UDC 552.161
MELTING OF ACID VOLCANITES IN CONTACT WITH A
SHALLOW BASITE MAGMA*
B. A. Litvinovskiy, Yu. Yu. Podladchikov, A. N. Zanvilevich,
and V. M. Dunichev
Geological Institute, Buryat Scientific Center, Siberian Division,
USSR Academy of Sciences, Ulan Ude, Geological Institute,
USSR Academy of Sciences, Moscow, and Sakhalin Pedagogic
Institute, Yuzhno-Sakhalinsk
There is evidence of melting of acid volcanites in the
exocontact zones around shallow basite dikes in the center of
Kunashir Island. Contact melting is found only in the altered pumice and rhyolites, which contain large amounts of secondary clay material and have 5-7 wt.X HZO. The thermal dehydration of the clay minerals increased PN,o considerably and
s 2-4 kbar. ~ o r r e i ~ o n d i n ~the
l~,
resulted in
= Ptot
temperature ofqmelting in the water-saturated s stem was reSimulation indicates that t e water presduced to 650-700'.
sure can attain such values under subsurface conditions. The
calculations confirm the hypothesis that the acid magtuas of
the Neogene subvolcanic intrusions on Kunashir could have been
formed by contact melting of pumice at depths of 1-2 km.
b
There is a vast literature on the formation of acid magmas at contacts between sialic rocks and injected basite magmas. It is generally accepted that
contact melting may be substantial only at large and medium depths. Here we
describe contact melting occurring at shallow depths and due to high water contents in the rocks that are melted. An interpretation is proposed. The same
phenomenon has been described for the Izu peninsula in Central Japan [I].
We examined the Cenozoic volcanic and subvolcanic rocks on Kunashir Island
(Kurile arc). In the central part of the island, acid volcanites of Middle
Pliocene age [ 2 , 31 commonly are cut by tholeiitic basalt, andesite-basalt, and
andesite dikes. The acid rocks at the contacts with the dikes have melted in
some cases. The contact melting has been examined in two areas: near White
Crags (to the north of Lake Lagunnoye) and near Vodorazdel'nyy Cape. In the
'dhite Crags region, the high-alumina basalt dikes cut through pale-gray kaolini z e d pumice, which has over 5 wt.2 water content (Fig, la, Table 1, specimen 1).
A zonai sequence i s found in the pumice towards the dike contact: annealing
and then partial melting in the pumice matrix and then almost complete melting.
The annealing is seen as a change in color (from pale gray to brown) and as
some consolidation. The onset of melting is indicated by the occurrence of
irregular areas, lenses, and patches of dark or almost black isotropic glass
(pale-brown and finely fluidal in fabric in thin section), whose amount increases
towards the contact. Directly at the contact, the groundmass is entirely converted to gl-ass,but the fragments of PZ, Q and K F e crystals and also of basalt
and rhyolite contained in the pumice show no signs of melting. Analyses for the
zones (Table 1, specimens 1-31 show that annealing and melting are not appreciably accompanied by migration of material. The glass has a composition very
*Translaced from Geokhlmiya, No. 6 , pp. 807-814,
1990.
Fig. 1 . Basalt and andesite dike6 in pumice and r h y o l i t e s i n the region of White
Crags (a) and Cape Vodorazdel'nyy (b): 1) pumice; 2) s l i g h t l y altered r h y o l i t e
(a) ar crushed and kaolinized r h y o l i t e (b); 3) basalt and andesite dikes; 4 and
5) zones of complete (4) and p a r t i a l (5) melting; 6) annealing zone i n pumice.
The numbers by the points are thore of the specimens i n Table 1 .
close to that of the groundmass of the pumice but contains less water (the higher silica concentrations in the glass occur because the composition was determined by microprobe, whereas the rock analyses were performed on overall sam0
cryples that contained not only groundmass but also 10-152 of small A n 4.0. - 5 .
stals). The glassy parts contain rounded and oval pores ranging in size from
10-20 pm up to tenths of a millimeter together with gas bubbles 5-10 um in
diameter. Their amount in volume X is around 10% by volume but in places it
reaches 50%.
