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The relationship between KAr ages and grain size in middle and upper Proterozoic shales from the Yanshan area, North China
Precambrian Research, 29 (1985) 175--181 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
175
THE RELATIONSHIP BETWEEN K--Ar AGES AND GRAIN SIZE IN MIDDLE AND UPPER PROTEROZOIC SHALES FROM THE YANSHAN AREA, NORTH CHINA
ZHANG XUEQI
Department of Isotope Geology, Tianjin Institute of Geology and Mineral Resources. Tianjin 300170 (People's Republic of China) (Received May 16, 1984; revision accepted August 14, 1984)
ABSTRACT
Zhang, X., 1985. The relationship between K--Ar ages and grain size in middle and upper Proterozoic shales from the Yanshan area, North China. Precambrian Res., 29: 175--181. Eight shale samples from the middle and upper Proterozoic succession of the Yanshan area of North China were dated using the K--Ar method. Each sample was divided into four grades of different grain size: > 2 u, 2--1 u, 1--0.4 u and < 0.4 u. On the basis of the results, the eight samples were classified into two models. In model 1 the potassium content and the age values show a regular change with different grain sizes. The age of the finest grains (< 0.4 u) may represent the age of sedimentation and the age of coarser grains shows the influence of an isotopic system inherited from the terrigenous source. In model 2 the ages yielded by materials of any grain size are always younger than that considered reasonable for these strata and show the influence of a re-set isotopic system. The relationships among the three isotopic systems, here called inherited, contemporaneous and re-set, are discussed in an attempt to explain their geological meaning.
INTRODUCTION
T h e d a t i n g o f s h a l e s is a d i f f i c u l t b u t i n t e r e s t i n g p r o j e c t f o r g e o c h r o n o l o gists. M u c h p r o g r e s s h a s b e e n m a d e in t h i s f i e l d r e c e n t l y ( B o n h o m m e , 1982). Bonhomme e t al. ( 1 9 7 8 ) s t u d i e d t h e e f f e c t o f an i n h e r i t e d a r g o n component on isotopic age and dated samples of Phanerozoic rocks from France. Clauer (1979) systematically summarized the new approach to R b - - S r d a t i n g o f s h a l e s . C l a u e r ( 1 9 8 1 } e x p l a i n e d in d e t a i l t h e p r i n c i p l e s of Rb--Sr and K--Ar dating of clay minerals and glauconite and gave examples from the middle and upper Proterozoic of Mauritania. This paper d i s c u s s e s t h e r e s u l t s o f K - - A r d a t i n g o f d i f f e r e n t g r a i n sizes f r o m s h a l e s o f m i d d l e a n d u p p e r P r o t e r o z o i c r o c k s o f t h e Y a n s h a n a r e a in H e b e i P r o v ince, North China. M i d d l e a n d u p p e r P r o t e r o z o i c r o c k s a r e w e l l e x p o s e d in t h e Y a n s h a n a r e a ; t h e s t r a t i g r a p h i c s u c c e s s i o n is w e l l u n d e r s t o o d . T h e s e a r e a s , p a r t i c u l a r l y
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
176
Lingyuan Xuanhua -t"
+ Kuancheng
J,'xian + +
Beijing "., 0
60km
[
Tainja'n
Fig. 1. Location o f samples in Yanshan area.
