I. Introduction
Kuruktag area is located in the northeast margin of Tarim Basin, and belongs to the uplift belt of Tarim block (Xinjiang Bureau of Geology and Mineral Resources, hereinafter referred to as Xinjiang Bureau of Geology and Mineral Resources, 1993). Precambrian basement is widely exposed, and the oldest is the Tuoge complex of deep metamorphic TTG series, which is covered by the unconformity of medium-deep metamorphic supracrustal rocks. In deep metamorphic rocks and granites, there are thousands of intermediate-basic dike groups with high density and alternating black and white, which form a special landform and look like zebras, so they are called "zebra" dike groups in the field (Xinjiang Bureau of Geology and Mineral Resources, 1993). These diabase walls have never been studied in detail in rock geochemistry and isotopic chronology. This book focuses on the systematic study of representative dykes in Kuokesutage area from the perspective of rock geochemistry and isotopic geochronology, in an attempt to provide geochronology and rock geochemistry basis for the study of tectonic evolution in this area.
Two. Petrological and petrochemical characteristics
The Kuokesutage basic dyke group is located on the north and south sides of Xingdi Fault (Figure 1- 1- 1), which is mainly composed of a series of diabase dykes with approximately equal intervals. The strike of rock wall group is about 330, almost vertical or slightly inclined to the southeast, with an inclination of about 78. A single dike is tens of centimeters to several meters wide and tens of meters to several hundred meters long. The middle part of the dike is of medium-fine grain crystal structure, and the edge has obvious condensation edge. The width of the condensation edge is proportional to the width of the dam, ranging from several centimeters to more than ten centimeters. The dike swarms intrude into Precambrian metamorphic rocks and granites, with straight boundaries and the characteristics of local tracking tension. Systematic thin section observation shows that the rocks forming the condensation edge of rock wall groups are generally homogeneous medium-grained semi-columnar and granular structures, and the mineral particles in the center of rock wall are coarse, mostly in sub-glow green structure. The main minerals that make up rocks are almost equal amounts of augite and plagioclase. Ordinary pyroxene is generally amphibole and epidote, while plagioclase is mostly weakly sericitized. In addition, it contains a small amount of chlorite and titanomagnetite, so rock samples are generally fresh.
Figure 1- 1- 1 Geological Schematic Diagram of Basic Rock Wall Groups in Kuokesutage Area
(Revised according to the geological map of Kuoksu1:200,000)
1- four yuan; 2-Yuangu language; 3- Taikoo Yu; 4- Granite rock mass; 5- rock wall formation; 6- Fault; 7— Sampling point
See table 1- 1- 1 for the chemical analysis results of main elements of rock wall. The percentage content characteristics of main oxides are as follows: ① The content of ①①SiO 2 is 40.68% ~ 53.34%, which belongs to basic rocks as a whole. ② The total iron content is 8.47% ~ 14.52%, with an average of 10.04%. ③ The total alkali content is 3.50% ~ 5.85%, with an average of 4.72%, in which w(K2O)/w(Na2O) is between 0.29 and 0.50. The variation range of TiO _ 2 is 1.07%-3.2%, generally 1.07%- 1.5%.
Irvine & Barragar (1971) discriminant diagram (figure 1- 1-2) is used to discriminate the rock series properties of the sample. The results show that all the samples except one belong to alkaline rock series. Generally speaking, subalkaline series can be further divided into calc-alkaline series and tholeiite series. Therefore, the above subalkaline samples were put into AFM diagram (Irvine&Baragar, 197 1) to further determine their properties. The mapping results show that (Figure 1- 1-3) belongs to the calc-alkaline series as a whole. Therefore, the dike rocks in this area generally belong to subalkaline series and have the chemical composition characteristics of calc-alkaline series.
Table 1- 1- 1 major element analysis results of basic rock wall group samples in kuokesutage (wB/%)
Note: 1 ~ 6 in the table was tested by the National Geological Experimental Testing Center of the Ministry of Geology and Mineral Resources; 7 ~ 9 are all quoted from (Xinjiang Bureau of Geology and Mineral Resources, 1993).
Fig.1-1-2 (Na2O+K2O)-SiO2 magmatic rock series discrimination diagram
(According to Owen & Baraga, 197 1)
Alk—- alkaline series; Subalkaline-Subalkaline Series
Fig.1-1-3feo *-(Na2O+K2O)-MgO magma series discrimination diagram
(According to Owen & Baraga, 197 1)
Th- tholeiite series; Calcium-calcium alkaline series
Three. Characteristics of Rare Earth and Trace Elements
The analysis results of trace elements and rare earth elements in rock wall are listed in table 1- 1-2. From the analysis results, it can be seen that the elements such as Sr, Ba, Ce, Zr, Sm are relatively rich relative to chondrite (Boynton, 1984) or N-type MORB(Pearce, 1984), while P, Ti, Y and Yb are relatively rich relative to N-type.
