Adirondack Geology - Faculty web pages

Adirondack GeologyIntroduction: Located within the larger Grenville province• U-Pb zircon and Sm-Nd ages indicate Adirondacks experiencedwidespread calcalkaline magmatism 1300-1220 Ma ( Daly andMcLelland, 1991; McLelland and Chiarenzelli, 1991; Marcantonioet al., 1990)• Identical rocks with identical ages are described from GreenMts., VT (Ratcliffe and Aleinikoff, 1990); northern Ireland(Menuge and Daly, 1991); Texas-Mexico (Patchett and Ruiz,1990); suggesting that a major collisional magmatic belt alongthe present southern flank of Grenville Province occurred 1300-1220 Ma

Adirondack GeologyIntroduction:• May have been related to assembly of supercontinent at thistime• Magmatism and its associated metamorphism represent Elzevirorogeny of the Grenville orogenic cycle

Topographically divided intothe Highlands (granulitefacies) and Lowlands(amphibolite facies)separated by a broad bandof high strain called theCarthage-Coulton mylonitezone• Highlands underlain byorthogneiss• Lowlands underlain byparagneiss rich in marbleRegion has experiencedmultiple metamorphic andintrusive events and largescaleductile structures arecommon. McLelland (2000)

GeochronologyMcLelland et al. (2002)

GeochronologyAfter McLelland (2002)U-Pb geochron indicates oldest igneous rocks exposed are~1350-1300 Ma tonalitic arc-related plutons intrusive into olderHighland paragneisses of uncertain ageOldest rocks in Lowlands are ~1200 Ma granodiorites intrusiveinto older paragneisses of uncertain age

GeochronologyMcLellandet al. (2002)In both Highlands and Lowlands, metapelitic migmatites >1200-1300 Ma contain anatectic material of ~1170 MaFollowing intrusion of granodiorites in Lowlands at 1207 Ma,leucocratic and tonalitic rocks were emplaced at ~1172 Ma andaccompanied by deformation and metamorphism associatedwith the latest, culminating phase of the Elzevirian orogeny,~1220-1160 Ma

GeochronologyMcLellandet al. (2002)From ~1160-1150 Ma, entire Adirondack-Frontenac region wasintruded by AMCG suite (anorthosite-charnockite-mangeritegranitemagmas) that are associated with four anorthositemassifs (Marcy, Oregon, Snowy and Carthage) recently datedby SHRIMP zircon techniques at 1155±10 Ma

Final Events of Adirondack Evolution1. Emplacement of Hawkeye granite suite at ~1095 Mafollowed by2. High-grade metamorphism resulting in vapor-absent, peakgranulite facies conditions in the Highlands (T~700°-800°C,P ~6-8 kbars) and associated widespread recumbent folding,isoclinal folding and development of intense penetrativefabrics. This granulite facies metamorphism anddeformation assigned to collisional Ottawan orogeny.3. Toward end of orogeny, much of Adirondack region wasintruded by late- to post-tectonic leucogranites (~1055 Ma)belonging to the Lyon Mt. Gneiss (Granite) and thought to berelated to the delamination and extensional collapse of theorogenElzevirian and Ottawan orogenies together comprisethe Grenville orogenic cycle (~1350-950 Ma)

Ottawan orogeny – 1100-1050 Ma• after 30 Ma of quiet, Adirondacks along with entireGrenville Province experienced onset of Ottawan orogeny ofGrenville orogenic cycle• 1090-1100 Ma hornblende granites of NW Highlands• These granite followed by deformation, high-grademetamorphism and emplacement of trondhjemitic (felsictonalite- plag+qtz±K-spar + biot) to alaskitic magnetite-richrocks such as Lyon Mt. Gneiss in N and E AdirondacksElzevir orogeny – 1300-1220 Ma• followed by 40-50 Ma of quiescence to 1170-1130 ma whenvoluminous anorogenic magmatism began• referred to as anorthosite-mangerite-charnockite-granite(AMCG) suiteAMCG suite – 1170- 1130 Ma• older ages are characteristic of AMCG magmatism inFrontenac terrane including the Lowlands while Highlandsshow ages of 1150-1130 Ma• Marcy massif and its associated granitoid envelope showemplacement ages of 1135 Ma

