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Geochronology And Geochemistry Of Cretaceous Mafic Volcanic Rocks From Zhejiang And Fujian Provinces, SE China: Petrogenesis And Geodynamic Implications For Lithospheric Extension

Posted on:2008-01-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:S C QinFull Text:PDF
GTID:1100360215950799Subject:Structural geology
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Systematic Ar-Ar dating and petrological and geochemical (major, trace element and Sr-Nd isotope compositions) investigations on Cretaceous mafic volcanic rocks and relative diabases in the Zhejiang and Fujian Provinces, SE China, have been presented in this paper, with main focuses on the magma origins (source characteristics and magmatic evolution processes) and their geodynamic implications for the extensional tectonics in the region. The major conclusions are summarized below:(1) Ar-Ar dating results of nine whole rocks show that mafic volcanic rocks from these two provinces were erupted during 114-109 Ma, in accordance with the peak erupting age of 113-100 Ma reported by previous work. Along the Jiangshan-shaoxing lithosphere-scale fault occurred alkali olivine basalts of 99 Ma. Diabases from Wencheng and Yongtai that intruded the Cretaceous volcanic rocks spanned an emplacement age of 94-87 Ma, representing the latest magmatic pulses in Mesozoic time.(2) On the basis of trace element patterns and Sr-Nd isotope compositions, the Cretaceous mafic volcanic rocks and diabases can be subdivided into three groups:①the mafic lavas from Wencheng and Yongtai, partly from Pujiang which show arc-like trace element features, e.g., highly large ion lithophile element (LILE, such as Ba, Rb, K and Sr) and light REE enrichment with Nb-Ta-Ti depletion. These rocks have highly radiogenic Sr (87Sr/86Sr(i) = 0.7066 0.7100 ) and non-radiogenic Nd (εNd(t) = -8.4~-3.7 ) compositions. (2) The alkali olivine basalts, which show oceanic island basalt (OIB)-type trace element characteristics, i.e., enriched in incompatible elements with no Nb-Ta depletion as well as highly fractionated LREE/HREE patterns. These basalts have weakly radiogenic Sr (87Sr/86Sr(i) = 0.7049~0.7050 ) and highly radiogenic Nd (εNd(t) =+4.4~+5.8) isotope com- positions.③Mafic rocks show transitional elemental and Sr-Nd isotope features between the above two types. These rocks include those samples collected from Yongding, Lishui, and a few from Pujiang, which generally exhibit LILE and LREE enrichment with weak-moderate Nb-Ta depletion and have moderately radiogenic Sr (87Sr/86Sr(i) = 0.7055~0.7073) and variably radiogenic Nd (εNd(t) =-5.4~+3.6 ) compositions. Combined elemental and Sr-Nd isotope data indicate that all three types of Cretaceous mafic magmas were produced by melting of mantle reservoirs with different proportional mixing between the convective asthenosphere and the underlying EM2-type subcontinental lithospheric mantle (SCLM) under an extensional regime. The source for Group 1 contains the highest ratio of the SCLM with the least contribution from the asthenosphere, whereas that for the second group comprises predominantly the convective asthenosphere (>90%). The third group of mafic rocks have the transitional geochemical features between the former two groups, its melting source also contains a proportion of SCLM in-between the other two groups.(3) Adakitic andesites from the Laocun Formation are characterized by enrichment in Al2O3, Na2O, and Sr, and depletion in Y and HREE (e.g. Yb), low 87Sr/86Sr(i) (= 0.7041~0.7044) and highε)(Nd)(t) (= +0.2~+2.2). Such features are similar to adakites in modern subduction settings. On the basis of the absence of coeval ridge subduction and the extensional regime in the region, we prefer to relate the petrogenesis of these adakitic andesites to remelting of Mesozoic juvenile crust.(4) The geochemical features of the mafic rocks are spatially correlated with the distribution of major faults. Mafic rocks having arc-type trace element features generally occur far from the lithosphere-scale faults, whereas the Jiangshan OIB-type basalts are just located on the Jiangshan-Shaoxing lithosphere-scale fault—a boundary being considered to separate the Yangtze from the Cathysian Block. The Group 3 rocks that have transitional geochemical features between the Group 1 and 2, are distributed along the major fault (e.g., the Zhenghe-Dafu fault). Such a correlation favors a progressive extensional model in response to lithospheric extension-thinning process. From 114 Ma, the beginning of lithosphere attenuation created the Group 1 lavas, while the occurrence of asthenosphere-derived OIB-type magma at 99 Ma signified the peak stage of lithospheric thinning, and the intrusion of 94-87 Ma diabases corresponded to the end of lithospheric thinning through the Mesozoic time.(5) Considering the Indo-Sinian intracontinental crustal deformation in SE China, and the following development of metamorphic core complexes and a series of rift basins and eruption of calc-alkaline mafic magmas, the general aspects of the southeastern offshore areas are analogues to those observed in the basin and range province in northwestern USA. Furthermore, the temporal consistence of extensive calc-alkaline magmatic activity throughout the eastern China continent (from NE China to SE China) suggests a uniform extensional regime. The Cretaceous mafic magmas in SE China were products of asthenosphere-lithosphere interaction caused by progressive lithospheric thinning process. This process was resulted from Indosinian post-orogenic collapse and amplified by resultant effects from surrounding plate interactions, including the rapid northward movement of the Palaeo-Pacific Ocean, compressional forces from the Tethyan tectonic belt and possibly the Indo-China Block.
Keywords/Search Tags:mafic volcanic rocks, Cretaceous, geochemical features, geodynamics, Zhejiang and Fujian, southestern China
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