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Emplacement time of Salai Patai carbonatite, Malakand, Pakistan, from fission track dating of zircon and apatite
0735-245X/91 $3.00+ .00 Pergamon Pressplc
Nucl. Tracks Radiat. Meas., Vol. 18, No. 3, pp. 315-319, 1991 Int. J. Radiat. Appl. Instrum., Part D
Printed in Great Britain
EMPLACEMENT TIME OF SALAI PATAI CARBONATITE, MALAKAND, PAKISTAN, FROM FISSION TRACK DATING OF ZIRCON AND APATITE A. A. QURESm,*K. A. BUTTSand H. A. KHAN* *SSNTD-Laboratory, N.E.D., Pinstech, P. O. Nilore, Islamabad, Pakistan and tAtomic Energy Minerals Centre, Lahore, Pakistan (Received 18 April 1989; in revised form 29 November 1990)
Abstract--Based on fission track dating of zircon and apatite, the emplacement history of Salai Patai carbonatite has been traced. It has been estimated that the carbonatite was emplaced along the thrust plane associated with the Indian-Eurasian plate collision during the Oligocene period followed by some thermal/tectonic episode during Early Miocene. This negates the previous proposal that all carbonatites found in Pakistan are a part of a 200 km long alkaline province associated with the rifting of Peshawar Valley during Late Cretaceous or Early Tertiary.
1. INTRODUCTION Fission track dating depends upon the ability of minerals to record the radiation damage (as tracks) caused by the spontaneous fission of U-238 (Fleishcer et al., 1975; Durrani and Bull, 1987). The fission tracks in minerals are stable over long geological periods and begin to accumulate quantitatively once the mineral has cooled down below a temperature known as the "track retention temperature." So fission tracks ages of various minerals indicate a particular time in the history of a rock at which those minerals cooled down below the characteristic temperature, which is also known as the "closure temperature". The minerals sphene, zircon and apatite denote the closure temperatures of 250+50°C, 200 _+ 50°C and 100 + 20°C, respectively (Gleadow et aL, 1984). The fission track ages of these minerals can therefore be used not only to determine ages but also are useful for looking back into elapsed time and reconstructing the cooling and tectonic histories of rocks. This approach enables the geologist to interpret geological ages and to construct quantitative models of thermal and tectonic histories of mountain ranges and sedimentary basins (Gleadow et al., 1983). This has been possible because of the revelation of divergent ages of two or more co-existing minerals such as zircon and apatite, sphene and apatite, and so on. The older age usually indicates the emplacement time of the igneous complex; and the younger age, that of a later tectonic or thermal episode. Based on fission track analysis of apatite, Gieadow et al. (1984), Moore et al. (1986) and Duddy et al. (1984) have successfully studied problems such as the uplift history of the Transantartic Mountains in the Dry Valleys area, southern Victoria Land, Antarctica; the
thermal evolution of rifted continental margins of southeastern Australia; and the provenance studies of the southwest margins of the Rockall Plateau in the north Atlantic. The fission track study of zircon was used by McGoldrick and Gleadow (1977) for the age and provenance determination of ancient sedimentary rocks of north central Victoria, Australia. All the carbonatite complexes occurring in northern Pakistan (Fig. 1) were formerly considered to be a part of one alkaline igneous province emplaced during the rifting of the Peshawar Valley in the Late Cretaceous or Early Tertiary (Kempe and Jan, 1970). Fission track dating studies, now carried out on zircon and apatite from one of the carbonatite complexes located at Salai Patai, Malakand, Pakistan, have now revealed that the carbonatitic magmatism activity in this area took place sometime in the Late Tertiary. It is believed that the zircon age (32.1 + 1.9 Ma) is the time of carbonatite emplacement along thrust planes associated with IndianEurasian plate collision, and the apatite age (22.7 _+ 1.1 Ma) is the age of re-setting of thrust at a later stage.
2. ALKALINE PROVINCE AND CARBONATITE COMPLEXES OF PAKISTAN Existence of an alkaline province consisting of alkaline granites, syenites, albitites and carbonatites in an arcuate fashion around Peshawar Valley has been reported by Kemp and Jan (1970, 1980). The alkaline rocks are emplaced along fault zones in Early Paleozoic metasediments over a 200 km long belt extending from Afghanistan through Khyber Agency, Momand Agency, Warsak, Salai Patai, Shahbazgarhi, Koga, Tarbela, and further across the 315
316
A . A . QURESHI et al. FIG. 1
HINDU KUSH ~7700 5700 A.
