内蒙古中部构造混杂带晚古生代-早中生代变质基性岩的地球化学特征及其大地构造意义

Geochemical characteristics and tectonic significance of Late Paleozoic-Early Mesozoic meta-basic rocks in the mélange zones,Central Inner Mongolia

内蒙古中部构造混杂带中的变质基性岩可分为南、北两带：南带位于乌兰沟-图林凯地区,被划分至温都尔庙群下部的桑达来因组,主要为一套变质玄武岩和辉长岩、辉绿岩,局部含有超基性岩和碳酸岩透镜体,其中变质基性火山岩以绿片岩相变质为主,局部保留枕状构造或发育蓝片岩,已有的锆石U-Pb年代学数据表明南带变质基性火山岩形成于晚古生代到早中生代;北带位于芒和特-二道井-红格尔一线,主要呈岩块状保存在由绢云绿泥石英片岩、硅质岩、含铁石英岩和少量的大理岩组成的基质中,岩石类型包括蓝片岩、冻蓝闪石片岩、阳起片岩、绿帘角闪片岩等。地球化学研究显示南、北两带的变质基性岩相对低Al（Al2O3=10.66%~14.97%）、低Ti（TiO2=1.27%~1.96%）、高Na（Na2O=1.02%~4.20%）、贫K（K2O=0.02%~0.71%）,具有拉斑玄武岩到碱性玄武岩系列的过渡特征,高的Na2O/K2O比值（6.89~454）暗示这些基性岩在变质作用前发生了细碧角斑岩化。不活动元素Zr与其他高场强元素（HSFE;Th、Nb、Hf、Ti）和稀土元素（REE）显示良好的线性关系,表明在变质过程中,高场强元素和稀土元素基本保持稳定,可以反映原岩的性质。根据稀土、微量元素配分型式和相关比值可以将内蒙中部构造混杂带中的变质基性岩分为两类：一类稀土含量相对较低（∑REE=46.00×10-6~78.08×10-6）、轻重稀土分异不明显（（La/Yb）N=0.50~1.04）,无明显Eu负异常,Hf/Ta=6.82~15.18,具有正常的大洋中脊玄武岩（NMORB）特征;另一类稀土含量相对较高（∑REE=58.66×10-6~151.3×10-6）、轻重稀土分异明显（（La/Yb）N=2.28~4.68）,无明显Eu负异常,Hf/Ta=2.06~4.70,与富集型洋中脊玄武岩（E-MORB）相似。部分变质基性岩样品轻微富集大离子亲石元素Rb和Ba,可能暗示原岩在就位过程中遭受了微弱的陆壳混染。以上地球化学特征表明这些变质基性岩的原岩可能形成于一个扩张规模有限的陆内洋盆环境。已有的古生物地理学研究表明古亚洲洋闭合后,到晚古生代早期,内蒙古中部地区成为佳-蒙地块的一部分。石炭纪期间整个内蒙古中部发育稳定的浅海相沉积,局部为造山后隆起环境,发育加里东I型花岗岩和花岗闪长岩。从晚石炭世-早二叠世起,内蒙中部开始处于伸展环境：二连浩特到东乌珠穆沁旗一带发育大量的碱性岩;华北克拉通北缘发育很多高钾钙碱性-碱性的花岗岩;内蒙中部地区广泛发育二叠纪大石寨组双峰式火山岩。到中二叠世开始裂解形成若干近东西向分布的海盆,发育哲斯组、林西组浅海相、泻湖相沉积。持续的伸展形成了有限洋盆,发育以温都尔庙群为代表的含铁硅质岩以及晚古生代-早中生代基性岩。由于早中三叠世华北板块与扬子板块全面碰撞和陆内造山过程的影响,有限洋盆最终在早中生代之后发生被动闭合,形成南、北构造混杂带,并导致该基性岩乃至整个内蒙中部的晚古生代沉积发生广泛绿片岩相变质作用,而局部蓝片岩的形成可能与有限洋盆的俯冲作用有关。

