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Geochemistry and SHRIMP U-Pb zircon chronology of basalts from the Yanbian Group in the Western Yangtze block

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  • Institute of Geology

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Basalts from the Huangtian Formation of the Yanbian Group, previously considered as ophiolite, were formed in a back-arc basin environment as is revealed by this study on their geochemistry and trace elements geochemistry. A SHRIMP U-Pb zircon age of 782±53 Ma was obtained for the magmatism emplacement, and an inherited metamorphic zircon age of 1837±14 Ma was obtained. The metamorphic zircon age may represent the metamorphic age of the basement in the Western Yangtze, which support the existence of a Paleoproterozoic metamorphic basement.
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:本文为中国地质调查局地质调查项目(编号 200313000061)资助的成果
收稿日期: 2005-02-02; 改回日期: 2005-08-10;责任编辑:刘淑春
作者简介:杜利林 ,, 1973年生 ,助理研究员事变质岩石学和岩石地球化学研究通讯地址: 100037,北京阜外百万庄 26,中国地质科
学院地质研究所; Emai l: d ulilin7310@ cag s. net. cn
扬子地台西缘盐边群玄武质岩石地球化学特征
SHRIMP锆石 U
-Pb年龄
杜利林 1, 2 ) ,元生 2) ,杨崇辉 2) ,王新社 2) ,任留2) ,周喜文 2) ,石玉若 2, 3) ,杨铸生 4 )
1) 中国科学院地质与地球物理研究所 ,北京 , 100029; 2) 中国地质科学院地质研究所 ,北京 , 100037
3) 北京离子探针中心 , 100037; 4) 川省地质矿产厅攀西地质大队 ,四川西昌 , 615000
内容提要:选择盐边群底部荒田组玄武质岩石 (原划为蛇绿岩 ) ,通过岩石化学和微量元素地球化学研究 ,认为
盐边群玄武质岩石可能形成于弧后盆地环境;通过 SHRIM P锆石 U-Pb年代学研究 ,获得玄武质岩石岩浆结晶年龄
782±53 M a ,其形成时代为新元古代同时获得其中继承性变质锆石年龄为 1837±14 M a变质锆石年龄可能代
表扬子地台西缘变质基底年龄 ,证明扬子地台西缘可能存在古元古代变质基底
关键词:盐边群;玄武质岩石 ;离子探针;弧后盆地 ;新元古代
  四川境内的扬子地台基底分为结晶基底与褶皱
基底 (四川省地质矿产局 , 1991)群作为褶皱基
底的一部分 ,前人曾进行过大量研究工作 (李继亮 ,
1981, 1 984; 李继亮等 , 1983, 骆耀南 , 1983, 1985;
明宗 , 1983; 从柏林 , 1988; , 1988; 刘朝基
, 1 988; , 1994a, 1 994b)对盐边群中的基性
岩曾提出了蛇绿岩 (继亮等 , 1983; 耀, 1983,
1985; 赖 明宗 , 1983; 李继亮 , 1984; 孙传, 1994a,
1994b )岛弧玄武岩 (从柏林, 1988; 朝 基 等 ,
1988;张旗等 , 2001; 沈渭洲等 , 2003)和地幔柱 (朱维
光等 , 2004)等不同的成因认识时代,较早的
同位素资料皆反映其形成于中元古代 (李继,
1981;李复汉等 , 1988)近年来 ,着扬子地台西
缘开展了许多同位素年代学研究 ,认为形成于新
太古古元古代的康定杂岩被证明形成于新元古
( Zho u et al. , 2002; Li et al. , 2003; 岳龙 等 ,
2004)同时 ,原划为盐边群蛇绿岩的层状堆晶岩
也分别获得了 840 M a (朱维光等 , 2004)936 M a
(渭洲等 , 2003)的同位素年龄,盐边群的形
成时形成环境 ,是有待解决的重要科学问题
本次工作针对近年来研究相对较少的盐边群底
部荒田组玄武质岩石进行了地球化学研究 ,并结合
SHRIM P锆石 U -Pb年代学工,其形成的构造
背景和形成时代提出了一些不同认识
1 地质背
边群主要出露于磨盘山断裂以西 ,金河
断裂东南 (, 1984 ) ,自下而上划分为荒田
渔门组小坪组和乍古组 (四川省地质矿产局 ,
1991)荒田组主要为变质玄武岩 ,质凝灰岩和
硅质板岩碧玉岩等 ,其中变质玄武岩多为块状构
,并见有海底喷溢的枕状构造和气孔杏仁状构造
(陈智梁等 , 1987)本次工作主要沿着攀枝花至渔门
公路进行玄武岩野外呈灰绿色 ,致密块状构造 ,
发生轻微蚀变 ,地段出露有褐色斑点状玄武岩
武岩上部与其平行不整合接触的为渔门组板岩 ,
具类复理石沉特征 (四川省地质矿产局 , 1991)
位置见图 1,具体采样点坐标:CX1 07-3( N: 26°
51. 946, E: 101°31. 11 6) ; CX 108-1 ( N: 26°51. 5 00,
E: 1 01°30. 982) ; CX109-1CX109-3CX109-4 ( N:
