:~t is strongly differentiated, varying in composition from troctolite dolerite to granophyre. ... Still higher, the gabbro-dolerite changes into ferro-gabbro and quartz.
P Y R O M E T A M O R P H I S M OF L I M E S T O N E S A N D T H E T E M P E R A T U R E OF B A S A L T I C M A G M A S V. V. R E V E R D A T I ' O
Reverdatto, V. V. 1970: Pyrmnetamorphism of limestones and the temperature of basaltic m.qgmas. J~/thos 3, 135-143. Calc-silicate rocks which have been st,~jected to therma.~ m e ~ m o r p h i s m (sanidinite or spurrite-merwinite fades) are descn'bed from three localities on the Siberian Platform. All have formed in the aureoles of subv©lcanic basaltic intrusions. T h e temperature of metamorphism, as estimated fr.~m the earlier published experimental data, is about 800-1000°C, whilst the temperature of the basaltic magma is believed to have been not less than 1200°C.
V. V. Re~erdatto, Lab. of Metamorphism, Institute of Geology and Geophysics, Not~siln'rsk, 90, U S S R
The Anakit district A calc-silicate mineral assemblage produced by high-grade thermal metamorphism corresponding to the sanidinite or spurrite-merwinite facies (Turner & Verhoogen 1960, Reverdatto 1965) was first de~cribed in the USSR by V. S. Sobolev (1935) from the Anakit district on the Lower Tunguska river. At that time this discovery was the fourth in the world and unique in the USSR. Rocks of this type are now known from more than 25 localities spread all over the world. Nevertheless, new finds are of great scientific importance. The contact metamorphosed rocks of the Anakit district have been produced around a differentiated dolerite (trapp) intrusion, which lies in the arch of an anticlinal dome composed of carbonate-bearing Paleozoic rocks. It is an intrusive sheet (or phacolith), which on one of the anticlinal limbs changes into a thick cross-cutting body, which was evidently the feeder dyke. The observed thickness of the intiusion is about 100 m. :~t is strongly differentiated, varying in composition from troctolite dolerite to granophyre. The lower part of the intrusion is composed of olivine and olivine-hypersthene gabbro-dolerite, with poorly defined patches of troctolite dolerite. At higher levels these rocks change into ordinary gabbro-dolerite consisting of labradorite and clinopyroxene with hypersthene, olivine, titanomagnetite, etc. Still higher, the gabbro-dolerite changes into ferro-gabbro and quartz gabbro-dolerite. Dolerite-pegmatite and granophyre are well developed in the uppermost part of the intrusior. The mineral series with relative enrichment of iron and alkalies in the late differentiates, and the distribution of minerals in the vertical sequence of the intrusion, clearly indicate differentiation processes at the time of magmatic crystallization. The Anakit intrusion resembles that of Skaergaard in Greenland (Wager & Deer 1939)
136
v.v.
REVERDATTO
Table l. Variation of composition of principal series in vertical section of Anakit intrusion Olivine
Orthopyroxene
Clinopy roxene
Fa
Fs
Fs
Troctolite dolerite
23-25
45
16-18
A~. ~,5
39--~'1
85--90
Olivine gabbro-dolerite
25-45
50
18-22
39-41
39 4A
80--85
Gabbro-dolerite
48-50
50
22-30
31-38
39-44
60-80
Quartz gabbro-dolerite Ferrogabbro Dolerite-pegmat~te G~anophyre
~ 50-70 ~ ~
~ ~ ~ ~
20-30 30-39 32-42 52-53
30-35 25-32 12-25 4- 5
40-43 39-47 39-48 44-45
48-59 42-53 31-44 21-31
En
Plagioclase
W~
An
Remark: T h e mineral compositions determined by optical data are denoted by the end members: Fa - fayalite, Fs - ferrosilite, En - enstatite, Wo - wollastonite, A n - anorthite, given as molecular per cents.
