the Liuhe syenite porphyries were derived from partial melting of the thickened lower crust rather than the enriched lithospheric mantle.Moreover,the Liuhe syenite porphyries featured by low oxygen fugacity and high water content have not the potential to form porphyry deposits.As for the low-grade metamorphic enclaves hosted in the Liuhe syenite porphyries,they have significantly different zircon U-Pb age,major-trace element and Sr-Nd isotopic characteristics from the host rocks,indicating that they were captured during the ascent of the host porphyric magmas and their involvement in the genesis of the host porphyries were negligible if any.Collectively,our results combined with previous data reflect that the distribution of the early Eocene alkaline magmatic rocks along the Jinshajiang-Ailaoshan tectonic belt does not show a younging trend from north to south as proposed before,and that this stage of magmatism was related to the lithospheric extrusion of the Tibetan Plateau in response to the India-Eurasian continental collision rather than the Eocene lithospheric delamination.