“Normal” and “inverted” metamorphic isograds and their relation to syntectonic proterozoic batholiths in the Wopmay Orogen, northwest territories, Canada—Reply

180

“NORMAL” RELATION WOPMAY

AND “INVERTED”

METAMORPHIC

ISOGRADS

AND THEIR

TO SYNTECTONIC

PROTEROZOIC

BATHOLITHS

IN THE

OROGEN, NORTHWEST

TERRITORIES,

CANADA-REPLY

M.R. ST-ONGE Geological Survey of Canada, Precambrian Diuision, 588 Booth St., Orfawn, Ontario HA 0E4 (Canada) (Received March 19, 1982; accepted May 27, 1982)

(1982) presents

Bhattacharyya verted

mineral

isograds

In reply to Dr. Bhattacharyya’s (1) In the St-Onge outlined

by mineral

a short

discussion

in the early Proterozoic discussion

on the documentation

Wopmay

the author offers the following

comments:

(1981) paper it is shown that the three metamorphic isograds,

are restricted

of in-

Orogen by St-Onge (1981).

to and are concordant

suites, as

(in map view) with

the two granitic batholiths. In the pelites of Zones 1, 2 and 3 of Wopmay Orogen, the regional metamorphic grade is characterized by the low-grade assemblage muscovite-chlorite-plagioclase-quartz over thousands of square kilometers. It is significant therefore locally high-grade

that the grade increases over a short distance garnet-sillimanite-cordierite-orthoclase-plagioclase-quartz-

(2-15

km) to

granitic pods assemblages only in the immediate vicinity of the two batholiths and that the change from andalusite-muscovite schists to the high-grade gneisses occurs over only l-7 km. This suggests the presence of a thermal aureole with the granites as the heat source leading

to the formation

of the isograds.

(2) The stratigraphy of the pelite belt is regionally upright (Hoffman et al., 1978). The inversion of the mineral isograds could not therefore have originated by a structural inversion as this would have shown up in the stratigraphy the composite down-plunge cross-section of St-Onge (198 1). (3) The highest stability quartz

temperature

fields of calibrated + plagioclase

granitic

liquid.

+ biotite

The inferred

estimates

mineral

in the study

assemblages,

+ sillimanite temperature

area,

documented

as derived

are for the products

G= garnet

+ cordierite

range is 725”-750°C

of excess water have been calibrated

at 650“-675’C

the

of the reaction + orthoclase

at pressures

4. 10’ Pa (Lee and Holdaway, 1977). These are maximum temperatures imply the unlikely condition of uH,o = 1.0. Minimum granite melting conditions

from

by

+

of 3 to

since they curves for

for pressures

of 3 to

4. 10’ Pa by Piwinskii (1968), Huang and Wyllie (1975, 1981) among others. These are minimum temperature values since they refer only to conditions of initial melting for a ‘f2p = 1.0. Maximum values of aHzO - 0.7 to 0.8, which are more realistic for graphltlc systems such as the Wopmay pelites (Ohmoto and Kerrick, 1977) have the effect of lowering the temperature stability of the highest grade assemblage (garnetcordierite-K-feldspar-granitic pods) found in the study area and of raising the temperature conditions of the granite solidus to a common temperature range 0040-195l/83/oooo-0/.$03.0(,

0 1983 Elsevier Scientific Publishing Company

181

(Robertson Wyllie,

and Wyllie,

1971; Kerrick,

1972; Lee and Holdaway,

198 1). The very fact then that the granites

the highest

temperatures

documented

in the metapelites

granites

did not form in situ at the structural

volume

of melt

conditions

represented

associated

granite

requires

generation

exposed. that

and

forming

is an indication

level presently

by the two batholiths

with the actual

1977; Huang

could have only started

at

that

the

Rather

the

the temperature

be higher.

Geobarometry

work on a garnet-sillimanite-biotite-plagioclase-quartz zenolith from the Hepburn Batholith suggests pressures of at least 5.0. 10’ Pa (St-Onge, in prep.). Geobarometry work on xenocrystic aluminous phases in the granites yields pressure determinations of 5 to 10. lo8 Pa (Pattison et al., in press.) in marked contrast to the 3 to 4. lo8 Pa determinations for the adjacent pelites (St-Onge, in prep.). This indicates that actual magma generation took place at depth and that the present erosion surface transects not the level of magma generation but rather the level of magma emplacement.

It is therefore

by the granitic

batholiths

of lower temperature

reasonable

during

isograds

to suggest that enough

ascent to generate

at the higher structural

(4) The author is well aware tacharyya (1981) but must point

heat was transferred

what are narrow

suites (aureoles)

levels.

of the papers by Thompson (1976) and Bhatout that Bhattacharrya (1981) was published in

March of 198 1 whereas St-Onge (198 1) was accepted for publication December 1980. In any case the author agrees with the fact that isograds and isotherms separate

entities.

inversion

of isograds

the isotherms

This is why in St-Onge and no equation

is, however,

(1981) the discussion

of isograds

an interesting

question

and isotherms

is restricted

12, are

to the

is made. The dip of

and to see what constraints

can be

formulated one can look at the inverted isograds shown in fig. 5 of St-Onge (198 1). In that figure it can be seen that the sillimanite isograd is inverted to the west to become vertical at depth. This isograd can be related to the polymorphic change of andalusite to sillimanite. Since AT/AP of the reaction on a P-T phase diagram is a negative value (Holdaway, 1971) the inversion of the isograd cannot be due to (a) a horizontal temperature gradient, (b) isotherms and isograd dipping in the same direction, dipping tacharyya

the dip of the isotherms in opposite

directions,

(1981) and illustrated

being

these

greater,

being

by Thompson

nor

the three

(c) isotherms

and

cases considered

(1976, p. 285). Rather

isograd

by Bhat-

the inversion

of the sillimanite isograd can only be brought about by isotherms and isograd dipping in the same direction (i.e. both being inverted) with the dip of the isograd being greater,

a case not mentioned

by Bhattacharyya

(1982).

The same argument is true for the granitic pods isograd in fig. 5 of St-Onge (198 1). This isograd is related to the reaction that produces in situ anatectic melt in a pelitic system. Calibrations of minimum granitic melt equilibria by Piwinskii (1968) Huang and Wyllie (1975, 1981) all show similar curves characterized by negative values of AT/AP in the pressure range of interest. An inversion of this isograd can only be brought about by isotherms and isograd dipping in the same direction (both inverted) with the dip of the isograd being greater.

182

In the Wopmay granitic

pods

(St-Onge,

Orogen

isograd

the dip of the sillimanite

is towards

the plutonic

1981). Due to the negative

the isograds, Batholith

with

inverted

isotherms

inversion

AT/AP

the dip of the associated

being

can only strengthen funnel

isograd

and

of the

Hepburn

the dip of the

values for the reactions

isotherms

the dip of the isograd

to the proposed

rocks

equated

is also towards

greater.

shape of the Hepburn

with

the Hepburn

The shallower

the case of relating

Batholith

dip for the

the cause of the isograd

Batholith.

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