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Synplutonic dikes in the wadi Um-Mara area, Sinai
Tectonophysics, 67 (1980) 35-44 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
SYNPLUTONIC
DIKES IN THE WADI UM-MARA
35
AREA, SINAI
YEHUDA EYAL department Beer-Sheva
of GeoIogy mineralogy, 84120 (Israel)
3e~-~ur~o~
university
of the Negev, PO Box 653
(Received August 8, 1978; revised version accepted June 22, 1979)
ABSTRACT Eyal, Y., 1980. Synplutonic 35-44.
dikes in the Wadi Urn-Mara area, Sinai’. Tectonophysics,
67:
Sets of parallel andesitic schistose dikes intruding granodiorite country rocks were themselves intruded by the same country rock. This mutual cross-cutting relationship is explained as synplutonic dikes that intruded the granodioritic crystal mush in its last stages of crystallizatjon. Four deformation phases affected the dikes as a result of which they became schistose and were folded, faulted, boudinaged and rotated. The kind of strain was dependent on relative competency of these dikes and the granodiorite which changes during the cooling history. All the deformation phases ceased before the end of crystallization of the granodioritic body. INTRODUCTION
The Urn-Mara gneiss is exposed in Wadi Urn-Mara about 40 km south of Eilat (Fig. 1). It is part of the metamorphic complex of Precambrian age, extending from Eilat southward to the Sinai Peninsula along the western side of the Gulf of Eilat (Eyal, 1976). The Urn-Mara granodioritic gneiss is intruded by quartz andesitic dikes, now schistose, which in turn are cut by granodioritic dikes and veins derived from the host rock. Similar relationships between dikes and country rocks were reported by Sederholm (1907), Eskola (1961), Davis (1963), Watson (1967), Pitcher and Read .(1960), and others. However, the Wadi Urn-Mara area differs from those previously described in that the schistosity of the dikes and their defo~ation occurred before their intrusion by the host rock. This paper describes and explains the mutual relationships between the schistose boudinaged and folded dikes and the gneissic country rock, and the deformation phases that affect both rock types. THE ROCK TYPES
The Urn-Mar-a gneiss (the Wadi Urn-Mara coarse-grained, mineralogically homogeneous 0040-1951/80/0000-0000/$02.25
country rock) granodiorite.
is a pale-grey, In some out-
0 1980 Elsevier Scientific Publishing Company
32”
2
f
P SINAI
ELAT,’
SAUDI
;--_-__
ARABIA
-
32”
Fig. 1. Location
340
36’
map,
crops the gneissosity is well developed whereas in others it is very weak. Irregular and tabular mafic bodies occur within the gneiss. The irregular bodies are commonly a few meters long, with considerable variation in thickness over short horizontal distances. Their contacts with the country rock may be sharp or gradational. They are heterogeneous in composition and their color is mostly lighter than that of the tabular bodies. In some places they continue into the light-colored gneiss as dark bands or sehlieren of biotite or biotite schist. In some outcrops the bodies which are commonly folded resemble
Fig. 2. Schistose quartz andesitic dike; note sharp was faulted and then rotated (see also Fig. 6b).
contacts
Fig. 3. Schistose gneissic country
was faulted
quartz andesitic dike. This rock along the fault plane.
dike
with
country
and then
rock.
intruded
The dike
by the
38
Fig. 4. Schistose dike gneiss were removed.
intruded
by the Urn-Mara
gneiss.
Note
perfect
fit if dike parts
of
39
schists, banded gneisses or migmatites. The features described above indicate the xenolithic or relict nature of the irregular bodies. The tabular bodies range in length from a few meters to tens of meters and in width from a few cm up to 0.5 m (Figs. 2 and 3). They appear in sets of parallel bodies, all of which have a homogeneous quartz andesitic composition. Laterally each body maintains uniform width and the contacts with the country rock are invariably sharp. When granodiorite dikes intrude the andesite the latter shows a corresponding dilatation of normal width (Fig. 4). Based on the characteristics of the tabular bodies it is concluded that they are synplutonic dikes as used by Pitcher and Berger (1972, p. 336). The degree of development of schistosity in the dikes is not even. Most commonly the schistosity is parallel or sub parallel to the walls of the body. The dikes underwent shearing, particularly near the boundary with the country rock as evidenced by strong schistosity near the dike walls and shearing of previous veins (Fig. 5). In some dikes the orientation of the schistosity in the central part is diagonal to the wall and becomes progressively more parallel to it as the contact is approached (Fig. 6a). In others the
Fig. 5. The contact between a schistose dike and its country rock. A folded vein with fanning axial plane gneissosity is seen in the dike. The shear of the dike near the contact is evidenced by the thinning of the gneissic vein as well as by a pronounced dark colored schistosity. The area shown is about 30 square centimeters.
