東北地區(qū)中生代火成巖Sr-Nd-Pb同位素填圖及其對(duì)區(qū)域構(gòu)造演化的

1、東北地區(qū)中生代火成巖Sr-Nd-Pb同位素填圖及其對(duì)區(qū)域構(gòu)造演化的通過(guò)對(duì)東北地區(qū)中生代火山巖系統(tǒng)的Sr-Nd-Pb同位素地球化學(xué)研究，結(jié)合近年來(lái)積累的區(qū)域中生代侵入巖同位素資料，本論文對(duì)東北地區(qū)中生代火成巖進(jìn)行了同位素省劃分，探討了其巖石成因與地殼增生、殼-幔相互作用的內(nèi)在聯(lián)系，為重塑東北地區(qū)構(gòu)造格架和演化提供了重要的同位素證據(jù)，論文取得了以下主要成果：1東北地區(qū)中生代火成巖的Sr-Nd-Pb同位素填圖結(jié)果顯示區(qū)域中生代火成巖可以劃分為4個(gè)Nd-Pb同位素?。捍笈d安嶺北區(qū)同位素省、大興安嶺南區(qū)-遼源-延吉同位素省、張廣才嶺-佳木斯同位素省及完達(dá)山外來(lái)地體。其中在完達(dá)山地體的饒河地區(qū)出露了一套異

2、常高Pb同位素比值的HIMU型洋島玄武巖，代表了中生代的新增生外來(lái)地體。除完達(dá)山外來(lái)拼貼地體的HIMU型OIB具有負(fù)的7/4值，東北地區(qū)中生代火成巖都為正的7/4和8/4值，顯示出親北方勞亞大陸的巖石圈屬性。2在大興安嶺南區(qū)林西-霍林河地區(qū)存在一條近似平行索倫-賀根山-小興安嶺古縫合帶的高_(dá)(Nd)(t)早白堊世中酸性火山巖組合。這些巖石具有低T_(DM)(0.420.64Ga)、高_(dá)(Nd)(t)(+4.0+5.9)和高Pb(206)pb/(204)Pb(i)=18.3618.56)和類似硅質(zhì)地球的Sr同位素組成(87)Sr/(86)Sr(i)=0.70420.7049)。其熔融源區(qū)主要由新

3、增生的地殼物質(zhì)組成，很可能為古生代的古亞洲洋殘余洋殼的構(gòu)造底墊作用相關(guān)。3在大興安嶺南區(qū)西烏旗發(fā)育一套具低_(Nd)(t)(-10.5-6.3)、高T_(DM)(1.221.51Ga)、中等放射成因Sr(87)Sr/(86)Sr(i)=0.70630.7066)和低Pb同位素比值(206)Pb/(204)Pb(i)=17.2917.33)的安山巖。該套安山質(zhì)火山巖的7/4為3.76.9，8/4為94104，具有典型的Pb同位素DUPAL異常，反映了古老大陸下地殼物質(zhì)對(duì)巖漿成因的重要貢獻(xiàn)。該套巖石的出現(xiàn)為證明東北地區(qū)存在古老大陸下地殼提供了重要證據(jù)。4張廣才嶺-佳木斯地區(qū)中生代花崗巖表現(xiàn)出低_(

4、Nd)(t)(-3.5+2.1)、高T_(DM)(1.01.27Ga)和高放射成因Sr(87)Sr/(86)Sr(i)=0.7050.711)和Pb同位素組成(206)Pb/(204)Pb=18.418.9)，反映了古亞洲洋遠(yuǎn)洋沉積物或變沉積巖為區(qū)域地殼的重要組分之一。5同位素填圖結(jié)果表明，大興安嶺北區(qū)基性火山巖表現(xiàn)出較高且變化小的(206)Pb/(204)Pb(i)比值；而南興安嶺-遼源-延吉地區(qū)中生代基性火山巖整體上表現(xiàn)出相對(duì)北區(qū)低(206)Pb/(204)Pb(i)同位素比值特征。北區(qū)和南區(qū)基性火山巖的Pb同位素差異很可能為南區(qū)靠近古老華北克拉通而有古老大陸下地殼物質(zhì)加入導(dǎo)致其低Pb同位

5、素特征，北區(qū)高Pb同位素特征暗示其熔融源區(qū)可能有大洋沉積物的加入。南北大興安嶺的同位素組成差異支持前人認(rèn)為的索倫山-賀根山-小興安嶺代表了古亞洲洋縫合帶的觀點(diǎn)。6東北地區(qū)中生代火成巖_(Nd)(t)(或T_(DM)和Pb同位素比值隨時(shí)間演化顯示出平穩(wěn)變化的趨勢(shì)，反映中生代尤其是巖漿活動(dòng)峰期(早白堊世)期間區(qū)域上并不存在大規(guī)模的地殼增生作用，其巖石成因暗示地殼物質(zhì)的再循環(huán)和分配作用是形成廣泛中酸性巖漿的主要機(jī)制The NE China fold belt is the eastern segment of Central Asian Orogenic belt (CAOB), which is

