岩石学报  2013, Vol. 29 Issue (4): 1450-1460   PDF    
缅甸硬玉岩地区的热液型钠长石岩
王静1, 施光海1, 王君1, 袁野1, 杨梦楚1     
地质过程与矿产资源国家重点实验室,中国地质大学,北京 100083
摘要: 产于俯冲带内的低温高压带的由单矿物构成的硬玉岩通常伴有钠长石岩,目前对于硬玉岩研究的关注度较高,而对于钠长石岩则相对较低,很少有相关论文报导。产于缅甸翡翠矿区的钠长石岩,经常与硬玉岩相伴而生,是良好的研究样品。钠长石岩的主要矿物成分是低温钠长石,其次含有硬玉、绿辉石、透辉石等辉石类矿物和钠透闪石、蓝透闪石、镁钠闪石等闪石类矿物,此外还有钠沸石等。钠长石沿着解理和裂隙交代硬玉,说明钠长石形成晚于硬玉岩。钠长岩中的主要组成矿物钠长石的形成温度小于300℃,且其形成压力小于0.5kb,推测是在硬玉岩抬升程中通过交代与沉淀作用形成。其内的透辉石有两种类型,一类可能是被交代的硬玉中的透辉石组分会渐进增加,最终形成透辉石。另一类是被绿辉石包裹的透辉石残留,其很有可能是早期来自地幔楔或者俯冲带岩石中的矿物残留,即异剥钙榴岩或辉石岩类,可以视作硬玉化绿辉石岩和硬玉化异剥钙榴岩的矿物学证据。热液型钠长石岩的存在进一步说明缅甸翡翠矿区钠化热液存在现象的普遍性与穿越性。
关键词: 缅甸     钠长石岩     硬玉岩     热液     低温高压    
Hydrothermal albitite from the Myanmar jadeite deposit
WANG Jing1, SHI GuangHai1, WANG Jun1, YUAN Ye1, YANG MengChu1     
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
Abstract: Albitites occur commonly in the jadeitite area. Jadeitite, a rock made up almost entirely of jadeitic pyroxene, forms under a relatively high-P/T condition in the subduction zones. Jadeitites have attracted remarkable attention of geologists and gemologists from all over the world and many localities of jadeitites have been well-documented. However, there has been little understanding about the albitites. Albitites in the Myanmar jadeite area, which are always associated with jadeitites, are perfect samples for understanding formation of the jadeitite and related events. Albitite is mainly composed of low-temperature albite, pyroxene minerals such as jadeite, omphacite and diopside and amphibole minerals such as richterite, winchite and magnesio-riebeckite, natrolite. Replacement of the jadeite by albite in the albitite along its cleavage and crack indicates that albite forms later than the jadeite. Albite record conditions of lower pressure than the jadeitite, and it was formed at temperature lower than 300℃, thus it is concluded that albite forms during uplift of the jadeitite through hydrothermal filling and precipiation. Diopside in the albitite consists of two types: one formed by replacement of jadeite which contains some components of diopside, the other is inferred to be the residual which was part of the replaced rocks like rodingite or pyroxenite belonging to the host serpentine melánge. The latter can be used as mineralogical evidence for confirmation of jadeitized pyroxenite and jadeitized rodingite. Existence of the abitite shows that Na-rich fluids are thorough and traversing.
Key words: Myanmar     Albitite     Jadeitite     hydrothermal     High-P/Low-T    
1 引言

钠长石岩是以单矿物钠长石为主要组成的一种岩石类型,世界上比较少见。最常见的钠长岩类型是由热液交代花岗岩而形成(Kovalenko, 1978; Cathelineau, 1988)。这种类型的钠长石岩通常形成于寄主花岗岩的裂隙中,以从原岩中淋滤出石英并且有新矿物沉淀为特征,伴随着明显的蚀变相(变正长岩)和/或铀的矿化(Cathelineau, 1987; Schoch and Scheepers, 1990; Petersson and Eliasson, 1997; Deng et al., 2009; Yang et al., 2009; Sun et al., 2010)。另一种是直接从富钠的岩浆中结晶而形成的钠长岩,具有典型的“石英雪球”结构,如我国宜春地区钽-锂花岗岩(Schwartz, 1992)。这些钠长石岩或伴有金、铜、银、铅、锌、铁、钡、铀、稀有元素等矿产,或是作为重要的生产瓷器的原料。然而,产于低温高压俯冲带内与超基性岩中翡翠矿相伴生的钠长石岩,其产状、矿物组成、成因等都与上述钠长石岩有显著不同。

与翡翠伴生的钠长石岩,主要产于缅甸、危地马拉、日本等地。其中危地马拉的钠长石岩是流体在较低的压力条件(3~8kb)下交代硬玉岩形成,和硬玉岩以脉状包体的形式出现在蛇纹岩基体中(Johnson and Harlow, 1999);日本的钠长石岩和硬玉岩是以硬玉岩-钠长石岩脉形式存在,脉体呈环带,由内到外依次是钠长石岩(含或不含石英)-白色硬玉岩-绿色硬玉岩-钠质、钠钙质角闪石-蛇纹石基质(Chihara,1999), 钠长石岩被描述为硬玉岩体的核心(Iwao, 1953; Shido, 1958; Chihara, 1971)。但是后来多位地质学家(Harlow and Sorensen, 2005; Shi et al., 2010, 2011, 2012; Tsujimori and Harlow, 2012)的实地考查和研究没有在岩体中找到钠长石核,他们认为所谓的钠长石核应该是钠长石带或钠长石层。缅甸硬玉岩地区的钠长石岩也多次被报道(Chhibber, 1934; Harlow and Olds, 1987; Shi et al., 2003)。缅甸的钠长石岩多以后期矿脉形式切割硬玉岩、绿辉岩、角闪岩和/或钠铬辉石岩,还可以粒间充填相的形式出现在硬玉岩和绿辉岩中(Shi et al., 2012)。