= E. O ~3 F.. t~ C
NANGA PARBAT[(
35"N
PALEOZOIC SHAHBAZ GAP
SHILMAN
WARSAK~.. PESHAWAR
"
"'
_,&Y ~
ISLAMABAD
\
o
~
,oo
7 •E
FIG. l. Geological sketch map of northern Pakistan and adjoining areas (After Tahirkheli, 1979). Kohistan Sequence:mainly composed of garnet granulites, amphibolites, pyroxene granulitcs with (+ + + ) acidic intrusions, (vvv) volcanics (:::) sediments and (UM) ultramafics at places. Poleozoic Metosediments: mainly composed of pclitic/psamatic schists, graphitic schists, marbles, slates, metaconglomerates with
granites, granite gneisses and alkaline intrusives. Alkaline rocks are at localites 1-7 and (*) carbonatites are exposed at places where locality name is underlined. Indus river to the Mansehra area. Occurrences of these rocks are restricted by the Main Boundary Thrust (MBT) in the south and by the Main Mantle Thrust (MMT) in the north. Within the alkaline belt, the carbonatite complexes are found in four places, namely Shilman, Salai Patai, Koga and Tarbela (Fig. 1). The carbonatites are intrusive carbonate-silicate rocks found in a region of crustal stability of rifting. They mainly contain calcite and/or dolomite, rarely ankerite, rhodochrosite or siderite, and lesser amounts of silicates, oxides and metallic minerals. Because of their newly recognized significance as large-scale potential sources of niobium of the cerium group, rare-earth elements, radioactive minerals, apatite, magnetite, barite and vermiculite, they are of considerable economic importance. The Shilman carbonatite complex consists of a 3 km long sheet emplaced in a fault zone with E-W trend and northerly dip. It is located in the north of Landi Kotal in Khyber Agency near PakistanAfghanistan (Jan et al., 1981; Qureshi et al., 1982). Pure sovite and biotite carbonatite occur as small patches within the main mass of amphibole carbonatite. It is underlain by Precambrian slates and phyllites and overlain by Paleozoic schists. Sixty km NE of Shilman, at Salai Patai in Malakand Agency, is another carbonatite complex (Fig. 2). This also occurs as an E-W trending, but southerly dipping, sheet in a thrust plane. The carbonatite is underlain by pellitic schist and quartzites
and overlain by granitic gneisses, psamatic schist and garnet mica schist (Ashraf and Chaudhary, 1977). The MMT located 20 km north of the carbonatite may be related to the thrust-containing carbonatite (Le Baset al., 1987). The Salai Patai carbonatite consists of biotite, apatite and sovite within brownish amphibole apatite and sovite. The carbonatite sheet is thin, with a maximum thickness of 20 m, but extends for more than 15 km in the lateral direction. Another carbonatite complex is exposed at Koga, which is located about 70 km SE of Salai Patai. Here, felspathoidai syenites are found, associated with granodioritic gneisses and alkaline syenites (Siddiqui
FIG. 2. Salai Patai-Malakand carbonatite complex (after Le Baset al., 1987).
E M P L A C E M E N T T I M E O F SALAI PATAI C A R B O N A T I T E et al., 1968). The nepheline syenite was intruded
by a swarm of carbonatites and ijolites. The carbonatites, mostly a few metres in dimension, are exposed over an area about 1 km long and 200 m wide. The carbonatites are composed of calcite, pyroxene, apatite and deep yellow sphene. Extensive fenitization has been noted in the enclosing nepheine syenite. About 50 km SE of Koga, at Tarbela, is another outcrop of alkaline rocks which consists of alkaline granites, microgranites, gabbros, dolerites, albitites and carbonatites (Kempe and Jan, 1980). These rocks are exposed over a 3.5 km long, 200 m wide area, and are intruded along a major fault which separates Precambrian Saikhala Series from Cambrain Tanol quartzites. Kempe and Jan (1980) suggested that the Peshawar plain, located in the south of the alkaline belt, is an irregular rift valley extending for about 200 km from the Pakistan-Afghanistan border to Tarbela, and perhaps further eastwards. It has been advocated that rifting took place in the Late Cretaceous or Early Tertiary with the generation of alkaline magma, which ultimately gave rise to carbonatites, syenites, ijolites and associated rocks. However, faults suggestive of doming or flexuring or dyke swarm, such as are often associated with rifting, are not known in this area. The Salai Patai carbonatite is an extensive igneous complex located in the centre of the alkaline belt. In order to check its association with the rifting of the Peshawar Valley, as suggested by Kempe and Jan (1980), and to trace the tectonic history of northern Pakistan, fission track analyses of zircon and apatite from carbonatite have been carried out. Suitable sphene crystals were not available in the carbonatite rocks for fission track studies. The carbonatite also contains uranium and some rare-earth elements such as Nb, Ta, Yt, etc. These economic minerals are also present in the Shilman carbonatite located at the western margin of the alkaline belt near the PakistanAfghanistan border. The fission track dating studies are also expected to yield useful information concerning the genetic relationship between Salai Patai and Shilman carbonatites for an overall evaluation of the economic significance of the carbonatite complexes of the area at some later stage when a mining venture on these rocks is underway. 3. EXPERIMENTAL DETAILS Zircon crystals up to a few millimetres in size were hand-picked from carbonatite outcrops, crushed and sieved to obtain nearly 200 ttm (62 mesh) sized grains. These were washed and dried to eliminate fine dust. The red-brown (which are metamict) and opaque grains were removed with the help of a binocular microscope to obtain a clear fraction. These clear grains were then mounted on teflon wafers, polished and etched for 20 h at 220°C using a K O H : N a O H eutectic mixture, as described by Gieadow et al. (1976). The counting was done at 1000x overall
317
magnification for the calculation of track density. For induced track density, external detector (Lexan) and re-polish methods, as described by Hurford and Green (1982), were used. Fluenee was calculated by means of Lexan and standard reference glass SRM 962 by irradiating for 3rain at 2 × 10~3n cm -2 S -I flux in a reactor at 5 M W power. The Lexan was etched in 6.5 M NaOH for 45 min at 50°C. Clear apatite grains were separated from an available pyrochlore + apatite concentrate, using a binocular microscope. These grains were mounted in epoxy resin, polished and etched in 10% HNO3 for 30s. The spontaneous fission track density was counted in relatively clear apatite grains, whereas induced track density was calculated using the repolish method. For induced density and fluence measurement the samples were irradiated in the reactor for 3 min at 2 x 1013n cm -2 S -m flux along with a standard reference glass SRM-962. More details are given in Table 1. 4. RESULTS Fission track ages determined on zircon and apatite are presented in Table 1. The ages have been calculated using values of constants given at the foot of the table. Out of the many zircon grains mounted on teflon wafers, only a few survived through prolonged polishing and etching steps for the final track analysis. The re-polish method seems to be more satisfactory in this case, since the results computed on this basis show very little scattering compared with the external detector method. The reason for this is not understood. The results are in the range of 32.0 Ma, close to results reported by Le Bas et al. (1987) as 31 + 2 Ma, based on K - A r analysis of the biotites extracted from these carbonatites. The development of tracks and their optical resolution were checked at various etching intervals. After 20h of etching at 220°C in the N a O H + K O H (8 + 11.5 g) eutectic, the track development and optical resolution were found to be suitable for counting at 1000 × magnification. It was noted that excessive etching made counting difficult due to an intermingling of tracks. Anisotropic etching behaviour was noted in some crystals. The apatite from Salai Patai carbonatite w a s found to be easy for mounting in epoxy, polishing and etching. The re-polish method was used to calculate induced track density for all apatite samples. The track density and optical resolution were found to be relatively uniform and better, both within one crystal and from crystal, compared with zircon. The age values in this case were concentrated in a narrow zone of 21.3-24.8 Ma, with an average value of 22.7 + 1.1 Ma. The result of uranium content determination in a few zircon grains is also presented in the table; it is found to be fairly uniform in all grains, with a ratio of about ! :2 between the minimum and the maximum values.