b
7
Fig. 6. a. The development of sigmoidal schistosity in the quartz andesitic dikes. The folding of the schistosity was caused by a later deformation phase with opposite shear direction. b. Two deformation phases following the development of schistosity. The first phase caused faulting of the schistose dike and during the second phase boudins were rotated.
angle between the schistosity and the wall is greater near the wall and may be normal or even more than 90”, to the direction expected by the sigmoidal form which implies later folding of the schistosity (Fig. 6b). The gneissosity of the country rock is mostly parallel to the direction of the dikes although in some cases it is diagonal. BOUDINS AND FOLDS
Most of the dikes underwent several phases of defo~ation after the one in which the schistosity was formed. During these subsequent deformations, which occurred during the crystallization of the granodiorite, the bodies underwent folding and faulting, mostly without any strong internal deformation. Some of the deformatio 1 phenomena, such as folds or small thrusts, indicate shortening whereas others such as normal faults and boundinage indicate extension. Most boudins are of rectangular or rhombohed~l form (Fig. 2) while a few are of normal pinch- and swell type. The planes between the rhombohedral boudins were the sites along which subsequent sliding or
41
rotation occurred (Rast, 1956). All these sliding planes and small faults separating the boudins were annealed by recrystallization of the rack. A specific history of shortening or extension cannot be determined in any one area due to the random distribution of shortening and extension structures and the presence of both types of structures in many outcrops.
extention
boudin
axes
shortening boudin axes and fold uxes mineral line0 tion pote to great circle (Paxis ) pote to white veins in the dikes Fig.
7. a. Density
lines of 102
poles to the planes of the schjstose dikes. 1, 3, 6 and 9%. hemisphere). b. The distribution of some of the poles according to their geographical location. c. 24 shortening axes, 19 extension axes and 8 poles to planes of the leucacratic veins and 3 mineral lineations. d, Density contour lines of the various axes.
(All plots axe equal area lower
42 VEIN PHENOMENA
There are two kinds of veins in the dikes: thick ones up to a few centimeters wide are identical to the country rock, and thin ones up to few millimeters -are mostly lighter in color than the country rock. The veins may or may not be folded. Unfolded veins are parallel or subparallel to the schistosity of the body, whereas the folded veins are commonly of high angles to the schistosity. The two kinds of veins belong to at least two generations of emplacement as shown by the fact that the thick veins mostly cut the thin ones and that deformation phenomena such as folds are abundant in the thin veins only. In one block found in Wadi Urn-Mara (Fig. 5) a granodioritic vein than intruded andesitic dikes was subsequently folded. In this case the gneissosity of the vein, which is parallel to the axial plane of the fold, is mainly characterized by prefered orientation of long quartz grains.