26°51. 186, E: 101°31. 045)
2 岩相学特征
镜下玄武岩样品 (CX1 07-3CX109-1CX 109-
3CX 109-4)主要由透闪石绿泥石斜长绿帘石
少量的钛铁矿 (或白钛矿 )组成 ,具变熔结凝灰结
石和绿泥石组矿物集合体 ,含量从30%
50% 不等 ,定向排列 ,态不规则 ,可能代表变
7 9 6
2 0 0 51 2          AC TA GEO LO G ICA SIN IC A V o l. 7 9 N o. 6
Dec .  2005
1 川西盐边群地 质略图 (据李复汉等 , 1988简化和修改 )
Fig . 1 Geo l ogic al m a p o f Ya nbia n Gro u p i n W est ern Sichu an ( m od ified af ter Li F H et al , 198 8)
( a): 1康定群; 2会理群 /峨边群 /盐边群 ; 3石英闪长岩; 4苏雄组; 5新元古代花岗岩; 6逆断层 /推测断层 ; ( b ): 1康定群; 2
田组 ; 3渔门组; 4乍古组; 5小坪组; 6上震旦统; 7冷水箐杂岩; 8花岗闪长岩; 9断层 ; 10不整合面; 11采样点
( a): 1Kangding Group; 2Huili G ro up / E 'bian Group /Yan bi an Group; 3quar tz dio rite; 4Sux i ong Fo rmati on; 5Ne opro terozo ic
gr anit e; 6ove rth r us t fa ul t /inferred fa ul t
; ( b): 1Kang ding Gro up; 2Huang tian Form ati on; 3Yume n Form ati o n; 4Zh ag u
Form atio n; 5Xiaopi ng Fo rma tion; 6U pp er Sinian ; 7Leng sh ui qi ng com plex; 8g ranodio ri te; 9faul t; 10unconf o rmit y; 11
sam pling localit y
质前的塑性晶屑或岩屑形态;质具交织,主要
由微晶斜长石和基性玻璃质组成 ,绿帘石化绿泥石
化强烈;钛铁矿含量小于 5% CX 108-1样品中 ,
下主要组成矿物由板状基性斜长隐晶质基质和
基性玻璃质组成 ,具间隐结构斜长石排不规则 ,
长可达 1 m m ,其间充填有隐晶质基质或基性玻璃
,局部黑云母化和绿泥石化较强烈同时 ,镜下该
样品中的钛铁矿含量稍高 ,可达 5% 8% 左右
根据岩相学初步观察认为 ,该组样品中大都不
具备典型玄武岩的结,结合下面的岩石化学分析
结果 ,暂将这些样品定名为玄武质岩石
3 地球化学特征
地球化学析样品被磨碎至 200以下分析
中国地质科学院国家地质测试中心完成:主量元
素和 Nb
Zr
Rb
Sr
Ba
Ga
Pb
V利用 X荧光光谱
仪测定 ,析误差为 3% 5% ; 稀土元素和 YCr
NiCoScTaThUHf利用 ICP-M S测定 ,分析误
差为 3% 8% 析结果见表 1
玄武质岩石的 SiO246. 52% 48. 88% , M g O
4. 07% 8. 26% , TiO21. 39% 3. 11% , Na2O
+K2O2. 52% 4. 0 2% 岩石化学分析中
806    2005
1 盐边群玄武质岩石地球化学分析表
Table 1 Geochemica l ana lysis of bas alts f rom
Yanbia n Group
样品号 CX 107
-3
CX 108
-1
CX 109
-1
CX109
-3
CX109
-4 9915
*9933
*9934
*
SiO248. 56 48. 88 46. 52 47. 63 48. 12 48. 13 48. 19 48. 12
TiO21. 88 3. 11 1. 87 1. 47 1. 39 1. 99 1. 86 2. 00
Al2O313. 74 11. 84 13. 79 14. 82 14. 99 12. 59 12. 39 12. 59
Fe2O35. 50 7. 34 3. 44 4. 41 3. 30 5. 06 4. 53 4. 45
FeO 7. 17 8. 07 8. 70 7. 67 8. 42 8. 75 9. 91 9. 87
M nO 0. 22 0. 29 0. 27 0. 22 0. 20 0. 19 0. 21 0. 21
M gO 6. 40 4. 07 7. 64 7. 94 8. 26 7. 49 7. 56 7. 62
CaO 9. 09 6. 65 10. 33 8. 82 7. 18 7. 78 7. 19 7. 43
Na2O2. 61 3. 64 1. 72 2. 93 3. 31 3. 27 3. 15 3. 17
K2O0. 75 0. 38 0. 80 0. 21 0. 48 0. 46 0. 47 0. 46
P2O50. 22 0. 51 0. 23 0. 18 0. 16 0. 33 0. 33 0. 30
H2O
+3. 38 3. 44 3. 46 3. 36 3. 50
CO20. 21 1. 44 0. 56 0. 30 0. 12
total 99. 52 99. 66 99. 33 99. 96 99. 43
LO I 2. 89 3. 91 3. 19 2. 87 3. 07 3. 43 3. 99 3. 97
La 8. 26 18. 0 8. 02 6. 18 5. 86 10. 17 10. 12 10. 32
Ce 20. 3 44. 9 20. 4 15. 5 14. 4 27. 79 27. 29 28. 25
Pr 3. 08 6. 81 3. 18 2. 36 2. 23 3. 705 3. 888 3. 905
Nd 15. 6 32. 8 15. 8 12. 0 10. 9 20. 05 19. 96 19. 85