The basalt dikes show aphanitic chilled zones from 5 to 30 cm wide, within
which the groundmass has an intersertal structure and contains up to 30-402
glass. The groundmass glass directly at the contact and up to 15 cm away from
it also contains
10-152 micropores with sizes up to 0.1-0.5 mm. The pores
are very unevenly distributed and tend to concentrate in bands parallel to the
contact. In many cases, the plagioclase microlites in the basalt groundmass
are oriented along the oval and rounded pores and as it will flow around the
latter. That orientation is seen only in a thin boundary layer and indicates
that the gas bubbles occurred when the magma was still liquid. The basalts have
constant mineral and chemical compoeitions in the central parts of the dikes
and in the endocontact zones (Table 1, specimens 4 and 5 ) , which shows that
there was no substantial reaction between the magma and the country rock. The
width of the contact melting zone is clearly correlated with the dike thickness:
at the contact with a dike 3 m thick, the zone of continuous melting of the
pumice groundmass was 25-30 cm wide, while the partial melting zone following
it (Fig. la) was up to 60 cm widei at a contact with a 1 m dike, the widths of
those zones were correspondingly 5 and 10 cm. A t 100 m from this area, where
basalt dikes intrude a stockwork body composed of unaltered dacites, the contact
of the latter with the dikes shows no signs of melting and no traces of thermal
action from the basite magma.
-
-
In the Cape Vodorazdel'nyy region, the cliffs show a series of andesite
150 m that intrude rhyolites (Fig. lb). In the
dikes over a distance of
southwest part of the exposure, the rhyolites have been extensively crushed and
kaolinized. The clay material accounts for 30-50% of the rock volume, and the
bound water content is up to 7.5 wt.% (Table 1, specimen 6). Outside the crushed zone, the rhyolites are almost unaltered, and at the contact with the dikes
(on the right in Fig. lb), there are no signs of thermal action, with the groundmass retaining its microfelsitic fabric, without signs of recrystallization.
Contact melting is observed only in the kaolinized rhyolites. Given the general
similarity in morphology of the contact zones to the zones of melting in the
pumice (unusually high microporosity in the eecondary rhyolite glass, and abundant pores in the endocontacts in the andesite dikes, with signs of remelting not
throughout the rock but only in the parts rich in clay, and correlation between
the width of the melting zone and the dike thickness), there are also some specific features
.
Firstly, there are signs of mixing between the andesite and the secondary
acid liquids. In the endocontact parts of the dikes, one can see pale veinletlike segregations and elongated lenses of rhyolite glass containing disintegrated
Table 1
Composition of Acid Volcanic Rocks, Their Melting Products, and Basalt Dikes
Note:
Lake Lagunnoye r e g i o n (1-5):
1 pumice w i t h o u t s i g n s o f a n n e a l i n g , 2 a n n e a l e d p u m i c e , 3 m e l t e d mass i n p u m i c e , 4 b a s a l t
Cape V o d o r a z d e l ' n y y r e g i o n (6-14):
6-10 a n d 11-12 d i k e s i n e x t e n s i v e l y c r u s h e d
from d i k e e n d o c o n t a c t , 5 b a s a l t from d i k e c e n t e r .
a n d k a o l i n i z e d v o l c a n i t e s : 6 a l t e r e d a c i d t u f f 2 m f r o m c o n t a c t . 7 t h e same, c o n v e r t e d t o d r y r e s i d u e , 8 t h e same m e l t e d a t c o n t a c t
w i t h d i k e , 9 d i k e e n d o c o n t a c t z o n e , 10 d i k e c e n t e r , 11 s l i g h t l y a l t e r e d a c i d t u f f i n c o n t a c t w i t h d i k e . 12 a n d e s i t e f r o m d i k e c u t 1 3 a n d 14 r h y o l i t e s f r o m s u b v o l c a n i c i n t r u s i o n s a t Lake
ting slightly altered tuffs.
See Fig. 1 f o r p o s i t i o n s of sampling p o i n t s ,
1.agunnoye (13) a n d Lake S e r e b r y a n n o y e ( 1 4 ) .
and partially separated 4 and P t crystals. The veinlets range from 5-6 up to
15-20 mm in length and from 0.2-0.3 up to 3 mm in width. The acid glass contains up to 40% micropores. Elevated small-pore levels occur also in the andesites directly at the contacts with the acid glass segregations. The boundaries
of the rhyolite veinlets are sharp and linear, while the andesites contain occasional quartz diacrysts, which indicates that magma mixing had begun, and complete homogenization was prevented only by the rapid cooling. The entry of the
rhyolite liquid into the andesite dikes has raised the silica level in the latter appreciably (Table 1, specimen 9).
