Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China

Journalof Southeast Asian EarthSciences,Vol. 10. No. 3/4, pp. 263-277, 1994 ElsevierScienceLtd 07~7(94)F..0002-U Printed in Great Britain

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Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China Jincheng Zhou*, Taiping Zhaot and Kerong Chen* *Department of Earth Sciences, Nanjing University, Nanjing 210008, China; tHenan Institute of Geological Sciences, Zhengzhou 450053, China (Received 23 January 1993; acceptedfor publication 31 December 1993)

Ala~raet--The Mesozoicvolcanicsequencefrom Lishuibasin in the LowerYangtzeregionis subdividedinto four formations which constitutethe associatedK-rich calc-alkalineseries and shoshoniteseries.The maficrocks in the Lishni shoshonite series are nearly saturated in silica. Various rocks of the whole shoshonite series are significantlyrich in K,O and have high Al203 and low TiO2. Fe content is not high, but has a high Fe203/FeO ratio. Moreover,the Lishnishoshoniteseriesare notablyenrichedin highlyincompatiblelithophileelements,such as Rb, Ba, Th, U, Zr, LREE, etc. which are generallyobserved in typicalshoshoniteassociation.Incompatible elements patterns show that the shoshoniteseries from Lishuiand other volcanic basins in the LowerYangtze region are, geochemically,similarto those of the East Africarift and distinctlydifferentfrom those of a typical island are. It is consideredthat the formationof the high-Kvolcanicseries from Lishuiand other volcanicbasins in the LowerYangtze region have no direct relationshipto the subductionof the Pacificplate but are connected with crustal thinning,mantleupweUingand the developmentof the YangtzeRiverdeep fracturezone in the area. Rb--Sr and Sm-Nd isotope data and model calculationsprovide constraintson the petrogenesisand evohitional relationship of the Lishui shoshoniteseries.

Introduction Tim Lowmt Yangtze region (Fig. 1) is a specific magmatic province of East China. The magmatism in Mesozoic volcanic faulted basins of the Luzhong, the Ningwu, etc. in the area have been studied previously (Pen et al. 1991; The Ningwu Research Project 1978, Xue et al. 1989). The Mesozoic volcanic sequences in Lishui, however, have not been studied thoroughly. The Mesozoic volcanic sequences were divided into calc-alkaline and alkali-calcic series by Wu et al. (1981). Recently, it has been confirmed by the authors that the two series should be assigned to the K-rich calc-alkaline and shoshonite series. In this paper, we will make a comparison of the petrographical, mineral--chemical and geochemical features of the two series. However, our discussion will only focus on the characteristics and petrogenesis of the shoshonite series.

Outline of the Geology The Lishui Mesozoic volcanic basin (Fig. 1) is located in the Lower Yangtze fault depression zone (or Mesozoic--Cenozoic rift zone, Mesozoic-Cenozoic quasi-rift zone and rift zone) according to Ren et al. (1991), Chen (1990), Zhai et al. (1992) and Wu (1987). The Lower Yangtze region was situated in the northeast flank of the Yangtze plate before the Triassic. The pre-Sinian metamorphic basements of the Yangtze plate in the region are made up of a hypometamorphic series with U-Pb ages of 2400-2900Ma (amphibolite facies, local granulite facies) and a epimetamorphic series with a U-Pb age of 1850 Ma (greenschist facies) (Zhai et aL 1992). The lower Paleozoic strata consist of thick marine sediments.

Absence of the middle and the lower Devonian series has been caused by Caledonian movement. Sandstones and shales in epicontinental littoral facies constitute the upper Devonian series. The Carboniferous, Permian and the middle to lower Triassic series are all composed of marine strata, except for the occurrence of a small amount of pyroclastic rocks in the CarboniferousPermian strata. From Sinian to middle Triassic, therefore, the Lower Yangtze region was relatively stable, tectonically. During this long period, thick marine sediments were deposited in extensional and fault-depressive tectonic settings. Among them are thick carbonate rocks which are widely distributed in the area. However, between the Upper Triassic and the Lower Jurassic strata in the area there is an obvious angular discordance. It is a result of the Indosinian movement caused by the collision between the North China plate and the Yangtze plate in the late Triassic (200-230 Ma). Moreover, the strata of the Sinian-Triassic systems in the area have all been folded. Recent studies indicate that the Dabieshan area on the north of the Lower Yangtze fault depression zone is a collision orogen generated by collision and merging of the North China plate with the Yangtze plate in the early Mesozoic (Xu et aL 1992). Since then, the Lower Yangtze fault depression zone began a new development stage as a tectonic active zone within Eurasian plate. During the Jurassic--Cretaceous Yenshan movement, intensive tectonic and magrnatic activities occurred in the Lower Yangtze fault depression zone. In the Early to middle Jurassic, intensive block faulting movements led to the formation of several faulted basins with volcanicity in the region. The Lishui basin is one such basin in the Lower Yangtze region (Fig. 1). At this time, fluviolaeustrine sediments were deposited in these basins. In the Late

263

.lincheng Zhou eta/.

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Fig. 1. Generalizedgeologicalmap of the Lishui volcanicbasin showingits location in the LowerYangtze fault depression zone (simplifiedafter geologicmaps of Jiangsu Bureau of Geologyand Mineral Resource 1984,and Qui t992). A. Luzhong volcanic basin, B. Ningwu volcanic basin, C. Study area: Lishui volcanic basin, D. Tan-Lu fault, E-E'. The location of generalized transverse section in Fig. 8, F. K2 E2 Strata, G. Jiashan formation, H. Guanshan formation, I. Dawangshan formation,J. Yunheshan formation,K. Longwangshanformation, L T2-J3basementstrata of the basin, M. LowerYangtze fault depression zone, N. The locations of samples (some locations of samples from Wu et al. 1981 and Deng et aL 1992). Jurassic, intensive faulting activities accompanied by large-scale volcanic eruptions and plutonic intrusions of intermediate-acidic magmas occurred. In the Early Cretaceous, further tectonic extension and deepening of faulting were followed by the eruptions and intrusions of intermediate-basic and -alkaline magmas. Furthermore, the large-scale magmatic activities led to the formation of many kinds of metallic and nonmetallic ore deposits in the area, such as those of Fe, Cu, Au and S, etc. Therefore, the Lower Yangtze fault depression zone became an important metallogenic province of Fe and Cu ore deposits in China. The Cretaceous volcanic sequences, from bottom to top, in the Lishui basin include Longwangshan, Yunheshan, Dawangshan, Guanshan and Jiashan formations. The Longwangshan formation is distributed in the middle and northwestern parts of the Lishui basin. Its major rock types are olivine pyroxene basalt, basaltic andesite, high-K andesite and related subvolcanic rocks. Alkaline olivine basalt and absarokite occur in the uppermost part of the Longwangshan formation. The Yunheshan formation consists of fluviolacustrine sediments, such as carbonate rocks, shale, siltstone, marl, jasperoid and tutIite. The Dawangshan formation distributed in the middle and eastern parts of the Lishui basin and around the Longwangshan formation com-

prises dominantly high-K andesite, high-K dacite, banakite and corresponding ignimbrites. The Rb-Sr whole rock and minerals isochron age of the high-K andesite porphyry is 125.3 + 7 Ma. The Guanshan formation is distributed in the southeastern part of Lishui basin and is composed of trachybasalt, shoshonite, banakite, trachyte, high-K trachyte and related subvolcanic rocks, such as syenite porphyry and aegirine-augite laurdalite porphyry. The Jiashan formation is distributed in the northern part of the Lishui basin and consists of rhyolite, rhyolitic ignimbrite and subvolcanic quartz porphyry. The Yunheshan formation represents the sediments of a period of volcanic inactivity. Other formations are the products of the four volcanic cycles, respectively.

