Basement Tectonics 13: Proceedings of the Thirteenth International Conference on Basement Tectonics held in Blacksburg, Virginia, U.S.A., June 1997
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VOLUME 7
The titles published in this series are listed at the end of this
volume.
BASEMENT TECTONICS 13
held in Blacksburg, Virginia, U .S.A., June 1997
Edited by
SPRINGER SCIENCE+BUSINESS MEDIA, B.V.
A C.I.P. Catalogue record for this book is available from the
Library of Congress.
ISBN 978-94-010-6015-8 ISBN 978-94-011-4800-9 (eBook) DOI
10.1007/978-94-011-4800-9
Cover: Schematic cross section of the Grenville basement in the
Blue Ridge of Virginia (from Sinha and Bartholomew, 1984 in The
Grenville Event in the Appalachians, Geological Society of America
Special Paper 194)
Printed on acid-free paper
Originally published by Kluwer Academic Publishers in 1999
Softcover reprint ofthe hardcover Ist edition 1999
No part of the material protected by this copyright notice may be
reproduced or utilized in any form or by any means, electronic or
mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright
owner.
CONTENTS
Geotectonics and Characteristic Features of Fertile and Non-Fertile
Younger Granites, Eastern Desert, Egypt F.S. Bakhit, H.A. Hussein,
M.M. Ali
...................................................................
1
Structural Associations of the Basement and Sedimentary Cover of
the Georgian Part of the Caucasus L.Basheleishvili
....................................................
........................................... 25
The Eastern Edge of the Rio de la Plata Craton: A History of
Tangential Collisions N. Campal, A.Schipilov
.......................................................
............................. 33
New Data and Interpretations for the Precambrian, Midcontinent USA
M.P. Car/son, S.B.Treves, R.J. Goble, A. Xu
...................................................... .49
Allochthonous Units in the Variscan Belt of NW Iberia: Terranes and
Accretionary History l.R.Martinez Catalan, R.Arenas, F.Diaz Garcia,
l.Abati .......................................... 65
Early Compression and Late Dextral Transpression Within the
Grenvillian Event of the Hudson Highlands, NY, USA A.E. Gates ....
...................................................................................................
85
Crust-Forming Processes: Basement and Basements in the Southern
Appalachian Orogen R.D. Hatcher
...................................................................................................
99
Petrogenesis of Pan-African Granitoids, Gabal Hamra Area,
Southwestern Sinai, Egypt H.A.Hussien, A.G.El Shazly, I.E. El Assy,
M.M. El Galy .................................... 119
Geology of the Blue River Gneiss, Eastern Arbuckle Mountains,
Oklahoma E.G. Lidiak, R.E. Denison
...............................................................................
139
vi
Zircon Ages of Basement Orthogneisses from the Northern Segment of
the Araguaia Belt, Brazil C.A. V.Moura, H.E.Gaudette
............................................................................
155
Damodar Graben - A Centre of Contrasting Magmatism in the Eastern
Indian Shield Margin D.Mukherjee, N.C.Ghose
................................................................................
179
A New Tectonic Belt in the Baltic Sea Region - Farther
Interpretation of the Deep Seismic Results from the "Baltic Sea"
and "Babel-B" Profiles A.A. Ostrovsky
.............................................................................................
203
A Geochemical Reconnaissance of the Roseland Anorthosite Complex,
Virginia, and Comparisons with Andesine Anorthosites from the
Grenville Province, Quebec B.E.Owens, R.F.Dymek
..................................................................................
217
Basement Tectonic Structures Delineated from Aeromagnetic Survey
Data South Eastern Desert, Egypt S.I. Rabie, A.F. Khalil..
.................................................................................
233
Appalachians in the Time Interval Between the Grenville Orogeny and
Variscan Collision N.Rast, J. W.Skehan
.......................................................................................
257
Geodynamics of the Early Precambrian: Evidence for the Baltic
Shield E. V.Sharkov
.................................................................................................
277
Lead Isotope Mapping of Crustal Reservoirs Within the Grenville
Superterrane: II. Adirondack Massif, New York A.K.Sinha,
J.M.McLelland
..............................................................................
297
Paleomagnetism of the Boot-Phantom Pluton and the Amalgamation of
the Juvenile Domains in the Paleoproterozoic Trans-Hudson Orogen,
Canada D. T.A.Symons, C.D.MacKay
..........................................................................
313
Dengying Formation Gas System of the Sichuan Basin, Southwest
China: Model for Precambrian Indigenous Hydrocarbon Accumulation
G.Zhang, A.B.Dickas, J.Song ......
....................................................................
333
Morpholog~ and Isotopic Age of Zircons from Shear-Zones Within
Granitoids of the Belomorian Tectonic Zone, Baltic Shield, Russia
T.F.Zinger, V.S.Baikova, B. V.Belyatsky, S. V. Klepinin, J.Gotze,
O.A.Levchenkov, I. K. Shuleshko ....
...........................................................................................
3 4 5
TRUSTEES International Basement Tectonics Association, Inc.
Chief Trustee Chairman of the Board of Trustees
M.Charles Gilbert School of Geology and Geophysics
University of Oklahoma Norman, OK 73019 USA
Trustee Treasurer
Urbana, IL 61801 USA
Virginia Polytechnic Institute and State University Blacksburg, VA
24061 USA
Trustee Marvin Carlson
Lincoln, NE 58588 USA
Past Chief Trustee MJ. Bartholomew
Earth Sciences and Resources Institute University of South Carolina
Columbia, SC 29208 USA
vii
FOREWORD
The Thirteenth International Conference on Basement Tectonics was
held on the campus of Virginia Polytechnic Institute and State
University in Blacksburg, Virginia from June 2 - 6, 1997. The oral
presentations and discussions over three days covered a wide range
of topics, and provided the international audience with a
perspective on scientific efforts underway around the world. The
conference participants were able to attend two separate field
trips: (I) a pre-conference trip guided by Professor Robert Hatcher
of the University of Tennessee, Knoxville, examined the Basement
rocks in the North Carolina - Tennessee region of the Appalachian
Mountains, and (2) a mid-conference field trip guided by A.K.
Sinha, convener of the conference, allowed participants to examine
the complex rock associations and structures of the> 1000 m.y.
old basement rocks in Virginia. Both the field trip guidebooks and
abstract volumes were published for the conference.
The meeting brought together scientists from more than 14
countries. Their participation, and the fiscal success of the
meeting would not have been possible without the support of the
Department of Geological Sciences, the College of Arts and Sciences
(VPI&SU) and the Basement Tectonics Association. Their support
is gratefully acknowledged.
As Chairman of the Organizing Committee, I would like to thank
Margie Sentelle, Jay Thomas, Peter Welch, and Barry Robinson for
the smooth operation of the conference. Jyl Smithson - Riehl,
Assistant Director of Program Development at VPI&SU provided
the support necessary to host an International meeting in a
university setting. Kim Bevan's ability to assemble and edit the
manuscripts that constitute this volume is gratefully acknowledged.
On behalf of the participants, I also thank the Trustees of the
International Basement Tectonics Association, Inc. for their
permission to hold the conference in Blacksburg. Finally, a thank
you to all the participants who have provided the manuscripts for
this volume.
A. Krishna Sinha, Convener Professor of Geology Virginia
Polytechnic Institute and State University Blacksburg, VA
24061
ix
GEOTECTONICS AND CHARACTERISTIC FEATURES OF FERTILE AND NON-FERTILE
YOUNGER GRANITES, EASTERN DESERT, EGYPT
F.S. Bakhit, H.A. Hussein and M.M. Ali Nuclear Materials Authority,
Cairo, Egypt. P.O. Box: 530 EI-Maadi - Cairo
Abstract: The present article is concerned with the geochemistry of
red-pink granites in the Late Orogenic Plutonites and their
relationship with uranium content in order to discriminate the
fertile from non-fertile types. Chemical analysis has been carried
out, including the major oxides, trace elements and REE, to
detennine their geotectonic affinity, and the relationship between
U and major oxides, trace elements and rare earth elements.
Geotectonically, the study revealed that the post orogenic red-pink
granites are either within-plate granites or volcanic arc granites.
Chemically, the granites are subalkaline and peraluminous to
metaluminous. Also, the younger granites occur in three phases. The
highly differentiated phase is the fertile type which is
characterized by high silica content, with respect to alumina,
impoverished in calcium, as well as in ferromagnesian elements
(Fe203+MgO) and is enriched in alkali elements (Na20+K20).
Moreover, the fertile type ;s characterized by high Rb and low Sr.
Also, direct relation between Si02 and Rb is obtained, while with
Ba and Sr do not correlated well. The REE illustrate that the
fertile granites possess high negative Eu value relative to the
normal type.
The relationship between U and La, Ce, Sm, Pb, Rb, Zr, F and Mo was
found to be direct relation, while Co, Sr, Eu, and Ba was found to
be a weak correlation ..
1. Introduction
The study area including six younger granitic plutons namely; Gabal
Hadrabiya, Gabal EI Dob, Gabal El Urf, Gabal El Erediya, Gabal EI
Missikat aild Gabal Kab Amiri. They lie within the limits
oflatitudes 26 18' and 26 43' N and longitudes 33 IS' and 33 40' E
in the Central Eastern Desert of Egypt. These six granite plutons
lie just at the north and south the midway of Qena-Safaga road
(Fig. I ).
