Phanerozoic Blueschist Subduction Zones
Samples 4-1 to 4-8 represent an apparently "upside-down" sequence of metamorphic rocks that crystallized within an inverted metamorphic gradient induced by the perturbation of the mantle geothermal gradient by the subduction penetration of a cold oceanic plate into 'hot' mantle. Rocks at the base of the section represent relatively low temperature hydrated oceanic material whereas rocks at the top of the section formed under conditions of high temperature and high pressure metamorphism. The fluids that were released as a result of the high-grade metamorphism moved upward into the overlying mantle to form titanium enriched magnesian amphiboles and micas.
4-1b; subduction zone: dark grey,irregularly foliated mudstone with fragments of fine grained, laminated carbonate. Note irregular indented borders of the fragments indicating brecciation under conditions of high water pressure = ‘argille scagliosa’; ‘argille a bloc’, ‘wildflysch’, ‘melange’.
4-2; subduction zone: dull green, dense, crystalline metamorphic rock composed entirely of nephrite (a jadeite-like [pseudo-jade] material); protolith indeterminate, likely a product of hydrothermal alteration of peridotite; black grains are chromite..
4-3-1; 4-3-1p; subduction zone: blue, massive to faintly foliated glaucophane schist - the blue, acicular (needle-like) mineral is the amphibole glaucophane, and the pale green material is the low grade metamorphic mineral pumpellyite or epidote; protolith is likely an oceanic basaltic rock that has been metamorphosed under conditions of high pressure (30km) and relatively low temperatures at the base of the subducted oceanic slab;
4-3-2; 4-3-2p; subduction zone: fragment of blueschist containing coarse, white lawsonite (CaAl2Si2O8.H2O) in a vug; metamorphosed under blueschist conditions towards the base of the subducted oceanic slab;
4-4; 4-4p; subduction zone: dull green, tectonically polished, lower greenschist/blueschist metamorphic rock mainly composed of chlorite and epidote; the highly polished surface may have formed during subduction transport; NOT due to cows or glaciers; note vein of glaucophane and phengite (muscovite) in the material.
4-5; subduction zone: dark green, well foliated, garnet-chlorite schist; garnet has grown at the expense of chlorite as temperature of metamorphism increases in the subduction zone; the protolith is likely a basaltic volcanic rock
4-6a; subduction zone: finely crystalline, dense gneiss composed of alternating layers of pale green epidote and black amphibole gneiss; transition from greenschist to amphibolite facies of metamorphism; likely protolith is a Ca-rich ferromagnesium mafic volcanic.
4-6b1; subduction zone: dark green and steel grey, actinolite-chlorite/black-amphibolite gneiss; relict layering from alternation of mafic ash (now actinolite-chlorite/amphibolite) and felsic (andesitic/dacitic) ash beds and laminae; amphibolite metamorphism follows greenschist metamorphism
4-6b2; subduction zone: massive amphibolite.
4-6b3; subduction zone: green, even textured, fine-grained chromiferous Mg-amphibolite; L-tectonite; protolith was perhaps an olivine and chromite-bearing, layered oceanic gabbro.
4-6c; 4-6cp;subduction zone: dark grey to black garnet amphibolite with faint compositional layering; rock contains small red garnets in a very fine-grained, non-schistose amphibolite matrix; high grade metamorphism of a mafic protolith located near the top of the subducted oceanic plate.
4-7a; 4-7ap; 3 samples; subduction zone: red/green, dense, massive, anhydrous eclogite - glassy red garnet porphyryblasts in a dark green, fine-grained groundmass of pyroxene (jadeite - diopside); high grade metamorphism of a mafic protolith, metamorphosed close to the slip boundary separating subducted low temperature oceanic crust from overlying high temperature mantle.
4-7b; 2 samples; subduction zone: large, rounded, red-green crystalline shear pods (‘fish’) of fine-grained, massive eclogite in silver grey to pale green sericite (phengite) schist.
4-8; 4-8p1; 4-8p2; subduction zone: compositionally layered partly serpentinized ultramafic mylonite - extreme grain size reduction of normally coarse grained mantle lherzolite (note small porphyroclastic ‘minnows’ of olivine). The brownish layers contain clino- and orthopyroxene, titaniferous amphibolite, titaniferous phlogopite (biotite), and garnet. The amphibole and phogopite have formed by reaction with high temperature fluids released from the underlying oceanic amphibolites, greeschists, and blueschists of the subducting plate. These rocks are located within the primary subduction slip zone separating hot mantle material from cold subducting oceanic crust. Sample collected from the sole of the Bay of Island ophiolite, Western Newfoundland.
Coarse grained plutonic granitic rocks
In contrast to the basaltic rocks of the subducting plate, the following suite of granitoid rocks represents the ultimate fractionation product of the magmas fed from the subduction zone into the overlying island arc or collision zone. The fractionation process within the arc leads to progressive enirchment in in quart and K-feldspar, and is represented by a change in rock type from diorite to alkali and other incompatible element (e.g. boron; REE; lithium; rubidium) enriched pegmatoid granite.