钠长石岩多产于翡翠矿床,与硬玉岩关系密切。目前硬玉岩的成因主要有两种模式:一种是斜长花岗岩、变质辉长岩和榴辉岩经变质交代作用而形成(Shigeno et al., 2005; Compagnoni et al., 2007; Mori et al., 2011; Tsujimori and Harlow, 2012);另一种是从流体中直接结晶(Harlow, 1994; Miyajima et al., 1999; Shi et al., 2005; Sorensen et al., 2006; Morishita et al., 2007; Tsujimori and Harlow, 2012)。对与硬玉岩相关的钠长石岩的成因也有相似的理解。例如,Harlow (1994)描述了危地马拉的两种钠长石岩,一种是经交代变质作用形成,另一种是从流体中直接结晶而形成。不论是哪一种成因的硬玉岩和钠长石岩,流体在其形成过程中扮演了重要的角色。有很多学者对高压低温条件下流体的成分和元素的行为进行了研究。Manning (1998)模拟了俯冲带中蓝片岩相变质作用的流体性质和成分,其认为俯冲带中流体的成分是由深度决定的(Manning, 2004)。对硬玉岩及其相关岩石如蛇纹岩、钠长石岩、异剥钙榴岩等的研究,有助于了解俯冲带中流体的成分和性质以及壳幔相互作用的过程。目前在缅甸翡翠矿区,对于蛇纹岩、辉石岩和异剥钙榴岩等已经开展了一定的研究(如施光海等, 2001; 易晓等, 2006; Wang et al., 2012),从中获得了较丰富的有关缅甸翡翠的成岩信息。然而,对于钠长石岩,目前的研究仍然开展甚少。

2 地质背景

缅甸硬玉岩位于印-缅岭(Indo-Myanmar Range)以东,缅中盆地(Central Myanmar Basin)北部,实皆走滑断裂(Sagaing Strike-Slip Fault)北端(其中西端一分支断裂经过岩体) (图 1)。其位于帕敢-道茂蛇绿杂岩体内。各地质单元的特征简述如下:

图 1 缅北大地构造略图(据Bender, 1983; Morley, 2004修改) Fig. 1 Simplified geological map the northern Myanmar (after Bender, 1983; Morley, 2004)

印-缅岭:地理上,该岭在缅甸东起缅中盆地,西至缅甸西部国界。岭的东界由不连续蛇绿岩和蛇绿岩衍生岩石块体构成。这一区域的岩石的堆积与形变发生于俯冲带环境,该俯冲带环境是由于印度板块最东部分在缅甸板块下的向东俯冲所引起。帕敢-道茂蛇绿杂岩可能是被实皆走滑断裂切断的印-缅岭内蛇纹岩的一部分,但他们之间的关系还没有确定。

缅中盆地:位于缅甸中部(从16°N到25°N, 94°E到97°E),由夹于印-缅岭和中-缅高地间的低洼部分构成。南北向展布的火山弧杂岩将缅中盆地分隔成一系列的雁列式、类型不同的狭长盆地。盆地以西被认为是弧前盆地,有厚达15km的沉积物(Pivnik et al., 1998)。缅中盆地内的火山弧含有印度洋壳板块向东俯冲于缅甸板块之下的证据(Swe, 1981; Bertrand and Rangin, 2003)。

实皆走滑断裂:是起于喜马拉雅山脉东角的一巨型南北向延伸超过1200km的右旋走滑断裂,至今可能已滑移了150~300km (Vigny et al., 2003)。这一断裂容纳了印度板块相对于欧亚板块约60%的向北运动(Bertrand and Rangin, 2003; Morley, 2004)。该断裂最北端表现为多个交角很小的断裂,近平行展布,经过硬玉岩出露区(Morley, 2004)。

帕敢-道茂蛇纹岩杂岩呈近N-S的带状分布,长约50km,宽5~10km。主要由以下岩石构成:蛇纹岩化的橄榄岩、含铬铁矿橄榄岩、铬铁矿块体、变辉长岩、玄武岩等,围岩主要为蓝片岩相和高角闪岩相的岩石,有以下几类:含蓝闪石多硅白云母的石英片岩、含黑硬绿泥石的石英岩、含透辉石大理岩、含石榴石斜长角闪岩、角闪岩类等(施光海等,2001)。硬玉岩主要呈脉状、团块状产于蛇纹岩中,其与蛇绿杂岩之间为钠质或钠钙质角闪石岩、钠铬辉石岩、绿辉石岩等。硬玉岩脉长几十至数百米,宽数十厘米至几十米,主要由较纯净的硬玉矿物组成,大多呈致密块状构造。在缅甸硬玉岩中,有时可见到钠长石脉。这些钠长石脉切割硬玉岩及其相关的角闪石边和钠铬辉石岩,为后期形成产物。

