CAJ | 학술논문

四棵树岩体位于北天山缝合带中,是侵入依连哈比尔尕岛弧带中最大的一个花岗岩体,具有“钉合岩体”的构造意义。组成该岩体的岩石类型较多,大体上可分为3个侵入期次,依次为石英闪长岩和花岗闪长岩、二长花岗岩和钾长花岗岩、花岗斑岩；其中第二侵入期次构成了该岩体的主体。岩石 SiO2含量为57.85%~74.28%,K2 O含量为1.95%~5.25%,Na2 O 含量为3.00%~4.50%,整体表现为高 K、Ca 碱性特征。∑REE=113.91×10-6~169.47×10-6,(La/Sm)N 为1.77~3.16,(Gd/Yb)N 为1.33~2.12,(La/Yb)N 为3.41~5.64,δEu 为0.37~0.82,表现为轻稀土元素富集的右倾型特征；岩石具有 Nb、Ta、Eu 的亏损以及 Th、Zr、Hf 相对富集。通过形成环境判别,第一期石英闪长岩和花岗闪长岩大部投在火山弧环境,板块碰撞前-后碰撞隆起阶段；第二期二长花岗岩和钾长花岗岩大部投在后碰撞阶段；第三期花岗斑岩主要投在晚造山阶段。通过 LA-ICP-MS 对该岩体中石英闪长岩、二长花岗岩和钾长花岗岩进行较为系统的锆石 U-Pb 测年,得出其加权平均年龄分别为(324.1±4.3)Ma、(314.9±4.1)Ma、(311.5±3.9)Ma。结合前人得出的巴音沟蛇绿岩中辉长岩的(344±3.4)Ma 年龄,可初步得出北天山缝合带的形成时代应该为344~311.5 Ma,碰撞造山作用主要发生在324~311 Ma,随后进入后碰撞伸展阶段,而四棵树岩体3期侵入岩可能分别代表了北天山造山带俯冲汇聚碰撞陆内造山各阶段构造岩浆作用的产物。
사과수암체위우북천산봉합대중,시침입의련합비이소도호대중최대적일개화강암체,구유“정합암체”적구조의의。조성해암체적암석류형교다,대체상가분위3개침입기차,의차위석영섬장암화화강섬장암、이장화강암화갑장화강암、화강반암；기중제이침입기차구성료해암체적주체。암석 SiO2함량위57.85%~74.28%,K2 O함량위1.95%~5.25%,Na2 O 함량위3.00%~4.50%,정체표현위고 K、Ca 감성특정。∑REE=113.91×10-6~169.47×10-6,(La/Sm)N 위1.77~3.16,(Gd/Yb)N 위1.33~2.12,(La/Yb)N 위3.41~5.64,δEu 위0.37~0.82,표현위경희토원소부집적우경형특정；암석구유 Nb、Ta、Eu 적우손이급 Th、Zr、Hf 상대부집。통과형성배경판별,제일기석영섬장암화화강섬장암대부투재화산호배경,판괴팽당전-후팽당륭기계단；제이기이장화강암화갑장화강암대부투재후팽당계단；제삼기화강반암주요투재만조산계단。통과 LA-ICP-MS 대해암체중석영섬장암、이장화강암화갑장화강암진행교위계통적고석 U-Pb 측년,득출기가권평균년령분별위(324.1±4.3)Ma、(314.9±4.1)Ma、(311.5±3.9)Ma。결합전인득출적파음구사록암중휘장암적(344±3.4)Ma 년령,가초보득출북천산봉합대적형성시대응해위344~311.5 Ma,팽당조산작용주요발생재324~311 Ma,수후진입후팽당신전계단,이사과수암체3기침입암가능분별대표료북천산조산대부충회취팽당륙내조산각계단구조암장작용적산물。
Sikeshu granitic pluton is located in the North Tianshan suture zone,the biggest gra-nitic body which intrudes the Yilianhabierga island arc belt,and has tectonic significance as one stitching pluton.Sikeshu pluton is composed of various types of rocks generally formed in three intrusive periods.In the first period,the pluton rocks are quartz diorite and granodiorite,while in the second period the major compositions are monzogranite and K-feldspar granite,and in the third period the pluton is made of granite porphyry.Noticeably,rocks occurred in the second intrusive period constitute the main body of the pluton.On the whole,the rocks are featured by high-K calc-alkaline characteristics, with SiO2 content varying between 57. 85 and 74. 28 wt.%,K2 O content from 1.95 to 5.25 wt.%,and Na2 O content from 3.00 to 4.50 wt.%. The total REE content is between 1 13.91×10 -6 ppm and 1 69.47×10 -6 ppm.In addition,the values of (La/Sm)N,(Gd/Yb)N and (La/Yb)N fluctuate from 1.77 to 3.1 6,1.33 to 2. 12,and 3.41 to 5.64 respectively,showing a right-inclined distribution pattern characterized by weakly negative Eu anomalies with δEu value from 0.37 to 0.82.Moreover,the rocks are enriched in Th,Zr,Hf and depleted in Nb,Ta and Eu.In terms of deformation environment, quartz diorite and granodiorite of the first period mainly fall into the volcanic arc area of the pre-collision to the post-collision stage;rocks of the second period,namely monzogranite and K-feld-spar granite,mainly formed in the post-collision stage;and graniteporphyry in the last period oc-curred in later orogenic stage. LA-ICP-MS zircon U-Pb dating results of quartz diorite, monzogranite and K-feldspar granite are 324.1±4.3 Ma,314.9±4.1 Ma and 31 1.5±3.9 Ma.Considering the age of Bayingou ophiolite,which is 344±3.4 Ma according to previous re-searches,we can conclude that the suture zone of North Tianshan formed between 344 to 31 1.5 Ma and the major collision orogenic process took place from 324 to 31 1 Ma,and then the suture moved into the post-collision stage after 31 1 Ma.To some extent,the three intrusive rock units of Sikeshu granitic pluton may be representative products of three tectonic-magmatic stages, which are subduction,collision and intracontinental orogenic of North Tianshan orogenic belt.