I
CORRELATIONS BETWEEN ZIRCON MORPHOLOGY AND CHEMICAL
CHARACTERISTICS OF ALKALINE MAGMATITES (GRANITES,
RHYOLITES, SYENITES) FROM TURC OAIA-PIATRA ROŞIE ZONE,
NORTH DOBROGEA
Ion Niculae HOBU. Lucia ROBU
IlIstit utul Geologic al României,
Q
st; r.C'aransebeş
nI'. J. i9678
Bucureşti
32.
Key words: Zircon. Morphology. Typology. Frequency. Prism. Pyramid. Elongat.ion. Alkaline magmatites.
Absh'act: Morphological variations of zircons from alkaline magmatites, TurcoaiaPiatra Roşie zone, North Dobrogea, empha~ized some aspects specific to the identified
petrotypes: riebeckite-aegil'ine granites, riebeckite granites, alkali granites and alkali
rhyolites, quartz syenites and syenites. AII investigated rocks have similar zircon
morphological types (D,Ps KI, Ss); the differences, when they exist, are determined
by (1) different degree of prism or pyramid face development, 01' (2) different frequency
of these types in each studied petrotype. A large variability of the crystal elongation
has been observed. The elongation is differentiated in studied petrotypes, varying as
follows: (1) very elongat.ed cryst.als (long prismatic habit) in alkali granites; (2) long,
medium and short in riebeckite-aegirine granites and rhyolites; (3) medium in alkaline
rhyolites; (4) medium and short in syenitic rocks. No significant differences exist
between the physical properties of the zircon crystals from different rocks: generally
dark or brown colour, no zoning, no cores, frequently translucent OI' opaque, including
mafic OI' opaque minerals. Morphological and physical properties of zircons point
to a common source of the studied alkaline magmatites, with a deep origin (upper
mantle-lower crust), including high alkali content, a variable water content, small U,
Th, Y, and REE amount, enough Zr to be fixed as zircon crystals characterizeel by
well-eleveloped (100) prism anei (101) pyramidal faces.
Intl'oduction
The influence of t.he crys!'allization environment.
on the growth of crystals is well known. This has
been proved by many researchers (Poldervaart , 1950;
Pupin,Turco, 1972; Pupin, 1980), who studied the
morphology of t.he zircon crystals and their probabie physical and chemical conditions of crystallization, accordirig t.o chemical features of the parental
magma.
This paper tries to establish (1) the connections
between the morphology of zircon crystals and chemical composit.ion of the rocks includ ing t.hem and (2)
t.he relat.ions between morphology of zircons , chemical feat.ures of the rocks and t.he magmatic processes
which generated, in given t.ime and space, identifiecl
pet.rotypes from an initial magma.
Geological Setting
The studied alkaline magmatites belong to the
Unit of the North Dobrogea, cropping out in
Măcin
the Turcoaia- Piatra Roşie zone.
They intruded and determinat.ed the thermic met.a1110rphism of the Paleozoic formations ancl are covered by Cretaceous and Quaternary seclimentary formations.
The alkaline magmatites form irregular boci ies,
composecl of granites and alkaline granites, in the central part, but rhyolites exist in the marginal zones of
granitoicl boclies.
The Paleozoic formations are represented by
the Carapelit (coarse- to fine-grainecl sandstones,
conglomerat.es, pelites) and Cerna (pelites, micasanclst.ones, lime-sanclstones) formations.
A Il observecl pet.rotypes are characterized by
high alkalinity, being rec6gnized: quartz syenit.es,
riebeckite-aegirine granites, granophyric alkali granites, alkali rhyolites, riebeckite - aegirine - alkali hornblende rhyolites (Table 1).
All previous petrographical and mineralogical
stuclies (Cantuniari, 1912; St.efan, in Seghecli, 1990)
I
I. N. ROBU, L. ROBU
Table 1
Petl'otypes and sample locatioll
No.
