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A Group of Rifting Events in the Terminal Paleoproterozoic in the North China Craton
Gondwana Research, Y 5, No. I, pp. 123-131. 0 2002 lnternational Association for Gondwana Research, Japan. ISSN: 1342-93ix
Gondwana Research
A Group of Rifting Events in the Terminal Paleoproterozoic in the North China Craton Lu Songnian, Yang Chunliang, Li Huaikun and Li Humin Tianjin Institute of Geology and Mineral Resources, China (Manuscript received February 26,2001; accepted May 21,2001)
Abstract Rifting events of the terminal Paleoproterozoic are well preserved in the North China Craton. They include an aulacogen and cratonic marginal rift basins, mafic dyke swarms, two anorthosite suites and rapakivi granites. All of these events took place after metamorphism, deformation and magmatism of the late Paleoproterozoic between 2.0 Ga and 1.85 Ga (or 1.80 Ga). The rifting events belong to an anorogenic event group, reflecting breakup of a late Paleoproterozoic united North China Craton. An intracratonic rift basin in the center of the North China Craton is named the Yanshan-TaihangshanAulacogen. A stratigraphic thickness of 9193 m was deposited within the basin, including three groups and twelve formations with time range ca. 1.8 Ga to 0.85 Ga. The oldest (Changcheng) group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates, terrestrial clastic rocks, and shallow marine sandstone and carbonates with super-high-potassium basalt-trachyte. The marginal basins occur around the northern, western and southern margins of the North China Craton, respectively located in Langshan-Baiyun Obo, Helanshan Mountain, and the adjacent regions between Henan, Shanxi and Shaanxi Provinces. The stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part, with a thick volcanic sequence with U-Pb age of 18402 14 Ma exposed in the southern marginal regions. Based on stratigraphic sequences, isotopic data and volcanic features, the initial rifts took place at about 1.85 Ga to 1.80 Ga. Mafic dyke swarms are largely developed in central parts of the North China Craton, where they intrude Archean granulite and high-amphibolite rocks. A new single-zircon U-Pb isotopic age of 1769k 2.5 Ma has been reported from a mafic dyke in east Henshan Mountain, northwestern Shanxi Province. An anorthosite suite (AGMS) is exposed along two deep fault belts in northern Hebei Province. Isochron Ages of 16832 193 Ma by Rb-Sr and 1735k 239 Ma by Sm-Nd were measured from an anorthosite body. The rapakivi granites crop out along the Miyun-Qiangzilu-XinglongFault in an E-W direction, and are characterized by megacrysts of alkali feldspars mantled by plagioclase, thus showing many of the petrographic features of classic rapakivi granites worldwide. A typical rapakivi body preserved in Shachang of Miyun County, Beijing, has a U-Pb age of 1716f 31 Ma. Super-highpotassium volcanics located in the upper Changcheng Group have been dated at 16832 67 Ma and 1625f 6 Ma. The time range for all magmatic events is between 1.72 Ga and 1.62 Ga. Key words: Mafic dyke swarms, anorthosites, rifting, Paleoproterozoic, North China Craton.
Introduction The North China Craton was terminally stabilized between ca. 2.0 Ga to 1.85 Ga (or 1.80 Ga). According to Sun and Lu (1990),the Proterozoic Eon in the North China Craton contains four important orogenies: about 2.5 Ga (Fuping), 2.3 Ga (Wutai), 1.85 Ga (Luliang-Zhongtiao) and 0.85 Ga (Jixian). Only three orogenies can be distinguished, however, based on new isotopic data: the terminal Archean Wutai Orogen (2.6-2.4 Ga), late Paleoproterozoic Zhongtiao Orogen (2.0-1.85 Ga) and the early Neoproterozoic Jixian (0.85 Ga). The first two orogenic periods involved metamorphism, deformation,
magmatism, but the last one is shown only by an uplift that resulted in about 0.30 Ga of missing stratigraphy between the early Neoproterozoic Qingbaikou Group and the middle part of the early Cambrian strata in a large portion of the craton. The late Paleoproterozoic Zhongtiao orogeny is very important for evolution of the North China Craton. Following it the terrain came to be more stable and a number of subsequent rifting events took place in the inland and marginal regions during the terminal Paleoproterozoic between ca. 1.8 Ga (or 1.85 Ga) to 1.6 Ga. The records of rifting events mainly include an aulacogen and cratonic margin basins, mafic dyke swarms,
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anorthosite suite (AGMS: anorthosite-gabbro-mangeritesyenite), rapakivi granites and super-high potassium volcanics. They belong to a group of anorogenic events reflecting breakup of the late Paleoproterozoic united North China Craton. Characteristics and geochronology of the rifting events and their temporal sequence are discussed in this paper.
Yanshan-TaihangshanAulacogen Aulacogen sedimentary records are preserved in Yanshan-Taihangshan Ranges in the central part of the North China Craton (Fig. 1). The most complete stratigraphic sequence is well exposed in the northern portion of Jixian County, Tianjin, where the sequence has been regarded as a type section of Proterozoic strata in China (Lee, 1939; Cheng et al., 1981). The Proterozoic sequence in Jixian can be subdivided into three groups and twelve formations with a total thickness of 9193 m
(Kao et al., 1934; Wang et al., 1980) and time range between ca. 1.8 Ga to 0.85 Ga (Lu et al., 1996). According to recent information, the Changcheng Group (ca. 1.81.6 Ga), the Jixian Group (ca. 1.6-1.0 Ga) and the Qingbaikuo Group (ca. 1.0-1.85 Ga) are respectively referred to as the terminal Paleoproterozoic, Mesoproterozoic,and early Neoproterozoic. The sequence rests upon Archean granite gneisses with an angular unconformity and is overlain by the middle part of early Cambrian strata overlying a para-unconformity in the type section. The Changcheng Group, as the first sedimentary cover, consists of four formations with 2762 m of thickness. The lowermost Changzhougou Formation rests unconformably upon the granite gneiss, which is recently dated at 2458+ 12 Ma by the single-zircon U-Pb method (Table 1 and Fig. 2). The formation is chiefly composed of basal conglomerate, pebble-bearing sandstone, and arkosic sandstone of alluvial facies in the lower portion and
Fig. 1. Sketch map showing terminal Paleoproterozoic rifting events in the North China Craton, 1- Mesoproterozoic sedimentary beds, 2 - Terminal Paleoproterozoic beds, 3 -Terminal Paleoproterozoic volcanics, 4 - Rapakivi granites, 5 - Gabbro, 6 - Anorthosite, 7 -Dyke swarm, 8 -Fault.
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TabIe 1. U-Pb zircon data of granite gneiss sample, from Jixian. Description
Sample weight (ug)
Concentration U Pb (ug/g) (ug/g)
Common Pb (ng)
Atomic ratios
zospb z04pb
zospb
199
0.1983
206pb
No.
