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Geochemical evidence for the southern China block being a part of Gondwana
Journal of Southeast Asian Earth Sciences, Vol. 9, No. 4, pp. 319-324, 1994 Copyright ~ 1994 Elteviet $deace Lid
Pergamon
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Geochemical evidence for the southern China block being a part of Gondwana ZHU BING-QUAN Instituteof Geochemistry,Guangzhou Branch, AcademiaSinica, Wushan, Guangzhou,510640, People's Republicof China AImtrnetmBased on Pb-Sr-Nd isotope and trace element comparison of mantle, crust and mineral deposits from Gondwana and Laurasia notably from southern China, Yangtze, Tibet and northern China blocks, the southern China block is found to be similar to Gondwana and Tibet, having ~ P b / ~ P b > 18.4, mDPb/~Pb > 38.4 and /~ > 8.5, evidently distinguishing it from the northern China block, Yangtze and Laarama. Therefore, the southern China block could be a part of Gondwana.
INTRODUCTION WHY DID Gondwana continental fragments always drift and converge towards Laurasia, which continuously grew? The mantle convection theory established by physical model does not give a satisfactory answer. Based on geochemical perspective, the driving forces for plate movement and continental growth are attributed to heterogeneous distribution of radioactive energy materials. Wide geochemical studies of mantle, crust and mineral deposits have revealed that there are lateral heterogeneities on a large scale and some isotopic and elemental ratios as well as their concentrations show coupled variation within crust and mantle between different terranes; for example, Pb isotopic ratios, Ce/Pb, U/Pb and Mg, Al, Cu, V concentration (Hart 1984, 1988; Zhu et al. 1988, 1990; Zhang 1989; Hofmann 1986). Therefore, these geochemical data, especially Pb isotopes, can be used for discriminating chemical heterogeneities of mantle and crust. Based on tectonic and geographic distribution of the lateral heterogeneities, we can regard them as significant geochemical differences between Gondwana and Laurasian continents. Some sharp transition zones in geochemistry between the two continents can be identified in, for example, the continent of China, Japan Arc, the Pacific side of Northern America, and western Europe. Here, some geochemical comparisons between the southern China block and typical Gondwana areas, and also the northern China block as a representative of Laurasia, are given, which reveal that the southern China block is similar to the Gondwana continent, but evidently differs from the northern China block. The Yangtze block, a transition area between the northern and southern China blocks, can also be distinguished. COMPARISON OF MANTLE GEOCHEMISTRY 1. Isotopic systematics
Pb-Sr-Nd isotopic data of Cenozoic basalts, indicate that there exist geochemical anomalies in the mantle on
a large scale beneath Gondwana (Hart 1984, 1988). These anomalies include two anomalous mantle endmembers; i.e. DUPAL (showing higher ~Sr/U"Sr, >0.705 and ~ P b / ~ P b , A - 208/204 defined by Hart > 60) and HU (showing higher U/Pb, ~ P b / ~ P b > 18.6 and evenup to 20-24), as well as a prevalent mantle end-member (PREMA) (~Pb/2°4Pb = 18.4- 18.6, S~Sr/USr0.703 - 0.7032, 143Nd/144Nd- 0.51295 - 0.51305) instead of a primitive mantle (PM) (Zindler 1982; Allege et al. 1981, 1987; W6rrner et al. 1986; Zhu et al. 1990, 1991; Foot and Hawkesworth 1988). Pb-Sr-Nd isotopic composition data of Cenozoic basalts from the southern China block, such as Hainan Island, Liezhou Peninsular, western Yunnan, Fujian and Zhejiang areas, show a PREMA component and transition trend towards HU and DUPAL end-members; ~ P b / ~ P b = 18.4-18.8, ~ P b / ~ P b -- 38.4 - 38.9, most of data yield A -- 208/204 > 60. SVSr/USr-- 0.7031 0.7060, 143Nd/144Nd-- 0.51290 - 0.51305 (Zhu et al. 1983, 1989, 1990), which is similar to the southern hemisphere but evidently different from those of the northern China block showing primitive mantle compositions (SVSr/~SSr= 0.7043 - 0.7047, teNd/t44Nd ffi 0.51265 - 0.51280, ~ P b / ~ P b = 17.2 - 17.7, 2~pb/ ~ P b = 3 7 . 1 - 37.9) (Liu et al. 1987, Peng et al. 1986, Basu et al. 1991). In order to distinguish the multi-isotopic deviations between Gondwana and Laurasian areas, a Pb-Sr-Nd isotopic topological diagram of five dimensions is given. The data of five isotopic systematics were adjusted to the same size ranges by using age vector (~1) for eliminating the influence from the differences of decay constants and parent/daughter of bulk earth:
?;= (I/,~)In[l + (I~= - It0)/Ri],
(I)
where I~, I~ and RI represent measured and primitive isotopic ratios,and parent/daughter values of bulk earth for isotopic systematics respectively.The coordinate Vx in the diagram represents evolution direction of primitive mantle, setting zero point at the earth age 4.57 Ga, corresponding to present values of primitive mantle; Vy represent the direction from PM to PREMA. The
319
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Fig. 1. Topological projection diagram in five-dimension space for Nd-Sr-Pb isotopic compositions of mantle from global major end members in the different blocks of China: 1. southern China block; 2. southern hemisphere and Gondwana; 3. the northern China block; 4. northern Pacific (Zhu 1991).