in Jixian C o u n t y (100 km north o f the city of Tianjin) are considered as the stratotype section for the middle and upper Proterozoic in North China (Chen et al., 1980) {Fig. 1). The Jixian section involves three systems and 11 formations consisting of 99% sedimentary rocks and 1% volcanics. It is a b o u t 9200 m thick. The sedimentary rocks consist of mudstones (8%), sandstones (18%} and carbonates (74%). Mudstones or shales occur mainly in the Jingeryu, Xiamaling, Tielin~ Hongshuizhuang, Chuanlinggou and Changzhougou Formations. Data for this paper were obtained from these formations {Fig. 2). Different amounts of terrigenous material are present in different formations. For example the average terrigenous content in shales of the Xiamaling, Chuanlinggou and Hongshuizhuang Formations is about 41, 30 and 20%, respectively. Quartz, mica and feldspar are reported as the main terrigenous fraction in these formations. It is probable that there are t w o isotopic systems (or radioactive clocks) in the shales. One is an inherited isotopic system, carried by the terrigenous fraction. Another was formed during sedimentation or diagenesis and is called the contemporaneous isotopic system. Only the latter can show the real age of the strata. If the t w o kinds of isotopic system are thoroughly homogenised during a geological event, a new isotopic system will be formed, called the re-set isotopic system and the age value represents the time of the geological event. If homogenisation is partial then the three kinds of isotopic system can exist within the same rock and the age obtained is random and has no geological meaning. If the true sedimentary age of the rock is sought, then the inherited isotopic system must be removed. In geological terms the terrigenous fraction has to be removed. For instance, a large amount of detrital mica from
177
the Xiamaling Formation yielded a K--Ar age o f 1443 Ma. This age is much older than what is considered for the Xiamaling Formation and shows the presence of an inherited isotopic system. After removing this detritus, the next step is to determine whether the isotopic system has been homogenised by subsequent geological events. ] Subdivisions
Eons
of
the
scale
Rock types
Sample
No, s
J
I
/ J
System
F o r m a t ; on d~ngeryu
r~ r~
qingboikou
Xiomollng Tieling
JiXiOn L)
Hongshuizhuang
NO. iSO - 7 J -It
~
"It
No. iso
*
*
Z81030
-
16
t
*
Z81028
Wu mlshan
I
0
Yangzhuang
L
i
o
Gaoyuzhuang
Q_
Dahongyu !
Chongchen9
!
Tuanshanzi Chuonllngg°u
I, ~ ~ * ~ *
Chongzhougou
Fig. 2. Major shale o c c u r r e n c e s *--shales; b l a n k - - c a r b o n a t e rock.
No. i s o - 1 5 NO. ~ s o - 1 4 No. iso - 1 S n 7 9 0 0 4
in the stratigraphic s u c c e s s i o n o f the Yanshan area.
DATING DIFFERENT SIZE FRACTIONS OF SHALES
To study the influence of the terrigenous fraction on isotopic age and relationship between grain size and isotopic age, several samples (see Table I) were divided into different size grades. Taking into account such things as palaeogeographic setting, field observations and petrographic studies, shale samples were selected from locations where the palaeoenvironment was rather stable and the terrigenous detrital c o m p o n e n t of the sediments was relatively small. All samples were washed and dried at a low temperature, then crushed. Crushing was carried out to separate the coarser terrigenous detritus from the clay minerals. In general the material was not crushed finer than 100 mesh. Each sample was divided into four grades: > 2 #; 2--1 ~; 1--0.4 p; < 0.4 p. The different size fractions were dated separately. A large a m o u n t of terrigenous detritus is present in the samples > 1 p and some in the samples < 1 ~.
178 A n e l e c t r o n m i c r o s c o p e w a s u s e d t o c h e c k w h e t h e r t h e g r a m size o f e a c h s a m p l e c o r r e s p o n d e d t o t h e e x p e c t e d v a l u e s . T h i s is v e r y i m p o r t a n t in explaining the results of dating. B a s e d o n X - r a y d i f f r a c t i o n s t u d i e s , t h e c o m p o s i t i o n o f all t h e s a m p l e s u s e d in t h i s s t u d y is v e r y s i m p l e . T h e m a i n c o m p o n e n t is i l l i t e o f 1M t y p e . In s a m p l e N o . i s o - 1 6 , in a d d i t i o n t o 1M i l l i t e , s o m e s m e c t i t e c h l o r i t e is p r e s e n t a n d in Z 8 1 0 2 8 , l e n n i t e w a s d e t e c t e d . T h e K - - A r d a t a a r e g i v e n in T a b l e I. TABLE I K--At data from shale samples of the Yanshan area Samples
Grade (p)
K%
' ° A r / ' ° K 10 -6 g/g
Age (Ma)
No. iso-7 Jingeryu Fro.
> 2 1--2 1-0.4 < 0.4
4.78 5.04 5.30 5.20
0.0768 0.0745 0.0686 0.0635
999 969 908 855
Lingyuan
No. iso-16 Xiamaling Fm.
