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The Geochemical Character Of Potassic Alkali Rocks Zone In The Ailaoshan-Jinshajiang And Its Tectonic Implications

Posted on:2008-04-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q C LinFull Text:PDF
GTID:1100360215950810Subject:Structural geology
Abstract/Summary:PDF Full Text Request
The alkali rocks include potassic alkali rocks and natric alkali rocks. And in the 19 century alkali rocks are often associated with Pacific style natric alkali rocks and mediterranean style potassic alkaic rocks. They were usually generated in deep-large-fault zone and closely connected with basic/ultra basic rocks in space. Therefore, alkali-rock is of significance in tectonic petrology. The researching area that is Ailaoshan-Jinshajiang potassic alkalic rock zone lies in the southwest of China. The zone lies at the joint of Yangtze plate and Zhongmian plate in Sanjiang fold series and many rocks' borderlines. It is composed of Himalaya period potassic alkalic magmatic rock. The zone is 3700 kilometres. The rock belt are composed of malignite, nepheline gabbro, shonkinite, diopside-sienite, hornblende sienite, amphibole-monzonite,granodiorite-granophyre, kaligranite-porphyry, fraidronite, kersanton, precious garnet, olivine alkali-lamprophyre, alkalic-pyroxenolite, alkalic-basalt, latite and trachyte. And the lithofacies of the potassic rock zone include plutonic intrusion, hypabyssal intrusion and eruption.The lithologic characters about the rock zone include ultra basic rocks, basic rocks, neutral rocks and acidic rocks, but the characters of all rocks are alkali. The bigest character is that the content of kalium is more than the natrium. The rock forming mineral of all rocks has diopside, and the nepheline is in the ultra basic rocks and basic rocks. The characters of geochemistry are rich alkali. And at the same time the content of calcium is high. They are rich in alkali, high in Potassium, enriched in LILE, LREE and Pb, depleted in HFSE and have a small Sr, Nd, Pb isotope range. Their source is probably a kind of even metasomatic mantle. This is consistent with the fact that there is kimberlite containing flogopite in the deep-source enclosure of Cenozoic super-K alkali lava in the southern part of this rock zone.The Zircon SHRIMP dating results of 36.8Ma, 36.8Ma, 34.1Ma, 38.3Ma yielded about the rocks in the south belt. This result is in late Eocene and belongs to Himalaya period. The rock-controlling structure of this rockmass may form before 40 Ma, much earlier than 45Ma-the collision time. At the same time the Ar-Ar and K-Ar dating results of 44Ma-0.37Ma yielded about the rocks in the north belt. The time shows the evolvement history and time of volcanic action about the North Tibet. And it reflects that the magmatism time of Ailaoshan-Jinshajiang potassic alkalic rock zone from east is younger than west magmatism time. The time is from the middle of Eocene to Miocene. The durative time of magmatism is 40Ma. This phenomenon is consistent with that about the collision between Europe slab and Asia slab. It hints that the cause of formation is inherent contact with the collision. Studying the information about geological structure, physical geography, mineral petrology, geochemistry, mineral and chronology, we know that the magma of this rock zone is originated from the partly melting of relatively shallower spinel mantle.
Keywords/Search Tags:tectonic implication, alkali-rock, SHRIMP U-Pb dating, Ar-Ar dating, geochemistry, Ailaoshan-Jinshajiang potassic alkalic rocks zone, Chinese
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