Structural
characteristics
of the schistose dikes in Wadi Urn-Mara
Three tectonic elements were measured in the Wadi Urn-Mara area: (a) the gneissosity of the country rock; (b) the orientation of the schistose dikes; and (c) the directions of shortening (normal to the fold axes or to the intersection line of dikes walls with reverse fault planes) or extension (normal to boudin axes or to the intersection line of dikes walls with the normal fault planes or joints. It is almost certain that the maximum shortening or extension was not exactly normal to these axes. Figure 7a shows density lines for 102 poles of dike planes and gneissosity. The poles fall on a great circle dipping 73”/183” with a 17” /3” axis. There is a geographical subdivision of the measurements, the dips in the eastern part of Wadi Urn-Mara are generally 44”/75”, in the central part 24”/360” and in the western part 35”/296” (Fig. 7b). In this area 23 extension axes and 21 shortening axes were measured (Fig. 7~). These axes were measured directly (fold axes) or indirectly (by measuring the dike plane and the fault or joint plane). From this figure one may infer that: (1) The extension and shortening axes are dispersed randomly and therefore no regional compression or tension directions can be defined. (2) Most axes fall near a great circle dipping 30” /29” (Fig. 7d). A few mineral lineations also fall on this circle. A small number of measured axes fall on another, not well defined, great circle dipping 60” /65”. The question now arises as to how compression and tension structures from the same generation were developed in the same direction. DISCUSSION
The geological history of the Wadi Urn-Mara areas as evidenced work is as follows: (a) emplacement of a granodioritic magma; (b) intrusion of andesitic dikes into the granodiorite;
from field
43
(c) intrusion of leucocratic veins from the granodiorite into the andesitic dikes; (d) fo~ation of gneissos~ty and schistosity in the country rock and the dikes respectively, with simultaneous folding or thinning of the leucocratic veins, according to their orientation in the dikes; (e) folding of the schistosity of the dikes; (f) folding, faulting and boudinaging of the dikes followed by intrusion of the granodioritic gneiss into the jointed dikes; (g) rotation of the boudins formed in stage “f” by a stress field differing in orientation from that responsible for the original formation of the boudins; (h) folding of the whole area around a N-S axis. Some of the leucocratic veins were emplaced after stage “d”. Rock relationships such as those described above, in which country rock is found as small dikes or veins in earlier dikes that intruded the same country rock have been described by many workers, as summarized by Mehnert (1968) and Pitcher and Berger (1972). Eskola (1961) called this phenomenon the “Sederholm effect”. The three main explanations for this phenomenon are: (a) remobilization (Wegmann, 1963; Watson, 1967); (b) granitization of country rock previously intruded by dikes or contained xenoliths (Goodspeed, 1955; Roddick and A~strong, 1959); and (c) intrusion of dikes into a crystallizing magma i.e. synplutonic dikes (Davis, 1963; Pitcher and Berger, 1972). The dikes of Wadi Urn-Mara differ from others described in the literature in that they underwent four or five deformation phases while the granodioritic country rock was crystallizing. By examining the effects of these deformation phases it is possible to see how the competencies of the country rock and dikes were progressively changed. Schistosity of the dikes was formed while the granodioritic crystal mush was the more competent whereas folding, faulting and rotation of the dike fragments occurred while the dikes were the more competent. It was concluded above that the tabular bodies are quartz-andesitic dikes and there is no evidence (absence of high grade met~orphism, flow structures, replacement or feldspathization of dikes, etc.) for remobilization or granitization of the Wadi Urn-Ivlara country rock. The evidence conforms with Pitcher and Berger’s (1972, p. 336) view that synplutonic dikes are emplaced in the late stages of crystallization of the plutonic host. STRUCTURAL
GENESIS
OF THE SCHISTOSE
DIKES IN WAD1 UM-MARA
The schistosity of the dikes and the gneissosity of the gneiss is not parallel at all outcrops. The differences in their direction may be explained in two ways or a combination of both: (a) By differences in competence during the first deformation (Ramsey, 1967, p. 407, figs. 7-72). (b) On the assumption that the gneissosity of the granodiorite reflects the end product of the sum total of the deformation phases whereas in the dikes schistosity was formed first, followed by subsequent phases of folding, jointing and boudinage. Examination of Fig. 7c shows that no regional compression or tension can
44
