The Petrologic Characteristics And Tectonic Implications For Early Carboniferous Intrusions From The Area Of Karamaili In Eastern Junggar

Granite is an important part of the continental crust. There are a series of problems about the granite formation and evolution, the genetic types of granite and its relation with tectonic environment are the hotter issues.The north of Xinjiang is the joint of Siberia plate, Kazakhstan plate and Sino Korean plate, which belongs to the Central Asian orogenic belt. The Junggar basin is located in between Tianshan and Altay orogenic belt. The Karamaili belt is located in the northeastern margin of eastern Junngar, which includes an important ophiolite melange zone, namely Karamaili ophiolite zone. Large areas of granite bodies are an ideal carrier for discussing post collisional magmatic evolution. Previous researchs are focused on the area of the north of Karamaili fault, including these granitoids such as Kubusu, Huang yangshan, Laoyaquan, Sujiquan, Bellekuduke et.al. The granite’206Pb/238U ages are between287Ma and314Ma, the genetic types of granites are A-type and tectonic setting of post-collision extensional environment.In addition, although there have been reported large areas of alkaline granites in the region, and many scholars also think that the granites are related to underplating of magma derived mantle, but the articles reported only focus on enclaves of magmatic mixing. The age of enclaves is between286Ma and300Ma, post-collisional mantle underplating body has not been reported, and so, it is necessary for searching mantle underplating body.Based on detailed geological survey, combined with petrography, geochemistry and isotope, there are many discussion about rock genesis, source characteristics and tectonic environment, the main research contents consist of the genetic types of granites, the characteristics of the source of granite and gabbro, and the relationship between the granites and tectonic background. The main realizations are as follows:Wucaicheng pluton is located in the south of Karamaili in the north of Wucaicheng scenery spot, which shows a characteristic of zonal distribution from NW-SE and is intruded in Songkaersu group of the Lower Carboniferous. The pluton is characterized by K-feldspar granite pluton and involves small-scaled quartz-syenite pluton. The mixture of mantle and crust is obvious, the dioritic enclaves are universal. Dishuiquan-Xumuban intrusion is between the Dishuiquan fault and Karamaili fault, which shows a characteristic of NWW zonal distribution and is intruded in Songkaersu group of the Upper Carboniferous or Beitashan group of Middle Devonian. The pluton is characterized by K-feldspar granite pluton and visible smaller hornblende-gabbro pluton. It is a stitching body and intrudes into the newly discovered Dishuiquan ophiolite. Hornblende gabbro pluton is visible in Xumuban or Yemazhan for the long axis of NE shape distribution, magma mixing phenomenon and K-feldspar metasomatism phenocrysts are obvious in the contact zone of granite and gabbro.LA-ICP-MS zircon U-Pb age shows that Wucaicheng pluton is formed the early stage of Early Carboniferous(K-feldspar granite’206Pb/238U ages are341±4Ma and341±5Ma), Dishuiquan-Xumuban pluton is formed the late stage of Early Carboniferous (K-feldspar granite’206Pb/238U ages is321±2Ma and hornblende-gabbro is319±3Ma).Through the rock associations and geochemical characteristics, Wucaicheng pluton consists of quartz-syenite+monzogranite+K-feldspar granite, low SiO2content (59.35%-69.51%), Na2O> K2O(average of1.54), A/CNK<1.1, a good negative correlation between SiO2and P2O5, the amphibole beingMain dark mineral, displaying the characteristics of I-type granites, the geochemical characteristics of the pluton shows enrichment in LILE (K、Rb、Ba),depletion in Nb、Ta、P and Ti, relatively high REE contents and negligible Eu anomalies(0.75-0.95), which is similar with Early Carboniferous post-collisional granites in western Junngar. Dishuiquan-Xumuban pluton has the bimodal rock associations, whose characteristics is similar with Huangyangshan A-type granite, such as high SiO2content (71.07-76.71wt%), K2O>Na2O(average of1.10), intensive enrichment in Ba、P and Ti, moderate-intensive depletion in Eu.Wucaicheng granites are characterized by positive εNd(t) values (+5.41-+5.95), a relative low initial Sr isotopic ratios(ISr=0.70289-0.70335) and young second Nd model ages (610Ma-650Ma), Dishuiquan-Xumuban granites are characterized by positive εNd(t) values (+5.52-+5.90)and young second Nd model ages (600Ma-630Ma), so the magma source is consensus with the young crust of the north of Xinjiang. Hornblende-gabbro are characterized by (87Sr/86Sr)i=0.70359, εNd(t)=+5.41, which shows source of depleted asthenosphereMantle, La/Ba-La/Nb diagram and enrichment in LILE(K、Sr、Ba), depletion in HFSE(Nb、Ta、2Th、 Zr、Ti)are characterized with wedge matasomasised by subducting slab.The magma mixing is universal in Wucaicheng pluton, the rock species contain quartz monzonite and quartz monzobiorite, taxitic structure and acicular apatite are visible; TheMagma mixing only occur in the contacts between granite and gabbro in Dishuiquan-Xumuban pluton, quartz diorite represents the rock species of magma mixing, in which plagioclase corrosion zoning is visible.The similar zircon U-Pb age and obvious magma mixing between gabbro and granite indicate that Dishuiquan hoRNblende-gabbro represents the product of underplating magmas. The finding of underplated hornblende-gabbro pluton in the late stage of Early Carboniferous provides a reliable geological basis for underplating of granitoids in Kalamaili.Wucaicheng granite is a typical Ⅰ-type granite, its emplacement time later the age of molasse formation, and at the same time the post-collision volcanic rocks are general; Dishuiquan-Xumuban intrusions are bimodal rock associations, the type of granite is transitive from I-type to A-type, there are intra-continental bimodal volcanic rocks at the same time. Combining with R1-R2tectonic discrimination diagrams, we can find that Wucaicheng pluton is formed in post-collision stage, Dishuiquan-Xumuban pluton is after collision. The zircon U-Pb age(341Ma) of Wucaicheng I-type granites in this paper is the eldest age of post-collisional granites in Karamaili, which manifests that the ocean basin of Karamaili has been closed before Early Carboniferous(Late Devonian?). The area of Karamaili has been transformed into intra-continental extensional stage after collision in the late stage Early Carboniferous(321Ma).