Petrography and Mineral Chemistry Microprobe analyses results of minerals in the rocks are listed in Tables 2-5. L o n g w a n g s h a n cycle

The basalt of the Longwangshan cycle is dominantly olivine pyroxene basalt which consists of plagioclase

Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China

265

Table I. Chemical compositions of the Lishui volcanic complex Formation Sample SiO2

TiO2 AI203 Fe203 FeO MnO MgO CaO Na20 K20 P2Os Hz O+ H20CO 2 LoI

Total Rock type

Longwangshan Ib

Dawangshan

2

3

4

5b

48.13 1.19 17.13 5.45 4.67 0.23 4.43 9.55 3.04 2.15 0.40

54.48 0.84 16.88 3.62 5.46 0.16 3.39 5.87 4.39 1.57 0.55

57.11 0.50 17.84 3.25 5.05 0.17 2.02 6.82 2.75 2.01 0.50

59.28 0.64 17.06 1.70 5.87 0.16 2.40 6.07 3.61 2.53 0.72

48.32 1.09 17.34 5.24 5.90 0.16 4.92 10.11 3.34 1.20 0.37 1.24

3.04 99.41 Abs

2.67 99.88 Hdp

1.83 99.85 HKa

0.64 99.73 Pdp

6

99.73 Aob

11

SiO2 TiO2 AI203

53.14 0.71 16.68 5.23 3.64 0.14 3.43 7.14 3.28 3.41 0.47

57.59 0.64 15.46 3.05 3.07 0.I0 1.95 6.40 3.53 3.79 0.29

58.30 0.59 15.77 2.33 3.13 0.15 1.80 5.80 3.65 4.05 0.28

57.69 0.96 20.77 7.74 0.33 0.06 0.28 0.84 3.60 6.30 0.32

63.32 0.47 18.56 2.03 0.58 0.07 0.29 1.14 4.52 6.82 0.45

2.17 99.44

4.18 100.05

Sho

Ban

3.67 99.52 Eanp

1.28 100.19 Ban

1.23 99.48 Tr

13

14

H~O-

Rock type

2.34 99.41 Bard

99.13

Trb Jiashan

10c

Total

2.74 99.32 HKd

Guanshan 12b

9b 50.74 1.51 17.08 4.75 5.14 0.13 3.40 7.99 3.82 2.02 0.49 1.62 0.44

3.68 99.48 HKa

Sample

CO2 LOI

8b

62.01 64.09 62.03 0.45 0.42 0.45 1 5 . 1 2 1 5 . 0 8 16.03 2.09 2.96 3.78 2.44 1.40 0.57 0.10 0.09 0.19 1.50 1.15 0.57 4.50 3.94 5.52 2.97 2.94 3.37 4.42 4.33 4.36 0.20 0.18 0.20

0.5

Formation

Fe203 FeO MnO MgO CaO Na20 K20 P205 H20 +

7

Guanshan

15" 65.30 0.33 15.53 3.89 0.21 0.03 0.22 0.11 0.72 11.37 0.16 0.88 0.49 0.15 99.39 HKtr

16

17

58.37 0.21 20.12 1.10 1.63 0.19 0.00 1.25 7.25 5.95 0.05

71.64 0.24 14.14 1.53 1.06 0.01 0.22 1.74 3.91 4.72 0.29

3.23 99.35 Aalp

0.59 100.09 Rh

Data resource: a. Deng et al. (1992), b. Wu

et al. (1987), c. Jiangsu Bureau of Geology and Mineral Resource (1984). Others were analyzed in Central Lab. of Dept. of Earth Sciences, Nanjing University. I. Abs, absarokite; 2. Hdp, hornblende diorite porphyry; 3. HKa, high-K andesite; 4. Pdp, pyroxene diorite porphyry; 5. Aob, alkaline olivine basalt; 6. HKa, high-K andesite; 7. HKd, high-K dagite; 8. Bard, banakitic ignimbrite; 9. Trb, trachybasalt; 10. Sho, shoshonite; I 1, 13. Ban, banakite; 12. Banp, banakite porphyry; 14. Tr, trachyte; 15. HKtr, high-K trachyte; 16. Aalp, aegirine-augite laurdalite porphyry; 17. Rh, rhyolite.

(labradorite, Anso-6s), auglte and olivine. Some basalts erupted later in the cycle are absarokite with less Kfeldspar in matrix and alkaline olivine basalt with olivine more than 5%. The intermediate rocks of Longwangshan cycle, chemically, are basalt-andesite, high-K andesite and a small amount of dacite. The andesite may be subdivided into hornblende andesite and pyroxene andesite. In hornblende andesite, most plagloclases are andesine, labradorite (An6s Ab30 Or2; Table 2) is not common. Pigeonite occurs occasionally in the matrix. In pyroxene andesite, most clinopyroxenes are auglte, salite is subordinate and hypersthene is rare. In dacite, SiO:-rich glass exists in the matrix. The subvolcanic rock related to hornblende andesite is hornblende diorite porphyry, in which plagloclase phenocryst is andesine (An39 Ab59 Orz) and plagloclases in matrix are oligoclase-andesine (Table 2). The subvolcanic rock related to pyroxene andesite is pyroxene diorite porphyry in which plagoclase is labradorite (An6j Ab37 O1"2; Table 2), Ca-rich auglte (Wo3s.70-39.z6 En4o.s~L63 F519.67-19.90; Table 2) and

salite (Wo4Lss En39.56 Fsls.59; Table 3) are major mafic minerals and hypersthene (W02.93-3.33 En60.1s-64.24 FS32.43_36.s9; Table 2) is rarely found.

Dawangshan cycle In high-K andesite of the Dawangshan cycle, andesine (An33 Ab61 Or6-An36 Ab59 Ors; Table 2), Mg-rich biotite (Table 5) and salite occur as phenocrysts. The matrix is composed of K-rich semicrystalline materials. In high-K dacite, andesine (An36 Abss Or6; Table 2), Mg-rich biotite and sometimes sanidine and salite occur as phenocrysts. The matrix of high-K dacite consists of microlitic sanidine, SiO2- and K20-rich glass (Table 6), aphanitic and semicrystalline materials. It was reported that silica content of matrix glasses in shoshonite is generally greater than 60% (Morrison 1980). Table 6 shows that silica contents in the glassy matrix of high-K dacite are as high as 75.55-75.90%. It is worthy of mention that there are considerable magma inclusions in

Jincheng Zhou et al

266

Table 2. Microprobe analyses of feldspars from the Lishui volcanic complex Ptagoclase Longwangshan I

2

3

M SiO2 TiO 2 A1203 FeO MnO

MgO CaO Na20 K20 Cr203 Total

52.27 0.11 30.37 0.67 0.05 0.00 11.62 3.85 0.33 0.26 99.53

Rock type PdP Si Ti AI Fe Mn Mg Ca Na K

Cr An Ab Or

Dawangshan 4

HKa

5

~

59.99 0.00 25.83 0.19 0.05 0.00 6.49 5.57 0.84 0.04 99,01

59.88 0.15 25.51 0.18 0.11 0.00 6.45 5.71 0.84 0.00 98.84

Guanshan

7

~

9

Dawangshan

10

11

!2

57.15 0.04 27.79 0.53 0.00 0.00 8.65 4.65 0.68 0.02 99.53

59.81 0.23 25.75 0.22 0.06 0.07 2.66 5.62 0,83 0.00 99.25

65.41 0.18 19.25 0.15 0.01 0.15 0.21 3.20 0.15 100.02

Ban

Tr

HKd

Guanshan 13

i~

~5

16

M

52.04 58,15 60.27 59.42 0.03 0,00 0.00 0.12 30.91 26,91 24.47 25.62 1.03 0,59 0.61 0.37 0.15 0.00 0.14 0.05 0.30 0.00 0.00 0.06 11.66 7.73 5.81 6.71 2.87 6.49 7.45 6.69 0.16 0.32 0.77 0.88 0.24 0.17 0.22 0.22 99.39 100.36 99.75 100.15

2.380 2.368 0.004 0.001 1.628 1.656 0.026 0.040 0.002 0.004 0.000 0.200 0.586 0.568 0.340 0.252 0.019 0,008 0.009 0.008 61 68 37 30 2 2