Fraction
1
10
2
Lilac (colour) Lilac
10
57
33
0.005
2981
0.3035
3
Lilac
15
59
33
0.021
1074
0.2167
4
Lilac
5
49
29
0.015
403
0.3289
58
39
0.079
Apparent age (Ma)
207pb 2 3 8 ~
0.4588 (122) 0.4536 (61) 0.4585 (30) 0.4232 (126)
zo7pb
L35u
206pb
10.05 (30) 10.06 (14) 10.11 (7) 8.67 (27)
0.1589
2ospb
zo'pb
207pb
238u
23Su
206pb
2434
2440
2444
2411
2440
2464
2433
2445
2455
2275
2304
2330
(18)
0.1608 (5) 0.1600 (1) 0.1486 (11)
Note 206Pb/204Pb corrected for blank Pb=0.050 ng, U=O.O02 ng and desaturator. Other atomic ratios are radioisotope. Errors are quoted at 20. Weighted average of apparent age for2n7Pb/Z06Pb (No. 1-3) is 2458+ 12 Ma and 207Pb/2n6Pb for No. 4 is 2330f 13 Ma.
Fig. 2. Concordia diagram of U-Pb age for granite gneiss from Jixian.
sandstone of marine facies in the middle-upper parts. The overlying Chuanlinggou Formation is mostly made of neritic shales passing transitionally upward into the dolomitic Tuanshanzi Formation, which contains minor volcanic layers in the upper part. The uppermost Dahongyu Formation contains littoral and neritic sandstone, shale and super-high-potassium basalt-trachyte, with cherty dolostone in the upper part of the formation. The volcanic beds occupy the central part of the aulacogen and extend along an E-W direction about 110 km long and 20 km wide. It is thought that the volcanic eruption is spatially and temporally associated with intrusive rapakivi granite complex (see following), with both of them controlled by the same extensional rift system. Gondwana Research, I? 5, No. I, 2002
The super-high-potassium volcanics are developed in the upper TuanshanziFormation and Dahongyu Formation of the Changcheng Group. The rocks show very high potassium content up to 11.08% of K,O and K,O/Na,O ranging from 10 to 100 (average for 14 samples). According to single-zirconU-Pb ages of 1683k 67 Ma and 1625i 6 Ma, the eruption of the super-high-potassium volcanics occurred at ca. 1.69 Ga to 1.62 Ga. More than 200 isotopic dates have been obtained since 1960, most of them from K-Ar and Rb-Sr methods, with only a few ages based on U-Pb isotopic results. Singlezircon U-Pb ages of 1683+ 67 Ma (Li et al., 1995) and 1625f 6 Ma (Lu and Li, 1991) have been measured from super-high-potassium volcanic rocks of the uppermost
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Tuanshanzi and Dahongyu Formations. The thickness between the volcanic beds of the Tuanshanzi Formation and the base of the Changzhougou Formation is about 2000 m, so 1.8 Ga is referred to as the basal time boundary of the Changcheng Group and, consequently, the time of onset of the initial rift basin in the Yanshan-Taihangshan Aulacogen.
Cratonic Margin Rift Basins Three marginal rift basins are developed a t the northern, western and southern margins of the craton. A northern marginal basin occurs in the Langshan-Baiyun Obo Rift System (Wang et al., 1992) of Inner Mongolia with a length of 500 km in an E-W direction. The basin is occupied by the Zhaertai Group in the south segment and by the Baiyun Obo Group in the northern part of the rift system. The two groups are very famous respectively for the very large Baiyun Obo Nb-REE-Fe ore deposits and a number of stratabound polymetal sulfide deposits. They are directly underlain by the early Precambrian granite gneisses and supracrustal rocks, with U-Pb ages of 1997k 6 Ma and 1999+ 19 Ma reported from the metamorphic basements (Wang et al., 1992). More than 8000 m of sedimentary cover rest unconformablyupon the basement, and alluvial conglomerate and coarse-grained clastic rocks occur in the lowermost sequence. Clastic rocks and carboniferous slates predominate in the two groups, with carbonates with stromatolites making up only 14.5% and 6.5% in the Baiyun Obo and Zhaertai Groups, respectively.
Only in Jixian are carbonates as much as about 75% of total thickness. Very rare volcanic rocks are present, so the bottom and top time boundaries of both groups are questionable. A single-zircon U-Pb age of 1728+/-5 Ma, however, has been obtained from a basalt sample from the lower formation of the Baiyun Obo Group (Table 2 and Fig. 3), and it is reasonable to infer ca. 1.8 Ga as the bottom time boundary of the group. A western marginal basin is preserved in the central segment of the Helan Mountain of Ningxia Hui Autonomous Region. Three Proterozoic sedimentary units are underlain by early Precambrian granite gneisses with 1839 Ma K-Ar age (Zhao et al., 1980). The Proterozoic sequence includes, in ascending order, the Huangqikou Group composed of clastic deposits, the Wangquankou Group consisting of Carbonates and yielding a K-Ar age of 1289 Ma from glauconite of its base. The youngest formation is the terminal Neoproterozoic Zhengmuguan diamictite Formation, which is disconformably overlain by the trilobite-bearing Cambrian Suyukou Formation. Deposits formed during initial rifting in the western marginal basin seem to be absent, and the time range of the sequence is also uncertain. A southern marginal basin named the Yu-Shaan Rift System is developed around adjacent regions between Henan, Shanxi, and Shaanxi Provinces. The terminal Paleoproterozoic unmetamorphosed beds (Xiyanghe Group or Xionger Group) are unconformably underlain by early Precambrian metamorphosed basement and overlain by Mesoproterozoic carbonate strata. The
Table 2. U-Pb zircon data of granite gneiss sample, from Baiyun Obo. Description
Sample weight
Concentration U Pb
Common Pb (ng)
No.
Fraction
CUR)
(UR/X)
(UR/R)
1
rosiness (colour)
5
136
103
0.28
66.0
0.2063
0.3061 (245)
0.1028 (15)
1722
1701
1676
2
rosiness
5
135
69
0.12
125
0.2133
0.2937 (118)
0.1062 (6)
1660
1694
1736
3
rosiness
5
136
63
0.086
166
0.2394
0.2906 (121)
0.1077 (6)
1644
1696
1761
4
rosiness
5
127
71
0.140
109
0.1960
0.3074 (127)
0.1012 (10)
1728
1689
1646
5
rosiness
5
170
76
0.089
196
0.2631
0.2897 (102)
0.1057 (5)
1640
1675
1726
6
achromatism
5
596
46
0.080
139
0.1008
0.05036 (256)
0.05662 (92)
31 6.7
33 6.6
47 6.6
Note 206Pb/204Pb corrected for blank Pb=50 Pg, U=2 Pg and desaturator. Other atomic ratios are radioisotope. Errors are quoted at 20. Weighted average of apparent age f ~ r * ~ ~ P b of / ~No. ~ ~1-5 P bis 1728+/-5 Ma, and for No. 6 is 316.7+/-16.2 Ma.
