Abstract
Important thermal metamorphic events in west-central Maine occurred at 400 Ma (M2), 394-379 Ma (M3), and 325 Ma (M5). Each is closely associated with emplacement of S-type granites, such that the isograd patterns produced in the surrounding pelitic schists generally follow plutonic outlines. From north to south, grade of metamorphism varies from chlorite to sillimanite-K-feldspar-muscovite.
Mineral-chemistry studies on M3 and M5 indicate the following: (1) Much staurolite-grade chlorite is retrograde as demonstrated by the lack of a consistent relation between biotite composition and presence or absence of chlorite, given the restricted range of XH2O allowed in these reduced graphitic rocks. Consequently, the first sillimanite-forming reaction for many pelitic schists in Maine does not involve chlorite. (2) Garnet zoning patterns are prograde in rocks of the staurolite zone and retrograde in rocks of higher grade. (3) Presence of graphite and nearly pure ilmenite suggests low Fe3+ in micas and allows for end-member calculations that include a “Ti biotite” (KTi□(Fe,Mg)AlSi3O10(OH)2) and a “Ti muscovite” (KTi(Fe,Mg)AlSi3O10(OH)2). (4) Staurolite contains about 3 H (48-oxygen basis) and shows subtle indications of nonideality in Fe-Mg KD relationships.
Garnet-biotite geothermometry indicates the following average temperatures for the first occurrence of minerals: staurolite—510 °C, sillimanite—580 °C, sillimanite-K-feldspar—660 °C. The muscovite-almandine-biotite-sillimanite (MABS) geobarometer is calibrated on the basis of an average M3 pressure of 3.1 ±0.25 kbar such that staurolite + muscovite break down directly to sillimanite + almandine + biotite immediately above the sillimanite-andalusite phase boundary. On the basis of this geobarometer, the M5 rocks inside the second sillimanite isograd crystallized at 3.8 kbar.
The muscovite-quartz dehydration boundary shows excellent agreement with garnetbiotite temperatures and MABS pressure for the second sillimanite isograd in M5 if reasonable models for behavior of fluids in reduced graphitic pelites are assumed. The staurolite-quartz dehydration boundary predicts temperatures about 60 °C above the garnet-biotite temperatures for the first sillimanite isograd in M3. However, this discrepancy can be partially eliminated by assuming a regular pseudobinary staurolite solution model with WG= 15 kJ per tetrahedral Fe site. Nonideality in staurolite solid solution may also explain staurolite KD relationships that involve reversals in KD values.
The progressive southward increase in pressure from early M2 (2.35 kbar) to M3 (3.1 kbar) to M5 (3.8 kbar) over a 75-m.y. period requires a net increase of about 5 km of rock during a time when much of New England was suffering erosion. This P increase is believed to have been the combination of two effects: (1) as the locus of metamorphic heat moved southward, the depth of metamorphism increased by 2 to 3 km over a distance of 125 km laterally, implying postmetamorphic tilting of the area and/or differences in original topography; and (2) at any given place, P increased with time because of development of a widespread Devonian volcanic highland and correlative emplacement of granites at shallow levels.
The extensive terrane of M5, which varies from K-feldspar-bearing to K-feldspar-absent rocks, marks incipient granite melting immediately below the present rocks at 3.8 kbar, 660 °C; local T variations probably reflect local differences in degree of mobilization of magma and fluids. This presumably occurred above the gently north-dipping contact of the Sebago batholith exposed south of the M5 rocks.
