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Lithogeochemistry Constraints on Assimilation and Fractional

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Lithogeochemistry Constraints on Assimilation and Fractional LithogeochemistryLithogeochemistry ConstraintsConstraints onon AssimilationAssimilation andand FractionalFractional CrystallizationCrystallization ProcessesProcesses inin thethe SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia MichaelMichael WhitbreadWhitbread,, ioGeochemistry, Brisbane, Queensland, Australia, [email protected] and: CliffCliff StanleyStanley,, Dept. of Geology, Acadia University, Wolfville, Nova Scotia, B4P 2R6, [email protected] SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia largestlargest peraluminousperaluminous ((SS-type,-type, ilmeniteilmenite seriesseries)) granitoidgranitoid inin thethe AppalachianAppalachian orogenorogen DevonianDevonian age,age, ~380~380 MaMa megacrystic,megacrystic, porphyriticporphyritic emplacementemplacement occurredoccurred inin twotwo stagesstages StageStage 1:1: maficmafic porphyry,porphyry, granodioritegranodiorite && BTBT monzogranitemonzogranite StageStage 2:2: MSMS-BT-BT monzogranite,monzogranite, leucomonzograniteleucomonzogranite && MSMS leucosyenograniteleucosyenogranite majormajor mineralsminerals quartz,quartz, plagioclase,plagioclase, alkalialkali feldspar,feldspar, biotite,biotite, muscovitemuscovite accessoryaccessory mineralsminerals aluminosilicate,aluminosilicate, cordierite,cordierite, garnet,garnet, tourmaline,tourmaline, topaztopaz hosthost toto significantsignificant Sn,Sn, Mo,Mo, UU && MnMn mineralization!mineralization! SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia Windsor Middleton Millet Brook101 Ross Creek Mn U Prospect Deposits Reeves Sn Walker Mo Prospect Prospect Halifax Long Lake Digby Mo Prospect 12 101 10 8 Atlantic Ocean Fine-grained leucomonzogranite, leucosyenogranite and porphyry Stage 2 Coarse-grained leucomonzogranite East Kemptville Muscovite-biotite monzogranite Sn Mine Coarse-grained biotite monzogranite and Stage 1 biotite granodiorite Duck Pond 0Sn 25 km Country rock Prospect CoarseCoarse Grained,Grained, PorphyriticPorphyritic StageStage 11 GaspereauGaspereau LakeLake BTBT GranodioriteGranodiorite MediumMedium Grained,Grained, EquigranularEquigranular StageStage 22 MurphyMurphy LakeLake LeucosyenograniteLeucosyenogranite DykeDyke (intruding(intruding StageStage 11 GaspereauGaspereau LakeLake BTBT granodiorite)granodiorite) FineFine Grained,Grained, EquigranularEquigranular StageStage 22 LakeLake LewisLewis MSMS LeucosyenograniteLeucosyenogranite 1 cm FineFine Grained,Grained, EquigranularEquigranular StageStage 22 TantallonTantallon Leucomonzogranite Leucomonzogranite HalifaxHalifax PlutonPluton Tantallon FG Halifax Leucomonzogranite Haifax CG Leucomonzogranite TA Harrietsfield BT-MS Monzogranite Sandy Bat/Peggy’s Cove BT Monzogranite Granodiorite HX HF N Peggy’s Cove 15 km Modified from MacDonald and Horne 1988 XenolithXenolithXenolith-Rich--RichRich PortionPortion ofof thethe SouthSouth MountainMountain BatholithBatholith Photo courtesy of Mike MacDonald, NSDNR HowHow diddid thethe variousvarious meltsmelts inin thethe SouthSouth MountainMountain BatholithBatholith becomebecome evolved?evolved? AFCAFC ProcessesProcesses (assimilation(assimilation && fractionalfractional crystallization)crystallization) SidewallSidewall BoundaryBoundary LayerLayer DifferentiationDifferentiation • Crystallization takes place at cool intrusion margin • Evolved residual melt forms between the crystals at this margin • Residual melt migrates into centre of magma chamber, mixing with less evolved melt, making the mixed melt more evolved • Additional crystallization leads to an even more evolved, mixed melt in the centre of the magma chamber • Process continues … sidewall • Magma chamber becomes boundary concentrically zoned layer ResidualResidual MeltMelt CompositionComposition SiO2 Water Saturated Quartz 1 kb 2 kb 5 kb 10 kb Albite K-Feldspar NaAlSi3O8 KAlSi3O8 FractionationFractionation DuringDuring CrystallizationCrystallization f (D−1) 100 D(1− f )+ f • Enrichment of incompatible Cliq ⎛ M liq ⎞ = ⎜ ⎟ elements in the Co ⎝ M o ⎠ residual melt can occur during sidewall 0.01 melt path boundary layer 10 crystallization o incompatible /C elements • Critical factor controlling l mineral deposit C 0.1 1 genesis D = 1 2 compatible 5 10 0.1 elements 010.5 F HowHow diddid thethe variousvarious meltsmelts inin thethe SouthSouth MountainMountain BatholithBatholith becomebecome evolved?evolved? (1)(1) SidewallSidewall BoundaryBoundary LayerLayer DifferentiationDifferentiation ((FractionationFractionation)) additionaddition ofof evolvedevolved meltmelt (essentially(essentially QZ,QZ, ALB,ALB, KSP)KSP) toto