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0.40
127
r
. 16.2 Ma
4
I 0.00
1
1
4.00
2.00
b/”’U
*07P
Xiyanghe or Xionger Groups have a maximum thickness of 8000 m and are composed of basal terrigenous clastic rocks and predominant volcanic layers containing porphyritic andesite, pelitic sandstone, basaltic andesite and basalt, andesitic agglomerate, basaltic andesite and rhyolitic ash flow tuff in ascending order. The main rock types are neither a typical calcalkalineseries nor a bimodal series. Jia Chengzao (1987) pointed out the volcanic series may be one of the products of plate subduction similar to that of modem active continental margins. In contrast, other geologists proposed that they formed in an intracontinental rift (Sun et a]., 1990). An age of 1840f 28 Ma by U-Pb SHRIMP from volcanic rocks of the Xiyanghe Group (Sun et al., 1993), may represent the age of the volcanic rocks. It is the most precise and oldest isotopic date of the Xiyunhe and Xionger Groups, so it is believed that the beginning of the Yu-Shaan Rift System was at ca. 1.85 Ga.
Mafic Dyke Swarms Mafic dyke swarms, stratiform layered mafic intrusions, and bimodal volcanic suites are thought to be three major igneous components of continental rift zones. Mafic dyke swarms are often coeval with major phases of rift-related mafic volcanism and layered intrusions. Activity of mafic dyke swarms can provide a reliable time marker for initial stage of continental rifting. This type of igneous activity Gondwana Research, V. 5,No. 1,2002
I
I
Fig. 3. Concordia diagram for basalt sample from Baoyun Obo.
is widely spread, especially in the central part of the North China Craton. The Wutai-Hengshan Mountains of the northwest Shanxi Province are located in the west central portion of North China Craton, where Archean supracrustal and granite gneisses are predominant. At least three episodes of Precambrian mafic magmatic activities can be identified. The first one is characterized by metamorphosed layered intrusions showing strong deformation and cutting across the Wutai supracrustal beds. The second phase consists of vertical dyke swarms that consist mainly of unmetamorphosed and undeformed diabases, which are covered by the Gaoyuzhuang Formation, slightly younger than 1.6 Ga. The third suite of dykes intrudes unmetamorphosed Mesoproterozoic sedimentary strata and is covered by the Cambrian beds. This paper discusses mainly the second dyke swarm. The second mafic dyke swarm occurs in the Wutai Group and is best exposed to the east of Wutai and Henshan Mountains. The dykes strike northwestsoutheast, especially in a 310”-340” direction, and have steep dips. Where the diabase is adjacent t o metamorphosed rocks, contacts between them are well defined but have irregular wavy margins. These dykes are generally 2 to 5 km long and 10 to 30 m wide, but the longest one is up to 24 km. Pyroxene and plagioclase are predominant minerals with minor amphibole and chlorite (Tian, 1991). Field investigation indicates that the
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emplacement of the second dyke swarms took place during the period younger than metamorphism and older than the overlying Gaoyuzhuang Formation. A more precise U-Pb age of 1769+ 2.5 Ma from mafic dykes of east Henshan Mountain was recently reported (Li et al., 2000). It is assumed the emplacement of the dykes occurred in the early terminal Paleoproterozoic.
Anorthosite Suite Two anorthosite (AGMS) suites crop out along two fault belts and are mainly composed of predominant anorthosite and minor gabbro, with mangerite as well as syenite. The northern anorthosite suite is preserved in Damiao, Chengde City, Hebei Province, with an area of 88 sq. km along an E-W deep fault (Fig. 4). The anorthosite intrudes Neoarchean gneisses and is covered by Mesozoic sedimentary-volcanic strata. According to Yu et al. (1996), at least three intrusive phases of the suite have been identified. The earliest one is composed of medium- to coarse-grained anorthosite characterized by plagioclases with cumulate texture. It is intruded by the second phase of norite and gabbro-norite in irregular or striped bodies. The latest phase consists of mangerite to quartz mangerite that cuts into the earlier anorthosite and norite.
Gabbroic rocks are largely made up of hypersthenite and hyperite. Minerals consist predominantly of Kenriched plagioclase, hypersthene, and clinopyroxene, with minor magnetoilmenite and apatite. The mangerite sheets intrude the main anorthosite and are generally characterized by porphyritic texture and a mineralogy of microperthite, plagioclase, ortho- and clino-pyroxenes,FeTi oxides and apatite. The antiperthite and microperthite in the separated anorthosite and mangerite of the Damiao Anorthosite suite demonstrate a more complete fractional crystallization of potassium, calcium and sodium in their magmatic evolution. The southern anorthosite suite is along the ChichenGubeikou Fault and is more than 150 km long. Five plutonic intrusions have been identified, each of them about 20 km by 1-3 km. Besides the anorthosite and gabbro, K-rich (up to 5.90%) granites and quartz syenite are also present along the southern belt. Only a few precise U-Pb ages have been reported from the northern anorthosite suite. On the basis of 1735+ 239 Ma of Sm-Nd isochron and 1683+ 193 Ma Rb-Sr isochron (Xie et al., 1988), the emplacement of the anorthosite took place about 1.7 Ga, but the exact time of intrusion is still uncertain. The quartz syenite and K-rich granite of the southern belt have been dated at 1697.3+/-0.9 Ma
Fig. 4. Sketch map showing distribution of magmatic events and deep faults in the central rifting system. 1 - Quaternary, 2 - Paleozoic and Mesozoic strata, 3 - Proterozoic strata, 4 - Archean rocks, 5 - Mesozoic granites, 6 - Damiao anorthosite suite, 7 - Chicheng-Gubeiko anorthosite suite, 8 - Paleoproterozoic granites, 9 - Super-high-potassium volcanics, 10 - Deep faults, 11- Faults, 12 - City or county.
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and 1696.72.8 Ga, respectively (Yu et al., 1996). Based on these measurements, it is reasonable to conclude that the emplacement of the AGMS took place around 1.7 Ga. The AGMS suite of the North China craton is comparable to the polycyclic gabbro-norite and anorthosite-mangerite-charnockite-granite(AMCG) suites, which are well developed in the Proterozoic Grenville Province of eastern Laurentia (Rivers, 1997). The first episode intrusions occurred from ca. 1.65 Ga to 1.61 Ga (Gower et al., 1992; Connelly and Heaman, 1993). Several large AMCG suites were also emplaced in the time range of 1.17 Ga-1.01 Ga. Both the Grenville AMCG suite and the North China AGMS suite contain nearly the same components except for charnockites in the Grenville suite. The AMCG suite was interpreted to have formed in response to delamination of the subcontinental lithosphere following crustal thickening (Corrigan and Hanmer, 1997), and both the AMCG and AGMS are generally related to anorogenic intracontinental rifting or incipient rifting (Windley, 1989).