A. A. Q U R E S H I et al.
318
Table 1. Fission track ages of zircon and apatite from Salai Patai carbonatite
Spontaneous tracks Area of Sample
Induced tracks
Counted Track density Counted Track density
Sample No.
Mineral
(10-(era 2)
tracks
(I06 cm -2)
tracks
(106 cm -2)
sP-I/l sP- 1/2. SP-l/3 SP-I/4 SP-I/5* SP-2/I SP-2/2 SP-2/3 SP-2/4" SP-2/5 SP-2/6 SP-2/7 SP-2/8 SP-2/9 SP-2/10*
Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon
17.47 2.54 4.90 6.81 1,83 2.21 2.72 3.23 7.67 3.88 2.93 3.82 5.71 6.19 2.96
1964 448 795 509 267 676 747 437 377 539 443 427 700 676 292
1.21 1.76 1.62 0.75 1,45 3.05 2.74 1.35 0.49 1.39 1.51 1.12 1.22 1.09 4.36
3111 644 1299 830 470 1381 1621 899 734 1069 899 816 1435 1381 2896
1.78 2.61 2.65 1,21 2.56 6.24 5.96 2.78 0.95 2.75 3.07 2.14 2.51 2.23 9.78
SP-3/I SP-3/2 SP-3/3 SP-3/4 SP-3/5 SP-3/6 SP-3/7 SP-3/8 SP-3/9 SP-3/IO
A aatite A mtite A )atite A )atite A ~atite A mtite A mtite A mtite A mtite A ~atite
2.91 3.85 7.12 2.61 3.91 4.21 2.95 5.81 2.31 6,91
58 115 85 63 99 109 143 97 224 195
0.20 0.30 0.12 0.24 0.25 0.26 0.48 0.17 0.97 0.28
185 364 275 207 316 329 443 281 645 772
0.63 0.94 0.39 0.79 0.81 0.78 1.50 0.48 2.79 0.84
Fluencc
Age
U content
(n cm -2)
(Ma)
(ppm)
9.9 x 10t4 33.6 281 9.9 x l0 t` 34.4 410 9.9 x 1014 30.3 429 9.9 x 10j4 30.7 192 9.9 x 1014 28.0 415 1,33 x l0 ts 32.5 -1,33 x 10Is 30.6 -1.33 x l0 ts 32.3 -1.33 x 10ts 34.3 -1.33 x 10Is 33.6 -1.33 x 10Ls 32.7 -1.33 x 10Is 34,8 -1.33 x l0 Is 32.3 -1.33 x 10Is 32.5 -1.33 x l0 ts 29.6 -Mean zircon age = 32.1 + 1.9 Ma 1.4 x i0 ts 22.2 -1.4 x l0 ts 22.3 -1.4 x l0 ts 21.5 -1.4 x 10~s 21.3 -1.4 x l0 ts 21.6 -1.4 x 10t5 23.3 -1.4 X i 0 Is 22.4 -1.4 x l0 ts 24.8 -1.4 x 10t5 24,3 -1.4 x 10ts 23.3 -Mean apatite age = 22.7 + 1.1 Ma
*The ages reported above have been calculated using the fission decay constant ).f = 8.4 x 10-t7 yr-J (Spadavacchia and Hahn, 1967), the uranium isotopic ratio ( ~ s U / m U ) 1 = 7.26 x 10-3; and the thermal-neutron fission cross-section (/~f) for U-235 as 5.8 x 10 -22 cm -2 (Durrani and Bull, 1987). The flucnc¢ was determined by counting tracks in Lcxan, induced by the standard reference glass No. SRM 962 of the National Btmmu of Standards (U.S.A.) by using Au-neutron dose calibration. The mineral samples and Lcxan-SRM 962 assembly were irradiated in the reactor for 3 rain at 2 x 10t3 era -2 sneutron flux. Annealing studies sufficient for the age interpretation of carbonatite were not carried out. Four zircon samples, marked by (*), were dated by the external detector method, whereas the rest of the samples were dated using the re-polish method. The uranium content was calculated by comparing track density in Lexan due to zircon grains and to SRM 962. The zircon was etched in a K O H : N a O H eutectic at 220°C for 20h; apatite in 10% HNO3 at 230°C for 30s; and Lexan in 6.5 M NaOH at 50°C for 45 rain.
