榴辉岩中灰闪石稳定性的相平衡研究——以苏北碱厂榴辉岩中灰闪石为例  

Thermodynamic modelling on the stability of nyb?ite in eclogite: A case from Jianchang, northern Jiangsu Province

作　　者：曾晓燕 陈安平 ZENG Xiao-yan;CHEN An-ping(State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;University of Chinese Academy of Sciences, Beijing 100049, China)

机构地区：[1]中国科学院地质与地球物理研究所,岩石圈演化国家重点实验室,北京100029 [2]中国科学院大学,北京100049

出　　处：《岩石矿物学杂志》2019年第4期535-552,共18页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金项目(41573029,41873040)~~

摘　　要：灰闪石(nyboite,Nyb)已在世界多处超高压榴辉岩中发现,但对于其稳定性及成因的研究结果存在较大分歧。本文以中国苏北碱厂榴辉岩中灰闪石为例,基于岩相学观察和相平衡模拟限定了灰闪石的稳定条件。在Na2O-Ca O-K2OFe O-Mg O-Al2O3-Si O2-H2O-Ti O2-Fe2O3(NCKFMASHTO)体系下对含有灰闪石榴辉岩进行的相平衡计算得到与实际观察一致的矿物组合,且模拟和实测的灰闪石成分基本一致。相平衡模拟结果表明,灰闪石稳定温压范围为t <500℃,p <1. 20 GPa,地温梯度范围为12~17℃/km。灰闪石的出现主要受控于全岩成分的XNa与XAl值,要求分别满足XNa=0. 58~0. 65和XAl<0. 15。碱厂灰闪石含有较高的F含量(1. 88%~2. 54%),由于目前没有含F的角闪石活度模型,文中根据前人研究结果讨论了F替代OH对灰闪石稳定性的影响。榴辉岩中出现灰闪石指示岩石经历了低温、中高压变质作用,其全岩主量元素特征表明原岩可能为低钾碱性玄武岩。Nyb?ite has been found in several sites of ultra-high pressure( UHP) eclogite in the world, but the p-T range of its stability is still controversial. In this study, the authors conducted a set of thermodynamic modelling in the system of Na2 O-Ca O-K2 O-Fe O-Mg O-Al2 O3-Si O2-H2 O-Ti O2-Fe2 O3( NCKFMASHTO) to constrain the stability of nyb?ite, on the basis of the Jianchang eclogite( Sulu UHP metamorphic belt, northern Jiangsu Province). Modelling results reproduce the observed mineral assemblage and show that nyb?ite is only stable at t < 500 ℃, p < 1. 20 GPa, and geothermal gradient of 12 ~ 17℃/km. The modeled composition of nyb?ite is broadly consistent with measured data. The formation of nyb?ite in eclogite is mainly controlled by XNaand XAlof the bulk composition, the condition of XNa= 0. 58 ~ 0. 65 and XAl< 0. 15 is necessary for the formation of nyb?ite. The nyb?ite investigated here is characterized by enrichment of F( 1. 88%~ 2. 54%), while F was not taken into account in the modelling for the lack of F-bearing amphibole thermodynamic model hitherto. However, the substitution F for OH seems to significantly affect the stability of nyb?ite. The occurrence of nyb?ite in eclogite indicates that the bulk composition is rich in Na and Al,possibly corresponding to an alkali basalt protolith, and the host rock underwent low temperature and intermediate to high pressure metamorphism.

关 键 词：灰闪石(nyboite) 榴辉岩 相平衡模拟 苏鲁超高压变质带 碱厂 

分 类 号：P578.955[天文地球—矿物学]