The meta-basic rocks in the melange zones from the Central Inner Mongolia are divided into southern and northern zones. The southern zone meta-basic rocks are distributed in Wulan Valley and Tulinkai areas and have been grouped into the lower part of the Ondor Sum Group, called Sangdalaiyin Formation. The rocks mainly include meta-basalt, meta-gabbro and meta-diabase intercalated with lenses of serpentinite and carbonate. The meta-basic rocks mainly underwent greenschist facies metamorphism, with blueschist developing and pillow structure preserved locally. Available zircon U-Pb age data suggested that the southern zone meta-basic volcanics were formed in the Late Paleozoic to Early Mesozoic. The northern zone meta-basic rocks are distributed from Manghete, eastwards to Erdaojing and Honger and mainly occur as tectonic blocks in a matrix consisting of sericite-chlorite-quartz schist, siliceous rock, ferrnginous quartzite and a small amount of marble. The rocks mainly include blueschist, barroisite schist, actinolite schist, epidote amphibole schist. Geochemical study shows that the meta-basic rocks in both the southern and northern zones are characterized by relatively low Al（Al2O3 = 10. 66% - 14. 97% ）, low Ti （TiO2 = 1.27% - 1.96% ）, high Na （Na2O = 1.02% -4. 20% ） and low K （K2O =0. 02% -0. 71% ）, suggesting a transitive feature from tholeiitic series to alkali basalt series. The high ratio of Na2O/K2O （6. 89 - 454） indicates that these basic rocks might have experienced spilitization before they underwent the regional metamorphism. As immobile element Zr shows good linear relationships with other high field strength elements （HSFE; Th, Nb, Hf, Ti） and rare earth elements （ REE）, it is concluded that most HSFE and REE in the recta-basic rocks remain stable during the metamorphic process, and thus can represent the characteristics of their protoliths. According to the distribution patterns of rare earth and trace elements and their ratios, the meta-basie rocks can be subdivided into two groups： Group I has a relatively low REE content （ ∑ REE =46. 00 × 10^-6 -78. 08 × 10^-6）, indistinctive REE fraetionation （（La/Yb） N =0. 50 - 1.04）, no obvious Eu negative anomaly and a relatively high Hf/Ta ratio （ Hf/Ta = 6. 82 - 15.18 ）, resembling normal mid-ocean ridge basalts （ N-MORB ） ; GroUp Ⅱ is characterized by relatively high REE content （ ∑REE = 58. 66 × 10^-6 - 151.3 ×10^-6）, intense fractionation of REE （（La/Yb） N = 2. 28 - 4. 68）, no obvious Eu negative anomaly and a relatively low Hf/Ta ratio （Hf/Ta = 2. 06 - 4. 70 ）, showing an affinity to enriched mid-ocean ridge basalts （E-MORB）. A few meta-basic rock samples are weakly enriched in large ion lithophile elements （LILE） such as Rb and Ba, probably reflecting a weak crustal contamination during emplacement of the basaltic magma. The above geochemical characteristics suggest that these meta-basic rocks may have formed in limited ocean basins. The previous paleobiogeography research suggested that the study region had been a part of the Jiamusi-Mongolia Paleo-eontinent in the early Late Paleozoic after closure of the Paleo-Asian Ocean in the Devonian. The Carboniferous sedimentary sequences in the Central Inner Mongolia were dominated by stable neritic facies, while in local areas were developed the Caledonian I type granites and granodiorltes which represented a post-orogenic uplift environment. From the Late Carboniferous-Early Permian on, the extensional tectonicenvironment was prevailed in the Central Inner Mongolia. This extensional setting was supported by the development of a large amount of Early Permian alkaline rocks distributed from Erenhot to East Ujimqin Banner, numerous high potassium tale-alkaline and alkaline granites in the northern margin of North China Craton, and the Early Permian bimodal volcanics grouped into the Dashizhai Formation widely-spreading in the Central Inner Mongoha. The regional extension might have resulted in the formation of sea basins since beginning of the Early Permian, in which developed the neritic sedimentary sequences of the Zhesi and Linxi formations. As a result of the continuous extension, limited ocean basins might have formed, in which developed the ferruginous siliceous rocks and the large amount of Late Paleozoic-Early Mesozoic basic rocks as represented by the Ondor Sum Group. These limited basins might have been passively closed in the Early Mesozoic probably due to the influence of the Early-Middle Triassic collision between the North China Craton and Yangtze plate, which resulted in the extensive greenschist facies metamorphism in the meta-basic rocks and all the Late Paleozoic sedimentary sequences from the Central Inner Mongolia and formation of melange zones and local blueschists as a consequence of subduction of the limited ocean basins.