【作者】 付长亮；

【导师】 孙德有；

【作者基本信息】 吉林大学 ， 矿物学、岩石学、矿床学， 2009， 硕士

【摘要】 本文对小西南岔地区花岗岩类进行了详细的岩石学、年代学、地球化学及锆石Hf同位素研究,确定了花岗岩类的形成时代和岩石组合,阐述了花岗岩类的成因演化,探讨了其形成的构造背景。小西南岔地区花岗岩类可分为4期:①晚二叠世英云闪长岩-花岗闪长岩(252~256Ma);②中三叠世石英闪长岩(240~241Ma);③早侏罗世二长花岗岩(183Ma);④早白垩世英云闪长岩和花岗闪长岩(105~112Ma)。该区花岗岩均为I型花岗岩,源岩为玄武质下地壳。其中,①晚二叠世花岗岩属于钠质钙碱性系列,在其演化过程中存在结晶分异作用,形成于古亚洲洋构造域活动陆缘环境;②早侏罗世花岗岩为钾质高钾钙碱性系列,形成于太平洋板块俯冲导致的伸展环境下的活动陆缘。③早白垩世花岗岩形成于太平洋板块斜向俯冲造成的岩石圈伸展背景下的活动陆缘环境,其中英云闪长岩属于钠质钙碱性系列,花岗闪长岩属于钾质高钾钙碱性系列。运用最新的花岗岩Sr-Yb分类方法对研究区显生宙花岗岩的源区深度、残留相等进行了限定,认为:①晚二叠世花岗岩形成于低压条件下,残留相为斜长石和角闪石;②早侏罗世花岗岩形成压力可能非常低,与地壳减薄有关;③早白垩世两类花岗岩形成深度明显不同,英云闪长岩的形成压力为研究区内所有花岗岩中最高,残留相为石榴石(±金红石),并首次鉴别出该类花岗岩为阿留申型埃达克岩;花岗闪长岩形成于中等压力下,残留相为石榴石和斜长石。首次发现该区中三叠世钠质钙碱性系列石英闪长岩为高镁闪长岩,其地球化学特征与赞岐岩相似,源区为含水地幔橄榄岩。Hf同位素特征显示,新元古代-显生宙期间该区存在幔源岩浆底侵的地壳增生作用,提出该区乃至延边地区在240Ma左右的中三叠世存在一次重要的地壳增生事件。更多还原

【Abstract】 The research region, located in Yanbian, Jilin province, belongs to the crossing zone between the ancient Asian Ocean tectonic belt in the eastern Xingmeng orogenic belt and the Mesozoic circum-Pacific Ocean tectonic belt. The evolution traces of both the ancient Asian ocean and the circum-Pacific Ocean tectonic belt have been found. There occur extensive Phanerozoic granitiod intrusions and abundant mineral resources of gold, copper, tungsten and molybdenum. Studies on the granitiod intrusions play an important role on discussing evolution of the two tectonic belt and mineral exploration in the district.1 Geochronology and rock combination of granitoidBased on the former data, detailed field geologic survey and lithology research as well as zircon LA-ICP-MS U-Pb Geochronology, granites occurring in this area can be classified into four periods:①Late Permian tonalite- granodiorite(252~256Ma);②Middle Triassic quartz diorite(240~241 Ma);③Early Jurassic monzogranite(183Ma);④Early Cretaceous tonalite and granodiorite(105~112Ma).2 Geochemistry and Hf isotope composition of granitoid2.1 Late Permian tonalite-granodioriteIn the K2O-Na2O scheme, granites formed in this period belong to the series of natrium calc-alkiline, and is located in the I-type granites zone completely. It is shown that P2O5 and A/NK value decreases with increasing of SiO2, whereas A/CNK value changes in the opposite direction. Moreover, such elements are enriched as Rb、La、Ce、Zr and Hf, while some are depleted as Ba、Nb、Ta、Sr、P and Ti elements. The tonalite is belonged to metaluminous type rock(A/CNK=0.91~0.92), the total REE content is lower(ΣREE=96.56~157.22ppm ) , showing the weak fractionation between LREE and HREE [(La/Yb)N=2.14~4.85],the value ofδEu is between 0.47 to 0.81. The granodiorite, however, is the weakly peraluminous rock (A/CNK=1.08)of, with the upper total REE content (ΣREE=171.23ppm) than the tonalite, the fractionation beteween LREE and HREE is strong [(La/Yb)N=14.17],and the value ofδEu is 0.80. The 176Hf/177Hf values of zircons from granites in this period is between 0.282738 to 0.283045, the value ofεHf(t) is between 4.38 to 14.99,and TDM2 is between 305 to 858Ma. 2.2 Middle Triassic quartz dioriteThe quartz diorite in this period are sodic calc-alkaline series and belongs to the metaluminous high-Mg diorite, the normalized curves of REE distribution are close to flat. There is an unclear fractionation between LREE and HREE. Rocks are enriched in Rb, depleted in Ba, Nb, Ta, P and Ti. The differences between different rock types are that, the fine granular quartz diorite have relatively high content of rare earth elements(ΣREE=82.43~129.43ppm),δEu=0.47~0.73, also enrich in K, Nd and Sm. The total amount of rare earth elements in mylonization quartz diorite are relatively lower(ΣREE=39.96~48.81ppm),δEu=0.91~1.03, also enrich in U. Zircon Hf isotopic analyses show that 176Hf/177Hf value in quartz diorite if from 0.282542 to 0.283085,εHf(t) from -3.52 to 15.95 and TDM1 from 246 to 1096Ma。