be inferred because shortening and extension axes are randomly mixed. It is impossible to explain this randomness by deformation of dikes with various dips (Ramsay, 1967, p. 118) because: (1) Almost all the dikes found in any small area are subparallel, although with both extension and shortening structures. (2) Extension and shortening structures only one meter apart may be found in the same dike. An alternative possible explanation is that the dikes were so oriented that in progressive deformation they developed compressive structures (folds, reverse faults) followed by extension structures (boudins and normal faults) as described by Ramsay (1967, p. 119). (b) At least two deformation phases with different stress directions acted upon the dikes, after the development of their schistosity. It may be that well defined regions of compression and tension axes were disturbed and became randomly dispersed in the subsequent deformation phases. However, this disturbance would have had to be completed prior to the last deformation phase in which the whole study area was folded on a N-S axis. ACKNOWLEDGEMENT
I thank all those who assisted me in the completion of this study. Special thanks are due to Prof. R. Freund who spend some days with me in the field, to Prof. R. Shagam, Prof. R. Freund and Dr. B. Kohn for critical reading of the manuscript and to Prof. Y.K. Bentor for his guidance of the doctoral dissertation on which this work is based. Appreciation is expressed to Dr. M. Eyal and Mr. J. Glass for helpful discussions and suggestions. REFERENCES Davis, G.A., 1963. Structure and mode of emplacement of Caribou Mountain pluton, Klamath Mountains California. Geol. Soe. Am. Bull., 74: 331-348. Eskola, P., 1961. Granitenstehung bei Orogenese und Epiorogenese. Geol. Rundsch., 50: 105-123. Eyal, Y., 1976. The Metamorphic and Structural History of the Precambrian Massif in Taba - Bir Swair Area (North East Sinai), Unpublished Ph.D. thesis, Hebrew University of Jerusalem, 147 pp. Goodspeed, G.E., 1955. Relict dikes and relict pseudodykes. Am. J. Sci., 253: 146-161. Mehnert, K.R., 1968. Migmatites and the Origin of Granitic Rocks. Elsevier, Amsterdam, 393 pp. Pitcher, W.S. and Berger, A.R., 1972. The geology of Donegal: a study of granite emplacement and unroofing. Wiley Interscience, New York, 435 pp. Pitcher, W.S. and Read, H.H., 1960. Early transverse dikes in the main Donegal granite. Geol. Mag., 97: 53-61. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York, 568 pp. Rast, N., 1956. The origin and significance of boudinage. Geol. Mag., 83: 401-408. Roddiek, J.A. and Armstrong, J.E., 1959. Relict dikes in the Coast Mountains near Vancouver, B.C. J. Geol., 67: 603-613. Sederholm, J.J., 1907. Om Granit och Gneis. Bull. Comm. Geol. Finlande N:O 23: l-40. English translation: Sederholm, J.J., 1967. Selected Works, Granite and Migmatites. Wiley, New York. Watson, J., 1967. Evidence of mobility in reactivated basement complexes. Proc. Geol. Assoc., 78: 211-236. Wegmann, C.E., 1963. Tectonic patterns at different levels. Trans. Geol. Sot. S. Africa. Annex to vol. 66, 78 pp.
SYNPLUTONIC
DIKES IN THE WADI UM-MARA
35
AREA, SINAI
YEHUDA EYAL department Beer-Sheva
of GeoIogy mineralogy, 84120 (Israel)
3e~-~ur~o~
university
of the Negev, PO Box 653
(Received August 8, 1978; revised version accepted June 22, 1979)
ABSTRACT Eyal, Y., 1980. Synplutonic 35-44.
dikes in the Wadi Urn-Mara area, Sinai’. Tectonophysics,
67:
Sets of parallel andesitic schistose dikes intruding granodiorite country rocks were themselves intruded by the same country rock. This mutual cross-cutting relationship is explained as synplutonic dikes that intruded the granodioritic crystal mush in its last stages of crystallizatjon. Four deformation phases affected the dikes as a result of which they became schistose and were folded, faulted, boudinaged and rotated. The kind of strain was dependent on relative competency of these dikes and the granodiorite which changes during the cooling history. All the deformation phases ceased before the end of crystallization of the granodioritic body. INTRODUCTION
The Urn-Mara gneiss is exposed in Wadi Urn-Mara about 40 km south of Eilat (Fig. 1). It is part of the metamorphic complex of Precambrian age, extending from Eilat southward to the Sinai Peninsula along the western side of the Gulf of Eilat (Eyal, 1976). The Urn-Mara granodioritic gneiss is intruded by quartz andesitic dikes, now schistose, which in turn are cut by granodioritic dikes and veins derived from the host rock. Similar relationships between dikes and country rocks were reported by Sederholm (1907), Eskola (1961), Davis (1963), Watson (1967), Pitcher and Read .(1960), and others. However, the Wadi Urn-Mara area differs from those previously described in that the schistosity of the dikes and their defo~ation occurred before their intrusion by the host rock. This paper describes and explains the mutual relationships between the schistose boudinaged and folded dikes and the gneissic country rock, and the deformation phases that affect both rock types. THE ROCK TYPES
The Urn-Mar-a gneiss (the Wadi Urn-Mara coarse-grained, mineralogically homogeneous 0040-1951/80/0000-0000/$02.25
country rock) granodiorite.