Sm 4. 84 9. 70 4. 65 3. 51 3. 32 5. 962 6. 009 5. 950
Eu 1. 70 3. 02 1. 61 1. 39 1. 18 2. 291 2. 332 2. 325
Gd 5. 8 10. 5 5. 27 4. 17 3. 73 7. 036 6. 924 6. 799
Tb 1. 03 1. 91 0. 98 0. 74 0. 68 1. 278 1. 363 1. 270
Dy 6. 75 12. 2 6. 36 5. 12 4. 47 8. 213 8. 207 7. 994
Ho 1. 45 2. 41 1. 29 1. 06 0. 86 1. 679 1. 643 1. 633
Er 4. 23 6. 67 3. 66 3. 08 2. 44 4. 455 4. 461 4. 225
Tm 0. 60 0. 88 0. 50 0. 44 0. 33 0. 632 0. 630 0. 647
Yb 3. 97 5. 18 2. 83 2. 77 2. 03 3. 962 3. 866 4. 134
Lu 0. 61 0. 66 0. 37 0. 41 0. 28 0. 579 0. 580 0. 589
V 329 292 311 281 281
Cr 120 5. 29 300 259 317
Co 35. 9 34. 2 39. 5 42. 9 37. 8
Ni 47. 1 4. 29 72. 1 69. 2 74. 2
Ga 17. 6 22. 4 18. 6 18. 3 17 18. 62 18. 02 17. 89
Rb 15. 5 13. 3 11. 7 7. 78 11. 2 6. 511 5. 510 5. 864
Sr 203 223 247 324 174 192. 9 206. 2 214. 2
Ba 159 475 255 62. 4 109 464. 2 449. 8 463. 1
Th 0. 44 0. 65 0. 43 0. 59 0. 47 0. 527 0. 537 0. 547
Sc 49. 7 39. 0 48. 9 46. 6 45. 9 51. 97 53. 47 54. 76
Pb 5. 41 4. 11 6. 02 7. 03 3. 30
U0. 15 0. 19 0. 13 0. 17 0. 07 0. 116 0. 095 0. 077
Zr 126 289 136 91. 4 83. 7 75. 49 84. 25 82. 35
N b 6. 4 11. 5 6. 5 6. 3 5. 0 3. 990 4. 087 4. 138
Hf 3. 69 7. 49 3. 66 2. 56 2. 36 2. 554 2. 430 2. 475
Ta 0. 28 0. 57 0. 31 0. 19 0. 20 0. 733 0. 358 0. 387
Y 37. 3 68. 7 35. 7 28 24. 7 41. 97 42. 58 43. 54
: * 为盐边群玄武岩样,引自沈渭洲, 2003; 分析单位:主量
素为% ,稀土元素和微量元素×10- 6
(1) ,玄武质岩石的烧失量为 2. 87% 3. 99% ,
明样品受到较强的蚀变作用影响 ,活动性组分 K
2 盐边群玄武质岩石 N b / Y -Zr /P2O5分类图
Fi g. 2 N b /Y-Zr / P2O5c la ss ificati o n o n basal t o f
Yan bian Gro up
◆— 1CX107-3CX109-4五个样品;■— 1
99159934三个样品
◆— Fiv e sam ples f rom CX107-3 to CX 109-4 in t he t able 1;
■— t hre e sampl es f ro m 9915 t o 9934 in th e t abl e 1
3 盐边群玄武质岩石 SiO2-FeO*/M g O 分类图
Fi g. 3 SiO2-FeO*/M g O class ificati o n o n ba salt o f
Yan bian Gro up
◆— 1CX107-3CX109-4五个样品;■— 1
99159934三个样品
◆— Fiv e samples f rom CX107-3 to CX 109-4 in t he t a ble 1;
■— t hre e sampl es f ro m 9915 t o 9934 in th e t abl e 1
Na能发生一定的,因而可能会对岩石分类有
影响采用惰性组分分类图解 N b /Y-Zr /P2O5判定
(Winchester et al. , 1976) ,荒田组玄武质岩石位于
拉斑玄武岩区 (2)SiO2-FeO
*/M gO 图解进一
区分钙碱性和拉斑玄武岩 (Miy a shi r o, 1974) ,
有的玄武岩样皆投于拉斑玄武岩区 (3)
田组玄武质岩石稀土元素总量低 ,般在 53
×10
- 698×10
- 6
之间 ,而样品 CX108-1稀土总量
偏高 ,156×1 0
- 6
在稀土元素球粒陨石标准化配
分图解中 (4) ,稀土相对略微富集 ,显示平坦或
具右倾的稀土分配模,Eu异常不明显 ,类似于
8076期 杜利林等:扬子地台西缘盐边群玄武质岩石地球化学特征及 SHR IM PU -Pb 年龄
中脊玄武岩 (Saunders, 1983)弧后盆地玄武岩
( Saunders et al. , 1979; 高 永军, 2 000;
, 2003)和岛弧拉斑玄武岩 (万渝生等 , 1997) ,而与
轻稀土富,轻重稀土强烈分异的洋岛玄武岩差别
明显 (Sun et al. , 1989)同时 ,球粒陨石标准化配
分图解中 (4) ,CX 109-4 CX108-1稀土总量
一定的变化,但各样品稀土配分模式相,分配曲线
于平 ,显示出结晶分异的演化特征La /Sm -La
解中 (图略 ) ,也显示分离结晶作用的趋
4 盐边群玄武质岩石稀土配分图解
(标准化值为 Leede y 球粒陨石 ,引自王仁民等 , 19 87)