> 2 1--2 1-0.4 < 0.4
4.14 4.66 5.16 5.31
0.1104 0.1043 0.0772 0.0732
1289 1246 996 956
Xuanhua
Z81030 Honphu~huang Fm.
> 2 1--2 1-0.4 < 0.4
5.60 5.60
0.0875 0.0877
1114 1116
Jixian
4.40
0.0980
1191
Z81028 Hongshumhuang FrrL
> 2 1--2 1-0.4 < 0.4
5.52
0.1037
1240
4.40 4.52
0.1006
1221
No. mo-15 Chuanlinggou Fr~
> 2 1--2 1---0.4 < 0.4
10.00 8.80 7.44 6.98
0.0613 0.0611 0.0762 0.0749
831 829 983 973
No. iso-1 Changzhougou Frn.
> 2 1--2 1--0.4 < 0.4
9.74 8.06 7.60 7.16
0.1078 0.1047 0.1138 0.1102
1276 1250 1326 1297
No. iso-14 Changzhougou
> 2 1--2
3.18 3.92
0.0645 0.0578
865
4.92
0.0616
792 834
< 0.4
6.00
0.0532
741
Sn79004
> 2
6.20
0.0881
Changzhougou Fro.
1--2 1----0.4 < 0.4
6.31 6.18 6.20
0.0893 0.0912 0.0890
1101 1112 1130 1109
Fm.
1--0.4
Location
Jixian
Xuanhua
Qianan
Xuanhua
Kuancheng
179 DISCUSSION A statistically significant correlation exists between the isotopic age and grain size. T he samples shown in Table I are divided into two groups. T h e first group includes No. iso-7 of the Jingeryu F o r m a t i o n , No. iso-1 o f the Xiamaling F o r m a t i o n and Z 81030 and Z 8 1 0 2 8 of the Hongshuizhang Formation. T h e age values obtained from t he smallest grade (< 0.4 ~} material from these samples corresponds well with those inferred from the stratigraphy and have been a dopt e d for the middle and upper Prot erozoi c time scale in the Yanshan area. The second group includes f our samples: No. iso-15 o f the Chuanlinggou F o r m a t i o n and No. iso-14, Sn 79004 and No. iso-1 of the Changzhougou F o r matio n . Th e age values o f all size grades from these samples are at variance with those suggested by their stratigraphic position. In the first group there is a clear influence of terrigenous detritus on the potassium c o n t e n t and the age values of the sample. In o t h e r words, the variation in potassium c o n t e n t and the age values are d e p e n d e n t on the grain size of the samples. X-ray diffraction data show that a large a m o u n t o f quartz is present in coarse grained samples {> 1 ~) and little quartz is present in the fine grained sample (< 1 ~). As quartz is removed, the potassium c o n t e n t increases. Conversely, the age values become reduced as in sample No. iso-16 and No. iso-7. A few orthoclase grains were noted in sample Z 81030; on removal of these, the potassium c o n t e n t was reduced and the age value increased. Obviously in this group, the inherited isotopic system carried in clastic materials o f continental origin, has greatly influenced the age obtained. After removal o f such material a real age value can be obtained. F o r the second group, the age values o f all size grades are far y o u n g e r than the ex p ect ed age. T he influence of the inherited isotopic system is greatly reduced. All samples appear to have suffered a y o u n g e r geological event. Whether the potassium c o n t e n t changes with grain size or not, the age values o f the different grain sizes are all a p p r o x i m a t e l y the same. This suggests that a high degree o f homogenization has taken place and a new isotopic system was imposed. T h e r e f o r e t he obtained age represents the time o f a geological event. T he fact that the potassium cont ent s and age values from four different grain sizes are nearly the same in sample Sn79004 shows that the degree o f homogenization is quite high. T he difference b e tween the oldest and the youngest age values is 29 Ma and the average value is 1113 Ma. This age probably reflects the time of the geological event, b u t there is no o t h e r evidence suggesting that a significant geological event occurred at that time. The nearest geological event was the intrusion of basic dykes at 1150- - 1250 Ma ago. The ages of the d y k e and of 2M illite from strata near the d y k e are 1220 and 1185 Ma, respectively. The Rb--Sr isochron age o f the same strata is about 1220 Ma and t h e y yielded a Pb-Pb isochron age of 1848+ 58 Ma (Zhang and Li, 1982). The Pb--Pb isochron
180
