【作者】 刘建峰；

【导师】 迟效国；

【作者基本信息】 吉林大学 ， 地球化学， 2006， 硕士

【摘要】 中国东北部的小兴安岭地区分布有兴东群和东风山群等前寒武纪地层单元,它们共同经受了后期的角闪岩相至麻粒岩相区域变质作用,空间上出现一套与该期变质作用紧密伴生的花岗岩侵入体。从前人研究成果和作者的野外工作来看,该区前寒武纪岩石组合及其变质变形特点均与东部佳木斯地区麻山群相似,这些地层单元与麻山群过去一直被认为是早元古代形成的变质岩系。近年来麻山群500Ma左右变质年龄和同期侵位的花岗岩的确定为兴东群和东风山群的变质时代及与之伴生的花岗岩的构造背景提出了疑问。本次研究的对象为与兴东群和东风山群伴生的花岗岩侵入体。年代学资料证明这些岩体侵位时代属于早古生代,按时间的先后顺序可大致分为三个期次。同时,岩石学和地球化学资料表明这三期早古生代花岗岩分别对应片麻状S型花岗岩、碰撞后I型高钾钙碱性块状花岗岩和造山晚期A2型碱长-碱性花岗岩三种岩石组合。三种岩石组合的出现,标志着小兴安岭地区存在早古生代造山事件,而该造山事件说明佳木斯板块与松嫩地块很可能在早古生代就已经拼贴到了一起。S型花岗岩组合具有“高温型”SP花岗岩的特点,表明该造山带具有类似海西造山带和澳大利亚拉克伦褶皱带的“高温型”碰撞模式。A2型花岗岩的出现则标志着该造山事件的结束和尾声。更多还原