Fi g. 4 R EE di st ribut i on pa tt er ns o n bas al t of
Yan bian G rou p( no r mali zi ng d at a a re Leedeys Cho nd rit e
fro m W an g Ren min et al. , 1987)
5 盐边群玄武质岩石微量元素配分图解
(标准化值引自王仁民等 , 19 87)
Fig . 5 Tr a ce elemen ts dis tr ib utio n p att er n s o n basal t
of Yan bian Gr ou p
(no rma lizi ng da ta f ro m Wa ng Renm in et a l. , 1 987)
玄武质岩石微量元素中 (样品 CX107-3CX109-
1CX 109-3Cx 109-4 ) , 相容性元素 VC r
Co
Ni普遍相对较高 (CX 108 -1Cr
Ni相对其
6 盐边群玄武质岩石构造环境判别图
Fi g. 6 Iden ti fica tio n d iagr a ms o f t ecto ni c set tin g o n
basal t o f Yan bian Gro up
(a): 引自 M esch ed e, 1986; A
Ⅰ — 板内碱性玄武岩 ;A
Ⅱ — 板内碱
性玄武岩和板内拉斑玄武岩; B富集型洋中脊玄武岩 ; C
拉斑玄武岩和火山弧玄武岩 ; D正常型洋中脊玄武岩和火山弧
玄武岩; ( b): 引自 W o od, 1980; A正常洋中脊玄武岩; B
型洋中脊和板内玄武岩; C板内碱 性玄武岩 ; D岛弧拉 斑玄武
;●— 1CX107-3CX109-4五个样品 ;19915
9934三个样品
( a ): Tecto nic s et ting dis c rmina tion of Zr /-Y-N b* 2 f or bas alt
( af t er M esche de, 1986): AⅠ — w it hin plate alka lic basalt; AⅡ —
wi thi n pla t e al kalic bas alt an d tho l eiit e; Be nrich ed m id dle
oceanic ridg e ba salt;C
w i thi n pla te th o leiit e and v olcanic arc
basal t; Dno rmal middle oc eani c ridg e basa lt a nd v olc anic a rc
basal t; (b ): t ecto nic set ting disc rmina t ion of T h-N b / 16-Hf / 3 f or
basal t ( aft e r W o o d, 1980 ): Anormal middl e o ceanic r idge
basal t; Benri ch ed middl e ocea ni c ridge bas alt and w ithin plate
basal t,C
w it hin pla t e alka lic basa l t;D
is land a rc th ol eiite;
●— f ive s am ples f r om CX 107-3 t o CX 109-4 in th e t abl e 1;
th ree s am ples f ro m 9915 to 9934 in t he table 1
他样品低 ) ,高场强元素相对于 N-型洋中脊玄武岩
( M O RB)含量当或略高 (CX108-1N bTa
ZrHf相对其他样品明显增高 )N -M O RB标准
化配分图解中 (5) ,大离子亲石元素相对略微富
,高场强元素分布趋势平坦 ,并无明显的亏,
类似于洋中脊玄武岩和弧后盆地玄武岩特
(Keleman et al. , 1990)样品 CX 107-3CX109-4
具轻微的 Ta负异常 ,可能与受到少量中下地壳物质
混染有关 ,型的岛弧玄武岩不同玄武岩样品
( 991599339934),具明显的 N b(Ta)Zr
Hf负异常 ,叠加了部分岛弧拉斑玄武岩特征在玄
武岩构造环境判别图解 Zr /4-Y-N b* 2(6a ) ,
品点分别投于正常洋中脊玄武岩区;T h-N b /
16-Hf / 3图解中 (6b ) ,部分玄武岩样品投于洋中
脊玄武岩区 ,而部分投于洋中脊玄武岩与岛弧拉斑
玄武岩的过渡
808    2005
7 盐边群玄武质岩锆石阴极发光图像
Fi g. 7 CL im ag es of zi r co n s o n ba salt o f Ya nbia n Gro u p
2 盐边群玄武岩锆石离子探针分析表
Ta ble 2 SHRIMP zircon analysis on bas alt f rom Yanbian Group
样点号 206 Pbc
(% )
U
(×10- 6 )
Th
(×10- 6 )
232 Th
238 U
206 Pb
*
(×10- 6 )
206 Pb
238 U
年龄 ( M a)
207 Pb
206 Pb
年龄 ( M a )
不谐
和性
(% )
207 Pb
*
206 Pb
*±%
207 Pb
*
235 U±%
206 Pb
*
238 U±%误差
相关性
CX 109-4-1. 1 4. 10 119 110 0. 95 6. 52 383 ±12 - 1310±1100 129 0. 0286 34 0. 241 35 0. 0611 3. 1 0. 090
CX 109-4-2. 1 0. 15 451 687 1. 57 48. 7 762 ±19 705 ±35 - 8 0. 0629 1. 7 1. 088 3. 1 0. 1255 2. 6 0. 846
CX 109-4-3. 1 1. 51 110 111 1. 04 12. 8 809 ±22 740 ±160 - 9 0. 0639 7. 8 1. 178 8. 3 0. 1337 2. 9 0. 348