Rapakivi Granites and Super-HighPotassium Volcanics Rapakivi granites are spread along the MiyunQiangzilu-Xinglong fault in an E-W direction (Fig. 4). A largest and typical plutonic body in Shangchang, Miyun County, Beijing, is called the Shangchang Rapakivi Granite Complex. It is 13 km long, 0.5-2.5 km wide, and cuts sharply across Archean metamorphic country rocks. The rapakivi granites are characterized by the plagioclasemantled alkali feldspar megacrysts showing typical features of the classic rapakivi around the world. Three intrusive phases of the complex have been distinguished: the first is porphyritic hornblende-biotite granite, the second is porphyritic biotite granite with scattered alkaline feldspar ovoids that cuts the first phase, and the third is fine-grained biotite granite and leucocratic muscovitebiotite granite. Only a few precise isotopic ages have been obtained from the Shangchang Rapakivi Complex. An age of 1683k 4 Ma by zo7Pb/206Pbfrom the porphyritic hornblende-biotite granite was reported by Yu et al. (1996). In addition, isotopic ages of roughly 1.70 Ga by zircon U-Pb and Pb-Pb methods were obtained from plutonic rocks associated with the rapakivi granites. Based on these isotopic data, the emplacement of the complex is inferred to be a little younger than 1.7 Ga, coeval with the super-high-potassium volcanic rocks in the YanshanTaihangshan Aulacogen. The rapakivi granites are typical of the terminal Paleoproterozoic over the world (Salop, 1983). The Gondwana Research, K 5,No. 1,2002
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rapakivi massifs and the related anorthosites are found in many regions and on every continent. Almost all of them are situated within older platforms and are confined to basement stabilized after the Luliang-Zhongtiao or orogenies of similar age elsewhere. Usually they are associated with taphrogenic strata of K-rich volcanics. The occurrence of the rapakivi granites, super-high-#volcanics, and the AGMS association of North China also display this same significance.
Discussion and Conclusion Rifting events are well preserved in the interior and marginal regions of the North China Craton. They include an aulacogen and cratonic marginal rift basins, mafic dyke swarms, anorthosite suite (AGMS), rapakivi granites and super-high-potassium volcanics, most of which developed between 1.8 Ga and 1.6 Ga. They are not directly related to metamorphism, deformation, and magmatism of the late Paleoproterozoicorogeny, but belong to an anorogenic group of rifting events. Based on present isotopic data, the temporal sequence of events may follow a pattern shown in table 3 and listed below. (1) A first phase of initial rifting to form basins. The earliest basin was apparently developed in the southern margin of the North China Craton at about 1.85 Ga. A thick sequence of volcanic rocks and alluvial conglomerate and pebble-bearing coarse-grained sandstone rest unconformablyupon the early-Precambrianmetamorphosed basement. In addition, the Yanshan-Taihangshan Aulacogen in the central craton and the north marginal basin began to accumulate sediments near 1.8 Ga or at a slightly younger age. The western marginal basin, however, may not have been formed during the terminal Paleoproterozoic, and its beginning is referred to be in the Mesoproterozoic. (2) The second phase consisted of intrusion of mafic dyke swarms and took place in the early stage of the terminal Paleoproterozoic (1769k 2.5 Ma) outside the Yanshan-Taihangshan Aulacogen, especially to the northwest and southwest of the basin. (3) A third phase consisted of AGMS rocks, rapakivi granites, and super-high-potassium volcanic rocks. They are concentrated in the Central Rift System, including the Yanshan-Taihangshan Aulacogen and a series of E-W directional deep faults. The absence of ophiolites and the presence of very thick carbonate strata show that all of these events took place within the craton or on most of its margins. All of the eruptive and intrusive magmatic events related to the rifting did not occur at the same time. They happened during the time interval between 1.85 Ga and
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Table 3. Major events of the Proterozoic and rifting events of the terminal Paleoproterozoic in the North China Craton. Tectonic stages
Tectonic movement
Paleozoic platform
Major rock association
Sedimentary environment
Tectonic background
Shale and carbonates
Carbonate shelf
Passive margin
Clastic rocks and limestone
Tidal and shallow marine
Intracratonic basin
Carbonate with abundant stromatolite
Shallow marine
Passive margin
Clastic rocks and minor volcanics
Alluvial to shallow marine
Graben and marginal rifts
Magmatic activity
0.85 or 0.80
Jixian movement Neoproterozoic platform
1.0
Qinyu uplift Mesoproterozoic platform Qinglong uplift Terminal Paleoproterozoic platform
K-rich volcanics Rapakivi anothosites Dykes Volcanics
Lower-middle grade metasediments, volcanics
Varying environments
ca.1.7 ca.1.7 1.77 1.84
Intracratonic and marginal basins
Mid-high ca. 2.5 metamorphism, deformation and TTG
Wutai orogeny
Archean Ortho-platform
1.6 1.6-1.7
Metamorphisn, deformation, and magmatism 1.85-2.0
Zhongtiao orogeny Paleoproterozoic Ortho-platform
Age (Gal
Middle-high grade supracrustals, TTG gneisses
1.6 Ga, first along the southern margin of the craton, then the central portion and northern margin, and the final events are concentrated in the central aulacogen and adjacent regions. The AGMS suite, rapakivi granites and K-rich volcanics in the central rift system are controlled by deep faults that are well exposed from the north to the south. They include the Damiao anorthosite suite located along the northernmost fault, a second belt of anorthosite suites along north-central Chicheng-Gubeikou Fault, the rapakivi granite complex along the south-central MiyunQiangzilu-Xinglong Fault and the super-high-potassium volcanics along the southernmost fault. There is no doubt that the extensional group of rifting events took place during the terminal Paleoproterozoic at the inner parts and margins of the North China Craton. Although they represent intracratonic extension, they may be responsive to supercontinental breakup events at this time,
Acknowledgment We wish to thank Prof. Rogers for his careful reviews and comments, which helped to improve the manuscript. This work is supported by NSFC grant [No. 40032010-C) and is a contribution to IGCP 386 and 440.
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China: new interpretation of Luliang Orogeny. Earth Sci., v. 25, pp. 15-20 (Chinese with English abstract). Li Huaikun, Li Huimin and Lu Songnian (1995) Grain zircon U-Pb age for volcanic rocks from Tuanshanzi Formation of Changcheng System and their geological implication. Geochimica, v. 24, pp. 43-48 (Chinese with English abstract), Lu Songnian and Li Humin, (1991) A precise U-Pb single zircon age determination for the volcanics of the Dahongyu Formation, Changcheng System in Jinxian. Chinese Acad. Geol. Sci. Bull., v. 22, pp. 137-145 (Chinese with English abstract). Lu Songnian, Yang Chunliang and Zhu Shixing (1996) Precambrian continental crust profile from eastern Hebei to Jixian, Tianjin. 30th IGC field trip guide, T105, Beijing, Geological Publishing House, pp. 42 (Chinese with English abstract). Rivers, T. (1997) Lithotectonic elements of the Grenville Province: review and tectonic implications. Precamb. Res., V. 86, pp. 117-154. Salop, L.J. (1983) Geological Evolution of the Earth during the Precambrian. Springer-Verlag,Berlin, Heidelberg, New York, 459p. Sun Dazhong and Lu Songnian (1990) Proterozoic tectonic evolution of the North China Platform. In: Naqvi, S.M. (Ed.), Precambrian Continental Crust and its Economic resources, Developments in Precambrian geology, 8, Elsevier, Amsterdam, pp. 503-522. Sun Dazhong, Hu Weixing, Tang Min, Zhao Fengqing and Condie, K.C. (1990) Origin of late Archean and early Proterozoic rocks and associated mineral deposits from the Zhongtiao Mountains, East-Central China. Precamb. Res., V. 47, pp. 287-306.