Secondly, the P t and 4 phenocrysts in the heating zone have been extensively crushed. On the whole, the rhyolites, including the cataclastic ones,
are characterized by idiomorphic and well-preserved phenocrysts, while near the
contacts with the dikes, the phenocrysts are very often aggregates of small
sharp-cornered fragments, which in places retain the crystal shape but elsewhere
may be deformed sometimes considerably. This deformation and disintegration
is observed only in areas where there is secondary acid glass with its finebanded fluidal texture, which indicates that the molten material has begun to
be displaced. In the veinlet-like acid-glass segregations in the dike endocontacts, there are mainly sharp-cornered phenocryst fragments.
The melting of the acid rocks at the contacts with these basite magmas
occurred near the surface. It occurred only in those rocks where there is 5-7
wt.X bound water in the clay minerals. The fresh and largely unaltered rhyolites at the contacts with the dikes do not even show signs of thermal metamorphism.
in the
The main reason for the contact melting is the sharp rise in
system, with the related reduction in melting point. Rapid contact heating dehydrates the clay minerals at 550-700°C [41. The melting point is reached in a
water-saturated system
P
) , a8 the water content is more than 5 wt. Z.
tot
At depths of 1-2 km, that content substantially exceeds the solubility of water
in an acid magma with those parameters [ 5 1 , so there is a free vapor phase in
equilibrium with the liquid. The negative slope of the melting curve means that
the melting point is appreciably reduced by the high steam pressure at the con1 kbar or 650-700' at 3-4 kbar [5]. The very high
tact, namely to 750" at P
H20
acid-glass porosity (tens of vo1.l) indicates the presence of free vapor, with
the liquid in a state of foam. The elevated fluid pressure is evident from the
distillation of part of the water in the contact zone (transvaporization), together with the crushing of the phenocrysts, and probably the injection of the
rhyolite foam into the andesite liquid.
-
An important feature of the procese is the formation of a zone of partial
melting along with the zone of complete melting, which in general is characteristic of systems having a considerable T,-Tt range and is difficult to explain
in water-saturated anchieutectic system. Model calculations (61 show that in
the system examined, to produce the observed thicknesses of the partial-melting
zones requires differences of some tens of degrees between the temperatures for
the start and end of melting. In an anchieutectic water-saturated system, that
temperature reduction in the melting zone substantially exceeds T a- - T r-, at 1-2
kbar and can be explained only from the corresponding increase in p H
Under
70
isochoric conditions, the maximum fluid pressure occurs at the melting front,
i.e., at the outer boundary of the zone of melting, where the proportion of acid
liquid is minimal. In the inner parts, some of the water dissolves in the liquid, which progressively reduces the pore pressure and correspondingly increases
the melting point towards the contact. That temperature and pressure pattern
persists while different pore pressures can be maintained in the solid skeleton.
Then the skeleton loses its strength, the pressure cannot be reduced further,
and then a slight temperature rise leads to complete melting. In the zone of
complete melting, the fluid pressure is minimal and the same throughout the
liquid (Fig. 2). The value can be estimated, since the mean porosity in the
acid glass is 20-302, which indicates that the volume gas content q in the liquid
was about the same. The solubility C H 2 0 ( P , T ) of water in an acid magma as a
function of pressure ( 5 1 indicates that the water content in the liquid + vapor
system at 700-750°C corresponding to the initial water content of 6-8 wt.Z in
the substrate can occur with P H
2 1 kbar; at higher pressures, the values
20
Fig. 2 . Distribution of temperature T, pressure P ,
and degree of p a r t i a l melting N in r e l a t i o n to d i -
L
stance from dike a x i s f o r a given time: I) i n j e c t ed body; 11) complete melting zone; 111) p a r t i a l
melting; IV) unmelted rock.
for the water content are unrealistically high*. Simulation based on models
analogous to [ 6 ] shows that one can obtain these melting-zone thicknesses if the
temperature at the melting front is 640-700°C (with the basite magma at 12001300eC and the rock at
100°C, in which case the temperature for complete
melting is estimated as
700-750°C). In contrast to [6], lower values were
used for the thermal conductivity and heat of melting for the rock on account
of the high porosity. The T - P melting diagram for the granite-H20 system [ 5 1
indicates that 650-700°C corresponds to
2-4 kbar, which agrees with the pressures required to crush the Q and PZ crystals at elevated temperatures 171.
The differences between P H
at the melting front and in the complete-melting
20
zone may thus be 2-3 kbar, which provides the necessary temperature differences
of some tens of degrees between the start and end of melting.
