华北克拉通中部造山带龙门钾质岩浆结晶条件:矿物化学和锆石微量元素的约束  

Crystallization conditions of magma from Longmen potassic pluton of the Trans-North China Orogen,North China Craton:Constraints from mineral chemistry and zircon trace element

作　　者：唐宗源 李杰 邓昌州 郑涵 刘少泽 TANG ZongYuan;LI Jie;DENG ChangZhou;ZHENG Han;LIU ShaoZe(Hebei Key Laboratory of Strategic Critical Mineral Resources,Shijiazhuang 050031,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi an 710069,China;State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China;School of Geosciences and Info-physics,Central South University,Changsha 410083,China;Huaxin College of Hebei Geo University,Shijiazhuang 050700,China)

机构地区：[1]河北省战略性关键矿产资源重点实验室,石家庄050031 [2]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [3]中国科学院地球化学研究所,矿床地球化学国家重点实验室,贵阳550081 [4]中南大学地球科学与信息物理学院,长沙410083 [5]河北地质大学华信学院,石家庄050700

出　　处：《岩石学报》2023年第5期1370-1386,共17页Acta Petrologica Sinica

基　　金：河北省自然科学基金(D2022403015)资助.

摘　　要：华北克拉通中部造山带发育的早白垩世钾质岩是研究华北克拉通中部幔源岩浆成因的天然样品,然而其成岩物理化学条件仍不明确。本文选取了涞源县龙门岩体中闪长岩和二长岩开展了矿物原位地球化学和锆石微量元素分析,以探讨矿物形成的温压条件及氧逸度与矿化的关系。分析结果显示,二长岩和闪长岩的锆石Th/U比值较高(>0.1),轻稀土亏损、重稀土富集,具明显的Ce正异常和Eu负异常特征,并具有明显的震荡环带,为典型岩浆成因锆石。电子探针结果显示,龙门岩体中斜长石主要为中性斜长石,黑云母为原生镁质黑云母,角闪石主要为钙铁闪石。闪长岩中角闪石结晶温度为793~842℃,压力为0.50~0.75GPa,深度范围为19~28km,具有较低氧逸度(log f O 2为-12.21~-11.38)和高的含水量(8.5%~10.75%);二长岩中黑云母结晶温度为764~788℃,具有更低的氧逸度(log f O 2=-15.14~-14.54);闪长岩和二长岩中锆石结晶温度为770~978℃,具有较低的氧逸度(ΔFMQ=-0.37~+1.70)。矿物化学分析表明,角闪石和黑云母虽都在地壳开始结晶,但角闪石形成于比黑云母更深的岩浆房。龙门岩体中角闪石、黑云母和锆石的结晶主要受岩浆分异控制,其地球化学动力学机制主要为岩浆上侵时减压熔融,进而引起岩浆氧逸度随岩浆演化而变化。随着岩浆上涌,闪长岩中角闪石随着温度降低和压力减少逐渐结晶,地幔交代作用导致二长岩中的黑云母具有较高的X Mg值,进而形成镁质黑云母。结合区域地质背景,古太平洋板块(伊佐奈琦板块)俯冲-后撤释放的流体交代上覆岩石圈地幔对中部造山带钾质岩(~141Ma)的形成有一定贡献,该过程导致了岩浆具有较高的含水量,但龙门岩体低的氧逸度和区域上较厚的岩石圈制约了成矿金属物质(Fe-Au等)的迁移和富集,因而不利于亲硫、亲铁元素聚集以及铁、金矿化的形成。The Early Cretaceous potassic outcrops in the Trans-North China Orogen provide natural samples for the studying on the origin of mantle-derived magmas.However,the chemical and physical conditions of diagenesis are still unknown.In this paper,we present in-situ mineral geochemistry and zircon trace element analysis for the potassic pluton from Longmen in Laiyuan County to help answer these questions and to constrain the relationship between temperature and pressure conditions for mineral formation,oxygen fugacity,and mineralization.All zircons in monzonite and diorite have a high Th/U ratio(>0.1),which is of a typical magmatic zircon characteristics,and exhibit strong regular oscillatory zoning as well as enrichment in heavy rare earth elements relative to light rare earth elements with a positive Ce anomaly and a negative Eu anomaly.According to the results of electron probe microanalysis,amphibole in diorite belongs mainly to ferrotschermakite,while plagioclase in monzonite and diorite is mainly andesine,and biotite in monzonite mainly magnesiobiotite.The temperature(793~842℃),pressure(0.50~0.75GPa),depth(19~28km),low oxygen fugacity(log f O 2=-12.21~-11.38),and high-water content(8.5%~10.75%)of the magma were calculated based on the compositions of the amphibole in the diorite.The temperature(764~788℃)and low oxygen fugacity(log f O 2=-15.14~-14.54)of the magma were calculated based on the compositions of biotite in the monzonite.The temperature(770~978℃)and low oxygen fugacity(ΔFMQ=-0.37~+1.70)of the magma were calculated based on the composition of zircons in the diorite and monzonite from Longmen.The mineral chemistry study shows that amphibole crystallizes in deep magma chambers at higher temperatures and pressures than biotite does in crustal environments.The crystallization of amphibole,biotite and zircon in the Longmen rock mass is mainly controlled by magmatic differentiation.It demonstrates that the oxygen fugacity of magma changes as a result of the decompression melting in the geochemical dyna

关 键 词：钾质岩体 锆石微量元素 矿物化学 氧逸度 中部造山带 

分 类 号：P575.1[天文地球—矿物学] P588.122[天文地球—地质学]