vectors ti are projected on the V~- V~ plane. It can be seen from Fig. 1 that southern China lies in the Gondwana area (Zhu 1991). 2. Major and trace elements
When we compare the concentrations of major elements of Cenozoic basalt between the southern and northern China blocks, the evident deviation is in the FeO (total Fe). Figure 2 is diagram of FeO-SiO2; the mean values of FeO against SiO2 are within a 1% range from 39 to 56%. It can be seen that the FeO concentrations in southern China are always lower, by about 2%, than northern China in every SiO2 range. This difference is not due to the influence of magma fractionation or partial melting of different degrees. The Cenozoic alkaline basalts from the southern China block show stable Ce/Pb(24), U/Pb(0.5) and Nb/U(49) ratios, which are typical geochemical features of ocean island basalts in the southern hemisphere (Sun and MeDonough 1989) but differ from the northern China block, which has lower U/Pb(0.28) and Ce/Pb(16).
As crustal materials have undergone large chemical fractionation, it is necessary to select the same kinds of rocks with the same age range for comparison. Here, the Mesozoic granites, especially S-type granites, are selected as representatives of the present upper crust. The Pb isotopic compositions of feldspars for these granites show coupled variations with the rocks derived from the mantle within various blocks, an important criterion for geochemical comparison between different terranes. For example, the 2°6pb/2°4pb, 2°7pb/2°4pb and 2°SPb/2°4pb in the southern China block are higher than 18.4, 15.55 and 38.4, respectively (Zhang 1989), in agreement with those of Cenozoic basalts from this block, and also are comparable with those of Mesozoic-Cenozoic granites in Tibet, as well as other Gondwana fragments (Fig. 3). They are distinguished from the Mesozoic granites in the Yangtze block (2°6pb/2°4pb = 17.8 -- 18.4, 2°7pb/2°4pb = 15.45- 15.55 and 2°Spb/2°4pb = 37.8- 38.4) and the northern China block (2°6pb/2°4Pb = 15.8 - 17.8, 2°Tpb/2°4pb = 15.10 - 15.45 and 2°Spb/2°4Pb= 36.7 37.8) (Li et al. 1991), which also lie in the range of Pb isotopic compositions of the mantle (Fig. 3). The trace elements of granites between the southern and northern China blocks also show large deviation. In order to eliminate the influences of different rock types and genesis, the Mesozoic granites with SiO2 of 71-73%
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Fig. 3. Histogram of ~Pb/2°4Pb for feldspar of Cenozoic granites (A), in comparison with those of mantle (B). 1 = NCB; 2 = Yangtze; 3 = SCB; 4 = Heilongjiang; 5 = Tibet
Southern China block and Gondwana
321
Table I. Mean concentrations (ppm) of some trace element in southern China (SC) and northern China (NC)
high grade metamorphic rocks show higher ~nPb/~Pb and p, (8.0-9.0) values and lie in the Gondwana U Th W Sn Ba Sr V Cr Co Be Rb area (Fig. 4). For example, the metamorphic rocks SC 7.1 29 2.4 7.7 385 135 17 9.4 2.8 5.0 314 containing garnet and pyroxene from Zhejiang give NC 2.2 13 1.0 1.9 1300 430 36 19 7.0 2.5 129 2°tPb/~Pb = 16.5 and anPb/~Pb--- 15.6, in comparison Li S. and Wang T., 1991. with the granulites from northern China with the same ~ P b / ~ P b , but having 2°TPb/~4pb usually lower than are selected for comparison. It can be seen in Table 1 15.3. that the mean concentrations of U, Th, Rb, Sn, Be and W in the southern China block are higher by a factor of 2-3, and Ba, Sr, V, Cr and Co are lower by a factor of MINERALIZATION 