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Tian Yongqing (1991) Geology and gold mineralization of Wutaishan-Hengshan greenstone belt. Shanxi Science and Technology Press, 244 p (Chinese with English abstract). Wang Ji, Li Shangqing, Wang Baoliang and Li Jiaju (1992) The Langshan- Langshan-Baiyun Obo rift system. Beijing University Press, Beijing, 132p (Chinese with English abstract). Wang Yuelin, Lu Zongbin, Xing Yusheng, Gao, Zhenjia, Li Weixing, Ma Guogan, Zhang Luyi and Lu Songnian (1980) Subdivision and correlation of the upper Precambrian in China. In: Sinian Suberathem in China, Tianjin Science and Technology Press, pp. 1-30. Windley, B.E (1989) Anorogenic magmatism and the Grenvillian Orogeny. Can. J. Earth Sci., v. 26, pp. 479-489. Xie Guanghong and Wang Junwen (1988) A preliminary study on the emplacement age of the Damiao Anorthosite complex. Geochemistry, No. 1, pp. 13-17. Yu Jianhua, Fu Huiqin, Zhang Fenglan, Wan Fangziao, Haapala, I., Ramo, O.T. and Vaasjoki, M. (1996) Anorogenic rapakivi granites and related rocks in the northern of North China Craton. Science and TechnologyPress, Beijing, 182p (Chinese with English abstract). Zhao Xiangsheng, Zhang Luyi, Zou Xianghua, Wang Shuxi and Hu Yunxu (1980) Sinian tillite in northwest China and their stratigraphic significance. In: Sinian Suberathem in China, Tianjin Science and Technology Press, pp. 164-185. Zheng Zhaochang, Li Yuzhen, Lu Songnian and L:, Huaikun (1994) Lithology, sedimentology, and genesis of the Zahengmuguan Formation of Ningxia, China. In: Deynoux, M., Miller, J.M.G., Domack, E.W., Eyles, N., Fairchild, I.J. and Young, G.M. (Eds.), Earth's glacial records, Cambridge Univ. Press, pp. 101-108.
Gondwana Research
A Group of Rifting Events in the Terminal Paleoproterozoic in the North China Craton Lu Songnian, Yang Chunliang, Li Huaikun and Li Humin Tianjin Institute of Geology and Mineral Resources, China (Manuscript received February 26,2001; accepted May 21,2001)
Abstract Rifting events of the terminal Paleoproterozoic are well preserved in the North China Craton. They include an aulacogen and cratonic marginal rift basins, mafic dyke swarms, two anorthosite suites and rapakivi granites. All of these events took place after metamorphism, deformation and magmatism of the late Paleoproterozoic between 2.0 Ga and 1.85 Ga (or 1.80 Ga). The rifting events belong to an anorogenic event group, reflecting breakup of a late Paleoproterozoic united North China Craton. An intracratonic rift basin in the center of the North China Craton is named the Yanshan-TaihangshanAulacogen. A stratigraphic thickness of 9193 m was deposited within the basin, including three groups and twelve formations with time range ca. 1.8 Ga to 0.85 Ga. The oldest (Changcheng) group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates, terrestrial clastic rocks, and shallow marine sandstone and carbonates with super-high-potassium basalt-trachyte. The marginal basins occur around the northern, western and southern margins of the North China Craton, respectively located in Langshan-Baiyun Obo, Helanshan Mountain, and the adjacent regions between Henan, Shanxi and Shaanxi Provinces. The stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part, with a thick volcanic sequence with U-Pb age of 18402 14 Ma exposed in the southern marginal regions. Based on stratigraphic sequences, isotopic data and volcanic features, the initial rifts took place at about 1.85 Ga to 1.80 Ga. Mafic dyke swarms are largely developed in central parts of the North China Craton, where they intrude Archean granulite and high-amphibolite rocks. A new single-zircon U-Pb isotopic age of 1769k 2.5 Ma has been reported from a mafic dyke in east Henshan Mountain, northwestern Shanxi Province. An anorthosite suite (AGMS) is exposed along two deep fault belts in northern Hebei Province. Isochron Ages of 16832 193 Ma by Rb-Sr and 1735k 239 Ma by Sm-Nd were measured from an anorthosite body. The rapakivi granites crop out along the Miyun-Qiangzilu-XinglongFault in an E-W direction, and are characterized by megacrysts of alkali feldspars mantled by plagioclase, thus showing many of the petrographic features of classic rapakivi granites worldwide. A typical rapakivi body preserved in Shachang of Miyun County, Beijing, has a U-Pb age of 1716f 31 Ma. Super-highpotassium volcanics located in the upper Changcheng Group have been dated at 16832 67 Ma and 1625f 6 Ma. The time range for all magmatic events is between 1.72 Ga and 1.62 Ga. Key words: Mafic dyke swarms, anorthosites, rifting, Paleoproterozoic, North China Craton.
Introduction The North China Craton was terminally stabilized between ca. 2.0 Ga to 1.85 Ga (or 1.80 Ga). According to Sun and Lu (1990),the Proterozoic Eon in the North China Craton contains four important orogenies: about 2.5 Ga (Fuping), 2.3 Ga (Wutai), 1.85 Ga (Luliang-Zhongtiao) and 0.85 Ga (Jixian). Only three orogenies can be distinguished, however, based on new isotopic data: the terminal Archean Wutai Orogen (2.6-2.4 Ga), late Paleoproterozoic Zhongtiao Orogen (2.0-1.85 Ga) and the early Neoproterozoic Jixian (0.85 Ga). The first two orogenic periods involved metamorphism, deformation,
magmatism, but the last one is shown only by an uplift that resulted in about 0.30 Ga of missing stratigraphy between the early Neoproterozoic Qingbaikou Group and the middle part of the early Cambrian strata in a large portion of the craton. The late Paleoproterozoic Zhongtiao orogeny is very important for evolution of the North China Craton. Following it the terrain came to be more stable and a number of subsequent rifting events took place in the inland and marginal regions during the terminal Paleoproterozoic between ca. 1.8 Ga (or 1.85 Ga) to 1.6 Ga. The records of rifting events mainly include an aulacogen and cratonic margin basins, mafic dyke swarms,
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anorthosite suite (AGMS: anorthosite-gabbro-mangeritesyenite), rapakivi granites and super-high potassium volcanics. They belong to a group of anorogenic events reflecting breakup of the late Paleoproterozoic united North China Craton. Characteristics and geochronology of the rifting events and their temporal sequence are discussed in this paper.