Aerial radiometric and magnetic surveys were carried out for an
area covering about 4500 km" including the six mentioned granitic
plutons in the studied area (Fig.2). The aerial survey indicated
the presence of significant radioactive anomalies scattered in the
area (Ammar, 1973). The subsequent field "'ork for ground checking
and follo,\ing these airborne anomalies was carried out and some
highly radioactiyc occurrences with visible uranium mineralization
were discovered ( EI-Kassas, 1974 ; Bakhit, 1978).
A. K. Sinha (ed.), Basement Tectonics 13, 1-24. @ 1999 Kluwer
Academic Publishers.
L E
G E
N D
R O
C K
4 F. S. BAKHIT ET AL.
Detailed studies showed that the highly radioactive bodies display
great similarities in their type of mineralization, host rocks,
country rocks and structures. The general geology, structure and
radioactivity are discussed in previous works by El-Shazly et al.
(1981), Bakhit, (1978), Bakhit and Guirguis (1983), Bakhit and
Mostafa (1987), Bakhit et al.(1985), Bakhit and El-Kassas (1989),
Abu Dief(1985), El-Kassas and Bakhit (1989 and 1992), EI-Tahir
(1985), Hussein et al. (1986), Meleik and Bakhit (1990), Abdel
Monem et al. (1990) and Bakhit and El-Kassas (1992).
The area surrounding these granitic plutons is an active region
where different mineralized deposits of copper, gold, molybdenite,
wolframite, iron-ores, tin and tungsten occur ( El-Kassas and
Bakhit, 1983).
The aerial radiometric survey revealed the presence of different
radioactive levels among the scattered younger granitic plutons.
The different radiometric levels attracted the authors' attention
to study this phenomenon. Many authors attributed differences to
magmatic differentiation. The main goal of this study is to explore
the exact fertile phase among the other different phases of younger
granites.
2. Geology and Structure
The selected six younger granite masses were emplaced during the
post-tectonic episode in Egypt, 603-575 Ma (Greenberg, 1981). They
were emplaced near the northern edge of the central tectonic block
of the Eastern Desert where rocks with strong oceanic affinity are
exposed (Stem, 1981 and Stem et al., 1984). The central tectonic
block is characterized by the presence of linear belts of
ultramafics and serpentinites that are interpreted as suture zones
resulting from island-arc-continental collision (Shackleton et
al., 1980; EI-Bayoumi, 1984; Hassanain, 1984 and Ries et al.,
1985). The presence of domal structures encompassing large crustal
sections (Migif Hafafit and Meatiq gneiss dome) are characteristic
features indicative of convergent continental margin and
collisional tectonics (Bentor, 1985 and Kroner, 1985). The six
younger granite masses are intruded into basement rocks comprising
parts of metamorphosed ophiolite-island-arc sequences (Fig. 1).
Gabal El-Missikat and Gabal EI-Erediya are of the six granite
masses associated with uranium-mineralization.
Structurally, the position of the radioactive chalcedony and
jasperoid veins is determined by open spaces along rock contact.
The uplift of igneous intrusive masses at the contact with the
country rocks may have resulted in mineralization as has been
documented throughout the Eastern Desert (such as at Gabal
El-Missikat and Gabal EI-Erediya), which is one of the most
important structural control in the Eastern Desert (Bakhit, 1987).
Dominant trends of the uranium-bearing veins are in the northeast
to easterly direction such as in Gabal El-Misslkat (Bakhit, 1978)
and less pronounced in the N-S and/or NNE as in Gabal El-Erediya
(El-Kassas, 1974), and as fracture - filling in the NW and NE
direction in Gabal Kab Amiri (Bakhit, 1987). The different
mineralizations of fissure-filling or shear zones occur in three
main sets of fractures, mainly in the NW-SE. ENE-WSW and N-S
directions ( EI-Kassas and Bakhit,1983).
FERTILE AND NON-FERTILE YOUNGER GRANITES
None of these uranium deposits occur far away from the edge of the
granite bodies. Moreover, they invariably lie within 50-200 meters
of the contact (Bakhit, 1987). Also, veins containing uranium are
commonly deformed during the introduction of gangue and ore
deposits. These processes produced a series of small-scale
fractures or breccias that may have localized any late uranium
minerals.
3. Geochemistry
3.1. CHEMICAL CLASSIFICATION
Twenty representative samples were analyzed for major, trace and
rare earth elements. The results of the chemical analysis of the
examined granites are listed in Table (1), together with CIPW-norm
values. The data are compared with corresponding geochemical data
of the Egyptian younger granites reported by Ali and Mostafa
(1984), Greenberg (1981), EI-Gaby (1975), as well as the World wide
average for granitic rocks ofLe Maitre (1976), Table (2).
3.2. ANALYTICAL TECHNIQUE
Quantitative chemical analysis for major oxides were carried out
following the method of Shapiro and Branock (1962). Si~, Al2~'
Fe2~, Ti~, P20 S and MnO were determined spectrophotometrically.
Na20 and K20 were determined by flame photometry. CaO and MgO were
determined using EDT A titeration and FeO was determined
volumetrically. Uranium was determined using laser fluourometric
technique which allow the determination of uranium with high
accuracy. Fluorine and chlorine were determined using
chromatography technique.
The other trace elements were determined by inductively couple
plasma emission spectroscopy which allow simultaneous
multi-elements analysis using HF and H2S04
opening technique. The analytical system used was Philips PV 821d.
(1.5 m airpath spectrometer) linked with Philips 852 computer. The
standards G-2 and GSPI were used for comparison with the data
obtained
3.3. MAGMA TYPE
- The An-Ab-Or ternary diagram (Fig.3 a) shows that these rocks
fall within the field of granite (O'Connor, 1965 and Barker,
1979).
- On the alkali-silica diagram (Fig.3 b) ofIrvine and Baragar
(1971), the examined granitic rocks are subalkaline.
- The AFM diagram (Fig.3 c) shows that these rock samples are
alkali rich and magnesium-iron poor. On the same diagram and
according to the dividing line of Irvine and Baragar (1971) they
are of calc-alkaline affinity.
- On Al20 3 / (CaO+NazO+K20) versus Al20 3 / (Na20+K20) molar
diagram (Maniar and Piccoli, 1989) these granitic rocks are of
peraluminous to metaluminous nature (Fig.3 d).
5
Ali
Ie)
50 5581.111 75 lOIS SiG2Iwt·'.)
FaG
3~------~--------------~
~l~----~~~--__________ ~ •• ct
Figure 3.
a. AN-Ab-Orternary diagram (O'Conner, 1965 and Barker, 1979). b.
SiOz vs NazO+KzO diagram (Irvine and Baragar, 1971). c. AFM ternary
diagram (dashed curve after Irvine and Baragar, 1971). d.
AlzO/(KzO+Nazo+caO) vs AlzOf{NazO+KzO) diagram
(Maniar and Piccoli, 1989). e. Rb-Ba-Sr ternary diagram (HI
bouseily and EI Sukkary, 1975)
A. Highly differentiated granites. B. Normal granites. C &. F.
Anomalous granites. D. Granodiorites. E. Quartz-diorites. --+
Differentiated trend.
9
10 F. s. BAKHIT ET AL.
- Rb-Sr-Ba ternary diagram (EI-Bouseily and EI-Sukkary, 1975)
indicates that the studied granitic masses fall within the nonnal
granites and anomalous granites field "F" (Fig.3 e). Table (1)
shows the ratios of Na20/CaO, Na20IK20, MgOlFeO*, MgOIMnO,
AhOy'(Na20+K20r mol. Granitoid rocks are described (using Shand's
index) as peraluminous; AlCNK 1.0, Metaluminous; AINK 1.0 and AlCNK
1.0 and peralkaline AINK 1.0, with all ratios being molar ratios
and A=Alz03, C=CaO, N=Na20 and K=K20. The studied granitic masses
are of peraluminous and metaluminous nature as indicated by Shand's
index.
CIPW-norms (Table 1) illustrate that the fertile granites contain
very little amounts of fluorite dispersed in the granite which
ranges from 0.01 to 0.09 %, while the non fertile type does not
have this phenomenon. Also, the fertile type contains high zircon
content relative to the non-fertile type as shown in Table ( 1 ).
In addition, the fertile granite is characterized by large negative
Eu anomalies, and high REE as compared with the non-fertile type
(Table 1). On the other hand, some of the trace and rare earth
elements can be used as indicators for the fertile type, such as
La, Sm, Zr, F, Pb and Mo which fonn direct relation with uranium
content while with Co, Sr, Eu, and Ba have not well correlation.
Also, the ratio of LaNIYbs in the fertile granite is markedly
higher than that in the non-fertile granite as shown in Table ( 1
).
3.4. TECTONIC SETTING
The tectonic setting of the studied granitoids could be deduced
from the following diagrams: - Si02 versus K20 diagram (Fig.4 a)
ofManiar and Piccoli (1989) indicates that the
studied granitic rocks fall in the field of island arc granitoids
(lAG), continental arc granitoids (CAG), continental collision
granitoids (CCG), continental epeirogenic uplift granitoids (CEUG),
and post orogenic granitoids (POG).
- Si02 versus FeO*/( FeO*+MgO) diagram (Fig.4b) of Maniar and
Piccoli (1989) shows that the analyzed samples fall in the field of
post orogenic granitoids (POG) and three samples fall in
rift-related granites and continental epeirogenic uplift granitoids
(RRG & CEUG).