Table 1- 1-2 Abundance of Trace Elements and Rare Earth Elements in Rocks of Dike Group (wB/ 10-6)
Note: The data in the table were tested by the National Geological Experimental Testing Center of the former Ministry of Geology and Mineral Resources.
The REE abundance of all samples and their chondrite normalized distribution curves show that the light REE is obviously enriched (La/Yb) n = 2.44 ~ 32.99, generally between 4 ~ 6; Except TG38-6 and TG38-7, all other samples have slight Eu loss (Δ EU = 0.71~ 0.84), which generally indicates that the primary magma has been separated and crystallized with plagioclase as the main crystal phase. It shows typical rare earth partition characteristics of alkaline or calc-alkaline basalt.
Figure 1- 1-4 Standardized distribution model of rare earth chondrites in the rocks of the Kuokesutage basic dyke group.
Fourth, the formation of the times.
Potassium and argon are measured with the same sample, and the sample is sampled by shrinking method to ensure the consistency of the sample as much as possible. Potassium was measured by flame photometer in lithium internal standard and sodium buffer solution. The repeatability of repeated determination is very good, and the relative error is generally less than 65438 0%.
Ar was determined by isotope dilution method. Put the sample into a degassed molybdenum crucible, put it into a system for extracting Ar, vacuum-pump, bake at a constant temperature of 200℃ overnight, and adsorb the released gas with molecular sieve. The whole extraction system was baked to 450℃ and evacuated by diffusion pump. The samples were melted by high frequency induction heating system, and purified by sponge titanium furnace, Cu-CuO furnace and zeolite produced by Pertersen company. The isotopic composition of argon is determined by RGA 10 mass spectrometer produced by VSS Company, and equipped with molecular pump for vacuum pumping. Vacuum conditions: the system vacuum is (6 ~ 7) × 10-7pa, and the mass spectrometer vacuum is (4 ~ 5) × 10-7pa. Background level: 40ar = (1.7 ~ 3.5 )×10-13mol, 38ar = (2.7 ~ 5.4 )×10/4mol, 36ar.
Table 1- 1-3 K-Ar diabase wall isochron dating results
Note: Analyst: K-Ar Isotope Analysis Room of Peking University Geology Department; Sample quality refers to the sample quality used for argon gas measurement.
The dike swarms in this area intruded into Precambrian geological bodies, and there is no direct evidence of geological conditions of its formation age. Therefore, four samples with weak alteration were selected for routine K-Ar dating, and their apparent ages were scattered. By using K-Ar isochron technique (Mu Zhiguo, 1990), a good isochron with linear correlation coefficient of 0.985 1 (Figure1-kloc-0/5) is obtained, with an isochron age of 282.35Ma and an initial ratio of 40Ar/36Ar. ISOPLOT program is used to process analysis data. The isochron age is (282 65438 05) Ma, and the confidence is 95%. The initial value of 40Ar/36Ar is 508. 1. The initial value is far from the modern atmospheric value (295.5), which leads to the deviation of the apparent age calculated by the modern atmospheric value, resulting in the apparent age of 455.2 ~ 673. 1ma. This is also in line with the geological fact that it occurred in deep geological bodies, that is, the emplacement is deep and the magma forming the rock wall is deep, which leads to the existence of excess argon, making the apparent age obviously older than its true emplacement age.
Figure1-1-5 (40ar/36ar)-(40k/36ar) Diabase wall isochron map of Kuokesutage area.
The isochron age represents the time after the rock reaches the argon closed system. The sealing temperature of basic rocks to argon is high, and the thickness of dikes there is small, and there are generally condensation edges, which indicates that the cooling rate is faster after magma intrusion. Therefore, the time from magma intrusion to condensation crystallization to argon sealing is not long, and the K-Ar isochron age can be used as the diagenetic age.
Verb (abbreviation for verb) Isotopic characteristics of helium and argon.