CX 109-4-4. 1 0. 04 825 52 0. 07 248 1934 ±44 1924. 6±8. 0 0 0. 11790 0. 45 5. 69 2. 7 0. 3498 2. 6 0. 986
CX109-4- 5. 1 1. 43 215 204 0. 98 14. 1 469 ±12 169 ±150 - 178 0. 0494 6. 6 0. 515 7. 1 0. 0755 2. 7 0. 384
CX 109-4-6. 1 0. 15 551 8 0. 02 175 2028 ±46 1842 ±13 - 10 0. 11264 0. 69 5. 74 2. 7 0. 3697 2. 6 0. 967
CX 109-4-7. 1 0. 25 482 526 1. 13 47. 3 695 ±17 691 ±36 - 1 0. 0625 1. 7 0. 982 3. 1 0. 1139 2. 6 0. 844
CX 109-4-8. 1 0. 44 586 554 0. 98 62. 5 752 ±19 897 ±34 16 0. 0690 1. 7 1. 176 3. 1 0. 1237 2. 6 0. 847
CX 109-4-9. 1 0. 45 1007 1068 1. 10 117 817 ±20 855 ±30 4 0. 06755 1. 5 1. 259 3. 0 0. 1352 2. 6 0. 873
CX109-4-10. 1 0. 09 363 6 0. 02 108 1910 ±44 1847 ±13 - 3 0. 11291 0. 73 5. 37 2. 8 0. 3449 2. 7 0. 964
CX109-4-11. 1 0. 17 569 9 0. 02 171 1929 ±44 1826 ±11 - 6 0. 11160 0. 60 5. 37 2. 7 0. 3488 2. 6 0. 975
:误差为 1σ, PbcPb
*分别代表普通铅和放射成因铅;标准校正误差为 0. 6 8% ( 1)普通铅校正用204 Pb测量值; ( 2)普通铅用推测的206 Pb /2 38
U-
207 Pb /235 T h一致性年龄校正; ( 3)普通铅用推测的206 P b /238U -
208 Pb /232 T h 致性年龄校正
4 锆石特征及 U-Pb年龄
本次选取玄武质岩石样品 CX109-4行年龄测
具体的处理过程是破碎洗和重液,之后
进行电磁分离 ,后对锆石进行手工挑纯然后将其
T EM 标样一起粘在树脂台上 ,打磨抛光 ,去掉约
锆石颗粒一半的厚度 ,可能得到横切颗粒中心的
剖面 ,制成样靶离子探针测试前 ,在电子探针上进
行阴极发光研究 ,以确定锆石的内部结构和成因
后再经清洗镀金 ,S HRIM P上进行同位素测试
( W illiam s, 19 98; 宋彪等, 2002 )应用锆石标样
T EM (年龄 4 17 M a )进行元素间的分馏校正 ,应用
SL13(年龄 572 Ma ,U238×10
- 6 )标定样品的
UThPb 含量据处理采Ludwig S Q U ID1. 0
IS PLO T程序普通铅应用实测204 Pb单次
测定数据点误差采用 1
σ
于较年轻的锆石 ( < 1.
21. 4 Ga ) ,年龄测定结果采用206 Pb /238 U,
于时代较老的( > 1. 21. 4 Ga ) , 测定结果
采用207 Pb /206 Pb数值 (与万渝生研究员交流 )其加
权平均值为 95% 置信度本次锆石阴极发光图像
在中国科学院质与地球物理研究所电子探针实验
室完成;SHRIM P U -Pb年龄测定在北京离
8096期 杜利林等:扬子地台西缘盐边群玄武质岩石地球化学特征及 SHR IM PU -Pb 年龄
8 盐边群玄武质岩锆石 U-Pb年龄谐和图
Fig . 8 Co nco rdia diag ram o f zirco n U -Pb a g es on ba salt o f Ya nb ian Gro u p
子探针中心的 S HRIM P上完
玄武质岩石中锆石呈柱状和不完整粒状 ,粒度
多为 50100μm在阴极发光 ( CL )图像中 (7) ,
石有两种:一种锆石颗粒中具有典型的岩浆韵律环
,内部无继承性锆石核 ,Th /U比值为 0. 981.
57 (2) ,具岩浆锆石特征 ( Row ley e t a l. , 1997;
Sue et al. , 1999; Hoskin et al. , 2000) ,个锆石年
龄测点206 Pb /238 U加权平均年龄为 782±53 M a(
于所挑选出的锆石颗粒太,测点少较分散 ,使年
龄误差偏大 ) ,可能代表了玄武质岩石的形成时代
(8b)另一种为具核幔结构的复合型锆石 ,锆石
内核较均匀弱发光 ,Th /U比值为 0. 020. 07(
2) ,具典型的变质锆石特征 ( Bradley et al. , 1998;
Claess on et al. , 2000; Hoskin et al. , 2000) ,
测点的锆石207 Pb /206 Pb 加权平均年龄为 1837±14
Ma(8a) ; 锆石幔部可能为岩浆结晶部分 ,
20μm ,离子探针无法进行年龄测定 (7)
5 讨论与结论
盐边群荒田组玄武质岩石为拉斑玄武岩 ,稀土
总量低 ,稀土配分模式表现为平坦或略具右倾的稀
土配分模式;在微量元素方面 ,容性元素含量高
在微量元素配分图解,部分样品表现为正常洋中
脊玄武岩特征 ,部分具有岛弧拉斑玄武岩特征 ,
形成于弧后盆地环境玄武岩相似 (Saunders et al. ,
1979)玄武岩中的 Ti与洋中脊玄武岩中的相当或
略高 ,可能更多是继承了源区的特征锆石特征方
,玄武质岩石中具有典型的变质继承性锆石 ,说明
岩浆在形成前或形成过程中可能受到地壳物质的混
在岩相学方面 ,玄武质岩石中变质晶屑和岩屑与
基质的含量随样品变化 ,同时样品 CX108-1,结构
其他样品明显不同 ,反映玄武质岩石形成存在陆
海相环境的变化
于盐边群的形成时代 ,前的同位素资料反
其形成于中元古代 (, 1981; ,