- Al20 3 I(CaO+Na20+K20) versus Al2031'(Na20+K20) molar diagram
(Fig.4c) of Maniar and Piccoli (1989) illustrates that the granitic
rocks fall within the field of post orogenic granitoids (POG),
continental arc granitoids (CAG) and continental collision
granitoids (CCG). '
Greenberg (1981) reported that the Egyptian granites are mildly
peralkaline and according to Maniar and Piccoli (1989), the studied
granitic masses are post orogenic granitoids. - Si~: versus Ah03
diagram (Fig.4 d) of Maniar and Piccoli (1989) indicates that
the
studied granitic samples fall in the field of post orogenic
granites (POG). - Y+Nb versus Rb diagram (Fig. Sa) of Pearce et al.
(1984) shows that the studied
granitic samples fall in the field of within plate granites
(WPG).
C Il
o ~ 21
i0 2
V B
F eO
*lF eO
l203/(K 20+
N a20+
C aO
d. S i0
12 F. S. BAKHIT ET AL.
- Y versus Nb diagram (Fig.5 b) of Pearce et aI. (1984) illustrates
that these granitic masses faIl within the field of within plate
granites (WPG) and two samples in the field of volcanic arc
granites (VAG) and syn-collision granites (syn-COLG).
- The plot diagram (Fig.Sc) Rl = [4Si-ll(Na+K)-2(Fe+Ti)] versus R2
= (6Ca+2Mg+Al) of Patchelor and Bowden (1985) shows that the
studied granitic masses fall within the field of post orogenic
granites.
Post orogenic granitoids are intruded during the end of an orogeny,
generally, after regional deformation has ceased. These granitoid
rocks are associated with the post orogeny in space and time
(Maniar and Piccoli, 1989). It has been suggested (Rogers and
Greenberg, 1981 a and b) that they represent the transitional phase
of the continental crust undergoing stabilization follo\\ing the
orogeny. - Rb versus Sr diagram (Fig.5 d) of Condie (1973)
indicates that all twenty samples of
the six granitic plutons fall within the field of thickness ranging
from 20-30 kIn. This conclusion is in concordance with many authors
(e.g. Takla et al., 1991 and El Nashar, 1991).
3.5. REE AND TRACE ELEMENTS GEOCHEMISTRY
Post-magmatic, deuteric and hydrothermal processes are shO\\on to
have great effect on the distribution of REE, because the fluids
generated at such stages are highly enriched in complexing agents
as C03 -, cr, and F -anions. Fluids rich in cr -anions are capable
of enriching LREE (La-Sm), whereas fluids rich in C03 - groups are
accompanied by extreme HREE enrichment (Tb-Lu) (Maclennan and
Taylor, 1979). Mineyev (1963), suggested that although the REE
group has similar ionic species, they form at high pressure
complexes in the form ofNa (REE) F4 , with the stability of the
HREE-complexes being greater than those for the LREE. Under such
conditions fluoride-rich fluids could deplete the rock in HREE.
Thus the application of REE geochemistry to hydrothermal systems
may be useful to elucidate its chemical history as well as the
mechanism responsible for metal transport.
It is generally recognized that a proportion of the REE do not
substitute in trace amounts in essential minerals in granites
(McCarthy and Robb, 1978). Jefferies (1985) demonstrated that
monazite is the principal host for LREE in biotite granite. HREE
are strongly partitioned into the accessory minerals such as
xenotime, apatite and zircon. The previously mentioned accessory
minerals are present in the six studied granitic plutons in very
small variable amounts. The behavior of the REE during granite
differentiation is controlled by the behavior of the radioactive
accessory minerals, (Jefferies, 1985). Pagel (1982) mentioned that
sphene plays a major role in controlling the REE geochemistry.
Also, thorite has little effect on the geochemistry of REE compared
with sphene or allanite.
The transfer of uranium from the more refractory accessory minerals
to uraninite during deuteric evolution is a critical step which not
only increases the uranium coment in the altered rocks, but also
makes the uranium more readily leachable by later hydrothermal
solutions (Cuney, 1978). The uraninite crystals are stable in
surface samples, as occur in EI-Erediya occurrence, probably due to
high thorium content.
10 00
10 0
10 00
til :J t= trJ ~ o a z ~ :J ~ ~ ~ I tn -w
14 F. S. BAKHIT ET AL.
The data of the REE elements are listed in Table ( 1 ). The
chondrite-normalized rare earth elements pattern for the fresh
samples of the six granitic masses, are shown in Figure (6).
The average total REE content for Gabal Hadrabiya, Gabal EI-Dob,
Gabal EI-Urf, Gabal EI-Erediya, Gabal El-Missikat and Gabal Kab
Amiri are 272.4,239.4,236,401, 430.1 and 318.9 ppm respectively as
shown in Table (1). The first three granitic masses are close to
the average of the nonnal granite (Hermann, 1970). The chondrite
nonnalized pattern (Fig. 6) indicates that the granitic rocks are
more enriched in LREE than HREE with average LawYbN ratios 5.94,
4.99, 5.43, 8.80,6.60, and 6.12; namely for Gabal Hadrabiya, Gabal
EI-Dob, Gabal EI-Urf, Gabal EI-Erediya, Gabal El-Missikat and Gabal
Kab Amiri respectively. Table (1) shows that the REE content is
generally less than 1 percent for all the studied granitic
masses.
Figure (6) shows that the fertile granitic masses of Gabal
EI-Erediya, Gabal EI Missikat and Gabal Kab Amiri have higher LREE
content than the non-fertile granite masses, namely; Gabal
Hadrabiya, Gabal EI-Dob and Gabal EI-Urf. In addition, the above
mentioned fertile granitic masses have large negative Eu anomalies,
as compared with the non-fertile granitic masses. This illustrates
that the fertile granites are highly differentiated. Eu is depleted
in this differentiation between level 8-1 ~ resulting in a decrease
in the negative Eu anomaly (which is conventionally expressed as
EulEu·) from 0.21 to 1.3.
3.5.1. Ba, Rb and Sr Relationships
Since the only one major element of comparable ionic size to barium
is potassium, barium is included as a trace element in K-feldspar
and biotite. The rules of trace element distribution in igneous
processes (Onuma et al., 1969) predict that Rb should replace K.
Rubidium forms no mineral of its own, being always incorporated in
potassic minerals; in granitic rocks it is in muscovite, biotite,
and K-feldspar. The size of strontium ion indicates that it can
proxy for either calcium or potassium, being partitioned most
strongly into calcic minerals. Mason and Moore (1982) indicated
that strontium in igneous rocks is present mostly in plagioclase
and K-feldspars.
The barium, rubidium, and strontium contents in granitic rocks were
examined on a variation diagram (Fig. 7 a). In figure (7 a), silica
content increases from 72.3 to 74.8; the sum of the other oxides
must then decrease from 27.7 to 25.2 % and if no differentiation
occurs except for the increase in silica, each oxide should
diminish in this ratio. In other words, Just from the method of
plotting, we should expect each oxide to show a decrease from left
to right 72.2% of its initial concentration; the reactions that
lead to differentiation will be significant only if they cause a
markedly smaller or larger change. Figure (7 a) shows a narrow
range of SiCh content ranging between 72.2 and 74.8 % without
obvious relation with Ba and Sr while the value of Si02 increases
by the increase of Rb.
Inspection of the diagrams show that Gabal EI-Erediya, Gabal
EI-Missikat and Gabal Kab Amiri possess high Rb and high silica
content as compared with the other
FERTILE AND NON-FERTILE YOUNGER GRANlTES 15
.... 1...,z.ftJ.,... . ..,......, ... .... .... (3) (b)
w~ ~ 1G
.: " c I J. .: : ~ ~ I .:. ~ ~ .3 ! " c ! J. .: ;s ~ & I .. ~ t
.3
-02 -03 -oc -t.s . ."~ . .,,. .... "'" (C) (d)
"" ~ . ~
, , J
, , : , , l " " c J J. .: 01 /! & I .:. ~ t .I ! " c oS . &
I .. ~ t .3
"'llQ2eQ).~ Jqt II() .... (c) ~
.. ~ ..
" " c J J. .: 01 /! & I .. ~ t J " " 0: I oS .: I /! & I ..
~ t J
",aD_OX( ......
"Figure 6" 'Df. (g) ~ REE patlems of granitic red. "1
a Gabal Hadrabiya ~ b. Gabal El Dob "" .."
C. Gabal El Urf 3 ~
d. Gabal Kab Amiri
~ e. Gabal El-Missikat f. Gabal El-&cdiya " ..., g. AVcr:lgcs
of each granite masses. : ,
J , ,1 I , I
16 F. S. BAKHIT ET AL.
a ••• b 19S + 12Cl .. • • •
i lOCI It
60 10 11)0 110 Me) 160 110 200 Rb(ppm)
SiOz(wt"/.)
71 72 13 7(, 75 76
14Cl Si02(Wrl.)
Rb(PP·)
70 • •• + ++ 100 100 300 400 500
Ba(ppm)
Figure 7.
a. Si02 vs Rh, Ba, and Sr variation diagrams. b. Sr vs Rh binary
diagram. c. Ba vs Rb binary diagram. d. Sr vs Ba binary
diagram.