2.3 Early Jurassic monzograniteGranites in this period are potassic high-K calc-alkaline series with weakly peraluminous (A/CNK=1.06). On the diagram of K2O-Na2O, all samples located in the region of I-type granite. The total amount of rare earth elements is 104.31ppm, REE distribution patterns are right-inclined type with obvious fractionation of HREE and LREE [(La/Yb)N=4.94],δEu=0.32. The spider diagram are strongly right-inclined and rich in incompatible elements. The rock is enriched in Rb, Th and K, depleted in Ba, U, Nb, Ta, Sr, P and Ti, characterized by strong depletion of Sr, P and Ti. The 176Hf/177Hf ratio of monzogranite range from 0.282407 to 0.282750,εHf(t) from -9.27 to 3.16, two-stage model age of the rock(TDM2) is 862 to 1495Ma.2.4 Early Cretaceous tonalite and granodioriteAll granites formed in this period are metaluminous, on the diagram of K2O-Na2O, all samples located in the region of I-type granite. Companying with the increasing of SiO2 content, P2O5 contents gradually decreased. The normalized curves of REE distribution patterns are strongly right-inclined type with obvious fractionation of HREE and LREE; Rocks are enriched in Rb, Zr and Hf, depleted in Ba, Nb, Ta, P and Ti; The differences between them are that, tonalite belongs to sodic calci-alkaline series, low the total amount of rare earth elements (ΣREE=67.42~112.23ppm ), Eu anomalyδEu=0.82~1.10, slightly enriched in Sr. Granodiorite belongs to potassic high-K calc-alkaline series, have relatively higher total amount of rare earth elements(ΣREE=83.97~148.22ppm),δEu=0.57~0.91, slightly depleted in Sr.3 Petrogenesis and tectonic setting of granitoid3.1 Late Permian tonalite-granodioriteLate Permian granites are I-type formed at low pressures, with plagioclase and hornblende in residual phase, pressure lower than 1.0Gpa, the corresponding crustal thickness is 30-40 km, fractional crystallization exists in the process of evolution. Analysis of major element and Hf isotope (εHf(t)=-3.52~15.95) shows that the source rock is mainly basaltic rocks (lower crust), with little old crustal materials.Granites at this period belong to sodic calc-alkaline series, and are metaluminous to weakly peraluminous I-type granite. They are composed of tonalite, belong to cordilleran I-type granite in active continental margin, and deplet Nb, Sr, P and Ti. Their characteristics above are similar to the granite in active continental margin. Based on the trace elements tectonic discrimination diagram, and compared with the trace element spider diagram formed in typical tectonic settings, Yanbian area are located in the surrounding of active continental margin in late Permian, the ancient-Asian Ocean was not closed completely.3.2 Middle Triassic quartz dioriteThe quartz diorite emplaced in middle Triassic is High-Mg diorite which wasderived from sanukitic HMA magmas, namely, the partial melting matter of mantle peridotite with water. Data from Hf isotopic composition also support the conclusion.Quartz diorite of this period is middle-low potassic calc-alkaline series, is depleted in Ba, Nb, Ta, P and Ti, etc,which shows the formation environment of the active continent. In the tectonic setting discrimination diagram of trace elements, the Quartz diorite belongs to the tectonic setting relating