Amphibole in UHP eclogite from the Sulu region, eastern China
Amphibole in UHP eclogite from the Sulu region, eastern China
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DOI：
10.2465/jmps.170420
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发表时间：
2018
影响因子：
0.7
通讯作者：
S. Yamasaki;D. Nakamura;T. Hirajima
中科院分区：
文献类型：
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作者：
S. Yamasaki;D. Nakamura;T. Hirajima
根据苏鲁地区东海青龙山和建昌地区超高压榴辉岩的矿物学资料，研究了角闪石在620-750℃、2.9-3.7 Gpa的超高压条件下的稳定性。所研究的样品主要由石榴石、绿辉石、角闪石、金红石、蓝晶石、石英、绿帘石和多硅白云母组成。根据角闪石的赋存状态，角闪石可分为三种类型：Amp I，石榴石和绿帘石中的包裹体；Amp II，基质相；Amp III，沿晶界发育的小颗粒(~10-30µm)。青龙山AMP I的NaB含量稍差(B点的Na含量为1.2a.p.f.u)，主要是钠长石-纤锌矿。Amp II是一种粗晶石，除了某些颗粒的边缘(NAB=1.1-1.4a.p.f.u；Si=7.0-7.5a.p.f.u)外，其余颗粒的成分基本都是均一的。并在各类型中显示最高的NAB值。AMP III的NAB明显较低(=0.3-1.15 a.p.f.u)，主要是绿辉石-蒲公英。另一方面，建昌的AmpⅡ和Amp III分别为Nyböite和Taramite，且建昌角闪石的(NAA+NAB)值之和明显大于青龙山角闪石。在青龙山的大片绿帘石颗粒中发现了柯石英和Amp I之后的多晶石英假象，并含有大块绿帘石成分。这些岩石学数据表明了以下的进化史。在榴辉岩的挤压过程中，具有较低NAB值的Amp I结晶并与其他矿物颗粒混杂，在峰期以稳定相的形式出现重晶石Amp II。最后，在减压阶段形成了NAB值低得多的AMP III。因此，我们得出结论，即使在超高压条件下，榴辉岩中也可以稳定地存在钙钠角闪石(AMP II)。此外，在青龙山样品中的重晶质角闪石中还检测到了大量的F(高达1wt%)fi。至少，研究区的一些榴辉岩在超高压条件下可能发生了含FfiflUID的交代作用。
We examined the stability of amphibole in ultrahigh – pressure (UHP) conditions of about 620 – 750 °C, 2.9 – 3.7 GPa based on the mineralogy of UHP eclogites collected from the Qinglongshan and Jianchang areas in Donghai County of the Sulu region. The samples studied are mainly composed of garnet, omphacite, amphibole, rutile, kyanite, quartz, epidote, and phengite. Amphibole can be classi fi ed into three types based on its mode of occurrence; Amp I, inclusions in garnet and epidote; Amp II, the matrix phase; Amp III, small (~ 10 – 30 µm) grains developed along grain boundaries. Amp I from Qinglongshan, mainly barroisite – taramite, is slightly poorer in NaB (Na contents in B site <1.2 a.p.f.u.) than Amp II. Amp II is barroisite with a mostly homoge-neous composition in each grain except for the margins of some grains (NaB = 1.1 – 1.4 a.p.f.u; Si = 7.0 – 7.5 a.p.f.u.) and shows the highest NaB values among the types. Amp III, mainly pargasite – taramite, shows clearly lower NaB (= 0.3 – 1.15 a.p.f.u.) than that of Amp II. On the other hand, Amp II and Amp III from Jianchang are nyböite and taramite, respectively, and the sum of the (NaA + NaB) values from the Jianchang amphibole is distinctly greater than that of the Qinglongshan amphibole. Polycrystalline quartz pseudomorphs after coesite and Amp I with barroisite compositions were found within a large epidote grain of the Qinglongshan samples. These petrological data suggest the following evolutionary history. Amp I with relatively low NaB values crystalized and was involved with other mineral grains during the compression of the eclogite, and then barroisitic Amp II occurred as a stable phase during the peak – P stage. Finally, Amp III with much lower NaB values was formed during the decompression stage. Thus, we concluded that a Ca – Na amphibole (Amp II) can be stably present in an eclogite even in the UHP conditions. In addition, signi fi cant amounts of F (up to 1 wt%) were detected from the barroisitic amphibole in the Qinglongshan samples. At the least, some eclogites from the studied areas may have undergone metasomatic in fi ltration of an F – bearing fl uid in the UHP conditions.