Definition and Geological Significance of Early Silurian Quartz Diorite Pluton in Langshan Area, Inner Mongolia
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摘要: 内蒙古狼山山脉西侧分布有零星的早古生代岩浆岩，早志留世侵入体主要出露于哈日音熬瑞-迈罕陶勒盖一带.该岩体岩性为石英闪长岩，LA-ICP-MS锆石U-Pb年龄显示，其206Pb/238U加权平均年龄为440±1 Ma.岩石富集大离子亲石元素Rb、K、Pb，不同程度地亏损高场强元素Nb、Ta、P、Ti；轻稀土富集，重稀土亏损，具有弱的负Eu异常，显示了岩浆弧的地球化学特征.εHf（t）值为-12.1~-8.8及TDM2在1 973~2 182 Ma之间，反映了源区以古元古代古老地壳为主，闪长质包体中的斜长石斑晶及矿物成因判别图解显示了岩体在形成过程中受到了岩浆混合作用.构造判别图解及Hf同位素特征表明该岩体可能形成于大陆边缘弧的构造背景.对比达茂旗、白乃庙地区新近发表的有关数据，早志留世石英闪长岩的岩石地球化学特征与之十分相似，由此可见，华北地块北缘早古生代俯冲带从白乃庙地区可以延伸到狼山地区.Abstract: The Early Paleozoic magmatites are sporadically distributed along the western side of the Langshan Mountains, Inner Mongolia.The Early Silurian rocks are mainly exposed in the zone of Hariyinaorui-Maihantaolegai.LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U weighted average age of quartz diorite is 440±1 Ma.The rocks show similar chondrite-normalized REE patterns, and display relatively high concentration of light rare earth elements (LREEs) but low contents of heavy rare earth elements (HREEs) with minor negative Eu anomalies. The overall chemical similarities of these rocks on the primitive mantle-normalized variation diagrams display affinity to arc signature; considering both tectonic discriminated diagrams and the Hf isotopic signature, it is proposed that the Langshan area was under the tectonic setting of continental margin arc during Early Silurian. It is concluded that the subduction zone of Early Paleozoic had been extended from the area of Damaoqi to Langshan, since these rocks show similar geochemical characteristics with the recently published data of Early Silurian quartz diorite in the area of Damaoqi and Bainaimiao.
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Key words:
- Inner Mongolia /
- Langshan area /
- Early Silurian /
- quartz diorite /
- continental margin arc /
- petrology
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图 1 测区大地构造位置(a)和测区地质简图(b)
Fig. 1. The tectonic location and simplified geological map of the survey area
图 2 那仁宝力格幅1:5万地质图
Q.第四系；K.白垩系；P1-2ds.大石寨组；C2a.阿木山组；Jxa.阿古鲁沟组；Chz.增隆昌组；Chs.书记沟组；Pt1By.宝音图岩群；zxηγT2.中三叠世中细粒二长花岗岩；zxπβηγP1.早二叠世中细粒似斑状二长花岗岩；xδοP1.早二叠世细粒石英闪长岩；xνP1.早二叠世细粒辉长岩；zβηγC2.晚石炭世中粒黑云母二长花岗岩; zψδοC2.晚石炭世中粒角闪石英闪长岩；zxηγS3.晚志留世中细粒二长花岗岩；xδοS3.晚志留世细粒石英闪长岩；zxδοS1.早志留世中细粒石英闪长岩；O2？.中奥陶世.据中国地质调查局天津地质调查中心(2016，据内蒙古查干呼舒庙等六幅1:5万区域地质矿产调查地质图)
Fig. 2. The geological map of the survey area
图 3 早志留纪侵入岩剖面图及采样点
据PM007实测剖面修改
Fig. 3. The profile and sampling point of Early Silurian intrusive rocks
图 4 石英闪长岩的野外露头照片及岩相学特征
a.石英闪长岩定向构造；b.石英闪长岩中的包体；c.石英闪长岩中的大理岩捕掳体；d.角闪石反应边结构；e.闪长质包体中的斜长石斑晶；f.次生白云母
Fig. 4. Outcrop photos and photomicrographs of quartz diorite and dioritic inclusion, showing the typical textures
图 5 石英闪长岩的矿物学特征及电子探针位置
Hbl.角闪石；Bt.黑云母；Pl.斜长石；Mic.微斜长石
Fig. 5. Mineralogical characteristics and electron probe position of quartz diorite
图 6 灰白色片麻状中细粒石英闪长岩的锆石CL图像(a)和锆石U-Pb平均年龄、谐和图(b, c)
Fig. 6. CL imaging of zircons from quartz diorite (a) 206Pb/238U average age and concordia plots of zircons from quartz diorite (b, c)
图 7 侵入岩的SiO2-(Na2O+K2O)图解(a)、AFM图解(b)和K2O-SiO2图解(c)
图a据Middlemost(1994); 图b据Kuno(1968); 图c据Peccerillo and Taylor(1976).图 7a中，1.橄榄辉长岩；2a.碱性辉长岩；2b.亚碱性辉长岩；3.辉长闪长岩；4.闪长岩；5.花岗闪长岩；6.花岗岩；7.硅英岩；8.二长辉长岩；9.二长闪长岩；10.二长岩；11.石英二长岩；12.正长岩；13.副长石辉长岩；14.副长石二长闪长岩；15.副长石二长正长岩；16.副长石正长岩；17.副长石深成岩
Fig. 7. SiO2-(Na2O+K2O) (a), AFM (b) and K2O-SiO2 (c) diagram of intrusive rocks
图 8 微量元素蛛网图(a)和稀土元素配分曲线(b)
Fig. 8. Primitive mantle-normalized trace element spidergrams (a) and chondrite-normalized REE patterns (b)
图 10 角闪石成因图解(a)和黑云母分类图解(b)
图a据谢应雯和张玉泉(1990)；图b据杨文金等(1986)
Fig. 10. Origin diagram of hornblende (a) and classification diagram of biotite (b) from quartz diorite
图 11 Sr/Y-Y判别图(a)和R2-R1图解(b)
图a据Defant and Drummond (1990), 图b据Batchelor and Bowden(1985).R1=4Si-11(Na+K)-2(Fe+Ti)，R2=6Ca+2Mg+Al.1.幔源花岗岩; 2.板块碰撞前消减地区花岗岩; 3.板块碰撞后隆起花岗岩; 4.晚造山期花岗岩; 5.非造山花岗岩; 6.地壳熔融的花岗岩(同碰撞); 7.造山后期花岗岩
Fig. 11. Tectonic setting determinant diagram of quartz diorite
图 12 测区与达茂旗、白乃庙地区的石英闪长岩岩石地球化学特征对比
达茂旗地区样品据Jian et al.(2008)；白乃庙地区样品据Zhang et al.(2013).图a中数字解释同7a；图f中数字解释同图 11b
Fig. 12. The comparison of petrochemical characteristics for the survey area, Damaoqi and Bainaimiao
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