本次研究中,选取了14块钠长石岩作为研究样品。样品产于帕敢翡翠矿区,其特点是均含有钠长岩。简要描述如下:样品Ab-Jd为含宽约6~7cm呈灰白色的钠长石脉的缅甸硬玉岩,脉体切穿周围的硬玉岩、角闪岩和钠铬辉石岩(图 2a)。选取钠长石脉及其与硬玉岩的过渡带,磨制探针片,编号依次为Ab-Jd1、Ab-Jd2、Ab-Jd3、Ab-Jd4、Ab-Jd5。样品2010WJ-1,2010WJ-2,2010WJ-3为钠长石岩与硬玉岩相伴而生(图 2b-d)。样品D-2为黄绿色的钠长石岩,结构较细(图 2d)。样品BS-1、BS-2、BS-3、BS-4、BS-5为较纯净的钠长石岩(图 2f),为白色-灰白色。

图 2 缅甸钠长石岩样品照片 Fig. 2 Albitites samples from jadeitite area in Myanmar
3 测试方法

X射线粉晶衍射实验在中国地质大学(北京)粉晶X射线衍射实验室完成,采用日本理学D/Max-RC粉晶衍射仪,测试条件:Cu靶(Cu Kα1=1.5406), 电压为40kV,电流为80mA,起始角度为3°,终止角度为70°,连续扫描速度为8度/分,狭缝DS=SS=1°,RS=0.15mm。

样品的矿物成分及电子背散射图像是在中国地质大学(北京)电子探针室用EMPA-1600探针仪获得的,测试条件:加速电压为15kV,电流为10nA,电子束直径小于10μm,采用ZAF修正法。随后在中国科学院地质与地球物理研究所用JXA-8100探针仪进行了后续研究,仪器运行条件同上。EMPA标样包含如下矿物:钙铁榴石测Si和Ca,金红石测Ti,刚玉测Al,赤铁矿测Fe,绿铬矿测Cr,蔷薇辉石测Mn,绿镍矿测Ni,方镁石测Mg,钠长石测Na,钾长石测K,重晶石测Ba。矿物分子式的计算是利用Minpet 2.0程序计算,辉石的Fe2+与Fe3+的比值是依据电价平衡计算的,以23个氧为标准来估计Fe2+与Fe3+的比值(Enders et al., 2000)。

4 岩相学及矿物化学 4.1 钠长石

钠长石岩有等粒变晶结构,颗粒大小在0.03~0.05mm,多呈半自形粒状,颗粒紧密相连,颗粒边界平直或稍弯曲。另有一些为不等粒变晶结构,可细分为连续不等粒变晶结构和斑状变晶结构。前者粒度多在0.02~0.20mm之间逐渐过渡,后者斑晶多为自形-半自形粒状钠长石,粒度约为0.50mm;基质为半自形-他形粒状钠长石,粒度多小于0.02mm。

钠长石岩的主要矿物组成为钠长石。其中钠长石含量可达90%,钠长石中Ab含量接近100% (表 1)。结合样品的X射线粉晶衍射数据(测试Ab-Jd3和BS-2),测定钠长石为低钠长石(图 3)。其中:Δθ1=2θ131-2θ131,Δθ2=2θ241-2θ241。低钠长石的Si、Al完全有序,Al主要集中在T1位。据Δd和钠长石的牌号在钠长石的形成温度图(图 4),估算钠长石的形成温度低于300℃。

表 1 钠长石岩中代表性钠长石的化学分析(wt%) Table 1 Representative chemical compositions (wt%) of albite in the albitite

图 3 斜长石Δθ1、Δθ2与成分和状态关系图 Fig. 3 Plagioclase ingredient and its structure

图 4 钠长石的形成温度图 Fig. 4 Chart of formation tempreture of albite
4.2 钠沸石

钠沸石晶体或生长于钠长石集合体中,边缘比较完整(图 5a),形成于钠长石之后。或沿硬玉的解理、颗粒间的裂隙交代硬玉(图 5b),形成于硬玉之后。钠沸石是最早发现(1758年)的沸石族矿物之一(齐进英和江绍英,1983)。用阴离子法(氧原子数为10)计算其平均化学式为(Na1.7536Ca0.0049Fe0.0041Mn0.0034Ti0.0003)1.769[Al1.8590Si3.1581O10]·2H2O,其中Na:Al≈1.0,(Al,Si):O≈1:2,为低硅钠沸石。

图 5 钠长石与其它矿物的关系图 (a)-钠沸石颗粒在钠长石基质中,颗粒边缘比较完整的偏光显微镜照片(+);(b)-钠沸石沿硬玉的解理、颗粒间的裂隙交代硬玉的电子背散射图像;(c)-钠长石沿硬玉的解理和裂隙交代硬玉的偏光显微镜照片(+);(d)-钠长石颗粒沿裂隙交代钠透闪石的偏光显微镜照片(+) Fig. 5 Relationships of albite and other minerals (a)-natrolite phenocryst in albitite (transmitted light, +); (b)-replacement of jadeite by natrolite along the cleavage and fracture of jadeite grains (BSE image); (c)-replacement of jadeite by albite along the cleavage and cracks of jadeite grains (transmitted light, +); (d)-replacement of richterite by albite along the cracks of richterite grains (transmitted light, +)
4.3 硬玉

