Geochemical Characteristics And Tectonic Significance Of Early Paleozoic Granites In Eastern Segment Of Xiao Xing'an Mountain

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.