U–Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA

Abstract

Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province in Virginia and Maryland. Lithology, structure, and U–Pb geochronology have been used to subdivide the gneisses into three groups. The oldest rocks, Group 1, are layered granitic gneiss (1153±6 Ma), hornblende monzonite gneiss (1149±19 Ma), porphyroblastic granite gneiss (1144±2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?). These gneisses contain three Proterozoic deformational fabrics. Because of complex U–Pb systematics due to extensive overgrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolution ion microprobe (SHRIMP), whereas all other ages are based on conventional U–Pb geochronology. Group 2 rocks are leucocratic and biotitic varieties of Marshall Metagranite, dated at 1112±3 Ma and 1111±2 Ma respectively. Group 3 rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077±4 Ma) and quartz-plagioclase gneiss (1077±4 Ma); (2) white leucocratic metagranite (1060±2 Ma), pink leucocratic metagranite (1059±2), biotite granite gneiss (1055±4 Ma), and megacrystic metagranite (1055±2 Ma). Groups 2 and 3 gneisses contain only the two younger Proterozoic deformational fabrics. Ages of monazite, separated from seven samples, indicate growth during both igneous and metamorphic (thermal) events. However, ages obtained from individual grains may be mixtures of different age components, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite (imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070, 1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granulite event at about 1035 Ma did not exceed about 750°C. Lack of evidence for 1110 Ma growth of monazite in porphyroblastic granite gneiss suggests that the Short Hill fault might be a Grenvillian structure that was reactivated in the Paleozoic. The timing of Proterozoic deformations is constrained by crystallization ages of the gneissic rocks. D1 occurred between about 1145 and 1075 Ma (or possibly between about 1145 and 1128 Ma). D2 and D3 must be younger than about 1050 Ma. Ages of Mesoproterozoic granitic rocks of the northern Blue Ridge are similar to rocks in other Grenville terranes of the eastern USA, including the Adirondacks and Hudson Highlands. However, comparisons with conventional U–Pb ages of granulite-grade rocks from the central and southern Appalachians may be specious because these ages may actually be mixtures of ages of cores and overgrowths.