In situ granitoid genesis and dehydration in the Adirondacks

Penelope J. Lancaster, Bin Fu, Frederick Z. Page, Noriko T. Kita, Marian E. Bickford, Barbara M. Hill, James M. McLelland, John W. Valley

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.
Original languageEnglish
Title of host publicationGSA annual meeting, Philadelphia, USA
Place of PublicationBoulder, Colorado
PublisherGeological Society of America
Pages343
Volume38(7)
Publication statusPublished - 2006
Externally publishedYes
Event2006 Philadelphia Annual Meeting - Philadelphia, United States
Duration: 22 Oct 200625 Oct 2006

Conference

Conference2006 Philadelphia Annual Meeting
Country/TerritoryUnited States
CityPhiladelphia
Period22/10/0625/10/06

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