CONDITIONS AND TIMING OF EMPLACEMENT OF THE MARCY ANORTHOSITE, NE ADIRONDACK HIGHLANDS, INFERRED FROM ANATECTITES IN ITS AUREOLE: PART 1, FIELD RELATIONSHIPS AND P-T ESTIMATES

Anatectic segregations in metagabbro (mGB), mangerite (MN) and charnockite (CH) of the New Russia gneiss complex, NE Adirondack Highlands, formed in response to heating caused by emplacement of the adjacent Marcy anorthosite. Field relationships indicate syn to posttectonic intrusion of the anorthosite. Gabbroic-anorthosite gneiss occurs within the New Russia complex and, in places, at the margin of the Marcy massif. At other locations undeformed metanorthosite cuts the fabric of the gneiss and contains gneiss xenoliths. The segregations in the New Russia complex are also syn to posttectonic features occurring as thin bands that parallel foliation in the granulite facies gneiss and as coarse-grained, undeformed lenses with igneous appearance that crosscut the gneissic fabric. Different assemblages in restite and segregation of each rock type indicate that the segregations formed by partial melting of the host rock, probably by dehydration melting reactions involving pargasitic hbl + feldspar ± qtz.

Metamorphic conditions during anatexis have been estimated by thermometry using ternary feldspar from segregations and by grt-opx and grt-hbl barometry. Temperature estimates range from 830 to 950 ±50 °C with an apparent increase in temperature towards the Marcy. Pressure estimates are 970 ±100 MPa (grt-opx-pl-qtz in MN) and 735 and 985 ±100 MPa (grt-hbl-pl-qtz in mGB and MN respectively). Estimates are consistent with partial melting experiments using metagabbro that have garnet growth in the restite if P > 700 MPa.

The connection between high-T, high-P metamorphism during anatexis with emplacement of the Marcy is inferred from 1. the very high temperature reached in the New Russia gneiss, more than 100 °C higher than estimates of peak regional temperature; 2. the metamorphic gradient within the complex as inferred from geothermometry; and 3. the syn to posttectonic nature of both anorthosite and anatectic segregations. The data indicate that the anorthosite intruded thickened crust at depths of at least 23 km and probably near 30 km.