The Geochronology, Geochemistry And Petrogenesis Of Granitoid From Xiaoxi'nancha In Hunchu Area, Jilin Province

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.