age is regarded as the real age of the strata (Changzhougou Formation). The age of the dyke and 2M illite represent the time of the younger event. During that period, new K--At and Rb--Sr isotopic systems were established. Part of the argon may have been lost so that the K--Ar age is slightly younger than the event. The age values of three size grades from sample No. iso-1 from the Chuanlinggou Formation are between 1200 and 1300 Ma. The geological event mentioned above may have affected this sample but the difference between the oldest age value and the youngest is 74 Ma. The degree o f homogenization was not as high as that in sample Sn79004. In samples No. iso-15 and No. iso-14, the differences between the oldest and youngest ages are more than 100 Ma with mean age values of 904 and 808 Ma, respectively. These t w o values are close to the time of Jixian movement in the Yanshan area. In other words,there are three isotopic systems in the coarse grained samples: inherited, c o n t e m p o r a n e o u s and re-set, and at least t w o systems in the fine grained samples. Thus the age values of the t w o samples are random and cover a wide range. In summary, the eight samples could be a c c o m m o d a t e d by two models. In model 1 the potassium content and age values o f the sample change systematically with the gram size. The age value of the smallest grade corresponds most closely to the true age of the strata. Typical examples are sample No. iso-6 and No. iso-7. In model 2 the potassium content and age value do not change, or only change slightly, with grain size. The age value is always younger than that expected from the strata and represents the time of a later geological event (e.g., sample Sn79004). The other samples fall between these two models. Model 1 is applicable in the upper Proterozoic and in the upper part of the middle Proterozoic; model 2 in the lower part o f the middle Proterozoic. The geological history of strata where model 2 applies is very different from that of strata with model 1. The strata with model 2 formed before the main geological event, e.g., sample Sn79004 from the Changzhougou Formation which was covered by a b o u t 8000 m o f strata, suffered a geological event with basic dyke intrusion a b o u t 1150--1250 Ma ago. So the influence o f the re-set isotopic system is quite strong. After the main geological event, for instance sample No. iso-1 from the Xiamaling Formation, covered b y a b o u t 250 m of younSer strata, had a simple geological history. Consequently the influence of the inherited isotopic system was strong. Through the study o f isotopic age models for clay minerals in Precambrian sedimentary rocks, not only can the real ages of strata be obtained, but the geological history can be more fully understood. For these reasons the method o f using different grain sizes for radiometric dating is worth further study.
181 ACKNOWLEDGEMENTS ! a m i n d e b t e d t o Mr. C h e n J i n b i a o f o r his h e l p a n d I a c k n o w l e d g e t h e assistance given by my colleagues: Yu Rongbing, Wang Ruzheng, Xu Caiyoa, Liu Xiuqin, Yang Yunying, Zhao Zhi and others.
REFERENCES Bonhomme, M.G., 1982. The use of Rb--Sr and K--Ar dating methods as a stratigraphic tool applied to sedimentary rocks and minerals. Precambrian Res., 18: 5--25. Bonhomme, M.G., Elsass, F. and Mosser, C., 1978. Argon isotopic geochemistry in clays. The influence of an inherited fraction. In: Short paps. 4th Int. Conf. Geochronology, Cosmochronology, Isotope Geology, U.S. Geol. Sure. 46-47. Chen Jinbiao, Zhang Huimin, Zhu Shixing, Zhao Zhen and Wang Zhengang, 1980. Research on Sinian Suberathem of Jixian, Tianjin. In: Research on Precambrian geology Sinian Suberathem in China, pp. 56--114. (in Chinese, English abstract). Clauer, N., 1979. A new approach to Rb--Sr dating of sedimentary rocks. In: E. J~ger and J.C. Hunziker (Editors), Lectures in Isotope Geology, Springer-Verlag, Berlin, Heidelberg, New York, pp. 30--51. Clauer, N., 1981. Rb--Sr and K--At dating of Precambrian clays and glauconites. Precambrian Res., 15: 331--352. Zhang Xueqi and Li Shunzhi, 1982. The geochronological problems of the phosphoric strata of Dongjiao Type in the middle Taihangshan mountains. Bull. Tianjin Inst. Geol. Min. Res., 5 : 6 9 - - 7 4 (in Chinese, English abstract).