Alkaline feldspar

Pya

HdP 2.594 2.699 0.000 0.000 1.415 1.292 0.022 0.023 0.000 0.005 0.000 0,000 0,369 0.279 0.561 0.647 0.018 0.044 0.006 0.009 39 29 59 67 2 4

59.35 5 9 . 4 7 57.84 0 . 0 9 0.16 0.08 25.34 26.81 27.02 0.11 0.21 0.49 0.00 0.00 0.23 0.00 0.00 0.05 6.69 7.23 7.95 6.74 6.49 6.62 0.94 0.80 0.87 0.14 0,08 0.02 99.40 101.34 100,58

Bani HKd HKa

HKa

2.652 2.684 2.687 2.664 0.004 0.000 0.005 0.003 1.348 1.364 1,349 1.340 0.014 0.008 0.007 0.004 0,002 0.002 0.004 0.000 0.004 0.000 0.000 0.000 0.321 0.312 0.310 0.320 0.579 0.484 0.497 0.588 0.050 0.048 0.048 0.052 0.006 0.000 0.000 0.003 34 37 36 33 61 57 58 61 5 6 ~ 6

2.680 0.010 1.400 0.010 0.000 0.000 0.036 0.560 0.040 0.000 36 59 5

Ban

2.581 2.564 2.672 0.003 0.001 0.008 1,421 1.472 1.356 0,018 0.020 0.008 0.009 0.000 0.004 0.003 0.000 0.004 0,380 0,416 0.320 0.527 0.404 0.488 0.046 0.040 0.048 0.001 0.001 0.000 37 48 20 58 47 73 5 5 7

11.37

2.962 0.006 t.027 0.003 0.000 0.010 0.010 0.281 0.657 0.000 I2 33.9 6&9

M: matrix plagioclase. Pya: pyroxene andesite; others same as Table 1. Analysis done in the Microprobe Lab. of China University of Geosciences.

Table 3. Chemical compositions of pyroxene from the Lishui volcanic complex Longwangshan

Formation Sample

SiO2 TiO2 AI20 ~ FeO F%O 3 MnO MgO CaO Na20 K20 Total Rock type Si Ti AI Fe 2+ Fe 3+ Mn Mg Ca Na K Wo En Fs

Dawangshan Guanshan

1

2

3

4

5

6~

7~

Aug

Aug

Sal

Hy

Hy

Sal

Sat

50.58 0.75 2.60 11.41 0.00 0.36 13.14 17.57 0.52 0.00 96.97

5 1 . 1 0 51.19 0.77 0.48 2.36 3.00 l 1.25 10.08 0.00 0.00 0.43 0.32 1 3 . 3 4 12.05 1 7 . 2 5 17.74 0.56 0.50 0.00 0.04 97.08 96.29

53.21 0.20 1.51 20.08 0.00 0.88 22.32 1.61 0.45 0.01 100.35

52.69 0.29 0.54 22.25 0.00 1.26 20.36 1,38 0.12 0.00 98.90

52.66 0.18 1.62 7.25 1.99 0.49 13.29 21.95 0.47 0.10 100.00

52.05 0.31 1.51 6.5 2.76 0.39 14.82 21.15 0.54 0.05 100.08

pdp

Pdp

Pdp

Pdp

Pdp

HKai

Banp

1.950 0.022 0.118 0.368 0.000 0.012 0.755 0.726 0.039 0.000

1.973 0.023 0.161 0.363 0.000 0.014 0.768 0.714 0.021 0.000

1.987 0.014 0.137 0.327 0.000 0.010 0.697 0.738 0.038 0.002

1.977 0.006 0.099 0.624 0.000 0,028 1.236 0.064 0.033 0.000

1.996 0.008 0.024 0.705 0.000 0.041 1.150 0.056 0.009 0.000

1.962 0.005 0.071 0.226 0.056 0.015 0.738 0.876 0.034 0.005

1.940 0.009 0.066 0.203 0.078 0.012 0.824 0.845 0.002 0.002

39.26 40.83 19.90

3 8 . 7 0 41.88 4 1 . 6 3 39.56 1 9 . 6 7 18.59

3.33 64.24 32.43

2.93 60.18 36.89

47.61 40.10 12.28

45.13 44.02 10.84

Data resource: a. Wu et al. (1981) (by chemical analysis), Others were analyzed by electron microprobe in the Microprobe Lab. of China University of Geosciences. Aug, augite; Sal, salite; Hy, hypersthene. HKai, high-K andesitic ignimbrite. Other rock abbreviations, same as Table i.

65.46 61.85 55.86 67.51 0.26 0 . 0 3 0 . 0 0 0.06 18.25 20.55 26.70 19.53 0.35 0.19 0 . 3 9 0.00 0.00 0 . 0 6 0 . 0 3 0.09 0.00 0 . 0 0 0.00 0.00 0,18 0 . 0 9 0 . 1 9 0.15 0.93 0.96 10.09 7.43 13.83 6 , 3 3 4 . 5 8 4.26 0.12 0 . 0 0 0.00 0.06 99.46 96.07 99.85 99.09 Tr

Aalp Aalp

Tr

3 . 0 0 8 2.880 2,600 3.000 0 . 0 0 9 0,000 0.000 0.002 0 . 9 8 8 1,120 1.440 1.024 0 . 0 1 4 0.000 0.000 0.000 0 . 0 0 0 0.000 0,000 0,004 0 . 0 0 0 0.000 0.000 0.000 0 . 0 0 9 0.000 0.000 0.008 0,083 0,840 0.920 0.642 0.810 0.360 0,280 0,240 0 . 0 0 4 0.000 0.000 0.004 1.0 04 0.8 0,8 9,2 62 3 7 6 . 4 72.0 89,8 3 7 . 3 2 2 . 8 27.2

M e s o z o i c s h o s h o n i t e series f r o m Lishui in the L o w e r Y a n g t z e region, C h i n a Table 4. Microprobe analyses of hornblende from the Lishui volcanic complex Formation

I

SiO2 TiO2 Al203 FeO MnO MgO CaO Na20 KzO Cr203 Total

plagioelase a n d o t h e r m i n e r a l p h e n o e r y s t s o f h i g h - K dacite. The solid phases in the m a g m a inclusions t r a p p e d in plagioclases include v a r i o u s glasses a n d fine, optically unidentifiable seed crystals o f K-feldspar(?) (Kc). T h e chemical c o m p o s i t i o n s o f the coexisting solid p h a s e s in inclusions have been d e t e r m i n a t e d b y m i c r o p r o b e before h o m o g e n i z a t i o n . Based on chemical c o m p o s i t i o n s , the glasses m a y be s u b d i v i d e d into rhyolitic (Rg), K feldspathic (Kg) a n d K - f e l d s p a r c o m p o n e n t - b e a r i n g silicic glass (Skg) (Table 6). T h e chemical c o m p o s i t i o n s o f coexisting solid phases in m a g m a inclusions in plagioclase p h e n o c r y s t s indicate the e v o l u t i o n a l t r e n d o f h o s t h i g h - K dacitic m a g m a f r o m one aspect. T h e b a n a k i t e in the D a w a n g s h a n cycle is chemically a n d m i n e r a l o g i c a l l y similar to those in the G u a n s h a n cycle (see below). In the D a w a n g s h a n cycle, besides v a r i o u s lavas, there are vitric crystal i g n i m b r i t e a b o u t 1 k m thick which, chemically, are h i g h - K andesitic, h i g h - K daeitic a n d banakitic. T e m p e r a t u r e estimates were calculated using the K u d o a n d Weill (1970) plagioclase t h e r m o m e t e r a n d the S t o r m e r (1975) t w o - f e l d s p a r g e o t h e r m o m e t e r . T h e quenching t e m p e r a t u r e c a l c u l a t e d for h i g h - K andesite is

Longwangshan

Sample

2

43.52 1.94 12.17 14.75 0.43 I 1.54 10.07 2.29 0.48 0.13 97.34

43.26 2.65 12.79 13.20 0.24 12.74 9.77 2.2O 0.55 0.02 97.41

Rock type

Hdp

Hdp

Si Ti A1 Fe Mn Mg Ca Na K Cr

6.452 0.219 2.128 1.829 0.058 2.553 1.599 0.656 0.092 0.012

6.360 0.288 2.220 1.622 0.035 2.795 1.541 0.633 0.104 0.002

Analysis done in the Microprobe Lab. of China University of Geosciences. Rock abbreviations, same as Table 1.

