TY - JOUR

T1 - Intracrystalline redistribution of Pb in zircon during high-temperature contact metamorphism

AU - McFarlane, Christopher R. M.

AU - Connelly, James N.

AU - Carlson, William D.

N1 - Funding Information: This work was supported in part by NSF Grant EAR-0087564 to J.N.C. and W.D.C., by the Geological Society of America, and by the Geology Foundation of the University of Texas at Austin. The authors also acknowledge the support of B. Ryan and W. Tuttle (Geological Survey of Newfoundland and Labrador), D. Lee and C. Mackenzie (Voisey's Bay Nickel), and K. Manser for all of their help in the field. Many thanks to M. Hamilton and T. Ireland for SHRIMP help. An earlier version of the manuscript benefited from helpful reviews by T. Geisler, A. Möller, and J. Hermann. [RR]

PY - 2005

Y1 - 2005

N2 - Zircons in the high-T (700-900 °C) contact aureole surrounding the Makhavinekh Lake Pluton (MLP), northern Labrador, were studied using conventional thermal ionization mass spectrometry (TIMS) and sensitive high-resolution ion microprobe (SHRIMP) geochronology to test for evidence of high-T Pb mobility. Metasedimentary gneisses in the country rocks (Tasiuyak Gneiss) contain ∼1850 Ma zircons that formed during regional (M1) metamorphism that were reheated in the aureole during emplacement of the MLP at 1322 Ma (M2). M1 zircons that experienced M2 temperatures <750 °C are concordant at ∼1850 Ma and were, thus, virtually unaffected by M2 contact metamorphism. In contrast to this well-established baseline, sector-zoned M1 zircons in samples that reached T>800 °C scatter along a discordant array between M1 and M2 are locally reversely discordant and commonly return younger apparent ages for lower-U cores than higher-U rims. These data collectively require widespread intracrystalline Pb redistribution during M2. Isometric M1 overgrowths and inherited magmatic cores in the same samples were unaffected, indicating that susceptibilities to subsequent Pb redistribution may have been controlled by variations in M1 zircon growth mechanisms. The scatter of U-Pb ages in sector-zoned M1 grains is consistent with Pb expulsion from low-U domains (e.g., broad bright-CL sector boundaries) that had accumulated high lattice strains prior to M2. Lattice strain is inferred to have resulted from a combination of high intrinsic growth defects compounded by self-induced internal stresses from expanded metamict high-U sectors. Recovery of strained domains occurred while high-U sectors were partly metamict, allowing Pb to accumulate in the remaining amorphous fraction.

AB - Zircons in the high-T (700-900 °C) contact aureole surrounding the Makhavinekh Lake Pluton (MLP), northern Labrador, were studied using conventional thermal ionization mass spectrometry (TIMS) and sensitive high-resolution ion microprobe (SHRIMP) geochronology to test for evidence of high-T Pb mobility. Metasedimentary gneisses in the country rocks (Tasiuyak Gneiss) contain ∼1850 Ma zircons that formed during regional (M1) metamorphism that were reheated in the aureole during emplacement of the MLP at 1322 Ma (M2). M1 zircons that experienced M2 temperatures <750 °C are concordant at ∼1850 Ma and were, thus, virtually unaffected by M2 contact metamorphism. In contrast to this well-established baseline, sector-zoned M1 zircons in samples that reached T>800 °C scatter along a discordant array between M1 and M2 are locally reversely discordant and commonly return younger apparent ages for lower-U cores than higher-U rims. These data collectively require widespread intracrystalline Pb redistribution during M2. Isometric M1 overgrowths and inherited magmatic cores in the same samples were unaffected, indicating that susceptibilities to subsequent Pb redistribution may have been controlled by variations in M1 zircon growth mechanisms. The scatter of U-Pb ages in sector-zoned M1 grains is consistent with Pb expulsion from low-U domains (e.g., broad bright-CL sector boundaries) that had accumulated high lattice strains prior to M2. Lattice strain is inferred to have resulted from a combination of high intrinsic growth defects compounded by self-induced internal stresses from expanded metamict high-U sectors. Recovery of strained domains occurred while high-U sectors were partly metamict, allowing Pb to accumulate in the remaining amorphous fraction.

KW - Granulites

KW - Makhavinekh Pluton

KW - Metamorphism

KW - Pb redistribution

KW - Zircon

U2 - 10.1016/j.chemgeo.2004.11.019

DO - 10.1016/j.chemgeo.2004.11.019

M3 - Journal article

AN - SCOPUS:15344339631

VL - 217

SP - 1

EP - 28

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

IS - 1-2

ER -