5. C O N C L U S I O N S O u t o f the four established c a r b o n a t i t e occurrences o f Pakistan, one, located at Salai Patai in M a l a k a n d Agency, has been d a t e d by fission track analyses o f zircon a n d apatite. O n the basis o f present w o r k it has been estimated t h a t this carbonatite, located in the centre o f the alkaline belt, was i n t r u d e d somewhere in the middle o f the Oligocene (zircon age, 32.1 + 1.9 M a ) as a sheet a l o n g t h r u s t plane associated with I n d i a n - E u r a s i a n plate collision. Because o f the changes in the etching characteristics o f minerals due to t h e r m a l effects, it would have been m o r e advisable to i n c o r p o r a t e the results o f the a n n e a l i n g studies o n zircon for a better age i n t e r p r e t a t i o n o f the c a r b o n a t i t e , b u t this could not be d o n e due to non-availability o f sufficient a n n e a l i n g d a t a o n zircon. However, there is n o evidence available from this area o f any t h e r m a l or tectonic event d u r i n g the Oligocene period, which could have a n n e a l e d the fission tracks f r o m zircon partially or totally, a n d re-set the fission track clock. So it looks safe
to assume t h a t zircon age is that o f c a r b o n a t i t e e m p l a c e m e n t a n d n o t t h a t o f any t h e r m a l or tectonic event. However, there seems to have been a mild t h e r m a l / t e c t o n i c episode at a later stage in the Early M i o c e n e which a n n e a l e d tracks only from co-existing apatite a n d n o t f r o m zircon (apatite age 22.7 + 1.1 Ma). This work does n o t agree with the p r o p o s a l of K e m p e a n d J a n (1980) t h a t the c a r b o n a t i t e s a n d associated alkaline rocks are associated with the Late Cretaceous or Early Tertiary rifting o f the P e s h a w a r Valley. However, the dates d e t e r m i n e d are in a c c o r d a n c e with the K - A r dates d e t e r m i n e d o n the biotites f r o m Salai Patai c a r b o n a t i t e by Le Bas et al. (1987). T h e fact t h a t the c a r b o n a t i t e m a g m a t i s m was a Late Tertiary activity is also s u p p o r t e d by the absence o f the nepheline-bearing rocks at Salai Patai, as at this stage, the m e t a s e d i m e n t s were thick e n o u g h to allow only the c a r b o n a t i t e m a g m a to pass t h r o u g h , n o t nephelinitic m a g m a ( H u a n g , 1962) because crustal thickening started
EMPLACEMENT TIME OF SALAI PATAI CARBONATITE much earlier in the Eocene, or during an even earlier epoch. The nepheline-bearing magma is unable to rise through the low density thick sequence of metasediments.
REFERENCES Ashraf M. and Chaudhary M. N. (1977) A discovery of carbonatite from Malakand. Geol. Bull. Punjab. Univ. 14, 89-90. Duddy I. R., Gleadow A. J. W. and Keen J. B. (1984) Fission track dating of apatite and sphene from paleogene sediments of deep sea drilling project LEG 81, Set. 555. Initial reports of the deep sea drilling project, Vol. LXXI, Washington (U.S. Government Printing Office). Durrani S. A. and Bull R. K. (1987) Solid State Nuclear Track Detection: Principles, Methods and Applications. Pergamon Press, Oxford. Fleischer R. L., Price P. B. and Walker R. M. (1975) Nuclear Tracks in Solids: Principals and Applications. University of California Press, Berkeley. Gleadow A. J. W., Duddy I. R. and Lovering J. F. (1983) Fission track analysis: a new tool for the evaluation of thermal histories and hydrocarbon potential. Aust. Petrol. Explor. Ass. J. 23, 93-102. Gleadow A. J. W., Hurford A. J. and Quaife R. D. (1976) Fission track dating of zircon: improved etching techniques. Earth planet. Sci. Lett. 33, 273-276. Gleadow A. J. W., Mckelvey B. C. and Ferguson K. U. (1984) Uplift history of the Transuntarctic Mountains in the Dry Valleys areas, southern Victoria Land, Antarctica, from apatite fission track ages. New Zealand J. Geol. Geophys. 27, 457--464. Huang W. T. (1962) Petrology, pp. 129-131. McGraw Hill, New York.
N T ISI3----C
319
Hurford A. J. and Green P. F. (1982) A user's guide to fission track dating calibration. Earth planet. Sci. Lett. 59, 343-354. Jan M. Q., Kamal M. and Qureshi A. A. (1981) Petrography of Shilman carbonatite complex, Khyber Agency. Geol. Bull. Univ. Peshawar 17, 61-68. Kempe D. R. C. and Jan M. Q. (1970) An alkaline igneous province in the NW Frontier Province, West Pakistan. Geol. Mag. 107, 395-398. Kempe D. R. C. and Jan M. Q. (1980) The Peshawar plain alkaline igneous province, NW Pakistan. Spec. Issue Geol. Bull. Univ. Peshawar 13, 71-78. Le Bas M. J., Main I. and Rex D. C. (1987) Age and nature of carbonatite emplacement in northern Pakistan. Geol. Randsch. 76, 31%323. McGoldrick P. J. and Gleadow A. J. W. (1977) Fission track dating of lower Paleozoic standstones at Tatong, north central Victoria. J. Geol. Soc. Aust. 24, 461--464. Moore E., Gleadow A. J. W. and Lovering F. J. (1986) Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatite from southeast Australia. Earth planet. Sci. Left. 78, 255-270. Qureshi A. A., Nawaz A. and Beg M. I. (1982) Radioactive carbonatite complex of Shihnan, Khyber Agency, Pakistan. PAEC-KfK Seminar, internal report, Atomic Energy Minerals Centre, Lahore, Pakistan. Siddiqui F. A., Chaudry M. N. and Shakoor A. B. (1968) Geology and petrology of feldspathoid syenites and the associated rocks of Koga area, Chamla Valley, Swat, West Pakistan. Geol. Bull. Punjab Univ. 7, 1-30. Spadavacchia A. and Hahn B. (1967) Spontaneous fission decay constant measurement of U23s.Heir. Phys. Acta 40, 1063-1068. Tahirkhali R. A. K. (1979) Geology of Kolustan and adjoining Eurasian and Indopakistan continents, Pakistan. Geol. Bull. Univ. Peshawar 11, 1-30.