Table 5. Microprobe analyses of biotite from the Lishui volcanic complex Formation Sample SiO2 TiO2 AI203 FeO MnO MgO CaO Na20 K20 CrzO 3 BaO Total Rock type Si TP v Tivl AItv AIv) Fe3+ Fe2+ Mn Mg Ca Na K Cr 8a

Longwangshan I 38.92 5.28 15.94 10.15 0,16 14.95 0.16 0.58 9.31 0.03 0.00 95.47 Pya

2 37.82 4.63 13.99 14.02 0.27 14.89 0.00 0.42 9.25 0.33 0.42 96.04 Pya

2.82 0.00 0.29 1.19 0.17 0.09 0.53 0.01 1.61 0.01 0.08 0,86 0.00 0.00

2.76 0.04 0.21 1.20 0.00 0.38 0.47 0.02 1.62 0.00 0.06 0.86 0.02 0.00

Dawangshan 3

Guanshan

4

38,53 4.45 14,44 16.20 0.36 13.58 0.04 0.64 8,75 0.I1 0.00 97.12 Bani

37.29 4.22 13.74 16.89 0.59 13.05 0.00 0.94 9.08 0.00 0.68 97.00 HKa On the basis 2.79 2.73 0.00 0.08 0.24 0.15 1.21 1.27 0.02 0.00 0.29 0.58 0.70 0.46 0.02 0.04 1.47 1.42 0.00 0.00 0.09 0.13 0.8l 0.85 0.01 0.00 0.00 0.04

5

6

38.27 37.35 5,57 6.14 14.44 14,50 10,79 11.06 0.01 0.10 16,79 16.26 0.01 0.18 0.67 0.70 9.15 8.72 0.11 0.10 0.00 0.00 9 5 . 7 9 95.10 Ban Ban of 11 oxygens 2.75 2.72 0.03 0.04 0.27 0.30 1.22 1.24 0.00 0.00 0.30 0.24 0.35 0.43 0.00 0.01 1.80 1.76 0.00 0.01 0.09 0.I0 0.84 0.81 0.01 0.01 0.00 0.00

7

8

9

37.58 5.13 13,99 14.66 0.30 14.20 0.00 0.65 9.14 0.00 0.88 96.53 Ban

37.47 5.44 14,08 14.92 0.08 14.46 0.04 1.06 9.10 0.01 0.37 97.03 Ban

33.74 6.02 15,78 16.32 0.05 12.67 0.09 0.55 9.21 0.00 0.00 98.43 Banp

2.74 0.06 0.22 1.20 0.00 0.37 0.53 0.02 1.55 0.00 0.09 0.85 0.00 0.03

2.72 0.07 0.23 1.21 0.00 0.33 0.58 0.01 1.57 0.0(3 0.15 0.84 0.00 0.01

2.52 0.09 0.25 1.39 0.00 0.55 0.48 0,00 1.41 0.01 0.08 0.88 0.00 0.00

Mg

60

60

54

54

66

64

58

58

52

A1vl + Fe3+ + TiT M

20

22

20

28

21

20

22

20

30

Fe2+ + Mn

20

18

26

18

13

16

20

22

18

Fe3+ and Fe2+ calculations carried out using the charge difference method of Zheng (1983). Analysis done in the Microprobe Lab. of China University of Geosciences. Rock abbreviations, same as in Tables I and 2.

267

268

Jmcheng Zhou ,:i ai, -Fable 6. Microprobe analyses of matrix of high-K dacite and banakite and the magma inclusions in their plagioclase phenocrys! Fo,mation

Guanshan

Dawallgshitl~

Rock type

11Kd

ftkd

MI

Ma

Rg

SiO~ TiO 2 A120~ FeO MnO MgO CaO Na, O K~O P205 Cr20s Total

Mi

Skg

75.44 0.26 12.65 0.47 0.10 0.00 0.06 0.87 7.61 0.00 0.04 97,49

Ma

Kc

1

2

83.20 0.05 9.59 0.17 0.00 0.01 0.13 0.47 5,75 0.00 0.08 99.45

85.38 0.25 7.16 0.29 0.00 0,00 0. t I (i.48 3,56 (/.05 0.13 9%42

Ban Mi

Kg

Kc

(7)

75.55 0 19 12,09 1,08 0.01 0.0(I 0.29 2.18 7,10 0,00 0.30 97.94

Rg

C)

6Z85 02 l 19,06 0.12 0,06 0,00 0.03 t.79 ll.89 0 10 0.05 101.16

67.96 0.00 18.44 0.54 0.00 0.00 0.11 1,73 11.28 0.00 0.00 100.47

75.90 0.07 13.01 1,22 0.04 0.00 0.91 2.80 5.52 0.00 0.07 99.52

63.38 0.13 18.59 0.47 0.02 0.09 0.63 2.57 10,73 0.00 0.12 96.74

71,97 0.11 14.88 0.50 0.00 0.00 0.34 1.27 8,63 0.00 0.65 98.36

Mi: magma inclusion; Ma: matrix; Rg: rhyolitic glass; Kg: K-feldspathic glass; Skg: K-feldspar component-bearing silicic glass; Kc: microlitic K-feldspar crystals; Rg, Kg, Skg, Kc are coexisting phases in magma inclusion. Rock abbreviation same as Table I. Analysis done in the Microprobe Lab. of China University of Geosciences.

1 159°C. The crystallization temperature of feldspar phenocrysts in high-level magma chamber (Pmo = l kb) calculated for high-K andesite is 934.6'C

interstitial alkali-rich glasses between microlitic plagioclases in the matrix. In shoshonite, andesine, hornblende, salite and biotite occur as phenocryst phases. The intervals between microlitic plagioclases in matrix have been filled by alkalirich glasses and aphanitic materials. In banakite, phenocryst phases include zoned andesine (An37 Ab58 Ors-An48 Ab47 Ors; Table 2), Mg-rich biotite and less K-feldspar and salite. The

Guanshan cycle Compared with alkaline olivine basalt of the Long wangshan cycle, the trachybasalt of the Guanshan cycle has plagioclase with lower An (andesine) and there exist

Ph

/

T

F ~,

©

10

8

tXt

/

+

b• II a&,s, a

/\ U1

A A-A

&

.-.

• •" ,

Z ,it

B

Z 0

R

I

Os

•

\

O2 Oa

•

'

~, B

, t

I

I

I

SiO2wt

A ~X

o I

Dy

c D

\ \\

l

\

I

%

Fig. 2. Total alkali-silica diagram of the Lishui Mesozoic volcanic complex (fields and Nomenclature after Le Bas et al. 1986). Pc: picritic basalt; B: basalt; OL: basaltic andesite; 02: andesite; Os: dacite; R: rhyolite; S~: trachytic basalt; S.,A: shoshonite; S2B: latite; T: traehyte; U t : basanite or tephrite; U2: phonotephrite; Us: tephritic phonolite; Ph: phonolite; F: feldspathoidite. Legend represents rocks of four formations: A. Longwangshan formation, B. Dawangshan formation, C, Guanshan formation, D. Jiashan formation. Double circles (X), average of shoshonites of Yellowstone Park, Wyoming, U.S.A. (after Joplin 1968) Square (Y), average of quartz banakite of Yellowstone Park. Wyoming, U.S.A. (after Joplin 1968).

Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China

and aegirine-augite occur as phenocryst phases. There exist orientated arrays of acicular anorthoclases and a small amount of nepheline in the matrix. In the volcanic complex of the Guanshan cycle, biotite is a ubiquitous marie phase. As mentioned before, the biotites in various volcanic rocks in the Longwangshan, Dawangshan and Guanshan cycles are all Mg-rich biotites. These biotites have higher MgO (13.05-16.79%), TiO2 (4.22-6.02%) and lower AI203 (13.74--15.94%) (Table 5) and the biotites in volcanic complex of Guanshan cycle have higher TiO2 than those of Dawangshan cycle. From petrochemical data (see below, Table 1), it is evident that TiO2 content in biotites increases with the increase in alkalinity of magma. The fugacity of biotite crystallization in magma was estimated using Wones and Eugster (1965) diagram. Based on the above, the calculated temperature for high-K andesitic magma (934.6°C) and Fe/Fe + Mg ratio of biotite in it, the fugacity of biotites crystallization in h~gh-K andesitic magma is about 10-1ZSatm. Pe-Piper (1984) pointed out that augite phenocrysts occur in rocks with intermediate K content, but salite occurs only in K-rich rocks. In the Lishui volcanic complex with similar features, augite phenocrysts occur in moderate K-rich volcanic rocks of the Longwangshan cycle. Salite is not common among them.

plagloclase phenocrysts also trapped some magma inclusions in which solid phases are composed of rhyolitic glass (Rg) and seed crystals of K-feldspar(?) (Kc) (Table 6). K-feldspar also occurs as the rim around plagioclase phenocrysts. The matrix of banakite is composed of microlitic plagloclase and interstitial alkali feldspar grains and alkali- and SiO2-rich glasses. In trachyte, phenocryst phases are Mg-rich biotite and almost equal amounts of plagioclases and alkali feldspars. Plagioclase is oligoclase (An20 Ab73 Or7, Table 2). Alkali feldspars are sanidine with less anorthoclase (Ano.s AbTz00r27.2; Table 2). The matrix consists of SiO2- and K2 O-rich allotriomorphic granular semicrystalline materials. In high-K trachyte, K20 content is as high as 11%. Phenocryst phases include Mg-rich biotite, K-feldspar and plagioclase. The Mg-rich biotite has been darkened. The amount of K-feldspars is more than plagioclase. Additionally, the amount of norm Or in K-feldspars is very large (AnL0 Abg.~ Ors~.8; Table 2). In matrix, orientated slender prismatic sanidines exhibit a trachytic texture. Subvolcanic rocks of the Guanshan cycle are banakite porphyry and biotite syenite porphyry. In addition, aeglrine-augite laurdalite porphyry, a special sodium-rich alkaline subvolcanic rock occurs (Table 1). Anorthoclase 12

•

1o

go

0

• °°°~

~

• /" ~

• /'-~

10 , 1 ~

• ,

L% . . ~

[

/

A A

++

o 0°~o o

L •

11

45

51

57

63

69

75

Si02g Fig. 3. K20-SiO2 diagram of the Lishuivolcaniccomplex(fieldsand nomenclaturesafter peccerillorand Taylor 1976). Legendsameas Fig. 2. I. low-K tholeiite, 2. low-K basalticandesite,3. Iow-K andesite, 4. low-K dacite; 5. Iow-K rhyolite, 6. basalt, 7. basalticandesite,8. andesite,9. dacite, 10. rhyolite, I 1. high-K basalticandesite, 12. high-Kandesite, 13. high-K dacite, 14. absarokite, 15. shoshonite, 16. banakite. 5F..AES 10/3-4--I

269

27(i

Jincheng Zhou ei a/.

However, the K-rich volcanic rocks of the Dawangshan and Guanshan cycles have salite phenocrysts while augite phenocryst has never been found in them. Bolh the occurrence of salite as a phenocryst mineral and the existence of sanidine in phenocryst and matrix of the volcanic complex, therefore, may be regarded as the mineralogical mark for the Lishui K-rich rock series. Jiashan cycle

Major rock types occurring in Jianshan cycle include rhyolite, rhyolitic ignimbrite and subvolcanic quartz porphyry. The rhyolite is nearly aphyric with perlitic and well-developed flow structures, which look like a sedimentary rock in the field outcrop.

Geochemistry Petrochemistry and volcanic rock series

The representative samples of the Lishui volcanic complex were selected for chemical analysis (Table I). The petrochemical data of 98 samples gathered from references (Wu et al. 1981, Jiangsu Bureau of Geology and Mineral Resource 1984, Deng et al. 1992) are together plotted in TAS and K20-SiO2 diagrams (Figs 2 and 3) which provided the basis for studying petrochemical features of the Lishui volcanic complex. A notable feature in the petrochemistry of the Lishui volcanic complex is that, with increasing SiO2, total alkali, especially K: O, increased significantly. Therefore, the K20-SiO2 diagram clearly shows the petrochemical characteristics of the volcanic complex (Fig. 3).

From Fig. 3, it is evident that most rocks ol the Dawangshan and the Guanshan cycles fall into the field of shoshonite association. We have reason, therefore, to consider that they should be classified as a shoshonite series. Most rocks of the Longwangshan cycle, however~ are located in the transitional area between the caicalkaline series and the high-K talc-alkaline series except for absarokite which erupted later in the Longwangshan cycle. Thus they have been named a K-rich calc-alkaline series by us. On the basis of experimental data iMeen 1990) and other studies (Keller 1974), the small amount of rhyolite of the Jiashan cycle may be considered as part of the shoshonite series. Petrochemically, typical features o[ the Lishui shoshonite series are high total alkalies ( N a , O + K~O = 5.31 13.69), K20/Na~O ratio (0.82 2.87 (Fig. 4), AI,O~ (14.14-20.77) and low TiO2 (0.21 1.21). Fe content is not high. Most data points on the AFM diagram are situated below the boundary between tholeiitic and talc-alkaline series (Fig. 5). But Fe20~/i--'eO ratio is high (0.67-3.5). Absarokite is nearly saturated in silica and its norm Q is less than 3. Whole rock major element data of the Lishui volcanic complex are consistent with the typical characteristics of shoshonite association (Morrison 1980). In contrast, K20-rich calc-alkaline series of l,ishui have lower K20, K : O + N a ~ O (Fig. 3) and Fe:O~/FeO, but are richer in Fe (Fig. 5 R E E and trace elements

REE abundances of the Lishui volcanic complex are listed in Table 7 and average EREE of each cycle is as follows: Longwangshan 111.9 ppm, Dawangshan

i [

2"5 I I 2.0 .5

O

SHO

A

B

Z 1.5 "~I

Oo

J'

•

%

L~

0 o ~

a

•

,t

,5

,5

0000

oo

A

0"5 t A j'

45 ¸

•

s;

CA

s'5

/0

7'0

Si02% Fig. 4. K20/Na:O-SiO 2 diagram of the Lishui volcanic complex. SHO, shoshonite series, CA, K-rich calc-alkaline series. Legend same as Fig. 2,

Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China F

A

means that the magma of the Guanshan cycle is not a differentiated product of the latter magma. On the other hand, the enrichment of alkali (especially K20) in volcanic magma of the Guanshan cycle relative to other cycles suggests that it may be derived from deeper source areas. Trace elements abundances of the Lishui volcanic complex are given in Table 8. From Tables 1, 7 and 8, it is obvious that, compared to K-rich calc-alkaline series, the Lishui shoshonite series are notably enriched in highly incompatible lithophile elements such as K, Rb, Ba, Th, U, LREE, etc. and, on average, have higher intermediate incompatible elements (e.g. Zr). The characteristics are generally observed in other shoshonite associations (Morrison 1980). The Lishui shoshonite series also has slightly higher compatible elements (e.g. Ni and Co) than island are shoshonitic andsite and dacite (Morrison 1980). To discern the tectonic setting of the Lishui Mesozoic volcanic magmas, the authors compared the trace elements and REE abundances of mafic rock from the Lishui shoshonite series with those of shoshonitic mafic rocks from different tectonic environments. This is on the assumption that mafic rocks represent relatively primitive melts which have not been significantly affected by other geological processes (Peceerillo et al. 1984). Figure 7 shows the incompatible element patterns normalized by hypothetical primordial mantle compositions (Wood 1979) for absarokites from Lishui and Luzhong basins in the Lower Yangtze fault depression zone (Ren et al. 1991), East Africa rift (Mitchell et al. 1976, Bell et al. 1969) and island arc (Gill 1970, Morrison 1980). Figure 7 indicates that the patterns of absarokites from the Lower Yangtze fault depression zone are distinctly different from that of the island arc. For example, the patterns of absarokite from Lishui and Luzhong have Rb, Th, Ce, Nd and Zr spikes (similar to that of the East Africa rift). In contrast, the pattern of the island arc has depletions for these elements. On the other hand, the patterns of absarokites from Lishui and Luzhong have

M

Fig. 5. AFM diagram of the Lishui volcanic complex. Dashed line is a boundary between tholeiitic and calc-alkaline series. Legend same as Fig. 2.

