Timing of anatexis in metapelites from the Adirondack lowlands and southern highlands:A manifestation of the Shawinigan orogeny and subsequent anorthosite-mangerite-charnockite-granite magmatism

摘要

Anatectic leucogranites are common in metapelites within both the highlands and lowlands terranes of the Adirondack Moun-tains of northern New York State.The forma-tion of these igneous bodies,which are folded in the lowlands and commonly mylonitized in the highlands,has been widely consid-ered an event accompanying the ca.1050 Ma Ottawan orogeny,during which metamor-phic grade reached granulite facies in the highlands,while the lowlands experienced amphibolite facies metamorphism.Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons separated from leuco-somes and melanosomes in both the southern highlands and the lowlands indicate that pri-mary anatexis occurred ca.1180-1160 Ma,and is thus a manifestation of heating during the earlier Shawinigan orogeny (ca.1210-1160 Ma) and associated anorthosite-man-gerite-charnockite-granite (AMCG) mag-matism (ca.1165-1150 Ma).The absence of Ottawan overgrowths on Shawinigan zircons in these leucosomes suggests that by Ottawan time the rocks were too dry for further melt-ing or zircon growth to occur.However,electron microprobe analyses of monazites from the southern highlands reveal mul-tiple age zones,including cores with ages of ca.1170-1180 Ma,consistent with primary growth during Shawinigan orogenesis,com-plex zones formed ca.1140-1155 Ma during AMCG magmatism,and ca.1050-1020 Ma formed during Ottawan orogenesis and high-grade metamorphism.Throughout the Adirondacks,leucosomes and melanosomes contain older,ca.1320 Ma,zircons that are considered to be remnant detrital zircons derived from arc rocks of the Elzevirian terrane.The apparent absence of Archean detrital zircons suggests that the protoliths of the metapelites were deposited in restricted basins that did not receive detritus from the Superior craton.