Nucl. Tracks Radiat. Meas., Vol. 18, No. 3, pp. 315-319, 1991 Int. J. Radiat. Appl. Instrum., Part D
Printed in Great Britain
EMPLACEMENT TIME OF SALAI PATAI CARBONATITE, MALAKAND, PAKISTAN, FROM FISSION TRACK DATING OF ZIRCON AND APATITE A. A. QURESm,*K. A. BUTTSand H. A. KHAN* *SSNTD-Laboratory, N.E.D., Pinstech, P. O. Nilore, Islamabad, Pakistan and tAtomic Energy Minerals Centre, Lahore, Pakistan (Received 18 April 1989; in revised form 29 November 1990)
Abstract--Based on fission track dating of zircon and apatite, the emplacement history of Salai Patai carbonatite has been traced. It has been estimated that the carbonatite was emplaced along the thrust plane associated with the Indian-Eurasian plate collision during the Oligocene period followed by some thermal/tectonic episode during Early Miocene. This negates the previous proposal that all carbonatites found in Pakistan are a part of a 200 km long alkaline province associated with the rifting of Peshawar Valley during Late Cretaceous or Early Tertiary.
1. INTRODUCTION Fission track dating depends upon the ability of minerals to record the radiation damage (as tracks) caused by the spontaneous fission of U-238 (Fleishcer et al., 1975; Durrani and Bull, 1987). The fission tracks in minerals are stable over long geological periods and begin to accumulate quantitatively once the mineral has cooled down below a temperature known as the "track retention temperature." So fission tracks ages of various minerals indicate a particular time in the history of a rock at which those minerals cooled down below the characteristic temperature, which is also known as the "closure temperature". The minerals sphene, zircon and apatite denote the closure temperatures of 250+50°C, 200 _+ 50°C and 100 + 20°C, respectively (Gleadow et aL, 1984). The fission track ages of these minerals can therefore be used not only to determine ages but also are useful for looking back into elapsed time and reconstructing the cooling and tectonic histories of rocks. This approach enables the geologist to interpret geological ages and to construct quantitative models of thermal and tectonic histories of mountain ranges and sedimentary basins (Gleadow et al., 1983). This has been possible because of the revelation of divergent ages of two or more co-existing minerals such as zircon and apatite, sphene and apatite, and so on. The older age usually indicates the emplacement time of the igneous complex; and the younger age, that of a later tectonic or thermal episode. Based on fission track analysis of apatite, Gieadow et al. (1984), Moore et al. (1986) and Duddy et al. (1984) have successfully studied problems such as the uplift history of the Transantartic Mountains in the Dry Valleys area, southern Victoria Land, Antarctica; the
thermal evolution of rifted continental margins of southeastern Australia; and the provenance studies of the southwest margins of the Rockall Plateau in the north Atlantic. The fission track study of zircon was used by McGoldrick and Gleadow (1977) for the age and provenance determination of ancient sedimentary rocks of north central Victoria, Australia. All the carbonatite complexes occurring in northern Pakistan (Fig. 1) were formerly considered to be a part of one alkaline igneous province emplaced during the rifting of the Peshawar Valley in the Late Cretaceous or Early Tertiary (Kempe and Jan, 1970). Fission track dating studies, now carried out on zircon and apatite from one of the carbonatite complexes located at Salai Patai, Malakand, Pakistan, have now revealed that the carbonatitic magmatism activity in this area took place sometime in the Late Tertiary. It is believed that the zircon age (32.1 + 1.9 Ma) is the time of carbonatite emplacement along thrust planes associated with IndianEurasian plate collision, and the apatite age (22.7 _+ 1.1 Ma) is the age of re-setting of thrust at a later stage.
2. ALKALINE PROVINCE AND CARBONATITE COMPLEXES OF PAKISTAN Existence of an alkaline province consisting of alkaline granites, syenites, albitites and carbonatites in an arcuate fashion around Peshawar Valley has been reported by Kemp and Jan (1970, 1980). The alkaline rocks are emplaced along fault zones in Early Paleozoic metasediments over a 200 km long belt extending from Afghanistan through Khyber Agency, Momand Agency, Warsak, Salai Patai, Shahbazgarhi, Koga, Tarbela, and further across the 315
316
A . A . QURESHI et al. FIG. 1
HINDU KUSH ~7700 5700 A.