FERTILE AND NON-FERTILE YOUNGER GRANITES 17
three granite plutons, in which Gabal Hadrabiya pluton has the
least Rb and silica content among all the plutons.
Rb-Sr binary diagram (Fig.7 b) shows a reverse relation as well as
Rb-Ba (Fig.7 c). The arrows on both figures represent the predicted
trend of differentiation.
Lehmann (1982) has regarded the Rb/Sr ratio as a measure of the
degree of magmatic differentiation, being higher with increasing
differentiation.
Ba-Sr binary diagram (Fig.7 d) does not show a significant trends
however the spaced clusters may represent different phases of
granite.
3.5.2. Relation between Uranium and some Trace and Rare Earth
Elements U versus Rb, Pb, Zr, Ce, Mo, F, Sm and La form direct
correlations with fertile
granite and indirect relation not well correlated \lith non-fertile
granite, (Figs. 8 and 9). U versus Ba, Sr, Co, and Eu form indirect
relation with the fertile type (Fig. 9). The above relations
suggest that in all cases of direct relations and/or indirect
relations that the fertile granites and non-fertile granites are
completely (or markedly) separated into two groups, as shown in
Figures (8 and 9). Also, each type, either fertile or non fertile,
mostly clustered into two groups. The younger granites are mostly
differentiated at least into three phases or more.
3.6. GRANITE DIFFERENTIATION
The late orogenic granites emit high gamma radioacti\ity (maximum
200 cps) in the Eastern Desert relative to the surrounding country
rocks of basement complex. Field radiometric work was carried out
by many authors in the Central Eastern Desert (El Kassas, 1969 and
1974, Boutros, 1973, EI-Ghawaby, 1973, Assaf, 1973, Meleik et al.,
1981, Bakhit, 1984 and Bakhit et al., 1984) and reYealed great
variations in the background radiometric levels (ranges between II
and 30 RIb) in the red-pink granite. Thus reflecting mostly the age
offormation (Bakhit et al., 1984). Accordingly, the writer
attributed these variations due to magmatic differentiation and/or
to their intrusion at different lapse of time of the same magma.
Many red-pink granite masses in the Central Eastern Desert possess
high background radiometric levels such as Gabal El-Missikat and
Gabal EI-Erediya, in which their arithmetic means (x) are 82.2 cps
and 85 cps respectively (Bakhit and Guirguis, 1983). Meanwhile,
some similar red pink granite masses scattered elsewhere in the
Central Eastern Desert recorded relatively low background, such as
Gabal El-Urf, Gabal EI-Dob and Gabal Hadrabiya (Fig. 1), with
arithmetic means 45.9 and 53.2 cps respectively (Bakhit et al.,
1984). The electronically adjusted scintillometer used in this
study is the Canadian type Gr- 101 (Scintrex). The younger granite
generally displays high magmatic characters. Magmatic
differentiation seems to control the further evolution of the
younger granites and also lead to the development of certain
alkaline characteristics probably during their late phase of
formation.
El-Ramly and Akaad (1960) separated the granites in the Central
Eastern Desert of Egypt into major groups namely; older grey
granite and younger granites of red-pink colours. These are
equivalent or belong to EI-Shazly's (1964) S)n-orogenic
plutonites
18 F. S. BAKHIT ET AL.
36 lea 34 Itl xx
x
E + E 24 ++
III • Q. + Q. Q. II Q. • E • " 60 • " • ... • oft • • t ~ ••
10
40 ~-- I··· · 6 .. ' 2
x Ie d' + • x
24 + III: ••• .. ,& 100 • ... .. 20 • eo -. • • 10
•• 60 111 10 0 10
U(pp ... ' UCpPIII)
• i 15 ++ ~ • 't. •• 10 • • .-.: . 5 ~,..l
0 10 20 0
Figure 8.
U vs La, Sm, Pb, 'h:, and Mo variation diagrams a. Gabal Hadrabiya
(e) b. Gaba! El Dob (A) c. Gabal El Urf (_) d. Gabal Kab Amiri
(.)
e. Gabal El-Missikat (+) e. Gabal El-Erediya (x)
FERTILE AND NON-FERTILE YOUNGER GRANITES 19
220 360 + D x b
+ 200 300 • Ie
";'160 • /:L - Q.
U .. • u: 11.0
a •• --- .. ~ 80 • dl250 va
UCppm' U(ppm' 8 22
e f • .. E 6 • _ 1-8 •• • /:L e • • /:L I Q. •
I. Q. ••
2 .- + + 1 + • • 0 10
UlppmJ 20 0 10
UlppmJ 20
Figure 9. U vs Ce, F, Ba, Sr, Co and Eu
20 F. S. BAKHIT ET AL.
and late-orogenic plutonites respectively. A much younger
post-orogenic granite group is distinguished by EI-Shazly (1964)
for the porphyritic granite of Aswan. Since then
much work was done on the Egyptian granite aiming to determine the
different parameters for the larger granite bodies in many
localities of Egypt. EI-Gaby (1975) introduced a trial to survey
the petrochemistry and geochemist!), of some granites from Egypt,
and he concluded that the studied granites represent one continuous
granite series. The early members are of tonalite composition,
whereas the content of potash feldspars increases progressively in
the late members. The last phases acquired alkaline or peralkaline
affinities. Generally, the early members of synorogenic granitoids
and the late members of the younger granites are quite distinct in
the field. El-Gaby (1975) considered the post orogenic granites as
the younger red- pink granites.
Akaad and Noweir (1980) recorded three different phases of the
younger granites, each phase possessing its own field, petrographic
and petrochemical characteristics. Phase I is of calc-alkaline,
hornblende-biotite granodiorite composition, rich in xenoliths
which are variably digested. Phase II is of alkaline character and
adarnellitic composition and contains fewer xenoliths which are
undigested. Phase III is of leucogranite with thick chilled margins
of rnicrogranite which is devoid of xenoliths and dykes. Greenberg
(1981) mentioned that the younger granites apparently were formed
at a time when north Eastern Africa was being converted from an
area of compressive tectonics and crustal instability to an area of
stable continental craton. The younger granite magmas appear to
have been derived by very limited partial fusion rather than as end
products of differentiation of calc-alkaline magma series. He also
stated that the younger granites are generally siliceous (65-77 %,
Si02) and rich in alkalies (7.8-9.3 %, Na20+K20). He classified the
Egyptian granites into three groups. Plutons near the Si02-rich end
point are classified as "Group I " (mainly hypersolvus) and include
EI-Erediya and EI-Sibai. Plutons near the mafic end point (65-70 %)
are classified here as " Group III" (mainly subsolvus) and include
Fawakhir, ... , .... They contain moderate amounts of biotite and
hornblende. Intermediate plutons are classified as "Group II "
(mainly transsolvus) and include Kadabora, .. , .... Group I is
separated from Group II on the basis of Ti02 and MgO values, with
Group I less than 0.1 Ti02 and less than 0.08 % MgO and Group II
values above those values. In general, Group I and Group II are
rich in K-Feldspar and have K20 in excess of Na20, while Group III
are more sodic. Typical Group III averages are 3.64 % K20 and 4.57
% Na20 while those of Group I are 4.71 % K20 and 4.04 % Na20.
Accordingly, the six studied granite plutons are equivalent to
Group II of Greenberg (1981) as shown from table (1).
Hussein et al. (1982) proposed a new classification of the Egyptian
granitoids, where the older granitoids "syn-orogenic" plutonites
are designated G I granites and denote subduction related granites
formed by partial fusion of the mantle wedge with little or without
crustal melt contributions. Moreover, the younger granite, red-pink
granites "late and post" orogenic plutonites are further classified
into two subgroups namely, G II and G III granites. G II granites
denote granitic type formed by partial melting of the lower
crust.
FERTILE AND NON-FERTILE YOUNGER GRANITES
G III granites on the other hand, denote granitic type fonned by
melting of pre existing crustal rocks (i.e. intraplate anorogenic
rocks).
21
Pagel (1982) found a decrease ofThIU ratio corresponding to slight
decrease of Th content whereas: there is an increase of uranium
content at the end of differentiation. This situation is more
comparable with the granite masses of Gabal El-Missikat, Gabal
EI-Erediya and Gabal Kab Amiri plutons in which Th content is
nearly absent as compared with the other studied granite plutons
(Table 1). Accordingly, the high radioactive granite among these
red-pink granites represent the end of differentiation.
Bakhit (1988) suggested the presence of at least three red-pink
granites phases or more, based on the radiometric levels. He
attributed the difference in the value of radiometric levels, as a
result of granite differentiation. Also, he mentioned that all the
three phases or types of younger granite without clear contact.
Besides, he compared his study with Akaad and Noweir (1980) on the
younger granites and concluded that Gabal El-Missikat, Gabal
EI-Erediya and Gabal Kab Amiri granite plutons (Fig.2 ) could
belong to phase II. As well as, he attributed that the three phases
of the younger granites represent the differentiation process of
Group II which mentioned by Hussein et al.(l982).
Takla et al. (1991) classified the younger granites
petrographically, beginning with the oldest, as biotite-hornblende
granites, perthitic leucogranites and graphic granites, while the
porphyritic granites occur occasionally on the outer zone of
perthitic leucogranites
Guirguis and Bakhit (in press) mentioned that the simultaneous rise
in Rb content together with drop in Sr content for these granites
is indicative of differentiation.