1988)渭洲( 2003)和朱维光等 ( 2004)用单颗
粒锆石 U-Pb法获得原划为盐边群蛇绿岩的冷水箐
杂岩 (村杂岩 )的同位素年龄分别为 936±7 Ma
840±5 Ma,者结果相差近 100 M a,较本次工作
获得同位素年偏大笔者认为可能与其中含有残
留的继承性锆有关 ,其年龄可能代表混合年龄
次利用 SHRIM P测定年龄结果为 782±5 3 M a,
较大 ,初步判定玄武质岩石的可能形成时代在 800
M a 左右 ,区域上构造岩浆事件的时代近于一致
玄武质岩石中1. 8 Ga 变质继承性锆石的发现
和年龄测定 ,证明扬子地台西缘可能存在古元古代
的变质基底
目前 ,关于扬子地台西缘新元古代构造环境有
两种观点:一种为具板块俯冲性质的岛弧环境 (颜丹
平等 , 2002; Zhou et al. , 2 002; 沈渭洲等 , 2003) ;
种 为 地 幔 (李献华等, 2001, 2002a, 2002b,
2002c; Li et al. , 2002; Li et al. , 2003)李献华等
( 2 001, 2002a , 2002b, 2002c)Li et al. ( 20 02)Li et
al ( 2003)根据地球化学和同位素资料认为 ,扬子西
860750 M a的中性岩浆活动是新元古代地
幔柱活动引起裂谷环境下的岩浆产物康滇地
区来看 ,新元古代火山岩 (定杂岩苏雄
河群等 ),酸性火山岩约占全部火山岩的
98% 以上 ,而基性火山岩非常少 ,与典型的地幔柱引
发的以基性岩主的大火成岩省 (如峨眉山玄武岩 )
有明显的差异本次工作中 ,盐边群荒田组玄武质岩
洋中脊和岛弧玄武岩的地球化学特,与地幔柱
810    2005
成因的玄武岩有很大的不同时扬子地台西缘从
北向南 600多千米范围内大量出现新元古代具有火
山弧性质的 I型花岗岩 (李献, 2002c; Zho u et
al. , 2002; 耿元生等 ,待发表资料 )
笔者认为 ,盐边群玄武质岩石的形成可能代表
一种弧后盆地环境新元古代板块俯冲体制下引
起的弧后拉张 ,时伴随具有 MO RB性质的地幔物
质上涌 ,喷发形成盐边玄武岩玄武岩的形成过程
中有少量中壳物质的加入 ,同时可能有俯冲板
片脱水形成的流体混入 ,因而使玄武岩中轻稀土
LIL E发生微富集 ,在玄武岩中也保留了残留
地壳的变质锆石年龄信息然而 ,扬子地台西缘在新
元古代究竟处于何种复杂的构造环境 ,还应该将野
外地质岩石学地球化学与同位素年代学紧密结
,综合分析
致谢:中国科学院地质与地球物理研究所郭敬
辉研究员在文章资料分析及成文过程中提出部分修
改建议;北京离子探针中心刘敦一研究员陶华工程
师及中心其他老师安排和准备离子探针测试工作;
中国地质科学院地质研究所伍家善研究员万渝
研究员简平研究员怀民研究员和王彦斌研究
,中国地质大学 (北京 )张招崇教授提出许多建议
和意见 ,几位审稿人提出了宝贵的修改建议此表
示衷心感谢
   
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Geochemistry and SHRIMP U
-Pb Zircon Chronology of Basalts f rom the
Yanbian Group in the Western Yangtze Block
DU Lilin1, 2) , GENG Yuansheng2) , Y AN G Cho ng hui2) , W AN G Xi nsh e2) , REN Li udong2)
ZHOU X iw en
2) , SHI Yur uo2, 3) , Y AN G Zhu sh eng4)
1) In sti tute of Geolog y an d Geop hy sics ,Ch inese Academy of S cien ces,Beij in g , 100029
2) In stitu te of Geolog y ,Ch inese Academy of Geo log ical Sci ences ,Beij ing; 100037
3) B ei jing S H R I M P Center ,Beij ing ; 100037
4) Pa nx i Geolog ical T ea m ,Sichuan B ureau of Geology a nd M ineral E x p loration and Developm ent ,X ichang ,Sichuan, 615000
Abstract
Basalt s f ro m th e Hu ang tia n For mat io n o f th e Ya nbian Gr o up , p revio us ly co nsid er ed as o phio li te , w er e
fo rm ed in a back-
arc basin envi ronmen t as is revealed by t his s tu dy on their geochemis t ry and t race
ele men ts g eo ch emis try . A S HRI M P U -Pb zir co n ag e of 7 82±53 M a w a s o b tained f o r th e m ag mat ism
em placem ent ,a nd a n i nhe ri ted m e tamo r phi c zir con ag e o f 1837±14 Ma w a s o b tained.Th e m etam o rphic
zi rco n ag e m a y re pres ent t he m etam o rph ic a g e of th e b aseme nt in t h e W es ter n Yang t ze, w hic h su ppor t th e
exi st ence of a Pa leo pro te ro zoic me tam or phic b asem en t.