= E. O ~3 F.. t~ C
NANGA PARBAT[(
35"N
PALEOZOIC SHAHBAZ GAP
SHILMAN
WARSAK~.. PESHAWAR
"
"'
_,&Y ~
ISLAMABAD
\
o
~
,oo
7 •E
FIG. l. Geological sketch map of northern Pakistan and adjoining areas (After Tahirkheli, 1979). Kohistan Sequence:mainly composed of garnet granulites, amphibolites, pyroxene granulitcs with (+ + + ) acidic intrusions, (vvv) volcanics (:::) sediments and (UM) ultramafics at places. Poleozoic Metosediments: mainly composed of pclitic/psamatic schists, graphitic schists, marbles, slates, metaconglomerates with
granites, granite gneisses and alkaline intrusives. Alkaline rocks are at localites 1-7 and (*) carbonatites are exposed at places where locality name is underlined. Indus river to the Mansehra area. Occurrences of these rocks are restricted by the Main Boundary Thrust (MBT) in the south and by the Main Mantle Thrust (MMT) in the north. Within the alkaline belt, the carbonatite complexes are found in four places, namely Shilman, Salai Patai, Koga and Tarbela (Fig. 1). The carbonatites are intrusive carbonate-silicate rocks found in a region of crustal stability of rifting. They mainly contain calcite and/or dolomite, rarely ankerite, rhodochrosite or siderite, and lesser amounts of silicates, oxides and metallic minerals. Because of their newly recognized significance as large-scale potential sources of niobium of the cerium group, rare-earth elements, radioactive minerals, apatite, magnetite, barite and vermiculite, they are of considerable economic importance. The Shilman carbonatite complex consists of a 3 km long sheet emplaced in a fault zone with E-W trend and northerly dip. It is located in the north of Landi Kotal in Khyber Agency near PakistanAfghanistan (Jan et al., 1981; Qureshi et al., 1982). Pure sovite and biotite carbonatite occur as small patches within the main mass of amphibole carbonatite. It is underlain by Precambrian slates and phyllites and overlain by Paleozoic schists. Sixty km NE of Shilman, at Salai Patai in Malakand Agency, is another carbonatite complex (Fig. 2). This also occurs as an E-W trending, but southerly dipping, sheet in a thrust plane. The carbonatite is underlain by pellitic schist and quartzites
and overlain by granitic gneisses, psamatic schist and garnet mica schist (Ashraf and Chaudhary, 1977). The MMT located 20 km north of the carbonatite may be related to the thrust-containing carbonatite (Le Baset al., 1987). The Salai Patai carbonatite consists of biotite, apatite and sovite within brownish amphibole apatite and sovite. The carbonatite sheet is thin, with a maximum thickness of 20 m, but extends for more than 15 km in the lateral direction. Another carbonatite complex is exposed at Koga, which is located about 70 km SE of Salai Patai. Here, felspathoidai syenites are found, associated with granodioritic gneisses and alkaline syenites (Siddiqui
FIG. 2. Salai Patai-Malakand carbonatite complex (after Le Baset al., 1987).
E M P L A C E M E N T T I M E O F SALAI PATAI C A R B O N A T I T E et al., 1968). The nepheline syenite was intruded
by a swarm of carbonatites and ijolites. The carbonatites, mostly a few metres in dimension, are exposed over an area about 1 km long and 200 m wide. The carbonatites are composed of calcite, pyroxene, apatite and deep yellow sphene. Extensive fenitization has been noted in the enclosing nepheine syenite. About 50 km SE of Koga, at Tarbela, is another outcrop of alkaline rocks which consists of alkaline granites, microgranites, gabbros, dolerites, albitites and carbonatites (Kempe and Jan, 1980). These rocks are exposed over a 3.5 km long, 200 m wide area, and are intruded along a major fault which separates Precambrian Saikhala Series from Cambrain Tanol quartzites. Kempe and Jan (1980) suggested that the Peshawar plain, located in the south of the alkaline belt, is an irregular rift valley extending for about 200 km from the Pakistan-Afghanistan border to Tarbela, and perhaps further eastwards. It has been advocated that rifting took place in the Late Cretaceous or Early Tertiary with the generation of alkaline magma, which ultimately gave rise to carbonatites, syenites, ijolites and associated rocks. However, faults suggestive of doming or flexuring or dyke swarm, such as are often associated with rifting, are not known in this area. The Salai Patai carbonatite is an extensive igneous complex located in the centre of the alkaline belt. In order to check its association with the rifting of the Peshawar Valley, as suggested by Kempe and Jan (1980), and to trace the tectonic history of northern Pakistan, fission track analyses of zircon and apatite from carbonatite have been carried out. Suitable sphene crystals were not available in the carbonatite rocks for fission track studies. The carbonatite also contains uranium and some rare-earth elements such as Nb, Ta, Yt, etc. These economic minerals are also present in the Shilman carbonatite located at the western margin of the alkaline belt near the PakistanAfghanistan border. The fission track dating studies are also expected to yield useful information concerning the genetic relationship between Salai Patai and Shilman carbonatites for an overall evaluation of the economic significance of the carbonatite complexes of the area at some later stage when a mining venture on these rocks is underway. 3. EXPERIMENTAL DETAILS Zircon crystals up to a few millimetres in size were hand-picked from carbonatite outcrops, crushed and sieved to obtain nearly 200 ttm (62 mesh) sized grains. These were washed and dried to eliminate fine dust. The red-brown (which are metamict) and opaque grains were removed with the help of a binocular microscope to obtain a clear fraction. These clear grains were then mounted on teflon wafers, polished and etched for 20 h at 220°C using a K O H : N a O H eutectic mixture, as described by Gieadow et al. (1976). The counting was done at 1000x overall