is a pale-grey, In some out-
0 1980 Elsevier Scientific Publishing Company
32”
2
f
P SINAI
ELAT,’
SAUDI
;--_-__
ARABIA
-
32”
Fig. 1. Location
340
36’
map,
crops the gneissosity is well developed whereas in others it is very weak. Irregular and tabular mafic bodies occur within the gneiss. The irregular bodies are commonly a few meters long, with considerable variation in thickness over short horizontal distances. Their contacts with the country rock may be sharp or gradational. They are heterogeneous in composition and their color is mostly lighter than that of the tabular bodies. In some places they continue into the light-colored gneiss as dark bands or sehlieren of biotite or biotite schist. In some outcrops the bodies which are commonly folded resemble
Fig. 2. Schistose quartz andesitic dike; note sharp was faulted and then rotated (see also Fig. 6b).
contacts
Fig. 3. Schistose gneissic country
was faulted
quartz andesitic dike. This rock along the fault plane.
dike
with
country
and then
rock.
intruded
The dike
by the
38
Fig. 4. Schistose dike gneiss were removed.
intruded
by the Urn-Mara
gneiss.
Note
perfect
fit if dike parts
of
39
schists, banded gneisses or migmatites. The features described above indicate the xenolithic or relict nature of the irregular bodies. The tabular bodies range in length from a few meters to tens of meters and in width from a few cm up to 0.5 m (Figs. 2 and 3). They appear in sets of parallel bodies, all of which have a homogeneous quartz andesitic composition. Laterally each body maintains uniform width and the contacts with the country rock are invariably sharp. When granodiorite dikes intrude the andesite the latter shows a corresponding dilatation of normal width (Fig. 4). Based on the characteristics of the tabular bodies it is concluded that they are synplutonic dikes as used by Pitcher and Berger (1972, p. 336). The degree of development of schistosity in the dikes is not even. Most commonly the schistosity is parallel or sub parallel to the walls of the body. The dikes underwent shearing, particularly near the boundary with the country rock as evidenced by strong schistosity near the dike walls and shearing of previous veins (Fig. 5). In some dikes the orientation of the schistosity in the central part is diagonal to the wall and becomes progressively more parallel to it as the contact is approached (Fig. 6a). In others the
Fig. 5. The contact between a schistose dike and its country rock. A folded vein with fanning axial plane gneissosity is seen in the dike. The shear of the dike near the contact is evidenced by the thinning of the gneissic vein as well as by a pronounced dark colored schistosity. The area shown is about 30 square centimeters.
b
7
Fig. 6. a. The development of sigmoidal schistosity in the quartz andesitic dikes. The folding of the schistosity was caused by a later deformation phase with opposite shear direction. b. Two deformation phases following the development of schistosity. The first phase caused faulting of the schistose dike and during the second phase boudins were rotated.
angle between the schistosity and the wall is greater near the wall and may be normal or even more than 90”, to the direction expected by the sigmoidal form which implies later folding of the schistosity (Fig. 6b). The gneissosity of the country rock is mostly parallel to the direction of the dikes although in some cases it is diagonal. BOUDINS AND FOLDS
Most of the dikes underwent several phases of defo~ation after the one in which the schistosity was formed. During these subsequent deformations, which occurred during the crystallization of the granodiorite, the bodies underwent folding and faulting, mostly without any strong internal deformation. Some of the deformatio 1 phenomena, such as folds or small thrusts, indicate shortening whereas others such as normal faults and boundinage indicate extension. Most boudins are of rectangular or rhombohed~l form (Fig. 2) while a few are of normal pinch- and swell type. The planes between the rhombohedral boudins were the sites along which subsequent sliding or
41
rotation occurred (Rast, 1956). All these sliding planes and small faults separating the boudins were annealed by recrystallization of the rack. A specific history of shortening or extension cannot be determined in any one area due to the random distribution of shortening and extension structures and the presence of both types of structures in many outcrops.
extention
boudin
axes
shortening boudin axes and fold uxes mineral line0 tion pote to great circle (Paxis ) pote to white veins in the dikes Fig.