4. Discussion and Conclusions
This study reveals that the fertile younger granites are post
orogenic granites and are one of the different phases of the
younger granites, characterized by high differential index, which
intum represents the latest stage of the successive younger granite
intrusions.
By correlation of the chemical analyses of the well kno\\n fertile
granites from different points of view, which include Gabal
El-Missikat, Gabal EI-Erediya and Gabal Kab Amiri, with the other
three non-fertile granite masses, namely; Gabal Hadrabiya, Gabal
EI-Dob and Gabal EI-Urf. It is observed that the chemical analyses
of the fertile granites indicate a saturation with respect to
silica and alumina. Moreover, the fertile granites tend to be
impoverished in Ca, as well as, in the ferromagnesian elements
(Fe203+MgO) and are enriched in alkali elements (Na20+K20).
Besides, it is clear from the analyzed samples including trace
elements that they are characterized by high Rb content with drop
in Sr content.
The fertile granites contain very little amounts of fluorites,
while the non-fertile type does not have this phenomenon. Also, the
fertile type contains high zircon content relative to the
non-fertile type. In addition, the fertile granite is characterized
by large negative Eu anomalies, high REE and high ratio of LaN/ ~
as compared to the non-fertile type. On the other hand, some of the
trace elements and REE elements can
22 F. S. BAKHIT ET AL.
be used as indicators for the fertile type, such as La, Ce, Sm, Pb,
Rb, Zr, F and Mo which form direct relation with uranium content
and reverse relation with Co, Sr, Eu and Ba.
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STRUCTURAL ASSOCIATIONS OF THE BASEMENT AND SEDIMENTARY COVER OF
THE GEORGIAN PART OF mE CAUCASUS
L.B.BASHELEISHVILI Geological institute of Georgian Academy of
Science MAlexidze Sir.] Build. 9 Tbilisi, 380093 Georgia
Abstract: Investigations in the Caucasian region have clearly shown
the two-staged structure of the upper part of the earth's crust:
the pre-Mesozoic crystalline basement, and the overlying thick
sedimentary cover composed of terrigenous, carbonate, volcanogenic,
and molasse formations of Mesozoic-Cenozoic age. Structurally, the
basement is characterized by mosaic block structure. Kinematics of
basement blocks and their relative vertical and horizontal movement
along faults are expressed in the sedimentary cover in the from of
zones of intensive structure formation. Structural associations
between tectonic forms in the basement and overlying sedimentary
cover are clearly visible. The study of morphology and
characteristic patterns of spatial interposition of tectonic forms
helps to determine kinematics of transverse faults and
consequently, the displacement of individual blocks of the
basement.
L Introduction
The upper part of the Earth's crust of the Caucasus provides good
evidence of a two stage division-the older unit represented by
pre-Alpine crystalline basement and overlying, thick
Mesozoic-Cenozoic sedimentary cover. The structure and composition
of the basement is known from its not numerous outcrops which
mainly are linked with the Transcaucasian transverse uplift
(Central Caucasian, Dzirula, Loki, Khrami salients).
The basement consists of the pre-Cambrian schists, gneisses.
amphibolites, migmatites, Lower and Middle Paleozoic metamorphites,
Hercynian age gneissic granitoids, Paleozoic basic rocks, and upper
Paleozoic volcano-sedimentary rocks of quartz porphyric
composition (Gamkrelidze 1984).
The sedimentary cover is represented by thick complex of
terrigenous-carbonate, volcanogenic, volcano-sedimentary, molassic
formations of Mesozoic-Cenozoic age.
A. K. Sinha (ed.), Basement Tectonics 13, 25-32. © 1999 Kluwer
Academic Publishers.
25
26 L. BASHELEISHVILI
Figure 1. Location of the main outcorps of pre-Jurassic crystalline
basement in Caucasian region. I-Zone of Transcaucasian Tramversai
uplift. 2-pre Jurassic ofbasement G.G-Greater Caucasus.
D-Dzirula. K- Khrami . L-Loki
2. Structural-cinematic features of basement and sedimentary
cover
Structurally, the basement is characterized on the whole by
mosaic-block structure due to the development of the fault systems,
predominantly meridional, latitudinal and near latitudinal
direction located at different depths of the lithosphere (fig.
2).
Figure 2. Scheme of the main structural units and faults of the
basement I -Alpine mollase depression. 2- pre-Jurassic Outcrops
ofbasement. 3- nappes.
4- faults in the basement observed by geologic-geophysical
data.
Numerous data have appeared lately (Ioseliani 1982, Basheleishvili
1987, 1989) which permit the creation of a physical model of the
present mOIphostructure of the surface of the pre-Jurassic
basement, especially within the molasse depressions of the
Transcaucasian median massif.
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS 27
According to seismic data the Ctystalline basement of the Kolkheti
depression subsides from east to west up to 8-9 kIn depth (in
coastal zone of the Black Sea). However, on the gradual westward
subsidence several local uplifts have been discovered through both
geophysical surveys and deep drilling. Missing stratigraphic
sections and substantial decrease in their thickness are associated
with these uplifts.
The seismogeologica1 profile Dzirula-Gldani gives the idea of the
structure of the sedimentaIy cover and basement within the Kartli
basin. The basement here has block type composition where three
blocks have been distinguished.
The largest dip angles (about 20 0) of basement surface are known
on the eastern slope of the Dzirula salient. An abrupt subsidence
is observed at the distance of 16-18 kIn. Then there is nearly
horizontal occurrence of basement lying at the depth of3.2-3.4
kIn.
Further east the basement is subject to rigid subsidence to the
depth of 6.5-7 kIn accompanying by a meridional normal fault to the
east of which (east of Thilisi) the basement again rises to the
depth of about 4 kIn.
The character of pre-Jurassic basement within the Kartli basin
shows that its surface forms a wide open to the east trough. From
the north it is delimited by large zone of long faults of Caucasian
(NW -SE) trend. Here, two different zones may be established in the
basement. The western one is characterized by complicated relief
with local structural forms elongated predominantly in meridional
direction. To the east, the relief of the basement is more gentle
and here gentle sloping sublatitudinal structures are developed.
Three an echelon deepening have been established in this part of
the basin indicating the step-like structure of the basement
subsiding from W to E.
According to gravitational modeling (Yusupkhodjaev et al, 1986) the
lithosphere with in the Crimea-Caucasian region along the different
fragments of the geotraverse Varna Tashkent is subdivided into 8
layers decreasing within the limits of the Black and Caspian seas
to 4-5 layers. As a result of this modeling it has been established
that the Moho boundary, along this profile (fig. 3), is
characterized by local rises under Central Black Sea and South
Caspian areas and relative subsidence beneath the Dzirula salient
(zone of Transcaucasian transversal uplift).
0
40
CASPIAN SEA
28 L. BASHELEISHVILI
The comparison of this profile with factual material concerning the
step-like subsidence of the crystalline basement to the west and
east from the Trans-caucasian transversal uplift alone, the listric
faults shows two plans of structural symmetry, of defonnations: I)
meridional-caused by compressional tectonics due to northward
propagation of the Afro Arabian projection; 2)
latitudinal-characterized mainly by tensional structures in the
form oflistric faults dipping toward maximum subsidence.
(Basheleishvili 1993).
Analysis of maps of total thicknesses of Meso-Cenozoic sediments
(Moshashvili, 1990) strongly promotes in the reconstruction of the
picture of inversional nature of blocks of basement and,
correspondingly, activity of faults delineating the latters.
Most of the Kolkhida depression is covered by the Quaternary
sediments and only in its flanks Cretaceous and Paleogene rocks in
carbonate and molasse facies are exposed. In the axial part of the
depression under the Quaternaty sands and conglomerates (about 300m
thick) drill-holes discovered Neogene sand-clayey sediments
unconfonnablv overlying Mesozoic rocks. Passing in the Cretaceous
rocks more than 2000m and about 450m in the Upper Jurassic
variegated shales the Chaladidi drill-hole was stopped at the depth
of 4450m. The Samtredia drill-hole shows somewhat reduced thickness
of the Mesozoic: at the depth of lOOOm it entered the
clayey-volcano-terrigenous sediments of the Cretaceous, at the
depth of 1700m - tuffogene Upper Jurassic, and at the depth of 2835
- Middle Jurassic porphiritic series continuing to the bottom of
the hole (3045m).
Thus, towards the Black Sea basin abrupt increases in thicknesses
of both the Neogene Quaternaty and Mesozoic sediments takes place.
It is characterized by the absence of the Paleogene from the
section. Towards the flanks of the depression in Abkhaz-Megrel (in
the north) and Guria (in the south) subzones the section of the
Tertiary, complex becomes more complete and thick (up to 4-5 km).
The intensity of rock defonnation is increasing in the same
direction.
In the Kartli basin from west to east noticeable increase of
thickness of the Cenozoic rocks at the expense of the Mesozoic is
observed. In the western periphery of the basin the thickness of
the Cenozoic does not exceed 500m (with complete absence of
Pliocene Quaternaty) and the total thickness of the Cretaceous
(drilled to the Upper Barremian) is more than 3 km. To the east in
the Shindisi reference hole the Cenozoic molasse is about 2.5 km
thick whereas the whole Cretaceous reduces up to 400m.
