Igneous sapphirine as a product of melt-peridotite interactions in the Finero Phlogopite-Peridotite Massif, Western Italian Alps
Igneous sapphirine as a product of melt-peridotite interactions in the Finero Phlogopite-Peridotite Massif, Western Italian Alps
复制标题
DOI：
10.1127/0935-1221/2013/0025-2251
复制
发表时间：
2013
影响因子：
2.1
通讯作者：
T. Giovanardi;T. Morishita;A. Zanetti;M. Mazzucchelli;R. Vannucci
中科院分区：
文献类型：
--
作者：
T. Giovanardi;T. Morishita;A. Zanetti;M. Mazzucchelli;R. Vannucci
蓝宝石通常被解释为高MgO-Al 2 O3岩石中的变质成因。火成岩碱，即从熔体中结晶出来的岩碱，非常罕见。我们研究了在菲内罗金云母-橄榄岩地块，意大利西部阿尔卑斯山的含Al 2 O3的岩浆脉，以调查可能的火成岩起源Al 2 O3从熔体中结晶，获得特别高的Al 2 O3含量通过熔体-岩石反应和分步结晶。蓝宝石局部产于白辉长岩脉和寄主橄榄岩之间的黑晶带中。浅色辉长岩脉以高角度切割地幔叶理和橄榄岩的岩性分层，橄榄岩的岩性分层由交替的富含金云母的方辉橄榄岩和辉石岩限定。沿着其独特的主元素矿物化学特征，表明淡色辉长岩脉与主地幔序列的渗透交代重结晶无关，记录了一个明显的熔体注入事件。在白纹静脉的两侧观察到黑色接缝。它们显示出明显的模式分区：一个斜方辉石丰富的区域上的主机橄榄岩（OPX区），而角闪石丰富的区域发生对淡色辉长岩脉（AMPH区）。蓝宝石通常包裹尖晶石或以离散颗粒形式出现，位于（1）AMPH带中的大的浅棕色闪长岩晶体内或（2）在AMPH带和OPX带中的浅棕色闪长岩之间。浅棕色的闪长岩也可以包裹不含镁铝尖晶石的尖晶石。为解释含橄榄石脉岩和寄主橄榄岩的岩石化学特征，提出了熔体-岩石反应和分离结晶的四阶段过程：第一阶段（A阶段），上升的SiO2饱和熔体与寄主橄榄岩相互作用，使橄榄岩中的橄榄石、角闪石和金云母被新形成的正辉石取代，Al 2 O3、TiO 2、FeO在寄主橄榄岩中富集，并在重结晶前沿以外富集。这种矿物学反应导致迁移熔体中的高Al 2 O3/SiO2和MgO/FeO比。改性的熔体在开放导管中结晶出富含Al 2 O3的深棕色的硅铝石（16.5重量% Al 2 O3）和磷灰石（阶段B）。蓝宝石/尖晶石饱和实际上是在后期，在阶段C中，在存在更分化的熔体的情况下实现的，该熔体与早期的深棕色闪长岩反应，局部产生由浅棕色闪长岩和尖晶石/尖晶石构成的假合晶岩结构。伴生的浅棕色闪长石（17.5wt%Al2O3）、金云母和尖晶石的组成表明，分离出富铝矿物的母熔体具有一种独特的富Al 2 O3成分，在局部，这种熔体也分离出含富铝矿物的OPX带。在阶段D期间，可能由于水力压裂导致AMPH区分裂后，最终分离出无氯苯胺的淡色辉长岩脉。
Sapphirine is generally interpreted to be of metamorphic origin in high-MgO-Al2O3 rocks. Igneous sapphirine, i.e. sapphirine crystallised from melt, is very rare. We examined sapphirine-bearing magmatic veins in the Finero Phlogopite- Peridotite Massif, Western Italian Alps, to investigate a possible igneous origin for sapphirine by crystallisation from a melt, acquiring particularly high Al2O3 content via melt-rock reaction and fractional crystallisation. Sapphirine locally occurs in melano- cratic zones placed between leucogabbroic veins and the host peridotite. The leucogabbroic veins cut at high angle the mantle foliation and the lithological layering of the peridotite massif, which is defined by alternating phlogopite-rich harzburgites and pyroxenites. This observation, along with their peculiar major-element mineral chemistry, indicates that leucogabbroic veins were unrelatedtothe pervasivemetasomatic recrystallisation ofthe hostmantle sequence,recordingalater,distinctevent ofmelt injection. Melanocratic seams are observed on both sides of the leucogabbroic veins. They show a marked zoning of the mode: an orthopyroxene-rich zone overgrows upon the host peridotite (OPX zone), whereas an amphibole-rich zone occurs towards the leucogabbroic vein (AMPH zone). Sapphirine commonly mantles spinel or occurs as discrete grains, located either (1) within large light-brown pargasite crystals in the AMPH zone or (2) interstitially, between light-brown pargasite in both AMPH zone and OPX zone. Light-brown pargasite can also enclose spinel that does not have sapphirine envelope. To explain the petrochemical features of the sapphirine-bearingveinsandthe host peridotitea four-stageprocessinvolvingmelt-rockreactionsandfractional crystallisation is hereproposed.Duringthefirststage(StageA),theinteractionbetweentheuprisingSiO2-saturatedmeltandthehostperidotitecaused thereplacementofperidotiteolivine,amphiboleandphlogopitebynewlyformedorthopyroxeneclosetothecontactandAl2O3,TiO2, FeO enrichments in the host peridotite beyond the recrystallisation front. This mineralogical reaction resulted in high Al2O3/SiO2and MgO/FeO ratios in the migrating melt. The modified melt crystallised Al2O3-rich dark-brown pargasite (16.5 wt% Al2O3) and apatite in the open conduit (Stage B). Sapphirine/spinel saturation was actually achieved later, in the Stage C, in presence of a more differentiated melt, which reacted with the early dark-brown pargasite locally producing pseudo-symplectite textures made by light- brown pargasite and spinel/sapphirine. A peculiar Al2O3-enriched composition for the parent melt segregating sapphirine is indicated bythecompositionoftheassociatedlight-brownpargasite(17.5wt%Al2O3),phlogopiteandspinel.Locally,thismeltpercolatedalso the OPX zone, segregating sapphirine-bearing mineral assemblages. The sapphirine-free leucogabbroic vein was finally segregated during the Stage D, after splitting of the AMPH zone likely due to hydraulic fracturing.