2-3 than those in the northern China block. It also indicates that the southern China block is similar to the The high-energy features (enriched in U, Th and K) in Himalaya area in Gondwana, showing high U (9 ppm) Gondwana and the mineralizations of Au, Cu, Pb-Zn, and Rb (360ppm) and low Ba (210ppm) and Sr W-Sn and U are stronger than those in Laurasia. Super(75 ppm) (Cuney et al. 1984). There also existed an large deposits of these metals frequently occur in Gondabundance of high heat producing granites (U, wana, showing ratios of reserves 20-10:1 between the 10-20 ppm) in the Gondwana area, which could drive southern and northern hemispheres. Their Pb isotopic mineralization of some super large deposits (Solomon compositions show high p features, mainly along the et al. 1992). growth curve suggested by Doe (1975) (/~ ffi 9.57), and show an evolution trend around PREMA (Zhu 1991). The Pb isotopic compositions of ores in Laurasia Pb ISOTOPIC FEATURE OF THE LOWER CRUST are distributed along a low p (7.8) growth curve (Zhu et al. 1984). The Paleozoic metal deposits (Ath As representative of lower crust, rocks with granulite Pb-Zn and W) occurring in the southern China block and high-degree amphibolite facies occurring in Gond- have Pb isotopic compositions of ~SPb/~Pb> 18.4, wana (Antarctic, southern Africa, southern America and ~pb/2°4Pb=15.55-15.90 and ~Pb/a~Pb>38.4, also India) usually show Pb isotopic features with high Pt along Doe's growth curve, and showing an evolution (8.5-15.8), while those occurring in Laurasia (northern ellipsoid in three-dimensional space of Pb isotopes around China, Lewisian and western Greenland) show low p~ the PREMA end-member (see Fig. 5). It is distinguished (7.0-8.0) (Rudnick 1990, Depaolo 1982, Montgomery et from the low ~ (7.8) Pb occurring in the northern China al. 1978; Tu and Zhu 1992). Although there are no block, which shows an evolution ellipsoid around typical granulites found in the southern China block, the PM, and also from the Yangtze block, which shows a
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Fig. 4. ~Pb/~Pb-z°Tpb/~Pb diagram for global Archaean granulites and granulite xenoliths from Cenozoic volcanic rocks. SA -- Southern Africa; An = Antarctic; In = India; L = Lewisian; GL = Greenland and Laborador; Q ffi Queensland in Australia; E = Eifei in Germany; C = Arizona in USA; N = Geronimo in New Mexico; K ,~ Kibourne Hole in New Mexico; LSA -- Lesotho in Southern Africa; T = East Africa; black circle-north China block, high-grade eros-metamorphic rocks in the southern China block (Tu et al. 1992).
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Fig. 5. Topologicaldiagramin three-dimensionspace for Pb isotopic compositionsof ore from SCB, NCB and Yangtze block. Fig. 6. Pb isotopicvariationsin three-dimensionspacefor golddeposit with the same mineralizationages from north to south in China, Q-Qinlingmountain,NH-northernHunan,WG-westernGuangdong, transition between the southern and northern China HA-Hainan.
block (Fig. 5). When we compare the gold deposits with the mineralization age of 100-200 Ma along a zone from north to south through Qinling mountain, northern Hunan Province, western Guangdong Province and Hainan Island, it can be found that the Pb isotopic ratios linearly increase ( ~ P b / ~ P b from < 17.0 to > 19.0) as can be seen in the topological diagram of Pb isotopes in three-dimensional space (Fig. 6), among which Guangdong and Hainan undoubtedly belong to Gondwana.
