Characteristics Of Granitic Rocks And Its Geological Significance In Taili Area Of Western Liaoning Province

Taili area of western Liaoning province is located in the eastern section of the northern margin of the North China Craton, part of the southeast margin Yanshan Proterozoic depressed zone-Shanhaiguan ancient uplift. There are large area granitic rocks in the region, which are generally regarded as Archean migmatite (Suizhong granite).But according to recent study the migmatite is not just Archean pluton, which is intruded by Phanerozoic granite. It provides a large number of clues for learning more about the features,formation and evolution of the Archean granite. This study is helpful to deep understand the Precambrian geological history and the crustal evolution of the North China Craton. The thesis takes granitic rocks as an example, reports its petrology, geochemistry and chronology, as well as a Hf isotope of zircons, and discusses the nature of granitic magma sources and its forming background.1. Petrologic characteristics of granitic rocks in Taili areaThe main rock types in the study area are granitic gneiss, porphyritic granitic gneiss and biotite adamellite. Granitic gneiss is located in the central of Taili, and its outcrop area is widest in the study area. Granitic gneiss develops gneissic structure, and the gneissic schistosity is close to an east-west direction. Rock body contains a large number of biotite plagioclase gneiss xenoliths, lineation of which is consistent with the surrounding rock .There is felsic gneiss developing in the granitic gneiss and biotite plagioclase gneiss. They are intruded by porphyritic granitic gneiss and biotite adamellite. Porphyritic granitic gneiss is granitic rock of vein invasion. It intrudes into granitic gneiss,biotite plagioclase gneiss and felsic gneiss. Porphyritic granitic gneiss is associated with vein-type granite aplite, and they interpenetrate with each other. The two together and granitic gneiss are intruded by Biotite adamellite. Biotite adamellite is located in the north and south sides of study region, the one in the south with massive structure, the other one with gneissic structure. Biotite adamellite intrudes into granitic gneiss and porphyritic granitic gneiss.2.Geochemical characteristics of granitic rocks in Taili areaAccording to characteristics of major elemen of tthree major granitic rocks in Taili area, we can know that granitic gneiss, porphyritic granitic gneiss and biotite adamellite are all high-k calc-alkaline sub-aluminous to weak peraluminous I -type granite. The characteristics of trace element are that: from granitic gneiss to Biotite adamellite, total rare earth is gradually reduced. This may be due to the granitic rocks in the region are from the partial melting of crustal rocks. The partition coefficient of rare earth elements is or less the same in rock and melt. The fractionation of light-heavy rare earth of biotite adamellite is the weakest, followed by the granitic gneiss, in which the porphyritic granitic gneiss is the strongest. Chondrite-normalized REE distribution patterns of granitic rocks show :Negative Eu anomalies of granitic gneiss is the most obvious, porphyritic granitic gneiss and biotite adamellite have no Eu anomalies or unconspicuous. Granitic gneiss, porphyritic granitic gneiss and biotite adamellite are all depleted in high field strength elements(HFSE)such as Ti, Nb, Ta, P and enriched in large ion lithophile elements(LILE)such as Ba, Sr.Geochemical characteristics of three granitic rock reflect the environment of crust when they formed. The original rock of granitic gneiss may formed in middle to shallow layer of normal crust;When porphyritic granitic gneiss formed,the thickness of crust was thicker than that when granitic gneiss formed.When biotite adamellite formed, crustal thickness was obvious thinner than previous period. 3.Chronology of granitic rocks in Taili areaThe emplacement age of granitic gneiss is about 2500Ma, porphyritic granitic gneiss is about 220Ma, biotite adamellite is about 150Ma. In which , granitic gneiss which age is 2500Ma is the form sign of crystalline basement of North China Craton in late Archean; Porphyritic granitic gneiss which age is 220Ma may be related with the North China Craton destruction in early Mesozoic. Biotite adamellite which age in 150Ma formed by magmatism in late Jurassic. It is the product of transformation effect of the North China Craton.4.Magma sources and their nature of granitic rocks in Taili areaAccording to zircon Hf isotopic study, the original magma of granitic gneiss is from lower crust which formed in 2800-3000Ma.The original magma of porphyritic granitic gneiss is from new lower crust which formed in 1800-2100Ma. Biotite adamellite is related with partial melting of the crust which model age of Hf is 2500-3000Ma. Three types of granitic rocks came from two types of lower crust which formed in different poriods of time. On the basis of zircon U-Pb isotopic chronology, Hf isotopic and rock geochemistry, we infer that from the late stage of the Archean to the early stage of the palaeoproterozoic, the region was in normal thickness of continental crust environment; From the palaeoproterozoic to the early stage of Mesozoic, continental crust was obvious thicken, it may be related with new crust that formed at 1800-2100Ma; The forming of Mesozoic granite(150Ma) reflect apparent thinning of continental crust.