to plate subduction. Research data show that the sanukitic High-Mg diorite are generated by the partial melting of a young and/or hot oceanic slab, probably be created in mantle wedge above subduction zone of slab. So we think the Quartz diorite is generated in mantle wadge contecting with subduction of slab. The conclusion is consistent with the former points - the ancient Asia Ocean had completely closed, and the mantle wadge formation the Quartz diorite may generate the hysteresis effect.3.3 Early Jurassic monzograniteGranites in this period are I-type rocks, which formed in the much lower pressure, maybe less than 0.8Gpa, and it is related to the lithospheric thinning event, the depth is less than 30Km.Main elements and Hf isotope(εHf(t)=-9.27~3.16)analysis indicates that the mother rock of this period granite is basaltic rock(lower crust), at same time, more older crust materials were involved in the evolvement. The granites are weakly peraluminous and belonging to potassic high-k calc-alkaline series. They consist only of monzogranite, strongly depleted Nb, Sr, P and Ti, which indicate that their characteristics are similar to the granite in active continental margin. The conclusion is accord with the one from the trace elements tectonic discrimination diagram, and the granites was formed at lower pressures, associated with thinning of the lithosphere. Based on the former data, it is concluded that the granites are generated active continental margin resulted from the subduction of Pacific Ocean slab.3.4 Early Cretaceous tonalite and granodioriteEarly cretaceous granites are I-type absolutely, the tonalite type belongs to Aleutian-type adacite, and the mother rock is the basaltic rock derived from the bottom of upper crust, the forming pressure is at least more than 1.0Gpa, general than 1.5Gpa, the same to, the corresponding depth is larger than 40Km, general than 50km. While the granodiorite is formed below the middling pressure, the forming depth is bewteen 40Km to 50Km, going with the forming pressure is bewteen 1.0Gpa to 1.5Gpa, and residual phase conclude garnet and plagioclase,so the mother rock must be the basaltic rock.The granites is metaluminous I type granite of sodic calci-alkaline series, include tonalite and granodiorite, belong to cordilleran I-type granite in active continental margin. Among them, the ranodiorite is depleted Nb, Sr, P and Ti, which shows similar characters to the granite in active continental margin. Tonalite is Aleutian-type adacite, depleted Nb, P and Ti, but weak enrichment of Sr, which is mainly relate to the forming depth of formation the granites magmas. Furthermore, the tonalite also were formed in active continental margin. Both the trace elements tectonic discrimination diagram and the trace element spider diagram formed in typical tectonic settings support upper conclusion. At the same time, abundant data indicate that the Northeastern China in Early-Cretaceous located in extensive background. A conclusion can be drawn that the granites were generated under extensive active continental margin resulted from the obliquely subduction of Pacific Ocean slab.4 Crustal growthHf isotopic characteristics demonstrate that there are great crust accretion working from mantle-source magma between the era of Neoproterozoic Era to Phanerozoic Eon in this area. Compared to the crystal age of 240Ma to 241 Ma, the least TDM1 age of Meso-Trias quartz diorite is 246Ma, which indicates that a great crust growth event took place about 240Ma in this region,even Yanbian area.更多还原