CONTINENTAL GROWTH Based on Nd model ages, it is indicated that Gondwanaland continuously experienced long-term continental growth (4.0-0 Ga) with a relative constant growth rate, whereas Laurasia mainly grew during 3.5-2 Ga, and after that time, the growth rate sharply decreased (Allegre 1984). The southern China block also shows a longer history of continental growth from 3.5 to 0.4 Ga based on Nd model ages, which can be comparable with Gondwana. Continent growth in the northern China block was almost terminated before 2.5 Ga. Based on Pb isotopic compositions of ores, Cenozoic basalts and Mesozoic granites, the boundary between
I
Gondwana and Yangtze can be established in the middle of Hunan along Xuefong mountain, northern Jiangxi and middle Zhejiang, approximately along latitude 29.2 N, northern Guangxi, south-western Guizhou and north-western Yunnan provinces (Fig. 7). The boundary between Gondwana and Yangtze may also stretch to the Japan Arc based on the Pb isotopic evidences from the Mesozoic-Neogene deposit, and igneous and metamorphic rocks (for example, Sato 1979; Sasaki 1982). As shown in Fig. 8, the outer zone of Japan including major parts of Shikoku, Kyushu and Hokkaido, and Hokkaido, and eastern part of Honshu is consistent with Gondwana and the southern China block. The inter zone of Japan could be a part of Yangtze block. The geochemical boundary of the southern China block with Gondwana features established by using Pb isotopes (Fig. 7) is generally consistent with the tectonic boundary, but the disconcordance between the two types of boundaries is still apparent. Although there are not enough geochemical data to establish an exact boundary, the basic difference between the two types of boundaries is that the former provides information on the deep crust and mantle, whereas the latter only
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9ff 10~ 108" 113" 118" Fig. 7. Contourlinediagramof Pb isotopicmapping(based on V~vector)in southernChina showingthe boundarybetween the southern China block and Yangtze block.
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Fig. 8. Pb isotopic mapping of Japan arc, black circle; Gondwana and SCB Pb; open circle; Yangtze lb.
Hofmann, A. W. et al. 1986. blb and Pb in oceanic basalts: new constraints on mantle evolution. Earth Planet. Sci. Lett. 79, 33-45. Li, S. and Wang, T. 1991. Geochemistry of Granitoida in TongbaishanDabieshan, Central China. China Univ. of Geosci. Press. Liu, B.-L. et al. 1987. The genesis of Cenozoic basalts in Jinbo Lake, Northeastern China, and its geochemical feature of mantle--evidencesfrom Sr, Nd, Pb isotopes and trace elements. Geochemistry 1, 9-19. Montgomery, C. W. and Hurley, P. M. 1978. Total rock U-Pb and Rb-Sr systematics in the Imataca series, Guyana shield, Venezuela. Earth Planet. Sci. Lett. 39, 291-290. Peng, Z. C. et aL 1986. Pb-, Sr- and Nd-isotopic systematics and chemical characteristics of Cenozoic basalts, Eastern China. Chem. Geol. 59, 3-33. Rudnick, R. L. et al. 1990. The Pb isotopic compositions of lower crustal xenoliths and evolution of lower crustal Pb. Earth Planet. REFERENCES Sci. Lett. 98, 192-207. Sasaki, A. et al. 1982. Isotopic composition of ore lead from the Japanese island. Mining Geology 32, 457-474. All~gre, C. J. and Rausseau, D. 1984. The growth of the continent through geological time studied by Nd isotopic analysis of shales. Sato, K. 1979. Lead isotopes of igneous materials. Science Council of Japan 4, 43-49. Earth Planet. Sci. Lett. 67, 19-34. Ail~gre,C. J. et ai. 1981. The subcontinental venus suboceanic debate, Solomon, M. and Heinrich, C. A. 1992. Is high-heat-producing granites necessary condition for the formation of Mount lsa and 1. Lead-neodymium-strontium isotopes in primary alkali basalts McArthur river super large Pb-Zn deposits. Explor. Mining Geol. 1, from a shield area: the Ahaggar volcanic suite. Earth Planet. Sci. Left. 52, 85--95. 85-91. Alh~gre, C. J. et al. 1987. Topology in isotopic multispace and origin Tu, X. L. and Zhu, B.