摘要
大兴安岭晚中生代岩浆活动在东北亚晚中生代构造岩浆演化研究中具有重要意义。本文首次报道了大兴安岭中南段甘珠尔庙地区5个晚中生代花岗岩体的锆石U-Pb年龄和地球化学研究结果。这些岩体形成于晚侏罗世(154Ma)和早白垩世(139~125Ma),显示高硅富碱特征,属高钾钙碱性系列,为高分异钙碱性I-A过渡型花岗岩,其中,早白垩世花岗岩分异程度高于晚侏罗世花岗岩。锆石Hf同位素特征显示,这些花岗岩源区以年轻物质为主,其中,晚侏罗世花岗岩εHf(t)值(+9.9^+15.9)高于该区底侵的新生下地壳,源区可能有更年轻的新底侵物质参与。早白垩世花岗岩εHf(t)值略低(+3.4^+13.9),其物源主要为底侵的新生下地壳和古生代俯冲增生杂岩混源,可能还混有少量更年轻的新底侵物质。结合区域资料分析,甘珠尔庙地区晚中生代花岗岩形成于后造山伸展背景,这种背景可能与蒙古-鄂霍茨克洋闭合碰撞后伸展有关。
The Da Hinggan Range （Great Xing＇ an Range） is located in the eastern section of the Central Asian Orogenic Belt （CAOB） and also regarded as the middle-western section of the Xing-Meng Orogenic Belt. It experienced strong magmatic activities that produced widespread granitoid intrusions during Late Mesozoic. Those granitoids have been studied intensively, but few of them have been reported from the Ganzhuermiao region in the middle-southern Da Hinggan Range. In this study, we first present the zircon U-Pb dating results and geochemistry data of five Late Mesozoic granitic plutons in the Ganzhuermiao region and discuss the origin and tectonic settings as well as the implications for understanding of the Late Mesozoic tectono-magmatic evolution in NE Asia together with other research.. These data show the granitoids were formed in Late Jurassic （154Ma） and Early Cretaceous （ 139 - 125Ma）. The time of emplacement consistents with the eruption time of volcanic rocks and peak time of the Late Mesozoic granitoids in NE Asia. The Late Jurassic granitic intrusions are biotite syengranites and the Early Cretaceous plutons are biotite monzogranite and granite porphyry. The Late Jurassic Tumufuzhou pluton with an area of 100km^2, located in southwest of the Ganzhuermiao region. They were emplaced in Permian and Jurassic strata, made up of dozens of stocks. The Early Cretaceous granitoids cover area over 180km^2. The early stage of the Early Cretaceous granitoids include Wulandaba, Xiaojingzi and Huajialaga plutons. The first two are situated in the center and southern of the study area, presenting as batholiths and intruding into the Permian and Jurassic strata. The Balimuhade pluton, intruded in the late stage of the Early Cretaceous, located in the western of the Ganzhuermiao region as large as 90km^2. The mainly body were emplaced in Jurassic volcanic rocks with clear boundaries between them, indicating this intrusion is subvolcanics or middle- shallow rocks. They are all characterized by high SiO2 （69. 96% - 75.85% ） and enrichment of K20 （4. 58% - 5.45% ）. Their A/ CNK ratios is from 0. 98 to 1.14, indicating the metaluminous and peraluminous granitoids. These granitoids also present low Sr （25. 1 × 10^-6 -262. 1 ×10^-6 with average of 84. 05 × 10^-6 ）, high Y （ 18. 1× 10 ^-6 - 46. 1 × 10^-6 with average of 29. 93× 10 ^-6 ） and low Sr/Y ratio （0. 97 - 12. 55 ）. The REE contents of them are between 82. 17 × 10^- 6 to 216. 6×10^-6, with δEu value of 0. 06 -0.58 and （La/Yb） n ratio of 3.42 to 11.48. Their REE patterns are characterized by enrichment of LREE relatively to HREE and negative Eu anomalies in chondrite-normalized REE diagram. Their trace elements relatively enrich large ion lithophile elements （ LILE）, high field strength elements （HFSE） and poor Ba, Sr, Ti, P, with notable depletion of Nb and Ta. The differentiation degree of the Early Cretaceous granitoids are higher than that of the Late Jurassic granitoids. All these indicate that these granitoids belong to the highpotassic calc-alkaline series and show features of highly differentiated I-type granitoids. The geochemical signatures above are similar to those of post-orogenic granitoids in the adjacent area. Model age of zircon Hf isotopic of these granitoids range from 174Ma to 680Ma, suggesting juvenile sources for the granitoids. Furthermore, the 8Hf（t） values （ ＋ 9. 9 - ＋ 15.9） of the Late Jurassic granitoids are higher than those of the underplated juvenile lower crust in study. This implies that sources of these granitoids may be more juvenile than underplated materials. The εHf （t） values （ ＋ 3.4 - ＋ 15.9） of the Early Cretaceous granitoids are slightly lower, which suggests that their sources may be derived from a mixture of underplated juvenile lower crust and Paleozoic subduction-accretionary assemblages. From Late Jurassic to Early Cretaceous, the εHf （t） values of zircons decreased which implies more younger underplated materials involved. The younger Hf isotope model ages of the granitoids imply that this area experienced crustal growth during Late Mesozoic. Combined with regional geology, we suggest that the Late Mesozoic granitoids in the Ganzhuermiao region were formed in an extensional setting which may be related to the post-ordgenic extension following the closure of the Mongolia-Okhotsk ocean to the northwest rather than the subduction of the Paleo-Pacifi6c plate to the east.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第7期1961-1981,共21页
Acta Petrologica Sinica
基金
国家973项目(2013CB429803)
国家自然科学基金项目(41002073
41372006
90714006)
中国地质调查局工作项目(1212010811033
12120113096500
12120113094000)联合资助