Sample
Petrotipe
1
3335
rlebeckite- aeglrine granite
2
3332
3
3331
4
3329
riebecklte - aegirine granite
5
6
3327
3326
riebeckite granite
Piatra
Roşie
alkali granite
Piatra
Roşie
7
3324
alkali granite
Tu rcoaia
8
3325
alkali rhyolite
Tu reoala
9
alkali rhyolite
qua rtz syenite
19 lici o a ra
10
3322
3333
11
3330
syenite
1acobdeal
rlebeckl~e
Loc ati on
Tu rcoai a
granite
Turcoaia
alkall granite
Turcoaia
--
Iacobde al
Turcoaia
Table 2
The optic and physical pl"Opel'ties of zircon crystals
No.
Sa m p le
Colour
Zone
Core
I ne 1usions
Transparenee
1
3335
3332
3331
3329
3327
3326
oB
no
no
O, M
t, o
LB, oB
no
no
mgt
t,o
L B, OB
no
no
t, o
L B, OB
no
no
OB
no
no
OB
no
no
LB, OB
no
no
CS
no
no
LB, OB
no
no
OB
no
na
OB
na
no
O, M
O,M
O, M
O. M
O, M
O, M
O, M
O, M
O, M
2
3
4
5
6
7
8
9
10
11
3324
3325
3322
3333
3330
-
t--
t, o
--- - -
t,O
t
t, o
t
ti o
a
tia
LB =light brown; OB = dark brown; CS = eolourless; 0= opaque inclusions; M =undetermined
mafie inc!usians; T = trans parent; t =translueent ; 0= opaque; mgt = magnetite
point. out. a genetical connection between granites,
syenites and rhyolites _
partial melting process followed by the small differentiated magmatic phenomena_
Chemi cal features of the rocks
Sample preparation and methods of study
Alkaline magmatites of the Thrcoaia - Piatra Roşie
zone present the following chemical characteristics
(Stefan, in Seghedi, 1990): • high Si0 2 and alkali
sum, up to 8 %; • a small CaO content (under 1%),
excepting quartz syenites; • high Zr-Nb- Y contents
and small Sr-Ba ones; • REE amounts point out a
The sample rocks were cI'ushed undeI' 1 mm ancl
zircolls were obtained using vibration table, Frantz
isodynamic magnetic separator and dense liquids.
CI'ystal faces were indexed according to Caruba's
abaque method (Caruba, Turco, 1971) and their morpho!ogical features were established in concordance
I
35
Z/HCONS OF TURCOAIA ALKALINE NIAGMATITE
o
• 2
A
3
o
2km
!
• it
Fig. - Geulllgic' ai sketch of TlIl"coaia-Piat.l"a
Ho~ie
Zone (acr.ording t.o Geological Map of Homania, 1:50000, Peceneaga) and
zir("oll t.ypology (zit'con morphology accol"ding to PlIpin. Turco. 1972) : 1, riebeckitc-aegirine granites; 2, alkali granites;
3, syenit.es: 4. I"hyolit.es.
J. N. ROB[ , L. TiGBU
36
with Pupin's typological method (Pupin, Turco , ones in a -AI-rich crystallization environment and at
low temperature (up to 500° e);
1972).
2. Low Zr, Hf contents ancl sma.1l U, Y, Th a.nd
_The optical and morphological properties of zircon
crystals were observed at the stereo-microscope (x oc. REE amounts (Benisek and Finger, HJ93; Va:vra,
12.5; x ob. 7) and Jenapol microscope (x oc.lO; x ob. 1990) point to favourable conditions for the (100)
prism faces;
20).
3. H:?O content of crystallization medium (Caruba,
Morphological and optical properties of
1975; Caruba et al., 1971 , fide Duchesne , J 984) deterzircons
mines size variations of crysta.l faces , namely a dry enThe morphological study of zircons (according to vironment favors the growth of zircons without. wellPupin , 1980) from alkaline rocks of the North Do- developed pyramidal faces.