In the vicinity of Tbilisi (eastern part of Kartli basin) the
thickness of only Pliocene Eocene sediments exceeds 4.7 km. There
are no data of the thickness of Mesozoic as only, upper 400m of
Cretaceous are drilled by holes.
The Alazani depression is located on the raised block of
granite-metamorphic basement which occurs here at the depth no more
than 2.5-3 km and is delineated by faults.
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS
The Kakheti depression is supposed to be made up of Mesozoic (5-6
Ian) and Tertiary (about 9 Ian) sequences of sedimentary
rocks.
29
Kinematics of blocks of the basement and their relative vertical
and horizontal displacement along faults are expressed in the
sedimentary cover as zones of intensive structure formation
strongly differing from the general style of deformations in the
region. Structural associations (disjunctive and folds) in basement
and sedimentary cover are mostly conditioned by realization of
dynamic-kinematic parameters along basic faults.
The Odishi block corresponding to the Megrelian syncline is limited
from all sides by basement faults having strike-slip character.
Some of them are dextral, such as Tsaishi fault (figA) whose nature
is conditioned by above-fault en echelon Vrta and Satandjo
anticlines, whereas sinistral character of the Poti-Abedati fault
are conditioned by above fault en echelon Eki, Nokalakevi and
Abedati anticlines.
Figure 4. The tectonic scheme of the Odishi block I-Quaternary. 2-
post-Eocene. 3- Cretaceous. 4- Bajossian volcanics.
5- reverse faults. 6- faults in the basement.
The establishment of sense of motion along faults with the help of
above-fault and folds has its experimentally supported basis. In
this case, strike-slip displacement in the basement induces two
Gmdients of deformation in overlying sedimentary stmta horizontal
and vertical. It is the existence of these two gradients that
determines the development of folds in the fault zone.
As it has been indicated by Camond 1.F. et al, (1984) geometry and
structure of folds formed by this manner are chamcterized by
following specific features: 1) en echelon pattern- 2) periclinal
closure ofbeds-, 3) certain position (at the angle of 30-40·) to
the basic fault
30 L. BASHELEISHVILI
In the case of disjunctive structures the situation is somewhat
different as may be seen at the example of the fault bounding the
Gurian depression in soutb-east. Detailed study of the near-fault
zone showed that the Chokhatauri thrust that earlier was considered
as a surface expression of the basement Fault (fig.5) in reality
consists of four en echelon faults of reverse fault-shear nature
replacing one another and steeply dipping (60-80 0 )
to south-east. Along these faults Middle Eocene volcanics of the
Adjara-Trialetian zone overthrust the Oligocene-Miocene clays and
marls.
The band ofleft-Iateral stepped-like en echelon reverse
fault-shears is controlled by right-lateral strike-slip
displacement in the basement.
v v v v v
O ~ fVVl 1 ~2 L.:!U3
04~5~6
Figure 5. The tectonic scheme of the Gurian depression and its
surrounding's. 1. Quaternary, 2- post-Paleogene. 3- Eocene
volcanics of the AIljara - Trialetian zone. 4- reverse faults and
reverse fauh-shears. 5- front of nappes. 6- faults in the
basement.
A typical example of interaction of structures in the basement and
sedimentary cover represents the large flexure extending along the
junction of the southern slope of the Greater Caucasus and the
Georgian mass (fig. 6). Here the rotation of blocks causes
formation of shear zones in narrow graben-like depression in the
sedimentary cover. Formation of the Racha graben-syncline is a
direct result of faulting in the basement. As for stepped-like
shape of the southern limb of this graben, it is caused by
syntectonic faults in the northward rising basement. Besides, the
southern limb is subject to secondary folding. One of the forms of
structural associations of the basement and sedimentary cover are
cloaking structures, or structures in the sedimentary cover
reflecting the morphologically expressed projections in basement.
Such conditions for the formation of tectonic structures existed in
the marginal zone of the Dziryla salient of the crystalline
basement (fig. 7).
SEDIMENTARY COVER OF THE GEORGIAN PART OF THE CAUCASUS
---~-~-~---~--------- ---------,
Figure 6. The tectonic scheme of the zone of junction of the fold
system of the southern slope of the Greater Caucasus and the
Georgian Block.
1- Miocene sediments. 2- flexures. 3- faults. 4- axes anticlines.
5- axes of secondary anticlines. 6-axes of secondary
synclines.
31
This salient developed in the Late Miocene-Quaternary time, and
during the general northward propagation of the Adjara-Trialetian
zone served as a rigid stamp bending which caused a different
character of displacement on the Northern Adjara-Trialetian fault.
It is also reflected in the character of smaller structures.
Systems of transversal dextral shears, zones of en echelon
near-fault folds, folds with vertical crest (horizontal folds) have
been formed here.
+ o 10 km + + '---L-.J + +
Figure 7. The scheme of the Dzirula salient and its surroundings.
l-postpPaleogene. 2-Eocene volcanics of the Adjara-Trialetian zone.
3-Mesozoic complex. 4-Paleozoic granites.
5-axes of main anticlines. 6-nappes and thrusts 7-reverse faults.
8-shears.
The Kakheti molasse depression is characterized by development of
south-vergent nappe-thrust structures that have subhorizontal slip
planes. Here, main displacement is connected with Oligocene-Lower
Miocene clayey sediments. Locally, on the background of horizontal
slips stepped-like deformations of this plane can be
observed.
32 L. BASHELEISHVILI
The latter are the direct consequence of those projections in
basement above which in the process of plastic mass material
injection narrow dyapiric anticlinal folds have fonned.
3. Conclusion
Studies carried out in different regions of the Caucasus showed the
presence of various defonnational tectonic forms in the upper crust
that is genetically connected with displacement of rocks of the
rigid substratum. These dislocations are caused by a great number
of factors.
Thus, through all the observations of combination of structures in
crystalline basement and overlying sedimentary cover, regarding
their genesis, morphology and dynamo kinematic parameters, give us
a possibility to suggest a structural associations or paragenesis
of the two regions of the Earth's crust. As a consequence, two
different positions may take place: 1 )activation and re-activation
of basement structures (predominantly, faults and blocks of
basement) inducing deformations and structure fonnation in the
sedimentary cover, 2) passive behavior of basement structures under
active deformations in the sedimentary cover.
4. References
Bsheleishvili, L.B. (1987), Kinematics of the alpine defonnation of
the Transcaucasian median massif II, national youth school, Sofia,
215-222.
Bsheleishvili, L.B. (1989), Tectonics of the zone of jWlction of
Adjara-Trialetian fold zone and the Georgian Block. Geotecionics,
NI, Moscow, 77-86.
Bsheleishvili, L.B. (1993), Extension structures of COOSQlidated
crust of the Transcaucasion intermontane depression (latitude
profile). Annales Geophysical. European Geophysical Society,
c56.
Gamkrelidze, I.P.(1984), The tectonic structure and Alpine
geodynamics of the Caucasus and acljacent areas. Otkmezuri., Z. W.
(edit), Tectonics and Metalogeny of the Caucasus. Tbilisi,
Metsniereba publish., 105-127.
Gamond, I.F., Odonne, F. (1984), Some identification criteria of
deep wrench fault induced folds: analog, model and field data.
Bulletin Sos. Geol. France, NI. tome XXII, 115-127.
IoseJiani, M.S. (1982), Deep geological structure of the
Intermontane depression of Georgia and adjacent areas on geophysic
data. Doclotare thesis Tbilisi. 1-29.
Yusupkbodjaev, Kh.l., Mindeli, P.Sh., Kartvelishvili, KM., (1986),
Lithosphere model of the Crimea-Caucasian Mediterranean region,
Dynamics and evolution oflithoshphere. Moscow, Nauka publish.,
129-139
Moshashvili, AB. (1990), Comparative analysis of geotectonical
evolution of the South Caspian and Black sea basins.
Doctorate1heses. Baku. 1-505.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON: A HISTORY OF
TANGENTIAL COLLISIONS
1. Abstract
N. CAMPAL and A. SCHIPILOV Universidad de la Republica Facultad de
Agronomfa - Cdtedra de Geologfa Av. Garzon 780 - 11900, Montevideo,
Uruguay.
The aim of this paper is to present the arguments in support of the
idea that the Rio de la Plata craton as a whole was not involved in
a Panafrican orogeny.
On the other hand, based on field relations, petrography, and only
a few available geochronological data, we began to find evidences
for the identification of a Middle Proterozoic (Grenvillian) event
involving sedimentation, metamorphism, and large-scale thrusting.
Intense dextral shear with the development of thick mylonitic bands
(up to 20 km wide), associated with this episode also suggest
oblique collision occurred here.
As we understand the evolution of the Uruguayan crystalline
basement, the assembly of the Rio de la Plata craton, and the
Panafrican (Brasiliano) terrane was completed during Cambrian and
after the deposition of the Arroyo del Soldado Group. The main
formations of this unit were considered up to now to belong to the
Panafrican molasse.
2. Introduction
The name Rio de la Plata craton was initially introduced by ALMEIDA
(1971) and more recently redefined by some with a different intent.
The word "Craton" has a strict tectonic-position sense in relation
to an orogenic belt. It thus was applied to the Rio de la Plata
crystalline basement on the assumption that these rocks were the
parts of the "Kratos" in relation to the Panafrican orogeny.