6、the largest accretion orogen between the North China Block and the Siberian Plate. Based on systematic Sr-Nd-Pb isotopic analyses of Mesozoic volcanic rocks and available data from Mesozoic intrusive plutons by previous studies in NE China, this paper presents a new isotope mapping of region, and di

7、scusses the origin of the extensive felsic magmatism and their relationship with the crustal growth and mantle-crust interacion. The results provide robust evidence for understanding the regional tectonic framework and the evolution of the CAOB. Major conclusions are summarized below.1 The Sr-Nd-Pb

8、isotope mapping results of Mesozoic igneous rocks in NE China identify four Nd-Pb isotope provinces: (1) the northern Hinggan Mountains; (2) the southern Hinggan Mountain-Yanji-Liaoyuan region; (3) the Zhangguangcai Range -Jiamusi block; and (4) the Wandashan massif. The Wandashan massif, in which o

9、ccurred HIMU-type OIBs with highly radiogenic Pb isotope compositions from the Raohe area, is a foreign terrane accreted to the CAOB. Except for HIMU-type OIB with negative7/4 ratios in the Wandashan massif, all other rocks have positive7/4 and8/4 values, a feature of the Laurasia continent in the N

10、orthern Hemisphere.2 In the southern Hinggan Mountains distributes a Cretaceous felsic igneous rock belt characterized by highly positive-_(Nd) (T_(DM) = 0.420.64Ga,_(Nd)(t) = +4.0+5.9) and radiogenic Pb (206)Pb/(204)Pb(i) = 18.36 18.56) isotope compositions. These melts were derived from a source c

11、omprising mainly juvenile crust with subordinate metasediments. Combined geochemical data and subordinate crustal growth during late Mesozoic time suggest that the tectonically entrained Paleo-Asian oceanic crustal relics were the likely candidatesfor the juvenile components.3 The west Ujimqin andes

12、ites in the southern Hinggan Mountains have unradiogenic Nd (_(Nd)(t) = -10.5-6.3, T_(DM) = 1.221.51Ga) and unradiogenic Pb (206)Pb/(204)Pb(i) = 17.2917.33) and moderately radiogenic Sr isotopic compositions (87)Sr/(86)Sr(i) = 0.70630.7066). These melts also have positive7/4 (3.76.9) and8/4 (94104)

13、values, a DUPAL Pb isotope signature as observed in the Indian Oceanic mid-ridge basalts. Such features are typical of ancient lower continental crust, e.g., Archean granulite terranes that experienced extensive U-Th loss and developed time-integrated low radiogenic Pb compositions. The occurrence o

14、f the west Ujimqin andesites indicates the existence of ancient continental lower crust beneath the NE China fold belt.4 The Mesozoic granitoids in the Zhangguangcai Range and Jiamusi block have lower Nd (_(Nd)(t) = -3.5+ 2.1, T_(DM) = 1.01.27Ga) and highly radiogenic Pb and Sr isotopic composition

15、(206)Pb/(204)Pb(i) = 18.418.9, (87)Sr/(86)Sr(i) = 0.7050.711), suggesting that the pelagic sediments of the Paleo-Asian Ocean or their metamorphosed derivates were an important component in the region.5 The Pb isotope mapping results show that the mafic rocks in the northern Hinggan Mountains have h

16、igher and weakly variable (206)Pb/(204)Pb ratios than those from the southern Hinggan Mountains and the Liaoyuan-Yanji area. Such a Pb isotope difference between the two regions supports the previous hypothesis that the Suolun-Hegenshan-Less Hinggan mountain belt represented the collisional suture b

17、etween the North China and the Mongolian Blocks. On the other hand, the higher radiogenic Pb compositions in the northern Hinggan Mountains mafic rocks were possibly attributed to the recycling of the pelagic sediments into the mantle source, whereas the less radiogenic Pb compositions in the southe

18、rn Hinggan Mountains and Liaoyuan-Yanji counterparts suggested the involvement of terrigneous sediments derived from the continental lower crust of the North China Block.6 The lack of systematic correlations between emplacement age and_(Nd)(t) (or T_(DM) and Pb isotope ratios and the absence of abrupt change in Sr-Nd-Pb isotope data at any given time throughout the Mesozoic indicate that the crustal growth was subordinate during Mesozoic time, especially at the peak stage (early Cretaceous) of magma