For the zircon studied populations, their 11l0rphobrogea region included zircOlis from alkali granites,
logical
characteristics el1lphasize some specific feariebeckite-aegirine granites, quartz syenites, syenit.es
tures
of
the crystallization environment, that will be
and alkali rhyoli tes (Table 1).
mentioned
bellow. CrystalIizat.ion of zircons espeThe typological analyses point out the same morcially
with
well-developed
(100) prism faces and (101)
phological types for each petrotype (Fig.).
pyramidal
one
points
to:
• AI anei high alkali conSo, the main type is D, which represents about
AI
and
alkali
variations
in eoncordance with
t.ents;
•
100% for granitic rocks and 98% for the alkali rhythe
temperature
changes;
•
a
deep
source for the iniolites; P 5 and S4 types were met only in the alkali
tial
material,
probably
upper
mantle-Iower
crust; •
rhyolites. Sometimes the (301) pyramidal face has
low
Zr,
Hf,
small
V,
Th,
contents;
•
high
water
conbeen observed which points to (Pupin, Turco, 1972)
tent
variation
,
responsible
for
the
high
variation
of
a very high alkaline environment for the growth of
the
face
growth
process,
determining
especialIy
the
the zircon crystals.
There are some obvious differences regarding the well-development of the prisl1l faces, when the cryselongation of the zircon crystals composing zir- tallization environment was a dry one; • a high temcon populations from different petrographical types perature, about 900° C, for zircon growth in grantwo zireon
(Fig.), as folIows: • alkali granites contain very elon- ites anei syel1ites, the rhyolities including
0
(about
900
C)
anei lower
types,
specifically
for
high
gated zircon .crystals; • riebeckite-aegirine granites
(about
650°
C)
temperatures;
•
the
same
physicoand syenites have alI types of the elongation (long,
crystallization
conditions
for
syenites
anei
chemical
medium and short zircon crystals); • alkaIi rhyolites
riebeckite-aegirine
granites.
are characterized by the medium elongated zircon
The changes in the morphological aspects of zircon
crystals; • syenites contain medium and short zircon
crystals
show two important moments of magma evocrystals.
lution:
1.
the variation of magma water content durSome physical properties (Table 2) are very siming
the
crystallization
of the mentioned petrotypes;
ilar for ali ~studied samples: dark or light brown
2.
obvious
temperature
decrease during the crystalcolour (sometimes colourless), no zoning, no cores,
lization
of
rhyolites,
or
some
low contamination profrequently translucent or opaque (sometimes transappearance
of two zircon
cesses,
emphasized
by
the
parent) inclusions of mafic or opaque mineral, no obtypes,
characteristic
of
low
and
high
temperatures.
vious dimension variations (between 0.15/0.10/0.10
and 0.0155/0.0125/0.0125 mm) .
Petrogenetic implicatiolls
It is well known now that the variations of some
physico-chemical parameters of the crystallization environment determine exchanges of zircon morphology. So, it could be mentioned, as follows:
1. Aluminium and alkali content variations in concordance with temperature changes determine (according to Pupin, 1980) a large variability of zircon
morphology, as (1) the well-developed (100) prism
face and (101) pyramidal one, crystallize in high alkaline medium and high temperature (up to 800°C),
reached at a deep level, probably at the upper mantIe
and lower crust, and (2) the appearance and development of the (110) prism face and (211) pyramidal
Conclusions
The study of zircon morphology points to genetical connections between alI alkali magmatites from
Turcoaia-Piatra Roşie Zone. They could form from
the same deep material (upper mantle-Iower crust),
which in the beginning (about 900°C) ha.ve been (1)
high alkaline, (2) sufficient Zr supersaturated for the
crystals with welI-developed (100) prism faces, (3)
wit.h a variable water content, and (4) sl1lall V, Y, Th
and REE amounts during zircon growth, but. higher
in the final magmatic stage evolution.
References
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ZIRCONS OF TURCOAIA ALKALlNE MAGMATITE
37
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