Afterward and without jUdging this assumption, different models
implying collision with eastward (RAMOS, 1988) or westward (FRAGOSO
CESAR et al., 1987) subduction or inclusively double subduction
(TOMMASI & FERNANDES, 1990) were applied to explain the geology
of Uruguay and southern Brazil.
All geological data involved in this work were obtained in Uruguay,
and although we propose some possible correlations, no discussion
about South American Geology is attempted.
To describe the observed geological features, we use the
nomenclature proposed by BOSSI & CAMP AL (1992), with the
subdivision of the Uruguayan crystalline basement
A. K. Sinha (ed.), Basement Tectonics 13, 33-48. © 1999 Kluwer
Academic Publishers.
33
34 N. CAMPAL AND A. SCHIPILOV
into three main units: Piedra Alta Terrane (TPA), Nico Perez
Terrane, (NPT) and Cuchilla Dionisio Belt (CDB) as shown in Fig.
1.
w [][] SYPL
-1780 My rapakivi batholith
ieo Perez terrane (Grenvillian ?)
[!]j (2)
Granitic intrusives (Cambrian)
Sicrra Ballcna lineament
Figure 1. Uruguay: geological sketch and main subdivision of the
crystalline basement
The word "terrane" is here used as an informal subdivision,
following the suggestions of CONEY et al. (1980). It will be used
to describe some geological entities that have stratigraphic
associations with different geological histories such as other
terranes or neighboring stable areas.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON 35
3. The Sarandi del Yi - Piriapolis Lineament (SYPL)
During 1987-1990 a detailed survey of an extensive mafic dike
produced an accurate geological (l :50,000) map with each dike
located on aerial 1 :20,000 photographs (BOSSI & CAMPAL, 1991)
(Fig. 2). More than 100 samples were petrographically analyzed, and
58 were geochemicaly (BOSSI et al., 1993a). The ages of these rocks
were determined by different methods and the results are shown in
table I.
TABLE I: KI Ar data of the "Rio de la Plata" basic dike swarm
Age Method Author 1604 ± 40 My KlAr whole rock GOMEZ RIFAS (1988)
1373 ±33 My KlAr whole rock GOMEZRIFAS (1988) 1393 ± 44 My KlAr
whole rock GOMEZ RIFAS (1988) 1786 ± 26 My KlAr contact's biotite
BOSSI et al. (1993a) 1828 ± 21 My KlAr contact's biotite BOSSI et
al. (1993a)
,r:: ~, :: ';~" j;'~.;~ '; .' / ., , ' 0 Piedra Alta terrane
(PAT)
',' '0 bn ~ Mafic dike swarm .... """""O:=~i!o!!""",;"=~ 1m
Phanerozoic sediments
Figure 2, Rio de la Plata mafic dike swarm and details of its
eastern edge, as can be seen in aerial photographs
As a consequence, the Sarandf del Yf - Piriapolis lineament was
identified and subsequent studies were conducted to find some
equivalent dikes in the eastern side of the lineament.
Nevertheless, in the detailed survey of this area, no dikes of this
age were found, but a rapakivi complex partially deformed and
sharply delimited by the SYPL was discovered (CAMPAL &
SCHIPILOV, 1995). Its age is broadly the same of the dike swarm,
but important differences in the geological context at both sides
of the lineament can be established and are summarized in table
II.
36 N. CAMPAL AND A. SCHIPILOV
TABlE II. Principal differences between PAT and NPT Piedra Alta
Terrane (PAT)
Rio de la·Plata dike swarm No equivalent rocks No magmatic activity
post basic dike swarm
Nico Perez Terrane (NPT)
No equivalent rocks llIescas rapakivi complex Intense Middle
Proterozoic and Cambrian magmatism
Post-dike swarm deformation is rare and brittle Widespread
mylonites and thrusting except at its eastern border post- rapakivi
granite emplacement Metamorphism of Transamazonian Pre-rapakivi
rocks of granulite facies and Pre-Transamazonian rocks from
including meta-Banded Iron Formations (BIF) greenschist up to
amphibolite facies with string perthites No Vendian sedimentary
cover identified Vendian sedimentary rocks widespread No Middle
Proterozoic-age rocks Common Middle Proterozoic metamorphic
rocks
This lineament seems to have evolved in at least two main stages.
The first one is a dextral shear and the sense can be well
identified considering the macro structures in the PAT as well as
in internal micro- and meso/structures. This deformation style can
be easily detected in a 20-kIn wide band in the PAT near to SYPL,
but is partially obliterated at NPT.
Geological evidence is that this stage is older than the Vendian
cover because basal conglomerates and sandstones of the lower
formation of "Arroyo del Soldado Group" (Vendian sedimentary
sequence) are in sedimentary contact with high temperature granitic
mylonites, which are the source of detrital elements in this
unit.
Discrete mylonitic and ultra-mylonitic bands 10,000m wide identify
a second stage with sinistral shear sense. These shear bands are
generalized on NPT. They characterize internal deformation of the
Brasilian Mobile Belt, and affect the Vendian meta/sedimentary
cover and also the intruded granites.
4. Piedra Alta Terrane (PAT)
The PAT consists of a central granitic-gneissic region and three
metamorphic belts that trend nearly EW. Except for some Rb/Sr ages
that are at the Archean-Proterozoic boundary, the tectono/thermal
event with higher age recorded in this Uruguayan basement portion
is the Transamazonian (ca. 2000 My, CINGOLANl et ai., 1997).
This basement would be correlated with the Tandil hills basement of
CINGOLANI et ai., (1997). In PAT, aside the small outcrop area of
the Piedras de Afilar Formation of uncertain age, neither
sedimentary deposition nor other metamorphic units were produced
after the "Rio de la Plata" mafic dike swarm intrusion, until the
deposition of Devonian sediments which uncomformably overlie
granitic rocks and are tectonically undisturbed.
5. Nico Perez Terrane (NPT)
This area seems to be the key to understanding the geological
evolution from the Early Proterozoic to the Cambrian because it
combines the best sedimentary units with well
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON 37
exposed structures. Until 1995, published papers treated this part
of the crystalline basement as a Transamazonian (Archean ?) complex
reworked by the Panafrican orogeny. Field work conducted during the
last two years allows the separation of a metamorphic complex that
will be discussed in detail below.
5.1. GEOLOGY OF THE NICO PEREZ TERRANE
The Nico Perez Terrane (NPT) has two major outcrops: one in North
Uruguay (Rivera's crystalline island), which is surrounded by
younger sedimentary rocks, and the other, the largest, in the
central part of the country with well exposed relationships with
neighboring geological units.
The central-Uruguay outcropping area comprise an old nucleus (Early
Proterozoic - Archean?) occupied by granulite-facies metamorphic
rocks, which are surrounded eastward and southward by a
medium-grade metamorphic complex, being the whole partially covered
by a Vendian to Cambrian metasedimentary unit.
The high-grade nucleous (named Valentines Complex by BOSSI et aI.,
1965) is made up of meta-BIF (banded iron formations, here
magnetite-augite quartzites), string perthite gneisses, and
pyroxenites. These rocks are polydeformed and were intruded by
granitic stocks of uncertain age.
The most important granitic intrusion is an extensive rapakivi
complex named "Illescas Batolite" by CAMPAL & SCHIPILOV (1995).
Their intrusion possesses several petrological facies, from
quartz-bearing syenite to granophyres and granites with high quartz
content. All of them have a distinctive and unequivocal feature:
iridescent bluish euhedral quartz as an early crystallization
phase. Rounded oligoclase-mantled potash feldspars is widespread;
these along with the few existing chemical analyses, help to define
the rapakivi nature of the intrusion.
The ages of this granite were obtained by whole rock Rb/Sr as 1760
± 32 My (Ro=0.704; BOSSI & CAMPAL, 1991); and by 207PbrPb on
zircon (1751 My, 5%
discordance); LARRY HEAMAN per. com., in CAMPAL & SCHIPILOV,
1995). The age, structure, and stratigraphy of the medium-grade
metamorphic complex as a
whole have not been discussed until now. As all previous writers
assumed that the NPT was the craton adjacent to a Brazilian
orogenic belt, the analysis was focused on the position and nature
of the margin of the belt. Consequently these rocks were mapped as
a part of the Brazilian mobile belt (Lavalleja Group) by BOSSI et
al. (1975) and PRECIOZZI et at. (1985); or as Transamazonian
reworked rocks (FERRANDO & FERNANDES, 1971; BOSSI et at, 1965;
FRAGOSO CESAR & SOLIANI, 1984; PRECIOZZI et aI., 1985; FRAGOSO
CESAR et at., 1987; FRAGOSO CESAR et at., 1993; HALINAN et aI.,
1993; MACHADO & FRAGOSO CESAR, 1987). In fact, the model
proposed by these authors implies a Transamazonian plate with no
tectonic activity existed between the Early Proterozoic and the
Neoproterozoic Brazilian orogeny. With this in mind, lithologies
that are similar to those founded in the Brazilian mobile belt were
mapped as Upper Proterozoic or inclusively considered as isolated
portions of hypothetical thrusts (FRAGOSO CESAR, 1986).
Near the eastern border of the NPT, medium-grade metamorphic rocks
and others corresponding to the Vendian metamorphic sequence (here
more folded) were not
38 N. CAMPAL AND A. SCHIPILOV
subdivided by previous authors. One of the most important features
of the NPT is the structure within thrust sheets
that verge toward the southeast (Fig. 3 and Fig. 4), and implies
the medium grade metamorphic complex and its basement. These
thrusts preceded the generation of wide dextral shear zones that we
consider simultaneous with the SYPL's first stage of development.