Helium isotope is determined in inert gas isotope laboratory of Institute of Deposit Geology, Chinese Academy of Geological Sciences. See related literature (Li Yanhe et al., 1997) for analysis method. Diabase used for He isotope analysis is all fresh whole rock samples, which are broken into small particles of about 6mm, and each sample weighs 500 ~ 800 mg. The sample was degassed by heating at 200℃ for 30 minutes and melted at 1500℃ for 40 minutes, so that the sample was completely melted and decomposed. The released gas is purified by sponge titanium pump and activated carbon cold trap for four times, and active gases and organic substances such as H2, N2, O2, CO2, CH4 and H2O are frozen and adsorbed. Pure He and Ne enter the analysis system. Trace impurity gases, such as H2 and Ar, which enter the analysis system together with he and Ne, are purified and removed again by titanium sublimation pump and liquid nitrogen. Helium isotope was measured by MI- 1200 1 IG inert gas mass spectrometer made in Ukraine. 4He is received by Faraday cup, and 3He is received by electron multiplier. The resolution of the multiplier is adjusted to 1200, so that 3He and HD+H3 peaks are completely separated without HD+H3 correction. Measure the standard gas before analyzing the sample, and calculate according to the measurement result of the standard gas. The working standard is the atmosphere in Beijing, and the 3He/4He value is 1.40× 10-6. The blank value of 4He is 2.129×10-11cm3stp, so it is generally unnecessary to correct the blank value of 4He. The measurement accuracy of the sample is 1% ~ 10%. The results are listed in table 1- 1-4.
The 3He/4He value of diabase in Kuluktag Kuokesu area, Xinjiang has little change, ranging from (2.03 ~ 7. 1) × 10-7. The value of 3he/4He is obviously greater than that of radioactive source and far less than that of mantle, which indicates that He isotope in rocks is not a single radioactive source. The 3He value changed little, ranging from (2.40 ~ 9.30) × 10- 12, and 3He/4He value changed even less, ranging from (1.09 ~1.41)×/kloc.
Table 1- 1-4 He and Ar test results of basic dikes
Note: Ar isotopes are determined by Liu Yulin of Peking University K-Ar Isotope Laboratory; Song, Song and Song of Inert Gas Isotope Laboratory, Institute of Deposit Geology, Chinese Academy of Geological Sciences, determined helium isotopes.
Fig. 1- 1-6 Helium Isotopic Composition of Diabase
P- primitive helium; M- mantle helium; R- radioactive helium; ☆— Altai diabase; ○—— The results in this respect
Table 1- 1-4 shows that the range of 40Ar/39Ar value is 803- 12 14, which shows that there is an obvious excess of 40Ar relative to air. 36Ar has little change, ranging from (1.60 ~ 3.29) × 10-8. The concentration of 40Ar has similar characteristics, and the range of variation is (1.94 ~ 3.58) × 10-5. Therefore, the influence of air pollution in the analysis process can be ruled out.
The 3He/36Ar value of diabase in this area is very low, ranging from (0.88 ~ 3.60) × 10-4, which is different from the 3He/36Ar value of lherzolite xenoliths in the Cenozoic basalts of Hannuoba [(0.14 ~1.24] The 3He/36Ar value is considered as the original isotope, and the rare gases in the mantle that are not radioactive are the original gases trapped in the process of earth material accumulation. The value of 3He/36Ar in all circles of the earth varies greatly, and there is no fixed value, which is related to the degassing of the earth and the source of rare gases. The 3He/36Ar value of diabase in Kuruktag area is very low, far less than the estimated 3He/36Ar value of mantle (1)(O'Nions et al., 1994), which may be due to the preferential loss of 3He in the transformation of diabase in the late stage of formation. 4He and 40Ar are radioactive, so it is difficult to give the 4He/40Ar characteristic values of the mantle at present. The 4He/40Ar value of Kuluktag diabase is 0.40 ~ 0.63, which is close to the estimated 4He/40Ar value of 2 ~ 3 upper mantle (O 'Nions et al., 1994).
It can be seen from the (3he/4He)-(40Ar/36Ar) diagram (figure1-kloc-0/-7) that the isotopic characteristics of He and Ar are the result of the mixing of the original mantle and radioactive sources, because the data points are basically distributed near the P-R mixing line.
