PETROGENETIC RELATIONS AND COMPOSITIONAL CHARACTERISTICS OF U-PB GEOCHRONOMETERS IN A MESOARCHEAN LATE MAGMATIC ZIRCON-APATITE VEIN: MANIITSOQ REGION, SW GREENLAND

A mm-scale vein, composed of megacrystic zircon and interstitial apatite with abundant monazite inclusions, within a Mesoarchean metagabbro in the Akia Terrane of SW Greenland has been investigated to determine petrogenetic relationships and compositional characteristics of the U-bearing minerals. Zircon are dominated by oscillatory-zoned interiors, locally cut by thin, CL-bright veinlets and rims. CA-ID-TIMS yields a concordia age of 3008.7 ± 0.75 Ma. LA-MC-ICPMS analyses yield slightly discordant intercept ages of 3008.4 ± 0.67 Ma for pristine interiors and 2996.8 ± 1.9 Ma for rims, whereas veinlet-bearing (mixed) analyses yield variably discordant ²⁰⁷Pb/²⁰⁶Pb dates between ~2975-2700 Ma. The Hf isotopic composition is homogeneous across all domains, with LA and solution MC-ICPMS analyses yielding ¹⁷⁶Hf/¹⁷⁷Hf_i = 0.280829 ± 10 and 0.280827 ± 11, respectively (εHf_i = -0.60 ± 0.38). The interior trace element composition is consistent with magmatic zircon, with rim composition trending toward a zircon fractionated, more H₂O-rich environment.

Apatite have decreasing Y, REE, Th and U from interior to rims, with trends suggesting variable alteration of primary apatite formed in a mafic magma. CL imaging shows a granular internal microstructure and halos surrounding monazite inclusions, interpreted as a dissolution-precipitation (D-P) network and zones of LREE-Th-U depletion associated with monazite exsolution, respectively. Apatite exhibit a spread in Pb₀/Pb*, correlating with REE concentration, and defining a discordia with lower intercept of 1808 ± 53 Ma and upper intercept ²⁰⁷Pb/²⁰⁶Pb of 1.103 (i.e. model Pb composition in the Mesoarchean). Monazite exhibit multiple age/compositional domains with ²⁰⁷Pb/²⁰⁶Pb dates ranging from ~2900 to 2700 Ma, forming a slightly discordant array with lower intercept ~2050 Ma. The results suggest crystallization of zircon and apatite from a slightly evolved magma at ~3009 Ma was followed by zircon recrystallization at ~2997 Ma, zircon veining and local Pb-loss, along with D-P-enhanced monazite exsolution from apatite between ~2900-2700 Ma, and resetting the U-Pb systems in apatite (and monazite to some degree?) between ~2050-1800 Ma. Thus, each chronometer records multiple aspects of a complex tectono-thermal history previously defined in the Akia Terrane.