在硬玉岩中可见钠长石沿着硬玉的解理裂隙交代硬玉(图 5c),也说明钠长石形成晚于硬玉。钠长石岩中硬玉成份相对比较纯净, 其端员组分NaAl[Si2O6]的含量为86.6%~99.2%(表 2图 6)。

表 2 钠长石岩中代表性辉石的化学成分(wt%) Table 2 Representative chemical compositions (wt%) of pyroxene in the albitite

图 6 钠长石玉标本中辉石的Ca-Mg-Fe和Na Pyr Jd-Ae-Q投影图 WEF-Wo (Wollastonite)、En (Enstatite)和Fs (Ferrosilite)三种矿物含量总和;Wo-硅灰石;En-顽火辉石;Fs-铁辉石;Ae-霓石;Jd-硬玉 Fig. 6 Diagram for the Ca-Mg-Fe and Na pyr Jd-Ae-Q of pyroxene in albitite WEF-the sum of Wo, En, Fs; Wo-Wollastonite; En-Enstatite; Fs-Ferrosilite; Ae-Aegirine; Jd-Jadeite
4.4 绿辉石和透辉石

绿辉石和透辉石多以短柱状包裹于钠长石集合体中(图 7a, b),部分绿辉石被硬玉交代成孤岛状(图 7c)。此外,还可观察到透辉石被绿辉石交代成残留体(图 7d)。上述矿物的形成顺序依次为:透辉石→绿辉石→硬玉→钠长石。

图 7 钠长石岩中辉石与其它矿物关系的电子背散射图像 (a)-透辉石和绿辉石以短柱状形式存在于钠长石颗粒中;(b)-镁钠闪石沿解理和裂隙交代透辉石;(c)-绿辉石被硬玉交代成孤岛状;(d)-透辉石被绿辉石交代成残留体 Fig. 7 Relationships of pyroxene minerals and other minerals in albitites (BSE images) (a)-stumpy diopside and omphcite in albite grain; (b)-replacement of diopside by magnesio-riebeckite along cleavage and cracks of diopside grains; (c)-isolated omphcite grains are surrounded by jadeite; (d)-diopside relicts in omphcite grain

钠长石岩中有两类绿辉石:一类绿辉石的SiO2含量高(达到59%),在辉石投影图(图 6)中位于绿辉石域的中部,M2位Ca2+, Na+的含量相当,属于P2绿辉石。另一类绿辉石中SiO2的含量相对较低,M2位Ca2+, Na+的含量变化大,在辉石投影图中位于绿辉石域上端近透辉石的端员部位,属于C2绿辉石(处于绿辉石和霓石-普通辉石的过渡状态)。

4.5 闪石类矿物

显微镜下可以观察到细小的钠长石交代钠透闪石(图 5d),说明钠长石形成于闪石之后。钠长石岩中的闪石类矿物主要为钠质和钠钙质角闪石类,包括镁钠闪石、钠透闪石和蓝透闪石(表 3)。

表 3 钠长石岩中代表性角闪石的化学成分(wt%) Table 3 Representative chemical compositions (wt%) of amphibole in the albitite
5 讨论 5.1 钠长石岩的成因探讨

钠长石岩与硬玉岩的关系一直是争论的焦点。李旭平和张立飞(2004)提出硬玉是由钠长石在埋藏深度在25~35km时分解形成硬玉和石英,形成的石英可被周围的超基性岩蛇纹石化过程中吸收。Shigeno et al. (2012)在日本Nishisonogi的翡翠矿区发现硬玉颗粒核部有石英包裹体,且石英所占体积分数(VQtz/(VJd+VQtz)接近0.27,认为硬玉颗粒核部的钠长石是由具有钠长石核心和硬玉边缘的颗粒在压力增大的条件下分解形成硬玉+石英。硬玉的微量元素组成(Sorensen et al., 2006; Morishita et al., 2007; Shi et al., 2008; Simons et al., 2010)和硬玉颗粒的成分环带,证明了含有饱和硬玉的脉状流体的存在;Harlow and Sorensen (2005)研究了来自世界主要产地的硬玉标本,没有发现原岩交代结构的残留。越来越多的研究也表明大多数产地的纯硬玉岩中的硬玉是从富Na-Al-Si的含水流体中直接结晶形成,仅有少数地区的硬玉岩是由变质交代作用而形成。

关于钠长石岩的成因,有几种不同的观点:即钠长石是由热液流体交代花岗岩而形成的(Kovalenko, 1978; Cathelineau, 1988),如伊拉克库尔德斯坦地区的一种由流体交代斜长花岗岩而形成的钠长石岩(Mohammad et al., 2007);从溶液中直接结晶而成(Harlow, 1994; Johnson and Harlow, 1999)和从富钠岩浆中直接结晶(Schwartz, 1992)。橄榄岩中的斜长花岗岩在经历了蛇纹石化和异剥钙榴岩化之后,可形成带有异剥钙榴岩黑边的钠长石岩,该成因类似于硬玉岩的成因。Harlow (1994)描述了危地马拉硬玉岩区两种钠长石岩,一种是交代硬玉而形成,另一种是直接从流体中结晶形成。