6-2; Polyphase_Intrusive_Complexes: dark grey, fresh, salt and pepper weathered, medium grained, equigranular, massive hornblende-diorite; mafic intrusive rock, no modal quartz;
6-3a1; 6-3a2; 6-3a3; 3 samples: Polyphase_Intrusive_Complexes: salmon pink to pink-grey, medium to coarse grained, equigranular K-spar megacrystic syenite; mafic minerals in decreasing order of abundance are hornblende, pyroxene, biotite (all not necessarily present in each sample); alkaline intrusive body; no modal quartz;
6-3b1; 6-3b2; 6-3b3; 3 samples: Polyphase_Intrusive_Complexes: pink to dark grey-pink, coarse grained, K-spar megacrystic rock; mafic minerals in decreasing order of abundance are hornblende, biotite, magnetite (all not necessarily in each sample); orthoclase phenocrysts are aligned during magmatic flow;
6-4a1; 6-4a2; 6-4a3; Polyphase_Intrusive_Complexes: quartz-porphyry.
6-4b; Polyphase_Intrusive_Complexes: dark, cloudy grey to pink, fine grained, equigranular, massive quartz syenite; mafic minerals are hornblende and lesser biotite; compositionally intermediate intrusive phase; note clear quartz, and the sub-aphanitic groundmass.
6-5a1; 6-5a2; 6-5a3; 6-5a4; 6-5a5; 6 samples; Polyphase_Intrusive_Complexes: salmon pink to light grey, fine to coarse grained, equigranular biotite granite; note the abundance of modal glassy quartz in the coarser samples, and the presence of xenoliths in the finer grained granite samples;
6-5b; 3 samples; Polyphase_Intrusive_Complexes: cloudy grey to grey-white, medium grained granite; the rock has been treated with a solution of colbaltinitrate causing the potassium feldspar minerals to stain a yellow colour;
6-5c; 6-5cp; Polyphase_Intrusive_Complexes: biotite plagioclase-megacrystic granite; zoned plagioclase;
6-6a; Polyphase_Intrusive_Complexes: cloudy grey, coarse-grained ‘graphic’ granite; the term ‘graphic’ refers to the pattern of intimate intergrowth of the feldspar and quartz;
6-6b; 2 samples; Polyphase_Intrusive_Complexes: dull grey, banal, K-feldspar-bearing pegmatite;
6-7-1; 6-7-2; 2 samples; Polyphase_Intrusive_Complexes: tan or beige, hard, siliceous, fine grained sugary textured aplite; rapidly cooled late stage felsic intrusive phase, commonly forms along the margins of plutons, or forms dykes which extend into the surrounding country rock from the pluton margin; often contains igneous crystals of garnet;
6-8a; Polyphase_Intrusive_Complexes: white, very coarse grained tourmaline-orthoclase pegmatite- note striations parallel to the length of the elongated tourmaline crystals; such pegmatites commonly occur as late-stage dykes associated with larger plutonic bodies;
6-8b-1; Polyphase_Intrusive_Complexes: green amazonite-bearing pegmatite;
6-8b-2; 6-8b-2p; Polyphase_Intrusive_Complexes: cloudy white to tan weathered pegmatite with patches of muscovite and purplish fluorite;
6-9a; 2 samples; Polyphase_Intrusive_Complexes: dull grey, coarse grained, equigranular biotite granite; sample 6-9a to d are from a single multiphase plutonic suite in the Slave Province, western Northwest Territories- mapping out phases in multi-phase plutons can be difficult (a granite is a granite is a granite, even if they are of different age, as is the case here where the 4 phases span an age of about 150 Ma (2.6-2.45 Ga)) - overall the pluton is intermediate to felsic in composition, but granite phases a and d can be distinguished by grain size and color; phase b is distinguished by modal quartz,and phase c is distinguished by the presence of 2 micas;
6-9b; 2 samples; Polyphase_Intrusive_Complexes: dull pink-grey, medium grained, equigranular hornblende quartz syenite - sample may have some interstitial magnetite;
6-9c; 2 samples; Polyphase_Intrusive_Complexes: pale pink to light grey, medium grained, equigranular 2-mica granite - white muscovite distinctive but much less abundant than biotite;
6-9d; 2 samples; Polyphase_Intrusive_Complexes: pink weathered, fine to medium grained granite - only a trace amount of biotite; clear quartz is distinctive;
6-10a; 2 samples; Polyphase_Intrusive_Complexes: cloudy grey, medium grained, equigranular peralkaline granite - the amphibole is very fresh, euhedral, and acicular; the amphibole is a Na-rich riebekite - note the contrast in colour of the two samples due to the variation in degree of alteration of the feldspar; late Proterozoic post-orogenic granite from Saudi Arabia;
6-10b; Polyphase_Intrusive_Complexes: peralkaline granite with coarse clear euhedral to subhedral quartz, and alkali hornblende (riebeckite); Silurian post-orogenic granite from Newfoundland
6-11a, b; Polyphase_Intrusive_Complexes: a) calcite veined calc-silicate rock composed of diopside and pyrite; b [20,792]) very dense garnet - galena rock . These two specimens are Ca-rich "skarn" rocks formed by contact metamorphism/metasomatism of limestones (dolomite) adjacent to granite plutons; their formation involves the introduction of large amounts of Si, Al, Fe and Mg (Holmes, 1920, p.211). Commonly associated with sulphide and iron deposits.
6-12; Polyphase_Intrusive_Complexes: dark pink K-feldspar-rich ferro-syenite with coarse hornblende crystals.
6-13; 6-13p; Polyphase_Intrusive_Complexes: rapakivi granite; K-feldspar megacrysts with white rims of oligoclase in a biotite-rich matrix.