Key words
:Yanbian Group;basalt;S HRIM P;back-
arc basin;N eop ro ter o zoic
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
十红滩砂岩型铀矿床层间氧化带的元素地球化学
蔡根庆 ,张子敏 ,李胜祥
核工业北京地质研究院 ,北京 , 100029
   本文根据含矿砂体氧化程度的不同 ,将吐哈盆地南缘十红滩砂
岩型铀矿床层间氧化带划分为完全氧化带不完全氧化带还原带和
原生带 4个地球化学亚带研究了各地球化学亚带的元素变化特征;
得出了与铀共同在还原障富集的元素有 Re, Mo, Se, Sr, S, RE E, C
机等 ;Re U在氧化还原过程中有相似的地球化学特征;M o
Se U
在还原环境下有相似的地球化学性质 ,在氧化环境下其地球化学
行为不同提出本区含矿层本身能为铀成矿提供一定铀源的认识
青藏高原第四纪泛湖期与古气候
郑绵平1, 2 , 3) ,袁鹤然1, 2, 3 ) ,赵希涛 3) ,刘喜1 , 2, 3)
1) 中国地质科学院盐湖与热水资源研究发展中心 ,, 100037; 2) 中国地质科学院矿产资源研究所 ,北京 , 100037
3) 中国地质科学院盐湖资源与环境研究重点开放研究实验室 ,, 100037
  本文对青藏高原不同位置的 17个湖区进行地质调,并结合卫
星照片和地形图解译 ,研究高原泛湖区形成的时间和范围及其古气
青藏高原第四纪最晚的 2次高湖面 (溢流面 )时间是约 4030ka
BP 和约 655 3ka B P;在该时段 高原为巨大的 相互连通泛湖 系所覆
,总面积约 36平方千米 ,湖水总体积约 53000万立方千米 ,分别
较现代湖泊的总面积和总体积大达 38倍和 659在该泛湖期之前
还有 3次高湖面: 132112k a B P; 11095ka BP 9172ka B P;
8375ka BP
说明青藏高原第四纪气候变化具有不稳定性和快速变
化特点4030ka BP高湖面还出现在青藏高原以北腾格里沙漠 ,
说明该时存在特强的南亚夏季风; 20ka 差周期太阳高 辐射变化对
地球低纬地带高海拔的青藏高原的特殊重要性30k a B P前后 ,
随青藏高原的快速抬升和古气候变,青藏高原周缘泛湖突然外泄 ,
在短时间内巨量冰冷湖水倾泄入印度洋和西太平洋该泛湖倾泄事
件已造成高原周缘江湖等环境变化
8136期 杜利林等:扬子地台西缘盐边群玄武质岩石地球化学特征及 SHR IM PU -Pb 年龄
... The tectonic setting in the Panxi belt, western Yangtze Block, are slightly different from the study area. There are many MORB-type and OIB-type basalts in the Yanbian (920-858 Ma) and Kangding (830-721 Ma) Groups (Du et al., 2005(Du et al., , 2007Sun et al., 2007; in the Panxi belt. Moreover, the 764-748 Ma adakitic granites, 860-740 Ma arc-related granites and intermediate-mafic plutons also were reported in this area (Du et al., 2009;Huang et al., 2009;Shen et al., 2003;Zhou, 2007a, 2007b;Zhou et al., 2002Zhou et al., , 2006aZhou et al., , 2006b. ...
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Gneissic granitoid rock suite in the Kangding Complex, located in western margin of the Yangtze Craton, comprises mainly tonalite, granodiorite, hoar fine-grained monzogranite with a little of pink coarse-grained monzogranite. SHRIMP zircon U-Pb isotopic dating revealed that the tonalite and granodiorite emplaced at 797 ∼795Ma, and a new SHRIMP zircon U-Pb chronological data yield a weighted average age of 206Pb/238U age of 767 ±24Ma for a hoar fine-grained monzogranite (sample Kd-18), interpreted as emplacement age of the hoar fine-grained monzogranite. Tonalites, granodiorites, hoar fine-grained monzogranites, comprising dioritic enclaves in the gneissic granitoids, are characterized by right-inclined chondrite-normalized REE patterns with high (La/Yb)n values and without Eu anomalies, and significantly negative Nb, Ta, P and Ti in primitive mantle normalized multi-element spider diagrams. Whereas, pink coarse-grained monzogranites show flat chondrite-normalized REE patterns, strongly negative Eu and Nb, Ta, Sr, P and Ti anomalies but LILEs enrichment Sm-Nd isotopes analyses reveal that the granitoids have εnd(t) = -0. 57 ∼ +5. 67, and most of the samples εnd(t) > 0. Integrated features of geology, petrology to petrochemistry and Sm-Nd isotopes, the magma of the tonalite, granodiorite and hoar fine-grained monzogranite may be derived from partial melting of juvenile basaltic rocks and greywacks with depleted mantle affinity under high pressure condition, and the magma of the pink coarse-grained monzogranite could come from supracrust materials at the relative lower pressure condition. Petrogenesis of these granitoids, combining with tectonic discrimination, suggests that the Neoproterozoic Kanding Complex in the western margin of the Yangtze Craton could be produced in a tectonic setting of the subduction-related active continental margin.