缅甸地区的钠长石岩呈脉状切穿硬玉岩和角闪岩,在偏光显微镜下可观察到钠长石沿硬玉解理交代。这种交代结构(图 5c)排除了钠长石是硬玉在压力降低条件下经压溶作用而形成的可能,从而,其形成与外来流体有关。缅甸翡翠矿床区钠长石岩的形成晚于硬玉岩,故其非硬玉岩的原岩。从其近纯净的脉状产状和交代早期辉石类和闪石矿物的结构来分析,其形成于富Na流体。在硬玉岩后期抬升过程中,压力降低,流体渗入可交代硬玉而形成交代型钠长石。在没有交代结构发育的钠长石岩中,目前不能排除钠长石从钠质流体中直接结晶形成的可能性。通过估算给出了缅甸钠长石的形成温度低于300℃,由于体系含水并伴生有钠沸石,依据Shi et al. (2012)中图 12,推测钠长石的形成压力应不高于0.5kb。

5.2 关于透辉石

透辉石在钠长石岩中发现,表明在缅甸翡翠形成之前有含透辉石的早期岩石。其有两种产状:呈短柱状和被绿辉石包裹的残留。据Harlow (1994),在钠长石交代硬玉的过程中,随着硬玉组分的消耗,形成产物为含Na更为纯净的钠长石,这种替代使透辉石中Ca2+增加(Sorensen and Harlow, 2001李旭平和张立飞,2004),本次研究中的呈短柱状透辉石与此种成因可能相同。对于被绿辉石包裹的透辉石残留,这种透辉石形成早于绿辉石,且透辉石和绿辉石都早于钠长石形成,因此这种透辉石不可能是钠长石交代硬玉的结果。被交代透辉石中的NaAl组分会渐进增加,最终形成了绿辉石,即包裹透辉石的绿辉石。由于缅甸翡翠的围岩为蛇纹杂岩,其中含透辉石的岩石可能为辉石岩和异剥钙榴岩。硬玉化的异剥钙榴岩(Wang et al., 2012)或辉石岩类(易晓等, 2006)的发现,与本次研究中发现的透辉石暗合,形成了很好的呼应。

5.3 钠质流体在俯冲带中的作用

钠长石岩和硬玉岩,无论是直接从流体中结晶形成,还是通过流体的交代作用而形成,都与富钠、铝、硅且贫钾的流体密切相关,但是这种流体的来源目前还不是很清楚。近年来,危地马拉、日本、缅甸等地的硬玉岩中钡矿物的发现(Harlow, 1995; Morishita, 2005; Shi et al., 2010), 缅甸硬玉中类似I型球粒陨石的发现(Shi et al., 2011)都表明形成硬玉岩的流体与蚀变的洋壳和洋壳沉积物有关。Sorensen et al. (2006)提出了几种可能的钠质来源:第一种是在低于300℃条件下被海水交代蚀变的岩石;第二种是经历了洋底热液系统交代的岩石,如细碧岩化的洋底玄武岩;第三种是与火成岩或地幔岩平衡的流体。但是目前对洋壳俯冲前所产生的钠质流体很少关注。有一种观点认为钠质流体可能来源于与蛇纹岩化有关的异剥钙榴岩化过程。蛇纹石化并不能产生钠质流体,在蛇纹石化过程中,单斜辉石分解产生的Ca2+不能被蛇纹石晶体所容纳而进入流体中,这种富Ca2+流体交代基性岩,形成异剥钙榴岩(李旭平等,2003)。异剥钙榴岩的原岩中含有钠(Austrheim and Prestvik, 2008; Bonev and Stampfli, 2009),但是异剥钙榴岩中却贫钠(李旭平和张立飞, 2004; Li et al., 2010)。在异剥钙榴岩化的过程中大量的钠和铝从原岩中转移,并且产生了富钠的流体。这种与硬玉成分类似的钠质流体随着洋壳俯冲被运送到俯冲带,在高压低温条件下形成硬玉。在抬升过程中可以发生二次蛇纹石化和异剥钙榴岩化作用(Li et al., 2007),由此产生的富钠流体可能成为后期钠质交代的流体来源(Shi et al., 2010; Wang et al., 2010, 2012)。此外,硬玉岩和钠长石岩受SiO2活度的影响很大,流体流向低压环境时富硅而贫钠和钾。在回返过程中,流体SiO2的活度由形成硬玉时的次饱和状态转变为饱和状态(Harlow, 1994; 易晓等, 2006),进而交代硬玉形成钠长石岩。

6 结论

缅甸钠长石岩的主要矿物成分为低温钠长石,其形成温度小于300℃,形成压力不高于0.5kb。对缅甸钠长石岩的形成过程作出如下推测:低温低压下蛇纹岩化,异剥钙榴岩化共同作用产生了富钠流体,这种流体可随着洋壳的俯冲被带入到俯冲带,在这里可能与洋壳变质脱水形成的流体共同构成了富Na流体。这种流体在高压低温条件下可形成硬玉岩。在抬升-降压过程中,超基性岩石还可能会发生蛇纹岩化和异剥钙榴岩化,导致产生富钠的流体。由于回返过程中温度变化不明显,压力降低可能会增加流体中的SiO2活度。这种富Na流体既可与硬玉发生交代反应(Jd+SiO2aq=Ab)形成钠长石岩,也可在硬玉岩的裂隙中直接呈脉状结晶。过程中,一些早期被交代岩石的组成矿物,如透辉石在后期被硬玉、钠长石交代过程中残留并被识别。