317
magnification for the calculation of track density. For induced track density, external detector (Lexan) and re-polish methods, as described by Hurford and Green (1982), were used. Fluenee was calculated by means of Lexan and standard reference glass SRM 962 by irradiating for 3rain at 2 × 10~3n cm -2 S -I flux in a reactor at 5 M W power. The Lexan was etched in 6.5 M NaOH for 45 min at 50°C. Clear apatite grains were separated from an available pyrochlore + apatite concentrate, using a binocular microscope. These grains were mounted in epoxy resin, polished and etched in 10% HNO3 for 30s. The spontaneous fission track density was counted in relatively clear apatite grains, whereas induced track density was calculated using the repolish method. For induced density and fluence measurement the samples were irradiated in the reactor for 3 min at 2 x 1013n cm -2 S -m flux along with a standard reference glass SRM-962. More details are given in Table 1. 4. RESULTS Fission track ages determined on zircon and apatite are presented in Table 1. The ages have been calculated using values of constants given at the foot of the table. Out of the many zircon grains mounted on teflon wafers, only a few survived through prolonged polishing and etching steps for the final track analysis. The re-polish method seems to be more satisfactory in this case, since the results computed on this basis show very little scattering compared with the external detector method. The reason for this is not understood. The results are in the range of 32.0 Ma, close to results reported by Le Bas et al. (1987) as 31 + 2 Ma, based on K - A r analysis of the biotites extracted from these carbonatites. The development of tracks and their optical resolution were checked at various etching intervals. After 20h of etching at 220°C in the N a O H + K O H (8 + 11.5 g) eutectic, the track development and optical resolution were found to be suitable for counting at 1000 × magnification. It was noted that excessive etching made counting difficult due to an intermingling of tracks. Anisotropic etching behaviour was noted in some crystals. The apatite from Salai Patai carbonatite w a s found to be easy for mounting in epoxy, polishing and etching. The re-polish method was used to calculate induced track density for all apatite samples. The track density and optical resolution were found to be relatively uniform and better, both within one crystal and from crystal, compared with zircon. The age values in this case were concentrated in a narrow zone of 21.3-24.8 Ma, with an average value of 22.7 + 1.1 Ma. The result of uranium content determination in a few zircon grains is also presented in the table; it is found to be fairly uniform in all grains, with a ratio of about ! :2 between the minimum and the maximum values.
A. A. Q U R E S H I et al.
318
Table 1. Fission track ages of zircon and apatite from Salai Patai carbonatite
Spontaneous tracks Area of Sample
Induced tracks
Counted Track density Counted Track density
Sample No.
Mineral
(10-(era 2)
tracks
(I06 cm -2)
tracks
(106 cm -2)
sP-I/l sP- 1/2. SP-l/3 SP-I/4 SP-I/5* SP-2/I SP-2/2 SP-2/3 SP-2/4" SP-2/5 SP-2/6 SP-2/7 SP-2/8 SP-2/9 SP-2/10*
Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon
17.47 2.54 4.90 6.81 1,83 2.21 2.72 3.23 7.67 3.88 2.93 3.82 5.71 6.19 2.96
1964 448 795 509 267 676 747 437 377 539 443 427 700 676 292
1.21 1.76 1.62 0.75 1,45 3.05 2.74 1.35 0.49 1.39 1.51 1.12 1.22 1.09 4.36
3111 644 1299 830 470 1381 1621 899 734 1069 899 816 1435 1381 2896
1.78 2.61 2.65 1,21 2.56 6.24 5.96 2.78 0.95 2.75 3.07 2.14 2.51 2.23 9.78
SP-3/I SP-3/2 SP-3/3 SP-3/4 SP-3/5 SP-3/6 SP-3/7 SP-3/8 SP-3/9 SP-3/IO
A aatite A mtite A )atite A )atite A ~atite A mtite A mtite A mtite A mtite A ~atite
2.91 3.85 7.12 2.61 3.91 4.21 2.95 5.81 2.31 6,91
58 115 85 63 99 109 143 97 224 195
0.20 0.30 0.12 0.24 0.25 0.26 0.48 0.17 0.97 0.28
185 364 275 207 316 329 443 281 645 772
0.63 0.94 0.39 0.79 0.81 0.78 1.50 0.48 2.79 0.84
Fluencc
Age
U content
(n cm -2)
(Ma)
(ppm)
9.9 x 10t4 33.6 281 9.9 x l0 t` 34.4 410 9.9 x 1014 30.3 429 9.9 x 10j4 30.7 192 9.9 x 1014 28.0 415 1,33 x l0 ts 32.5 -1,33 x 10Is 30.6 -1.33 x l0 ts 32.3 -1.33 x 10ts 34.3 -1.33 x 10Is 33.6 -1.33 x 10Ls 32.7 -1.33 x 10Is 34,8 -1.33 x l0 Is 32.3 -1.33 x 10Is 32.5 -1.33 x l0 ts 29.6 -Mean zircon age = 32.1 + 1.9 Ma 1.4 x i0 ts 22.2 -1.4 x l0 ts 22.3 -1.4 x l0 ts 21.5 -1.4 x 10~s 21.3 -1.4 x l0 ts 21.6 -1.4 x 10t5 23.3 -1.4 X i 0 Is 22.4 -1.4 x l0 ts 24.8 -1.4 x 10t5 24,3 -1.4 x 10ts 23.3 -Mean apatite age = 22.7 + 1.1 Ma
*The ages reported above have been calculated using the fission decay constant ).f = 8.4 x 10-t7 yr-J (Spadavacchia and Hahn, 1967), the uranium isotopic ratio ( ~ s U / m U ) 1 = 7.26 x 10-3; and the thermal-neutron fission cross-section (/~f) for U-235 as 5.8 x 10 -22 cm -2 (Durrani and Bull, 1987). The flucnc¢ was determined by counting tracks in Lcxan, induced by the standard reference glass No. SRM 962 of the National Btmmu of Standards (U.S.A.) by using Au-neutron dose calibration. The mineral samples and Lcxan-SRM 962 assembly were irradiated in the reactor for 3 rain at 2 x 10t3 era -2 sneutron flux. Annealing studies sufficient for the age interpretation of carbonatite were not carried out. Four zircon samples, marked by (*), were dated by the external detector method, whereas the rest of the samples were dated using the re-polish method. The uranium content was calculated by comparing track density in Lexan due to zircon grains and to SRM 962. The zircon was etched in a K O H : N a O H eutectic at 220°C for 20h; apatite in 10% HNO3 at 230°C for 30s; and Lexan in 6.5 M NaOH at 50°C for 45 rain.