146.9ppm, Guanshan 131.6ppm, Jiashan 236.8ppm. (La/Lu)N values indicate relative enrichment of LREE. The volcanic rocks of the Dawangshan cycle have weak Eu negative anomalies (Fig. 6). But for the volcanic complex of other cycles, Eu negative anomalies are all not prominent. Moreover, there are no correlations between Eu anomalies and both CaO variation in rocks and plagioclase crystallization. It suggests that the oxygen fugacity of magma in the process of mineral crystallization is high and Eu occurs as Eu 3+ rather than Eu 2+. For the volcanic complex from the Longwangshan, Dawangshan to Jiashan cycles, LREE/HREE [or (La/Lu)N] increase gradually. These imply that fractional crystallization was dominant. With fractional crystallization of LREE-depleted orthopyroxene, clinopyroxene and amphibole, the evolved magmas became richer in LREE. In addition, Table 7 shows that YREE and LREE/HREE for the volcanic complex of the Guanshan cycle are less than those of the Dawangshan cycle. It

Table 7. REE analyses of the Lishui volcanic complex (ppm) Formation Sample La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Y YREE (La/Lu)s LREE/HREE Eu/Eu* Rock type

Longwangshan

271

Dawangshan

Guanshan

I

2

3

4

5

6

7

19.50 42.50 6.30 24.70 5.20 1.40 4.60 3.70 2.20 2.20 0.34 19.70 112.6 6.1 7.6 0.86 HKa

28.17 55.21 6.04 20.10 5.03 1.36 4.07 2.99 1.81 1.75 0.28 15.56 126.8 10.9 10.6 0.89 Pdp

16.65 36.69 4.91 19.46 4.93 1,30 4.65 3.46 2.03 1.85 0.29 17.39 96.3 6.8 6.8 0.82 Hdp

32.68 61.12 6.85 23.65 4.85 1.06 3.96 3.16 1.87 1.88 0.23 16.76 141.3 15.3 1!.7 0.72 HKa

36.21 66.52 7.46 24.83 5.14 1.07 3.96 3.09 1.87 1.92 0.30 16.36 152.4 13.1 12.7 0.70 Ban

31.10 60.10 6.05 25.40 5.01 1,26 4,19 3.18 1.78 1.69 0.27 16.20 140.0 12.2 11.6 0.82 Ban

28.10 56.60 5.51 22.20 4.48 1.21 3.99 3.00 !.60 1.51 0.23 15.00 128.4 12.9 1i.4 0.86 Banp

Jiashan 8

9

27.00 66.07 57.20 103.60 4.92 11.81 21.30 36.36 4.42 6.61 1.18 1.31 4.03 4.48 3.11 3.18 1.64 1.67 !.61 1,43 0.24 0.23 16.00 14.60 126.6 236.8 12.3 31.4 10.9 20.5 0.84 0.69 Banp Rh

REE analysis by ICP in the Modern Analytic Center, Nanjing University. Rock abbreviations, same as Table I.

~.,~""~

Jincheng Zhou et al.

3~

oi

9,

\

10z i

t °.h..N.. --..:

10 z

- ~ 1

,o!i 4 /

~,~e~'.

,

,

La

,

i

i

t

~

~ "-'---.-..-

...........

i

i

I

Ce P r Nd S m Eu Gd T b Dy

1

~

Y

tto

S ~ ,

"-. ° ~ ° A o b ( C )

i

i

"i ' " ' - i Th ~C

Er T m Y b Lu

Fig. 6. Chondrite-normalized REE patterns of the Lishui volcanic complex (normalizing values of Boynton 1984). U D G. J. represent rocks of Longwangshan, Dawangshan, Guanshan and Jiashan formations respectively, Sample number same as Table 7. The patterns of the Cenozoic tholeiite [Th(C), Zhou 1983] and alkaline olivine basalt [Aob(C), Deng 1992] are also shown for comparison.

depletions for Ba, Sr and P (also similar to that of the East Africa rift), that of the island arc, however, has spikes for these elements. In Fig. 7, in general, the patterns of Lishui and Luzhong are situated between those of the East Africa rift and the island arc, and

keep basically synchronous variations with that of the former. It appears that some similarities in geochemical features exist between two high-K mafic rocks from the Lower Yangtze fault depression zone and the East Africa rift.

Table 8. Trace elements abundances of the Lishui volcanic complex (ppm) Longwangshan 1~ Rb Ba Th U Sr Zr Sc V Cr Ni Co Rock type

2

3

4

Dawangshan 5b

6

7

8

Guanshan 9

l0 b

11

12

13

Jiashan 14~

84.6 68.0 32.0 54.7 148.0 158.0 122,0 166.0 68.8 227.0 216,0 2 1 4 . 0 431.0 542.0 602.5 520.9 411.8 308.8 660.4 647.5 518.1 738.3 550.1 1030.0 971,2 985.9 1490.0 7.5 20,5 1.4 2,3 492.0 162.5 620.2 541.9 870.2 538.3 278.7 291.5 576.6 988.6 212.7 97,6 87.1 25.6 269.0 115.0 135.5 81.4 138.2 124.4 112.9 148.6 222.0 258.4 180.5 2 3 6 . 6 154.0 9 0 8 . 0 22.4 8.5 13.6 16.9 58.2 9.7 8.7 5.8 8.7 45.4 10.6 15.9 7.3 6.2 62.5 97.3 159.9 3 4 0 . 4 86.9 70.8 79.6 84.8 207.3 120.5 143.5 72.8 30.7 30.0 105.6 58.9 78.2 39.1 28.2 114.6 88.6 98.1 88.0 89.7 43.8 35.6 6.3 87.9 41.1 58.4 25.4 27.0 83.8 59.1 51.7 41.3 39.9 29.3 59.2 10.0 16.6 21.5 21.2 12.4 8.2 4.9 12.9 23.5 8.9 18.9 5.9 5.1 Abs

HKa

Pdp

Hdp

Aob

HKa

HKa

HKd

HKa

Trb

Ban

Ban

Tr

HKtr

15

16

0.0 30.0 13.0 1311,2 319.6 193.0 1.0 23.1 68.3 48.4 2.7 Aa!p

418.9 3.1 37.3 70. l 54.8 35 Rh

Data resource: a: Deng (1992), b: From personal communication with H. N. Chen, Others were determinated by XRF in the Modern Analytic Center, Nanjing University. 1~, average value of two samples. Rock abbreviations, same as Table 1.

Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China •

•

273 1

10:

\

' \

Ill

M

,,

X .

,,

,, .,o"

a

\o.-"

.N t.

/,,'%. o

',, .... "\

//

10

\, %,

'\

.

\ ;° !]\\ •

\,,

O

~e

Rb

Ba

Ce

Sr

Nd

Zr

Sm

:j;,"

Ti

Fig. 7. Incompatible elements patterns normalized by hypothetical primordial mantle compositions for absarokites from Lishui and Luzhong of the Lower Yangtze fault depression zone, East Africa rift and island arc area. 1. East Africa rift (Mitchell et al. 1976, Bell et aL 1979). 2. Island arc (Gill 1970, Morrison 1980). 3. Luzhong (Ren 1991). 4. Lishui.