致谢 感谢毛骞博士在测试过程给予的支持和审稿人建设性的审稿意见!
参考文献
[] Austrheim H, Prestvik T. 2008. Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north-central Norway. Lithos, 104(1-4): 177–198. DOI:10.1016/j.lithos.2007.12.006
[] Bender F. 1983. Geology of Burma. Berlin: Borntraeger, 1-293
[] Bertrand G, Rangin C. 2003. Tectonics of the western margin of the Shan plateau (central Myanmar): Implication for the India-Indochina oblique convergence since the Oligocene. Journal of Asian Earth Sciences, 21(10): 1139–1157. DOI:10.1016/S1367-9120(02)00183-9
[] Bonev N, Stampfli G. 2009. Gabbro, plagiogranite and associated dykes in the supra-subduction zone Evros Ophiolites, NE Greece. Geological Magazine, 146(1): 72–91. DOI:10.1017/S0016756808005396
[] Cathelineau M. 1987. U-Th-Ree mobility during albitization and quartz dissolution in granitoids: Evidence from southeast French Massif Central. Bulletin de Mineralogie, 110: 249–259.
[] Cathelineau M. 1988. Accessory mineral alteration in peraluminous granites at the hydrothermal stage: A review. Rendiconti della Società Italiana di Mineralogia e Petrologia, 43: 499–508.
[] Chhibber HL. 1934. The Mineral Resources of Burma. London: Macmillan and Co. Ltd.: 1-309.
[] Chihara K. 1971. Mineralogy and paragenesis of jadeites from the Omi-Kotaki area, Central Japan. Mineralogical Society of Japan, Special Paper, 1: 147–156.
[] Chihara K. 1999. Jadeite in Japan. Journal of the Gemological Society of Japan, 20: 5–21.
[] Compagnoni R, Rolfo F, Manavella F, Salusso F. 2007. Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps. Per. Mineral., 76(2-3): 79–89.
[] Deng J, Yang LQ, Gao BF, Sun ZS, Guo CY, Wang QF, Wang JP. 2009. Fluid evolution and metallogenic dynamics during tectonic regime transition: Example from the Jiapigou gold belt in Northeast China. Resource Geology, 59(2): 140–152. DOI:10.1111/rge.2009.59.issue-2
[] Enders M, McCammon CA, Maresch WV, Speer D. 2000. Ferric/ferrous iron ratios in sodic amphiboles: Mössbauer analysis, stoichiometry-based model calculations and the high-resolution micro-analytical flank method. Contributions to Mineralogy and Petrology, 140(2): 135–147. DOI:10.1007/s004100000179
[] Harlow GE, Olds EP. 1987. Observations on terrestrial ureyite and ureyitic pyroxene. Am. Mineral., 72: 126–136.
[] Harlow GE. 1994. Jadeitites, albitites and related rocks from the Motagua Fault zone, Guatemala. Journal of Metamorphic Geology, 12(1): 49–68. DOI:10.1111/jmg.1994.12.issue-1
[] Harlow GE. 1995. Crystal chemistry of barian enrichment in micas from metasomatized inclusions in serpentine, Motagua fault zone, Guatemala. Eur. J. Mineral., 7(4): 775–789. DOI:10.1127/ejm/7/4/0775
[] Harlow GE, Sorensen SS. 2005. Jade (Nephrite and Jadeitite) and serpentinite: Metasomatic connections. International Geology Review, 47(2): 113–146. DOI:10.2747/0020-6814.47.2.113
[] Iwao S. 1953. Albitite and associated jadeite rock from Kotaki District, Japan: A study in ceramic raw material. Reports of the Geological Survey of Japan, 153: 26.
[] Johnson CA, Harlow GE. 1999. Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone. Geology, 27(7): 629–632. DOI:10.1130/0091-7613(1999)027<0629:GJAAWF>2.3.CO;2
[] Kovalenko VI. 1978. The genesis of rare metal granitoids and related ore deposit. In: Štemprok M, Burnol L and Tischendorf G (eds.). Metallization Associated with Acid Magmatism. Prague, Czech Geological Survey, 3: 235-247
[] Li XP, Zhang LF, Ai YL. 2003. Discovery and geological implication of ophiolites at Changawuzi, western Tianshan, China. Progress in Natural Science, 13(7): 754–760.
[] Li XP, Zhang LF. 2004. Relationship between jadeitite and rodingite in serpentinite complex. Acta Petrologica Sinica, 20(6): 1477–1484.
[] Li XP, Zhang LF, Wei C, Ai YL, Chen J. 2007. Petrology of rodingite derived from eclogite in western Tianshan, China. Journal of Metamorphic Geology, 25(3): 363–382. DOI:10.1111/jmg.2007.25.issue-3