Fig.1-1-7 (3he/4he)-(40ar/36ar) diagram
P- mantle plume; Air; M- mid-ocean ridge mantle; R- radioactive source; C- crust; ○—— The results in this respect
He and Ar isotopic geochemical data of diabase in Kuruktag area indicate that its magma originated from the mantle. At present, the isotopic composition of helium shows that it is a mixture of mantle-derived helium and radioactive helium or crustal helium. The initial value of argon isotope is less concerned, mainly because: first, the existence of initial argon is recognized late; Second, the initial value of argon varies greatly. Kaneoka & Takaoka (1985) studied the initial values of 40Ar/36Ar and 3He/4He from different sources, and distinguished four sources: mid-ocean ridge basalt (MORB), mantle plume, continental crust and atmosphere. The reference values of 3He/4He and 40Ar/36Ar of the four end-member components are 1. 1× 10-5 and 2× 104, respectively. 6× 10-5、350; 4× 10-7、 1500; 1.4× 10-6、295.5。
The initial value of 40Ar/36Ar of basic dike in Kuruktag area is 507, which is the closest to that of mantle plume. It may be that the magma caused by mantle plume was polluted by crustal materials during upwelling, and the initial value was improved.
According to the analysis of main elements, trace elements and their rare earth elements in rock samples (Zhang Zhicheng et al., 1998), the basic dikes in this area are calc-alkaline series basalts with low Al2O3, high FeO* and CaO. Light rare earth elements and large ion pro-MagmaElemental elements are obviously enriched, while some transition metal elements are lacking. It also reflects that a large number of continental crustal materials have been melted and replenished.
To sum up, the Kuruktag basic dyke group has mantle plume origin, which may be related to early Permian rifting in Tarim and Tianshan tectonic belts. At the same time, it is also suggested that rifting in the above areas may be controlled by deeper tectonic activities and may be the result of the influence of crust-mantle boundary activities.
Constructive meaning of intransitive verbs
According to the analysis of main elements, trace elements and their rare earth elements in rock samples, the basic rock walls in this area are calc-alkaline series basalts with low Al2O3, high FeO* and CaO. Light rare earth elements and large ion pro-MagmaElemental elements are obviously enriched, while some transition metal elements are lacking.
The 3He/4He value of diabase is (2.03 ~ 7. 1) × 10-7. The value of 3he/4He is obviously greater than that of radioactive source and far less than that of mantle, which indicates that He isotope in rocks is not a single radioactive source. The range of 40Ar/36Ar value of dike group is 803 ~ 12 14, which has obvious 40Ar surplus compared with air. 36Ar has little change, ranging from (1.60 ~ 3.29) × 10-8. The concentration of 40Ar has similar characteristics, ranging from (1.94 ~ 3.58) × 10-5, and the initial ratio of 40Ar/36Ar is 507. The high initial ratio of 40Ar/36Ar is obviously higher than atmospheric argon, higher than mantle plume type (P type) and lower than MORB type (M type), which reflects the information of deep-source argon (Kaneoka&Takaoka, 1985), thus revealing that the magma of basic dikes may come from mantle. He and Ar isotopic characteristics of diabase in Kuruktag area are the result of the mixing of primitive mantle and radioactive source or crust source, which may be related to early Permian rifting in Tarim and Tianshan tectonic belts. At the same time, it also implies that rifting in the above areas may be controlled by deeper tectonic activities.
The basic dike group is the product of large-scale extensional tectonism (Fahrig,1987; Chen Xiaode et al., 1994), the consolidation depth is generally 5 ~ 15 km (Chen Xiaode et al., 1983), that is, the middle crust, which is strictly controlled by the tectonic stress field. The determination of early Permian basic dike swarms reflects the existence of early Permian large-scale extensional structures in this area. It is also consistent with the development time of basic volcanic rocks and dike swarms in other areas of Tarim Basin during this period (Yang Shufeng et al., 1996). It shows that the whole northern Tarim basin experienced a large-scale extension at the end of late Paleozoic. The extension in this period may be related to the post-extension under the background of the uplift of Tianshan orogeny at the end of Paleozoic. At the same time, it also shows that the Kuruktag area has risen by 5 ~ 15 km since the early Permian, which is consistent with the geological evidence that the Precambrian basement in this area is widely exposed.
refer to
Chen Xiaode, Shi Lanbin. 1983. Preliminary study on Wutai-Taihang diabase wall group. Science bulletin, 28 (16):1002 ~1005.
Chen Xiaode, Shi Lanbin. 1994. Basic dikes in extensional structures. In: Qian Xianglin. Study on extensional structure. Beijing: Geological Publishing House, 7 1 ~ 74.
, Zhang,,, etc. Helium isotopic composition of eclogite in Dabie Mountains and its geological significance [J]. Science Bulletin, 43 (4): 43 1 ~ 434.
, Song, et al 1997. Helium isotope characteristics of eclogite in Dabie-Sulu area and its geological significance [J]. Journal of Earth Sciences, 18 (Suppl.): 77 ~ 79.