Isotopic compositions

The whole rock and minerals Rb-Sr isotope values of high-K andesite porphyry of the Dawangshan cycle are given in Table 9, which yields an isochron age of 125.3 5:7 Ma with e,r(125.3 Ma) = 41.2. The whole rock Sm-Nd isotope values of high-K andesite porphyry of the Dawangshan cycle and hornblende diorite porphyry of the Longwangshan cycle are listed in Table 10. The volcanic complex of four cycles has been dated by the mineral K-Ar method (Wu et al. 1981): the pyroxene diorite porphyry of the Longwangshan cycle yields pyroxene age of 124.6 Ma, the banakite of the Dawangshah biotite age of 121.0 Ma, the banakite of the Guanshan biotite age of 105 Ma, and quartz porphyry of Jiashan biotite age of 99.3 Ma. These age data are comparable to the timescale of four cycles of magmatism in the nearby Ningwu basin (The Ningwu Research Project 1978). In the light of the above K-At and Rb-Sr data, the time-span of volcanic activities of the four cycles in the Lishui basin has been defined as 124.6-99.3 Ma (early Cretaceous volcanism). The timespan of isotopic geochronology is different from that Table 9. Rb-Sr isotopic analyses of the Lishui volcanic complex Rocks and minerals (Dawangshan cycle) Hkap Hkap Hkap Hkap Plagioclase Biotite

Rb (ppm) Sr (ppm) S?Rb/~'Sr S?Sr/~Sr 0.77649 0.49434 0.77120 0.49079 0.02748 0.82692

0.36997 0.64325 0.28038 0.59689 1.43514 0.32690

2.09883 0.76850 2.75054 0.82224 0.01915 2.52963

0.71057 0.70869 0.71194 0.70848 0.70716 0.71197

T = 125.3 + 7 Ma, Isr =0.70713 + 1.8 x 10-4, r =0.9935, %r(T) = 41.2. Determination done in Institute of Geology and Mineral Resources of Chengdu. HKap, high-K andesite porphyry.

of biochronology 03, Longwangshan cycle; K~, Dawangsahn, Guanshan and Jiashan cycles, Wu et al. 1981). In addition, the initial Sr isotope ratios (0.70713) in Table 9 and 8ND(T) (--4.0 to --7.9) in Table 10 suggest that some crust materials were involved in the shoshonitic magma which was originally derived from the mantle, and the magma of the Dawangshan cycle assimilated more crust materials than that of the Longwangshan cycle.

Discussion The classification of rock series

The Lishui shoshonite series was previously considered to be alkali-calcic series (Wu, 198 I). It is apparent that this classification could not reflect essential characteristics of the Lishui high-K volcanic complex. Petrographic and geochemical data show that the Lishui high-K rocks have typical features of the shoshonite series. Jakes et al. (1972) estimated that the shoshonite association are composed mainly of basic rocks (50%), with intermediate rocks (40%) and less dacitic rocks (10%). In the Lishui and other volcanic basins of the Lower Yangtze fault depression zone, the high-K rock series is dominated by intermediate-acidic rocks, with fewer basic rocks. For example, intermediate rocks make up 70% of the high-K volcanic rock series in Luzhong basin, leading some workers to conclude that the Mesozoic high-K volcanic rock series from Luzhong belongs to the latite series rather than shoshonite series (Yu et al. 1981). In fact, this involves an understanding of the characteristics and petrogenesis of the shoshonite series. The essential features of the shoshonite series are unusual enrichment of K20 and a moderate saturation of

274

Jincheng Zhou et al. 1 able 10. Sm. Nd isotopic analyses of the Lishui volcanic complex Formation 1 2

Rock type Sm (ppm)Nd (ppm) 147Sm/~44Nd *a3Nd/l*~Nd ~No(T)

Dawangshan Longwangshan

Hkap Bdp

4.023 4.023

~,,.""136 I9,070

0.109929 0.127599

0,51262 0.512368

-- 7.9 ~4.(1

Determination done in Institute of Geology and Mineral Resources of Chengdu, Rock abbreviations, same :~s Tables I, ~)

SiO2. The display of the feature in mafic rock is that absarokite is nearly saturated in silica and thus rarely has normative Ne or Q. There exists a certain amount of special mafic rock, absarokite, in the shoshonite series from the Lishui, Luzhong and Ningwu basins in the Lower Yangtze fault depression zone. On the other hand, related experiments (Meen 1987, 1990) show that basalts, which reservoired beneath normal continental crust (about 35 km thick) and evolved by fractional crystallization in chambers near the Moho, can evolve to absarokitic and shoshonitic magmas. On this hypothesis, if the basaltic magmas evolving along the liquid line of descent on Or-O1-Qz pseudoternary phase diagram (Meen 1990) are erupted before the singularity is attained (relatively lower degrees of fractional crystallization), then the erupted products will be composed mainly of mafic and intermediate members of the shoshonite series (i.e. absarokite and shoshonite). Alternatively, if the evolved basaltic magmas are erupted after the singularity is passed through (relatively higher degrees of fractional crystallization), then the eruptive products will comprise, dominantly intermediate-acidic banakite, high-K dacite or even rhyolite. On this petrogenetic supposition, the amount of mafic members in the shoshonite series should not be regarded as an identification criterion for the rock series. Therefore, the high-K volcanic rock series from the Lishui and other basins in the Lower Yangtze fault depression zone should be classified as shoshonite series regardless of the amount of mafic rocks within them, as are those of Sierra Nevada (Joplin 1968).

Origin and evolution of magma

The high-K volcanic rock series in the Lower Yangtze fault depression zone were considered to be the products of subduction of the Pacific plate (Deng et al. 1992). This interpretation is not supported by geological evidence. In fact, the relative movement between the Asia plate and the Pacific plate during the Mesozois era led to the formation of a NE-trending megafracture, i.e. Tan-Lu fault in East China (Fig. 1). Left-lateral shear movement of the Tan-Lu fault caused the east part of the Lower Yangtze fault depression zone to be extended considerably (Xu 1990) and to form a Yangtze river deep fracture zone. Geophysical data show that the Lower Yangtze region is at a crust-thinning and mantle-upwelling region with high magnetic and gravity anomalies (Fig. 8). These anomalies coincide with the domains of volcanic basin and outcropping and buried igneous complexes in the area. Some important buried fractures and intrusive rocks, therefore, have been deduced from the anomalies. The boundary fractures of the Lower Yangtze fault depression zone (or rift zone) shown in regional magnetic and gravity anomaly maps have been confirmed by a large amount of drilling data. The K-rich calc-alkaline and shoshonitic series of Lishui are obviously connected with intraplate deep fracture zones rather than compressive plate margins. The similarities of geochemical characteristics of high-K mafic rocks from the Lower Yangtze region and the East Africa rift also support this hypothesis.

u[0o

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-40

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South Dabie

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20 l

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Anhui

4okm ,

Fig. 8. Generalized transverse section of the Lower Yangtze fault depression zone (modified from Wu, 1987). The location of the section is shown on Fig. 1.

Mesozoic shoshonite series from Lishui in the Lower Yangtze region, China

275

Table 11. FC and AFC models for the Lishui volcanic complex

Parent

Fractional crystallization F

Aob

(A) Rb Ba

54.7 308.8

0.9

0.8

0.7

0.6

60.5 339.4

67.7 377.4

76.8 425.5

88.9 488.5

Parent Trb

(B) Rb Ba

68.8 550.1

0.5

Contaminant sgms

0.4

0.3

0.2

105.8 130.9 172.1 253.3 575.6 703.1 910.7 1310.5

Fractional crystallization F 0.9

0.8

0.7

76.0 604.0

85.0 671.7

96.5 756.4

0.6

0.5

Assimilation-fractional crystallization F (r -0.25)

296.0 697.0

0.9

0.8

0.7

0.6

0.5

0.4

71.2 361.3

91.7 433.2

117.8 519.4

152.4 632.7

200.5 787.1

271.4 1018.1

Contaminant sgms 0.4

0.3

Assimilation-fractional crystallization F (r = 0.5) 0.9

1 1 1 . 8 132.9 164.3 216.0 868.1 1022.1 1247.1 1612.9

296.0 697.0

108.6 673.3

0.8

0.7

1 5 7 . 5 219.5 825.7 1015.5

0.6 301.1 1260.8

0.5

0.4

413.6 578.6 1590.9 2066.2

(A) Model fractional crystallization uses alkaline olivine basalt (Aob) as a starting composition and the fractional assemblage of Ol + Aug + PI + Mt in the ratios 10:20:62:8. Model assimilation-fractional crystallization uses same starting composition and spinel-garnet-mica schists (sgms) xenolith in Precambrian granitoids from Jiangnan ancient island arc of Yangtze plate (Xu 1989) as assimilant, r = 0.2 (mass assimilated/mass fractionated). (B) Trachybasalt (Trb) is used as starting composition. The fractional assemblage of O1 + Aug + P1 + Mt in the ratios 8:22:63:6. r = 0.5. Contaminant is same as (A). Partition coefficients of Rb, Ba in model calculations taken from Ferguson et al. (1992). Trace elements in ppm.