[] Li XP, Zhang LF, Wilde SA, Song B, Liu XM. 2010. Zircons from rodingite in the western Tianshan serpentinite complex: Mineral chemistry and U-Pb ages define nature and timing of rodingitization. Lithos, 118(1-2): 17–34. DOI:10.1016/j.lithos.2010.03.009
[] Manning CE. 1998. Fluid composition at the blueschist-eclogite transition in the model system Na2O-MgO-Al2O3-SiO2-H2O-HCl. Schweizerische Mineralogische and Petrographische Mitteilungen, 78(2): 225–242.
[] Manning CE. 2004. The Chemistry of subduction-zone fluids. Earth and Planetary Science Letters, 223(1-2): 1–16. DOI:10.1016/j.epsl.2004.04.030
[] Miyajima H, Matsubara S, Miyawaki R, Ito K. 1999. Itoigawaite, a new mineral, the Sr analogue of lawsonite, in jadeitite from the Itoigawa-Ohmi district, central Japan. Mineralogical Magazine, 63(6): 909–916. DOI:10.1180/002646199548213
[] Mohammad YO, Maekawa H, Lawa FA. 2007. Mineralogy and origin of Mlakawa albitite from Kurdistan region, Northeastern Iraq. Geosphere, 3(6): 624–645. DOI:10.1130/GES00081.1
[] Mori Y, Orihashi Y, Miyamoto T, Shimada K, Shigeno M, Nishiyama T. 2011. Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan. Journal of Metamorphic Geology, 29(6): 673–684. DOI:10.1111/jmg.2011.29.issue-6
[] Morley CK. 2004. Nested strike-slip duplexes, and other evidence for Late Cretaceous-Palaeogene transpressional tectonics before and during India-Eurasia collision, in Thailand, Myanmar and Malaysia. Journal of the Geological Society, 161(5): 799–812. DOI:10.1144/0016-764903-124
[] Morishita T. 2005. Occurrence and chemical composition of barian feldspars in a jadeitite from the Itoigawa-Omi district in the Renge high-P/T-type metamorphic belt, Japan. Mineralogical Magazine, 69(1): 39–51. DOI:10.1180/0026461056910237
[] Morishita T, Arai S, Ishida Y. 2007. Trace element compositions of jadeite (+omphacite) in jadeitites from the Itoigawa-Ohmi district, Japan: Implications for fluid processes in subduction zones. Island Arc, 16(1): 40–56. DOI:10.1111/iar.2007.16.issue-1
[] Petersson J, Eliasson TH. 1997. Mineral evolution and element mobility during episyenitization (dequartzification) and albitization in the postkinematic Bohus granite, southwest Sweden. Lithos, 42(1-2): 123–146. DOI:10.1016/S0024-4937(97)00040-6
[] Pivnik DA, Nahm J, Tucker RS, Smith GO, Nyein K, Nyunt M, Maung PH. 1998. Polyphase deformation in a fore-arc/back-arc basin, Salin subbasin, Myanmar (Burma). American Association of Petroleum Geologists Bulletin, 82(10): 1837–1856.
[] Qi JY, Jiang SY. 1983. On thermal transformation and genesis of natrolite. Chinese Science Bulletin, 28(3): 162–165.
[] Schoch AE, Scheeper R. 1990. The distribution of uranium and thorium in the cape columbine granite from the southwestern cape province, South Africa. Ore Geology Reviews, 5(3): 223–246. DOI:10.1016/0169-1368(90)90012-C
[] Schwartz MO. 1992. Geochemical criteria for distinguishing magmatic and metasomatic albite-enrichment in granitoids: Examples from the Ta-Li granite Yichun (China) and the Sn-W deposit Tikus (Indonesia). Mineralium Deposita, 27(2): 101–108.
[] Shi GH, Cui WY, Liu J, Yu HX. 2001. Petrology of jadeite-bearing serpentinized peridotite and its country rocks from Northwestern Myanmar (Burma). Acta Petrologica Sinica, 17(3): 483–490.
[] Shi GH, Cui WY, Tropper P, Wang CQ, Shu GM, Yu HX. 2003. The petrology of a complex sodic and sodic-calcic amphibole association and its implications for the metasomatic processes in the jadeitite area in northwestern Myanmar, formerly Burma. Contributions to Mineralogy and Petrology, 145(3): 355–376. DOI:10.1007/s00410-003-0457-y
[] Shi GH, Tropper P, Cui WY, Tan J, Wang CQ. 2005. Methane (CH4)-bearing fluid inclusions in the Myanmar jadeitite. Geochemical Journal, 39(6): 503–516. DOI:10.2343/geochemj.39.503
[] Shi GH, Cui WY, Cao SM, Jiang N, Jian P, Liu DY, Miao LC, Chu BB. 2008. Ion microprobe zircon U-Pb age and geochemistry of the Myanmar jadeitite. Journal of the Geological Society, 165(1): 221–234. DOI:10.1144/0016-76492006-119