having similar differentiation trends as well as mineral composition and distributio~a of rocks in the vertical sequence. The country rocks of the Anakit massif are of variable composition: Silurian banded marls, marly limestones and limestones containing interbeds of dolomite, dolomitized limestone and thin discontinuous bands of chert nodules. Ordovician calciferous siltstones and sandstones which compose the central part of thc anticlinal dome are less widespread. Assemblages containing merwinite, spurrite, tilleyite, melilite, pyrrhothe and recrystaliized calcite are f~rmed in the exocontact z o n e near to the feeder dyke and in rocks which originally consisted mainly of marly limestones and marls. This mineral assemblage replaced more argillaceous interbeds contained in the marls which form bands, lenticles, nodules and veinlets in what are now fine- to medium-grained marbles. These patches are darker than the marls and are clearly seen against the light grey background of these rocks. They are more resistant to weathering than the calcite rocks and form 'niggerheads' on the marble surface. The inner structure of the patches is conspicuous. Their central parts are usually composed of merwinite forming roughly isometrical tabular and irregular polysyntheticaily twinned grains often with sinuous boundaries. The optical constants of merwinite are: Np - 1.705 zk 0.(102, Ng----1.722 :~--0.002, 2v -~ + 73 °. Merwinite always contains rather numerous poikilitic inclusions of prismatic and rounded gr~tins of melilite of akermanitic composition. Peripheral parts of the patches are composed of elongate tabular or irregular polysynthetically twinned grait~s of spurrite, which ~also contain inclusions of melilite, but in smaller ~mounts. The optical constants of Sl:,urrite are: Np -- 1.640 i 0.003, Ng ---- 1.679 _~ 0.003, 2v .......39 '~. It can be seen that m'..'rwinite formed hter than melilite since
PYROMETAMC)RPHXSM
137
Fig. 1. Reaction rims of tilleyite (Ti) around spurrite grains (Sp) in calcit/c matrix, x 26.
the latter has often been corroded and replaced by merwinite to give a poikilitic texture. It should be noted that the content of mel~lite increases from the periphery of the patches to the cemre. This trend persists even within the merwinite core; sometimes almost monomineealic accumulations of melilite are enclosed in a merwinite-melilite associat on. All the abovementioned minerals contain inclusions of pyrrhotite, which is more or less uniformly distributed throughout the rock, occurring both iI~ the patches of calcic silicates and in the marbles. Spurrite is sometimes replaced in part by tilleyite, which forms large porphyroblasts or rims arovnd the grains of spurrite (Fig. 1). Merwinite is not corroded by tilleyite. The chert (chalcedonic-calcitic) nod~Jles were almost compL:tely replaced by microgranular aggregates of wollastonite during metamorphism. A replacement of the same type was described by Tilley (1929) in the contact zone of the Scawt Hill dolerites. In this case wollastonite pseudomorphs are formed, which fully preserve the outlines of the nodules (Fig. 2). Tile most strongly siliceous material is only partly replaced by wolla~onite and comprises a fine-grained tridymite-wc, llastonitic aggregate of irregular and lamellar grains, which are rarely larger than some hundredths of a millimetre. The distribution of tridymite in the woliastonitic rr~arrix is not uniform. Knotty, lentieular and fusiform tridymifie spots are usually observed penetrating the pseudomorph either randomly or roughly oriented parallel to its margins. Tridymite grains teem with rounded inelt~sions of wollastonite and graphite. The nodules repiaeed by wollastonite are always surrounded by rims of tabular spurrite grains, lying radial to the margin
138
v.v. REVERDATTO
of the concretion. The peripheral parts of these rims are composed of aggregates of tabular merwinite grains. Melilite is associated with both spurrite and merwinite; the amount is variable but again tends to be highest in association with merwinite. Pyrrhotite ios ubiquitous. In the inner part of the rim close to the wollastonitic pseudomorphs, spurrite is frequently replaced by tilleyite, forming rather large (up to 1 centimetre) roughly isometric porphyroblasts, containing relics of resorbed spurrite grains. The optical constants of this rare calcic silicate are: Np = 1.611 + 0.002, Ng --= 1.653 ~ 0.002, 2v =- + 88 °.
$ ~-Ti Cu
Cu
Sp. Ti
M
Fig. 2. W o l h , s t o n i t i c p s e u d o m o r p h a f t e r a silicic c o n c r e t i o n , s u r r o u n d e d by con,zentrie zonal r i m s o f rare t a l c silicates, i/. o f full size. Wo -- woUastonite, G -- garnet, Cu euspidine, Sp.'ri -~ spurrite-tilleyite aggregate, M - - m e r w i n i t e - m e l i l i t e aggregate, M a - - m a r b l e .
