THE GRENVILLE OROGENIC CYCLE (CA 1350-1000 MA) - AN ADIRONDACK PERSPECTIVE

The Adirondack Mountains are characterized Joy three major events that took place during the interval ca. 1350-1000 Ma. The earliest of thes e is the are-related Elzevirian Orogeny (ca. 1350-1185 Ma) during whic h substantial volumes of juvenile calc-alkaline crust were added to th e Adirondacks as well as to the northwest segment of the Central Metas edimentary Belt. Data from the southwestern United States as well as f rom Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Gren ville Province, the Elzevirian Orogeny culminated at ca, 1185 Ma when accretion of all outboard terranes was completed. Compressional orogen y related to this convergence resulted in overthickened crust and lith osphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca, 1180-1130 Ma, L ocalized compressional events within neighboring parts of the Grenvill e Province emphasize the continued existence of contraction during thi s interval, although crustal extension caused local in sedimentary bas ins in which were deposited the Flinton and the St. Boniface Groups. T he Adirondacks have not yet provided any record of events within the i nterval ca. 1125-1100 Ma, although there is evidence of contraction el sewhere in the southwestern Grenville Province at that time. At 1100-1 090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the re sult of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following , at ca, 1090 Ma, the global-scale continental collision of the Ottawa n Orogeny was initiated. Strong convergence, deformation, and metamorp hism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event, During the waning stages of the Ott awan Orogeny, overthickened crust and lithosphere delaminated and the orogen underwent collapse, Large extensional faults such as the Cartha ge-Colton-Labelle shear zone developed and rapidly exhumed granulite f acies rocks in the mobile core of the orogen which centers on the Adir ondack-Morin terranes and extends southeastward into the New York-New Jersey Highlands. Extensional faulting along the Carthage-Colton mylon ite zone dropped the amphibolite facies Lowlands down to the west and into juxtaposition with granulite facies rocks of the Highlands, U-Pb cooling ages from garnet, monazite, and titanite exhibit a sufficientl y broad spectrum to accommodate an initial rapid rate of rebound-relat ed cooling followed by a slower, erosion-controlled cooling history. D uring delamination, late- to post-tectonic granites of the Lyon Mt. Gn eiss (ca. 1070-1045 Ma) were emplaced. The youngest member of this sui te is an undeformed fayalite granite dated at ca. 1045 Ma which crossc uts all older rocks and fabric. High-potassium, post-tectonic granites of similar age are common in other parts of the southwestern Grenvill e Province. Renewed contraction and metamorphism at ca. 1030 Ma demons trate that the Ottawan Orogen was still experiencing convergence well after the peak of orogeny. However, most of the manifestations involve reactivation of older thrust faults, including the Grenville Front Te ctonic Zone. The intrusion of small bodies of anorthosite at ca. 1015 Ma (i.e., Labrieville) provide further evidence for the emplacement of these rocks within collisional orogens, albeit in their collapsing ph ase.