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首页> 外文期刊>Contributions to Mineralogy and Petrology >Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka
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Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

机译：片麻岩的交代转换和部分融化中组分的流动性：来自斯里兰卡的一个例子

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摘要

Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite-hornblende gneiss from the Kurunegala district, Sri Lanka (hornblende (N_(Mg) = 38-42) + biotite (N_(Mg) = 42-44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite) was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO_2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700-750 deg C, 5-6 kbar, and a_(H_2O)~(fl)= 0.52-0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (N_(Mg) = 4548) + K-feldspar (Or_(70-80)) m quartz + plagioclase (An_(28)) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the C02-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns:- a transition zone with the assemblage biotite (N_(Mg) = 49-51) + hornblende (N_(Mg) =47-50) + plagioclase + quartz + K-feldspar t ilmenite + magnetite;- a KPQ (K-feldspar-plagioclase-quartz) zone with the assemblage K-feldspar + plagioclase +ortho-pyroxene (N_(Mg) = 45-48) quartz + ilmenite + magnetite:- a charnockite core with the assemblage K-feldspar + plagioclase + orthopyroxene (N_(Mg) = 39-41) + biotite (N_(Mg) = 48-52) + quartz + ilmenite + magnetite.Systematic changes in the bulk chemistry and mineralogy across the four zones suggest that along with metasomatic transformation, this process may have been complicated by partial melting in the charnockite core. This melting would have been coeval with metasomatic processes on the periphery of the charnockite patch. There is also good evidence in the charnockitic core that a second mineral assemblage, consisting of orthopyroxene (N_(Mg) = 36-42) + biotite (N_(Mg) = 50-51) + K-feldspar (Or_(70-80)) + quartz plagioclase (An_(28-26)), could have crystallised from a partial melt during cooling from 720 to 660 deg C at decreasing a_(H_2O)~(fl) from 0.67 to 0.5. Post-magmatic evolution of charn6ckite at T < 700 deg C resulted in fluids being released during the crystallisation of the charnockitic core. These gave rise to the formation of late stage rim myrmekites along K-feldspar grain boundaries as well as late stage biotite, cummingtonite, and carbonates.
机译：反应质地，流体包裹体和交代区带与热力学计算相结合，使我们能够估算出斯里兰卡库伦加拉地区的黑云母-角闪石片麻岩（角闪石（N_（Mg）= 38-42）+黑云母（N_ （Mg）= 42-44）+斜长石+石英+钾长石+钛铁矿+磁铁矿）沿剪切带和叶面转变成霞石块。主要流体包裹体数据表明，在炭化作用事件和随后的尖晶石质变质演化过程中，两种不混溶的流体共存，即碱性超临界盐水和几乎纯净的CO_2。这些变质流体在700-750℃，5-6 kbar和a_（H_2O）〜（fl）= 0.52-0.59的峰值变质条件下，沿着剪切带穿过闪石岩片麻岩迁移并进入围岩。这导致形成霞石斑块，除了沿钾长石微静脉外，还含有邻位邻苯二酚（N_（Mg）= 4548）+钾长石（Or_（70-80））m石英+斜长石（An_（28））。石英和斜长石晶界。富含CO 2的流体的残余物被捕获为单独的流体包裹体。硫霞石斑块显示出以下交代带状分布模式：-具有黑云母组合的过渡带（N_（Mg）= 49-51）+角闪石（N_（Mg）= 47-50）+斜长石+石英+钾长石钛铁矿+磁铁矿;-KPQ（钾长石-斜长石-石英）带，组合有钾长石+斜长石+邻辉石（N_（Mg）= 45-48）石英+钛铁矿+磁铁矿：-菱沸石岩芯钾长石+斜长石+邻苯二甲酚（N_（Mg）= 39-41）+黑云母（N_（Mg）= 48-52）+石英+钛铁矿+磁铁矿。四个区域的整体化学和矿物学的系统变化表明伴随着交代转化，这个过程可能已经由于菱镁矿岩心的部分熔化而变得复杂了。这种融化将与夏诺特贴片外围的交代过程同时发生。在霞石质岩芯中也有很好的证据表明，第二种矿物组合是由邻二甲苯（N_（Mg）= 36-42）+黑云母（N_（Mg）= 50-51）+钾长石（Or_（70-80 ））+石英斜长石（An_（28-26））可能在a_（H_2O）〜（fl）从0.67降低到0.5的过程中从720到660摄氏度的冷却过程中从部分熔体中结晶出来。在T <700摄氏度时，风霞石后岩浆的演化导致流体在风化岩心结晶过程中被释放。这些导致沿钾长石晶界形成晚期边缘黑闪石，以及晚期黑云母，卡明石和碳酸盐。

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