Dolomitic marl in the contact zone transforms into a montk:ellitemelilite-spurrite marble containing some tilleyite and pyrrhotite. MorMcellite, which is sometimes the only component of the rock apart from calcite, forms smaU, scattered, irregular grains or roughly prismatic crystals. The optical constants of monticellite are: Np =: 1.649 ~z 0.003, Ng := 1.661 ::i:: 0.003. Melilite present in this association is almost pure akermanite. Minerals tbrmed at a thermally early stage in the metamorphism have sometimes been replaced later by cuspidine and garnet in the metasomatic stage. Cuspidine replaces spurrite and tilleyite, forming irregular or rounded grains which often contain a great number of relict inclusions. Garnet (of grossular-andradite composition) is fi,rmed a little later than cuspidine and is developed mainly in place of the earlier formed ealcic silicates locally -eplaeing spurrite, tilleyite, cuspidine and others. On the boundary between the woltastonitic pseudomorphs (replacing chert) and their spurritetilleyite rims, cuspidine and garnet form thin, concentric rims, separated from each other (Fig. 3). In some parts of the exocontact of the Anakit
PYROMETAMORPHISM
139
Fig. 3. D e t a i l o f a w o i l a s t o n i t i c p s e u d o r n o r ~ h w i t h calc silicate r i m in t h i r section. × 26. Wo i
wollastonite, C u - -
cuspidine, Ti --
tilleyite, S p - - s p u r r i t e .
intrusion, garnet is well developed, forming monomineralic or garnetelinopyroxene lenses and veins. Hornfels consisting of wollastonitc, plagioclase, minerals of the diopsidehedenbergitc series, tridymite (or quartz), pyrrhotite and magnetite is the product of thermal metamorphism of calciferous siltstones ant sandstones. The rocks thus formed have a banded or spotted appearance and microgranoblastic texture with spots of poikiloblastic or porphyroblastic texture. Traces of a elastic texture often ren~ain in the metamorphosed calciferous sandstones, particularly among the quartz grains. Quartz sometimes converts to an aggregate of irregular minute grains of tridymite, lnterlayers of argillaceous rocks are transformed into spotted hornfels confisting of a microgranoblastic aggregate of phlogopite, sanidine, diopside anti pyrrhotite; andalusite (?) and calcic plagioclase are sometimes present in thi:~ association. The composition and distribution of metamorphic minerals in exocor. tactic rocks depends entirely on tLe distribution and composition of the original carbonate-argillocherty fractic,ns in the unmetamorphosed sedimentary rocks. Apparently, redistribution of the matter b'¢ metamorphism took place only in a microscale, the greatest distance of travel being only several millimetres. This statement is supported by preservation of the barding in marls, sandstones, and siltstones, and by the preservation of alI the details of the outlines of the chert nodules and of the carbonate-argillaceous bands in spite of the complete recrystallization into a variety of calksilicate minerals; it is further justified by the preservation of the pattern of distribution and composition of matter before and after metamorphism.
140
v.v. I~EVma)ATTO
This assemblage of high-temperature minerals was formed in the contact zone of the intrusion as a result of a series of reactions, mainly in solid phases, in which clay components, free silica, calcite and dolomite were the initial minerals. It should be pointed out that the composition and structure of the patches of calc-silicate minerals from exocontact marbles are a reflection of the primary composition of the initial sedimentary rocks. The compositional and structural variation observed in the patches of calcsilicate minerals from the exocontact marbles are a direct reflection of variations in the sediments prior to metamorphism. The patches which now consist of spurrite margins and a core enriched in melilite and merwinite were originally carbonate clay lenses which had a core rich in clay (kaolin and hydromicas) and dolomite (up to 10 %) whilst their rims were large calcite but possibly contained admixtures of dispersed free silica as chalcedony or opal. Zoning of another type is seen around the nodules which were origina31y chert. In this case the free silica has been replaced by wollastonite during metamorphism, whilst spurrite rims around wollastonite 'cores' have formed as a result of the reaction between calcite and silica (free and in clay minerals). It is to be noted that carbonate material around the silica concretions always contains increased quantities of finely dispersed chalcedony or quartz, as compared to the parts more distant from the concretion. Nevertheless, the bound~:ries of these concretions are always well defined. T h e K o c h u m d e k river area In the course of investigations into the conditions of formation of the high-teraperature minerals produced by the contact metamorphism, the ,uthor visited the Kochumdek river area (a tributary of Podkamennaya Tung~lska river), where spurrite-bearing rocks were recently discovered by N. S. Malich and V. V. Grigor'ev about 17 km from the mouth on the right bank of the Koehumdek river. A series of small-scale basic dykes, which correspond in composition to dolerite-pegmatite, occur here. These rocks consist of intermediate plagioclase, clinopyroxene, fayalite, and bluegreen hornblende in a quartzo-feldspathic matrix. At the contact between one d,:ke and the Silurian marly limestones there is a zone of tl'ermal metar~orphism 0.5 m thick. It is characterized by the formation of st,urrite anti melilite in association with calcite. The contact metamorphosed Inarble is a h~terogeneous grey colour with a fine- to medium-grained texture and roughly banded structure. The cale-silicate minerals are not evenly disseminated in the marble, but form narrow (not thicker than 1 cm) subparallel banded and lenticular segregation with indistinct outlines. Melilite is often concentrated in the central portior~ of the segregations, sometimes formi,:lg monomineralic areas, whereas spurrite is usually confined to the peripheral portions. Spurrite forms colourless, tabular or irregular grains, often ,vith poikilitic inclusions of calcite. The size of the grains is approxi-.