Liu, Xu Yongchang. Isotopic characteristics and gas source tracing of natural gas in Ordos Basin [J]. Science Bulletin, 46 (22): 1902 ~ 1905.
Liu Yulin, Guo, and so on. K-Ar isochron dating of Kuluktag basic dike group and discussion on related issues [J]. geological journal of china universities, China, 5 (1): 54 ~ 58.
Mu Zhiguo. 1990.K-Ar isochron dating method and its application. Geological Science, Vol.25, No.4: 367 ~ 376.
, leaves,,. 1997. He and Ar isotopic characteristics of eclogite in Dabie Mountains [J]. Journal of Geology, 18 (supplement). ): 80 ~ 82.
Wang, Chen Yuchuan, Li Hongyang, et al. Helium isotope study on mantle degassing in Altai orogenic belt [J]. Science Bulletin, 43 (23): 254 1 ~ 2544.
Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region. Regional Geology of Xinjiang Uygur Autonomous Region, Beijing: Geological Publishing House, 1 ~ 20.
Xu Sheng, Liu Congqiang. 1997. Helium isotopic composition and mantle geochemical evolution model of mantle xenoliths in eastern China [J]. Science Bulletin, 42 (11):1190 ~1/93.
Xu, and so on in 2003. He and Ar isotopic characteristics of mantle megacrysts in Nvshan and Yingfengling and their geological significance [J]. Science Bulletin, 48 (10):1087 ~1091.
Yang Shufeng, Chen Hanlin, Dong, et al. Relationship between late Paleozoic volcanic rock distribution and oil and gas evolution in Tarim basin. See: edited by Tong, Liang Digang and Jia Chengzao. New progress in petroleum geology research in tarim basin. Beijing: Science Press, 150 ~ 158.
Anderson D. L.1998 Helium paradox. Geophysics, 95(9):4822~4827
Boynton West V. 1984. Cosmic chemistry of rare earth elements: meteorite research. In: Geochemistry of Rare Earth Elements edited by Henderson. Elsevier, 9 1
Curtis G. H.1981. Guide to dating methods for determining the last time of faults. National Technical Information Service Center, Washington, 93~ 1 14.
fah rig W f 1987。 Tectonic setting of continental mafic dike swarms: fault arm and early passive edge. In: Halls H C and Farig W F (eds.), mafic dykes. Geological Association Specification Papers 34, 33 1~348
Hanan B B, Graham D W. 1996. Lead and helium isotopic evidence from marine basalt: common deep source of mantle plume. Science, 272(5264):99 1~995
Harper Jr, Stein B et al .. 1996. Rare gas and earth accretion. Science, 273 (5283):1814 ~1818
Hilton D R, Hammerschmidt K, Locke G, et al ..1993. Helium and argon isotopic systematics in the central Lau Basin and Valu Fa Ridge: evidence of crust-mantle interaction in back-arc basins [J]. Journal of Geochemistry and Cosmology, 57 (12): 2819 ~.
Hilton D R, Hammerschmidt, Teufel S et al .. 1993. Helium Isotopic Characteristics of Geothermal Fluids and Lava in Andes [J]. Geoscience Newsletter, 120:265~282.
irine T N & Baragar W R N 197 1。 Guide to chemical classification of ordinary volcanic rocks. Journal of Earth Science. ,8:523~548
On the K, Qiaojin K and Matsuda J. I. 1998. Constraints of inert gases on the evolution of the atmosphere-mantle system [J]. Journal of Geochemistry and Cosmology, 62 (13): 2311~ 2322
Kaneoka I & Gorkon N. 1985. The state of inert gas inside the earth-some restrictions on the current state. Chemical Geology (Isotope Earth Science), 52:75~95
Li B and Manul O K. 1994. Inert gas identification technology of mantle and crustal materials and its application in sunspot deposits [J]. Acta Geochemistry, 28:47~69
Marty B，Upton B G J，Ellam R M. 1998。 Helium Isotopes in Paleogene Basalt in Northeast Greenland: Evidence for 58Ma Mantle Plume Activity in Volcanic Area of North Atlantic ~ Iceland [J]. Geology, 26(5):407~4 10
O' Nions R K, Tolstihin I 1994. Behavior and residence time of lithophiles and rare tracers in the upper mantle [J].EPSL,124 (1/4):131~138.
Pierce judicial district 1984. "Guide to Use" for basalt identification diagram. Foreign geology, (11):1~12.
, Guo, Liu Shuwen. Age and tectonic significance of mafic dykes in Kuruktag area, Xinjiang [J]. Journal of Geology, 72( 1):29~36
(Guo Liu Yulin)