-Q. 1992. Pb isotopic features of granulite facies zone in North China, in comparison with globe, submitted to of chemical heterogeneities. 81, 319-337. Science in China. Basu, A. R. et al. 1991. Major element, Ree, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: W6rrner, G. et al. 1986. Sr, Nd and Ph isotope geochemistry of Tertiary and Quaternary alkaline volcanics from west German. implications for their origin from suboceanic-typemantle reservoirs. Earth Planet. Sci. Lett. 79, 107-119. Earth Planet. Sci. Lett. 105, 149-169. Cuney, M. and Fort, P. L. 1984. U and Th geochemistry and Zhan8, B. R. et al. 1991. Geotectonic subdivision and Paleozoic subduction and coUisioo orogeny of east Qinling and its adjacent mineralogy of Manashu granites (Nepal, Himalaya area). Proc. regions: evidence from geochemical research. J. China Univ. Geosci. Inter. Symposium on granites and its relationships with mineralization, Nanjing, pp. 584-600. 1-17. DePaolo, D. J. et al. 1982. Sm-Nd, Rb-Sr and U-Th-Pb systematics Zhang, L. G. 1989. Petrogenetic and Minerogenetic Theories and Prospecting, Beijing Univ. of Tech. of granulite facies rocks from Fyfe Hill, Enderby land, Antarctic. Zhu, B.-Q. et ai. 1983. Isotopic and geochemical evidence for the Nature, Land. ~ g , 614-618. origin of Plio-Pleistocene volcanic rocks near Indo-Eurasian Foot, P. L. and Hawkesworth, C. 1988. Origin of Deccan Trap lavas: collisional margin at Tengchong, China. Earth Planet. Sei. Lett. 68, evidencesfrom combined trace element and Sr-, Nd-, and Pb-isotope studies. Earth Planet. Sci. Lett. 91, 89-104. 263-275. Hart, S. R. 1984. A large-watle isotope anomaly in the southern Zhu, B.-Q. 1988. Nd-Sr-Pb isotopic feature for main end member of mantle beneath the continent of south China. 3rd Conhemisphere mantle. Nature, Land. 309, 753-757. ference of Chinese Society of Miner. Petrol. and Geochem. Hart, S. R. 1988. Heterogeneous mantle domains: signatures, genesis Abstract 51. and mixing chronologies. Earth Planet. Sci. Left. 90, 273-296.
reflected the signature of tectonic strata. Mixing of materials from different blocks occurs in the boundary areas through change of erosion sources of strata in the collision zone, and magmatism in the subduction zone. Therefore geochemical data for a variety of rocks and deposits must be considered, the mantle and lower crustal rocks being especially important. The geochemical criteria for boundary division and its relationships with tectonic, paleontological and paleomagnetic evidences should be further studied.
SEAES 914--C
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ZHU BING-QUAN
Zhu, B.-Q. et al. 1989. Nd-Sr-Pb isotopic evidence for volcanism with MORB-OIB transition mantle in Liequong area. Geochemistr)' 3, 193-2O 1 Zhu, B.-Q. 1990. Three-component mixing and tbur-system recycling models for explaining Nd-Sr-Pb isotopic correlations of suboceanic and subcontinental mantles. Science in China 33, 757 76,~
Zhu. B.-Q 1991. Evidence of isotopic systematics from crus| mantle for chemical heterogeneities of terranes. Chinese ~ci. 36, I27%.1282. Zindler. A. et al. 1982. Nd, Sr and Pb isotopic systematics three-component mantle: a new perspective. ,Vature. Lond. 519 523.
and Bull
m a 289,
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Printed in Great Britain..All fights reserved 0743-9547/94 $7.00 + 0.00
Geochemical evidence for the southern China block being a part of Gondwana ZHU BING-QUAN Instituteof Geochemistry,Guangzhou Branch, AcademiaSinica, Wushan, Guangzhou,510640, People's Republicof China AImtrnetmBased on Pb-Sr-Nd isotope and trace element comparison of mantle, crust and mineral deposits from Gondwana and Laurasia notably from southern China, Yangtze, Tibet and northern China blocks, the southern China block is found to be similar to Gondwana and Tibet, having ~ P b / ~ P b > 18.4, mDPb/~Pb > 38.4 and /~ > 8.5, evidently distinguishing it from the northern China block, Yangtze and Laarama. Therefore, the southern China block could be a part of Gondwana.