PYROMErAMORPHISM
141
mately 0.3 - 0 . 4 mm. The refractive indices of spurrite are: Np = 1.640 40.003, Ng = 1.680 4- 9.003, 2v = --39 °. Melilite forms small ( 0 . 2 - 0.3 mm), ~.rregular, or short prismatic grains. The refractive indices are variable: Nm = 1.665- 1.658 4- 0.003, Np = 1.660 - 1.653 4- 0.003. Uniaxial. Negative. The optical properties of the mineral provide evidence of its close affinity to gehlenite. Pyrrhotite is present as an insignificant impurity in the rock. T h e K u z m o v k a locality The third manifestation of high-temperature contact metamorphism of lialestones was identified 4-5 km d~wnstream from Kuzmovka village on the right bank of the Podkamennaya Tunguska river. A basic sill of considerable size (150 m thick) is found here. The total outcrop of the sill is not less than 80-100 sq.km. It is wtakly stratified as a resJlt of slight differentiation. The bottom part consists of olivine-hype~sthene doierite, overlain by normal medium-grained dolerite ,~ith hyperstt ene, whilst near the roof there appear small, indistinct patches of dolerit~-pegmatite with hortonalite, iron-rich augite and considerable amounts o~ a quartzo-feldspathic matrix. At the contact with the country rock there is a 0 . 5 - 0.7 m chilled margin. The roof of the sill is in contact with marly Silurian beds, which are contact metamorphosed. The zoae of high-temperature met:Jmorphism is no more than 1.5 m thick. The structural-t, extural features of the marble are analogous with those described above for the marble from the Kochumdek area. However, the mineralogical composition of the calc-silicate segregations formed as a result of metamorphism of the carbonaceous clay streaks in the marl is somewhat different. The assemblage is characterized by almost complete absence of spurrite. The latter occurs only in rare grains in immediate contact with the sill. The mineral forms small (0.1 - 0.2 mm) irregular, polysynthetically twinned grains. Its refractive indices are: Np -- 1.640 4- 0.003, Ng = 1.679 4- 0.003, 2v = --39 °. Tilleyite is developed in significant amounts. This mineral forms irregular, colourless grains. Its optical properties are identical with those for tilleyite from the Anakit district. Melilite is very abundant. It forms small, stumpy prismatic grains, poikilitically enclosed in large calcite crystals. Most of the melilite is optically isotropic. Its refractive index is 1.675 - 1.677 4- 0.003. Pyrrhotire is usually closely associated with melilite. Evaluation of pressure and t e m p e r a t u r e The mineralogy and textural relationships of calc-silicate minerals in thermally metamorphosed calcareous rocks can be more readily understood when studied in conjunction with unmetamorphosed samples of the same sediments. It becomes apparent that the spatial arrangement of these I0 - - Lithm 3:2
142
v.v. aEvE~ATro
minerals is merely a stronger expression of a primary arr~rlgement present in the original sediments. Spurrite-merwinite, spurrite-melilite, monticellite, ti~leyite-melilite and other marbles are the final high-temperature products of a series of progressive contact metamorphic conversions of primary matter. Ti~leyite is formed at lower temperatures out of spurrite in the pre;ence of CO2. The appearance of the tilleyite reaction tim around the wollastonite pseudomorph between wollastonitic 'core' and the spurrite rim (~~hich was described in the Anakit rocks) is due to increased microfissuring of this zone, which has resulted in a free penetration of COs. The formation of garnet and cuspidine rims occurs in a similar way. However, in the basin of the Podkamennaya Tunguska river (~t the locality of Kochumdek river and near the Kuzmovka village), tilleyite apparently developed during a gradual increase of temperature, directly t'rom the original material, as there is no indication of the replacement of ,.