7. a. Density
lines of 102
poles to the planes of the schjstose dikes. 1, 3, 6 and 9%. hemisphere). b. The distribution of some of the poles according to their geographical location. c. 24 shortening axes, 19 extension axes and 8 poles to planes of the leucacratic veins and 3 mineral lineations. d, Density contour lines of the various axes.
(All plots axe equal area lower
42 VEIN PHENOMENA
There are two kinds of veins in the dikes: thick ones up to a few centimeters wide are identical to the country rock, and thin ones up to few millimeters -are mostly lighter in color than the country rock. The veins may or may not be folded. Unfolded veins are parallel or subparallel to the schistosity of the body, whereas the folded veins are commonly of high angles to the schistosity. The two kinds of veins belong to at least two generations of emplacement as shown by the fact that the thick veins mostly cut the thin ones and that deformation phenomena such as folds are abundant in the thin veins only. In one block found in Wadi Urn-Mara (Fig. 5) a granodioritic vein than intruded andesitic dikes was subsequently folded. In this case the gneissosity of the vein, which is parallel to the axial plane of the fold, is mainly characterized by prefered orientation of long quartz grains.
Structural
characteristics
of the schistose dikes in Wadi Urn-Mara
Three tectonic elements were measured in the Wadi Urn-Mara area: (a) the gneissosity of the country rock; (b) the orientation of the schistose dikes; and (c) the directions of shortening (normal to the fold axes or to the intersection line of dikes walls with reverse fault planes) or extension (normal to boudin axes or to the intersection line of dikes walls with the normal fault planes or joints. It is almost certain that the maximum shortening or extension was not exactly normal to these axes. Figure 7a shows density lines for 102 poles of dike planes and gneissosity. The poles fall on a great circle dipping 73”/183” with a 17” /3” axis. There is a geographical subdivision of the measurements, the dips in the eastern part of Wadi Urn-Mara are generally 44”/75”, in the central part 24”/360” and in the western part 35”/296” (Fig. 7b). In this area 23 extension axes and 21 shortening axes were measured (Fig. 7~). These axes were measured directly (fold axes) or indirectly (by measuring the dike plane and the fault or joint plane). From this figure one may infer that: (1) The extension and shortening axes are dispersed randomly and therefore no regional compression or tension directions can be defined. (2) Most axes fall near a great circle dipping 30” /29” (Fig. 7d). A few mineral lineations also fall on this circle. A small number of measured axes fall on another, not well defined, great circle dipping 60” /65”. The question now arises as to how compression and tension structures from the same generation were developed in the same direction. DISCUSSION
The geological history of the Wadi Urn-Mara areas as evidenced work is as follows: (a) emplacement of a granodioritic magma; (b) intrusion of andesitic dikes into the granodiorite;
from field
43
(c) intrusion of leucocratic veins from the granodiorite into the andesitic dikes; (d) fo~ation of gneissos~ty and schistosity in the country rock and the dikes respectively, with simultaneous folding or thinning of the leucocratic veins, according to their orientation in the dikes; (e) folding of the schistosity of the dikes; (f) folding, faulting and boudinaging of the dikes followed by intrusion of the granodioritic gneiss into the jointed dikes; (g) rotation of the boudins formed in stage “f” by a stress field differing in orientation from that responsible for the original formation of the boudins; (h) folding of the whole area around a N-S axis. Some of the leucocratic veins were emplaced after stage “d”. Rock relationships such as those described above, in which country rock is found as small dikes or veins in earlier dikes that intruded the same country rock have been described by many workers, as summarized by Mehnert (1968) and Pitcher and Berger (1972). Eskola (1961) called this phenomenon the “Sederholm effect”. The three main explanations for this phenomenon are: (a) remobilization (Wegmann, 1963; Watson, 1967); (b) granitization of country rock previously intruded by dikes or contained xenoliths (Goodspeed, 1955; Roddick and A~strong, 1959); and (c) intrusion of dikes into a crystallizing magma i.e. synplutonic dikes (Davis, 1963; Pitcher and Berger, 1972). The dikes of Wadi Urn-Mara differ from others described in the literature in that they underwent four or five deformation