INTRODUCTION WHY DID Gondwana continental fragments always drift and converge towards Laurasia, which continuously grew? The mantle convection theory established by physical model does not give a satisfactory answer. Based on geochemical perspective, the driving forces for plate movement and continental growth are attributed to heterogeneous distribution of radioactive energy materials. Wide geochemical studies of mantle, crust and mineral deposits have revealed that there are lateral heterogeneities on a large scale and some isotopic and elemental ratios as well as their concentrations show coupled variation within crust and mantle between different terranes; for example, Pb isotopic ratios, Ce/Pb, U/Pb and Mg, Al, Cu, V concentration (Hart 1984, 1988; Zhu et al. 1988, 1990; Zhang 1989; Hofmann 1986). Therefore, these geochemical data, especially Pb isotopes, can be used for discriminating chemical heterogeneities of mantle and crust. Based on tectonic and geographic distribution of the lateral heterogeneities, we can regard them as significant geochemical differences between Gondwana and Laurasian continents. Some sharp transition zones in geochemistry between the two continents can be identified in, for example, the continent of China, Japan Arc, the Pacific side of Northern America, and western Europe. Here, some geochemical comparisons between the southern China block and typical Gondwana areas, and also the northern China block as a representative of Laurasia, are given, which reveal that the southern China block is similar to the Gondwana continent, but evidently differs from the northern China block. The Yangtze block, a transition area between the northern and southern China blocks, can also be distinguished. COMPARISON OF MANTLE GEOCHEMISTRY 1. Isotopic systematics
Pb-Sr-Nd isotopic data of Cenozoic basalts, indicate that there exist geochemical anomalies in the mantle on
a large scale beneath Gondwana (Hart 1984, 1988). These anomalies include two anomalous mantle endmembers; i.e. DUPAL (showing higher ~Sr/U"Sr, >0.705 and ~ P b / ~ P b , A - 208/204 defined by Hart > 60) and HU (showing higher U/Pb, ~ P b / ~ P b > 18.6 and evenup to 20-24), as well as a prevalent mantle end-member (PREMA) (~Pb/2°4Pb = 18.4- 18.6, S~Sr/USr0.703 - 0.7032, 143Nd/144Nd- 0.51295 - 0.51305) instead of a primitive mantle (PM) (Zindler 1982; Allege et al. 1981, 1987; W6rrner et al. 1986; Zhu et al. 1990, 1991; Foot and Hawkesworth 1988). Pb-Sr-Nd isotopic composition data of Cenozoic basalts from the southern China block, such as Hainan Island, Liezhou Peninsular, western Yunnan, Fujian and Zhejiang areas, show a PREMA component and transition trend towards HU and DUPAL end-members; ~ P b / ~ P b = 18.4-18.8, ~ P b / ~ P b -- 38.4 - 38.9, most of data yield A -- 208/204 > 60. SVSr/USr-- 0.7031 0.7060, 143Nd/144Nd-- 0.51290 - 0.51305 (Zhu et al. 1983, 1989, 1990), which is similar to the southern hemisphere but evidently different from those of the northern China block showing primitive mantle compositions (SVSr/~SSr= 0.7043 - 0.7047, teNd/t44Nd ffi 0.51265 - 0.51280, ~ P b / ~ P b = 17.2 - 17.7, 2~pb/ ~ P b = 3 7 . 1 - 37.9) (Liu et al. 1987, Peng et al. 1986, Basu et al. 1991). In order to distinguish the multi-isotopic deviations between Gondwana and Laurasian areas, a Pb-Sr-Nd isotopic topological diagram of five dimensions is given. The data of five isotopic systematics were adjusted to the same size ranges by using age vector (~1) for eliminating the influence from the differences of decay constants and parent/daughter of bulk earth:
?;= (I/,~)In[l + (I~= - It0)/Ri],
(I)
where I~, I~ and RI represent measured and primitive isotopic ratios,and parent/daughter values of bulk earth for isotopic systematics respectively.The coordinate Vx in the diagram represents evolution direction of primitive mantle, setting zero point at the earth age 4.57 Ga, corresponding to present values of primitive mantle; Vy represent the direction from PM to PREMA. The
319
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Fig. 1. Topological projection diagram in five-dimension space for Nd-Sr-Pb isotopic compositions of mantle from global major end members in the different blocks of China: 1. southern China block; 2. southern hemisphere and Gondwana; 3. the northern China block; 4. northern Pacific (Zhu 1991).
vectors ti are projected on the V~- V~ plane. It can be seen from Fig. 1 that southern China lies in the Gondwana area (Zhu 1991). 2. Major and trace elements
When we compare the concentrations of major elements of Cenozoic basalt between the southern and northern China blocks, the evident deviation is in the FeO (total Fe). Figure 2 is diagram of FeO-SiO2; the mean values of FeO against SiO2 are within a 1% range from 39 to 56%. It can be seen that the FeO concentrations in southern China are always lower, by about 2%, than northern China in every SiO2 range. This difference is not due to the influence of magma fractionation or partial melting of different degrees. The Cenozoic alkaline basalts from the southern China block show stable Ce/Pb(24), U/Pb(0.5) and Nb/U(49) ratios, which are typical geochemical features of ocean island basalts in the southern hemisphere (Sun and MeDonough 1989) but differ from the northern China block, which has lower U/Pb(0.28) and Ce/Pb(16).