--
-
Calculations confirm that such high pressures can occur under near-surface
conditions, where infiltration mechanisms provide effective reduction in the
pore pressure to the lithostatic value. In the quasistationary approximation,
the pore pressure P at the melting front is described by
Here p o and D are the density and dynamic viscosity of the pore fluid at the
low temperature, pn is the country rock density, k the permeability, L the
distance to the pressure relief level (the melting depth), K porosity, c the
weight content of volatiles in the rock, v d e = a/Zde the velocity of the dehydration front ( Z d e
qyt is the thickness of the dehydration front), a- lOk,
at 500°C, and k , the rock thermal diffusivity. The solution for PI ,=0
is
-
&
.
in which
*The pressure has been estimated In each case from tabulated values f o r Cn,o(P,TI, pi(P. TI,
(l+CE,o)pL,in which p+ and P L are the d e n s i t i e s of
and PL used in c - 9 p i + ( l - q ) C t 1 , 0 p ~ / q p , + ( I - + )
the aqueous f l u i d
the vapor phase and of the l i q u i d .
Fig. 3. Behavior of pore p r e s s u r e P a t contact between zones I11 and I V (Fig. 2) and behavior of
melting f r o n t f o r L I < ld ( .12 (1) and ld < Z2 ( 2 ) .
I n case 1, t h e malting is l i m i t e d by the heat cont e n t of t h e i n j e c t e d body, while i n case 2, i t i s
r e s t r i c t e d by t h e p r e s e u r e drop; l l m i n i m u m i n j e c t ed body e i z e providing c o n t a c t melting, l2 c r i t i c a l
s i z e above which t h e melted zone thickness c e a s e s
t o be dependent on t h e s i z e of the i n j e c t e d body.
Fig. 4. I l l u s t r a t i o n of how t h i c k n e s s of melted
zone 18 dependent on depth L and dimensions ld of
i n j e c t e d body under hypabyssal conditions. The
melted-zone t h i c k n e s s i s o l i n e s a r e shown in nomin a l u n i t e : I ) no c o n t a c t m e l t i n g ; 11) region in
which t h e ~ v l t i n gzone t h i c k n e s s i s p r o p o r t i o n a l t o
td f o r a given L; 111) r e g i o n in which t h e melted
zone thicknee.
is p r o p o r t i o n a l t o L and independent
of
-
Zd.
-
As the dehydration zone expands, the pressure at first increases but then
in a time
fallsi P , = P . ~ l + 2 A
is attained at I d e
'max
Here P.=cpn /xyi is the outgassing pressure under isochoric conditions (without
vapor loss into the surrounding medium) , and A =y,p,kl ( 2 y , ' u 2 p ) .
For t<t,,,
, the rapid formation of vapor (caused by the high rate of displacement of the dehydration front,
) is ineffective because of the smaii
pressure difference, so the pore fluid increases in density and the pressure
increases:
-la
the velocity of the degassing front decreases rapidly, while the
For t>L,,
rapid infiltration does not cease until the pore pressure has fallen to the
hydrostatic level: P - l l ( l + x ) .
With C= 0.06, @,-I 1 g/cm3, p , - 2 2 g/cm3,
2
X0.1, cz E 10-' m /sec, and k , = 10-7 m2/sec, k 2 10-l6 m2 , y2/7, =: 0.05, y, ,"
0.2 g/cm3 skbar, which gives P, z 10 kbar and ,P
10/71+4.lo-" k[m21 kbar, with
2
2
A =2 x
[m I , z
= ( 1 0 - l ~ :[rn
~ IIL.
A pressure of several kbar can be attained with realistic permeabilities
of 4 = l ~ ' ~ ~ - l ~ - "m2 . That pressure rise occurs only around t m a x , and a
second necessary condition for contact melting is that the melting--pointas a
function of pressure is attained at that time. The cooling time t c required
to produce temperatures less than the melting point at 2-5 kbar should be less
than t m a x . If in this case, that time is of the order of l;/kT, the melting
condition is
in which Id is the dike half-width and I I is the miniri dimension of the injected body to provide contact melting. - ~ q o n dt m a x ,the pressure falls again,
and the meltin eventually ceases (Fig. 3). Thus, when (1) is met with increase
in thickness o the intrusion, the increase in thicknesses of the melting zones
ceases as a value l2 that is proportional to the depth and related to the pressure drop (Fig. 4). In the present case, for example, I, z L/lOa. We conclude that
large basite bodies (thicknesses 500 m or more) injected into acid volcanites
at depths of 3-5 km can produce contact melting zones 30-40 m wide.
!
These data provide a basis for discussing how that melting has affected
the formation of the rhyolitic subvolcanic intrusions on Kunashir, which occur
in the same part of the island as the dikes and are of the same age: the intrusions at Lake Lagunnoye and Serebryannoye, and at Capes Spiridonov and Stolbchatyy and so on. The rhyolite bodies are from a few hundred meters to a
kilometer across. They have abundant regions and zones where they have a porous
structure, which indicates a high gas content in the magmas.