5. C O N C L U S I O N S O u t o f the four established c a r b o n a t i t e occurrences o f Pakistan, one, located at Salai Patai in M a l a k a n d Agency, has been d a t e d by fission track analyses o f zircon a n d apatite. O n the basis o f present w o r k it has been estimated t h a t this carbonatite, located in the centre o f the alkaline belt, was i n t r u d e d somewhere in the middle o f the Oligocene (zircon age, 32.1 + 1.9 M a ) as a sheet a l o n g t h r u s t plane associated with I n d i a n - E u r a s i a n plate collision. Because o f the changes in the etching characteristics o f minerals due to t h e r m a l effects, it would have been m o r e advisable to i n c o r p o r a t e the results o f the a n n e a l i n g studies o n zircon for a better age i n t e r p r e t a t i o n o f the c a r b o n a t i t e , b u t this could not be d o n e due to non-availability o f sufficient a n n e a l i n g d a t a o n zircon. However, there is n o evidence available from this area o f any t h e r m a l or tectonic event d u r i n g the Oligocene period, which could have a n n e a l e d the fission tracks f r o m zircon partially or totally, a n d re-set the fission track clock. So it looks safe
to assume t h a t zircon age is that o f c a r b o n a t i t e e m p l a c e m e n t a n d n o t t h a t o f any t h e r m a l or tectonic event. However, there seems to have been a mild t h e r m a l / t e c t o n i c episode at a later stage in the Early M i o c e n e which a n n e a l e d tracks only from co-existing apatite a n d n o t f r o m zircon (apatite age 22.7 + 1.1 Ma). This work does n o t agree with the p r o p o s a l of K e m p e a n d J a n (1980) t h a t the c a r b o n a t i t e s a n d associated alkaline rocks are associated with the Late Cretaceous or Early Tertiary rifting o f the P e s h a w a r Valley. However, the dates d e t e r m i n e d are in a c c o r d a n c e with the K - A r dates d e t e r m i n e d o n the biotites f r o m Salai Patai c a r b o n a t i t e by Le Bas et al. (1987). T h e fact t h a t the c a r b o n a t i t e m a g m a t i s m was a Late Tertiary activity is also s u p p o r t e d by the absence o f the nepheline-bearing rocks at Salai Patai, as at this stage, the m e t a s e d i m e n t s were thick e n o u g h to allow only the c a r b o n a t i t e m a g m a to pass t h r o u g h , n o t nephelinitic m a g m a ( H u a n g , 1962) because crustal thickening started
EMPLACEMENT TIME OF SALAI PATAI CARBONATITE much earlier in the Eocene, or during an even earlier epoch. The nepheline-bearing magma is unable to rise through the low density thick sequence of metasediments.
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Hurford A. J. and Green P. F. (1982) A user's guide to fission track dating calibration. Earth planet. Sci. Lett. 59, 343-354. Jan M. Q., Kamal M. and Qureshi A. A. (1981) Petrography of Shilman carbonatite complex, Khyber Agency. Geol. Bull. Univ. Peshawar 17, 61-68. Kempe D. R. C. and Jan M. Q. (1970) An alkaline igneous province in the NW Frontier Province, West Pakistan. Geol. Mag. 107, 395-398. Kempe D. R. C. and Jan M. Q. (1980) The Peshawar plain alkaline igneous province, NW Pakistan. Spec. Issue Geol. Bull. Univ. Peshawar 13, 71-78. Le Bas M. J., Main I. and Rex D. C. (1987) Age and nature of carbonatite emplacement in northern Pakistan. Geol. Randsch. 76, 31%323. McGoldrick P. J. and Gleadow A. J. W. (1977) Fission track dating of lower Paleozoic standstones at Tatong, north central Victoria. J. Geol. Soc. Aust. 24, 461--464. Moore E., Gleadow A. J. W. and Lovering F. J. (1986) Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatite from southeast Australia. Earth planet. Sci. Left. 78, 255-270. Qureshi A. A., Nawaz A. and Beg M. I. (1982) Radioactive carbonatite complex of Shihnan, Khyber Agency, Pakistan. PAEC-KfK Seminar, internal report, Atomic Energy Minerals Centre, Lahore, Pakistan. Siddiqui F. A., Chaudry M. N. and Shakoor A. B. (1968) Geology and petrology of feldspathoid syenites and the associated rocks of Koga area, Chamla Valley, Swat, West Pakistan. Geol. Bull. Punjab Univ. 7, 1-30. Spadavacchia A. and Hahn B. (1967) Spontaneous fission decay constant measurement of U23s.Heir. Phys. Acta 40, 1063-1068. Tahirkhali R. A. K. (1979) Geology of Kolustan and adjoining Eurasian and Indopakistan continents, Pakistan. Geol. Bull. Univ. Peshawar 11, 1-30.