thethe originaloriginal melt,melt, causingcausing itit toto becomebecome moremore evolvedevolved enrichmentenrichment ofof incompatibleincompatible elementselements inin thethe moremore evolvedevolved phasesphases PartialPartial MeltMelt CompositionComposition SiO2 Water Saturated Quartz 1 kb 2 kb 5 kb 10 kb Albite K-Feldspar NaAlSi3O8 KAlSi3O8 AssimilationAssimilation –– ThereThere isis evidenceevidence forfor it,it, butbut muchmuch ofof itit maymay bebe “physical”“physical” FractionationFractionation DuringDuring AssimilationAssimilation Cliq 1 • Enrichment of incompatible 100 = C D + F()1− D elements can occur in o res res the partial melt during partial assimilation, but only 0.01 melt path if assimilation if 10 incomplete o incompatible /C elements • If assimilation is complete, l no enrichment can C 0.1 occur unless the 1 D = 1 assimilant is enriched • Critical factor preventing 2 5 compatible mineral deposit elements genesis! 0.1 10 010.5 F Average concentrations (ppm) of Meguma Group Pelites & Psammites, and the granodiorite/monzogranite phases of the SMB Pelite Psammite SMB Rb 533 398 187 F 2440 1050 620 Li 122 81 76 U 24 6 4 Sn 21 17 7 Nb 14 9 12 Ta 41 1 W 75 1 Meguma Group average concentrations courtesy of Paul Smith, NSDNR HowHow diddid thethe variousvarious meltsmelts inin thethe SouthSouth MountainMountain BatholithBatholith becomebecome evolved?evolved? (2)(2) AssimilationAssimilation partialpartial meltingmelting ofof hosthost rockrock (essentially(essentially QZ,QZ, ALB,ALB, KSP)KSP) andand subsequentsubsequent additionaddition ofof thisthis granitegranite minimumminimum meltmelt compositioncomposition incompatibleincompatible elementselements inin hosthost rockrock cancan partitionpartition intointo partialpartial melt,melt, butbut likelylikely won’twon’t becomebecome enrichedenriched becausebecause partialpartial meltingmelting isis usuallyusually completecomplete (and(and thusthus fractionationfractionation isis limited)limited) additionaddition ofof assimilatedassimilated hosthost rockrock cancan increaseincrease incompatibleincompatible elementelement concentrationsconcentrations byby simplesimple mixing,mixing, butbut thisthis enrichmentenrichment willwill bebe limitedlimited byby thethe hosthost rockrock compositioncomposition CanCan oneone distinguishdistinguish betweenbetween evolvedevolved granitesgranites thatthat havehave undergoneundergone predominantlypredominantly fractionalfractional crystallizationcrystallization fromfrom thosethose thatthat havehave undergoneundergone predominantlypredominantly assimilation?assimilation? NovaNova ScotiaScotia RadiometricRadiometric MapMap New Ross Pluton Halifax Pluton Davis Lake Pluton SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia Windsor Middleton 101 New Ross Halifax Pluton Digby 12 Halifax 101 10 Pluton 8 Atlantic Ocean Fine-grained leucomonzogranite, leucosyenogranite and porphyry Stage 2 Coarse-grained leucomonzogranite Davis Lake Muscovite-biotite monzogranite Pluton Coarse-grained biotite monzogranite and Stage 1 biotite granodiorite 0 25 km Country rock NSDNR Lithogeochemical Database 5,000,000 4,980,000 4,960,000 4,940,000 Northing4,920,000 (m) 4,900,000 4,880,000 4,860,000 270,000 290,000 Davis Lake Pluton New Ross 310,000 Pluton 330,000 350,000 Halifax Pluton 370,000 Easting (m) 390,000 GD BMG 410,000 MBMG CLMG FLMG 430,000 MLSG 450,000 470,000 SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia 400 GD 350 BMG MBMG 300 CLMG FLMG MLSG 250 200 Zr (ppm) 150 100 50 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TiO2 (wt. %) SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia 300 160 GD Sr140 BMG V 250 MBMG 120 CLMG FLMG 200 MLSG 100 150 80 V (ppm) Sr (ppm) 60 100 GD BMG MBMG 40 50 CLMG FLMG 20 MLSG 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TiO2 (wt. %) TiO2 (wt. %) 70 30 GD BMG 60 La Sc 25 MBMG CLMG 50 FLMG 20 MLSG 40 15 La (ppm) 30 Sc (ppm) GD 10 20 BMG MBMG CLMG 5 10 FLMG MLSG 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TiO2 (wt. %) TiO2 (wt. %) SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia 1,000 Average Meguma Abundances…… GD 900 Pelite Psammite Rb 533 398 BMG F 2440 1050 MBMG 800 Li 122 81 U246 Sn 21 17 CLMG Nb 14 9 FLMG 700 Ta 4 1 W7 5 MLSG 600 500 Rb (ppm) 400 300 200 100 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TiO2 (wt. %) SouthSouth MountainMountain Batholith,Batholith, NovaNova ScotiaScotia 1,000 10,000 GD GD 900 9,000 LiBMG F BMG MBMG MBMG 800 8,000 CLMG CLMG 700 FLMG 7,000 FLMG MLSG MLSG 600 6,000 500 5,000 F (ppm) Li (ppm) 400 4,000 300 3,000 200 2,000 100 1,000 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 TiO2 (wt. %) TiO2 (wt. %) 50 200 GD GD 45 180 USnBMG BMG MBMG MBMG
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