Yanshan-TaihangshanAulacogen Aulacogen sedimentary records are preserved in Yanshan-Taihangshan Ranges in the central part of the North China Craton (Fig. 1). The most complete stratigraphic sequence is well exposed in the northern portion of Jixian County, Tianjin, where the sequence has been regarded as a type section of Proterozoic strata in China (Lee, 1939; Cheng et al., 1981). The Proterozoic sequence in Jixian can be subdivided into three groups and twelve formations with a total thickness of 9193 m
(Kao et al., 1934; Wang et al., 1980) and time range between ca. 1.8 Ga to 0.85 Ga (Lu et al., 1996). According to recent information, the Changcheng Group (ca. 1.81.6 Ga), the Jixian Group (ca. 1.6-1.0 Ga) and the Qingbaikuo Group (ca. 1.0-1.85 Ga) are respectively referred to as the terminal Paleoproterozoic, Mesoproterozoic,and early Neoproterozoic. The sequence rests upon Archean granite gneisses with an angular unconformity and is overlain by the middle part of early Cambrian strata overlying a para-unconformity in the type section. The Changcheng Group, as the first sedimentary cover, consists of four formations with 2762 m of thickness. The lowermost Changzhougou Formation rests unconformably upon the granite gneiss, which is recently dated at 2458+ 12 Ma by the single-zircon U-Pb method (Table 1 and Fig. 2). The formation is chiefly composed of basal conglomerate, pebble-bearing sandstone, and arkosic sandstone of alluvial facies in the lower portion and
Fig. 1. Sketch map showing terminal Paleoproterozoic rifting events in the North China Craton, 1- Mesoproterozoic sedimentary beds, 2 - Terminal Paleoproterozoic beds, 3 -Terminal Paleoproterozoic volcanics, 4 - Rapakivi granites, 5 - Gabbro, 6 - Anorthosite, 7 -Dyke swarm, 8 -Fault.
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TabIe 1. U-Pb zircon data of granite gneiss sample, from Jixian. Description
Sample weight (ug)
Concentration U Pb (ug/g) (ug/g)
Common Pb (ng)
Atomic ratios
zospb z04pb
zospb
199
0.1983
206pb
No.
Fraction
1
10
2
Lilac (colour) Lilac
10
57
33
0.005
2981
0.3035
3
Lilac
15
59
33
0.021
1074
0.2167
4
Lilac
5
49
29
0.015
403
0.3289
58
39
0.079
Apparent age (Ma)
207pb 2 3 8 ~
0.4588 (122) 0.4536 (61) 0.4585 (30) 0.4232 (126)
zo7pb
L35u
206pb
10.05 (30) 10.06 (14) 10.11 (7) 8.67 (27)
0.1589
2ospb
zo'pb
207pb
238u
23Su
206pb
2434
2440
2444
2411
2440
2464
2433
2445
2455
2275
2304
2330
(18)
0.1608 (5) 0.1600 (1) 0.1486 (11)
Note 206Pb/204Pb corrected for blank Pb=0.050 ng, U=O.O02 ng and desaturator. Other atomic ratios are radioisotope. Errors are quoted at 20. Weighted average of apparent age for2n7Pb/Z06Pb (No. 1-3) is 2458+ 12 Ma and 207Pb/2n6Pb for No. 4 is 2330f 13 Ma.
Fig. 2. Concordia diagram of U-Pb age for granite gneiss from Jixian.
sandstone of marine facies in the middle-upper parts. The overlying Chuanlinggou Formation is mostly made of neritic shales passing transitionally upward into the dolomitic Tuanshanzi Formation, which contains minor volcanic layers in the upper part. The uppermost Dahongyu Formation contains littoral and neritic sandstone, shale and super-high-potassium basalt-trachyte, with cherty dolostone in the upper part of the formation. The volcanic beds occupy the central part of the aulacogen and extend along an E-W direction about 110 km long and 20 km wide. It is thought that the volcanic eruption is spatially and temporally associated with intrusive rapakivi granite complex (see following), with both of them controlled by the same extensional rift system. Gondwana Research, I? 5, No. I, 2002
The super-high-potassium volcanics are developed in the upper TuanshanziFormation and Dahongyu Formation of the Changcheng Group. The rocks show very high potassium content up to 11.08% of K,O and K,O/Na,O ranging from 10 to 100 (average for 14 samples). According to single-zirconU-Pb ages of 1683k 67 Ma and 1625i 6 Ma, the eruption of the super-high-potassium volcanics occurred at ca. 1.69 Ga to 1.62 Ga. More than 200 isotopic dates have been obtained since 1960, most of them from K-Ar and Rb-Sr methods, with only a few ages based on U-Pb isotopic results. Singlezircon U-Pb ages of 1683+ 67 Ma (Li et al., 1995) and 1625f 6 Ma (Lu and Li, 1991) have been measured from super-high-potassium volcanic rocks of the uppermost
LU SONGNIAN ET AL.
126
Tuanshanzi and Dahongyu Formations. The thickness between the volcanic beds of the Tuanshanzi Formation and the base of the Changzhougou Formation is about 2000 m, so 1.8 Ga is referred to as the basal time boundary of the Changcheng Group and, consequently, the time of onset of the initial rift basin in the Yanshan-Taihangshan Aulacogen.
Cratonic Margin Rift Basins Three marginal rift basins are developed a t the northern, western and southern margins of the craton. A northern marginal basin occurs in the Langshan-Baiyun Obo Rift System (Wang et al., 1992) of Inner Mongolia with a length of 500 km in an E-W direction. The basin is occupied by the Zhaertai Group in the south segment and by the Baiyun Obo Group in the northern part of the rift system. The two groups are very famous respectively for the very large Baiyun Obo Nb-REE-Fe ore deposits and a number of stratabound polymetal sulfide deposits. They are directly underlain by the early Precambrian granite gneisses and supracrustal rocks, with U-Pb ages of 1997k 6 Ma and 1999+ 19 Ma reported from the metamorphic basements (Wang et al., 1992). More than 8000 m of sedimentary cover rest unconformablyupon the basement, and alluvial conglomerate and coarse-grained clastic rocks occur in the lowermost sequence. Clastic rocks and carboniferous slates predominate in the two groups, with carbonates with stromatolites making up only 14.5% and 6.5% in the Baiyun Obo and Zhaertai Groups, respectively.
Only in Jixian are carbonates as much as about 75% of total thickness. Very rare volcanic rocks are present, so the bottom and top time boundaries of both groups are questionable. A single-zircon U-Pb age of 1728+/-5 Ma, however, has been obtained from a basalt sample from the lower formation of the Baiyun Obo Group (Table 2 and Fig. 3), and it is reasonable to infer ca. 1.8 Ga as the bottom time boundary of the group. A western marginal basin is preserved in the central segment of the Helan Mountain of Ningxia Hui Autonomous Region. Three Proterozoic sedimentary units are underlain by early Precambrian granite gneisses with 1839 Ma K-Ar age (Zhao et al., 1980). The Proterozoic sequence includes, in ascending order, the Huangqikou Group composed of clastic deposits, the Wangquankou Group consisting of Carbonates and yielding a K-Ar age of 1289 Ma from glauconite of its base. The youngest formation is the terminal Neoproterozoic Zhengmuguan diamictite Formation, which is disconformably overlain by the trilobite-bearing Cambrian Suyukou Formation. Deposits formed during initial rifting in the western marginal basin seem to be absent, and the time range of the sequence is also uncertain. A southern marginal basin named the Yu-Shaan Rift System is developed around adjacent regions between Henan, Shanxi, and Shaanxi Provinces. The terminal Paleoproterozoic unmetamorphosed beds (Xiyanghe Group or Xionger Group) are unconformably underlain by early Precambrian metamorphosed basement and overlain by Mesoproterozoic carbonate strata. The
Table 2. U-Pb zircon data of granite gneiss sample, from Baiyun Obo. Description
Sample weight
Concentration U Pb
Common Pb (ng)
No.
