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Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire

Geochimica et Cosmochimica Acta

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Abstract

U-Pb analyses of zircons separated from two Concord-type plutons near Sunapee and Dixville Notch, New Hampshire, reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry. The Sunapee pluton contains only slightly more Zr than required to saturate the melt at the peak temperature of 700 ?? 30??C. Traces of inherited zircon in this separate are inferred to be present as small, largely resorbed grains. In contrast, the Long Mountain pluton, near Dixville Notch, contains about 240% more Zr than required to saturate the melt. Thus, more than half of the Zr existed as stable, inherited zircon crystals during the partial fusion event, consistent with the observation of substantial inheritance in all grain size fractions. Ion probe intra-grain analyses of zircon from the Long Mountain pluton indicate a complex pattern of inheritance with contributions from at least two Proterozoic terrenes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component. Ion probe analyses of zircons from the Sunapee pluton reveal clear evidence of U loss which results in incorrect apparent conventional U-Pb ages. Ages of crystallization for the Long Mountain and Sunapee pluton are ~350 and 354 ?? 5 Ma, respectively. A Sm/Nd measurement for the Long Mountain pluton yields a depleted mantle model age of 1.5 Ga, consistent with the observed inheritance pattern. In contrast, a Sm/Nd model age for the Sunapee pluton is improbably old due to minor monazite fractionation. ?? 1987.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire
Series title:
Geochimica et Cosmochimica Acta
Volume
51
Issue:
9
Year Published:
1987
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
2549
Last page:
2558
Number of Pages:
10