Differential response of zircon U-Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone, southeastern Massachusetts, USA

Article Abstract:

Zircons from two units of Ponaganset gneiss, chemically and mineralogically different, were analyzed to determine the effects of metamorphism and mylonitization on the uranium-lead (U-Pb) isotopic composition of zircons. Results showed that zircon from gray gneiss are neither corroded nor overgrown while those from pink gneiss are usually coroded and overgrown. In terms of U-Pb systematics, zircons from the two units sampled were both discordant with those from pink gneisses exhibiting up to 70% discordance. Further studies revealed that during metamorphism, the degree of isotopic changes is related to chemistry and interaction between fluids and rocks.

Environmental aspects, Metamorphism (Geology)

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Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

Article Abstract:

Zircon is used for determining U/Pb magmatic crystallization age. Analyses conducted on the oxygen isotope ratios of zircon megacrysts from kimberlites in southern Africa and Siberia showed a variation of 4.7 to 5.9 in the standard per mil notation relative to standard mean ocean water (VSMOW). Through the use of lasers, subtle pipe-to-pipe differences were discovered. The zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamonds.

Isotope geology, Kimberlite

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U-Pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite: evidence for magma mingling

Article Abstract:

The similarity in morphology of zircon from microgranitoid enclaves and granite host rocks indicates magma mingling origin of microgranitoid enclaves. The zircon cores from Lachlan Fold Belt and Wilson's Promontory Batholith exhibit ages varying between 395 and 1700 Ma. The zircon population within microgranitoid enclaves contains large numbers of small, anhedral or elongated crystals.The zircon age distribution data for granite reveals that around 12% of the total zircon is inherited.

Australia, Natural history, Magmatism, Granite, Inclusions

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