HAFNIUM ISOTOPIC COMPOSITIONS OF ZIRCON FROM ADIRONDACK AMCG SUITES: IMPLICATIONS FOR THE PETROGENESIS OF ANORTHOSITES, GABBROS, AND GRANITIC MEMBERS OF THE SUITES

摘要

Zircon crystals separated from selected samples of Adirondack (New York) anorthosite, gabbro, and granitic members of the AMCG suite, previously dated by SHRIMP U-Pb methods, have been analyzed for Hf isotopic composition. The resulting calculated Hf T_dm values for both granitic rocks and anorthosites are ca. 1350-1400 Ma, consistent with previously determined Nd Tdm, significantly greater that the U-Pb crystallization ages of 1155-1160 Ma. They indicate involvement of older lithosphere. The range of initial ent for anorthosites falls within the range observed for non-anorthosite lithologies (ca. +3 to +8) and suggests some consanguinity in their petrogenesis. On the basis of bulk compositions, it is likely that granitic members of the suite were formed by partial melting of pre-existing crust or mantle lithosphere (or both). Anorthosite may have formed from fractionation of basalt derived from previously enriched mantle lithosphere. The fact that the Hf isotopic characteristics (e.g., T_dm) of zircon from the chamockites, mangerites, and granites entirely overlap those of zircon from the anorthosites and gabbros suggests that the sources (e.g., enriched mantle and lower crust) were characterized by similar Hf isotopic compositions and a limited range of Lu、Hf at ~1100 Ma. Such a relationship might be expected if ~1350 Ma arc development produced relatively homogeneous crust and sub-arc mantle in terms of Hf isotopic composition and Lu/Hf values. This is feasible because changes ofεHf or ε_Nd in such a crust-mantle system in only ~150-200 Ma (difference between T_dm and U-Pb ages) would be less than 1_εHf unit.