[] Shi GH, Jiang N, Wang YW, Zhao X, Wang X, Li GW, Ng E, Cui WY. 2010. Ba minerals in clinopyroxene rocks from the Myanmar jadeitite area: Implications for Ba recycling in subduction zones. European Journal of Mineralogy, 22(2): 199–214. DOI:10.1127/0935-1221/2010/0022-1998
[] Shi GH, Zhu XK, Deng J, Mao Q, Liu YX, Li GW. 2011. Spherules with pure iron cores from Myanmar jadeitite: Type-I deep-sea spherules?. Geochimica et Cosmochimica Acta, 75(6): 1608–1620. DOI:10.1016/j.gca.2011.01.005
[] Shi GH, Harlow GE, Wang J, Wang J, Enoch NG, Wang X, Cao SM, Enyuancui W. 2012. Mineralogy of jadeitite and related rocks from Myanmar: A review with new data. European Journal of Mineralogy, 24(2): 345–370. DOI:10.1127/0935-1221/2012/0024-2190
[] Shido F. 1958. Calciferous amphibole rich in sodium from jadeite-bearing albitite of Kotaki, Niigata Prefecture. Journal of the Geological Society of Japan, 64(758): 595–600. DOI:10.5575/geosoc.64.595
[] Shigeno M, Mori Y, Nishiyama T. 2005. Reaction microtextures in jadeitites from the Nishisonogi metamorphic rocks, Kyushu, Japan. Journal of Mineralogical and Petrological Sciences, 100(6): 237–246. DOI:10.2465/jmps.100.237
[] Shigeno M, Mori Y, Shimada K, Nishiyama T. 2012. Jadeitites with metasomatic zoning from the Nishisonogi metamorphic rocks, western Japan: Fluid-tectonic block interaction during exhumation. European Journal of Mineralogy, 24(2): 289–311. DOI:10.1127/0935-1221/2012/0024-2195
[] Simons KK, Harlow GE, Brueckner HK, Goldstein SL, Sorensen SS, Hemming NG, Langmuir +CH. 2010. Lithium isotopes in Guatemalan and Franciscan HP-LT rocks: Insights into the role of sediment-derived fluids during subduction. Geochimica et Cosmochimica Acta, 74(12): 3621–3641. DOI:10.1016/j.gca.2010.02.033
[] Sorensen SS and Harlow GE. 2001. The jadeitites of Nansibon, Myanmar: Records of the geochemistry of subduction zone fluids. Eleventh Annual V. M. Goldschmidt Conference, 3800
[] Sorensen SS, Harlow GE, Rumble D. 2006. The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence. American Mineralogist, 91(7): 979–996. DOI:10.2138/am.2006.1949
[] Sun X, Deng J, Zhao ZY, Zhao ZH, Wang QF, Yang LQ, Gong QJ, Wang CM. 2010. Geochronology, petrogenesis and tectonic implications of granites from the Fuxin area, Western Liaoning, NE China. Gondwana Research, 17(4): 642–652. DOI:10.1016/j.gr.2009.09.008
[] Swe W. 1981. A major strike-slip fault in Burma. Contribution to Burmese Geology, 1(1): 63–72.
[] Tsujimori T, Harlow GE. 2012. Petrogenetic relationships between jadeitite and associated high-pressure and low-temperature metamorphic rocks in worldwide jadeitite localites: A review. European Journal of Mineralogy, 24(2): 371–390. DOI:10.1127/0935-1221/2012/0024-2193
[] Vigny C, Socquet A, Rangin C, Chamot-Rooke N, Pubellier M, Bouin MN, Bertrand G, Becker M. 2003. Present-day crustal deformation around Sagaing fault, Myanmar. Journal of Geophysical Research, 108(B11): 2533. DOI:10.1029/2002JB001999
[] Wang CM, Deng J, Zhang ST, Yang LQ. 2010. Metallogenic province and large scale mineralization of volcanogenic massive sulfide deposits in China. Resource Geology, 60(4): 404–413. DOI:10.1111/rge.2010.60.issue-4
[] Wang X, Shi GH, Qiu DF, Wang J, Cui WY. 2012. Grossular-bearing jadeite omphacite rock in the Myanmar jadeite area: A kind of jadeitized rodingite?. European Journal of Mineralogy, 24(2): 237–246. DOI:10.1127/0935-1221/2011/0023-2157
[] Yang LQ, Deng J, Guo CY, Zhang J, Jiang SQ, Gao BF, Gong QJ, Wang QF. 2009. Ore-forming fluid characteristics of the Dayingezhuang gold deposit, Jiaodong gold province, China. Resource Geology, 59(2): 181–193. DOI:10.1111/rge.2009.59.issue-2
[] Yi X, Shi GH, He MY. 2006. Jadeitized omphacitite from Myanmar jadeite area. Acta Petrologica Sinica, 22(4): 971–976.
[] 李旭平, 张立飞, 艾永亮. 2003. 新疆西天山长阿吾子蛇绿混杂岩中与榴辉岩伴生的异剥钙榴岩的发现及其地质意义. 自然科学进展, 13(7): 754–760.
[] 李旭平, 张立飞. 2004. 蛇纹岩体中的硬玉岩与异剥钙榴岩. 岩石学报, 20(6): 1477–1484.
[] 齐进英, 江绍英. 1983. 钠沸石的热转变及其成因. 科学通报, 28(3): 162–165.
[] 施光海, 崔文元, 刘晶, 于海侠. 2001. 缅甸含硬玉的蛇纹石化橄榄岩及其围岩的岩石学研究. 岩石学报, 17(3): 483–490.
[] 易晓, 施光海, 何明跃. 2006. 缅甸硬玉岩区的硬玉化绿辉石岩. 岩石学报, 22(4): 971–976.