STRUCTURAL PETROLOGY OF THE LATE ORDOVICIAN APPINITE COMPLEXES OF THE CORTLANDT INTRUSIVE SUITE (CIS) AND YOUNGER EARLY SILURIAN(?) GRANITES ALONG CAMERONS LINE

Composite stitching plutons of the CIS have shared igneous structures, chemistry, and petrology that require a common petrogenesis at about 450 ma shortly after Taconian arc /continental collision. From northwest to southeast, the Cortlandt, Croton Falls, Brookfield and Bedford plutons intrude Laurentian basement and cover, the accretionary wedge and collisional arc. Structural- petrologic similarities are: 1.multiple injection of lamprophyre, alkali pyroxenite, diorite, microdiorite, monzonite and granite in composite plutons and dikes; 2. Mutually intrusive mafic/felsic phases; 3. Commingling of poorly mixing intrusive phases; 4. Igneous- cemented autolithic breccias; and 5. Lit- par- lit injections in ductile shear zones. The CIS in the west cuts regional west-vergent Ordovician F1 and F2 structures but is syntectonic in a 4 km-wide zone of east vergent F3 fold- thrust structures that overprint Camerons line. This deformation zone is mapped for at least 50 km from the New Milford massif to the south end of the Brookfield pluton. The associated Siscowit and Candlewood granites intrude lit- par- lit and include screens of the D-3 ductiley sheared rocks. Available zircon ages indicate similar compressional deformation, high grade metamorphism, migmitization and plutonism in this deformation zone from about 455 ma to as young as 443 ma the age of the Candlewood granite. Oceanic- slab and or continental-slab detachment occurred at collision as arc volcanism was shut down. Segmentation of the over-thickened down- going continental slab allowed for the across-grain intrusion along the southern edge of the New York promentory as eastward vergence developed in the hinterland. The marked change from orogen-parallel late Taconic plutonism in western New England to across –grain intrusion at about 450 ma was the result of consumption of an irregular Laurentian plate margin. Pb and Nd-Sm isotopic signatures of sub-lithospheric Laurentian mantle might be expected in CIS plutons, if the model proposed is correct, this should differ with the isotopic signatures of late Taconic orogen- parallel plutons of New England of the same age