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Petrogenesis Of Mesozoic Intrusive Rocks In Northwest Fujian And South Jiangxi Province And Their Geodynamic Implications

Posted on:2017-01-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:G C WangFull Text:PDF
GTID:1220330485965712Subject:Mineralogy, petrology, ore deposits
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Origin and evolution of the Mesozoic magmatism of the Southeast China is a topic of international interest. It is now generally accepted that Early Mesozoic magmatism was related to the Tethyan tectonic regime and the Late Mesozoic magmatism occurred under the Pacific tectonic domain. However, the transition time of this two tectonic regimes and the style and initial time of subduction of the Paleo-Pacific plate remain controversial. Despite intensive research on the Mesozoic magmatism in SE China, there are yet some mafic and granitic plutons lacking detailed geochronologic and geochemical data, which would limit further understanding of the origin and evlution and the geodynamic mechanism of the regional magmatism.During the Mesozoic time, the magmatism widely occurred in northwest Fujian province and south Jiangxi province, including many granitic plutons. However, the granitic plutons in northwest Fujian province are rarely studied. In particular, the Early Mesozoic mafic pluton in northwest Fujian province is identified for the first time in this study. Furthermore, the Early Jurassic mafic pluton and A type granites in the south Jiangxi province are also identified in this study. The study on the origin and tectonic background of these plutons are vital importance to further understanding the basic principles of magma evolution and the geodynamic mechanism of the Mesozoic magmatism in SE China.This paper reports systematically petrological and SHRIMP or LA-ICP-MS zircon U-Pb dating, whole-rock geochemical and Sr-Nd-Hf isotopic research for five plutons in northwest area of Fujian province (Guangze, Zhima, Waitun, Shipi and Dayuan) and four granitic and mafic plutons in south Jiangxi province (Chenglong, Chebu, Zhulanbu and Keshubei). We discussed the magmatic evolution, source characteristics, tectonic and geodynamic settings of these plutons. All these results was aim to provide mew model for the long-debated tectonic evolution of Mesozoic magmatism in SE China. Our study shows that:1. The Zhima pluton in northwest Fujian province was emplaced in the Late Triassic Carnian (-228 Ma), composed of monzogabbro and quartz monzonite. Zhima mafic rocks show geochemical characteristics of shoshonitic series. They are enrich in alkalis, have high K2O contents and K2O/Na2O ratios(>0.5), and are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). They have initial 87Sr/86Sr ratios of 0.7078-0.7093 and εNd(t) of -4.9 to -9.5. These geochemical features are similar to the shoshonitic magma occurs in post-collisional arc tectonic setting instead of continental arc or inter-continental tectonic settings. Elemental and isotopic data suggest that Zhima mafic rocks formed in a post-collisional regime of the Tethyan tectonic domain and derived from the partial melting of the previous enriched metasomatic mantle trigged by the upwelling of the asthenosphere.2. The Chenglong and Chebu plutons in the south Jiangxi province were emplaced in the Early Jurassic Sinemurian (~197-191Ma). They consist of hornbende nortite and horn-blende gabbronorite, respectively. Both of them are sodic (K2O/Na2O<0.5). However, the chenglong mafic rocks belong to alkaline series, show positive Ta-Nb anomalies, and have relatively depleted Sr-Nd isotopic compositions (they have initial 87Sr/86Sr ratios of 0.7043-0.7047 and 8Nd(t) of 4.5 to 5.2.). Whereas the Chebu mafic rocks belong to calc-alkaline series, show negative Ta-Nb anomalies, And have relatively enriched Sr-Nd isotopic compositions (they have initial 87Sr/86Sr ratios of 0.7064-0.7065 and d(t) of -0.7 to 0.6.). Elemental and isotopic data suggest that the Chenglong and Chebu mafic rocks were formed in an oceanic-slab subduction setting coupled with subduction of the Palaeo-Pacific plate. These sodic magma were derived from partial melting of asthenospheric mantle with added sedimental-derived hydrous fluid. The Chebu mafic rocks show a large proportion of slab-derived components in their source while the Chenglong mafic rocks show less proportion of slab-derived components.3. The Guangze in the northwest Fujian province (~189Ma) and Zhulanbu (~188Ma) and Keshubei plutons (~189Ma) in the south Jiangxi province were