175
THE RELATIONSHIP BETWEEN K--Ar AGES AND GRAIN SIZE IN MIDDLE AND UPPER PROTEROZOIC SHALES FROM THE YANSHAN AREA, NORTH CHINA
ZHANG XUEQI
Department of Isotope Geology, Tianjin Institute of Geology and Mineral Resources. Tianjin 300170 (People's Republic of China) (Received May 16, 1984; revision accepted August 14, 1984)
ABSTRACT
Zhang, X., 1985. The relationship between K--Ar ages and grain size in middle and upper Proterozoic shales from the Yanshan area, North China. Precambrian Res., 29: 175--181. Eight shale samples from the middle and upper Proterozoic succession of the Yanshan area of North China were dated using the K--Ar method. Each sample was divided into four grades of different grain size: > 2 u, 2--1 u, 1--0.4 u and < 0.4 u. On the basis of the results, the eight samples were classified into two models. In model 1 the potassium content and the age values show a regular change with different grain sizes. The age of the finest grains (< 0.4 u) may represent the age of sedimentation and the age of coarser grains shows the influence of an isotopic system inherited from the terrigenous source. In model 2 the ages yielded by materials of any grain size are always younger than that considered reasonable for these strata and show the influence of a re-set isotopic system. The relationships among the three isotopic systems, here called inherited, contemporaneous and re-set, are discussed in an attempt to explain their geological meaning.
INTRODUCTION
T h e d a t i n g o f s h a l e s is a d i f f i c u l t b u t i n t e r e s t i n g p r o j e c t f o r g e o c h r o n o l o gists. M u c h p r o g r e s s h a s b e e n m a d e in t h i s f i e l d r e c e n t l y ( B o n h o m m e , 1982). Bonhomme e t al. ( 1 9 7 8 ) s t u d i e d t h e e f f e c t o f an i n h e r i t e d a r g o n component on isotopic age and dated samples of Phanerozoic rocks from France. Clauer (1979) systematically summarized the new approach to R b - - S r d a t i n g o f s h a l e s . C l a u e r ( 1 9 8 1 } e x p l a i n e d in d e t a i l t h e p r i n c i p l e s of Rb--Sr and K--Ar dating of clay minerals and glauconite and gave examples from the middle and upper Proterozoic of Mauritania. This paper d i s c u s s e s t h e r e s u l t s o f K - - A r d a t i n g o f d i f f e r e n t g r a i n sizes f r o m s h a l e s o f m i d d l e a n d u p p e r P r o t e r o z o i c r o c k s o f t h e Y a n s h a n a r e a in H e b e i P r o v ince, North China. M i d d l e a n d u p p e r P r o t e r o z o i c r o c k s a r e w e l l e x p o s e d in t h e Y a n s h a n a r e a ; t h e s t r a t i g r a p h i c s u c c e s s i o n is w e l l u n d e r s t o o d . T h e s e a r e a s , p a r t i c u l a r l y
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
176
Lingyuan Xuanhua -t"
+ Kuancheng
J,'xian + +
Beijing "., 0
60km
[
Tainja'n
Fig. 1. Location o f samples in Yanshan area.