As crustal materials have undergone large chemical fractionation, it is necessary to select the same kinds of rocks with the same age range for comparison. Here, the Mesozoic granites, especially S-type granites, are selected as representatives of the present upper crust. The Pb isotopic compositions of feldspars for these granites show coupled variations with the rocks derived from the mantle within various blocks, an important criterion for geochemical comparison between different terranes. For example, the 2°6pb/2°4pb, 2°7pb/2°4pb and 2°SPb/2°4pb in the southern China block are higher than 18.4, 15.55 and 38.4, respectively (Zhang 1989), in agreement with those of Cenozoic basalts from this block, and also are comparable with those of Mesozoic-Cenozoic granites in Tibet, as well as other Gondwana fragments (Fig. 3). They are distinguished from the Mesozoic granites in the Yangtze block (2°6pb/2°4pb = 17.8 -- 18.4, 2°7pb/2°4pb = 15.45- 15.55 and 2°Spb/2°4pb = 37.8- 38.4) and the northern China block (2°6pb/2°4Pb = 15.8 - 17.8, 2°Tpb/2°4pb = 15.10 - 15.45 and 2°Spb/2°4Pb= 36.7 37.8) (Li et al. 1991), which also lie in the range of Pb isotopic compositions of the mantle (Fig. 3). The trace elements of granites between the southern and northern China blocks also show large deviation. In order to eliminate the influences of different rock types and genesis, the Mesozoic granites with SiO2 of 71-73%
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Southern China block and Gondwana
321
Table I. Mean concentrations (ppm) of some trace element in southern China (SC) and northern China (NC)
high grade metamorphic rocks show higher ~nPb/~Pb and p, (8.0-9.0) values and lie in the Gondwana U Th W Sn Ba Sr V Cr Co Be Rb area (Fig. 4). For example, the metamorphic rocks SC 7.1 29 2.4 7.7 385 135 17 9.4 2.8 5.0 314 containing garnet and pyroxene from Zhejiang give NC 2.2 13 1.0 1.9 1300 430 36 19 7.0 2.5 129 2°tPb/~Pb = 16.5 and anPb/~Pb--- 15.6, in comparison Li S. and Wang T., 1991. with the granulites from northern China with the same ~ P b / ~ P b , but having 2°TPb/~4pb usually lower than are selected for comparison. It can be seen in Table 1 15.3. that the mean concentrations of U, Th, Rb, Sn, Be and W in the southern China block are higher by a factor of 2-3, and Ba, Sr, V, Cr and Co are lower by a factor of MINERALIZATION 2-3 than those in the northern China block. It also indicates that the southern China block is similar to the The high-energy features (enriched in U, Th and K) in Himalaya area in Gondwana, showing high U (9 ppm) Gondwana and the mineralizations of Au, Cu, Pb-Zn, and Rb (360ppm) and low Ba (210ppm) and Sr W-Sn and U are stronger than those in Laurasia. Super(75 ppm) (Cuney et al. 1984). There also existed an large deposits of these metals frequently occur in Gondabundance of high heat producing granites (U, wana, showing ratios of reserves 20-10:1 between the 10-20 ppm) in the Gondwana area, which could drive southern and northern hemispheres. Their Pb isotopic mineralization of some super large deposits (Solomon compositions show high p features, mainly along the et al. 1992). growth curve suggested by Doe (1975) (/~ ffi 9.57), and show an evolution trend around PREMA (Zhu 1991). The Pb isotopic compositions of ores in Laurasia Pb ISOTOPIC FEATURE OF THE LOWER CRUST are distributed along a low p (7.8) growth curve (Zhu et al. 1984). The Paleozoic metal deposits (Ath As representative of lower crust, rocks with granulite Pb-Zn and W) occurring in the southern China block and high-degree amphibolite facies occurring in Gond- have Pb isotopic compositions of ~SPb/~Pb> 18.4, wana (Antarctic, southern Africa, southern America and ~pb/2°4Pb=15.55-15.90 and ~Pb/a~Pb>38.4, also India) usually show Pb isotopic features with high Pt along Doe's growth curve, and showing an evolution (8.5-15.8), while those occurring in Laurasia (northern ellipsoid in three-dimensional space of Pb isotopes around China, Lewisian and western Greenland) show low p~ the PREMA end-member (see Fig. 5). It is distinguished (7.0-8.0) (Rudnick 1990, Depaolo 1982, Montgomery et from the low ~ (7.8) Pb occurring in the northern China al. 1978; Tu and Zhu 1992). Although there are no block, which shows an evolution ellipsoid around typical granulites found in the southern China block, the PM, and also from the Yangtze block, which shows a
