Geochemistry Characteristics of the Proterozoic Meta-Basalt in Southern Tarim Plate: Evidence for the Meso-Proterozoic Breakup of Paleo-Tarim Plate
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摘要: 超大陆裂解是当今地学研究最前沿课题.通过研究塔里木南缘铁克里克构造带前寒武纪变质火山岩的岩石化学特征、微量元素特征、稀土元素特征及Sm-Nd同位素特征, 探讨古塔里木板块的中元古代大陆裂解构造事件.该套变质火山岩组合下部以变质酸性火山岩(流纹岩夹部分晶屑凝灰岩)为主, 夹少量玄武岩, 上部为玄武岩夹少量条带状大理岩.基性火山岩w(SiO2)为42.34%~49.55%, w(Na2O+K2O)为2.63%~8.81%.下部玄武岩w(Na2O)/w(K2O)值为0.54~1.5, 为钾玄岩系列, 稀土总量高, 且轻稀土富集, 平均w(La)n/w(Yb)n值为3.45, δ(Eu)为0.69~0.91, 具弱至中等的Eu负异常, 微量元素具有碱性板内玄武岩特征; 上部玄武岩w(Na2O)/w(K2O)为5.11~17.89, 为拉斑系列, 轻重稀土没有明显分异, δ(Eu)在0.81~1.13之间, 基本没有Eu异常, w(La)n/w(Yb)n平均为1.69, 配分曲线与大洋拉斑玄武岩一致, 微量元素显示LILE(如Rb、Ba、Th)富集, HFSE与大洋拉斑玄武岩一致.岩石地球化学反映从早到晚由碱性-钙碱系列逐渐向拉斑系列演化.在上部玄武岩中获得(1 200± 82.3)Ma的Sm-Nd等时线年龄.认为该套变质火山岩记录了古塔里木板块在中元古代大陆裂解的构造过程.Abstract: The study of supercontinent-breakup is a modern frontier project of earth science. Based on the lithochemistry, the trace element, REE and Sm-Nd isotope characteristics of the Precambrian metamorphic volcanic rocks of Tiekelike belt in Southern Tarim plate, this paper studies the tectonic events for the Meso-Proterozoic breakup of paleo-Tarim plate. The meta-volcanic rocks were composed of meta-rhyolite (the lower part) and meta-basalt interspersed by thin-layered marble (the upper part). The basalt has low w(SiO2) arranging from 43.8% to 49.26% and averaging 47.29%, it has high alkaline and w(Na2O+K2O) is between 2.6% and 4.9%. The lower basalt has w(Na2O)/w(K2O) arranging from 0.54 to 1.50 and belongs to shoshinite which has high ∑REE and LREE, its w(La)n/w(Yb)n averages 3.45 and δ(Eu) is between 0.69 and 0.91 which shows middle negative anomality. The upper basalt has w(Na2O)/w(K2O) arranging from 5.11 to 17.89 and belongs to tholeiite, it has low ∑REE, δ (Eu) is between 0.82 and 1.13, its REE spider pattern (normalized by C1 chondrite) is similar with that of the oceanic tholeiite, its trace elements shows characteristics of oceanic tholeiite and the lower basalt is similar with that of intro-plate basalt. Sm-Nd isochron of the upper basalt is (1 200±82.3) Ma. Concluded on above studies, we put forward that the Precambrian metamorphic volcanic rocks recorded the whole breakup tectonic course in Meso-Proterozoic of the southern paleo-Tarim plate.
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Key words:
- southern Tarim /
- Precambrian volcanic rock /
- geochemistry
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图 1 铁克里克构造带前寒武系火山岩分布
Pt2.中元古宇; Pt2-3.中新元古宇; PZ2-MZ上古生界-中生界; 1.中元古代(双峰式)火山岩; 2.砂板岩; (1) 昆仑造山带; (2) 喀喇昆仑造山带; F1.塔里木南缘山前推覆断层; F2.康西瓦断裂; F3.喀喇昆仑断裂; F4.阿尔金断裂; 实心圈示下部玄武岩的取样位置, 空心圈示上部玄武岩的取样位置, 下同
Fig. 1. Sketch geological map of Tiekelike tectonic belt, western Kunlun
图 3 玄武岩w(A)-w(F)-w(M)图解(a)、火岩岩w(MnO)-w(TiO2)-w(P2O5)图解(b)
OIT.大洋岛屿拉斑玄武岩; MORB.洋中脊玄武岩; IAT.岛弧拉斑玄武岩; OIA.大洋岛屿碱性玄武岩; CAB.钙碱性玄武岩
Fig. 3. Diagram of w(A)-w(F)-w(M) (a) and w(MnO)-w(TiO2)-w(P2O5) (b)
图 4 玄武岩稀土元素(a)、微量元素N-MORB (b) 配分模式
Fig. 4. REE (a) and trance element (b) diagram of the basalt
图 5 玄武岩w(Th)-w(Hf/3)-w(Ta)图解(a)、w(Th)-w(Hf/3)-w(Nb/16)图解(b)
A.正常洋中脊玄武岩(N-MORB); B.富集型洋中脊玄武岩(P-MORB); B-C.板内玄武岩(WPB); D.岛弧拉斑玄武岩(IAT)
Fig. 5. w(Th)-w(Hf/3)-w(Ta) (a) and w(Th)-w(Hf/3)-w(Nb/16) (b) diagram of the basalt
表 1 火山岩岩石化学成分
Table 1. Chemical composition of the Proterozoic volcanic rocks
表 2 火山岩稀土元素分析成果
Table 2. REE composition of the volcanic rocks
表 3 火山岩微量元素分析成果
Table 3. Trace element composition of the volcanic rocks
表 4 玄武岩Sm-Nd同位素分析
Table 4. Sm-Nd isotope composition of the basalt
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