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The Evolvement Of The Minerals In Bengbu Granite:Contrains On The Formation Mechnism Of The Jurassic Granite In The North China Craton

Posted on:2014-07-11Degree:DoctorType:Dissertation
Country:ChinaCandidate:L J XuFull Text:PDF
GTID:1260330395489284Subject:Institute of Geochemistry
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In this study, we analysed the Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton which can be subdivided into three types:(ⅰ) the host granite,(ⅱ) mafic biotite and garnet rich enclave, and (ⅲ) aplite. The Sr-Nd-O-Li isotope for whole rock, the major-, trace-element, oxygen isotope for garnet, the major-, trace-element for allanite, the major element for white mica, feldspar and the zircon U-Pb dating in different rocks were analysed in our study.1)In-situ U-Pb dating and trace element analyses on zircons from different garnet-bearing samples show inherited cores and fine-scale oscillatory zoned magmatic rims. The dark-luminescent magmatic rims all have Jurassic concordia ages (~160Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of210to236Ma and occasionally neoproterozoic ages. The age data suggest that the different types of garnet, allanite, white mica may be genetically related and modified by cogenetic magmatic events. Based on the major element, trace elelemnt especially the majore element for the whole rock, the Jingshan granite is leucogranite. The Triassic and Neoproterozoic inherited zircon and the Sr-Nd-O isotope for whole rock suggested that the source of the Jingshan granite is the Dabie-Sulu gneiss, which is the subducted continental crust of the North China Craton. The Li isotope of the whole rock proved that the source of the Jingshan granite maybe from Su-Lu or the south Dabie gneiss in advance.2) Base on the major element, trace element, oxygen isotope and zoning textures of the three kinds of garnet, we suggest peritectic garnets and magmatic garnets were found in the granite. Combined the Jurassic concordia ages in a zircon inclusion in the garnet from the mafic enclaves and the similar minerals association with the host granite, we suggest that the mafic enclaves are restites which were fromed after the melt abstraction during the process of the minerals dehydration melting. This also imply that the Grt Ⅰ is peritectic garnet. Relative to Grt Ⅰ, Grt Ⅱ has higher MREE and HREE content, and Grt Ⅲ is enriched in Mn, but depleted in Ca. Grt Ⅲ has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ18O values of<3.8‰in most of Grt Ⅰ,3.8to4.7‰for most Grt Ⅱ (for inclusion-free garnets), and typically>4.7‰for Grt Ⅲ. Some of the Grt Ⅱ and Grt Ⅲ display two distinct zonings with cores having similar major and trace element compositions to Grt Ⅰ. The magmatic garnet is probably the results of magmatic dissolution-precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite.3) Based on the ramman shift and major element data for the white mica and the major, trace element analysis for the allanite in Jingshan granite, different gneiss of allanites and white micas were also found in granite and restite-maybe peritectic and magmatic minerals. The peritectic white micas in the restite are metamorphic phengite. The formation pressure for the granite is1.0-1.3Gpa based on the Si atomic number, which implied that the exhumation depth for the subducted South China Craton to the North China Craton is30-40Km at least, and represent the middle and lower crust level of the North China Craton.4) The peritectic minerals are not crstallizied from the mgma directly. The percent of the peritectic minerals in the granite are less than10%, thus the melt content is higher than90%. The magma of the Jingshan granite is magmatic flow, although it has the lineation of the sub magmatic flow.5) We also analysed the zircons in different rocks from the contact boundary of the Jingshan intrusion. Based on the petrology and the zircon U-Pb data, the restite of the Jingshan granite is found in the contact boundary. The Jurassic magmatic zircon rims and Paleoproterozoic inherited zircon cores are found in the granitic dykes from the contact boundary of the Jingshan intrusion. However, the granite manily exists along the gap of the Paleoproterozoic amphibolite. Thus, the Paleoproterozoic inherited zircon may be captured by the granite as it ascent, and the Paleoproterozoic amphibolite was not melted. The2.7-2.8Ga magmatic zircon cores were found in the amphibolite from Bengbu uplift, and the1.8Ga metamorphic zircon rims in the amphibolite implied that it underwent HP granulite facies metamorphism at c.1.8Ga.
Keywords/Search Tags:granite, isotope geochemistry, peritectic minerals, garnet, zirconchronology
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