【Abstract】 Xiao Xing’an Mountain is situated in the eastern segment of the giganticCentral Asian-Xing’an-Mongolian orogenic belt, which is the joint of Paleo-Asiaand west Pacific tectonic territories according to geotectonic position, and is one ofthe orogens that undergoes the longest history of development and possessescomplex tectonic-magmatic activities in northeast China. Phanerozoic granitesdistribute extensively in this region. With the development of domestic and foreignresearches on granites recently, much important progress has been made in thestudy on the granites in Xiao Xing’an Mountain, but there are still many significantgeological subjects left to be resolved, and one of which is the question aboutwhether there exist early Paleozoic granites in the area, and if that is true, what arethe spatial distribution characteristic and tectonic attribute of the granites. In thispaper we will systemically research the spatial-temporal distribution, petrologicaland geochemical characteristics, tectonic setting of the early Paleozoic granites ineastern segment of Xiao Xing’an Mountain on the basis of previous data andresearch findings.On the basis of field geological investigation, we chose typical plutons withdifferent structure and texture characteristics to study and confirmed the existenceof tectonic-magmatic activity around 500Ma in the eastern segment of XiaoXing’an Mountain through reliable high precise isotopic dating, which is the sameas the Jiamusi massif to the east of Mudanjiang fault. The plutons that are dated areJinlin pluton(482±19Ma), Dongfengshan pluton(500±3Ma), Tangwanghepluton(471±3Ma) and Jiling pluton(499±1Ma) respectively. According to previousdata, in both Jiamusi massif and Xiao Xing’an mountain, the magmatism in thisperiod was accompanied with the metamorphism of the pre-Cambrian strata ofMashan、Dongfengshan and Xingdong groups. The protolith associations of thosepre-Cambrian strata are quite similar, representing the sedimentary environment ofcontinental margin. The “Heilongjiang group” between Jiamusi massif and XiaoXing’an mountain possesses the attribute of ophiolitic mélange, which mayrepresent their collision suture 500Ma ago.Considering both geochronologic and petrologic characteristics of theearly-Paleozoic granites in Xiao Xing’an Mountain, we can further divide theminto three types:①Early gneissic granite assemblage (508～500Ma). This assemblage occurredwith Xingdong group, Dongfengshan group and other regional metamorphic rockgroups closely in space, and contacted with them at some place. The shapes of therock-masses are irregular, most of which “float” on later granites as tectonic sliceswith their metamorphic wall rock. The rock-masses contain residues of wall rockgenerally, and both the rock-masses and residues have accordant occurrence. Themain rock association includes gneissic granodiate、large-porphyritic monzograniteand K-feldspar granite. Taking Dongfengshan pluton as an example, the rocks arecharacterized by pervasive gneissic and porphyritic structure. The phenocryst isK-feldspar, with general granular size being 3×1cm, and 5×2cm in maximum;darkenclaves and stripes which are composed by biotite, quartz and plagioclase welldeveloped in matrix.②Middle massive high-K calk-alkaline granite assemblage (499～492Ma).This assemblage often intruded or captured Cambrian or Ordovician strata, havingapparent intrusive relationship with them. Hornfelization, skarnization andsilication phenomena are often seen in wall rock. There are many stratum xenolithnear contact zone, forming wide contaminative zone at the edge of plutons. Themain rock association consists of medium-coarse granular quartzdiorite,granodiorite, monzogranite and K-feldspar Granite. Typical rock body of thisassemblage is Jiling pluton, which is distinguished by its general massive structurefrom gneissic granites assemblage in the field occurrence, the weathering surface ofthe rock looks like leopard-skin. Main minerals comprise alkaline feldspar,plagioclase and quartz. The reddish alkaline feldspar phenocrysts often have thegeneral granular size of 2×1.5cm, and 3×3cm in maximum. Dark minerals aremainly amphibole and biotite.③Late alkali-feldspar granite and alkaline granite assemblage(475～445Ma).This assemblage is mainly composed of medium-coarse granular granite,syenogranite and alkaline granite. Typical plutons are Tangwanghe pluton andChaoxiantun pluton. The Tangwanghe pluton is characterized by its wine rock,coarse inequi-granular texture and massive structure. Main minerals are alkalinefeldspar and quartz. The alkaline feldspar is reddish, with general size being 2×1cm,and 3×2cm in maximum. The content of alkaline feldspar is about 40-50%. Quartzin the granite takes on anhedral granular shape and dark color,with averagegranular size being 0.5-1cm and content being 35-40%. Matrix comprises ofplagioclase, biotite and hornblende. Cataclastic texture well developed in the rock,there are many micro-fissions in feldspar phenocrysts, and quartz developedrecrystallization. The typical alkaline granite pluton is Chaoxiantun pluton, whichis characterized by its alkaline dark mineral-riebeckite.According to geochemical characteristics, the gneissic granite assemblage haswide range of SiO2 content, and has higher TiO2, P2O5, K2O and MgO contents andlower NaO2 content compared with the other two types of granite assemblages,which reflects different chemical composition. This assemblage shows S-typegranite characteristics in different geochemical discrimination diagrams. Throughcontrast with post-collisional granites of typical orogenies summarized bySylverster(1998), the gneissic S-type granite assemblage shows the characteristicsof post-collisional “high-temperature” SP (strongly peraluminous) granites inHercynides and Lachlan Fold Belt (LFB) . From the occurrence relationships ofgranites and wall-rock and the abundant enclaves and stripes in the rocks, it can beinferred that the granite originate from the partial melting of the immature felsicmaterials with the source area being the pre-cambrian strata of Dongfengshangroup and Xingdong group.During the same variation of w(SiO2), the contents of TiO2, P2O5 and MgO inmassive granite and alkaline granite assemblages are less than those of gneissicgranite assemblage, while the contents of Na2O and Al2O3 are larger. DuringHarker diagrams of oxides both assemblages show obvious linear evolutionarytendency, which may reflect they belong to comagmatic evolution series. In variouselement discrimination diagrams massive granite and alkaline granite assemblagesbelong to post-colliosional I-type granite and late-orogenic A2-type graniterespectively. Furthermore, both of them display certain genetic relation in thegeochemical diagrams, which reflect that there is no distinct boundary betweenI-type and A2-type granites, and they originated from similar magmatic sourcesthrough different degree fractional crystallization of the magma formed by differentdegree partial melting. The formation ages of both kinds of I-type and A2-typegranite are later than S-type granite, which also reflects they are formed fromextension environment after collision and the magma may originate from partialmelting of the intermediate-basic lower crust.The three kinds of granite assembalges above represent an early-Paleozoiccontinent-continent collision orogeny in eastern segment of Xiao Xing’an mountian.Combined with stratigraphy, paleontology and tectonics data in the region, wethink Jiamusi massif and Songnen massif have collaged together through thecollision orogeny described above in early-Paleozoic.更多还原