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The spatial and temporal distributions of copper deposit and gabbros in the Lala copper deposit area (Hui-li, Sichuan) are closely related indicates the petrogenesis and tectonic setting of the gabbros are of great significance in understanding the genesis of the deposit. This study analyzed the contents of major elements, trace elements and rare earth elements as well as Sm-Nd isotopic constitute of the gabbros and investigated the genesis of the gabbros. The results showed that the gabbros is alkaline basalt and was formed in the Rift Valley about 850Ma ago, the gabbros has a relatively flat REE distribution pattern, a "hump" shape primitive mantle-normalized pattern of trace elements, a low primitive mantle-normalized ratio of Th/Nb and a high &epsiNd(t) ratio (0. 8 - 4. 1) , in terms of discriminating evaluation system of the high field strength elements (HFSE) , the values of all gabbros samples fall into the range of oceanic island basalt (OIB) , and are similar with those of continental basalt, which were formed by mantle plume of the same period. It may suggest that the gabbros were formed by Neoproterozoic magmatic event in the western margin of the Yangtze craton, and that there may have closely thermodynamical connections between the gabbros and the deposit.
Article
Located at the central of the Anning River discordogenic fault, the binding site of the Songpan-Ganze orogenic belt and the Yangtze block, Dashuigou schiefer has attracted increasing attention of researchers regarding to its special tectonic site and the occurred tellurium deposit. In order to provide chronologic evidences for reconstructing tectonic and magmatic geochronic evolution of areas around Dashuigou and the western margin of the Yangtze block in Shichuan, as well as for locating time of Dashuigou schiefer, zircon ages of the greenschist from Dashuigou were determined by SHRIMP U-Pb dating technique. Five age groups of the zircons with different characteristics of inner texture and cosmetics are obtained and subdivided from the greenschist of Dashuigou schiefer which should contain great proportions of sediments in the protolith. Ages of 2467-2430 Ma from remained detrital magma zircons shows the mass transportation of Archaean-early proterozoic basement, whereas ages of 790.5-762.5 Ma probably represent the magmatic event around Dashuigou on the background of Rodinia crack and upwelling mantle in early neoproterozoic. Metamorphism and magmation generated by orogenics and post orogenics are documented by ages of 696.8-642.9 Ma from the magmatic zircons. Zircons of 262.0-220.0 Ma ages are inferred to have been originated from alkalic rock nearby related to the Emeishan movement. 216.5-167.1 Ma ages of typical new growth and re-crystallized zircons caused by hydrothermal alteration reveal the post-magmatic hydrothermal activity. Dashuigou schiefer is probably allochthonic thrust sheet, and ages from magma zircons and those thermal genetics bound its general locating time to the interval from 220.0 Ma to 167.1 Ma.
Article
The history of the past 25yr research on the geochemistry of ocean basin rocks is reviewed and the REE characteristics of the major basalt types are described. The basalts are considered according to setting and the distinctions between 'normal' and 'anomalous' ocean ridges, ocean islands and ocean plateaus are emphasized. The importance of REE for interpreting the differences in geochemistry between these types is outlined, and alternative models based on differences in partial melting regimes versus mantle heterogeneity are reviewed. It is concluded that mantle heterogeneity must be invoked to account for the chemical diversity of oceanic basalts, and models that also involve mantle metasomatism are favoured.-B.W.D.Yardley
Article
The geochemical and Nd isotopic data were reported for the Neoproterozoic Suxiong bimodal volcanic rocks in the Kangdian Rift, western Sichuan. Most samples of basaltic rocks were characterized by high positive εNd (T) values ( + 5 to + 6), pronounced enrichment in the incompatible trace elements, smooth LREE-enriched and "humped" trace element distribution patterns. They closely resembled the alkali basalts in the OIB and CFB provinces, such as the Hawaii OIB and the Ethiopian CFB. These features suggested that these basaltic rocks were most probably derived from an OIB-like mantle source. The differentiated basalt and trachyandesite samples showed relatively low εNd (T) values ( + 1.7 to + 2.4) and Nb-Ta depletion due to contamination by the mafic lithosphere and/or crustal materials. The rhyolite and dacite samples had small positive εNd (T) values ( + 1.1 to + 2.6), general enrichment in most incompatible trace elements but significant depletion of Nb, Ta, Ba, Sr, P, Eu and Ti. They shared geochemical characters of A2-type granites, and were interpreted to be formed through assimilation of OIB-type basaltic magma followed by fractional crystallization in a 'double diffusive' magma chamber at the middle crust level. The geochemical and Nd isotopic characters of the Suxiong bimodal volcanic successions formed within a typical continental rift environment, similar to those volcanics erupted in the Ethiopian Rift. The Kangdian Rift was likely part of a wider continental rift system triggered by a Neoproterozoic mantle plume beneath South China during the breakup of Rodinia.
Article
The Gaojiacun mafic-ultramafic intrusive complex in the Yanbian area, Sichuan Province, is a stratiform intrusive body that has undergone intensive magmatic differentiation. This intrusive body involves two magmatic accumulating cycles. Systematic U-Pb dating of single zircon grains and40Ar/39Ar dating of hornblende were conducted, and the results showed that the age of hornblende gabbro, which was formed at the main phase of intrusion of the Gaojiacun intrusive complex, is 840–5 Ma, casting doubt on the concept of “Yanbian Ophiolite”. It is believed that the formation of the Gaojiacun intrusive complex seems to be related to a super-mantle plume underneath the super-continent Rodinia. The above research results are helpful for us to get a better understanding of the characteristics of Neoproterozoic tectonic evolution of the Yanbian area in Sichuan Province. Keywordsmafic-ultramafic intrusive complex-geochronology-Gaojiacun
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