As mentioned above, the Mesozoic volcanic activities in the Lishui basin occurred in the early Cretaceous epoch. The basic member of the Lishui shoshonite series is rich in incompatible elements (Table 8; Fig. 7; Deng et al. 1992). It suggests that the primary magma of Lishui is the product of partial melting of enriched mantle. Besides, the chemical compositions of every rock type and their space distributions and positions in volcanic sequences provided a constraint on the genetic relationship of the various volcanic complexes. The earliest erupted products (olivine pyroxene basalt, basaltic andesite, high-K andesite, etc.) constitute K-rich calc-alkaline series of Longwangshan cycle. The mafic magmas derived from mantle become richer in alkalies with an increasing degree of tectonic extension and increasing depth of source area. Therefore, lavas which erupted at a late stage of the Longwangshan cycle are absarokite and alkaline olivine basalt. The fractional crystallization of alkaline olivine basalt combined with the crustal contamination produced magmas of the Dawangshan cycle. The trachybasaltic magma of the Guanshan cycle was derived from a deeper source area than the alkaline olivine basalt of the Longwangshan cycle. The fractional crystallization of the trachybasalt magma with contamination of crustal material produced latite, trachyte and high-K trachyte. The evolutional relationships of the Lishui volcanic magmas suggested by field and laboratory studies are also supported by model calculation (see below). Moreover, petrographic evidence indicates that aegirine-augite laurdalite porphyry at a late stage of the Guanshan cycle was produced by fractional crystallization of alkaline feldspars and biotite of trachytic magma. As to the rhyolite of the Jiashan cycle, its space distribution and major and trace element chemistry also reveal some genetic information. Table 8 shows that rhyolite of the Jiashan cycle has lower K20, Rb, Zr and higher Cr and Ni than the average composition of the Guanshan cycle. The rhyolite of the Jiashan cycle, thus, could not be the product of fractional crystallization of high-K magma of the Guanshan cycle. According to the

field occurrence of rhyolite (e.g. a small outcropped area and its distribution around the volcanics of the Dawangshan cycle) and geochemical coherence (e.g. variations in MgO vs K,O, MgO vs Ba) with volcanics of the Dawangshan cycle, the rhyolite of the Jiashan cycle may be the product of the fractional crystallization of high-K dacite magma of the Dawangshan cycle with contamination of crustal material. Petrogenetic modeling It was reported that some metamorphic minerals, such as kyanite and staurolite, were found in accessory minerals of pyroxene andesite of the Longwangshan cycle (Wu 1981). This seems to suggest that evolved calcalkaline and shoshonite magmas were contaminated by crustal rocks. Our trace element model calculations will Rb 300 ,AFC

~"

FC

200

n,

I00

(X I00 ) P P M Fig. 9. Rb vs Ba for high-K andesite and high-K dacite of the Dawangshan formation. Fractional crystallization (FC) and assimilation fractional crystallization (AFC) curves are calculated by the method of De Paoio (1981).

276

Jincheng Zhou et ai.

verify whether assimilation fractional crystallization (AFC) processes, which cause magmatic diversity, had occurred. In the first model calculation (A) (Table I I ), alkaline olivine basalt, which has the modal composition ofplagioclase (P1) + olivine (O1) + augite (Aug) + magnetite(M t) in the ratio 62:10:20:8 and bulk partition coefficients DRb = 0.0478, D~ = 0.1018, is used as a starting composition. The modal composition is taken as fractional assemblage in the calculation. Spinet-garnet mica schist (sgms) enclave (Table l I) in the Precambrian granites from Jiangnan ancient island arc in the Yangtze plate (Xu 1989) is used as an assimilant with the mass assimilated/mass fractionated ( r ) = 0.25. Variations of both Rb and Ba selected as representatives of incompatible elements have been calculated by the method of De Paolo (1981). Calculated results given in Table 11 and plotted in Fig. 9 suggest that, at a late stage of the Longwangshan cycle, about 40% fractional crystallization of alkaline olivine basalt magma assimilating some crustal materials (AFC) may have produced high-K andesite and high-K dacite of the Dawangshan cycle. The calculated result of fractional crystallization (FC) is also given for comparison. The mineral assemblage and bulk partition coefficients DRb and DB, of trachybasalt of the Guanshan cycle are similar to those of alkaline olivine basalt of the Longwangshan cycle. Trachybasaltic magma through AFC (based on sgms as assimilant, fractional assemblage of PI + Ol + Aug + Mt in the proportions 63 : 8 : 22 : 6, mass

Rb

/ •

AFC

//

//

400

,

/

/

//

300 //

/

/

/

//

//

m,

/ FC

assimilated/mass fractionated ratio of 0.5t may produce banakitic and trachytic magma (about 30% lYactiom ation) and high-K trachytic magma (about 50%, frac-. tionation). Fractional crystallization (FC) alone could not cause such evolutions of magma compositions (Fig. 10).

Conclusions

The Lishui Mesozoic volcanic sequences consist of four formations which are the products of four cycles of volcanic activities. Most rocks of the Longwangshan cycle are classified as K-rich calc-alkaline series. Absarokite erupted at a late stage of the Longwangshan cycle and all rocks of the Dawangshan. the Guanshan and the Jiashan cycles constitute a shoshonite series. The hishui shoshonite series comprises dominantly banakite, high-K dacite, trachyte, high-K trachyte and rhyolite with less absarokite and shoshonite. The Lishui shoshonite series is significantly rich in incompatible lithophile elements such as K, Rb, Ba, U, Th, Zr and LREE, etc. which are generally observed in typical shoshonite associations. For absarokites from Lishui and the nearby Luzhong basins, the incompatible elements patterns are similar to that of the East Africa rift and distinctly different from that of a typical island arc environment. It is suggested that the formation of the high-K volcanic sequences in Lishui and other Mesozoic volcanic basins of the Lower Yangtze region have no direct relation to the subduction of the Pacific plate, but are connected to the crustal thinning, mantle up-welling and the development of the Yangtze River deep fracture zone in the area. Rb--Sr and Sm-Nd isotope data show that the Lishui K-rich calc-alkaline series and shoshonite series are the products of partial melting of enriched mantle and contaminated by crustal rocks. On the basis of model calculation, at a late stage of the Longwangshan cycle, about 40% fractional crystallization of alkaline olivine basalt magma assimilating some crustal materials may have produced the high-K andesite and high-K dacite of the Dawangshan cycle. Similarly, trachybasaltic magma of the Guanshan cycle through AFC may have produced banakitic, trachytic and high-K trachytic magmas.

~.~ 200! Acknowledgements--We are grateful to Senior Engineer H N. Chen of Nanjing Institute of Geology and Mineral Resource and colleagues of Jiangsu Bureau of Geology and Mineral Resource for their help in field work and providing some d a t a

i

100~

References i J

iI 5

7

9

n

13

15

17

Ba

(XIOO) P P M Fig. 10. Rb vs Ba for banakite, trachyte (full cycles) and high-K trachyte (full square) of Guanshan formation. FC and AFC curves are calculated by the method of De Paolo (1981 }.

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