in Jixian C o u n t y (100 km north o f the city of Tianjin) are considered as the stratotype section for the middle and upper Proterozoic in North China (Chen et al., 1980) {Fig. 1). The Jixian section involves three systems and 11 formations consisting of 99% sedimentary rocks and 1% volcanics. It is a b o u t 9200 m thick. The sedimentary rocks consist of mudstones (8%), sandstones (18%} and carbonates (74%). Mudstones or shales occur mainly in the Jingeryu, Xiamaling, Tielin~ Hongshuizhuang, Chuanlinggou and Changzhougou Formations. Data for this paper were obtained from these formations {Fig. 2). Different amounts of terrigenous material are present in different formations. For example the average terrigenous content in shales of the Xiamaling, Chuanlinggou and Hongshuizhuang Formations is about 41, 30 and 20%, respectively. Quartz, mica and feldspar are reported as the main terrigenous fraction in these formations. It is probable that there are t w o isotopic systems (or radioactive clocks) in the shales. One is an inherited isotopic system, carried by the terrigenous fraction. Another was formed during sedimentation or diagenesis and is called the contemporaneous isotopic system. Only the latter can show the real age of the strata. If the t w o kinds of isotopic system are thoroughly homogenised during a geological event, a new isotopic system will be formed, called the re-set isotopic system and the age value represents the time of the geological event. If homogenisation is partial then the three kinds of isotopic system can exist within the same rock and the age obtained is random and has no geological meaning. If the true sedimentary age of the rock is sought, then the inherited isotopic system must be removed. In geological terms the terrigenous fraction has to be removed. For instance, a large amount of detrital mica from
177
the Xiamaling Formation yielded a K--Ar age o f 1443 Ma. This age is much older than what is considered for the Xiamaling Formation and shows the presence of an inherited isotopic system. After removing this detritus, the next step is to determine whether the isotopic system has been homogenised by subsequent geological events. ] Subdivisions
Eons
of
the
scale
Rock types
Sample
No, s
J
I
/ J
System
F o r m a t ; on d~ngeryu
r~ r~
qingboikou
Xiomollng Tieling
JiXiOn L)
Hongshuizhuang
NO. iSO - 7 J -It
~
"It
No. iso
*
*
Z81030
-
16
t
*
Z81028
Wu mlshan
I
0
Yangzhuang
L
i
o
Gaoyuzhuang
Q_
Dahongyu !
Chongchen9
!
Tuanshanzi Chuonllngg°u
I, ~ ~ * ~ *
Chongzhougou
Fig. 2. Major shale o c c u r r e n c e s *--shales; b l a n k - - c a r b o n a t e rock.
No. i s o - 1 5 NO. ~ s o - 1 4 No. iso - 1 S n 7 9 0 0 4
in the stratigraphic s u c c e s s i o n o f the Yanshan area.
DATING DIFFERENT SIZE FRACTIONS OF SHALES
To study the influence of the terrigenous fraction on isotopic age and relationship between grain size and isotopic age, several samples (see Table I) were divided into different size grades. Taking into account such things as palaeogeographic setting, field observations and petrographic studies, shale samples were selected from locations where the palaeoenvironment was rather stable and the terrigenous detrital c o m p o n e n t of the sediments was relatively small. All samples were washed and dried at a low temperature, then crushed. Crushing was carried out to separate the coarser terrigenous detritus from the clay minerals. In general the material was not crushed finer than 100 mesh. Each sample was divided into four grades: > 2 #; 2--1 ~; 1--0.4 p; < 0.4 p. The different size fractions were dated separately. A large a m o u n t of terrigenous detritus is present in the samples > 1 p and some in the samples < 1 ~.
178 A n e l e c t r o n m i c r o s c o p e w a s u s e d t o c h e c k w h e t h e r t h e g r a m size o f e a c h s a m p l e c o r r e s p o n d e d t o t h e e x p e c t e d v a l u e s . T h i s is v e r y i m p o r t a n t in explaining the results of dating. B a s e d o n X - r a y d i f f r a c t i o n s t u d i e s , t h e c o m p o s i t i o n o f all t h e s a m p l e s u s e d in t h i s s t u d y is v e r y s i m p l e . T h e m a i n c o m p o n e n t is i l l i t e o f 1M t y p e . In s a m p l e N o . i s o - 1 6 , in a d d i t i o n t o 1M i l l i t e , s o m e s m e c t i t e c h l o r i t e is p r e s e n t a n d in Z 8 1 0 2 8 , l e n n i t e w a s d e t e c t e d . T h e K - - A r d a t a a r e g i v e n in T a b l e I. TABLE I K--At data from shale samples of the Yanshan area Samples
Grade (p)
K%
' ° A r / ' ° K 10 -6 g/g
Age (Ma)
No. iso-7 Jingeryu Fro.
> 2 1--2 1-0.4 < 0.4
4.78 5.04 5.30 5.20
0.0768 0.0745 0.0686 0.0635
999 969 908 855
Lingyuan
No. iso-16 Xiamaling Fm.
> 2 1--2 1-0.4 < 0.4
4.14 4.66 5.16 5.31
0.1104 0.1043 0.0772 0.0732
1289 1246 996 956
Xuanhua
Z81030 Honphu~huang Fm.