Close to these bodies, there are subvolcanic intrusions of basalt, andesite, and andesite-dacite compositions: Cape Ginmeling, Rogachev, Kruglyy,
and so on. The range from rhyolites to basalts was formed in the Middle to
Late Pliocene era [2, 31. The two magmas were contemporaneous because there
are clear-cut signs of their mixing, which we have found in certain subvolcanic
bodies, particularly in the andesite intrusion in Cape Rogachev [ 8 ] .
Because of the closeness in space and time between the basalts and rhyol i t e ~ ,the high gas contents in the acid magmas, and the similarities in composition between the subvolcanic-body rhyolites and the contact melting products
(Table 1, specimens 3 and 8 or 13 and 14) all testify to the possibility that
the rhyolite magmas here may have been formed by the above mechanism. Pumice
or pumice tuffs in the Alekha formation form the most likely substrate that
melted in contact with the basalt magma, as their thickness is over 500 m. The
succession and the paleogeographic setting [ 2 , 31 indicate that at the end of
the Middle Pliocene, the acid rhyolites were at a depth of not less than 1-2 km.
The current erosion level indicates that the sizes of the basalt bodies may have
been more than 1-2 km across. When the basalt magma entered the hydrated acid
volcanites, the latter melted under conditions where
=
The thickness
Ptot*
of the melted layer is estimated as 30-40 m, and the voiumes with a cross section
of several square kilometers would be quite sufficient to produce subvolcanic
intrusions comparable in size with those now observed.
This hypothesis does not conflict with the crystallization differentiation
model, which many consider may play the decisive part in the volcanic stages.
We have tried to show that island-arc acid magmas may arise by various mechanisms under various thermodynamic conditions.
REFERENCES
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2.
3.
4.
5.
6.
7.
8.
Ito Tanio, Matsumoto Ryo, Kano Kenichi, et al., 1984. Tishitsugaku Zasshi,
J. Geol. Soc. Japan, v. 90, No. 3, 191.
Dunichev, V. M., 1983. Vulkanizm Bol'shoy Kuril'skoy dugi [Volcanism in
the Greater Kurile Arc], Nauka, Moscow.
Frolova, T. I. and T. K. Zlobin, 1987. Vestnik MGU, Ser. 4, Geologiya,
No. 1. 3.
Brown. G. (editor). 1965. X-Ray Examination Methods and Clay-Mineral
Structures [Russian translationl. Mir, Moscow.
Huang, W.-L. and P. T. Wyllie, 1975. J. Geol., v. 83, No. 6, 737.
Sharapov, V. N. and A. N. Cherepanov, 1985. Dinamika differentsiatsii
magm [Dynamics of Magmatic Differentiation], Nauka, Novosibirsk.
Clark, S. M. (editor), 1969. Handbook of the Physical Constants of Rocks
[Russian translation], Mir, Leningrad.
Litvinovskiy, B. A., A. N. Zanvilevich and V. M. Dunichev, 1989. Zap. VMO,
No. 1, part 118, 26.
UDC 552.323.6
TYPE ASSEMBLAGES OF KlMBERLlTE ILMENITES*
Yu. V. Malov, Ye. A. Oveyannikov and B. M. Ostrovskiy
All-Union Research and Development Institute for Geological,
Geophysical, and Geochemical Information Systems, Moscow
Analyses of kimberlite ilmenites from diamond-bearing
regions in Yakutia, Africa and America have been processed
by a computer. Arguments are presented on the methods of
describing the diagnostic chemical characteristics of these
ilmenites for use in exploration.
The distinctive chemical characteristics of magnesian ilmenites (picroilmenites) from kimberlites is of interest from the point of view of using them
to construct mineralogical criteria for prospecting for kimberlite bodies. If
each pipe is characterized by its own type of that mineral, it is possible to
detect the traces of unknown eources of detritus in an exploration region where
several dispersal aureoles are present against a background of minerals derived
from known pipes.
Ilmenite differentiation is usually based on a triangular diagram using
the three components ilmenite + geikielite + hematite. The compositional vari~ M+ ~ and
~ +heterovalent (~e~', Pfg2+) 4
ations are ascribed to isovalent ~ e +
+ 2~e~'isomorphien. A classification has been given [ll for kimberlite
*Translated from
Geokhimiya, No. 6, pp. 815-822, 1990.