PETROLOGY AND AGE OF THE MECHANIC-SETTLEMENT PLUTON, AVALON TERRANE, SOUTHERN NEW-BRUNSWICK

The Mechanic Settlement Pluton, located at the northern margin of the Caledonian Highlands in southern New Brunswick, is composed of rocks r anging from ultramafic (Iherzolite, plagioclase-bearing Iherzolite) th rough mafic (mainly olivine gabbronorite and gabbro) to intermediate ( quartz diorite and monzodiorite). Spatial distribution of these lithol ogies, textural features, and geochemistry are consistent with evoluti on of a tholeiitic mafic parent magma by crystal fractionation process es, with some evidence for magma mingling between evolved gabbroic and quartz dioritic magmas. The dioritic rocks form most of the southwest ern (upper?) part of the pluton, whereas the varied gabbroic rocks wit h ultramafic layers form the northeastern part. U-Pb (zircon) dating o f a quartz diorite sample from the southwestern part of the pluton ind icates crystallization at 557 +/- 3 Ma. Amphibole and phlogopite in tw o Iherzolite samples from the northeastern part of the pluton gave Ar- 40/Ar-39 dates of 550 +/- 5 and 539 +/- 5 Ma, respectively, indicating that the pluton cooled rapidly through the closure temperature for am phibole, with subsequent slower cooling to the time of phlogopite clos ure. The pluton is interpreted to be the intrusive equivalent of basal tic units in the host Coldbrook Group, analogous to granitic plutons e lsewhere in the Caledonian Highlands which appear to be the intrusive equivalents of felsic volcanic rocks in the group. These plutonic and volcanic rocks represent a major, short-lived (ca. 560-550 Ma), domina ntly bimodal igneous event, apparently related to late Precambrian ext ension within the Avalon terrane of southern New Brunswick.