> 2 1--2 1-0.4 < 0.4
5.60 5.60
0.0875 0.0877
1114 1116
Jixian
4.40
0.0980
1191
Z81028 Hongshumhuang FrrL
> 2 1--2 1-0.4 < 0.4
5.52
0.1037
1240
4.40 4.52
0.1006
1221
No. mo-15 Chuanlinggou Fr~
> 2 1--2 1---0.4 < 0.4
10.00 8.80 7.44 6.98
0.0613 0.0611 0.0762 0.0749
831 829 983 973
No. iso-1 Changzhougou Frn.
> 2 1--2 1--0.4 < 0.4
9.74 8.06 7.60 7.16
0.1078 0.1047 0.1138 0.1102
1276 1250 1326 1297
No. iso-14 Changzhougou
> 2 1--2
3.18 3.92
0.0645 0.0578
865
4.92
0.0616
792 834
< 0.4
6.00
0.0532
741
Sn79004
> 2
6.20
0.0881
Changzhougou Fro.
1--2 1----0.4 < 0.4
6.31 6.18 6.20
0.0893 0.0912 0.0890
1101 1112 1130 1109
Fm.
1--0.4
Location
Jixian
Xuanhua
Qianan
Xuanhua
Kuancheng
179 DISCUSSION A statistically significant correlation exists between the isotopic age and grain size. T he samples shown in Table I are divided into two groups. T h e first group includes No. iso-7 of the Jingeryu F o r m a t i o n , No. iso-1 o f the Xiamaling F o r m a t i o n and Z 81030 and Z 8 1 0 2 8 of the Hongshuizhang Formation. T h e age values obtained from t he smallest grade (< 0.4 ~} material from these samples corresponds well with those inferred from the stratigraphy and have been a dopt e d for the middle and upper Prot erozoi c time scale in the Yanshan area. The second group includes f our samples: No. iso-15 o f the Chuanlinggou F o r m a t i o n and No. iso-14, Sn 79004 and No. iso-1 of the Changzhougou F o r matio n . Th e age values o f all size grades from these samples are at variance with those suggested by their stratigraphic position. In the first group there is a clear influence of terrigenous detritus on the potassium c o n t e n t and the age values of the sample. In o t h e r words, the variation in potassium c o n t e n t and the age values are d e p e n d e n t on the grain size of the samples. X-ray diffraction data show that a large a m o u n t o f quartz is present in coarse grained samples {> 1 ~) and little quartz is present in the fine grained sample (< 1 ~). As quartz is removed, the potassium c o n t e n t increases. Conversely, the age values become reduced as in sample No. iso-16 and No. iso-7. A few orthoclase grains were noted in sample Z 81030; on removal of these, the potassium c o n t e n t was reduced and the age value increased. Obviously in this group, the inherited isotopic system carried in clastic materials o f continental origin, has greatly influenced the age obtained. After removal o f such material a real age value can be obtained. F o r the second group, the age values o f all size grades are far y o u n g e r than the ex p ect ed age. T he influence of the inherited isotopic system is greatly reduced. All samples appear to have suffered a y o u n g e r geological event. Whether the potassium c o n t e n t changes with grain size or not, the age values o f the different grain sizes are all a p p r o x i m a t e l y the same. This suggests that a high degree o f homogenization has taken place and a new isotopic system was imposed. T h e r e f o r e t he obtained age represents the time o f a geological event. T he fact that the potassium cont ent s and age values from four different grain sizes are nearly the same in sample Sn79004 shows that the degree o f homogenization is quite high. T he difference b e tween the oldest and the youngest age values is 29 Ma and the average value is 1113 Ma. This age probably reflects the time of the geological event, b u t there is no o t h e r evidence suggesting that a significant geological event occurred at that time. The nearest geological event was the intrusion of basic dykes at 1150- - 1250 Ma ago. The ages of the d y k e and of 2M illite from strata near the d y k e are 1220 and 1185 Ma, respectively. The Rb--Sr isochron age o f the same strata is about 1220 Ma and t h e y yielded a Pb-Pb isochron age of 1848+ 58 Ma (Zhang and Li, 1982). The Pb--Pb isochron
180