Fraction
CUR)
(UR/X)
(UR/R)
1
rosiness (colour)
5
136
103
0.28
66.0
0.2063
0.3061 (245)
0.1028 (15)
1722
1701
1676
2
rosiness
5
135
69
0.12
125
0.2133
0.2937 (118)
0.1062 (6)
1660
1694
1736
3
rosiness
5
136
63
0.086
166
0.2394
0.2906 (121)
0.1077 (6)
1644
1696
1761
4
rosiness
5
127
71
0.140
109
0.1960
0.3074 (127)
0.1012 (10)
1728
1689
1646
5
rosiness
5
170
76
0.089
196
0.2631
0.2897 (102)
0.1057 (5)
1640
1675
1726
6
achromatism
5
596
46
0.080
139
0.1008
0.05036 (256)
0.05662 (92)
31 6.7
33 6.6
47 6.6
Note 206Pb/204Pb corrected for blank Pb=50 Pg, U=2 Pg and desaturator. Other atomic ratios are radioisotope. Errors are quoted at 20. Weighted average of apparent age f ~ r * ~ ~ P b of / ~No. ~ ~1-5 P bis 1728+/-5 Ma, and for No. 6 is 316.7+/-16.2 Ma.
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0.40
127
r
. 16.2 Ma
4
I 0.00
1
1
4.00
2.00
b/”’U
*07P
Xiyanghe or Xionger Groups have a maximum thickness of 8000 m and are composed of basal terrigenous clastic rocks and predominant volcanic layers containing porphyritic andesite, pelitic sandstone, basaltic andesite and basalt, andesitic agglomerate, basaltic andesite and rhyolitic ash flow tuff in ascending order. The main rock types are neither a typical calcalkalineseries nor a bimodal series. Jia Chengzao (1987) pointed out the volcanic series may be one of the products of plate subduction similar to that of modem active continental margins. In contrast, other geologists proposed that they formed in an intracontinental rift (Sun et a]., 1990). An age of 1840f 28 Ma by U-Pb SHRIMP from volcanic rocks of the Xiyanghe Group (Sun et al., 1993), may represent the age of the volcanic rocks. It is the most precise and oldest isotopic date of the Xiyunhe and Xionger Groups, so it is believed that the beginning of the Yu-Shaan Rift System was at ca. 1.85 Ga.
Mafic Dyke Swarms Mafic dyke swarms, stratiform layered mafic intrusions, and bimodal volcanic suites are thought to be three major igneous components of continental rift zones. Mafic dyke swarms are often coeval with major phases of rift-related mafic volcanism and layered intrusions. Activity of mafic dyke swarms can provide a reliable time marker for initial stage of continental rifting. This type of igneous activity Gondwana Research, V. 5,No. 1,2002
I
I
Fig. 3. Concordia diagram for basalt sample from Baoyun Obo.
is widely spread, especially in the central part of the North China Craton. The Wutai-Hengshan Mountains of the northwest Shanxi Province are located in the west central portion of North China Craton, where Archean supracrustal and granite gneisses are predominant. At least three episodes of Precambrian mafic magmatic activities can be identified. The first one is characterized by metamorphosed layered intrusions showing strong deformation and cutting across the Wutai supracrustal beds. The second phase consists of vertical dyke swarms that consist mainly of unmetamorphosed and undeformed diabases, which are covered by the Gaoyuzhuang Formation, slightly younger than 1.6 Ga. The third suite of dykes intrudes unmetamorphosed Mesoproterozoic sedimentary strata and is covered by the Cambrian beds. This paper discusses mainly the second dyke swarm. The second mafic dyke swarm occurs in the Wutai Group and is best exposed to the east of Wutai and Henshan Mountains. The dykes strike northwestsoutheast, especially in a 310”-340” direction, and have steep dips. Where the diabase is adjacent t o metamorphosed rocks, contacts between them are well defined but have irregular wavy margins. These dykes are generally 2 to 5 km long and 10 to 30 m wide, but the longest one is up to 24 km. Pyroxene and plagioclase are predominant minerals with minor amphibole and chlorite (Tian, 1991). Field investigation indicates that the
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LU SONGNIAN ET AL.
emplacement of the second dyke swarms took place during the period younger than metamorphism and older than the overlying Gaoyuzhuang Formation. A more precise U-Pb age of 1769+ 2.5 Ma from mafic dykes of east Henshan Mountain was recently reported (Li et al., 2000). It is assumed the emplacement of the dykes occurred in the early terminal Paleoproterozoic.
Anorthosite Suite Two anorthosite (AGMS) suites crop out along two fault belts and are mainly composed of predominant anorthosite and minor gabbro, with mangerite as well as syenite. The northern anorthosite suite is preserved in Damiao, Chengde City, Hebei Province, with an area of 88 sq. km along an E-W deep fault (Fig. 4). The anorthosite intrudes Neoarchean gneisses and is covered by Mesozoic sedimentary-volcanic strata. According to Yu et al. (1996), at least three intrusive phases of the suite have been identified. The earliest one is composed of medium- to coarse-grained anorthosite characterized by plagioclases with cumulate texture. It is intruded by the second phase of norite and gabbro-norite in irregular or striped bodies. The latest phase consists of mangerite to quartz mangerite that cuts into the earlier anorthosite and norite.
Gabbroic rocks are largely made up of hypersthenite and hyperite. Minerals consist predominantly of Kenriched plagioclase, hypersthene, and clinopyroxene, with minor magnetoilmenite and apatite. The mangerite sheets intrude the main anorthosite and are generally characterized by porphyritic texture and a mineralogy of microperthite, plagioclase, ortho- and clino-pyroxenes,FeTi oxides and apatite. The antiperthite and microperthite in the separated anorthosite and mangerite of the Damiao Anorthosite suite demonstrate a more complete fractional crystallization of potassium, calcium and sodium in their magmatic evolution. The southern anorthosite suite is along the ChichenGubeikou Fault and is more than 150 km long. Five plutonic intrusions have been identified, each of them about 20 km by 1-3 km. Besides the anorthosite and gabbro, K-rich (up to 5.90%) granites and quartz syenite are also present along the southern belt. Only a few precise U-Pb ages have been reported from the northern anorthosite suite. On the basis of 1735+ 239 Ma of Sm-Nd isochron and 1683+ 193 Ma Rb-Sr isochron (Xie et al., 1988), the emplacement of the anorthosite took place about 1.7 Ga, but the exact time of intrusion is still uncertain. The quartz syenite and K-rich granite of the southern belt have been dated at 1697.3+/-0.9 Ma
Fig. 4. Sketch map showing distribution of magmatic events and deep faults in the central rifting system. 1 - Quaternary, 2 - Paleozoic and Mesozoic strata, 3 - Proterozoic strata, 4 - Archean rocks, 5 - Mesozoic granites, 6 - Damiao anorthosite suite, 7 - Chicheng-Gubeiko anorthosite suite, 8 - Paleoproterozoic granites, 9 - Super-high-potassium volcanics, 10 - Deep faults, 11- Faults, 12 - City or county.