Another element that contributes to difficult the
tectonostratigraphy of the NPT's rocks, is the intense Cambrian
sinistral shear strain that affects preexisting structures: these
effects can be clearly detected in the Vendian cover and its
magmatic intrusions.
NW
A
D Piedra aha lerrane (PAT)
~ Rio de la PlaIa mafic dike swarm
I@l Nico Perez lerrane (high grade rocks)
~'\\\ I Nico Perez lerrmc (medium grade rocks)
t.>:i Blue quanz rapakivi balholilh
o 10 20
Vendian sedimenlary cover
"<;.' ~ Cuchill. Dionisio beh (CDB)
# d iTranseurrcnl faults I Thrusts Grenvillian: dextral shear &
SE vergence Bruilian : sinistral shear &, W 'Vergence
Figure 3. Schematic cross section ofNico Perez terrane showing the
relation between older core, the medium grade metamorphic complex
and the Vendian cover.
SE
Some local cross sections will be described to illustrate the
nature of the more common lithological associations (locations are
showed in Fig. 4):
5.1.1. Location 1: Puntas del Santa Luda 1- muscovite quartzite
interbedded with metaconglomerate and muscovite
schist. 2- recrystallized BIF's 3- gray-yellowish to pinkish
carbonate, highly recrystallized, interbedded with
muscovite schist. At the top of this unit are 100 m of
stromatolitic limestones with synsedimentary breccias that may have
been caused by storms or tsunamis. SPRECHMANN et al. (1994) defined
three types of stromatolites in this unit:
- planar stromatolites (cryptalgallaminites). - laterally linked
hemispheroids (LLH). - vertically stacked hemispheroids (SH-V) with
variable basal radii .
A WRAMIK (1992) pointed out that the relatively high diversity is
typical of the Early and Middle Proterozoic.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON
x
x
x
Valenlines complex
D Peralumious graniles (ca. 1200 My) --an o D
ieo Perez mafic dike swarm (co 600 My)
Brazilian-age graniles (03 550 My)
Vendian sedimentary cover
39
33'
Ultramanc melamophie rocks (Ialc-schisl) 0 Low Crelaceous basalts
Grenvillian: dextral shear & SE vergence
Grenvillian.agc graniccs ~.J Transcurrent faults I Thrusts
Brasilia" : sinistral shear & NW vergence
Figure 4. Geologic map of the studied area and location of
available geochronological data
40 N. CAMPAL AND A. SCHIPILOV
5.1.2. Location 2: Piraraja 1- muscovite quartzite 2- Strongly
deformed quartz-matrix metaconglomerate. This rock has
elongated
pebbles with geometric relations between major and minor axes of
511 to 1011, and constitute a marker horizon in the complex
stratigraphy that permits better resolution and structural geometry
in this block.
3- red clastic limestone with variable amount of metamorphic
muscovite. 4- leucocratic gneiss.
5.1.3. Location 3: Maria Albina 1- garnet-bearing biotite-muscovite
schist 2- staurolite-bearing biotite-muscovite schist 3-
garnet-bearing grunerite BIF 4- muscovite-biotite gneiss 5-
muscovite quartzite 6- muscovite limestone
5.1.4. Location 4: Manso's Quarry 1- biotite - muscovite schist
interstratified with garnet-bearing impure
limestone. 2- tremolite schist 3- spilitic metabasalt and
metakeratophyre.
In this work we are proposing a Middle Proterozoic Grenvillian age
for this metamorphic sequence on the basis of:
A- Muscovite KlAr data from the peraluminous synkinematic granite
yield an age of 40 -6 ccSTP
1253.1 ± 32.2 My; K (%) = 8.7500; 75%ERR=1.9038; Arrad (10
'g)=614.61; Aralm
(%) = 6.16; Err = 2.6% (CINGOLANI, C. per. com.).
B- The only one post-intrusion thermal record obtained by 39 Arr Ar
in the "Rio de la Plata mafic dike swarm" yielded a Middle
Proterozoic age (RENNE & TEXEIRA, per. com., in CAMPAL et al.,
1995)
C- Vergence of all observed structures systematically indicates
mass transportation toward the southeast. These structures are
intimately related to dextral shear zones, deduced from macro- and
mesolkinematic indicators (Fig. 5). This fact contrast with the
vergence and shearing from the Brasiliano mobile belt.
D- It is very difficult to explain how rocks systematically
displaying medium-grade metamorphism ~;;d at least 3 phases of
deformation could be the source area for the Arroyo del Soldado
Group with a palaeontological age of about 570 My (see E), and can
be a part of the Brasiliano mobile belt, which indicates broadly
the same age for the main metamorphic peak and granitic intrusions
(700 - 570 My; GOODWIN, 1991). This is specially relevant, taking
into account the nature of the Arroyo del Soldado sequence.
THE EASTERN EDGE OF THE RIO DE LA PLATA CRATON 41
Figure 5. (a) aerial photo of a macro kinematic indicator developed
in the first stage of movement
P~~_LI.!..._~d::J~:c.;;;;::;;===::=====-- of Grenvillian age. (b)
detail of a protomylonite ~ in this zone.
E- The ages obtained for the carbonate deposition of the
Grenvillian metamorphic complex on the basis of the stromatolite
morphology yield values between the Early Proterozoic and Middle
Proterozoic.
In fact, the Brasilian ages of these metamorphic rocks were assumed
by previous authors solely on the basis of RblSr or KiAr
geochronological data, obtained on intrusive granitic stocks andlor
highly deformed rocks that were supposed to be synorogenic with the
Brasilian orogenic cycle. These rocks are really synkinematic with
a transpressional accretional episode that happened at the end of
the Cambrian Period.
In most places of the NPT the vergence, tectonic style, and
lithologies of the medium grade metamorphic complex, give us
reasonable criteria for its separation from both the Vendian cover
and the Transamazonian basement, despite the imbrication of these
units and the low relief of the area. Deformation and slab
structure with basement rocks increass eastward into the Vendian
group. Much field work to determine how these structures developed
is needed.
Until now, the eastern edge of the Nico Perez terrane has not been
well defined. The Sierra BaHena Lineament (SBL) is certainly the
most important structure that separates Nico Perez terrane and
CuchiHa Dionisio belt, but there is a thin belt of rocks between
them for which there is no criterion that defines its age and
origin.
5.2. GEOCHRONOLOGICAL DATA (NPT)
One of the principal contributions of this work has been a detailed
geological survey at locations where samples for geochronological
analysis were gathered by other authors. Consequentelly some
isolated data acquired a new geological significance (Fig.
4).
42 N. CAMPAL AND A. SCHIPILOV
TABLE m. Available geochronological data in Nico Perez terrane.
Point 1: 515 ± 10 My (UMPIERRE & HALPERN, 1971) Rb/Sr-
whole·rock Point 2: 510 ± 35 My (UMPIERRE & HALPERN, 1971)
Rb/Sr- whole·rock Point 3: 1253.1 ± 32.2 My (CINGOLANI, Per. com.,
KlAr - muscovite Point 4: 665 ± 203 My (RIV ALENTI et al. , 1995)
Rb/Sr - whole-rock Point 5: 550 ± 15 My (UMPIERRE & HALPERN,
1971) Rb/Sr - whole-rock & isolated mineral Point 6: 532 ± 16
My (CINGOLANI et aI., 1990) KI Ar - illite Point 7: 518 ± 15
My(CINGOLANIetal.,1990)KlAr-illite PointS: 1786 ± 26 My (BOSSIetal.
, 1993a)KlAr-contact'sbiotite Point 9: 520 ± 2 My (BOSSI et aI.,
1993b) Rb/Sr - whole-rock
Some ages deserve special emphasis; data 1 belong to a granitic
mylonite. This rock was considerated by BOSSI et al. (1975) and
PRECIOZZI et at. (1985) as an intrusive granitic body. Using
detailed structural analysis we can confirm that the protolith of
this mylonite is the -1800 My rapakivi granite, which has as a
special feature: euhedral blue quartz crystals. The blue coloration
is due to the presence of fine rutile needles and disappears when
the rock is subject to hard strain (CAMPAL & SCHIPILOV, 1995).
In the mylonite in the nuclei of quartz ribbon we can recognize
small pieces of the original blue rutilated quartz.
At the point 2 (Cerro Palma) the age corresponds to a mylonite just
recognized by PRECIOZZI et al. (1985). At the point 3 the age
obtained by KlAr on muscovite (CINGOLANI Per. com.) from a
garnet-muscovite-bearing granite, synkinematic with overthrusting
in the Grenvillian basement. Fig. 6 is a geologic cross section
near the sample location.
w granite E Amphibolite Garnet-mica-schist
Figure 6. Schematic cross section near sample 3 location.
Point 4 will be discussed later, it being a mafic dike swarm (Nico
Perez dike swarm) that was studied by RIV ALENTI et al.
(1995).
The age at point 5 (Polanco granite; UMPIERRE & HALPERN, 1971)
establishes the upper limit for Vendian-Cambrian sedimentation. In
fact, this granite intrudes the basal formations of the Arroyo del
Soldado Group, producing a 5 to 10m wide contact metamorphic