摘要
三江带在新生代印亚陆陆碰撞过程中形成了一条近南北向展布长度超过1500km的钾质岩浆岩带,并在北段形成了玉龙超大型斑岩成矿带,在中段、南段也形成零星分布的斑岩型矿床。本文选取三江岩浆岩带中段、南段3个新生代贫矿钾质岩体中未发生蜕晶化的岩浆锆石进行SIMS水含量和氧同位素、LA-ICP-MS微量元素及Lu-Hf同位素分析,对比玉龙带同期的钾质成矿斑岩,综合分析控制钾质岩浆大规模斑岩成矿的关键因素。分析结果显示,贫矿岩体锆石ε_(Hf)(t)值为-6.00~2.70,平均-2.94,显著低于玉龙带内多个成矿斑岩体的锆石(-3.37~6.63,平均2.68),表明贫矿的岩浆源区含有更多古老地壳物质,而北段的玉龙成矿带斑岩含更多新生下地壳物质。3个贫矿岩体锆石水含量及OH^(-)含量较高(平均(734±184)×10^(-6)和(14±4)apfu×1000),与北段玉龙带中超大型斑岩矿床的锆石水含量相似(平均(640±213)×10^(-6)和(12±5)apfu×1000),且显著高于中型斑岩矿床的锆石水含量(平均(335±181)×10^(-6)和(5.3±3)apfu×1000),表明三江带中段、南段的区域挤压应力作用相比北段的更强、贫矿岩浆储存深度更深,锆石结晶时的压力更高。贫矿岩体的锆石中未见原生流体包裹体,表明高压抑制了岩浆早期出溶流体,不利于萃取金属与成矿。我们提出富含新生下地壳物质的岩浆源区、挤压-伸展转换的构造应力背景、岩浆富水且在演化早期出溶流体是控制三江带北段新生代钾质岩浆形成大规模斑岩成矿带的重要因素;而三江带中段、南段的岩浆源区含更多古老地壳物质,处于挤压应力背景,可零星成矿,但难以形成斑岩矿床成矿带。此外,我们的研究还表明锆石水含量受结晶时岩浆水含量、岩浆储存深度与区域应力场控制,由于不同构造背景的区域应力条件不同,锆石水在不同构造背景岩浆水含量对比的应用需谨慎。
During the Cenozoic Indo-Asian continental collision,a 1500km long,north-south oriented potassium-rich magmatic belt emerged in the Eastern Tibet-Sanjiang region.In this belt’s northern sector,magmatic activity produced the extensive Yulong porphyry ore belt,while only sporadic deposits appeared in the central and southern sectors.This study employs SIMS water and oxygen isotope analyses,LA-ICP-MS trace element analyses,and Lu-Hf isotope analyses on primary zircon grains from three potassium-rich,barren rocks located in the middle and southern parts of the Sanjiang Belt.By comparing these with contemporaneous ore-forming porphyries in the Yulong ore belt,we delve into critical factors influencing large-scale porphyry mineralization of potassium-rich rocks in the Eastern Tibet-Sanjiang Belt.Our results reveal that zirconε_(Hf)(t)values(-6.00 to 2.70,average-2.94)from the barren rocks are significantly lower than those from fertile porphyries(-3.37 to 6.63,average 2.68),indicating that magmatic sources in the central and southern sectors have more ancient crustal material,while the northern sector’s source is enriched with juvenile crustal material.Zircon grains of the three barren rocks display high H_(2)O and OH^(-)contents(average(734±184)×10^(-6)and(14±4)apfu×1000),akin to those from the giant porphyry deposit(average(640±213)×10^(-6)and(12±5)apfu×1000),and much higher than those from the medium-sized porphyry deposit(average(335±181)×10^(-6)and(5.3±3)apfu×1000)in the Yulong belt.Elevated zircon H_(2)O contents in the barren rocks are primarily influenced by high pressures during zircon crystallization,which resulted from the regional high-pressure stress background and deep magmatic storage depths.These factors inhabited early fluid exsolution from the magmas,as indicated by the absence of fluid inclusions in zircons from the barren rocks,subsequently impeding metal extraction and porphyry mineralization.In summary,we propose that juvenile crust-rich sources,extensive regional stress,and early fluid exsolution of water-rich magmas collectively facilitated the large-scale porphyry mineralization of Cenozoic potassium-rich magmas in the Sanjiang Belt’s northern segment.Additionally,the inputs of mantle-derived ore-rich potassic magmas controlled the porphyry mineralization and its spatial distribution in the belt’s central and southern segments.Moreover,our approach suggests that zircon H_(2)O is regulated by regional stress fields,storage depths,and magmatic water content.Zircons from different tectonic setting formed under different regional stresses,thus caution should be exercised when interpreting the zircon water contents.
作者
黄文婷
王惊涛
夏小平
次罗
洛桑尖措
杨洋
朱吉涛
梁华英
HUANG WenTing;WANG JingTao;XIA XiaoPing;CI Luo;LUOSANG JianCuo;YANG Yang;ZHU JiTao;LIANG HuaYing(School of Jewelry,City Polytechnic of Shenzhen,Shenzhen 518116,China;Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;Hubei Key Laboratory of Petroleum Geochemistry and Environment,College of Resources and Environment,Yangtze University,Wuhan 430100,China;No.6 Geological Team of the Geological and Mineral Exploration and Development Bureau of Xizang Autonomous Region,Lhasa 851400,China;Tibet Xianglong Mining Co.,Ltd.,Xigaze 858532,China;Qinghai Xiwang Mining Development Co.,Ltd,Dulan 816100,China)
出处
《岩石学报》
北大核心
2025年第2期693-710,共18页
Acta Petrologica Sinica
基金
第二次青藏高原综合科学考察研究(2019QZKK0806)
国家自然科学基金面上项目(42072088)联合资助.