~purrite by tilleyite. Tridymite is known to be forrr~ed beyond the limits of its stability field by devitrification of glass. However, in contact metamorphism, formation of this mineral takes place in the region of its stability, that is to say, at a temperature above 870°C, which thus establishes a lower temperature limit of metamorphism for rocks of the exocontact near to the feeder dyke of the Anakit intrusion. These rocks were not, however, formed under atmosphere pressure. By summing the thickness of t~e Paleozoic deposits and the subvolcanic intrusions of the Anakit region, a figure of 1200-1300 m is obtained, corresponding roughly to a pressure of 350 arm., which would raise the lower limit of the temperature of metamorphism ~o 940~950°C. The formation of spurrite according ~o Turtle & Harker (19S7) and Harker (1950) must have taken place at temperatures not less tha~ 970~1020°C, the pressure being the same. Tilleyite can be formed at such pressure or~ly when the temperature drops to 950-970°C (Harker 1959.) On the basis of these data it can be concluded that the lower temperature limit of mel:amorphism in exocontact rocks of the Anakit intrusion (near to the feecier dyke) is approximately 1000°C. The thickness of the overlying sediments at the time of i~jection of ba:fic magma (Lower Triassic) was nearly 700 m in the area which is now the basin of the Podkamennaya Tunguska river. This caused lithostatic pressure of about 200-220 atm. In the presence of ~uch pressure, spurrtte could be synthesized at a temperature not lower than 930-940°C (Tutfle & Harker 1957, Harker 1959). This figure sets the lower temperature limit of the contact metamorphism and seems to be quite feasible. The temperature of the contact metamorphism at Kochumdek river was eviden~:ly somewhat higher than the temperature of the exocontact of the sill nearby the Kuzmovka village, since in the latter locality spurrite is only present in rare grains, whereas tilleyite, a lower temperature mineral, ha.,; I ather extensive development. It was formed here without replacement of ~;purrite, i.e. it must have been formed progressively.
PYROMErAMORPmSM
143
As could be expected, the temperatures in the vicinity, of Anakit massif and near to the dykes of the Kochumdek area were relatively higher than those around the sill near Kuzmovka v i l l ~ . The thermal conditions near to the feeder channel and the dykes were likely to approach a stationary regime of heat transfer to the country rocks, since in this case the dissipated heat loss was largely compensated by a constant addition of a new hot basaltic magma in the channels. The temperature of this contact metaraorphism, therefore, came near to the temperature of the basaltic magma: 1100-1200°C. The temperatu re around the sill was undoubtedly somewhat lower, since this magma was eUectively stationary and the thermal conditioEs therefore approached a non-stationary regime of heat transfer. The temperature of magma calculated for the sill by Jaeger (1957, 1959) was 1200°C, but allowing for heat loss due to dissipation it must be reckoned that the temperature of the contact rocks was never more than 800-900°C, which is also the temperature established for the contact metamorphism on the basis of the observed mineralogy. The above-cited examples of py ometamorphism of limestones clearly provide favourable conditions for indirect determination of the temperature of the basaltic magma. ACKNOWLEDGEMENTS. - The author would like to thank Dr. N. A. Chernova (Institute of Geology & Geophysics of Siberian Branch of Academy of Sci., of USSR) and Mrs. B. Jensen (Mineralogical-Geological Museum, University of Oslo) for improving the literary etyle of this manuscript.
REFERENCES Harker, R, I. 1959: The synthesis and stability of tilleyite, Ca~.~',~,,~()7(C~):~)~./liner Joker. of Sci. 257, 656-67. Jaeger, J. C. |957: The temperature in the neighbourhood o! a cooling intrusive sheet. Amer. your. of Sci. 255. 306-18. Jaeger, J. C. 1959: Temperatures outside of a cooling intrusive sheet. Amer. Jour. of Sci.
257, 44--54. Reverdatto V. V. 1965 (1964): Paragenetic analysis of caroonate rocl~ of the spurrite. merwinite facies. Geoehera. Internat. 5, 1038-53. Sobolev, V. S. 1935: The rare type of contact metamorphism of limestonea. Note~ of Miner. Soc. USSR (Zapisky Vseross. Miner. Obshchestva), Vol. 64, 1,2-5. (in Russian). Tilley, C. E. 1929: On larnite (calcium erthosilicate, a new mineral) and its associated minerals from the lime.~tone contact-zone o¢ Scawt Hill, Co. Antrim. Miner Magazine 22, 77-86. Turner, F. J. & Verhoogen J. 1960: Igne6u$ and Metamorphic Petrology. 2nd ed. McGrawHill Book Co., New York-London. Tuttle O. F. & Harker, R. I. 1957: Synthesis of spurrite and the reaction: wol~astonite qcalcite -~ spurrite + carbon dioxide. Amer. flour, of Sci. 255, 226-34-. Wager, L. R. & Deer, W, A. 1939: The petrology of Skaergaard intrusion. Medd. om Gr#nland. Bd. 105, No. 4.
Final manuscript accepted September 1969
Printed April 1970