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322
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block (Fig. 5). When we compare the gold deposits with the mineralization age of 100-200 Ma along a zone from north to south through Qinling mountain, northern Hunan Province, western Guangdong Province and Hainan Island, it can be found that the Pb isotopic ratios linearly increase ( ~ P b / ~ P b from < 17.0 to > 19.0) as can be seen in the topological diagram of Pb isotopes in three-dimensional space (Fig. 6), among which Guangdong and Hainan undoubtedly belong to Gondwana.
CONTINENTAL GROWTH Based on Nd model ages, it is indicated that Gondwanaland continuously experienced long-term continental growth (4.0-0 Ga) with a relative constant growth rate, whereas Laurasia mainly grew during 3.5-2 Ga, and after that time, the growth rate sharply decreased (Allegre 1984). The southern China block also shows a longer history of continental growth from 3.5 to 0.4 Ga based on Nd model ages, which can be comparable with Gondwana. Continent growth in the northern China block was almost terminated before 2.5 Ga. Based on Pb isotopic compositions of ores, Cenozoic basalts and Mesozoic granites, the boundary between
I
Gondwana and Yangtze can be established in the middle of Hunan along Xuefong mountain, northern Jiangxi and middle Zhejiang, approximately along latitude 29.2 N, northern Guangxi, south-western Guizhou and north-western Yunnan provinces (Fig. 7). The boundary between Gondwana and Yangtze may also stretch to the Japan Arc based on the Pb isotopic evidences from the Mesozoic-Neogene deposit, and igneous and metamorphic rocks (for example, Sato 1979; Sasaki 1982). As shown in Fig. 8, the outer zone of Japan including major parts of Shikoku, Kyushu and Hokkaido, and Hokkaido, and eastern part of Honshu is consistent with Gondwana and the southern China block. The inter zone of Japan could be a part of Yangtze block. The geochemical boundary of the southern China block with Gondwana features established by using Pb isotopes (Fig. 7) is generally consistent with the tectonic boundary, but the disconcordance between the two types of boundaries is still apparent. Although there are not enough geochemical data to establish an exact boundary, the basic difference between the two types of boundaries is that the former provides information on the deep crust and mantle, whereas the latter only
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Fig. 8. Pb isotopic mapping of Japan arc, black circle; Gondwana and SCB Pb; open circle; Yangtze lb.
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reflected the signature of tectonic strata. Mixing of materials from different blocks occurs in the boundary areas through change of erosion sources of strata in the collision zone, and magmatism in the subduction zone. Therefore geochemical data for a variety of rocks and deposits must be considered, the mantle and lower crustal rocks being especially important. The geochemical criteria for boundary division and its relationships with tectonic, paleontological and paleomagnetic evidences should be further studied.
SEAES 914--C
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Zhu, B.-Q. et al. 1989. Nd-Sr-Pb isotopic evidence for volcanism with MORB-OIB transition mantle in Liequong area. Geochemistr)' 3, 193-2O 1 Zhu, B.-Q. 1990. Three-component mixing and tbur-system recycling models for explaining Nd-Sr-Pb isotopic correlations of suboceanic and subcontinental mantles. Science in China 33, 757 76,~
Zhu. B.-Q 1991. Evidence of isotopic systematics from crus| mantle for chemical heterogeneities of terranes. Chinese ~ci. 36, I27%.1282. Zindler. A. et al. 1982. Nd, Sr and Pb isotopic systematics three-component mantle: a new perspective. ,Vature. Lond. 519 523.
and Bull
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