Ottawan orogeny – 1100-1050 Ma• Zircon U-Pb ages for these rocks fall 1050-1080Ma – peak of granulite facies metamorphism whencrust at surface currently was at ~25 km depthRegional picture:• None of igneous suites gives evidence of apre~1600 Ma crust in Adirondack region• Entire region appears to have come intoexistence in Middle to Late Proterozoic• Suggested that the Highlands and Lowlandswere in proximity to each other at ~1300 Mabased on Sm-Nd ages• Suggested that Carthage-Colton mylonite zoneis interpreted as W-dipping extensional normalfault that formed during the Ottawan orogeny inresponse to crustal thickening by crustal stacking

McLelland et al. (2002)• Extensional motion along the C-C zone helps explainjuxtaposition of amphibolite and granulite facies assemblagesacross this zone• Downward displacement of 3-4 km of Lowland block wouldjuxtapose lower metamorphic grade of Lowlands terrane withgranulite facies of Highlands

Cooling HistoryOldestmonazite ages,1220-1230 Madetrital ? –ElzevirianorogenyIntrusion ofAMCG suite,1160- 112 Maearly contactmetamorphismorhydrothermalactivityassociatedwith intrusion

ProgradeOttawanmetamorphism1088-1112 MamonazitegrowthCooling HistoryMonazitegrowth duringmeltcrystallization1028-1048 MaMonazite ageof 923 maprobablymonazitegrowth duringpegmatiteintrusion

Adirondacksunconformablyoverlain byPotsdam Ss. inearly CambriantimeCooling HistoryMust havebeen atsurface at thattime40Ar/39ArthermochronologyshowscomplexPaleozoichistory, Max T= 400°CAFT agessuggest uplift~180-100 MaStorm and Spear (2002)

Adirondackgeologichistory

McLelland et al. (2002)Principle rocktypes:TonalitesAMCG suite-AnorthositeJotuniteMangeriteCharnockiteGraniteMetagabbroHornblendegranitesLeucogranitesLyon MtgneissPegmatitesMetasediments

Tonalites-E-W belts in S AdksAMCG suitewithinHighlands in Marcymassif, Oregon andSnowy Mt. domeAnorthosite, gabbroicanorthositeanorthositic gneiss –plag = andesine tolabradorite, cpx, opx, Tirichmagnetite andilmeniteMcLelland et al. (2002)Jotunite – ferrodioriteand monzodiorite gneissgarnet-opx-cpx-plagrocks w/minor horn, qtz,K-spar, ilm, mag

Mangerite –Qtz monzonitegneiss – K-spar, qtz,Fe-rich cpx and opxw/fine-grained plagCharnockite –opx- and cpxbearinggranitesGranite – microcline,Qtz, horn, Fe-Tioxides, apatite,zircon, plagMcLelland et al. (2002)

Metagabbro – E andS AdksPlag – andesinedominantOlivine in reactionw/plag forms coronatextures of cpx andgarnetHornblende granites– restricted to NWHighlandspink, streaky rocksw/horn, biot, 2zircon fractions:1123-1154 Ma and1095 MaMcLelland et al. (2002)

Leucogranites:Lyon Mt gneiss – 1080-1050 Ma zirconsmicrocline, qtz, plag,biot, horn, cpxLots of magnetite,sphene and apatitePegmatitesMcLelland et al. (2002)Meta-sediments –S Adks – qtzites andmetapelitesC Adks – calcitemarbles andcalcsilicatesNE Adks – calcsilicategranulites, qtzites, andcalcite marbles withminor garnet-silmetapelites