(Ruby--Sapphire)--Chromian Mica--Tourmaline Rocks from Westland, New Zealand |
| |
Authors: | GRAPES RODNEY; PALMER KEN |
| |
Institution: | 1RESEARCH SCHOOL OF EARTH SCIENCES AND ANALYTICAL FACILITY, VICTORIA UNIVERSITY OF WELLINGTON PO BOX 600, WELLINGTON, NEW ZEALAND
2ANALYTICAL FACILITY, VICTORIA UNIVERSITY OF WELLINGTON PO BOX 600, WELLINGTON, NEW ZEALAND |
| |
Abstract: | Boulders of the assemblage rubysapphire corundum, chromianmuscovite, margarite, tourmaline (chromian chlorite, ZnMnchromite and MnTi magnetite) occur in glacial moraineand rivers of north Westland, South Island of New Zealand. Thelocation, Cr-rich composition of the boulders and the presenceof rare serpentinite rinds indicate that they are derived fromultramafic rocks (Pounamu Ultramafics) that occur within AlpineSchist of the Southern Alps. The largest sample is progressivelyzoned outwards from a corundummargarite core, throughan intermediate zone of Cr-muscovite, to an outer zone of Cr-chloritethat is in contact with serpentinite. Most finds consist oferosion-resistant corundum-rich cores. In the corundum, Cr2O3content ranges from 0.5 to 13%, with red coloration becomingmore intense with increasing Cr. In addition to the dominantCr3+ Al3+ substitution, those of (Fe, V)3+ Cr3+ and (Ti4++Fe2+) 2Cr3+ result in spectacular colour zoning from colourlessto deep ruby red-carmine and pale blue to dark blueviolet.Corundum has grown by replacement of the micaceous matrix thatconsists of chromian muscovite (0.104.10% Cr2O3) andchromian margarite (0.461.20% Cr2O3). Both micas containa significant paragonite component (up to 21.5% in muscoviteand up to 40.8% in margarite). Late phase muscovite is Ba richwith up to 4.77% BaO, and margarite has up to 0.66% SrO. Tourmalineoccurs as veins, vein outgrowths and larger poikilitic crystalsthat replace the mica matrix. Chromium content ranges between0.82 and 3.6% Cr2O3. High bulk rock Al (up to 78% Al2O3), K,Ca, Cr and Na, and low Si (14.523.1% SiO2), suggest thatthe corundumCr-silicate rocks are the products of extrememetasomatic alteration of quartzofeldspathic schist enclavesin serpentinite. Isocon analysis indicates that conversion ofthe schist to the micaceous matrix of the corundum rocks involvesconservation of Ca, Al, K, volatiles and Sr, a mass loss of59% and a volume reduction of 69% consequent on removal of 7080%Si and all other elements (most >80%), with enrichment ofbetween 900 and 1800% Cr. The formation of corundum from themica matrix involved a further massvolume reduction anddecrements in Si, Ca, K, volatiles and Sr from reaction sites.Concentric mineral zonation in single rock samples and zoningreplacementin minerals, e.g. Cr in corundum and chromite, Ti, Fe2+ in corundum,Ba in muscovite, Sr in margarite, and Mn and Zn in chromiteand magnetite, imply element redistribution during metasomatism.Experimental reaction between quartzofeldspathic schist andserpentinite at 450C and 2 kbar produced reaction sequencescontaining newly formed Ca-plagioclasephlogopitic micachloriteand muscovitechlorite that in terms of composition areanalogous with the observed (corundummargarite)muscovitechloritezonation. The temperature of metamorphism of garnet zone rocks(45020C) that contain the corundumCr-silicate rocksis well below that of the breakdown of muscovite and margariteto form corundum and indicates the importance of fluid composition,particularly the cationhydrogen variables aCa2+/H+, aK+/H+and aS1O2. Introduction of boron into the schist (from serpentinite),and boron released from the breakdown of original tourmalinein the schist, resulted in tourmaline veining and reaction ofthe mica matrix to form tourmaline that invoved both a massand volume increase and addition of Fe, Mg together with B. KEY WORDS: corundumCr-silicate rocks; metasomatism; New Zealand; Southern Alps
*Corresponding author. |
| |
Keywords: | |
本文献已被 Oxford 等数据库收录! |
|