THE OTTAWAN OROGENY Himalayan style crustal channel?

GOALS To present a Himalayan model for the development of the Grenville Orogenic Event. To apply this model to the a portion of the Canadian (Ontario) and adjacent Adirondack (New York) terrane assemblage. To examine this model with particular reference to the nature and timing of activity along the major terrane-bounding shear zones, periods of igneous intrusive activity, and timing of metamorphic events.

ADK. HIGHLANDS ADK. LOWLANDS FRONTENAC MSZ RLSZ SHARBOT LAKE MAZINAW ccsz ELZEVIR CMBBZ BANCROFT BSZ CENTRAL GNEISS BELT Adapted from Davidson (1998) 40km

Terranes and shear zones MSZ RLSZ CCSZ LL HL Adirondack Highlands Sharbot Lake-Frontenac/Adirondack lowlands Mazinaw Elzevir Bancroft Anorthosite (AMCG Suite)

Ma Igneous Events

Ma MSZ RLSZ AMCG etc.

Ma HAWKEYE

Ma CMBBTZ LYON MOUNTAIN

Observations There is a shift in the locus of igneous activity through time (generally W to E) reflecting the assembly of Grenvillian Terranes along eastern Laurentia. The anorogenic/rift AMCG event magmas intrude the Adirondack (ADK) Highlands, Lowlands, Frontenac, Sharbot Lake and Mazinaw Terranes but are focused in the ADK Highlands Post AMCG pre Ottowan Orogney; Hawkeye granitoids are primarily found in the ADK Highlands. Lyon Mountain Granitoids rim the ADK Highlands are common along or adjacent to the CCSZ and scattered through the other terranes. (Pressure-release A-type magmas: )

OTTAWAN THERMAL EVENT

Ma ADK H ADK LL F SL MAZ EL B CGB

Ma

Ma

Observations Metamorphic events Adirondack Highlands and the Mazinaw terranes record the Ottawan Event. Adirondack Lowlands record a weak Ottawan Event focused along the CCSZ Frontenac and Sharbot Lake Terranes do not record the Ottawan Event. Terranes west of the CMBBZ record pluses of metamorphism at a slightly later time and cool quickly. From Davidson (1998) An overview of the Grenville Province Geology, Canadian Shield.

ccsz Dana Hill metagabbro Diana Syenite Body ADK HL ADK LL Diana Syenite (ADK lowlands) Dana Hill Metagabbro (ADK highlands) 1020 Ma /-1.7Ma After (Streepey et al. 2001) Sphene U/Pb Hbl 39Ar/40Ar 989 +/- 1.7Ma Hbl 39 Ar/ 40 Ar 979+/-8.6Ma

CCSZ Early 30+m wide shear zones in the DHMG record recrystallization temperatures in excess of 700 o C, sphene replacement of Fe-Ti Oxide minerals, and a complete lack of scapolite. Late (sub-meter wide) shear zones in the DHMG record recrystallization temperatures ranging from o C and widespread repalcement of plagioclase by scapolite. Shear zones (meter + and sub meter widths) in the Diana Syenite Body span a wide range of temperature conditions (sub 550 o C a ), and most show sphene replacement of Fe-Ti oxide minerals Shear zones contain widespread scapolite replacement of plagioclase. a Lamb (1993) (Streepey et al. 2001: Johnson et.al (2004)).

CCSZ Large early-formed shear zones in Dana Hill Metagabbro (high temp) NO SCAPOLITE Scapolite replacement of plagioclase in (sub meter wide shear zones) Dana Hill Metagabbro Body Dana Hill Metagabbro (Highlands) Sphene cooling through 650 o C at 1020 Ma. 39 Ar/ 40 Ar cooling through closure for hornblende (550 o C) ~ Ma Scapolite replacement of plagioclase in shear zones (DSB) Diana Syenite Body (DSB) Sphene-growth in shear zones at ~ 1041 Ma. Thermal resetting/growth of sphene in DSB at Ma age. Zircon records ~ 1150 Ma AMCG age (McLelland et al (2003). 39 Ar/ 40 Ar cooling through closure tor hornblende (550 o C) ~ Ma

CCSZ Thermal Profile CCSZ distance (km) Temperature o C CCSZ ~1040Ma ~1020 Ma ~990Ma

Fluid Inclusion data (syn-post orogenic)_ LYON MOUNTAIN GRANITE

Adapted from: Beaumont, R.A. Jamieson, M.H. and Lee B. (2001) Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation: Nature, 414, pp o C MOHO 700 o C TEMPERATURE CCSZ 800 o C Channel Flow Thermal Gradients 100 km

A’ TECTONIC CROSS-SECTIONS

MSZ RLSZ CCSZ LL FLINTON GROUP Crustal detachment HL Old Mt Holly Arc Trench roll back ~ Ma AMCG main phase Or delamination

Ma MSZ RLSZ CCSZ LL Crustal detachment HL Old Mt Holly Arc Ma arc FLINTON GROUP HAWKEYE GRANITOIDS AMCG late phase-Hawkeye intrusive suite

FLINTON GROUP HAWKEYE GRANITOIDS LYON MOUNTAIN GRANITOIDS Ma OTTAWAN PHASE MSZ RLSZ CCSZ LL HL Amazonia?

Extrusion of the low viscosity granulite core of the orogen Ma ADK HL F.T. SLT MAZADK LL Granulite core Zone of active footwall deformation Lyon Mtn A-type Granitoid

ADK HL F.T. SLT MAZADK LL Granulite core Zone of active footwall deformation Lyon Mtn A-type Granitoid

Conclusions The Shawinigan- AMCG- pre-Ottawan development for this part of the Grenville can be explained via tectonic switching and or collision along an Andean-type margin (first proposed by Hanmer et al. (2000)). AMCG magmas intrude into an evolving back-arc basin at a time when the ADK Highlands are structurally beneath the ADK LL/Frontenac/Sharbot Lake Terranes. All of these terranes receive AMGG magmas. (Flinton Group deposited) Continental-Continenal Collision occurs at or near to 1090Ma (Ottawan Orogeny)

Conclusions During Ottawan compression, the ADK H.L. Terrane is mobilized and extruded along a tectonic channel. This extrusion is oblique thrust in character and focused along the CCSZ. Thermal gradients established across the CCSZ with localized heating of the adjacent ADK lowlands (resets isotopic systems in the ADK lowlands near to the CCSZ: Dahl (2002)). Major un-named shear zones in the eastern Adirondack Highlands my represent the base of this channel. Extrusion is aided by the formation of pressure release melts (syn- post Ottawan Lyon Mountain Granitoids). Fluid inclusion data from Lyon Mountain Granitoids adjacent to the CCSZ record nearly isothermal uplift.

Conclusions Uplift results in gravity-driven collapse of cover rocks (ADK L.L. Frontenac over Sharbot Lake Terranes) to the W- NW. Extrusion of the granulite core with deflation of the mid-lower crust, rotates the Mazinaw Terrane triggering amphibolite facies metamorphism. Continued convergence reactivates shear zones to the west driving short tectonothermal events in terranes to the west (Central Gneiss Belt).