phases while the granodioritic country rock was crystallizing. By examining the effects of these deformation phases it is possible to see how the competencies of the country rock and dikes were progressively changed. Schistosity of the dikes was formed while the granodioritic crystal mush was the more competent whereas folding, faulting and rotation of the dike fragments occurred while the dikes were the more competent. It was concluded above that the tabular bodies are quartz-andesitic dikes and there is no evidence (absence of high grade met~orphism, flow structures, replacement or feldspathization of dikes, etc.) for remobilization or granitization of the Wadi Urn-Ivlara country rock. The evidence conforms with Pitcher and Berger’s (1972, p. 336) view that synplutonic dikes are emplaced in the late stages of crystallization of the plutonic host. STRUCTURAL
GENESIS
OF THE SCHISTOSE
DIKES IN WAD1 UM-MARA
The schistosity of the dikes and the gneissosity of the gneiss is not parallel at all outcrops. The differences in their direction may be explained in two ways or a combination of both: (a) By differences in competence during the first deformation (Ramsey, 1967, p. 407, figs. 7-72). (b) On the assumption that the gneissosity of the granodiorite reflects the end product of the sum total of the deformation phases whereas in the dikes schistosity was formed first, followed by subsequent phases of folding, jointing and boudinage. Examination of Fig. 7c shows that no regional compression or tension can
44
be inferred because shortening and extension axes are randomly mixed. It is impossible to explain this randomness by deformation of dikes with various dips (Ramsay, 1967, p. 118) because: (1) Almost all the dikes found in any small area are subparallel, although with both extension and shortening structures. (2) Extension and shortening structures only one meter apart may be found in the same dike. An alternative possible explanation is that the dikes were so oriented that in progressive deformation they developed compressive structures (folds, reverse faults) followed by extension structures (boudins and normal faults) as described by Ramsay (1967, p. 119). (b) At least two deformation phases with different stress directions acted upon the dikes, after the development of their schistosity. It may be that well defined regions of compression and tension axes were disturbed and became randomly dispersed in the subsequent deformation phases. However, this disturbance would have had to be completed prior to the last deformation phase in which the whole study area was folded on a N-S axis. ACKNOWLEDGEMENT
I thank all those who assisted me in the completion of this study. Special thanks are due to Prof. R. Freund who spend some days with me in the field, to Prof. R. Shagam, Prof. R. Freund and Dr. B. Kohn for critical reading of the manuscript and to Prof. Y.K. Bentor for his guidance of the doctoral dissertation on which this work is based. Appreciation is expressed to Dr. M. Eyal and Mr. J. Glass for helpful discussions and suggestions. REFERENCES Davis, G.A., 1963. Structure and mode of emplacement of Caribou Mountain pluton, Klamath Mountains California. Geol. Soe. Am. Bull., 74: 331-348. Eskola, P., 1961. Granitenstehung bei Orogenese und Epiorogenese. Geol. Rundsch., 50: 105-123. Eyal, Y., 1976. The Metamorphic and Structural History of the Precambrian Massif in Taba - Bir Swair Area (North East Sinai), Unpublished Ph.D. thesis, Hebrew University of Jerusalem, 147 pp. Goodspeed, G.E., 1955. Relict dikes and relict pseudodykes. Am. J. Sci., 253: 146-161. Mehnert, K.R., 1968. Migmatites and the Origin of Granitic Rocks. Elsevier, Amsterdam, 393 pp. Pitcher, W.S. and Berger, A.R., 1972. The geology of Donegal: a study of granite emplacement and unroofing. Wiley Interscience, New York, 435 pp. Pitcher, W.S. and Read, H.H., 1960. Early transverse dikes in the main Donegal granite. Geol. Mag., 97: 53-61. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York, 568 pp. Rast, N., 1956. The origin and significance of boudinage. Geol. Mag., 83: 401-408. Roddiek, J.A. and Armstrong, J.E., 1959. Relict dikes in the Coast Mountains near Vancouver, B.C. J. Geol., 67: 603-613. Sederholm, J.J., 1907. Om Granit och Gneis. Bull. Comm. Geol. Finlande N:O 23: l-40. English translation: Sederholm, J.J., 1967. Selected Works, Granite and Migmatites. Wiley, New York. Watson, J., 1967. Evidence of mobility in reactivated basement complexes. Proc. Geol. Assoc., 78: 211-236. Wegmann, C.E., 1963. Tectonic patterns at different levels. Trans. Geol. Sot. S. Africa. Annex to vol. 66, 78 pp.