age is regarded as the real age of the strata (Changzhougou Formation). The age of the dyke and 2M illite represent the time of the younger event. During that period, new K--At and Rb--Sr isotopic systems were established. Part of the argon may have been lost so that the K--Ar age is slightly younger than the event. The age values of three size grades from sample No. iso-1 from the Chuanlinggou Formation are between 1200 and 1300 Ma. The geological event mentioned above may have affected this sample but the difference between the oldest age value and the youngest is 74 Ma. The degree o f homogenization was not as high as that in sample Sn79004. In samples No. iso-15 and No. iso-14, the differences between the oldest and youngest ages are more than 100 Ma with mean age values of 904 and 808 Ma, respectively. These t w o values are close to the time of Jixian movement in the Yanshan area. In other words,there are three isotopic systems in the coarse grained samples: inherited, c o n t e m p o r a n e o u s and re-set, and at least t w o systems in the fine grained samples. Thus the age values of the t w o samples are random and cover a wide range. In summary, the eight samples could be a c c o m m o d a t e d by two models. In model 1 the potassium content and age values o f the sample change systematically with the gram size. The age value of the smallest grade corresponds most closely to the true age of the strata. Typical examples are sample No. iso-6 and No. iso-7. In model 2 the potassium content and age value do not change, or only change slightly, with grain size. The age value is always younger than that expected from the strata and represents the time of a later geological event (e.g., sample Sn79004). The other samples fall between these two models. Model 1 is applicable in the upper Proterozoic and in the upper part of the middle Proterozoic; model 2 in the lower part o f the middle Proterozoic. The geological history of strata where model 2 applies is very different from that of strata with model 1. The strata with model 2 formed before the main geological event, e.g., sample Sn79004 from the Changzhougou Formation which was covered by a b o u t 8000 m o f strata, suffered a geological event with basic dyke intrusion a b o u t 1150--1250 Ma ago. So the influence o f the re-set isotopic system is quite strong. After the main geological event, for instance sample No. iso-1 from the Xiamaling Formation, covered b y a b o u t 250 m of younSer strata, had a simple geological history. Consequently the influence of the inherited isotopic system was strong. Through the study o f isotopic age models for clay minerals in Precambrian sedimentary rocks, not only can the real ages of strata be obtained, but the geological history can be more fully understood. For these reasons the method o f using different grain sizes for radiometric dating is worth further study.
181 ACKNOWLEDGEMENTS ! a m i n d e b t e d t o Mr. C h e n J i n b i a o f o r his h e l p a n d I a c k n o w l e d g e t h e assistance given by my colleagues: Yu Rongbing, Wang Ruzheng, Xu Caiyoa, Liu Xiuqin, Yang Yunying, Zhao Zhi and others.
REFERENCES Bonhomme, M.G., 1982. The use of Rb--Sr and K--Ar dating methods as a stratigraphic tool applied to sedimentary rocks and minerals. Precambrian Res., 18: 5--25. Bonhomme, M.G., Elsass, F. and Mosser, C., 1978. Argon isotopic geochemistry in clays. The influence of an inherited fraction. In: Short paps. 4th Int. Conf. Geochronology, Cosmochronology, Isotope Geology, U.S. Geol. Sure. 46-47. Chen Jinbiao, Zhang Huimin, Zhu Shixing, Zhao Zhen and Wang Zhengang, 1980. Research on Sinian Suberathem of Jixian, Tianjin. In: Research on Precambrian geology Sinian Suberathem in China, pp. 56--114. (in Chinese, English abstract). Clauer, N., 1979. A new approach to Rb--Sr dating of sedimentary rocks. In: E. J~ger and J.C. Hunziker (Editors), Lectures in Isotope Geology, Springer-Verlag, Berlin, Heidelberg, New York, pp. 30--51. Clauer, N., 1981. Rb--Sr and K--At dating of Precambrian clays and glauconites. Precambrian Res., 15: 331--352. Zhang Xueqi and Li Shunzhi, 1982. The geochronological problems of the phosphoric strata of Dongjiao Type in the middle Taihangshan mountains. Bull. Tianjin Inst. Geol. Min. Res., 5 : 6 9 - - 7 4 (in Chinese, English abstract).