emplaced in the early Jurassic Pliensbachian. They are composed of syenogranites and alkali-feldspar granites. Their geologic and geochemical features strongly suggest an A-type granite affinity. In petrography, the dark-colored mineral (e.g. biotites) are commonly anhedral and interstitial to both feldspar and quartz. In geochemistry, they are enriched in alkalis and potassium, REEs, depleted in Eu, Ba, and Sr, and show high Ga/Al ratios (10000*Ga/A1>2.6) and Zr+Nb+Ce+Y> 350 ppm. Thses granites occurred as a NNE-trend and constituted the Early Jurassic A-type granite belt in SE China. They show initial 87Sr/86Sr ratios of 0.7070-0.7283 and εNd(t) of -10.2 to 0.1. Our study results suggest that these granites were most likely formed in an extensional regime caused by slab break-off and derived by partial melting of Precambrian metaigneous and metasedimentary rocks with different ratios.4. The Waitun pluton in the northwest Fujian province is an intrusive complex consist of the Middle Jurassic Bathonian (~168Ma) monzogranite and alkali-feldspar granite and the Late Jurassic Oxfordian (~161Ma) granodiorite and monzogranite. The Shipi pluton is also an intrusive complex, which are comprised by the Late Jurassic Kimmeridgian (~156Ma) alkali-feldspar granite and monzogranite and the Early Cretaceous Albian (~109 Ma) granodiorite and monzogranite. The Dayuan pluton was emplaced in the Early Cretaceous Valanginian (~134Ma), composed of alkali-feldspar granites. All these granites are composed of high-K calc-alkaline I-type granites with a great diversity in elemental and isotopic compositions. The granites have SiO2 contents of 68.3-78.5 wt.%, showing a gradual decrease in initial 87Sr/86Sr ratios (0.7181 to 0.7091) and increase in εNd(t) (-16.7 to -8.1) and εHf(t)(in-stu zircon) (-20.6 to -6.9) with decreasing emplacement ages, suggesting multiple origins. Geochemical data suggest that the Middle Jurassic (~168 Ma) Waitun granites are of purely crustal origin, derived by partial melting of a mixed source of Paleoproterozoic metaigneous (~78%) and metasedimentary (~22%) rocks at a depth of 30-40 km triggered by underplating of basaltic magma. Mixing of such crustal melts with about 10% basaltic magma could account for the origin of the Late Jurassic (~161 Ma) Waitun granites. The Late Jurassic Shipi and Early Cretaceous Dayuan granites belong to the high fractionated I-type granites. They were produced by extensive fractional crystallization of primary crustal melts, the source of which consist of metaigneous rocks (~82%) and metasedimentary rocks (~18%) at a depth of 30-40 km. The Early Cretaceous (~109 Ma) Shipi granites were generated by partial melting of a mixed source of Paleoproterozoic metaigneous (~92%) and metasedimentary (-8%) rocks at a depth of~30 km plus additional (-15%) input from coeval basaltic magma. Our study also suggests that these granites were formed in a continental arc setting induced by northwestward subduction of the Paleo-Pacific plate. Progressive slab rollback since the Middle Jurassic from inland to the coastal area is the most likely mechanism for the generation of the granites.5. In this study, we also identified other four A-type granite belts in SE China by integrating the published data except for the abovementioned the Early Jurassic A-type granite belt. They are the Late Triassic, Late Jurassic, Early Cretaceous, and Late Cretaceous A-type granite belts. Late Triassic (229-221 Ma) A-type granites occur as an ENE-trending belt, Late Jurassic (163-153 Ma) occur as an NEE-trending belt and located further inland. Early Cretaceous (136-124 Ma) A-type granites occur as an NEE-trending belt and almost overlaps the Early Jurassic A-type granites belt. Late Cretaceous (101-91 Ma) A-type granite belt occurs NEE-trending in the coastal area of SE China. Integrating these studies, we suggest that the Early Mesozoic magmatism in SE China belong to the Tethyan tectonic domain. The emplacement of Late Triassic Zhima mafic rocks were coincident with the forming of Late Triassic A-type granites belt, suggesting that the tectonic background of SE China have switched to post-collisional tectonic regime in Late Triassic Carnian. Whereas the late Mesozoic magmatism in SE China is belong to the pacific tectonic domain. The origin and evolution of magmatism could attribute to the repeated slab-advance-retreat model of Palaeo- Pacific plate.
Keywords/Search Tags:Granite, Mafic rocks, Late Mesozoic Magmatism, Paleo-Pacific plate, Slab advance-retreat, SE China
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