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and 1696.72.8 Ga, respectively (Yu et al., 1996). Based on these measurements, it is reasonable to conclude that the emplacement of the AGMS took place around 1.7 Ga. The AGMS suite of the North China craton is comparable to the polycyclic gabbro-norite and anorthosite-mangerite-charnockite-granite(AMCG) suites, which are well developed in the Proterozoic Grenville Province of eastern Laurentia (Rivers, 1997). The first episode intrusions occurred from ca. 1.65 Ga to 1.61 Ga (Gower et al., 1992; Connelly and Heaman, 1993). Several large AMCG suites were also emplaced in the time range of 1.17 Ga-1.01 Ga. Both the Grenville AMCG suite and the North China AGMS suite contain nearly the same components except for charnockites in the Grenville suite. The AMCG suite was interpreted to have formed in response to delamination of the subcontinental lithosphere following crustal thickening (Corrigan and Hanmer, 1997), and both the AMCG and AGMS are generally related to anorogenic intracontinental rifting or incipient rifting (Windley, 1989).
Rapakivi Granites and Super-HighPotassium Volcanics Rapakivi granites are spread along the MiyunQiangzilu-Xinglong fault in an E-W direction (Fig. 4). A largest and typical plutonic body in Shangchang, Miyun County, Beijing, is called the Shangchang Rapakivi Granite Complex. It is 13 km long, 0.5-2.5 km wide, and cuts sharply across Archean metamorphic country rocks. The rapakivi granites are characterized by the plagioclasemantled alkali feldspar megacrysts showing typical features of the classic rapakivi around the world. Three intrusive phases of the complex have been distinguished: the first is porphyritic hornblende-biotite granite, the second is porphyritic biotite granite with scattered alkaline feldspar ovoids that cuts the first phase, and the third is fine-grained biotite granite and leucocratic muscovitebiotite granite. Only a few precise isotopic ages have been obtained from the Shangchang Rapakivi Complex. An age of 1683k 4 Ma by zo7Pb/206Pbfrom the porphyritic hornblende-biotite granite was reported by Yu et al. (1996). In addition, isotopic ages of roughly 1.70 Ga by zircon U-Pb and Pb-Pb methods were obtained from plutonic rocks associated with the rapakivi granites. Based on these isotopic data, the emplacement of the complex is inferred to be a little younger than 1.7 Ga, coeval with the super-high-potassium volcanic rocks in the YanshanTaihangshan Aulacogen. The rapakivi granites are typical of the terminal Paleoproterozoic over the world (Salop, 1983). The Gondwana Research, K 5,No. 1,2002
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rapakivi massifs and the related anorthosites are found in many regions and on every continent. Almost all of them are situated within older platforms and are confined to basement stabilized after the Luliang-Zhongtiao or orogenies of similar age elsewhere. Usually they are associated with taphrogenic strata of K-rich volcanics. The occurrence of the rapakivi granites, super-high-#volcanics, and the AGMS association of North China also display this same significance.
Discussion and Conclusion Rifting events are well preserved in the interior and marginal regions of the North China Craton. They include an aulacogen and cratonic marginal rift basins, mafic dyke swarms, anorthosite suite (AGMS), rapakivi granites and super-high-potassium volcanics, most of which developed between 1.8 Ga and 1.6 Ga. They are not directly related to metamorphism, deformation, and magmatism of the late Paleoproterozoicorogeny, but belong to an anorogenic group of rifting events. Based on present isotopic data, the temporal sequence of events may follow a pattern shown in table 3 and listed below. (1) A first phase of initial rifting to form basins. The earliest basin was apparently developed in the southern margin of the North China Craton at about 1.85 Ga. A thick sequence of volcanic rocks and alluvial conglomerate and pebble-bearing coarse-grained sandstone rest unconformablyupon the early-Precambrianmetamorphosed basement. In addition, the Yanshan-Taihangshan Aulacogen in the central craton and the north marginal basin began to accumulate sediments near 1.8 Ga or at a slightly younger age. The western marginal basin, however, may not have been formed during the terminal Paleoproterozoic, and its beginning is referred to be in the Mesoproterozoic. (2) The second phase consisted of intrusion of mafic dyke swarms and took place in the early stage of the terminal Paleoproterozoic (1769k 2.5 Ma) outside the Yanshan-Taihangshan Aulacogen, especially to the northwest and southwest of the basin. (3) A third phase consisted of AGMS rocks, rapakivi granites, and super-high-potassium volcanic rocks. They are concentrated in the Central Rift System, including the Yanshan-Taihangshan Aulacogen and a series of E-W directional deep faults. The absence of ophiolites and the presence of very thick carbonate strata show that all of these events took place within the craton or on most of its margins. All of the eruptive and intrusive magmatic events related to the rifting did not occur at the same time. They happened during the time interval between 1.85 Ga and
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Table 3. Major events of the Proterozoic and rifting events of the terminal Paleoproterozoic in the North China Craton. Tectonic stages
Tectonic movement
Paleozoic platform
Major rock association
Sedimentary environment
Tectonic background
Shale and carbonates
Carbonate shelf
Passive margin
Clastic rocks and limestone
Tidal and shallow marine
Intracratonic basin
Carbonate with abundant stromatolite
Shallow marine
Passive margin
Clastic rocks and minor volcanics
Alluvial to shallow marine
Graben and marginal rifts
Magmatic activity
0.85 or 0.80
Jixian movement Neoproterozoic platform
1.0
Qinyu uplift Mesoproterozoic platform Qinglong uplift Terminal Paleoproterozoic platform
K-rich volcanics Rapakivi anothosites Dykes Volcanics
Lower-middle grade metasediments, volcanics
Varying environments
ca.1.7 ca.1.7 1.77 1.84
Intracratonic and marginal basins
Mid-high ca. 2.5 metamorphism, deformation and TTG
Wutai orogeny
Archean Ortho-platform
1.6 1.6-1.7
Metamorphisn, deformation, and magmatism 1.85-2.0
Zhongtiao orogeny Paleoproterozoic Ortho-platform
Age (Gal
Middle-high grade supracrustals, TTG gneisses
1.6 Ga, first along the southern margin of the craton, then the central portion and northern margin, and the final events are concentrated in the central aulacogen and adjacent regions. The AGMS suite, rapakivi granites and K-rich volcanics in the central rift system are controlled by deep faults that are well exposed from the north to the south. They include the Damiao anorthosite suite located along the northernmost fault, a second belt of anorthosite suites along north-central Chicheng-Gubeikou Fault, the rapakivi granite complex along the south-central MiyunQiangzilu-Xinglong Fault and the super-high-potassium volcanics along the southernmost fault. There is no doubt that the extensional group of rifting events took place during the terminal Paleoproterozoic at the inner parts and margins of the North China Craton. Although they represent intracratonic extension, they may be responsive to supercontinental breakup events at this time,
Acknowledgment We wish to thank Prof. Rogers for his careful reviews and comments, which helped to improve the manuscript. This work is supported by NSFC grant [No. 40032010-C) and is a contribution to IGCP 386 and 440.
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